The role of summer surface wind anomalies in the summer Arctic sea ice extent in 2010 and 2011

The seasonal evolutions of Arctic sea ice extent (SIE) during the summers of 2010 and 2011 are contrasted with that in 2007. The June SIE in 2010 was lower than that in 2007 and was the lowest for that calendar month in the 32-year (1979–2010) record. The September SIE in 2010 would have set a new record low had it not been for the fact that the ice retreated more slowly during the summer months in that year than it did in 2007. Hence from early July onward, the SIE in 2010 remained at levels above those observed in 2007. The SIE minimum in September 2010 proved to be the third lowest on record, eclipsed by values in both 2007 and 2008. In spring and summer of 2011, the Arctic SIE was as low as it was in 2007, but the SIE in September 2011 did not reach record low levels. The SIE minimum in 2011 proved to be the second lowest on record for the period of 1979–2011.

Summertime atmospheric conditions play an important role in controlling the variations in Arctic SIE. In a previous study based on statistical analysis of data collected prior to 2007, we showed that anticyclonic summertime circulation anomalies over the Arctic Ocean during the summer months favor low September SIE. We also found that the record-low ice summer year 2007 was characterized by a strong anticyclonic circulation anomaly, accompanied by an Ekman drift of ice out of the marginal seas toward the central Arctic and eventually toward the Fram Strait, as evidenced by the tracks of drifting buoys. Here we assess the extent to which year-to-year differences in summer winds over the Arctic might have contributed to the differing rates of retreat of ice during the summers of 2007, 2010, and 2011.

Our results show that the patterns of 925-hPa wind anomalies averaged over May-June (MJ) and July-August-September (JAS) in 2010. The MJ pattern is characterized by strong anticyclonic wind anomalies over the Arctic Ocean. The corresponding pattern for JAS is dominated by a cyclonic gyre centered over the Kara Sea. The corresponding patterns for 2007 are weak in MJ and strongly anticyclonic in JAS.

The September SIE in 2007 was the lowest of any year in the period 1979–2011. The retreat of sea ice during summer 2007 showed evidence of a substantial Ekman drift, based on the tracks of drifting buoys relative to the monthly mean sea level pressure contours. The anomalous cross-isobar drift acted to enhance the sea ice transport across the Arctic Ocean toward and out of the Fram Strait. The MJ pattern in 2010 is dominated by strong anticyclonic wind anomalies with the flow directed toward the Fram Strait, consistent with the rapidly decreasing SIE during these months. In contrast, the JAS pattern in 2010 is not characterized by anticyclonic anomalies that would have favored a strong transport of sea ice toward and out through the Fram Strait. The JJA pattern in 2011 is characterized by anticyclonic wind anomalies over the Arctic directed toward the Fram Strait, whereas the September pattern exhibits wind anomalies directed away from the Fram Strait across the central Arctic Ocean toward the Chukchi Sea. The corresponding patterns for 2007 are strongly anticyclonic and directed toward the Fram Strait in both JJA and September. In the absence of the late season push by the winds, the ice did not retreat quite as far in 2011 as it did in 2007.

We have shown evidence that low level winds over the Arctic, play an important role in mediating the rate of retreat of sea ice during summer. Anomalous anticyclonic flow over the interior of the Arctic directed toward the Fram Strait favors rapid retreat and vice versa. We have argued that the relative rankings of the September SIE for the years 2007, 2010 and 2011 are largely attributable to the differing rates of decrease of SIE during these summers, which are a consequence of year-to-year differences in the seasonal evolution of summertime winds over the Arctic.