Windinduced ice cover variability

A more fundamental question is how crucial wind-driven ice motion is for a less ice anomaly in the Pacific sector of the Arctic Basin. Once a southerly wind pushes sea ice away from the coastal region in summer, solar radiation effectively heats the ocean and accelerates ice melting. This effect is considered to be a major one for the record low in summer of 2007 (J. Wang, personal communication 2008).

Winter

Winter

CONTOUR INTERVAL = 5,000E-02

1950 1960 1970 1980 1990 2000 Year

CONTOUR INTERVAL = 5,000E-02

1950 1960 1970 1980 1990 2000 Year

Fig. 6. The second EOF of sea level pressure in winter north to 70°N.

Fig. 7. Sea ice anomalies in three regions without 3-year smoothing. Positive ADMs are correlated with low ice in the Beaufort & Chukchi Sea with a 1-2 year lag.

Fig. 7. Sea ice anomalies in three regions without 3-year smoothing. Positive ADMs are correlated with low ice in the Beaufort & Chukchi Sea with a 1-2 year lag.

Once the atmospheric circulation is analyzed, we usually rely on the first EOF. This mode basically corresponds to fluctuation between intense and weaker phases of the Polar Vortex and called NAM/AO. Although it has effects on the ice cover in the Pacific sector (section "Introduction"), the direct wind effects work on zonal ice motion. Then, we extended analyses to the second EOF, as done by Wu et al. (2006), and found a horizontal feature with two poles shown in Fig. 6, where the domain was restricted to north of 70oN. This mode is called the Arctic Dipole Mode (ADM) in this paper and much stronger in winter (Dec.-Feb.) than the other seasons. During the positive phase, meridional winds effectively push sea ice seaward in the Pacific sector. The opposite tendency appears in the Barents-Kara Sea. The time series presents decadal and year-to-year variability with a slow negative trend.

A correlation is inspected visually between the sea ice cover and the ADM in Fig. 7, in which the ice cover time series is lagged or shifted leftward by 1.5 years. Here, the Fanning filter is not applied so that the year-to-year variability is clearly shown. A negative correlation is found between the ADM and the Beaufort-Chukchi Sea ice cover in summer with 1 year or 2 year lags: i.e., the ice cover decrease appears in summer 1.5 or 2.5 years after the positive winter ADM. This correlation became significant after 1970 particularly. The ice cover in the Barents-Kara Sea shows a positive correlation.

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