CHAPTER 21 WALSH ET AL. 21.1

Chapter 21

100 Years of Progress in Polar Meteorology

JOHN E. WALSH International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, Alaska

DAVID H. BROMWICH Byrd Polar and Climate Research Center, The Ohio State University, Columbus, Ohio

JAMES.E.OVERLAND NOAA Pacific Marine Environmental Laboratory, Seattle, Washington

MARK C. SERREZE National Snow and Ice Data Center, University of Colorado Boulder, Boulder, Colorado

KEVIN R. WOOD Joint Institute for the Study of the Atmosphere and Oceans, University of Washington, Seattle, Washington

ABSTRACT

The polar regions present several unique challenges to meteorology, including remoteness and a harsh en- vironment. We summarize the evolution of polar meteorology in both hemispheres, beginning with measure- ments made during early expeditions and concluding with the recent decades in which polar meteorology has been central to global challenges such as the ozone hole, weather prediction, and climate change. Whereas the 1800s and early 1900s provided data from expeditions and only a few subarctic stations, the past 100 years have seen great advances in the observational network and corresponding understanding of the meteorology of the polar regions. For example, a persistent view in the early twentieth century was of an Arctic Ocean dominated by a permanent high pressure cell, a glacial anticyclone. With increased observations, by the 1950s it became apparent that, while anticyclones are a common feature of the Arctic circulation, cyclones are frequent and may be found anywhere in the Arctic. Technology has benefited polar meteorology through advances in in- strumentation, especially autonomously operated instruments. Moreover, satellite remote sensing and com- puter models revolutionized polar meteorology. We highlight the four International Polar Years and several high-latitude field programs of recent decades. We also note outstanding challenges, which include un- derstanding of the role of the Arctic in variations of midlatitude weather and climate, the ability to model surface energy exchanges over a changing Arctic Ocean, assessments of ongoing and future trends in extreme events in polar regions, and the role of internal variability in multiyear-to-decadal variations of polar climate.

1. Introduction an acceleration of observations and understanding of polar meteorology. In addition to the establishment of The development of polar meteorology has faced new observing stations, technology has benefitted polar unique challenges, including the remoteness and the harsh meteorology through advances in instrumentation, es- environments of the Arctic and Antarctic. Whereas the pecially autonomously operated instruments. Moreover, 1800s and early 1900s provided data from expeditions and spatial coverage from satellites and computer models rev- only a few subarctic stations, the past 100 years have seen olutionized polar meteorology, which has emerged over the past half century as a widely recognized subdiscipline Corresponding author: John E. Walsh, [email protected] of atmospheric and climate science. In this review, we

DOI: 10.1175/AMSMONOGRAPHS-D-18-0003.1 Ó 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). 21.2 METEOROLOGICAL MONOGRAPHS VOLUME 59 summarize the evolution of polar meteorology in both The ill-fated 1879 expedition of the USS Jeannette, hemispheres, beginning with measurements made dur- which set out to reach the North Pole by following a hy- ing early expeditions and concluding with the recent pothetical ‘‘thermometric gateway’’ through Bering Strait decades in which polar meteorology has been central to an open polar sea (Bent 1872; Hayes 1867), was perhaps to global challenges such as the Antarctic ozone hole, the last to be motivated in large part by speculative geo- weather prediction, and climate change. graphical notions about the Arctic, including the possi- bility of an ice-free polar ocean. Today the Jeannette expedition, or more directly the part of its wreck that 2. Review of the pre-1919 period (before the turned up years later in Greenland, is known as an in- establishment of the American Meteorological spiration for Fridtjof Nansen’s attempt to drift with the Society) sea ice across the North Pole in the Fram (Nansen 1898). The development of polar meteorology in the nine- While expeditions like those carried out with the Jeannette teenth century is inextricably linked to the engines of and the Fram certainly pressed the frontier of discovery— commerce, territorial expansion, and geographic explo- often at a high cost—and produced extremely valuable ration. From an American perspective, these begin with results, the underlying story of scientific progress is per- the U.S. South Seas Exploring Expedition (also known as haps best revealed in the sustained, even routine, work to the Wilkes Expedition, or simply the U.S. Ex. Ex.) during measure, describe, and map the lands and oceans, their 1838–42 (Wilkes 1845a,b), followed by the lesser-known resources, and the weather and climate. Innovation was a U.S. North Pacific Exploring and Surveying Expedition key factor from the beginning, as new tools for observing (Ringgold–Rodgers Expedition) of 1853–56 (Ringgold the deep sea and the upper atmosphere were constantly and Rodgers 1950; U.S. National Archives 1964), both being developed, along with more capable ships (and U.S. Navy expeditions. The U.S. Navy, often with private later aircraft) for operation in harsh polar conditions. The support, contributed to the search for the missing British value to science of the vast archive of data that was dili- expedition of Sir John Franklin in the Arctic islands north gently collected by hundreds of people over these years is of Canada, and to a number of other early explorations still being realized. along the west coast of Greenland. These efforts added a. Early investigations of weather and ice to early knowledge of Arctic meteorology, mainly by providing observations (e.g., Kane 1854; Kane and Schot The earliest sustained and systematic investigations of 1859; Tyson and Howgate 1879; Bessels 1876)forcom- the weather, climate, and oceanography of the Arctic by parison with modern observational data and also de- the United States came with the Alaska Purchase (https:// scriptions of the atmospheric (as well as ice and ocean) www.loc.gov/rr/program/bib/ourdocs/Alaska.html). The phenomena they encountered. Steep inversions and as- U.S. Coast Survey and the Revenue Cutter Service (pre- sociated mirages, ice fog, sea ice ridges and leads, and decessor of the U.S. Coast Guard) began with an initial floating ice islands are examples. The first documented reconnaissance in 1867 with a view toward collecting in- measurements of surface-based inversions were actually formation necessary for the production of navigational made by measuring temperatures from the ‘‘crow’s nest’’ charts and for the Coast Pilot of Alaska (U.S. Coast Survey at 32 m above sea level on Nansen’s Fram expedition 1869). Starting in 1880 the Revenue Cutter Service/Coast (Palo et al. 2017). The Army Signal Service, the Coast Guard made annual summer cruises to the northern Be- Survey, and the Smithsonian Institution frequently sup- ring and Chukchi Seas, and in the process collected a ported observers, supplied meteorological instruments, nearly unbroken series of marine-meteorological and sea and provided expert data reduction and publishing as- ice observations that extends to the present day (Fig. 21-1). sistance to these endeavors (e.g., Abbe 1893). The U.S. Navy’s Bureau of Navigation also issued The Wilkes Expedition reached Antarctica, but there Findlay’s (1869) Directory for the Behring’s Sea and Coast were no follow-up scientific expeditions to this region of Alaska, a compendium of previously published infor- organized in the United States until the first of the Byrd mation about the region, including a review of weather expeditions in 1928 (Riffenburgh 2006). In the Arctic, and sea ice conditions in the Arctic recorded by earlier however, the development of the whaling industry in the explorers over the previous 100 years, back to Cook and Chukchi and Beaufort Seas beginning in 1848, the pur- Clerke’s voyages in 1778 and 1779 (Beaglehole 1967). chase of Alaska from Russia in 1867, and the rise of At the same time, the U.S. Army garrisoned Sitka collaborative scientific exploration of the polar regions (New Archangelsk) and a few other former Russian as demonstrated by the landmark first International outposts, forming the beginning of the station network in Polar Year (IPY; 1881–84), provided steady impetus for Alaska that would later be developed by the Army Signal exploration and research in the far north. Service (as the first official weather agency, then known as CHAPTER 21 WALSH ET AL. 21.3

FIG. 21-1. The USCGC (Coast Guard Cutter) Bear moored to sea ice in 1918. The Bear was initially purchased by the Navy for the Greely Relief Expedition in 1884 (Schley 1887) and subsequently served with the Revenue Cutter Service/Coast Guard in Alaska until 1928, then on Admiral Byrd’s expeditions to Antarctica from 1933 to 1940, and finally with the Navy on the Greenland Patrol during World War II. It was decommissioned for the last time in 1944. (The photograph was provided by the Coast Guard Museum Northwest in Seattle, Washington.) the Division of Telegrams and Reports for the Benefit of Adjacent Waters, published as Appendix 16 of the Annual Commerce and Agriculture). A break in operations oc- Report of the Superintendent of the U.S. Coast and Geo- curred in 1886, and all Signal Service work in Alaska was detic Survey (Dall 1882). Both are exhaustive examinations abandoned the following year (Henry 1898). In 1890 the of the data available from earlier times, especially from meteorological duties of the Signal Service were trans- RussianandBritishsourcesdatingbacktothe1820s,and ferred to the U.S. Weather Bureau, newly organized as a included new observations collected by the Coast Survey, civilian agency within the U.S. Department of Agriculture. the Medical Department of the Army, and the Signal Ser- The Weather Bureau began to rebuild the Alaska station vice. Information was also compiled from whaling ship network in the late 1890s, with coverage of the coasts of captains and other sources, both published and from origi- Alaska beginning to fill in by 1920, marked by the rees- nal logbooks. Dall assembled and published in Coast Pilot tablishment of a station at Point Barrow (Weather Bureau of Alaska, Appendix I: Meteorology, a bibliography and list 1925), initially occupied by the Signal Service for the first of charts containing more than 4000 titles. IPY in 1881. The development of the station network For Coast Pilot of Alaska: Meteorology (1879), Dall between 1867 and 1921 is shown in Fig. 21-2. Observations produced the first set of synoptic-scale charts of mean from these stations have become an important part of the annual and monthly barometric pressure for the Pacific record used to understand long-term climate trends—in- Arctic region, which provided a reasonable character- sights that depend on data ‘‘since record-keeping began.’’ ization of the Aleutian low. Dall (1882) notes: The first thorough synthesis studies of the meteorol- The most striking feature presented by the curves of ogy and oceanography of the Pacific Arctic to be pro- mean annual pressure is a region of depressed barometer, duced in the nineteenth century were made by William extending from Unimak Pass to Kadiak [Kodiak] Island, over Dall of the U.S. Coast Survey. These were Coast Pilot of which area, so far as the material permits of generalization, Alaska, Appendix I: Meteorology (Dall 1879)andReport a mean pressure is exerted of only 29.65 inches. This area on the Currents and Temperatures of Bering Sea and the of depression, which I shall term the Kadiak area, was first 21.4 METEOROLOGICAL MONOGRAPHS VOLUME 59

FIG. 21-2. The meteorological station network developed by the U.S. Army Signal Service and the Weather Bureau in Alaska, 1867– 1921. The IPY stations at Fort Conger, on Ellesmere Island, and at Fort Chimo (Kuujjuaq, Nunavit) are also included. The IPY period is marked by gray lines. The collapse of the Signal Service network in 1887 is apparent. CHAPTER 21 WALSH ET AL. 21.5

FIG. 21-3. Ferrel’s map in Meteorological Researches for the Use of the Coast Pilot (Ferrel 1875) ‘‘showing by isobaric lines the mean pressure of the atmosphere for January in millimeters, reduced to the gravity of the parallel of 458, and by arrows the prevailing directions of the wind, for the Northern Hemisphere.’’ Although the center of action in the Pacific (Aleutian low) is placed too far north, as his colleague Dall noted, the resemblance to modern maps is unmistakable (see, e.g., Hurrell et al. 2003, their Figs. 1 and 2).

indicated by Mr. Ferrel (1875), but from incompleteness of of work by Teisserenc de Bort (1883), Exner (1913), data in his possession, it was located somewhat too far north. Walker (1923), and others. Figure 21-3 shows the Northern Hemisphere sea level pressure and prevailing winds for Ferrel, at the time with the Coast Survey and subse- January from his analysis. Dall’s (1879) regional map for quently with the Signal Service, outlined the general cir- thesamemonth(Fig. 21-4, top panel) shows a more ac- culation of the atmosphere based on physical principles curate placement of the Aleutian low based on station data (Abbe 1892), including the Coriolis force, well in advance that were unavailable to Ferrel, and it provides an example 21.6 METEOROLOGICAL MONOGRAPHS VOLUME 59

FIG. 21-4. (top) Dall’s (1879) regional map of barometric pressure in January showing a split Aleutian low (referred to by Dall as the Kadiak area in general, with the Kamchatka area appearing in the case of split development). Dall recognized that the lack of data from the western Aleutians left this question ambiguous, but today it is seen to be the correct interpretation (e.g., Rodionov et al. 2005). (bottom) Dall’s (1879) map of summer sea surface isotherms and main ocean currents. The average extent of sea ice in summer is also shown and is generally consistent with what is known about ice distribution in the early satellite era and before (e.g., Danske Meteorologiske Institut 1900–1939, 1946–1956; U.S. Hydrographic Office 1946). of the characteristic west–east split of the Aleutian low. Dall also documented general outlines of other im- Simultaneous international observations supported this portant features of the regional climate in the areas of interpretation (e.g., Bulletin of International Meteorological meteorology, oceanography, and biology. These include Observations, 1875–87, from the U.S. Army Signal Office). mean annual and monthly air temperature patterns and It is now understood that, in winter, the positions of the one prevailing winds; ocean currents and sea surface tem- versus two centers of the Aleutian low are more important peratures; the summer distribution of sea ice, winds, and with respect to influence on the Bering Sea environment temperatures over boreal and tundra regions (Fig. 21-4, than its central pressure (e.g., Rodionov et al. 2005). bottom panel); and associated plants and animals. The CHAPTER 21 WALSH ET AL. 21.7

FIG. 21-5. (top) Map of the Bering Strait region showing surface isotherms and sea ice observed by the U.S. Coast Survey schooner Yukon in August/September 1880, and (bottom) the hydrographic section obtained on 5 September 1880 (Dall 1882). oceanography of the Bering Sea is dealt with in more Jeannette would have followed into the Arctic. At the detail in Dall’s subsequent work. same time, the USRC (Revenue Cutter) Corwin was In his Report on the Currents and Temperatures of Be- searching the area around Wrangel Island for signs of the ring Sea and the Adjacent Waters, Dall (1882) turned his missing ship, last seen the previous September in the ice attention to questions that are still relevant today. What near Herald Island (Hooper 1881). Unbeknownst to both ocean currents pass between the Pacific Ocean into the Dall and Captain Hooper of the Corwin, Commander De Bering Sea and thence into the Arctic by way of Bering Long and the officers of the Jeannette had already ex- Strait, or from the Arctic to the south? What are the ploded two of the prevailing myths that inspired their temperatures of these currents and what effect do they expedition: there was no such thing as a thermometric have on the climate, including the distribution of sea ice? gateway, and Wrangel Land was an island and not a large As he did in his work on meteorology for the Coast Pilot, landmass extending across the Arctic (De Long 1884). Dall scoured the literature (and primary sources) from Dall’s hydrographic transect, combined with the gen- around the world for data and collected new oceano- eral survey of the region, yielded a number of particular graphic observations as well in his role as assistant-in- insights. He found that the current through the Bering charge of the Coast Survey vessels Yukon and Humbolt. Strait is mainly to the north, although reversible by the 2 Of particular note is the hydrographic transect of the wind, and that the northward flow is around 1 ft s 1— 2 Bering Strait completed in 1880, likely the first ever ob- corresponding to a total flow of 42 289 425 ft3 s 1 (1.2 Sv; 2 tained (Fig. 21-5). In part, the motivation for the transect 1Sv[ 106 m3 s 1), which corresponds well to modern was to test the hypothesis that a branch of the warm Kuro measurements (e.g., Woodgate et al. 2005). The tempera- Siwo (Kuroshio) passed through Bering Strait, creating ture structure resolved by the Yukon transect in September a ‘‘thermometric gateway’’ (Bent 1872) that the USS shows the warm Alaska Coastal Current (ACC) on the 21.8 METEOROLOGICAL MONOGRAPHS VOLUME 59 eastern side of the strait and the cold Siberian Coastal at Lady Franklin Bay, Ellesmere Island, and another at Current (Weingartner et al. 1999)onthewesternside.The Point Barrow, Alaska. Lieutenant Adolphus W. Greely presence of sea ice at East Cape and southward seems (an early member of the American Meteorological So- unusual when compared with recent data, but this was ciety) took command of the former expedition, and once a common occurrence (e.g., Danske Meteorologiske Lieutenant Patrick Henry Ray commanded the latter. Institut 1900–1939, 1946–1956). Otherwise the temperature The results of the first IPY were mixed. Lieutenant range found by Dall is fairly typical. As to the source of Greely’s expedition to Lady Franklin Bay was marred ocean heat present in the region, Dall observed that it was by the loss of all but seven members to deprivation and primarily due to local solar radiation rather than to heat other causes. Abbe (1893) stated that transported into the area from the Pacific Ocean as sug- the large volumes and results of the two Signal Service in- gested by Bent (1872), a result consistent with the recent ternational polar stations, as well as the work of the Polaris findings by Timmermans et al. (2018). and Florence expeditions, have contributed not a little to advance our knowledge of the immense country lying to the b. The first International Polar Year north of the United States; in fact, the great importance of this work becomes more and more evident as other gov- The first IPY is notable as the first attempt to extend ernments publish their own contributions to this year of a wide meteorological network into the Arctic, and cooperative research, and thus enable us to take a compre- to collect simultaneous observations with similar well- hensive survey of the atmospheric conditions at that time. calibrated instruments and methods. The first IPY was inspired by the Austro-Hungarian naval officer and sci- The full publication of the synchronous observations entist Karl Weyprecht (Wood and Overland 2006). The unfortunately took 25 years—it was not completed until idea for a coordinated international expedition arose from 1910, and the data were never analyzed all together as his experience as co-commander of the Austro-Hungarian Weyprecht had envisioned. Polar Expedition of 1872–74. After returning home, he The meteorological observations of the first IPY were reflected on the value of the thousands of meteorological recently transcribed, digitized, and assimilated by modern measurements made during the expedition and noted: retrospective analysis (reanalysis) systems (e.g., Compo et al. 2011) and in this sense have finally fulfilled their in- But whatever interest all these observations may possess, tended purpose (Wood and Overland 2006). The greater they do not possess that scientific value, even supported by legacy of the first IPY may be that its successful demon- a long column of figures, which under other circumstances stration of international collaboration in polar science might have been the case. They only furnish us with a picture of the extreme effects of the forces of Nature in the carried on to three subsequent iterations: the second IPY Arctic regions, but they leave us completely in the dark of 1932–33, the International Geophysical Year (or third with respect to their causes (Weyprecht 1875). IPY) of 1957–58 (IGY), and the recent IPY of 2007–09. To answer that question, he understood that large-scale c. Arctic work of the Weather Bureau synchronous data collection was required just as it is now. Weyprecht’s address to a meeting of German naturalists The Alaska Section of the Weather Bureau was offi- and physicians in 1875 included an enduring assessment: cially started in 1898 with the establishment of the Climate ‘‘The entire meteorology of our day rests upon compari- and Crop Service and set up of a first-class weather station son. All the successes of which it can boast—the laws of at Sitka, under the direction of H. L. Ball (Ball 1898). From storms, the theories of winds—are the result of synchro- the end of the Signal Service years until the 1920s, much of nous observations’’ (Wood and Overland 2006). the meteorological data for the region was collected by The Second International Meteorological Congress, volunteer observers. Aside from the Sitka station, 10 new held in Rome in 1879, supported Weyprecht’s conception subsidiary stations were also expected to be operated by of a coordinated international polar research effort and volunteers. Henry (1898) also noted, ‘‘It is hoped that established a commission to put it into effect. It was to be, those to whom instruments have been issued from time to as Abbe (1893) described it, ‘‘a simultaneous invasion of time in previous years will also revive their interests and the polar regions from all sides.’’ International partici- report to [Ball].’’ Of 18 volunteer stations listed by Henry pation was invited, and in due course 11 nations estab- that were issued instruments by the Weather Bureau, the lished 14 polar research stations: 12 in the Arctic and two most successful were located at Coal Harbor (1889–1911) in the subantarctic. A number of auxiliary stations were and Killisnoo (1881–1910). Other efforts were not as suc- also established, including several in Alaska. Participa- cessful. Instruments sent to observers in the Northwest tion by the United States was the responsibility of the Territories (Canada) were seized, and in another case the Army Signal Service, which established two stations: one observer, a missionary, was murdered and the records CHAPTER 21 WALSH ET AL. 21.9 were lost. Further development by the Weather Bureau in because there was open water right to the coast, providing Alaska in the early twentieth century was spurred by easy access for their ship. The meteorological records economic development around the gold rush and the es- from 1912–13 revealed the most intense sustained wind tablishment of radio and cable communications (Jessup regime on Earth (Madigan 1929). The anemometer was 2007), as well as the increased need for aviation weather recalibrated because of doubts about the extreme condi- services beginning in the 1920s (see Encyclopedia Arctica, tions experienced, and it now appears that the revision 1947–51, https://collections.dartmouth.edu/arctica-beta/). was overly conservative. The uncorrected records reveal 2 The Weather Bureau’s further contributions to polar an annual average wind speed of 22 m s 1, with over 60% meteorology followed a similar pattern as in previous of all hourly wind speed reports falling in the range of 15– 2 years, although on very small scale. Between 1893 and 30 m s 1 (Parish and Walker 2006). The easy summer ac- 1902 Evelyn Briggs Baldwin, a Weather Bureau observer, cess to the coast was caused by the intense katabatic winds took part in three privately supported Arctic adventures: blowing the sea ice offshore to create coastal polynyas Peary’s North Greenland Expedition in 1893–94, the (Morales Maqueda et al. 2004), and therefore choosing this Second Wellman Expedition to Franz Josef Land in 1898– location turned out to be an unfortunate choice in retro- 99, and the Baldwin–Ziegler Arctic Expedition 1900–02. spect. A similar sequence of extreme katabatic wind events This would be the only polar activity directly related to the was experienced in 1912 by a satellite party of the Scott Weather Bureau until the 1920s (Encyclopedia Arctica, Antarctic Expedition at Terra Nova Bay (758S, 1658E) 1947–51, https://collections.dartmouth.edu/arctica-beta/). (Bromwich and Kurtz 1982; Bromwich et al. 1993). d. Early Antarctic observations e. A modern renaissance in historical climatology While efforts by the United States were focused on The advent of sparse-input reanalysis and reanalysis- the Arctic, important work in the Antarctic was being forced modeling and reconstruction techniques in recent carried out, especially by other nations. Major meteo- years has brought new interest in data that were col- rological studies in Antarctica commenced with two lected in the past but never integrated into modern historical expeditions. The first was in conjunction with large-scale datasets [e.g., the International Compre- Robert F. Scott’s attempt (1910–13) to be the first to hensive Ocean–Atmosphere Data Set (ICOADS); the reach the South (geographic) Pole. Scott’s Party peri- International Surface Pressure Databank (ISPD)]. A shed in 1912 on the Ross Ice Shelf after having arrived at surprisingly large amount of marine-meteorological and the Pole 1 month after Roald Amundsen. The role sea ice data collected in the polar regions by the U.S. played by weather in this tragedy remains controversial Navy, Revenue Cutter Service/Coast Guard, and other to this day (Solomon 2001; Fogt et al. 2017). Detailed federal vessels since the 1880s has never been extracted meteorological observations were collected during from primary sources and compiled. This deficit, how- 1911–12 at the base location of Cape Evans on Ross ever, is steadily being reduced through collaborative Island by George C. Simpson, who later became Di- data recovery projects organized under the Atmospheric rector General of the United Kingdom’s Meteorological Circulation Reconstructions over the Earth (ACRE) ini- Office. The reporting and analysis of the observations tiative (Allan et al. 2011) and with support from citizen- were delayed by World War I, but appeared in a series of scientists participating in Old Weather (http://www. volumes published in India (Simpson 1919, 1921, 1923). oldweather.org)andsimilarprojects(Freeman et al. 2016). Important was that the analysis suggested the origin of Of particular note in this regard are the sea ice ob- ‘‘pressure waves’’ in West Antarctica (Loewe 1967), servations collected in the nineteenth and early twenti- which became a prime motivation for the establishment eth century. Some of these data were used in a few early of Byrd Station (808S, 1208W) during the IGY (1957). studies (e.g., Page 1900; Simpson 1890), and from 1900 Although the observations have not been continuous, to 1939 as occasional contributions to the Danish Me- the early observations from the Byrd Station location teorological Institute’s annual publication State of the have enabled recent studies to demonstrate large annual IceinArcticSeas(Danske Meteorologiske Institut 1900– temperature increases since the IGY: 2.2861.38C from 1939, 1946–1956). This publication remains a primary 1958 to 2010 (Bromwich et al. 2013, 2014). source of sea ice data for the period in modern datasets, for The second expedition of major meteorological im- example, the Hadley Centre’s Sea Ice and Sea Surface portance was led by Douglas Mawson (the Australasian Temperature Dataset, version 2 (Titchner and Rayner Antarctic Expedition 1911–14), whose experiences were 2014; Walsh and Chapman 2001), and reanalyses that as- outlined in a well-known book entitled The Home of the similate ice information [e.g., the European Centre for Blizzard (Mawson 1915). In an ironic twist of events, Medium-Range Weather Forecasts (ECMWF) twentieth thepartycameashoreatCapeDenison(678S, 142.78E) century reanalysis (ERA-20C); Poli et al. 2016]. Reanalyses 21.10 METEOROLOGICAL MONOGRAPHS VOLUME 59

FIG. 21-6. Officers of the USRC Thetis on the Arctic cruise of 1903 (the photograph was provided by the Coast Guard Museum Northwest). require a good characterization of the ice edge to establish created (Polyakov et al. 2003). The 1920s also saw reports appropriate boundary conditions. Moreover, more com- of a loss of sea ice in the subpolar North Atlantic Ocean, plete recovery of available ice observations provides an together with early conjectures that reduced sea ice cov- invaluable baseline reference to understand the dramatic erage should contribute to changes in cyclone activity loss of sea ice taking place in the Arctic today. Ice obser- (Wiese 1924). In a report that would not have been out of vations from whaling ships for the period 1850–1913 have place in the early 2000s, the American consul in Bergen, been extracted (Bockstoce and Botkin 1983; Mahoney Norway, provided the following report to the U.S. State et al. 2011) and compiled into a sea ice dataset, the His- Department in October of 1922: torical Sea Ice Atlas (Walsh et al. 2016). However, the data- The Arctic seems to be warming up. Reports from fish- rich federal logbooks have only recently been addressed ermen, seal hunters and explorers who sail the seas around comprehensively by Old Weather citizen-scientists and Spitsbergen and the eastern Arctic, all point to a radical applied in current research (Schweiger et al. 2018, manu- change in climate conditions, and hitherto unheard-of high script submitted to J. Geophys. Res. Oceans). Thus, thou- temperatures in that part of the earth’s surface . . . The sands of sea ice observations from more than a century ago oceanographic observations have, however, been even have been gleaned from the logbooks of the Bear, Corwin, more interesting. Ice conditions were exceptional. In fact, Thetis, Northland, and other federal vessels and are being so little ice has never before been noted. The expedition all 0 put to new uses that were unimaginable to the officers who but established a record, sailing as far north as 81829 in ice- originally recorded them (Fig. 21-6). free water. This is the farthest north ever reached with modern oceanographic apparatus (Ifft 1922).

3. From 1919 to the 1940s a. Second International Polar Year (1932–33) Systematic aircraft-based observations of the Arctic Increased interest in the Arctic during this period led began in 1929, when the Soviet Polar Aircraft Fleet was to the second IPY held in 1932–33. A major goal was to CHAPTER 21 WALSH ET AL. 21.11 investigate how observations in the polar regions could improve the accuracy of weather forecasts and, as a result, the safety of air and sea transport. The second IPY was also motivated in part by the recognition that the electromagnetic processes in the polar regions were affecting telegraph, telephone, and electric power lines. In addition, the availability of new instruments such as the radiosonde as well as aircraft and motorized vehicles for sea and land transport provided new op- portunities for measurements, including below the surface. Altogether a total of 94 meteorological sta- tions operated in the Arctic for at least part of the FIG. 21-7. Time series of annual (October–September) air tem- perature anomaly averaged over 608–908N (blue curve) and the second IPY (Laursen 1959). This period provided the globe (red curve). Anomalies are relative to corresponding means first systematic upper-air measurements in the Arctic for 1980–2010. Both the Arctic and the global time series are based by radiosonde and pilot balloons. Plans for a network on surface air temperature measurements from land stations ar- of Antarctic stations never came to fruition because of chived in the CRUTEM4 dataset (https://crudata.uea.ac.uk/cru/ the global financial crisis of the 1930s. In the summer of data/temperature/). [Source: after Fig. 1 from Overland et al. (2017); see also ftp://ftp.oar.noaa.gov/arctic/documents/ArcticReportCard_ 1932, the Russian icebreaker Sibriyakov completed a full_report2017.pdf.] transit of the Northern Sea Route from Arkhangelsk to the Far East (Barr 1978). Although World War II prevented the planned archival of all the data at the well as atmospheric reanalyses). Much of our early Danish Meteorological Institute, much of the data knowledge of the surface energy budget of the central eventually found its way into a world data center that Arctic Ocean was built on surface flux measurements was created under an organization that eventually be- made at NP stations (e.g., Fletcher 1965), as was in- came known as the World Meteorological Organiza- formation on cloud conditions (e.g., Vowinckel and tion (Barr and Lüdecke 2010). Orvig 1971) and cloud radiative forcing. Even after the first stage of NP observations ended in the early 1990s, b. Russian North Pole stations the NP measurements formed the basis for studies of A major milestone of the period between the two surface–atmosphere interactions in the Arctic Ocean. world wars was the Soviet Union’s establishment of the For example, NP data showed that cloud-radiative first North Pole Drifting Station (NP-1). Established on forcing is negative for two to three months in the sum- pack ice near the North Pole in May of 1937, the ice mer, with a strong dependence of the surface radiative station drifted more than 2800 km before its abandon- fluxes on cloud fraction (Walsh and Chapman 1998). ment 9 months later. This was the first of many such Although the second IPY targeted Arctic observa- stations (from NP-1 through NP-31) deployed by the tions and measurements to improve forecasts, the 1930s Russians prior to the breakup of the Soviet Union. also saw the first attempts to document and understand A resumption of deployments in 2003 has included sta- understanding the warming of the Arctic during the tions from NP-32 through NP-40. These stations, occu- 1920s and 1930s. The Ifft (1922) report was among the pied for periods typically ranging from several seasons first to point to this notable climate event. As shown in to several years, provided the first multiyear records of Fig. 21-7, the early twentieth-century Arctic warming atmospheric, oceanic, and sea ice variables from the was followed by several decades of cooling, then by the central Arctic Ocean. In addition to standard surface strong warming of recent decades. These variations are and upper-air (sounding) meteorological observations apparent in the global as well as the Arctic time series of at regular intervals each day, the NP stations provided Fig. 21-7, which illustrates the tendency for variations of surface radiation (solar, longwave, and spectral albedo) global temperature to be amplified in the Arctic (section measurements, total ozone and UV measurements, teth- 5i). While various recent studies have placed the early ered balloon measurements in the lowest 2 km, and at- twentieth-century warming into a framework of climate mospheric composition measurements. These data are drivers, several notable observational reports and di- invaluable in the construction of twentieth-century cli- agnostic studies addressed the warming while it was matologies for atmospheric variables as well as snow and ongoing or shortly thereafter. Scherhag (1936) noted ice thickness. The NP data have also been widely used in that warming of the North Atlantic Subarctic region was the validation of historical simulations of the central accompanied by a retreat of sea ice that was consistent Arctic Ocean by global and regional climate models (as with anomalous wind forcing in the region. A role of the 21.12 METEOROLOGICAL MONOGRAPHS VOLUME 59 ocean, including a shoaling of the halocline (eerily similar wind and temperature on Greenland at Station Centrale to discussions of Arctic Ocean change in the past few de- (70.98N, 40.68W, 2965 m elevation) (e.g., Bedel 1954). cades), was proposed by Brooks (1938), Carruthers (1941), The station, near the location of Alfred Wegener’s and Manley (1944). The Second World War led to a hiatus ‘‘Eismitte’’ (1930–31), was close to, but downslope of, in the debate about the Arctic’s early twentieth-century the crest of the ice sheet. Analysis of profiles collected warming. However, interest resurfaced in the early under strong temperature inversion conditions allowed twenty-first century (e.g., Bengtsson et al. 2004; Wood and Schwerdtfeger (1972) to infer that the sloped-inversion Overland 2010; Yamanouchi 2011). While there is evi- pressure gradient force arising from the presence of cold dence that internal variability played a key role in the early air over sloping terrain, which was developed to explain twentieth-century warming (Fyfe et al. 2013), there is still the behavior of the wind field in the high interior of debate about the precise roles of the atmospheric circu- Antarctica, also applied to interior Greenland, indicating lation and the ocean. The most recent IPCC assessment that the governing dynamics were the same. (AR5) explicitly states, ‘‘There is still considerable dis- c. Early work on Antarctica cussion of the ultimate causes of the warm temperature anomalies that occurred in the Arctic in the 1920s and Following the historical Antarctic expeditions in the 1930s’’ (Bindoff et al. 2013, p. 907). early 1900s, meteorological studies entered a period with slow progress. Richard E. Byrd led three expedi- tions to Little America on the eastern edge of the Ross 4. From the 1940s to the 1970s (the Cold War Ice Shelf, starting with the base location to stage the first period) aircraft flight over the South Pole in 1929. All of these a. The Second World War featured extensive meteorological programs that in- cluded upper-air observations. Perhaps the most im- The Second World War led to rapid expansion of portant advance came in 1946 before the U.S. Navy was meteorological services. In 1939, the focus in Canada demobilized after World War II. The 1946–47 U.S. Navy was to meet the growing needs of Trans-Canada Air- Antarctic Expedition, designated as Operation High- lines. The onset of war brought added needs, especially jump (Byrd 1947), was conceived to map almost the to support the Royal Canadian Air Force (RCAF), the entire periphery of the Antarctic continent for the first British Commonwealth Air Training Plan, and the U.S. time. Led by Rear Admiral Byrd, it involved many navy Army Air Force for ferrying activities over the Atlantic ships and aircraft. This information and the associated Ocean and to Alaska. In northern Canada, the United photographs helped to set the stage for establishing the States assisted in establishing observing stations and network of Antarctic coastal stations for the 18-month forecast offices (Thomson 1948; Thomas 1971). Starting (1957–58) IGY, which marked the start of sustained in 1940, after the German occupation of Denmark, a instrumental observations from Antarctica and thus the number of stations were set up along the coast of Green- beginning of many climatic records from this remote land; these included weather stations in places like Thule continent. and Scoresbysund. This action resulted from an agreement with the Danish Ambassador of Denmark for the United d. Glacial anticyclones States to defend Danish colonies in Greenland. In 1941, While the need for climate and weather information when Germany attacked the Soviet Union, the Barents over the North Atlantic and Alaska remained critical Sea gained great strategic importance, leading to a series throughout the war, the climate and weather of the cen- of efforts by Germany, the United Kingdom, and Norway tral Arctic remained understudied, and data were sparse. to gain control of Svalbard, critically situated to pro- A persistent view was of an Arctic Ocean dominated by a vide data for forecasting weather in central Europe and largely permanent anticyclonic cell. First put forth by von for attacking Atlantic convoys headed for Murmansk, Helmholtz (1888), the idea was elaborated on by Hobbs Russia. In this ‘‘war for weather,’’ the Germans established (1910, 1926), in his ‘‘glacial anticyclone’’ theory and several secret stations in Svalbard as well as in north- subsequently gained traction. Jones (1987) notes that eastern Greenland and Franz Josef Land (https://www. charts from the U.S. Historical Weather Map Series, spitsbergen-svalbard.com/). prepared during the Second World War, contained con- siderable positive pressure biases over the Arctic Ocean b. Early work on Greenland up to 1930 and lesser errors up to 1939. It seems that these From September 1949 to August 1951, the meteorol- maps were compiled by relatively untrained analysts ex- ogists of the French Polar Expeditions under the di- trapolating pressures into the data-poor central Arctic rection of Paul-Emile Victor carried out soundings of with the preconceived notion of a high pressure cell. CHAPTER 21 WALSH ET AL. 21.13

Hobbs maintained his ‘‘Greenland glacial anticyclone’’ an effort by Canada to assert sovereignty in the high theory (Hobbs 1945), involving a persistent high pressure Arctic because of the region’s perceived strategic im- cell over the Greenland ice sheet with strong influences portance. As part of this effort, the Canadian Govern- on weather in midlatitudes. Although other investigations ment forcibly relocated Inuit from northern Quebec to found little support for the idea (Loewe 1936; Dorsey Resolute (and to Grise Fiord). By 1947, Canada and the 1945; Matthes 1946; Matthes and Belmont 1950), the United States had already built a weather station at thinking of anticyclones as dominant features of the cen- Resolute, as well as an airstrip. This was followed in tral Arctic Ocean persisted (e.g., Pettersen 1950; Rae 1949 by the establishments of a Royal Canadian Air 1951). Pettersen’s (1950) maps depict most of the Arctic Force base. Ocean, in both summer and winter, as a ‘‘quiet zone of Another major driver of the improved observational minimum cyclonic activity.’’ Such views may have been network in Canada was the establishment during the influenced by Otto Sverdrup’s observations during the 1950s of the Distant Early Warning (DEW) Line Maud expedition (1918–25) of the frequent passage of (Fig. 21-8). The DEW Line was a system of radar sta- cyclones along the fringes of the Arctic Ocean. tions installed in a line across Arctic Canada (some at existing villages, such as at Cambridge Bay in 1955), e. The growing data network intended to provide early warning of a Soviet bomber With the deployment of a series of the Soviet NP attack. Additional stations were built along the northern drifting stations on the Artic sea ice, U.S. drifting sta- coastline and Aleutian Islands of Alaska as well as in tions, the Ptarmigan series of aircraft overflights, the Greenland, Iceland, and the Faroe Islands. establishment of weather stations in the Canadian Arctic, and studies prompted by the IGY in 1957, the f. Evolving thought observing network started to improve. A key need was better coverage over the Arctic Ocean. The Soviet NP-2 Following World War II, two major Canadian research station, led by Mikhail Mikhailovich Somov (Hero of groups emerged at McGill University: a radar meteorol- the Soviet Union and recipient of three Orders of ogy group led by J. Stewart Marshall and R. H. Douglas Lenin), was deployed in April of 1950, and NP-3 as- in the Department of Physics and an Arctic meteorology sumed duties in 1954. Starting in 1954, from one to three group within the Department of Geography led by F. K. NP stations began operating simultaneously each year, Hare. The two groups merged in 1959 to form the De- collecting meteorological data of all types including at- partment of Meteorology. McGill became a dominant mospheric soundings from radiosondes. The United force in studies of Arctic meteorology and climate during States maintained a number of drifting stations, notably this period. By 1958 (before the merger) the McGill Arctic T-3 (also called Fletcher’s Ice Island, named after Col- meteorology research group had already published a onel Joseph O. Fletcher who discovered it). Starting in number of key reports on Arctic meteorology that took 1952, T-3 was used as a scientific drift station and in- advantage of the growing observational network (e.g., cluded huts, a power plant, and a runway for wheeled Wilson 1958; Hare and Orvig 1958). aircraft. T-3 was a tabular iceberg that presumably broke However, it is noteworthy that in the Soviet Union a off from the small ice shelves along the northern coast of mature view of the circulation over the central Arctic Ocean Ellesmere Island. The NP Stations were located variously had emerged as early as 1945. In a remarkable accom- on ice islands (tabular icebergs) and thick floes of sea ice. plishment, especially given the very trying wartime con- Ptarmigan was a series of aircraft reconnaissance missions ditions, Dzerdzeevskii (1945) correctly concluded that conducted by the U.S. Air Force over the period from 1950 cyclone activity was common in the central Arctic Ocean, to 1961. The missions included collecting soundings in the especially during summer. His study took advantage of lower troposphere over the Arctic Ocean from dropsondes data from the Russian drifting icebreaker Sedov,the that descended by parachute (Kahl et al. 1992). drifting ice island NP-1, and other high Arctic stations In terms of land-based stations, Eureka, on Ellesmere (Jones 1987). Island, then part of the Northwest Territories, Canada, Western scientists may have been unaware of this was established in April of 1947. Weather station Alert, work; indeed, even in 1958, the idea of a quiescent on the northern end of Ellesmere Island, was established Arctic Ocean persisted in some circles. For example, in 1950, and a military station was set up in 1958. The the quiet central zone [of the Arctic Ocean] in summer station is named after the HMS Alert, which wintered coincides fairly closely with the permanent pack ice of near the site of the station in 1875–76. The community at the Arctic Sea. Although a few frontal cyclones appear to Resolute Bay, on Cornwallis Island, was created in 1953 cross it, the prevailing state is one of monotonously slack as part of the ‘‘high Arctic relocation efforts.’’ This was and ill-defined circulation, appropriate enough to what is 21.14 METEOROLOGICAL MONOGRAPHS VOLUME 59

FIG. 21-8. The network of radar stations established during the 1950s and known as the DEW line. (Source: http://military.wikia.com/wiki/ Distant_Early_Warning_Line; photograph taken by Technical Sergeant Donald L. Wetterman, U.S. Air Force).

certainly the world’s largest quasi-homogeneous surface. of the Arctic circulation, especially in winter and over Only along the flanks does cyclonic cloud and rainfall land areas, cyclones are also frequent and, depending become at all common (Hare and Orvig 1958, p. 69). on the season, may be found anywhere in the Arctic It is clear, however, that by the late 1950s there was an (Keegan 1958; Reed and Kunkel 1960). As a sufficient epiphany. A series of studies emerged in rapid-fire suc- number of soundings began to reach the 25-hPa level, it cession that form a framework for our modern view of became possible to investigate stratospheric dynamics, the Arctic atmospheric circulation. As noted by the and the McGill University group played a leading role pioneering meteorologist Jerome Namias, (e.g., Hare 1960a,b, 1961) as did the Institute of Mete- orology at the Free University of Berlin under Richard the present generation of meteorologists is especially for- Scherhag (Scherhag 1960). tunate in being able to realize earlier hopes of obtaining Interest grew about the nature of Arctic air masses a reasonably accurate synoptic picture of the three- and Arctic fronts. Any synoptic analysis will reveal high- dimensional structure of the atmosphere over the entire latitude weather fronts and associated jet streams, but polar area. These data have brought to light many phe- can an Arctic frontal zone, separate from the polar nomena of circulation and weather hardly suspected in for- frontal zone, be identified? Some early studies that were mer years and thereby have been vital in the development of é scientific long-range prediction (Namias 1958,p.46). based on prevailing conceptual views (e.g., Palm n 1951; Palmén and Newton 1969) did not include a separate Although long-term prediction (a topic of great in- high-latitude Arctic frontal zone. Nevertheless, early terest to Namias) has remained an elusive goal, the new Canadian analysis schemes (Anderson et al. 1955; data certainly enabled a much better definition of the Penner 1955) adopted a three-front model, with the structure of the circumpolar vortex and features of the northernmost (in any season) representing individual surface circulation. It quickly became clear that while Arctic fronts. The Meteorological Branch of Canada anticyclones are common and often persistent features prepared routine synoptic charts showing the location of CHAPTER 21 WALSH ET AL. 21.15 three fronts on the 850-, 700-, and 500-hPa levels. Using but that changes in frontal strength will be largely re- these data, Barry (1967) examined the location of the stricted to June, when earlier snowmelt sharpens land– Arctic frontal zone over North America for January, ocean temperature contrasts. April, July, and October. Shapiro et al. (1987) more g. NWP and climate models recently presented clear evidence in winter of Arctic jet streams with tropopause folds between the lower Arctic By the 1940s, through the work of Bjerknes, Rossby, troposphere to the north and the higher Arctic tropo- and others, the physical mechanisms controlling weather sphere to the south. These fields are associated with processes were fairly well understood, enabling some what are now known as tropopause polar vortices skill in forecasting, which was critical to the wartime (Cavallo and Hakim 2009, 2010, 2012). effort. The widely studied ‘‘D-day’’ weather forecasts A prominent climatological feature of the Arctic are a prime example of the importance of meteorology summer is the thermal contrast between the Arctic to the wartime effort. However, successful numerical Ocean and the surrounding land areas. There has long prediction had to await the advent of digital computers. been interest in the concept of a summer Arctic frontal The first successful effort in the United States was in zone, separate from frontal activity in midlatitudes. 1950, when a team led by Jule Charney and John Dzerdzeevskii (1945) was the first to present evidence von Neumann used the Electronic Numerical Integrator for its existence. Reed and Kunkel (1960) subsequently and Computer (ENIAC) to solve the barotropic vor- looked at the issue in more detail. They noted the exis- ticity equation (https://en.wikipedia.org/wiki/History_of_ tence, in summer only, of a band of high frontal fre- numerical_weather_prediction). In the United Kingdom, quencies extending along the northern shores of Siberia the first numerical model forecast was made in 1952. Op- and Alaska and southeastward across Canada, and erational numerical forecasting in the United States started stated that it is ‘‘abundantly clear that the polar front in 1955, and the United Kingdom followed suit in 1965 remains separate from, and well to the south of, the (https://www.metoffice.gov.uk/research/modelling-systems/ Arctic frontal zone.’’ Bryson (1966) demonstrated that history-of-numerical-weather-prediction). That same year, the modal position of the summer Arctic frontal zone Norman Phillips completed a 2-layer, hemispheric, quasi- over North America coincided closely with Reed and geostrophic computer model that is generally regarded as Kunkel’s (1960) analysis as well as the position of the the first atmospheric general circulation model (AGCM; tree line. This led to a recurring notion of a vegetation Phillips 1956). link. Bryson (1966) proposed that the summer frontal The year 1955 also marked the birth of the first con- position might be important in determining the distri- tinued effort under the U.S. Weather Bureau to focus bution of forest versus tundra, but other investigators on the development of AGCMs (Smagorinsky 1983). (Hare 1968; Hare and Ritchie 1972) instead argued that Smagorinsky’s laboratory, initially located in Suitland, the tundra–forest boundary actually helps to control the Maryland, moved to Washington, D.C., and in 1968 position of the frontal zone in summer because of con- gelled at Princeton University as the Geophysical Fluid trasts in albedo, evaporation, and aerodynamic rough- Dynamics Laboratory (GFDL). Syukuro Manabe, who ness. However, it has now been clearly established that joined Smagorinsky’s group in 1959, was a pioneer in a primary control on the summer Arctic frontal zone model development (Manabe et al. 1965). In a seminal is differential heating between the land and ocean paper published in 1975, it was shown that the temper- (Serreze et al. 2001; Crawford and Serreze 2015), an idea ature response to a doubling of atmospheric carbon di- first advanced as early as 1945 by Dzerdzeevskii (1945). oxide would be magnified in high latitudes as a result of Arctic frontal activity, in particular the summer Arctic the recession of the snow and sea ice boundaries and the frontal zone, remains an active research area. Using an thermal stability of the lower troposphere that limits analog approach, Day and Hodges (2018) argue that, vertical mixing (Manabe and Wetherald 1975). because of increasing land–ocean temperature con- By the mid-1960s, climate model development was trasts, the summer Arctic frontal zone will sharpen, and being led by several groups in addition to GFDL: the that Arctic cyclones are likely to become more frequent University of California, Los Angeles, Department of and intense as the Arctic continues to warm. However, Meteorology; the Lawrence Livermore Laboratory; and work by Crawford and Serreze (2016) show the summer the National Center for Atmospheric Research. By the Arctic frontal zone is not in itself a region of cyclogen- 1970s, this had expanded to include the RAND corpo- esis, but rather acts to intensify cyclones that pass ration, the National Aeronautics and Space Adminis- through it. Based on coupled climate model simulations, tration (NASA) Goddard Institute for Space Sciences, Crawford and Serreze (2017) argue that the frontal zone and the Australian Numerical Meteorological Research will remain a significant cyclone intensifier in the future, Centre. The Arctic was not a primary consideration in 21.16 METEOROLOGICAL MONOGRAPHS VOLUME 59

FIG. 21-9. Locations of Antarctic stations operated during the IGY. The flag at each location denotes the nation that operates the station. [Source: Tom Woolley Illustration (https://www.tomwoolley.com/portfolio/map-infographics-for-the-polar-museum/)/Scott Po- lar Research Institute (https://www.spri.cam.ac.uk/museum/exhibitions/previous.html), University of Cambridge.] the development of the atmospheric component of models, The IGY marks the start of sustained instrumental ob- although credible simulations of sea ice and snow cover servations from Antarctica, and thus the beginning of were recognized as important to realistic simulations of many climatic records from this remote continent, such the albedo–temperature feedbacks. as are available from the Met READER database (https:// legacy.bas.ac.uk/met/READER/data.html). An interna- h. The International Geophysical Year (third tional analysis center was established at the Little America International Polar Year) V station to produce the first surface and upper-air TheIGY,alsoreferredtoasthethirdIPY,tookplace weather maps for Antarctica and the Southern Ocean from July 1957 through December 1958. The IGY was an (Moreland 1958) that were broadcast once a day. Several international effort to coordinate the collection of geo- of the participants (e.g., H. van Loon and P. D. Astapenko) physical data from around the world, including both polar subsequently made major advances in Antarctic meteo- regions. It marked the beginning of a new era of scientific rology. The launch of the first satellites during the IGY discovery at a time when many innovative technologies presaged the start of the comprehensive satellite network were appearing. While Greenland and the upper atmo- that today is a foundation for modern numerical weather sphere were emphases of Arctic activities, the IGY was a prediction in high southern latitudes. A symposium on watershed event for the Antarctic. A continentwide dis- Antarctic meteorology, held in Melbourne in February tribution of weather stations was established (Fig. 21-9). 1959, highlighted the coming explosion of meteorological CHAPTER 21 WALSH ET AL. 21.17 knowledge stimulated by the IGY. One contribution was computer modeling, satellite remote sensing and auton- the seminal effort of Ball (1960), who formulated a simple omous instrumentation. Below we highlight these ad- set of equations describing the first order behavior of the vances, together with several globally significant weather Antarctic surface winds. Once Antarctic terrain elevations and climate challenges in which these advances have been were determined with sufficient accuracy, this system of essential for scientific understanding and, in at least one equations was exploited by Parish and Bromwich (1987) to case (the Antarctic ozone hole), mitigation actions. derive a realistic depiction of the Antarctic katabatic winds a. The Global Weather Experiment: The First GARP and their concentration into a small number of conflu- Global Experiment ence zones such as the one that sustains the ‘‘Home of the Blizzard’’ at Cape Denison. In the early 1970s, the Global Weather Experiment, Prior to the IGY, seven countries claimed parts of initially known as the First Global Atmospheric Re- Antarctica, with some of the claims overlapping, while search Program (GARP) Global Experiment (FGGE), eight other countries made no assertions of sovereignty; led to major progress in numerical weather prediction. the latter included the United States, which did not rec- To paraphrase Hollingsworth (1989), the primary goals ognize the seven claims but reserved the right to make its of FGGE were to describe the global behavior of the owninthefuture(https://www.state.gov/t/avc/trty/193967. atmosphere for one full year, to greatly enhance nu- htm). To preserve the continent for cooperative scientific merical weather prediction on the global scale, and to study and peaceful purposes that characterized the IGY, design an optimal observing system for this purpose: ‘‘In the Antarctic Treaty was signed at the National Academy practice, the goal of the observational programme was of Sciences in Washington, D.C., on 1 December 1959 by to describe the dynamics and thermodynamics of the the 12 nations whose scientists had been active in and atmosphere with a horizontal resolution of about 500 km around Antarctica during the IGY. The Antarctic Treaty for the whole year, and with as good a vertical resolution set aside the issue of territorial claims but did not in- as possible. The main focus of the experiment was on the validate them. The treaty came into force in 1961. It has tropics, and on the Southern Hemisphere.’’ now been acceded to by 53 nations and governs interna- The resources required for the experiment were sub- tional activities south of 608S. The Scientific Committee stantial. For the first time, there was a global constella- on Antarctic Research (SCAR) that was established at tion of meteorological satellites consisting of ‘‘five the same time provides scientific advice to the Antarctic geostationary spacecraft and two polar orbiters. In ad- Treaty System and has, for example, been a leading pro- dition, extensive deployments of ships, aircraft with ponent of the Year of Polar Prediction (Jung et al. 2016) dropsonde capability, high-level and low-level super- that is under way at the time of writing (section 5k). pressure balloons, and drifting buoys in remote ocean Several efforts resulting primarily from the IGY led areas (especially in the Southern Ocean), along with to notable advances in meteorological knowledge of the greatly enhanced rawinsonde and synoptic station cov- Southern Ocean and Antarctica. Harry van Loon, Jan erage, both in space and time, were implemented’’ (from J. Taljaard, and colleagues were leaders in laying out the Hollingsworth 1989 with edits). ECMWF was founded basic characteristics of the atmospheric circulation, cul- in 1975 to exploit the anticipated advances in global minating in the Meteorology of the Southern Hemisphere numerical weather prediction up to 10 days ahead fol- (Newton 1972) monograph. One topic emphasized by van lowing from the Global Weather Experiment. Loon was the elucidation, explanation, and consequences b. Discovery and understanding of the Antarctic of the semiannual oscillation in atmospheric pressure and ozone hole wind so prevalent over the circumpolar ocean surround- ing Antarctica (e.g., van Loon 1967). Rusin (1964) focused The stratospheric Antarctic ozone hole was discovered on the radiation and surface energy budget of Antarctica, in the mid-1980s by scientists from the British Antarctic primarily using observations from Russian stations. Survey (Farman et al. 1985) by using total ozone amounts Schwerdtfeger (1970) presented a synthesis of Antarctic that were derived from ground-based Dobson spectro- climate that included detailed surface climatic descrip- photometer measurements at Halley and Argentine Is- tionsfor25stations,manybasedonadecadeofobser- lands stations that started in the IGY. This severe ozone vations starting from the IGY. depletion was subsequently confirmed to be an Antarctic- wide phenomenon in the austral spring by instruments on the Nimbus-7 satellite that had been operating since 1978 5. 1970s to the present (the modern/satellite era) (Stolarski et al. 1986); until the publication of the Farman In the period since 1970, progress in polar meteorology et al. paper, overly conservative processing of the Nimbus- has greatly accelerated, largely as a result of advances in 7 ozone retrievals had hidden the ozone hole’s presence. 21.18 METEOROLOGICAL MONOGRAPHS VOLUME 59

FIG. 21-10. Sequence of late-winter maps of ozone concentration (in Dobson units) illustrating the ozone holes (deep blue and purple) during each year of the 2006–13 period, taken from Newman et al. (2014, their Fig. 6.9). The occurrence of an ozone hole in each of these years contrasts with (top left) 1979, during which ozone concentrations were much higher.

Subsequently, satellite measurements have provided com- network of automatic data buoys to provide synoptic- prehensive mapping of the Antarctic ozone hole and its scale fields of sea level pressure, surface air temperature, rate of change. Figure 21-10 shows that ozone depletion and ice motion (Thorndike and Colony 1981). From a was modest in 1979 but extreme in the 2000s. Direct recommendation of the National Academies of Sciences, measurements of the stratospheric chemistry started in the Arctic Ocean Buoy Program began its deployments 1986 with the National Ozone Experiment (NOZE) at of buoys on the sea ice surface in early 1979 in support of McMurdo Station. This led to the explanation that het- the Global Weather Experiment (section 5a). Coordi- erogeneous chemical reactions involving anthropogenic nated by the University of Washington Applied Physics chlorofluorocarbons (CFCs) and polar stratospheric clouds Laboratory’s Polar Science Center, the program in 1991 release atomic chlorine gas that catalyzes the destruction of became known as the International Arctic Buoy Pro- ozone (e.g., Solomon et al. 1986; Douglass et al. 2014). In gramme (IABP; http://iabp.apl.uw.edu), with funding 1987, the international Protocol on Substances that De- provided by U.S. agencies and various other nations. As plete the Ozone Layer was signed in Montreal to phase out the buoy program approaches four decades of operation, CFC emissions. Direct confirmation that the reductions in its uses have included the real-time support of operations, CFC emissions have led to the recovery of the Antarctic ingestion into reanalyses, and diagnostic studies encom- ozone hole was reported by Strahan and Douglass (2018). passing the time scales of weather, the seasonal cycle, The Antarctic ozone hole has a major impact on the tro- interannual variability, and climate change. pospheric circulation by strengthening the circumpolar The first buoys were sheltered instruments, deployed westerly winds over the Southern Ocean (Thompson and on the ice surface to measure atmospheric pressure, air Solomon 2002) and moving them poleward. The varying temperature, and position. Interrogated by satellite at strength of these circumpolar westerlies, known as the frequent (approximately hourly) intervals, the atmo- southern annular mode (SAM), represents the extra- spheric measurements have always been available in tropical Southern Hemisphere’s dominant mode of large- near–real time for ingestion into models used for weather scale atmospheric variability and has many climatic impacts forecasts or reanalyses. Changes in a buoy’s location over (e.g., Thompson et al. 2011; Wang and Cai 2013). time enable the calculation of ice velocity. With 20–30 buoys operating over the Arctic Ocean during much of c. The International Arctic Buoy Programme the IABP’s first few decades (Fig. 21-11), more-accurate A major milestone for monitoring the weather and sea fields of sea level pressure and ice velocity were con- ice in the Arctic Ocean was the establishment of a structed. Such fields dating back to 1979 at daily intervals CHAPTER 21 WALSH ET AL. 21.19

FIG. 21-11. Sample distribution (9 Jan 2011) of buoys operating as part of the IABP network. Gray lines show buoy tracks over the previous 60 days (source: IABP; from an earlier version of http://iabp.apl.washington.edu/maps.html). are available from the Polar Science Center (e.g., over the Arctic Ocean. Most are placed on sea ice, but Thorndike and Colony 1981). some are placed in open water. Buoys have an average life In the past two decades, measurement capabilities of span of 18 months. In the future, IABP hopes to increase the buoys have been expanded to include subsurface the average life span to 3–4 years. variables such as ice and ocean temperature and salinity. The data collected are used for real-time operations Some buoys include ice mass balance (IMB) measure- and research. Real-time operations include collecting ments and an ice-tethered profiler (ITP) system. The data for meteorological predictions. IABP buoys have IMB buoys consist of a series of thermistors spaced helped to predict the trajectory of storms off the coast of 10 cm apart from just above the sea ice down 3–5 m into Alaska that otherwise would have been difficult to de- the ocean and acoustic pingers to measure snow depth termine. Data collected by IABP buoys are also impor- on sea ice and ice thickness from below, in addition to tant for forecasting sea ice conditions, which are crucial the fundamental surface air pressure and temperature to for coastal Alaskans, for those engaged in subsistence support the IABP. The ITP buoys consist of a small fishing, and those who work in the coastal commercial surface capsule that sits atop an ice floe and supports a industry. Shipping traffic in the Arctic region has in- plastic-jacketed wire rope tether extending through the creased in recent years with the retreat of sea ice. A ice and down into the ocean, ending with a weight combination of sea level pressure, air temperature, and (intended to keep the wire vertical). A cylindrical un- sea ice motion help forecasters to predict better the derwater apparatus mounts on the tether and cycles ver- movement of Arctic Ocean sea ice. tically along it, carrying oceanographic sensors through IABP buoys are also used to validate satellite prod- the water column. Water-property data are telemetered ucts and complement the capabilities of satellite remote from the ITP to shore in near–real time. The IABP now sensing. The National Weather Service and the National maintains more than 100 buoys of varying sophistication Snow and Ice Data Center use buoy data for weather 21.20 METEOROLOGICAL MONOGRAPHS VOLUME 59 predictions and ice charting. IABP data are also used for Schweiger and Key 1994; Curry et al. 1996). Much of the atmospheric reanalysis studies. To date, more than 800 early work on the radiative impacts of clouds over the scientific papers have been written using data from the Arctic Ocean was based on the radiation measurements IABP. Much of the data collected supports efforts for and cloud observations from the Russian drifting ice the World Climate Research Programme and the World stations (Marshunova and Mishin 1994; Walsh and Weather Watch. Chapman 1998). Cloud radiative properties are strongly Contributors to the U.S. section of IABP include the dependent not only on their elevation, as in lower lati- U.S. Coast Guard, the U.S. Department of Energy, tudes, but also on the phase (liquid vs ice) of the cloud NASA, the U.S. Navy, the National Science Foundation particles (Shupe et al. 2015). (NSF), and researchers from academic institutions such Research on Arctic clouds accelerated during the as the Woods Hole Institution, the U.S. Army’s Cold 1990s and 2000s with several major field programs (see Regions Research and Engineering Laboratory, and the the appendix). The Surface Heat Budget of the Arctic Polar Science Center. Researchers from private and (SHEBA), a yearlong field experiment centered on a public organizations from the United States as well as ship intentionally frozen into the Arctic pack ice during France, Norway, China, Canada, Japan, South Korea, 1997–98, showed that supercooled liquid water droplets India, and Russia contribute to the IABP. are surprisingly frequent over the Arctic Ocean. Recent estimates have indicated that liquid water is present in d. Antarctic automatic weather stations 10%–80% of Arctic clouds, depending on the season Until 1980, direct surface and upper-air meteorolog- and location (Shupe et al. 2011; Cesana et al. 2012). ical observations were provided by nearly the same SHEBA was followed in the early 2000s by the De- network of staffed locations as established for the IGY partment of Energy’s Atmospheric Radiation Mea- (Fig. 21-9). Charles R. Stearns from the University of surement/North Slope of Alaska (ARM/NSA) program, Wisconsin–Madison led the implementation of the which included the deployment of a variety of in- satellite-transmitting automatic weather station (AWS) strumentation for measuring radiation and clouds on the network (Lazzara et al. 2012). Funded largely by the northern Alaskan coast at Barrow and, more recently, NSF, the AWS network in Antarctica now consists of Oliktok Point. The ARM program targeted improve- about 60 active sites maintained primarily by the Uni- ments on model formulations of cloud/radiative pro- versity of Wisconsin. The AWS units typically measure cesses as one of its key objectives. Over the years since pressure, temperature, winds, and atmospheric moisture 2000, the ARM program has included a wide variety of at 2–3 m above the surface at intervals of a few minutes, manned and remote-controlled airborne measurements but the variables observed continue to expand. The (McFarquhar et al. 2011; Schmid et al. 2016), including sensors now include acoustic depth gauges to measure the Mixed-Phase Arctic Cloud Experiment (M-PACE; changes in the snow surface height. Because the AWS Verlinde et al. 2007), during which Arctic cloud particles are deployed in remote and challenging locations with at were sampled extensively. Another notable field study most annual maintenance visits, data outages do occur; was the Arctic Summer Cloud Ocean Study (ASCOS), therefore, data analysis requires care. The AWS net- which took place in 2008 and utilized the Swedish ice- work has expanded to more than 100 locations across breaker Oden (Tjernstrom et al. 2014). ASCOS targeted Antarctica (Fig. 21-12) through sites provided by many the physical and chemical processes responsible for the nations, including Australia, France, the United King- formation of the low-level clouds that are pervasive over dom, China, Japan, and Italy as well as the United the Arctic Ocean during summer. ASCOS measure- States. Observations from AWS sites are critical input ments have been used to improve model simulations of for Antarctic numerical weather prediction, global re- late-summer Arctic clouds (e.g., Hines and Bromwich analyses, and innumerable weather and climate studies. 2017). Another Arctic field campaign, the Indirect and Semi-Direct Aerosol Campaign (ISDAC) focused on e. Arctic clouds the impact of aerosols on Arctic clouds (McFarquhar Arctic clouds and their radiative interactions have et al. 2011). From these various field programs, it has emerged as a critical component of the climate research become apparent that atmospheric radiation is impacted agenda. Clouds have a strong warming influence on the much more by clouds containing liquid water than by surface during much of the year in the Arctic, and a ice-crystal clouds (Shupe and Intrieri 2004). While cooling effect for a short period in the summer. The clouds containing liquid are in near–radiative equilib- period of negative cloud radiative forcing ranges from a rium with the surface, thin clouds composed primarily of few weeks in the central Arctic Ocean to several months ice allow considerable surface-emitted longwave radia- over the subarctic land areas (Curry et al. 1993; tion to escape to space (Stramler et al. 2011). CHAPTER 21 WALSH ET AL. 21.21

FIG. 21-12. AWSs operated in Antarctica during 2018. The symbols denote the institutions that operate each station (legend in lower right). [Source: the Antarctic Meteorological Research Center of the University of Wisconsin–Madison Space Science and Engineering Center (http:// amrc.ssec.wisc.edu/aws/); created by Sam Batzli under NSF Grant ANT-1543305.]

Advances in remote sensing have also led to progress used to assess weather prediction models simulations in documenting Arctic cloud characteristics and their of clouds (Candlish et al. 2013) and radiative fluxes radiative effects. While surface–cloud contrast limita- (Zygmuntowska et al. 2012). tions inherent in visible and infrared sensors hindered Despite the importance of Arctic clouds and their early uses of satellite products in the Arctic, lidar and composition, models still have difficulty in producing the radar profilers on the CloudSat and Cloud–Aerosol correct Arctic cloud types (de Boer et al. 2012) and, for Lidar and Infrared Pathfinder Satellite Observations that reason, poorly represent Arctic surface energy (CALIPSO) satellites have been used to obtain Arctic fluxes (Tjernstrom et al. 2008; Pithan et al. 2014). For cloud climatologies that differ in some ways from earlier simulations of climate, these deficiencies have serious depictions. For example, Liu et al. (2012) showed that implications for surface temperatures and cryospheric cloud frequencies derived from radar and lidar profilers change (Persson 2012). As noted in section 6, the Arctic on CloudSat and CALIPSO have seasonal maxima and surface energy budget and its relation to clouds remain minima in autumn and winter, respectively. Climatol- major challenges of polar meteorology and polar climate ogies based on surface observations generally showed science. This realization has driven the upcoming Multi- maximum frequencies in summer (e.g., Vowinckel and disciplinary Drifting Observatory for Studies of Arctic Orvig 1971). The lidar and radar profile results also Climate (MOSAiC) program, an international Arctic drift showed that about 25% of Arctic clouds are multilay- expedition planned for the marginal ice zone in 2019–20 ered. CloudSat and CALIPSO products have also been (Shupe et al. 2016; https://www.mosaic-expedition.org/ 21.22 METEOROLOGICAL MONOGRAPHS VOLUME 59

fileadmin/user_upload/MOSAiC/Documents/MOSAiC_ spanning geophysics, ecology, human health, social sci- Implementation_Plan_April2018.pdf). ences, and the humanities. This increased breadth in- dicates that modern atmospheric science has become f. Satellite-derived soundings of the atmosphere multidisciplinary. In many cases, there was a significant Radiation emitted by Earth’s atmospheric gases and by atmospheric component to IPY projects carried out in clouds is recorded by polar-orbiting satellites in the infrared topic areas such as ice, ocean, land, people, and others and microwave wavelengths. These emissions can be used (Krupnik et al. 2011, p. 137). Purely atmospheric topics to infer the temperature and moisture content of the broad included the International Arctic Systems for Observing atmospheric layers from which they originate using emis- the Atmosphere (IASOA) observing network; radiation sion weighting functions. Profiles of atmospheric tempera- measurements from Spitzbergen; aerosol measurements ture and water vapor amounts from across the globe in the Arctic and Antarctic; investigations of Antarctic are provided by spaceborne observations for numerical polar stratospheric clouds and associated ozone de- weather prediction. These profiles are especially valuable in pletion; initiation of the regional synthesis of the physical the polar regions, where the network of rawinsonde stations components of Arctic climate known as the Arctic Sys- has large gaps over the polar oceans and even over land. tem Reanalysis (section 5h); investigations of Arctic The era of satellite sounding of the atmosphere pri- weather phenomena and their forecastability as a prelude marily started with the launch of the TIROS-N space- to the Polar Prediction Project (section 5k); the Con- craft in 1978 (https://science.nasa.gov/missions/tiros) for cordiasi Project over Antarctica that featured develop- the Global Weather Experiment. It included the first ment of more effective assimilation of radiances from TIROS Operational Vertical Sounder (TOVS) that hyperspectral infrared and microwave sounders over consisted of the High Resolution Infrared Radiation snow and ice and also featured dropsondes launched re- Sounder (HIRS), the Microwave Sounding Unit (MSU), motely from stratospheric superpressure balloons, in part and, on some satellites, the Stratospheric Sounding Unit to improve numerical weather prediction and also as a (SSU). Satellite sounding has advanced tremendously run-up to the Polar Prediction Project; and investigations since the 19 channel HIRS era. A recent example is of air pollutants impacting the Arctic. the Infrared Atmospheric Sounder Interferometer–New h. Reanalyses and the polar regions Generation (IASI-NG) sounder that has 16 920 channels spanning the infrared from 3.62 to 15.50 mm. Designed Global reanalyses provide valuable tools for investigating for temperature and humidity sounding, ozone profiling, climate variability and change in the data-sparse polar re- and total-column or profiles of greenhouse gases, it is gions, for example, exploring the spatial and temporal planned for flight on the European MetOp series of variability of Antarctic snow accumulation (Medley et al. polar orbiters starting in the 2021 time frame (https:// 2013). These reanalysis datasets are produced by merging www.wmo-sat.info/oscar/instruments/view/206). a short-term numerical weather prediction with a wide variety of ground-based, aircraft, and satellite-based ob- g. The fourth International Polar Year (2007–09) servations of the atmosphere while taking into account According to Krupnik et al. (2011), uncertainties in both the prediction and the observations. However, there are some important issues in using rean- the International Polar Year (IPY) of 2007–2008 co- alyses to investigate polar climate change. Although the sponsored by ICSU and WMO became the largest co- ordinated research program in the Earth’s polar regions, data assimilation system and the forecast model do not following in the footsteps of the IGY. An estimated 50,000 change, artificial shifts/trends can arise because of the researchers, local observers, educators, students, and sup- changing observing system. This sensitivity is heightened in port personnel from more than 60 nations were involved in high southern latitudes because of limited direct meteoro- the 228 international IPY projects (170 in science, one in logical observations prior to the Global Weather Experi- data management, and 57 in education and outreach) and ment in 1979 (e.g., Bromwich and Fogt 2004). For example, related national efforts. The IPY generated intensive re- the introduction of satellite atmospheric sounding data in search and observations in the Arctic and Antarctica over a late 1978 produced a jump in the Antarctic precipitation two-year period, 1 March 2007–1 March 2009, with many minus evapotranspiration (P 2 E) simulated by the ERA- activities continuing beyond that date. All IPY projects 40 global reanalysis (e.g., van de Berg et al. 2005). Even included partners from several nations and/or from in- during the modern satellite era (after 1978), the assimilation digenous communities and polar residents’ organizations. of radiances from the Advanced Microwave Sounding Unit Although meteorology was the major focus of the first (AMSU) in the late 1990s introduced a pronounced jump IPY (1882–83), the 2007–09 IPY was far broader in its into the precipitation forecast by the MERRA global re- scientific projects and involved a large range of disciplines analysis (e.g., Bromwich et al. 2011a). Global reanalyses are CHAPTER 21 WALSH ET AL. 21.23 less problematic in high northern latitudes as a result of of the processes and feedbacks challenges observational extensive surface and upper-air observations collected from assessments (and indeed has motivated special field pro- the land areas surrounding the Arctic Ocean. As for the grams such as SHEBA, described in the appendix,and Antarctic, the greatest challenges arise for those variables MOSAiC). For this reason, comparative evaluations of that are not observed but depend on the model physics for key feedbacks involved in Arctic amplification have re- their generation—namely, clouds, precipitation, radiative lied largely on global climate models (e.g., Taylor et al. fluxes, and surface energy fluxes (Lindsay et al. 2014). 2013; Pithan and Mauritsen 2014). In the latter study, the A regional reanalysis for the ‘‘greater’’ Arctic (pole- largest contributions to Arctic amplification were found ward of ;408N) has been produced for 2000–12 to to arise from 1) the surface albedo (snow and ice) feed- provide a more refined tool to investigate rapid climate back and 2) the different vertical structure of the warming change happening in Arctic latitudes. Two versions of the in high and low latitudes (lapse-rate effect). The next Arctic System Reanalysis (ASR) are available at 30-km largest contribution to Arctic amplification in the climate (version 1) and 15-km (version 2) grid spacing with a high models is the Planck effect, which arises because the Arctic vertical resolution (e.g., Bromwich et al. 2018), and the is colder at the top of the atmosphere than the subtropics latter is being updated to the present. The strengths of and radiates less energy to space. While the water vapor this high-resolution regional reanalysis reside in its more feedback is positive in the Arctic, it actually opposes Arctic accurate reproduction of surface variable behavior (10-m amplification in climate models (Pithan and Mauritsen wind, 2-m air temperature, etc.), and in realistically cap- 2014, their Fig. 2). If relative humidity stays nearly constant turing topographically forced winds. The ASR also pro- in climate models, then the Clausius–Clapeyron equation vides an improved depiction of cyclones relative to dictates a larger increase of water vapor in the tropics than coarser global reanalyses, including polar lows (Smirnova in the polar regions, thereby countering polar amplifica- and Golubkin 2017), although approximately one-third tion. The net role of clouds in the models was found small, of the polar lows are not analyzed by the ASR. For such but can be large in a particular season of a particular purposes, the effective resolution of the ASR is about 7 year. The largest and only substantial negative feedback in times the 15-km resolution (Skamarock 2004). the models is ocean heat transport, which decreases as the Arctic warms, thereby reducing the Arctic warming. i. Arctic amplification and the recent Arctic warming The different contributions vary among the global climate Arctic amplification, which refers to the observation that models, and the range of uncertainty is especially large in the Arctic warms and cools faster than the rest of the the ocean transport feedback. The feedbacks associated Northern Hemisphere and the global mean, has become a with clouds and atmospheric transport also have wide major topic of climate research. Figure 21-7 illustrates this ranges. behavior by showing the annual values of the Arctic and Atmospheric transport has also been a key contribu- global temperatures since 1900. In recent decades the tor to recent extreme warming events. For example, Arctic has warmed at twice the rate of the global and during January 2016, the Arctic-wide averaged tem- Northern Hemispheric mean temperatures. Arctic ampli- perature anomaly was 2.08C above the previous record fication, a long-expected Arctic response to climate warm- of 3.08C(Fig. 21-13a; Overland and Wang 2016; Kim ing and evident in simulations from even the earliest et al. 2017). This event caught the public’s attention with generation of global climate models (e.g., Manabe and reports of temperatures warming to near the freezing Wetherald 1975), started to clearly emerge toward the end point at the North Pole. The event was caused by ad- of the twentieth century (Serreze et al. 2009; Screen and vection of heat and moisture into the Arctic on Atlantic Simmonds 2010). One of the major drivers of observed and Pacific pathways as shown by contour directions Arctic amplification is sea ice loss; open water areas de- of the 700-hPa geopotential height field for January/ velop earlier in spring, allowing for more absorption and February 2016 (Fig. 21-13b). Northward advection of storage of solar energy in the ocean mixed layer through the temperature and moisture also results in an increase summer. As the sun sets in autumn and winter, this stored of downward longwave radiation, further warming the heat is released upward to the atmosphere. This heat loss surface and reducing sea ice buildup (Cullather et al. mechanism helps to explain why the Arctic amplification 2016; Binder et al. 2017; Rinke et al. 2017). A similar signal tends to be stronger in autumn than in summer. event occurred in the winter 2017/18 (Overland and However, it is increasingly recognized that Arctic Wang 2018). Consistent with the recent Arctic warming amplification has other causes in addition to sea ice loss. and the temperature–albedo feedback, the Arctic has Alexeev et al. (2005), for example, showed that polar experienced record low sea ice during multiple winters amplification arises in climate models of ‘‘aquaplanets’’ (2015–18), as shown in Fig. 21-14 for 2017. Arctic sea (i.e., systems with no sea ice or snow cover). The complexity ice has shifted from mostly multiyear thick (.3m)to 21.24 METEOROLOGICAL MONOGRAPHS VOLUME 59

FIG. 21-13. Mean January and February 2016 spatial patterns of (a) near-surface (925 hPa) air temperature anomalies and (b) averaged geopotential height at 700 hPa. The data are from the NOAA–NCAR reanalysis using the NOAA/ESRL online plotting routines. [The data and image were provided by the NOAA/OAR/ESRL Physical Sciences Division (https://www.esrl.noaa.gov/psd/).] mostly thin (,1 m) sea ice that formed in the previous early parts of the forecast that are most accurate (after winter (Meier et al. 2014). Both Arctic temperatures and 12 h of spinup from a cold start) has led to substantial sea ice conditions are now well beyond previous expe- advances in understanding of Antarctic atmospheric rience from the twentieth century. processes, including the surface winds (e.g., Nigro and Cassano 2014), storm-generation mechanisms in the j. Regional modeling of the polar regions most active cyclogenesis region in the Southern Hemi- sphere (Bromwich et al. 2011b), Antarctic precipitation 1) THE ANTARCTIC MESOSCALE PREDICTION (Schlosser et al. 2016), and the climate of West Antarc- SYSTEM tica (Nicolas and Bromwich 2011). The need for optimized numerical weather prediction 2) REGIONAL CLIMATE MODELING for Antarctica became apparent in 1999 when Dr. Jerri Nielsen was stranded at Amundsen–Scott South Pole During the last two decades, there have been substantial Station with a serious medical condition. The key ques- efforts devoted to regional atmospheric modeling of both tion was, When would the temperature warm up enough polar regions. The polar version of the Fifth-generation for planes to land safely to evacuate her in the austral Pennsylvania State University–National Center for At- spring? No convincing forecast capability existed to mospheric Research Mesoscale Model (Polar MM5; e.g., provide this information. As a result, the Antarctic Me- Cassano et al. 2001) and its successor the polar version of soscale Prediction System (AMPS) started in 2000 as a the Weather Research and Forecasting (WRF) Model collaboration between the National Center for Atmo- (Polar WRF; e.g., Bromwich et al. 2009) were developed spheric Research (NCAR) and the Byrd Polar Research to better characterize the high-latitude environments such Center of The Ohio State University to provide an opti- as sea ice areas, extensive ice sheets, and tundra regions. mized forecasting capability for the U.S. Antarctic Pro- These models have been applied to a wide variety of gram (Powers et al. 2012). It featured much higher spatial weather and climate problems, such a simulating the cli- resolution than existing global models, physical parame- mate of the Laurentide Ice Sheet during the last glacial terizations optimized for the Antarctic environment, and period (Bromwich et al. 2004), Antarctic numerical regional data assimilation. The current configuration of weather prediction via AMPS [section 5j(1)], the surface nested grids is shown in Fig. 21-15 with the finest reso- winds near Greenland (e.g., DuVivier and Cassano 2013), lution of 0.9 km around Ross Island where the extensive and conditions causing summer melting of ice shelves in U.S. aircraft operations are focused near McMurdo Sta- the Amundsen Sea embayment of West Antarctica (Deb tion, and an 8-km grid covering all of Antarctica. Every et al. 2018). Other major efforts have involved the Re- AMPS forecast is archived at NCAR. Exploration of the gional Atmospheric Climate Model (RACMO; e.g., Noël CHAPTER 21 WALSH ET AL. 21.25

FIG. 21-14. Evolution of Arctic sea ice area in recent years (colored lines) relative to the historical range (gray shading: 1979–2006 average 6 1 std dev). Recent years (2015–18) have had the lowest winter ice areas of the post-1979 period of record. [Source: Nansen Environmental and Remote Sensing Center (http://web.nersc.no/WebData/arctic-roos.org/observation/ssmi1_ice_area.png); after a fig- ure from the Arctic Regional Ocean Observing System (ArcticROOS; https://arctic-roos.org/).]

et al. 2015)andtheModèle Atmosphérique Régional atmospheric circulation (Vavrus et al. 2017) and the global (MAR; e.g., Fettweis et al. 2017) and have focused in climatic impacts of Arctic sea loss (Tomas et al. 2016). particular on the mass balance of the Greenland and k. Polar Prediction Project and the Year of Polar Antarctic ice sheets and their contribution to sea level rise Prediction, 2017–19 (e.g., Vernon et al. 2013). The Regional Arctic System Model (RASM; Cassano et al. 2017), is starting to be used The Polar Prediction Project (PPP; http://www. to explore atmosphere–ocean–land coupled problems in polarprediction.net/) is a 10-yr initiative (2013–22) of high northern latitudes (e.g., DuVivier et al. 2016). RASM the World Weather Research Programme of the World includes a regional ocean model that can be run at reso- Meteorological Organization (Jung et al. 2016). The lutions of several kilometers. Another regional ocean-ice mission of PPP is to ‘‘promote cooperative international model, the Pan-Arctic Ice Ocean Modeling and Assimi- research enabling development of improved weather and lation System (PIOMAS), has been used to simulate the environmental prediction services for the polar regions evolution of the Arctic sea ice cover, including an Arctic (Arctic and Antarctic), on time scales from hours to sea- sea ice volume reanalysis (Schweiger et al. 2011)thatis sonal.’’ Its core activity is the Year of Polar Prediction updated in near–real time. The global NCAR Community (YOPP) that will take place from mid-2017 to mid-2019. Earth System Model has been applied with a wide range YOPP focuses on improving the polar observing system, of bipolar climate change problems, such as the impact facilitating field programs, implementing better represen- of Arctic sea ice losses on the (northern) midlatitude tation of key polar processes in coupled and uncoupled 21.26 METEOROLOGICAL MONOGRAPHS VOLUME 59

FIG. 21-15. June 2018 nested grid configuration of AMPS. The outermost grid resolution is 24 km, Antarctica and the adjacent Southern Ocean are at 8 km, the 2.7-km grid spans from the South Pole to the Ross Sea, and the 0.9-km innermost grid is focused on McMurdo Station. A 2.7-km grid covers the Antarctic Peninsula. (The image is provided through the courtesy of NCAR.) models, enhancing assimilation of polar observations forecast improvements will be implemented to advance into models, analyzing the predictability of sea ice on regional and global numerical weather prediction for various time scales, evaluating the linkages between the both polar regions. polar regions and midlatitudes, developing forecast verification approaches optimized for the polar regions, 6. Priorities/opportunities for the next decade and exploring the linkage between the providers and users of polar weather and ice information. YOPP fea- It is apparent from the preceding review that there have tures four special observing periods of enhanced obser- been tremendous advances in polar meteorology over the vations and modeling, namely February–March 2018 in past 100 years. The study of polar weather and climate has the Arctic, July–September 2018 in the Arctic, mid- benefitted from advances in technology as well as a rap- November 2018–mid-February 2019 in the Antarctic, idly increasing cadre of scientists. The recent warming of and February–March 2020 in the Arctic to overlap with the Arctic and the diminished coverage of sea ice and MOSAiC drift across the Arctic Ocean (see section 6). snow have brought prominence to the Arctic as a sentinel YOPP participants include the academic community and of global change, and the Antarctic’s ozone hole and its operational forecast centers (including ECMWF and anticipatedrecoverycontinuetomaketheAntarctica NCEP), greatly enhancing the likelihood that YOPP focus of monitoring and research. CHAPTER 21 WALSH ET AL. 21.27

Among the priorities that have emerged in polar Finally, work remains to be done in assessing and an- meteorology and climate research are the linkages ticipating the role of internal variability in polar climate between polar and midlatitude weather and climate. and weather. As discussed in section 3b, internal variability Relationships between Arctic warming and extreme has been at least partially responsible for multidecadal weather and climate events in midlatitudes have been temperature variations in the Arctic and most likely the suggested (Francis and Vavrus 2012; Cohen et al. 2014), Antarctic as well. The magnitude of internal variations but the robustness of the linkages has been questioned exceeds the changes arising from external forcing over and the mechanisms are unclear (Barnes and Screen decadal time scales (Hodson et al. 2013); anticipation of 2015; Screen et al. 2018). YOPP (section 5k) is an effort changes over the yearly to decadal time scales relevant to to advance understanding and operational forecasting planners and decision-makers will have to contend with capabilities in this regard. Systematic assessments of the this issue. This challenge extends to the anticipation of impacts of polar data on forecasts in both hemispheres changes not only in atmospheric variables such as tem- can contribute to a firmer understanding of the impacts perature but in associated system components such as sea of the polar regions on midlatitude weather and climate. ice, snow cover, and terrestrial surface variables. Extreme events in the polar regions represent another emerging research topic. While changes in extreme Acknowledgments. The authors thank three anony- weather events in midlatitudes have been documented, mous reviewers for their constructive comments, which especially increases of heavy precipitation events and resulted in substantial improvements to the original high-temperature occurrences, comprehensive assess- manuscript. ments of changing extremes in the polar regions are lacking. Of particular interest in this regard are extreme high-temperature events (Fig. 21-7), which are favorable APPENDIX for high-impact rain-on-snow events. Other high-impact events, such as Arctic cyclones and their dynamical Significant Field Programs precursors, tropopause polar vortices (e.g., Hakim and a. AIDJEX (1970–78) Canavan 2005; Cavallo and Hakim 2010), are poorly documented and inconsistently simulated by weather The Arctic Ice Dynamics Joint Experiment (AIDJEX) and climate models. Moreover, short-term sea ice vari- was the first major western sea ice experiment con- ability has been linked to Arctic cyclones (Simmonds and structed specifically to answer emerging questions about Keay 2009; Simmonds and Rudeva 2012; Zhang et al. how sea ice moves and changes in response to the in- 2013; Parkinson and Comiso 2013; Kriegsmann and fluence of the ocean and atmosphere. In this respect, it Brümmer 2014), although the jury is still out on whether was a scientific sequel to the voyages of the Jeannete and Arctic cyclone activity will increase in the future. On the the Fram, as well as the drifting NP ice camps of the one hand, increases in cyclone frequency and/or intensity Soviet Union. AIDJEX was also the first major scientific will be favored by larger land–sea temperature contrasts effort conducted in the Arctic by U.S. agencies [NSF, in high latitudes during summer (Day and Hodges 2018); the National Oceanic and Atmospheric Administration on the other hand, polar amplification will reduce the (NOAA), and the Office of Naval Research] since the overall north–south baroclinicity of the midlatitudes, IGY in 1957–58 (Untersteiner et al. 2009). Pilot studies which are the source regions for many cyclones reaching in 1971 and 1972 were followed by the main AIDJEX the Arctic during the nonsummer months. field program from March 1975 to May 1976. The main The surface energy budget of the Arctic—in particular, experiment consisted of a central camp surrounded the role of polar clouds and radiation—continues to by three satellite camps arranged in a 150-km triangle. challenge weather and climate prediction models. Biases Surrounding these staffed camps was a polygon of eight in the surface radiative fluxes in global models are larger buoys at a distance of about 300 km from the main camp. than changes in those fluxes associated with changes in AIDJEX was timely in that a new generation of ob- sea ice cover. Clouds and their radiative properties un- serving technology was coming on line, including satel- doubtedly contribute to these biases. Model tuning is lite navigation, battery-powered buoys capable of complicated by the ongoing transition from a multiyear real-time transmission of position and other data via sea ice cover to a seasonal sea ice cover over the central satellite, accurate temperature–salinity probes for the Arctic Ocean. The upcoming MOSAiC program (Shupe ocean in place of Nansen bottles, quartz-oscillator ba- et al. 2016) offers promise as a coordinated effort to im- rometers, and laser equipment to measure ice defor- prove understanding and model simulation capabilities mation over scales of several kilometers to several tens with regard to the drivers of the surface energy budget. of kilometers (Untersteiner et al. 2009). That climate 21.28 METEOROLOGICAL MONOGRAPHS VOLUME 59 models were in need of more-realistic ice dynamics researchers at Ice Station SHEBA. SHEBA collected a formulations added to the timeliness of AIDJEX. complete annual cycle of observations, over spatial scales One unexpected result from AIDJEX was the dis- from meters to tens of kilometers, of albedo; snow covery that mesoscale eddies are widespread in the up- properties; melt ponds; ice growth and melt; radiation per layers of the Arctic Ocean. These eddies, which were fluxes; turbulent heat fluxes; cloud height, thickness, and shown to be baroclinic in nature, have diameters of 10– other properties; and ocean salinity, temperature, and 20 km and are found in in the uppermost 50–300 m of the currents. Persson et al. (2002) describe the atmospheric water column. Current speeds in their high-speed cores measurements made during the SHEBA field program. 2 are as large as 50–60 cm s 1, about 5 times that of the SHEBA data are still being widely used today. Scientific surrounding water. A total of 146 eddies were crossed by legacies of SHEBA include the realization that super- the AIDJEX station array during the 14-month main cooled clouds are surprisingly frequent over the Arctic observation period. Ocean (section 5e), the recognition of the importance of The successful use of automatic data buoys to de- Arctic cloud microphysical parameterizations in climate termine air stress and ice deformation paved the way for model simulations, and the discovery that an elevated expanded uses of buoys in the Arctic Ocean. AIDJEX temperature inversion is often associated with Arctic led directly to establishment of the Arctic Buoy Pro- clouds (Pithan et al. 2014). The latter complements the gram in 1978, followed by the extension to the IABP in earlier discovery that surface-based temperature in- 1991 (see section 5c). Another legacy of AIDJEX, versions are widespread in the Arctic (section 2). stemming from the use of sequential Landsat imagery c. CHAMP (early 2000s) from the main observing period of AIDJEX, is the use of satellite data for ice kinematic information, including The Community-wide Hydrologic Analysis and deformation rates and changes in leads and ridges. Monitoring Program (CHAMP) was a program to study Synthetic aperture radar data from satellites are now Arctic hydrology and its role in global change. CHAMP routinely processed into kinematic sea ice data for had its origins in a September 2000 workshop supported testing modeled ice motion and for assimilation into by NSF to assess the existing state-of-the-art in Arctic model hindcasts. AIDJEX model development has a systems hydrology and to identify research priorities that dual legacy in the state-of-the-art sea ice models, in- could lead to improved predictive understanding of feed- cluding those used in Earth system models: the ice backs arising from changes to the Arctic water cycle. thickness distribution and a plastic failure criterion. CHAMP was organized around three interacting compo- Both these features of sea ice models were foci of the nents: 1) compilation of data to better enable monitoring AIDJEX measurement program. and historical analysis of elements of the hydrologic sys- tem, 2) field observations and focused process studies, and b. SHEBA (1997–98) 3) the development of models operating over multiple The Surface Heat Budget of the Arctic, funded by temporal and spatial scales. CHAMP [ultimately funded NSF and the Office of Naval Research, was a field as the Freshwater Integration (FWI)] proved to be highly project designed to quantify energy transfer processes successful effort. The hydrologic cycle was shown to be that occur between the Arctic Ocean and the overlying intimately connected to all major processes defining the atmosphere. Planning for SHEBA started with a series character of the Arctic system as a whole. CHAMP of workshops held in the early 1990s. SHEBA was based therefore provided a platform for collaboration between on the premise that addressing climate feedbacks and scientists from diverse disciplines (Vörösmarty et al. 2002). improving the ability to model the Arctic system required Among the many accomplishments of CHAMP was a improved understanding of the surface energy budget much better understanding of the stocks and fluxes that and atmosphere–ocean–ice interactions. SHEBA was in constitute of Arctic hydrologic cycle, the freshwater bud- part driven by emerging observations that the Arctic was get of the Arctic Ocean, processes leading to variability in the midst of rapid change (Uttal et al. 2002). Phase I of and change in river discharge, and nutrient transports. SHEBA involved analysis of historical data, preliminary d. BOREAS (1990s–2000s) modeling studies and development of instrumentation to be used in the field program. Phase II of SHEBA—the The Boreal Ecosystem–Atmosphere Study (BOREAS) field element—got under way on 2 October 1997 when was a large-scale international interdisciplinary experi- the Canadian Coast Guard icebreaker Des Groseilliers ment in the boreal forests of central Canada. While most came to a halt in the Beaufort Sea and was allowed to be large field programs from the 1970s through the 1990s frozen in. Thus began a yearlong drift that lasted until addressed interactions of the atmosphere with sea ice and 11 October 1998. At any given time, there were 20–50 the ocean, BOREAS focused on the exchanges of CHAPTER 21 WALSH ET AL. 21.29 radiative energy, sensible heat, water, carbon dioxide, and rapid rate of change in northern Eurasia than in other other trace gases between the boreal forest and the lower parts of the world. Changes include an earlier spring atmosphere. Key questions targeted by BOREAS in- onset and longer warmer summers, which have in- cluded the following: What processes control the ex- creased the strength and duration of extreme events changes of gases and energy between the boreal forest and (drought, heavy precipitation, and extreme tempera- the atmosphere? How will climate change affect the for- tures) (Groisman and Gutman 2013). The increasing est? How will changes in the forest affect weather and temperatures and warm season duration are associated climate? The most intensive field experiments took place with decreases in river ice duration (Shiklomanov and in the mid-1990s, although additional measurements, Lammers 2014), changes in icing events (Bulygina 2015), analysis, and applications to model improvement contin- permafrost thaw (Streletskiy et al. 2015), and shrinkage ued into the 2000s. BOREAS integrated ground, tower, of Russian glaciers (Khromova et al. 2014). Future airborne, and satellite measurements of the interactions projections with climate and vegetation models point to between the forest ecosystem and the lower atmosphere. accelerated change in climate and land surface state Among the key findings was the fact that atmospheric (degrading permafrost) as well as the potential for exchanges were affected at least as much by processes in northward biome shifts of taiga vegetation into tundra the soil as by processes in the trees of the boreal forest. regions (Shuman et al. 2015). The impacts of such Annual carbon exchanges were found to be sensitive to changes on ecosystems and human activity have made summer temperatures and especially to the timing of the NEESPI region a target for integrated assessment snowmelt and soil freeze/thaw, which affect soil de- modeling (Monier et al. 2017). composition by microbial activity. While carbon uptake f. SEARCH (2010s) by photosynthesis is also dependent on the timing of snowmelt and spring/summer temperatures, net primary The ongoing Study of Environmental Arctic Change production was found to be generally more stable than (SEARCH) is a coordinated effort to observe, un- heterotropic respiration (Hall 2001). Flux measure- derstand, and guide responses to changes in the Arctic ments showed that the net ecosystem exchange of car- system. Motivated by the recognition that interrelated bon in boreal wetlands is a small residual between the environmental changes in the Arctic are affecting eco- much larger uptake and respiration rates. However, systems and living resources and are having an impact on sensitivities are such that a warming trend accompanied local and global communities and economic activities, the by permafrost thaw could change the boreal forest SEARCH program is supported by eight federal agencies from a long-term carbon sink to a climatically impor- in the United States. The NSF has overseen much of the tant carbon source (Hall 2001). BOREAS contributed planning and organization. The framing of SEARCH is to improved parameterizations of fluxes of water provided by a set of overarching science questions, which (evapotranspiration) and trace gases in climate models. are intended to bridge research and societal response: BOREAS measurements of forest albedo also led to How predictable are different aspects of the Arctic sys- improvements of weather forecasts by the ECMWF tem? How can improved understanding of predictability model (Viterbo and Betts 1999). facilitate planning, mitigation, and adaptation? What are the Arctic system’s tipping points—the abrupt changes e. GEWEX/NEESPI (2000s) that are most consequential for ecosystems and humans? The ongoing Global Energy and Water Exchanges How will the critical intersections between human and Project (GEWEX) program is a coordinated suite of natural systems in the Arctic change over the next several activities to improve understanding of the water cycle decades? What are the critical linkages between the and its interactions with the atmosphere and the land Arctic system and the global system? and ocean surfaces. Within its global framework, Among the specific targets of the early phases of GEWEX includes the Northern Eurasia Earth Science SEARCH is improved understanding of changes in sea Partnership Initiative (NEESPI), which addresses cli- ice, permafrost, and land ice with their implications for mate and environmental change in northern Eurasia global sea level. The goals of SEARCH also include within the water and energy cycle framework. NEESPI analysis of the societal and policy implications of Arctic targets not only the regional manifestations of change environmental change. An example of a SEARCH ac- but also the impacts of these changes on the global Earth tivity with societal applications is the Sea Ice Outlook, system. Over a period of a decade beginning in the early which has included seasonal forecasts of summer sea ice 2000s, NEESPI’s contributions include a wide range minima by several dozen research and operational groups of atmospheric, terrestrial, cryospheric, and socioeco- (Stroeve et al. 2014, 2015). The Sea Ice Outlook has nomic topics, the aggregate of which point to a more stimulated work to improve forecasts of sea ice over 21.30 METEOROLOGICAL MONOGRAPHS VOLUME 59 monthly and longer time scales (e.g., Stroeve et al. 2014; Rep., Government Printing Office, 986 pp., https://archive. Guemas et al. 2016). 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