Anomalies and Trends of Sea-Ice Extent and Atmospheric Circulation in the Nordic Seas during the Period 1864–1998

Abstract

The extent of ice in the Nordic Seas measured in April has decreased by ∼33% over the past 135 yr. Retrospective comparison indicates that the recent decrease in the ice extent is within the range of variability observed since the eighteenth century. Temporal, monotonically reduced extreme events occur with intervals of 12–14 yr, suggesting that series longer than ∼30 yr should be considered to obtain statistical significance regarding temporal changes. Otherwise, decadal temperature variation is also found in the northbound warmer ocean currents. The temperature in the upper layers of these currents seems moreover to have increased by the order of 1°C since the cooling during the Little Ice Age. This temperature increase accounts for most of the ice extent reduction since ∼1860. A strong negative correlation is found between the larger North Atlantic oscillation (NAO) winter index and the Nordic Seas April ice extent, and a corresponding positive correlation is observed for the Newfoundland–Labrador Sea. It is not until the warming of the Arctic, 1905–30, that the NAO winter index shows repeated positive values over a number of sequential years, corresponding to repeated northward fluxes of warmer air over the Nordic Seas during the winter. An analog repetition of southward fluxes of colder air during wintertime occurs during the cooling period in the 1960s. Concurrently, the temperature in the ocean surface layers was lower than normal during the warming event and higher than normal during the cooling event. Northward atmospheric winter fluxes are observed after the enhanced global warming after ∼1970, and, for the first time over the period considered, a positive correlation is observed between atmospheric and oceanic reducing effects on the ice extent. The enhanced global warming over the past two decades seems also to be manifest in an intensified winter circulation at higher latitudes, rather than a contemporary change in the Arctic Ocean surface temperature.