Changes in the air-sea temperature difference of the North Atlantic for the past 50 years

Surface observations in the vicinity of Bermuda, as well as from the one-degree version of ICOADS, and from the NCEP-NCAR Reanalysis data set, suggest that a decline in the sensible heat flux from the ocean to the atmosphere has taken place since at least 1960. At the same time, the observational record shows an increase in sensible heat fluxes in the subpolar region, between about ~45N–60N. Analysis of the surface data suggests a lessening of the air-sea temperature difference in the subtropical region of the North Atlantic (~20N–45N). The decline in sensible heat fluxes (SHF) in this region occurs despite an increase in the anticylonic wind circulation associated with the semi-permanent North Atlantic subtropical high-pressure system. The decline in the “pseudo SHF” (defined simply as the product of the wind speed and the sea-minus-air temperature difference) in the North Atlantic subtropics amounts to ~25% drop relative to the long-term mean value of the “pseudo SHF”. In the subpolar regions, the increase in pseudo SHF is approximately 10% of its long-term mean. The decline (increase) in the SST-AT for the subtropical (subpolar) regions is approximately the same (~0.3 C). A longer-term record of surface air temperature in Bermuda, compared to adjacent SST values, suggests these changes began after about 1950, with the largest SST-AT differences taking place in the first couple of decades of the 1900s. We present an assessment of the different data sets, which lead us to conclude that large-scale changes in SHF have taken place in the last few decades, and we discuss possible mechanisms and hypotheses to account for these changes.