DOCSLIB.ORG

The 3.6-Ma Aridity and Westerlies History Over Midlatitude Asia Linked with Global Climatic Cooling

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
The 3.6-Ma Aridity and Westerlies History Over Midlatitude Asia Linked with Global Climatic Cooling The 3.6-Ma aridity and westerlies history over midlatitude Asia linked with global climatic cooling Xiaomin Fanga,b,c,1, Zhisheng Anc,d,e,1, Steven C. Clemensf, Jinbo Zana,b, Zhengguo Shic,g, Shengli Yangh, and Wenxia Hani aCenter for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, 100101 Beijing, China; bKey Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, 100101 Beijing, China; cState Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, 710075 Xi’an, China; dCenter for Excellence in Quaternary Science and Global Change, Chinese Academy of Sciences, 710061 Xi’an, China; eInterdisciplinary Research Center of Earth Science Frontier, Beijing Normal University, 100875 Beijing, China; fEarth, Environmental, and Planetary Sciences, Brown University, Providence, RI 02912; gOpen Studio for Oceanic-Continental Climate and Environment Changes, Pilot National Laboratory for Marine Science and Technology (Qingdao), 266061 Qingdao, China; hKey Laboratory of Western China’s Environmental Systems (Ministry of Education), College of Earth and Environmental Sciences, Lanzhou University, 730000 Lanzhou, China; and iShandong Provincial Key Laboratory of Water and Soil Conservation & Environmental Protection, School of Resource and Environmental Sciences, Linyi University, 276000 Linyi, China Contributed by Zhisheng An, August 10, 2020 (sent for review December 27, 2019; reviewed by Joseph A. Mason and Jule Xiao) Midlatitude Asia (MLA), strongly influenced by westerlies-controlled the MLA interior (1), with approximately half of the <20 μm fine climate, is a key source of global atmospheric dust, and plays a particles being transported by the upper-level westerly jet out of significant role in Earth’s climate system . However, it remains un- the region to the Pacific Ocean and beyond (1), even joining the clear how the westerlies, MLA aridity, and dust flux from this region global circulation (2, 4, 8) (Fig. 1A). Most of the coarse fraction is evolved over time. Here, we report a unique high-resolution eolian transported by the lower-level westerly winds generally southward dust record covering the past 3.6 Ma, retrieved from the thickest and deposited proximally, chiefly along the central southern loess borehole sequence (671 m) recovered to date, at the southern Taklimakan desert and on the northern slope of the West Kunlun margin of the Taklimakan desert in the MLA interior. The results Shan (Mountains), accumulating as thick loess deposits carpeting show that eolian dust accumulation, which is closely related to arid- various geomorphic surfaces (mountain slopes, foothill fans, river ity and the westerlies, indicates existence of a dry climate, desert terraces, and basin rims) (12–14) (Fig. 1B). Both modern mete- area, and stable land surface, promoting continuous loess deposi- orological data and numerical modeling demonstrate clearly that EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES tion since at least ∼3.6 Ma. This region experienced long-term step- both the lower and upper winds over the Tarim basin are con- ∼ wise drying at 2.7, 1.1, and 0.5 Ma, coeval with a dominant trolled chiefly by the westerlies under modern and peak glacial periodicity shift from 41-ka cyclicity to 100-ka cyclicity between conditions (Fig. 1A and SI Appendix,Fig.S1). This loess, like that 1.1 Ma and 0.5 Ma. These features match well with global ice vol- in the Chinese Loess Plateau, bears a wealth of direct information ume variability both in the time and frequency domains (including about the MLA aridification and westerlies change. the Mid-Pleistocene Transition), highlighting global cooling-forced The thickest and most continuous loess is found widely dis- aridity and westerlies climate changes on these timescales. Numer- tributed across almost all geomorphic surfaces between the al- ical modeling demonstrates that global cooling can dry MLA and titudes of 2,500 and 4,500 m (12) around the Hetian−Yutian intensify the westerlies, which facilitates dust emission and trans- area to the south of the basin, with its core showing the highest port, providing an interpretive framework. Increased dust may have promoted positive feedbacks (e.g., decreasing atmospheric CO2 con- centrations and modulating radiation budgets), contributing to fur- Significance ther cooling. Unraveling the long-term evolution of MLA aridity and westerlies climate is an indispensable component of the unfolding We recovered the world’s thickest continuous loess record from mystery of global climate change. the southern margin of the Taklimakan desert, a global-scale dust source area. The continuous high-resolution grain size and dust emission | Taklimakan loess sequence | Asian inland aridification | flux records of dust emission, reflecting histories of aridity and global cooling | Plio-Quaternary westerlies climate, indicate an extant dry climate, desert area, and stable land surface supporting continuous loess deposition at least since ∼3.6 Ma, and that global cooling, rather than Tibet he vast arid regions in midlatitude Asia (MLA) are among the uplift, modulated the histories of aridity and westerlies climate most prominent landscapes on Earth’s surface, spanning over T changes in inland Asia since ∼3.6 Ma. Moreover, our study may nine countries and hosting about 0.6 billion people, most suffering suggest potential positive linkages and feedback among dust from poverty exacerbated by increasing environmental stress due emission, marine biogeochemical activity, atmospheric CO2,and to aridification. Aridity in MLA has long been thought to have global cooling, which might provide insights into dynamics of strong impacts on Pacific Ocean primary productivity, global Earth’s climate system and improve predictions for the future. geochemical cycling, and climate change by influencing the global radiation budget (1–4) and atmospheric CO2 variability (5–7) Author contributions: X.F., Z.A., and S.C.C. designed research; X.F., J.Z., and S.Y. per- (Fig. 1A). Uplift of the Tibetan Plateau and global cooling have formed research; J.Z., Z.S., S.Y., and W.H. analyzed data; X.F., Z.A., and S.C.C. wrote the been thought to control the aridification of the MLA interior, and paper; and J.Z. and S.Y. drilled the core. global climates as well, through changes in the westerlies (9–11). Reviewers: J.A.M., University of Wisconsin−Madison; and J.X., Chinese Academy of Sciences. However, lack of detailed aridity records hinders our under- The authors declare no competing interest. standing of which mechanisms drive MLA aridification and This open access article is distributed under Creative Commons Attribution License 4.0 changes of the westerly jet, and hence the linkages between dust (CC BY). emission from the MLA interior and Pacific Ocean biogeochem- 1To whom correspondence may be addressed. Email: [email protected] or fangxm@ ical processes and global cooling. itpcas.ac.cn. 2 B The Tarim basin (560,000 km ) in northwest China (Fig. 1 ), This article contains supporting information online at https://www.pnas.org/lookup/suppl/ containing the world’s largest active dune field, the Taklimakan doi:10.1073/pnas.1922710117/-/DCSupplemental. desert, provides nearly two-thirds of the total dust generated in First published September 21, 2020. www.pnas.org/cgi/doi/10.1073/pnas.1922710117 PNAS | October 6, 2020 | vol. 117 | no. 40 | 24729–24734 Downloaded by guest on September 29, 2021 Fig. 1. (A) The long-distance transport of a dust storm in NH MLA on April 6–9, 2001, via the westerly jet as indicated by the strongest wind speeds in the 500-hPa map of the potential height and vector winds. Red numbers are dates. Black solid lines stand for the potential height, and arrows stand for wind direction (modified from ref. 8). (B) Physical geography and schematic atmospheric circulation pattern of Asia with the locations of the loess drilling site. Note that MLA is the largest arid interior region in a temperate continent (pale area) and that East Asia is characterized by a monsoonal humid region (green area). The solid black line outlines the 3,000-m-altitude region. stable platforms with loess surface elevations of ∼3,300 m to on the highest fan surface, with the loess platform surface elevation 3,200 m, ∼2,000 m above the basin floor (SI Appendix, Fig. S2). at 3,300 m, at the central southern margin of the Tarim basin The current circulation, dust storm track/deposition, and dust−loess (36°11′58.4″N, 81°20′15.4″E). The drilling extended down to 207 m geochemical and geological comparative analyses show that the depth. The paleomagnetism of the core determined its basal age to lower westerly winds flow over the lower divides of the Pamir and be ∼1 Ma (18). We recently drilled a second core at the same site through the Turpan Wind Pass, between the Tian Shan and the using a more powerful drilling rig. This latter campaign completely Bogda Mountains (15), generating and carrying dust from the penetrated the loess sequence and reached the fan surface of the desert to the southern rim of the basin and the northern slope of Xiyu (Conglomerate) Formation at the depth of 671 m (SI Ap- the West Kunlun Shan, forming a continuous loess deposit (12, pendix,Fig.S2), with a recovery of 96.1%. This drilling provides 16, 17) (Fig. 1B). Thus, this loess deposit archives the continuous evidence that central Asia has a thick loess sequence, which is 2 to histories of change in the westerly climate, the drying of the Asian 3 times thicker than the loess on the Chinese Loess Plateau and is interior, and dust emission in the Tarim basin. Analysis of these the thickest known in the world. sections will shed light on whether these deposits are driven by the This loess deposit shows similar lithologic characteristics to uplift of the Tibetan Plateau and contribute to and impact global those we see on the Loess Plateau (SI Appendix, Figs.
Load more

Recommended publications
  • Ocean Currents and the Pacific Garbage Patch
    ocean currents and the Pacific Garbage Patch photo: Scripps Institution of Oceanography The image above shows a net tow sample from the Pacific Garbage Patch. The sample contains small fish and many small pieces of plastic. The Garbage Patch is primarily composed of this small plastic “confetti” suspended throughout the surface water of the North Pacific Gyre, and is not a island of trash as suggested by some media outlets. The region where the trash converges in the center of the North Pacific Gyre, in a region where surface currents are weak and convergent, thus concentrating large amounts of trash in an area estimated to be close to the size of Texas. With this slide I often show a video clip about the Garbage Patch. Although the links may change, here is one from ABC that I’ve used several times: www.youtube.com/watch?v=OFMW8srq0Qk this video is a bit over the top but it gets the point across. There is additional information from Scripps Institution of Oceanography SEAPLEX experiment: http://sio.ucsd.edu/Expeditions/Seaplex/ and several videos on youtube.com by searching the phrase “SEAPLEX” Pacific garbage patch tiny plastic bits • the worlds largest dump? • filled with tiny plastic “confetti” large plastic debris from the garbage patch photo: Scripps Institution of Oceanography little jellyfish photo: Scripps Institution of Oceanography These are some of the things you find in the Garbage Patch. The large pieces of plastic, such as bottles, breakdown into tiny particles. Sometimes animals get caught in large pieces of floating trash: photo: NOAA photo: NOAA photo: Scripps Institution of Oceanography How do plants and animals interact with small small pieces of plastic? fish larvae growing on plastic Trash in the ocean can cause various problems for the organisms that live there.
    [Show full text]
  • NWS Unified Surface Analysis Manual
    Unified Surface Analysis Manual Weather Prediction Center Ocean Prediction Center National Hurricane Center Honolulu Forecast Office November 21, 2013 Table of Contents Chapter 1: Surface Analysis – Its History at the Analysis Centers…………….3 Chapter 2: Datasets available for creation of the Unified Analysis………...…..5 Chapter 3: The Unified Surface Analysis and related features.……….……….19 Chapter 4: Creation/Merging of the Unified Surface Analysis………….……..24 Chapter 5: Bibliography………………………………………………….…….30 Appendix A: Unified Graphics Legend showing Ocean Center symbols.….…33 2 Chapter 1: Surface Analysis – Its History at the Analysis Centers 1. INTRODUCTION Since 1942, surface analyses produced by several different offices within the U.S. Weather Bureau (USWB) and the National Oceanic and Atmospheric Administration’s (NOAA’s) National Weather Service (NWS) were generally based on the Norwegian Cyclone Model (Bjerknes 1919) over land, and in recent decades, the Shapiro-Keyser Model over the mid-latitudes of the ocean. The graphic below shows a typical evolution according to both models of cyclone development. Conceptual models of cyclone evolution showing lower-tropospheric (e.g., 850-hPa) geopotential height and fronts (top), and lower-tropospheric potential temperature (bottom). (a) Norwegian cyclone model: (I) incipient frontal cyclone, (II) and (III) narrowing warm sector, (IV) occlusion; (b) Shapiro–Keyser cyclone model: (I) incipient frontal cyclone, (II) frontal fracture, (III) frontal T-bone and bent-back front, (IV) frontal T-bone and warm seclusion. Panel (b) is adapted from Shapiro and Keyser (1990) , their FIG. 10.27 ) to enhance the zonal elongation of the cyclone and fronts and to reflect the continued existence of the frontal T-bone in stage IV.
    [Show full text]
  • Migration: on the Move in Alaska
    National Park Service U.S. Department of the Interior Alaska Park Science Alaska Region Migration: On the Move in Alaska Volume 17, Issue 1 Alaska Park Science Volume 17, Issue 1 June 2018 Editorial Board: Leigh Welling Jim Lawler Jason J. Taylor Jennifer Pederson Weinberger Guest Editor: Laura Phillips Managing Editor: Nina Chambers Contributing Editor: Stacia Backensto Design: Nina Chambers Contact Alaska Park Science at: [email protected] Alaska Park Science is the semi-annual science journal of the National Park Service Alaska Region. Each issue highlights research and scholarship important to the stewardship of Alaska’s parks. Publication in Alaska Park Science does not signify that the contents reflect the views or policies of the National Park Service, nor does mention of trade names or commercial products constitute National Park Service endorsement or recommendation. Alaska Park Science is found online at: www.nps.gov/subjects/alaskaparkscience/index.htm Table of Contents Migration: On the Move in Alaska ...............1 Future Challenges for Salmon and the Statewide Movements of Non-territorial Freshwater Ecosystems of Southeast Alaska Golden Eagles in Alaska During the A Survey of Human Migration in Alaska's .......................................................................41 Breeding Season: Information for National Parks through Time .......................5 Developing Effective Conservation Plans ..65 History, Purpose, and Status of Caribou Duck-billed Dinosaurs (Hadrosauridae), Movements in Northwest
    [Show full text]
  • Surface Circulation2016
    OCN 201 Surface Circulation Excess heat in equatorial regions requires redistribution toward the poles 1 In the Northern hemisphere, Coriolis force deflects movement to the right In the Southern hemisphere, Coriolis force deflects movement to the left Combination of atmospheric cells and Coriolis force yield the wind belts Wind belts drive ocean circulation 2 Surface circulation is one of the main transporters of “excess” heat from the tropics to northern latitudes Gulf Stream http://earthobservatory.nasa.gov/Newsroom/NewImages/Images/gulf_stream_modis_lrg.gif 3 How fast ( in miles per hour) do you think western boundary currents like the Gulf Stream are? A 1 B 2 C 4 D 8 E More! 4 mph = C Path of ocean currents affects agriculture and habitability of regions ~62 ˚N Mean Jan Faeroe temp 40 ˚F Islands ~61˚N Mean Jan Anchorage temp 13˚F Alaska 4 Average surface water temperature (N hemisphere winter) Surface currents are driven by winds, not thermohaline processes 5 Surface currents are shallow, in the upper few hundred metres of the ocean Clockwise gyres in North Atlantic and North Pacific Anti-clockwise gyres in South Atlantic and South Pacific How long do you think it takes for a trip around the North Pacific gyre? A 6 months B 1 year C 10 years D 20 years E 50 years D= ~ 20 years 6 Maximum in surface water salinity shows the gyres excess evaporation over precipitation results in higher surface water salinity Gyres are underneath, and driven by, the bands of Trade Winds and Westerlies 7 Which wind belt is Hawaii in? A Westerlies B Trade
    [Show full text]
  • Waves in the Westerlies
    Operational Weather Analysis … www.wxonline.info Chapter 9 Waves in the Westerlies Operational meteorologists track middle latitude disturbances in the middle to upper troposphere as part of their analysis of the atmosphere. This chapter describes these waves and highlights the importance of these waves to day-to-day weather changes at the Earth’s surface. The Westerlies Atmospheric flow above the Earth’s surface in the middle latitudes is primarily westerly. That is, the winds have a prevailing westerly component with numerous north and south meanders that impose wave-like undulations upon the basic west- to-east flow. This flow extends from the subtropical high pressure belt poleward to around 65 degrees latitude. A glance at any upper level chart from 700 mb upward to 200 mb shows that the westerlies dominate in the middle and upper troposphere. The term westerlies will refer to this layer unless otherwise specified. Upper Level Charts The westerlies are easily identified on charts of constant pressure in the middle to upper troposphere. That is, charts are prepared from upper level data at specified pressure levels (called standard levels). Traditionally, lines are drawn on these charts to represent the height of the pressure surface above mean sea level, temperature, and, on some charts, wind speed. With modern computer workstations, any observed or derived parameter may be drawn on an upper level chart. Standard levels include charts for 925, 850, 700, 500, 300, 250, 200, 150 and 100 mb. For our discussion of the westerlies, only levels from 700 mb upward to 200 mb will be considered.
    [Show full text]
  • The Interactions Between a Midlatitude Blocking Anticyclone and Synoptic-Scale Cyclones That Occurred During the Summer Season
    502 MONTHLY WEATHER REVIEW VOLUME 126 NOTES AND CORRESPONDENCE The Interactions between a Midlatitude Blocking Anticyclone and Synoptic-Scale Cyclones That Occurred during the Summer Season ANTHONY R. LUPO AND PHILLIP J. SMITH Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 20 September 1996 and 2 May 1997 ABSTRACT Using the Goddard Laboratory for Atmospheres Goddard Earth Observing System 5-yr analyses and the Zwack±Okossi equation as the diagnostic tool, the horizontal distribution of the dynamic and thermodynamic forcing processes contributing to the maintenance of a Northern Hemisphere midlatitude blocking anticyclone that occurred during the summer season were examined. During the development of this blocking anticyclone, vorticity advection, supported by temperature advection, forced 500-hPa height rises at the block center. Vorticity advection and vorticity tilting were also consistent contributors to height rises during the entire life cycle. Boundary layer friction, vertical advection of vorticity, and ageostrophic vorticity tendencies (during decay) consistently opposed block development. Additionally, an analysis of this blocking event also showed that upstream precursor surface cyclones were not only important in block development but in block maintenance as well. In partitioning the basic data ®elds into their planetary-scale (P) and synoptic-scale (S) components, 500-hPa height tendencies forced by processes on each scale, as well as by interactions (I) between each scale, were also calculated. Over the lifetime of this blocking event, the S and P processes were most prominent in the blocked region. During the formation of this block, the I component was the largest and most consistent contributor to height rises at the center point.
    [Show full text]
  • Contrasting Recent Trends in Southern Hemisphere Westerlies Across
    RESEARCH LETTER Contrasting Recent Trends in Southern Hemisphere 10.1029/2020GL088890 Westerlies Across Different Ocean Basins Key Points: Darryn W. Waugh1,2 , Antara Banerjee3,4, John C. Fyfe5, and Lorenzo M. Polvani6 • There are substantial zonal variations in past trends in the 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA, 2School of Mathematics magnitude and latitude of the peak 3 Southern Hemisphere zonal winds and Statistics, University of New South Wales, Sydney, New South Wales, Australia, Cooperative Institute for Research in • Only over the Atlantic and Indian Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA, 4Chemical Sciences Laboratory, National Oceans have peak winds shifted Oceanic and Atmospheric Administration, Boulder, CO, USA, 5Canadian Centre for Climate Modelling and Analysis, poleward, with an equatorward (yet Environment and Climate Change Canada, Victoria, British Columbia, Canada, 6Department of Applied Physics and insignificant) shift over the Pacific • Climate model simulations indicate Applied Mathematics, Columbia University, New York, NY, USA that the differential movement of the westerlies is due to internal atmospheric variability Abstract Many studies have documented the trends in the latitudinal position and strength of the midlatitude westerlies in the Southern Hemisphere. However, very little attention has been paid to the Supporting Information: longitudinal variations of these trends. Here, we specifically focus on the zonal asymmetries in the southern • Supporting Information S1 hemisphere wind trends between 1980 and 2018. Meteorological reanalyses show a large strengthening and a statistically insignificant equatorward shift of peak near‐surface winds over the Pacific, in contrast to a fi Correspondence to: weaker strengthening and signi cant poleward shift over the Atlantic and Indian Ocean sectors.
    [Show full text]
  • Lecture 21 Scale Interaction
    MET 200 Lecture 21 Previous Lecture – Lost at Sea: Synoptic-Planetary Scale Interaction Hurricane Force Wind Fields and High Impact Weather and the North Pacific Ocean Environment The Global and Synoptic context of High Impact Weather Systems 1 2 Typhoon Haiyan Typhoon Haiyan Last Friday’s super typhoon Haiyan struck the Philippines. Officials estimate that 10,000 or more people were killed by Haiyan, washed away by the churning waters that poured in from the Pacific or buried under mountains of trash and rubble. But it may be days or even weeks before the full extent of the destruction is known. A 6-meter (20-feet) storm surge swept through Tacloban, capital of the island province of Leyte, which saw the worst of Haiyan’s damage. While the storm surge proved deadly, much of the initial destruction was caused by winds blasting at 235 kilometers per hour (147 mph) that occasionally blew with speeds of up to 275 kph (170 mph), howling like jet engines. 3 4 Typhoon Haiyan Typhoon Haiyan 5 6 Typhoon Haiyan Typhoon Haiyan 7 8 Typhoon Haiyan Why Wasn’t the Population better Protected? The Philippines, which sees about 20 typhoons per year, is cursed by its geography. On a string of some 7,000 islands, there are only so many places to evacuate people to, unless they can be flown or ferried to the mainland. The Philippines’ disaster preparation and relief capacities are also hampered by political factors. It lacks a strong central government and provincial governors have virtual autonomy in dealing with local problems.
    [Show full text]
  • Weather and Climate Science 4-H-1024-W
    4-H-1024-W LEVEL 2 WEATHER AND CLIMATE SCIENCE 4-H-1024-W CONTENTS Air Pressure Carbon Footprints Cloud Formation Cloud Types Cold Fronts Earth’s Rotation Global Winds The Greenhouse Effect Humidity Hurricanes Making Weather Instruments Mini-Tornado Out of the Dust Seasons Using Weather Instruments to Collect Data NGSS indicates the Next Generation Science Standards for each activity. See www. nextgenscience.org/next-generation-science- standards for more information. Reference in this publication to any specific commercial product, process, or service, or the use of any trade, firm, or corporation name See Purdue Extension’s Education Store, is for general informational purposes only and does not constitute an www.edustore.purdue.edu, for additional endorsement, recommendation, or certification of any kind by Purdue Extension. Persons using such products assume responsibility for their resources on many of the topics covered in the use in accordance with current directions of the manufacturer. 4-H manuals. PURDUE EXTENSION 4-H-1024-W GLOBAL WINDS How do the sun’s energy and earth’s rotation combine to create global wind patterns? While we may experience winds blowing GLOBAL WINDS INFORMATION from any direction on any given day, the Air that moves across the surface of earth is called weather systems in the Midwest usually wind. The sun heats the earth’s surface, which warms travel from west to east. People in Indiana can look the air above it. Areas near the equator receive the at Illinois weather to get an idea of what to expect most direct sunlight and warming. The North and the next day.
    [Show full text]
  • Synoptic Meteorology
    Lecture Notes on Synoptic Meteorology For Integrated Meteorological Training Course By Dr. Prakash Khare Scientist E India Meteorological Department Meteorological Training Institute Pashan,Pune-8 186 IMTC SYLLABUS OF SYNOPTIC METEOROLOGY (FOR DIRECT RECRUITED S.A’S OF IMD) Theory (25 Periods) ❖ Scales of weather systems; Network of Observatories; Surface, upper air; special observations (satellite, radar, aircraft etc.); analysis of fields of meteorological elements on synoptic charts; Vertical time / cross sections and their analysis. ❖ Wind and pressure analysis: Isobars on level surface and contours on constant pressure surface. Isotherms, thickness field; examples of geostrophic, gradient and thermal winds: slope of pressure system, streamline and Isotachs analysis. ❖ Western disturbance and its structure and associated weather, Waves in mid-latitude westerlies. ❖ Thunderstorm and severe local storm, synoptic conditions favourable for thunderstorm, concepts of triggering mechanism, conditional instability; Norwesters, dust storm, hail storm. Squall, tornado, microburst/cloudburst, landslide. ❖ Indian summer monsoon; S.W. Monsoon onset: semi permanent systems, Active and break monsoon, Monsoon depressions: MTC; Offshore troughs/vortices. Influence of extra tropical troughs and typhoons in northwest Pacific; withdrawal of S.W. Monsoon, Northeast monsoon, ❖ Tropical Cyclone: Life cycle, vertical and horizontal structure of TC, Its movement and intensification. Weather associated with TC. Easterly wave and its structure and associated weather. ❖ Jet Streams – WMO definition of Jet stream, different jet streams around the globe, Jet streams and weather ❖ Meso-scale meteorology, sea and land breezes, mountain/valley winds, mountain wave. ❖ Short range weather forecasting (Elementary ideas only); persistence, climatology and steering methods, movement and development of synoptic scale systems; Analogue techniques- prediction of individual weather elements, visibility, surface and upper level winds, convective phenomena.
    [Show full text]
  • Shifting Westerlies to Shift After the Last Glacial Period? J
    PERSPECTIVES CLIMATE CHANGE What caused atmospheric westerly winds Shifting Westerlies to shift after the last glacial period? J. R. Toggweiler he westerlies are the prevailing ica-rich deep water must be drawn to the sur- winds in the middle latitudes of face. As mentioned above, silica-rich deep Earth’s atmosphere, blowing water tends to be high in CO . It is also T 2 from west to east between the high- warmer than the near-freezing surface pressure areas of the subtropics and waters around Antarctica. the low-pressure areas over the poles. A shift of the westerlies that draws They have strengthened and shifted more warm, silica-rich deep water to poleward over the past 50 years, pos- the surface is thus a simple way to sibly in response to warming from explain the CO2 steps, the silica pulses, rising concentrations of atmospheric and the fact that Antarctica warmed carbon dioxide (CO2) (1–4). Some- along with higher CO2 during the two thing similar appears to have happened steps. Anderson et al.’s two silica pulses 17,000 years ago at the end of the last ice occur right along with the two CO2 steps, age: Earth warmed, atmospheric CO2 in- which implies that the westerlies shifted creased, and the Southern Hemisphere wester- early as the level of CO2 in the atmosphere lies seem to have shifted toward Antarctica began to rise. Had the westerlies shifted in (5, 6). Data reported by Anderson et al. on response to higher CO , one would expect to Southward movement. At the end of the last ice 2 page 1443 of this issue (7) suggest that the see more upwelling and more silica accumula- on October 23, 2009 age, the ITCZ and the Southern Hemisphere wester- shift 17,000 years ago occurred before the lies winds moved southward in response to a flatter tion after the second CO2 step when the level of warming and that it caused the CO2 increase.
    [Show full text]
  • On the Dynamics of Tropical Cyclone Structural Changes
    Quart. J. R. Met. SOC. (1984), 110, pp. 723-745 551.515.21 On the dynamics of tropical cyclone structural changes By GREG. J. HOLLAND. and ROBERT T. MERRILL Department of Atmospheric Science, Colorado State University (Received 16 March 1983; revised 17 January 1%) SUMMARY Tropical cyclone structural change is separated into three modes: intensity, strength and size. The possible physical mechanisms behind these three modes are examined using observations .in the Australiadsouth-west Pacific region and an axisymmetric diagnostic model. We propose that, though dependent on moist convection and other internal processes, the ultimate intensity, strength or sue of a cyclone is regulated by interactions with its environment. Possible mechanisms whereby these interactions occur are described. 1. INTRODUCTION Most of the work on tropical cyclone development in past years has been concerned with ‘genesis’ and ‘intensification’, even though no precise definition exists for either term. The result has been confusion in interpreting some aspects of cyclone structural changes, and the neglect of other aspects altogether. More precise and comprehensive interpretations are possible if we separate tropical cyclone structural changes into three modes: intensity, strength, and size (Merrill and Gray 1983). These are illustrated in Fig. 1 as changes from an initial tangential wind profile. ‘Intensity’ is defined by the maximum wind, or by the central pressure if no maximum wind data are available, and has received the most attention in the literature. ‘Strength’ is defined by the average relative angular momentum of the low-level inner circulation (inside 300 km radius). ‘Size’ is defined by the axisymmetric extent of gale force winds, or by the radius of the outermost closed isobar.
    [Show full text]