The Abyss of the Nordic Seas Is Warming

Abstract

Over the past decade, the multiyear oceanographic time series from ocean weather station Mike at 66°N, 2°E indicate a warming by about 0.01°C yr?1 in the deep water of the Norwegian Sea. The time of onset of this warming is depth dependent, starting at 2000-m depth in 1987 but not at the 1200-m level until 1990. The warming abruptly halts around 1993 for a couple of years before it culminates in the absolute maximum temperatures in the end of the 50-yr-long record. This warming is attributed to variations in the amount and direction of interchanges between the three deep basins of the Nordic seas: the Arctic Ocean, the Greenland Sea, and the Norwegian Sea. The reduction of deep convection in the Greenland Sea from the early 1980s and the increased horizontal exchange with the relatively warm deep waters of the Arctic Ocean are the proximate cause of the warming, leading to a constant rise in the temperature of the ?parent? Greenland Sea deep water (GSDW) from the early 1980s. These changing GSDW characteristics were passed on to the deep Norwegian Sea via the Jan Mayen Channel through Mohn Ridge, entering at a depth determined by the sill depth of this passage (2200 m) and propagating to shallower depths thereafter. The cessation of deep warming in the Norwegian Sea from 1993 to 1995?not shown by the GSDW?is attributed to the reversal of flow in the Jan Mayen Channel as GSDW production was greatly reduced, as confirmed by direct current measurements. The importance of the Arctic Ocean?Nordic seas system to global climate emphasizes the importance of identifying and understanding the mechanisms that control the interbasin dynamics of the Nordic seas and simulating them realistically in models.