Wintertime nordic seas cyclone variability and its impact on oceanic volume transports into the Nordic seas

Abstract

The variability of the Nordic Seas wintertime storm tracks has been investigated using a Lagrangian approach. For the tracking algorithm, the NCEP/NCAR reanalysis has been used to identify individual wintertime storm tracks during the last 50 years. Both the number and intensity of the cyclones extending into the Nordic Seas exhibit strong interannual and decadal variability. The decadal variability was especially pronounced from the mid-1970s to the mid-1990s. On average over the 50-year period, both the cyclone intensity and number were highly correlated with the Arctic Oscillation/North Atlantic Oscillation (AO/NAO). The relationship with the two-point NAO Index broke down in the 1960s, indicating that the NAO Index should be used with caution as an index for describing the mean Nordic Seas storm track variability. In the absence of long observational time series, output from an OGCM was used to investigate the relationship between the Nordic Seas wintertime storm tracks and the ocean volume transports into and out of the Nordic Seas across the Greenland-Scotland Ridge. To the extent that the temporal variability of the modelled transports are realistic, variability in the oceanic volume fluxes over the 50-year period shows a relatively strong co-variability with both the Nordic Seas mean cyclone intensity and the number of cyclones. As noted in several studies, the relationship between the atmospheric flow variability and the variability in the ocean transports was found to be stronger from the mid-1970s and onward. Decomposing the mean Nordic Seas storm track variability into frequency and time space shows that a possible reason for this increase in co-variability during the last 25 years might be related to nonlinear effects of the increased storm track variability and possibly to the increased persistence of the storm tracks.