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Recent Changes of Multiyear Ice Coverage and the Likely Causes by Igor v. Polyakov, John E. Walsh, and ronald kWok

Fig. 1. satellite-based Arctic multiyear ice (MYi) coverage. Composite time series shows MYi area on 1 Jan each year. Maps show fraction (part of a unit) of MYi. [Adapted from Kwok and Untersteiner (2011).]

ECEnt CHAnGEs of ARCtiC iCE CoVERAGE. AffiliAtions: Polyakov and Walsh—International Arctic Changes in the Arctic ice Research Center, University of Fairbanks, Fairbanks, R cover over the past several decades have been Alaska; kWok—Jet Propulsion Laboratory, California Institute of remarkable. Over the period 1979–2010, Northern Technology, Pasadena, California Hemisphere sea ice extent for September—the end of CoRREsPonDinG AUtHoR: Igor V. Polyakov, International the summer melt season—is characterized by a linear Arctic Research Center, University of Alaska Fairbanks, P.O. Box rate of decline of more than 11% per decade, and 757335, Fairbanks, AK 99775 the trend appears to be steeper for the last decade. E-mail: [email protected] For every year since 1996, the September ice extent DOI :10.1175 / BAMS - D -11- 0 0 070.1 has fallen below the 1979–1999 mean. The four ©2012 American Meteorological Society lowest September ice extents, including the record minimum in 2007, have all occurred during the past

aMerICaN MeTeOrOLOGICaL SOCIeTy February 2012 | 145 Unauthenticated | Downloaded 09/29/21 08:50 AM UTC when the area increased by more than 6 × 105 km2. Since 2007, the recovery of MYI coverage has been moderate and is localized in the eastern ; of particular interest is that MYI coverage resembles the bathym- etry defining the (a combination of the Amundsen and Nansen basins, shown in Fig. 4), pointing to the possible importance of oceanic processes affected by the shelf Fig. 2. Arctic ocean MYi coverage and export (black and green, 103 km2), atmospheric thermodynamic forcing (blue), and normalized break. In this article, we consider the Atlantic Water (AW) temperature anomalies (red). Atmospheric potential roles of both the atmosphere forcing is expressed as average fast-ice thickness anomalies (cm) and the ocean in the decline of MYI from six Arctic stations flanking the . MYi area export is coverage in the Arctic Ocean. from Kwok (2009). the normalized AW temperature anomalies are obtained by averaging data derived from four continental slope obser- iCE EXPoRt AnD AtMo - vation sites located at ~30°, 105°, 125°, and 142°E and normalized by sPHERiC DYnAMiCs. It is con- corresponding standard deviations (reverse vertical axis is used). ceivable that a large fraction of the MYI area loss is due to wind-driven four years. Winter ice extents are also declining, but export of sea ice through the straits connecting the at a slower rate, and the 2011 winter maximum ice Arctic Ocean with the subpolar basins. On average, extent was close to the lowest in the satellite record. ~700,000 km2 of sea ice (or ~10% of the area of the The observed decrease of ice extent is accompanied ) is exported annually via Strait by thinning. Using a combination of submarine and (for geographical notations, see Fig. 4), the major satellite records, Kwok and Rothrock (2009) found gateway of arctic ice export. Much of the export a ~1.8-m decrease in mean winter ice thickness in is MYI. Data collected by the International Arctic the central Arctic since 1980, with the steepest rate Buoy Programme suggest that the thinning of the of sea ice thickness decline, 0.10–0.20 m yr-1, during sea ice in the early to mid-1990s was attributable to the last five years. an increase in ice area export through . In addition to the diminishing extent and thin- Through that time period, this increased export has ning, the ice cover has become younger. At the end been linked to the positive phase of the Arctic Oscil- of the 2010 summer, only 15% of the ice remaining lation, which increases the cross-strait gradient in in the Arctic was more than two years old, compared sea level pressure. More recent atmospheric circula- to 50%–60% during the 1980s. There is virtually tion anomalies dominated by a dipole pattern (which none of the oldest (at least five years old) ice remain- is different from the Arctic Oscillation) also seem ing in the Arctic (less than 60,000 km2 compared favorable to increased advection of sea ice toward to 2 million km2 during the 1980s). Between 2005 Fram Strait. On the other hand, longer-term satellite and 2008 (Fig. 1), the Arctic Ocean lost 42% of its observations show that over the past 30 years there multiyear ice (MYI = ice which survives at least one has been a negligible increase in the measured ice Arctic summer) coverage. area export through Fram Strait. The decrease of The decline of the MYI winter coverage of the ice concentration at the strait compensated for the Arctic Ocean during the last decade has not been increase in the sea level pressure gradient across monotonic. QuikSCAT/ASCAT satellite records for Fram Strait, resulting in a statistically insignificant 1999–2009 show that since 1999, MYI area has de- trend in the Fram Strait area flux (discussed in a creased, with a total loss of more than 1 million km2 2009 Journal of Climate article by Kwok, p. 2,438; (~14% of the Arctic Ocean area) as evidenced by see also Fig. 2). Moreover, recent work has shown comparing the MYI area in each of the last three that decreases in MYI area in the due years of the record with that in each of the first three to the melting of mobile summer ice is another fac- years (Fig. 1). Over this period there were short-term tor that should be considered as a contributor to the recoveries of MYI coverage between 1999 and 2002, observed loss in coverage.

146 | February 2012 Unauthenticated | Downloaded 09/29/21 08:50 AM UTC Fig. 3. (left) Arctic annual surface air temperature (sAt) trends based on meteorological station data. statisti- cally significant trends are marked by larger circles.t he strongest warming has occurred in areas surrounding the Arctic ocean in accordance with the concept of polar amplification of global warming. Green dots denote stations where fast-ice thickness data were collected. Black circles denote locations of three meteorological stations for which time series are depicted in the right column. (Right) time series of sAt anomalies from three meteorological stations flanking the Eurasian Basin and from central Arctic (>80°n); the latter time series was derived from the output of the ERA interim reanalysis. Despite a strong background warming trend (red lines), the last several years show a cooling tendency, consistent with MYi coverage changes.

Interannual variability in sea ice export through Notably, there is an ongoing build-up of MYI in the Fram Strait is significant, with a low of 516,000 km2 Eurasian Basin. Analysis of satellite-derived ice motion in 1984–85 and a high of 1,002,000 km2 in 1994–95. showed that the inflows of ice into the Eurasian Basin An anomalously large wind-driven export event from the western Arctic Ocean in 2004–09 were too (for example, the peak 1994–95 export) could have small (< 0.04 × 106 km2) relative to the total MYI area a long-lasting impact on the survival of the MYI ice change to account for the observed increase. Therefore, cover, especially when large export events are super- it seems that the observed increase in MYI in this re- imposed on a warming trend. With lower annual gion must be due to the survival of seasonal ice. replenishment of the MYI area or reduced survival of seasonal ice through the summer, episodic large AtMosPHERiC tHERMoDYnAMiCs AnD outflows could be detrimental to the maintenance MElt. Changes of MYI coverage are tightly coupled of MYI coverage in the Arctic. Regional ice melt/ to the observed changes in surface air temperature growth complemented by ice flux through other (SAT) (Fig. 2). Positive SAT trends over recent years passages (like Nares Strait) has also contributed to (1990–2008) are the strongest in the maritime zone the observed MYI area decline. Thus, the role of flanking the Arctic Ocean (Fig. 3). Warming in the wind-driven export on the loss in MYI coverage Arctic since 1987 is evident in the time series of SAT remains an open question. anomalies from the three coastal stations. Strong

aMerICaN MeTeOrOLOGICaL SOCIeTy February 2012 | 147 Unauthenticated | Downloaded 09/29/21 08:50 AM UTC over the areas of reduced sea ice coverage suggest that SAT rises are partially a response to—rather than a driver of—declining ice coverage. Furthermore, SAT changes account for only part of the overall atmospheric thermo- dynamic forcing of the ice cover, which is orchestrated by a complex combination of various atmospheric param- eters and feedbacks. Fast-ice (motionless sea- sonal sea ice anchored to the sea floor and/or the shore that melts and reforms each year) thickness measurements are invaluable for isolating the impact of atmospheric warm- ing. Records of fast-ice thick- ness provide annual measures of ice growth due almost en- tirely to atmospheric thermo- dynamic forcing over the vast and shallow Siberian shelves, where there is negligible deep ocean influence on local ice formation. Fast-ice thickness Fig. 4. Vertical cross sections of water temperature (°C) from the Eurasian records are available through Basin of the Arctic ocean. the five series of cascaded plots show tempera- 2009 from several locations tures measured at the five locations shown by yellow lines on the map. in each along the Siberian coast (see section, the horizontal axis shows distance (km) and the vertical axis shows the map showing these lo- depth (m). note that the horizontal and temperature scales vary from one cations in Fig. 3). Using a cascaded section to another. the midocean nansen–Gakkel Ridge divides composite record of fast-ice the Eurasian Basin (not marked) into the and the Amundsen Basin. [Adapted from Polyakov et al. (2010).] thickness over the last several decades from 15 locations along the Siberian coast, it warming in the central Arctic (> 80°N) is also evident was shown that added surface melting in the eastern in fields from the ERA-Interim reanalysis http://( Arctic due to atmospheric thermodynamics caused data-portal.ecmwf.int/data/d/interim_mnth), which ~0.3 m of ice thickness loss. In particular, the updated are constrained by satellite retrievals, radiosonde thickness records from six fast-ice locations around the profiles, surface observations, and other data sources Laptev Sea show fluctuations in concert with changes (Fig. 3). Between 1999 and 2009, the overall decrease of MYI area (Fig. 3). These correlated changes suggest of MYI area was consistent with the general warming that the atmospheric thermodynamic forcing plays an trend shown in SAT records. The moderate increase of important role in shaping Arctic MYI coverage. MYI area in the Eurasian Basin fit well with the lower temperatures (relative to 2007) during the last several oCEAn HEAt. In addition to atmospheric ther- years (see time series in Fig. 3). While large-scale modynamics, the Arctic ice cover is affected by the high-latitude warming helps drive the observed re- thermal state of the Arctic Ocean. Enhanced upper- duction of sea ice extent and thickness, local warming ocean solar heating through openings in the ice and

148 | February 2012 Unauthenticated | Downloaded 09/29/21 08:50 AM UTC consequent ice bottom melting were observed in the Beaufort Sea EchoEs in summer 2007. This is a manifes- these rivers breathe a lot of carbon.” tation of the ice-albedo feedback —davId butman, a doctoral student at the Yale School of Forestry and mechanism, in which warming Environmental Studies, who coauthored a recent article published in leads to a reduction of ice coverage Nature Geoscience showing that rivers and streams in the “ are “supersaturated” with carbon dioxide (CO2) compared to the and decreasing albedo, resulting in atmosphere, releasing an amount of CO2 equivalent to a car burning further sea ice retreat. The sea ice 40 million gallons of gasoline (enough to fuel 3.4 million car trips to the reduction in the Canadian Arctic moon). Butman and coauthor Pete Raymond, a Yale professor, measured that began in the late 1990s as a temperature, alkalinity, and pH from samples of more than 4,000 U.S. result of increased influx of warm rivers and streams, and also studied the morphology and surface area summer waters of Pacific origin of the waterways. They fed this data into a model to determine the flux of CO from the water and found that the amount of CO given off by clearly shows the thermodynamic 2 2 rivers and streams “is significant enough for terrestrial modelers to take coupling between the Arctic ice note of it,” according to Butman. The study revealed that the CO2, after and the ocean interior. being released by decomposing plants, is making its way from the ground Further, oceanographic obser- into the rivers and streams. The researchers also determined that an vations carried out in the Eurasian increase in precipitation caused by climate change will create a cycle that

Basin of the Arctic Ocean suggest leads to increasing amounts of CO2 in the waterways and subsequently in that the thermal state of the Arctic the atmosphere. (sourcE: Yale University) Ocean interior has an impact on the Arctic ice cover. Observational and modeling results suggest that gradual warming of environment. The resulting turbulent heat fluxes from intermediate waters of Atlantic origin—the so-called the AW layer in the Arctic Ocean interior are small— Atlantic Water (AW) of the Arctic Ocean—helped less than 1 W m-2. There is, however, a wide range of precondition the polar ice cover for the extreme ice flux magnitudes that vary, depending on geographi- loss observed in recent years. In a 2010 Journal of cal location, from just a fraction of 1 W m-2 in the Physical Oceanography article, Polyakov et al. argued Canadian Basin to O (100) W m-2 north of Svalbard. that, on a time scale of several decades, ice thick- A double-diffusive mechanism resulting from the ness losses due to anomalous ocean heat flux could different diffusivities of heat and salt provides an im- be comparable to losses due to local atmospheric portant alternative to turbulent mixing in the eastern thermodynamic forcing. Observations in the 2000s Eurasian Basin. Diffusive instability (cold and fresh documented a new pulse of AW warming. Based on water above warm and salty water) maintains staircase all data prior to 1999, the most extreme 2007 tem- structures formed by layers of near-uniform water perature anomalies of up to 1°C and higher relative to temperature and salinity interleaved with strong- climatology were observed in the interior of the Eur- gradient thin interfaces. Double-diffusive heat fluxes asian and Makarov basins (see Fig. 2 from Polyakov in the lower halocline of the Eurasian Basin interior et al. 2010). On the other hand, observations carried based on ice-tethered profiler data are ~1–2 W m-2. out during several recent years showed cooling along Double-diffusive heat fluxes across several diffusive the Eurasian Basin margins (Fig. 4). layers occupying the 150–250-m depth range and The patterns of temporal AW temperature changes overlying the AW core from the eastern Eurasian in the Arctic Ocean’s Eurasian Basin and of MYI Basin are ~8 W m-2. These fluxes provide a means coverage are in good agreement (Fig. 2), suggesting for transferring AW heat upward over more than a a plausible role for anomalous oceanic heat in recent 100-m depth range toward the upper halocline. The changes of the Arctic ice cover. Unfortunately, no intermittent nature of staircases in the upper halocline direct evidence for such a link can be found in large- makes it difficult to evaluate the double-diffusive heat scale long-term measurements of oceanic heat fluxes. fluxes; a specialized field experiment may be neces- Does evidence exist to suggest a link between the sary to complete such an evaluation. AW heat and the state of arctic sea ice over the recent decade? The Arctic Ocean interior, away from the lACK of MYi RECoVERY sinCE 2007. boundary and upper mixed layer, is considered to be The absence of substantial MYI recovery between a low-energy and, correspondingly, a weakly mixing 2008 and 2009—despite the cooler conditions in aMerICaN MeTeOrOLOGICaL SOCIeTy February 2012 | 149 Unauthenticated | Downloaded 09/29/21 08:50 AM UTC 2009—implied by the change in fast-ice thickness scribed here is the potential contribution of oceanic points to a role of the large thermal inertia of the thermodynamic forcing to the recent changes of the ocean compared to the atmosphere. Specifically, high-latitude MYI coverage. Available observations heating of the near-surface and intermediate waters suggest a thermodynamic coupling between the heat of the deeper basins over the last decade, where MYI of the ocean interior and the sea ice. In the Canadian has historically formed, may be delaying the autumn Basin, the impact of Pacific water warmth has recently freeze-up, thereby reducing the length of time for been documented. While vertical AW heat fluxes are ice growth to occur, and hence the maximum ice negligible in the Canadian Basin, turbulent mixing thickness. This role of atmospheric thermodynamic may be strong enough in the western Nansen Basin forcing includes the effect of the albedo-temperature to produce a sizable effect of AW heat on sea ice. In feedback, which was triggered by a combination of the eastern Eurasian Basin, double diffusion provides higher air temperatures and wind patterns condu- an important alternative to weak turbulent mixing cive to ice retreat in years such as 2007. As noted for upward AW heat transport. However, this con- previously, additional warming added as heat from tribution to sea ice loss remains uncertain pending the Atlantic and Pacific layers may also have contrib- new field experiments that will provide estimates of uted to the reduction of ice thickness. The reduction upward AW heat fluxes. of ice thickness decreases the likelihood that the The fact that the rate of MYI recovery observed in ice will survive the following summer’s melt and recent years shows a delay relative to thermodynamic become MYI. In this respect, the reduction of MYI forcing indicates that MYI is resistant to recovery. is driven by, and perhaps even contributes (via the However, the relative roles of dynamic and thermo- reduced ice thickness) to the albedo-temperature dynamic factors in recent changes of the arctic MYI feedback. Thus, the rapid loss of sea ice in response cover remains to be determined. Quantifying these to the atmospheric and oceanic forcing of the past roles is a high priority if we are to develop reliable decade appears to have introduced some degree of forecasts of the future state of arctic ice coverage. irreversibility—at least over time scales of several years—into the loss of MYI. ACKnoWlEDGMEnts. This study was sup- ported by JAMSTEC (IP), NOAA (IP, JW), NASA (grant ConClUsions. This article addresses prob- NA06OAR4600183, IP, RK), and NSF (IP, JW) grants. We able causes of the observed reduction of the Arctic thank J. Moss for help with the graphics and three anony- Ocean’s coverage of MYI over the past decade. There mous reviewers for their useful comments. is evidence of the increasingly important role of at- mospheric thermodynamic forcing in shaping recent changes of the arctic MYI. In addition to direct MYI melt due to high-latitude warming, the impact of For FurthEr rEading enhanced upper-ocean solar heating through numer- Bekryaev, R. V., I. V. Polyakov, and V. A. Alexeev, 2010: ous leads in decaying arctic ice cover and consequent Role of polar amplification in long-term surface air ice-bottom melting has resulted in an accelerated rate temperature variations and modern arctic warming. of sea ice retreat via a positive ice-albedo feedback J. Climate, 23, 3888–3906. mechanism. The pan-Arctic role of this feedback is Belchansky, G. I., D. C. Douglas, and N. G. Platonov, yet to be quantified. Analysis of satellite ice motion 2008: Fluctuating arctic sea ice thickness changes suggests that the role of ice export through straits estimated by an in situ learned and empirically forced connecting the Arctic Ocean with subpolar basins neural network model. J. Climate, 21, 716–729. may be elusive. This situation probably differs from Comiso, J. C., 2002: A rapidly declining perennial sea the situation that existed in the early to mid-1990s, ice cover in the Arctic. Geophys. Res. Lett., 29, 1956, when enhanced ice export through Fram Strait was doi:10.1029/2002GL015650. caused by anomalous winds associated with the ——, 2006: Abrupt decline in the arctic winter positive Arctic Oscillation phase. The possible long- sea ice cover. Geophys. Res. Lett. 33, L18504, lasting impact of anomalous winds such as those in doi:10.1029/2006GL027341. 2004–05 or 2007 (especially when superimposed on ——, C. L. Parkinson, R. Gersten, and L. Stock, 2008: Ac- a warming trend) on the state of MYI should not be celerated decline in the Arctic sea ice cover. Geophys. ruled out. An intriguing feature of the scenario de- Res. Lett., 35, L01703, doi:10.1029/2007GL031972.

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