A new mechanism for ocean-atmosphere coupling in midlatitudes

The role of moist convection in 'transferring' upward surface ocean conditions throughout the troposphere is studied using reanalysis data for the extratropical Northern and Southern Hemispheres in winter. It is found that conditions for the development of a convective air column from the sea surface to the tropopause are met frequently over all major western boundary currents and their extension in the oceanic interior (sometimes for as much as 50% of the time). These frequent occurrences are shown to be jointly controlled by oceanic advection of warm waters and, on the atmospheric side, the downward displacement of the tropopause associated with synoptic weather systems. Based on these results, it is proposed that the oceans can influence the atmosphere directly through convection in midlatitudes, as is commonly thought to occur in the Tropics. Analysis of the Richardson number R-i found at low levels suggests that moist symmetric instability (0 < R-i <= 1) is a key process involved in linking surface ocean temperatures to atmospheric lapse rates, in addition to standard upright convection. These low R-i processes are not currently parametrized in climate models, which raises the possibility that the extratropical oceanic influence on climate might be underestimated in the current generation of models. Copyright (C) 2011 Royal Meteorological Society