IGOR POLYAKOV

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Variability of the intermediate Atlantic Water of the Arctic Ocean over the last 100 years

I. V. Polyakov, G. V. Alekseev, L. A. Timokhov, U. S. Bhatt, R. L. Colony, H. L. Simmons, D. Walsh, J. E. Walsh, V. F. Zakharov

Details of this research may be found in our paper, [download pdf - Polyakov et al., 2004]

We examine long-term variability of the Atlantic Water (AW) using high-latitude hydrographic measurements starting in the late 19th century. Despite gaps in the early part of the record, our analysis provides evidence that AW variability is dominated by multi-decadal fluctuations with a timescale of 50-80 years (low-frequency oscillation = LFO, Polyakov and Johnson 2000). Associated with this variability, the Atlantic Water temperature record shows two warm periods in the 1930-40s and in recent decades and two cold periods in the earlier century and in the 1960-70s (Figure 1, top). Over recent decades, the data show a warming and salinification of the Atlantic layer, accompanied by its shoaling and, probably, thinning. Our estimate of the Atlantic Water temperature variability shows a general warming trend which for 1893-2002 was about 0.13+/-0.04 standard deviation of the annual mean AWCT per decade. However, over the 100-year record there are periods (including the recent decades) with short-term trends strongly amplified by multi-decadal variations.

Observational data provide evidence that Atlantic Water temperature, arctic surface air temperature and pressure, ice extent and fast ice thickness in the Siberian marginal seas, sea level (Dvorkin et al. 2000), and Barents Sea salinity (Polyakov and Johnson 2000) display coherent low-frequency fluctuations (Figure 1). The resemblance between variability of the AW and other key climatic parameters is striking, suggesting a close connection between large-scale atmospheric circulation and arctic ice and oceanic conditions.

In addition to documenting the variability we would like to determine the extent to which observed fluctuations in AW structure and properties represent a 'static' thermodynamic response and to what extent they are a dynamical response to winds driving the circulation. Clearly, the observed shoaling of the AW layer in the 1990s (Figure 2) is heavily influenced by the dynamics, while temperature, salinity and heat content fluctuations may be either dynamically or thermodynamically controlled. While discerning the detailed causes of the observed variability will require further investigation of observational and modeling data, our analysis suggests a possible negative feedback mechanism through which changes in density act to moderate the inflow of Atlantic Water to the Arctic Ocean, and hence a potential local source for fluctuations in AW inflow. This negative feedback may in part explain the decrease in warm Atlantic Water temperature anomalies evident in the late 1990s (Figure 2).

Sustained concerted phases of warming/salinification and cooling/freshening in the Arctic Ocean and Greenland and Norwegian seas, and the opposition of the variability in these basins and the Labrador Sea are likely due to air-sea interaction on larger spatial scales. And, indeed, the striking resemblance between the variability of the North Atlantic SST and AWCT (Figure 3) supports a possible linkage of low-frequency variations occuring in the Arctic and North Atlantic. The coordinated set of changes in the Arctic air-sea-ice system and the North Atlantic SSTs emphasizes the possible importance of high-latitude regions to global climate, since evidently ``a means exists of transferring the 'signal' of high-latitude climate change to the deep and abyssal headwaters of the global thermohaline circulation'' (Dickson et al. 2002).

This final AWCT time series is available online [download the time series now].

References

Dickson, R. R., I. Yashayaev, J. Meincke, W. R. Turrell, S. Dye, and J. Holfort, 2002: Rapid freshening of the deep North Atlantic Ocean over the past four decades, Nature, 416, 832-837.

Dvorkin, E. N., S. Yu. Kochanov, and N. P. Smirnov, 2000: The North Atlantic Oscillation and long-term changes in the level of the Arctic Ocean, Russian Meteorology and Hydrology, 3, 59-64.

Polyakov, I. V. and M. A. Johnson, 2000: Arctic decadal and interdecadal variability, Geophys. Res. Lett., 27, 4097-4100 [download pdf].

Polyakov, I. V., G. V. Alekseev, L. A. Timokhov, U. Bhatt, R. L. Colony, H. L. Simmons, D. Walsh, J. E. Walsh, and V. F. Zakharov, 2004: Variability of the intermediate Atlantic Water of the Arctic Ocean over the last 100 years, J. Climate, 17(23), 4485-4497 [download pdf].

Figure 1. Linkage between key components of the Arctic climate system. Composite time series of the Arctic surface air temperature (SAT) anomalies (top), anomalies of fast ice thickness in the Kara Sea (Hice, middle), and intermediate Atlantic Water Core Temperature (AWCT) anomalies (dashed segments represent gaps in the record, bottom) are shown (all curves are smoothed using 6-yr running mean). The time series show striking resemblance. Adopted from (Polyakov et al., 2004).

Figure 2. The depth of the Atlantic Water core temperature (left panels) and the integrated 0-800m density (right panels) averaged over the 1970s (top panels), 1990s (center panels), and their difference (bottom panels). Note that some smoothing based on the Laplacian operator is applied. Starting from the 1970s, the increased inflow of salty dense North Atlantic water induced positive density anomalies in the central Arctic Ocean. These anomalies suggest a geostrophic anomaly current (arrows) which acts to reduce inflow into the Arctic (i.e. negative feedback).

Figure 3. Linkage between processes in the Arctic and North Atlantic. The AWCT (red), normalized 6-yr running mean 10m water temperature anomalies from ocean weather station ``Mike'' at 66N, 2E (Norwegian Sea, blue), and normalized North Atlantic SST anomalies from the region limited by 0-90N, 290-30E (green) are shown.

Appendix 1: Method

The data used in this study consolidates several datasets. Measurements made by Nansen (1902) during his famous drift on-board the ``Fram'' began the era of deep-sea observation in the Arctic Ocean. Occasional ship-based observations near Fram Strait occurred starting in the beginning of the 20th century. Systematic oceanographic observations began only in the 1930-40s, when the Russians started a monitoring program consisting of manned ice-drift stations and winter aircraft surveys complemented by ship-based studies during summer. In 1955-56 the first Russian basin-scale aircraft surveys were conducted. A few observations are available starting in the 1960s, but the 1970s were an exceptional period in the history of high-latitude exploration, with seven Russian winter aircraft surveys (1973-79) and 1034 oceanographic stations during this period. Most measurements during the 1980s were made within limited, local areas. In the 1990s, the intensive use of ice-breakers and submarines opened a new phase in the history of arctic observation. Vast areas of the Arctic Ocean, previously limited to ship-based sampling in light ice conditions, were now within reach of these powerful scientific observational platforms. Data from only a few oceanographic casts carried out in the 2000s are available.

Because maximum AWCT variability is found in the Eurasian Basin, with variability decreasing toward the Beaufort Sea, a direct comparison of AWCT fluctuations from different Arctic Ocean regions is problematic. However, we can obtain comparable regional quantities by reducing AWCTs from different regions to their anomalies and normalizing these anomalies by their respective regional standard deviations. This approach was first proposed by G. Alekseev and has been used for analysis of the AWCT Alekseev et al. (2003). Alekseev et al. were first who showed the existence of multi-decadal mode of variability in the AWCT data. In our study, the Arctic Ocean is divided into 10 boxes (regions) of approximately equal areas (Fig. A1). Individual (snapshot) measurements over the ten regions were averaged within a given year and region to produce ten regional time series of composite AWCT. The length of the regional composite records is shown in Table. Based on these composite values, regional means and standard deviations were calculated (Table). Each regional composite value was then reduced to an anomaly relative to its regional mean and normalized by its regional standard deviation. The final time series in Figure 1 represents normalized regional AWCT anomalies averaged over these ten regions. This time series

Figure A1. Map of the Arctic Ocean. The locations of oceanographic stations used in this study are shown by red dots. Boxes delineate regions used in the analysis of the ocean temperature. The pathways of Atlantic Water generally follow topographic contours keeping shallow regions on their right, and are shown schematically by grey arrows.

Table: AW core temperature regional means (Mean), standard deviations (SD), and the length (N) (number of years with data) used to calculate these statistics

Last modified: February 07, 2005. 11:43:27 am