IGOR POLYAKOV

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Long-term ice variability in arctic marginal seas

I. Polyakov, G. V. Alekseev, R. V. Bekryaev, U. Bhatt, R. Colony, M. Johnson, V. P. Karklin, D. Walsh, and A. V. Yulin

[for more details read the paper Polyakov et al, 2003b]

Introduction

Numerous changes have occurred in the Arctic over the last few decades: Arctic surface air temperature (SAT) exhibits rapid increases (Martin et al. 1997; Rigor et al. 2000), and sea-level pressure (SLP) has decreased and atmospheric cyclonicity increased in the late 1980s through the early 1990s, relative to any time in the past several decades (Walsh et al. 1996). Concurrent with these atmospheric changes are reductions in arctic ice extent and a decrease of ice thickness. Observations show an average decrease in ice cover over the whole Arctic by 2.9% per decade during 1979-96, with a stronger reduction of 4-6% per decade in summer and a much smaller decline of 0.6% per decade in winter (Deser et al. 2000 and references therein). Depending on the time period and geographical location, some investigators have reported thinning of arctic ice (Wadhams 1994), whereas other studies find no convincing evidence of a trend (McLaren et al. 1994; Shy and Walsh 1996). Summarizing available ice draft data from submarine cruises, Rothrock et al. (1999) documented a 1.3m reduction of ice thickness in the central Arctic in the 1990s relative to the 1950-1970s. They also pointed to continued thinning of arctic ice in the 1990s at a rate of 0.1 m/yr. However, Winsor (2001), extending the Rothrock et al. data through the 1990s with data from three additional submarine cruises, argued that there was no decrease in ice thickness during the 1990s. Later, Tucker et al. (2001) reported thinning of sea ice in the western Arctic in the recent decades, but found no evidence of a similar trend near the North Pole between the mid-1980s and early 1990s. This diversity of conclusions may indicate a fundamental statistical sampling problem in analyzing highly variable (both spatially and temporally) arctic ice thickness using existing time series. For example, Polyakov and Johnson (2000) and Holloway and Sou (2002) using coupled ice-ocean models showed that most ice loss in the recent decades was due to wind forced ice export from the central Arctic.

The goal of this research is to assess long-term arctic sea-ice variability, with a special focus on multidecadal fluctuations (Low-Frequency Oscillation, or LFO) with a period of 50-80 years, which has played an important role in recently observed changes in the Arctic environment (Polyakov and Johnson 2000).

Data

In this study, we use August ice extent (1900-2000) and fast-ice thickness (1936-2000, motionless sea ice anchored to the sea floor and/or the shore) measurements from five locations in the Kara, Laptev, East Siberian, and Chukchi seas (Figure 1). Measurements of fast-ice thickness are especially invaluable because they provide an opportunity to separate, to some extent, the contribution of thermodynamical and dynamical factors in the formation of arctic ice since they measure ``pure'' thermodynamical ice growth. While these data may have substantial errors, they are unique in indicating important changes in the arctic environment since the dawn of the industrial era.

Variability of ice extent and fast-ice thickness

Examination of records of ice extent (Figure 2) and fast-ice thickness (Figure 3) provide evidence that the ice variability in the arctic seas is dominated by a multidecadal, low-frequency oscillation (LFO) and (to a lesser degree) by higher-frequency decadal fluctuations. The time series show lower values prior to the 1920s, in the late 1930s-40s, and in recent decades, and higher values in the 1920s - early 1930s and in the 1960-70s. This is consistent with the multiyear LFO found in instrumental records of arctic SAT and SLP (Polyakov et al. 2003a), observed and simulated salinity in the upper Barents Sea (Polyakov and Johnson 2000), and observed ice extent in the Nordic Seas (Vinje 2001). There are, however, some important regional differences in ice variability. The LFO amplitude tends to decrease from the Atlantic sectors toward Pacific sectors of the Arctic. In the Kara Sea, which is closer to the North Atlantic, the LFO signature is most pronounced (Figures 2, 3). In the Chukchi Sea, the furthest of the seas from the North Atlantic, ice variability is dominated by a relatively high-frequency decadal mode and there is no evidence of the LFO. This spatial pattern is consistent with the air temperature/NAO index correlation pattern, with maximum correlation in the near-Atlantic region, which decays toward the North Pacific (Polyakov et al. 2003a).

Ice extent and fast-ice thickness long-term trends

Analysis of records (Figures 2, 3) also shows that long-term ice trends are small and generally not statistically significant (at 95% level), while trends for shorter records are not indicative of the long-term tendencies due to large-amplitude LFO. Over the entire Siberian marginal-ice zone the century-long ice-extent trend is only -0.5% per decade. In the Kara, Laptev, East Siberian, and Chukchi seas the ice-extent trends are not large either: -1.1, -0.4, 0.3, and -1.0% per decade, respectively. Moreover, statistical analysis based on the bootstrap technique shows that these trends, except for the Chukchi Sea, are not statistically significant. The fast-ice records do not show a significant trend either (Figure 3). In the Kara and Chukchi seas trends are positive, and in the Laptev and East Siberian seas trends are negative. In all of the seas the trends are relatively small, about 1 cm/decade, close to the resolution of the measurements. These trends are not statistically significant at the 95% confidence level. Thus, using these data we cannot conclusively identify the possible moderating role of sea ice in the apparent lack of polar amplification of global warming in the century-long arctic SAT time series (see for details Polyakov et al. 2002).

What drives major long-term changes in ice cover in the arctic marginal seas?

Previous studies showed that at time scales of up to decades sea-ice conditions are controlled by changes in the atmospheric circulation pattern. Our study extends this result, suggesting that even at interdecadal time scales winds remain the major contributor to ice-extent variation in the Siberian marginal-ice zone. Table 1 displays correlations of ice-extent/SAT and ice-extent/SLP gradients. It follows from Table 1 that dynamical factors (wind or surface currents) are at least of the same order of importance as thermodynamical factors in the Laptev, East Siberian, and Chukchi seas (compare raws 2 and 3). In the Kara Sea, ice growth/decay out-weighs dynamical factors. Note that prevailing easterly winds over the Chukchi Sea cannot contribute much to northward advection of ice into the Arctic Ocean, reflecting a weak correlation between winds and ice extent. However, northward surface currents fed by Pacific waters entering the Chukchi Sea through Bering Strait provide an effective mechanism of ice transport to the Beaufort Sea.

Conclusions

Examination of records of fast-ice thickness and ice extent from four arctic marginal seas (Kara, Laptev, East Siberian, and Chukchi) indicates that long-term trends are small and generally statistically insignificant, while trends for shorter records are not indicative of the long-term tendencies due to strong low-frequency variability in these time series, which places a strong limitation on our ability to resolve long-term trends. Ice variability in the arctic marginal-ice zone is dominated by the multidecadal LFO and, to a lesser degree, by decadal fluctuations. The LFO signal decays eastward, and is strongest in the Kara Sea, whereas in the Chukchi Sea, ice-extent and fast-ice variability is dominated by decadal fluctuations, and there is no evidence of the LFO.

Sensitivity analysis shows that dynamical forcing (wind or surface currents) is at least of the same order of importance as thermodynamical forcing for the ice-extent variability in the Laptev, East Siberian, and Chukchi seas. Prevailing easterly winds over the Chukchi Sea cannot contribute much to ice advection across the open boundary, but surface currents provide an effective mechanism of ice transport from the sea into the central Arctic Ocean. In the Kara Sea, thermodynamical factors out-weigh dynamical factors in controlling ice-extent variability. This analysis implies that deficiencies of present-day models, such as the oversimplification of ice dynamics, make simulation of fundamental ice-albedo feedback most difficult.

Acknowledgments. This project was supported by grants from the International Arctic Research Center, University of Alaska Fairbanks and the National Science Foundation's Office Of Polar Programs. We thank the Frontier Research System for Global Change (FRSGC) for financial support.

References

Deser, C., J. E. Walsh, and M. S. Timlin, 2000: Arctic sea ice variability in the context of recent atmospheric circulation trends, J. Climate, 13, 617-633.

Holloway, G., and T. Sou, 2002: Has arctic sea ice rapidly thinned? J. Climate, submitted.

Martin, S., E. A. Minoz, and R. Drucker, 1997: Recent observations of a spring- summer surface warming over the Arctic Ocean, Geophys. Res. Lett., 24, 1259-1262.

McLaren, A. S., R. H. Bourke, J. E. Walsh, and R. L. Weaver, 1994: Variability in sea-ice thickness over the North Pole from 1958 to 1992, in The Polar Oceans and Their Role in Shaping the Global Environment: The Nansen Centennial Volume, Geophys. Monogr. Ser., vol. 85, edited by O. M. Johannessen, R. D. Muench, and J. E. Overland, pp. 363-371, AGU, Washington, D.C.

Polyakov, I. V., G. V. Alekseev, R. V. Bekryaev, U. Bhatt, R. Colony, M. A. Johnson, V. P. Karklin, A. P. Makshtas, D. Walsh, and A. V. Yulin, 2002: Observationally-based assessment of polar amplification of global warming, Geophys. Res. Lett., 29, 1878, doi:1029/2001GL011111 [Download PDF file].

Polyakov, I. V., R. V. Bekryaev, G. V. Alekseev, U. Bhatt, R. Colony, M. A. Johnson, A. P. Makshtas, 2003a: Variability and trends of air temperature and pressure in the maritime Arctic, 1875-2000, J. Climate, 16(12), 2067-2077 [Download PDF file].

Polyakov, I., G. V. Alekseev, R. V. Bekryaev, U. Bhatt, R. Colony, M. A. Johnson, V. P. Karklin, D. Walsh, and A. V. Yulin, 2003b: Long-term ice variability in arctic marginal seas, J. Climate, 16(12), 2078-2085 [Download PDF file].

Polyakov, I., and M. A. Johnson, 2000: Arctic decadal and interdecadal variability, Geophys. Res. Lett., 27, 4097-4100 [ Download PDF file].

Rigor, I. G., R. L. Colony, and S. Martin, 2000: Variations in surface air temperature observations in the Arctic, 1979-1997, J. Climate, 13, 896-914.

Rothrock, D. A., Y. Yu, and G. A. Maykut, 1999: Thinning of the arctic sea-ice cover, Geophys. Res. Lett., 26, 3469-3472.

Shy, T. L., and J. E. Walsh, 1986: North Pole ice thickness and association with ice motion history 1977-1992, Geophys. Res. Lett., 21, 2975-2978.

Tucker, W. B. III, J. W. Weatherly, D. T. Eppler, L. D. Farmer, and D. L. Bentley, 2001: Evidence for rapid thinning of sea ice in the western Arctic Ocean at the end of the 1980s, Geophys. Res. Lett., 28, 2851-2854.

Vinje, T., 2001: An oscillation in the Barents sea climate system of period 70 years: Indication of a colder climate ahead, J. Climate, submitted.

Wadhams, P., 1994: Sea ice thickness changes and their relation to climate, in The Polar Oceans and Their Role in Shaping the Global Environment: The Nansen Centennial Volume, Geophys. Monogr. Ser., vol. 85, edited by O. M. Johannessen, R. D. Muench, and J. E. Overland, pp. 337-361, AGU, Washington, D.C..

Walsh, J. E., W. L. Chapman, and T. L. Shy, 1996: Recent decrease of sea level pressure in the central Arctic, J. Climate, 9, 480-486.

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Figure 1. Map of the Arctic Ocean, with colors denoting ice extent analysis regions. Locations of sea level pressure observations are shown by red dots. Red stars denote stations where ice thickness data were collected. Red lines denote locations of cross sections used for analysis of the modeling ice and water transports.

Figure 2. (Left panels) Time series of the ice-extent anomalies (1000km^2) in four arctic seas. Dotted lines show annual means from observations (blue) and modeling (red), solid lines show six-year running means from observations (blue) and modeling (red), green dashed lines show linear trends (quoted limits represent 95% confidence levels). Note axis scalings are not uniform. (Right panels) Wavelet transform of annual ice extent using the Mexican Hat (m=2) wavelet (Torrence and Compo 1998). Vertical axes show the period in years. The black contours are the 95% (thick) and 68% (thin) confidence levels, the black cross-hatched area denotes the region of the wavelet spectrum in which edge effects become important. The time series and wavelet transform indicate two periods of minimum ice extent associated with positive LFO phases (and warming in the 1930-40s and late 1980s-90s) and two periods of maximum ice extent associated with negative LFO phases (and cooling prior to the 1920s and in the 1960-70s).

Figure 3. Left panels. Time series of ice-thickness anomalies (cm) at five locations. Dotted blue lines show annual means and solid blue lines show six-year running means, green dashed lines show linear trends (quoted limits represent 95% confidence levels). Right panels. Wavelet transform of annual fast-ice thickness using the ``Mexican Hat'' (m=2) wavelet. Vertical axes show the period in years. The black contours are the 95% (thick) and 68% (thin) confidence levels, the black cross-hatched area denotes the region of the wavelet spectrum in which edge effects become important (hence statistical significance is low). The time series and wavelet transform at Dikson and Sterlegov (Kara Sea) and Chetirekhstolbovii (East Siberian Sea) indicate two periods of minimum ice extent associated with positive LFO phases and one period of maximum ice extent associated with negative LFO phase.

Last modified: May 06, 2004. 14:58:18 pm