Association of the North Atlantic Surface Turbulent Heat Fluxes with Midlatitude Cyclones

Atmospheric mechanisms leading to the formation of very strong turbulent air-sea heat fluxes in the North Atlantic are analyzed using the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) for the winter periods from 1979 to 2010. Surface turbulent flux extremes were quantified by considering both absolute and relative extremeness of these fluxes. For all cases of very strong surface turbulent fluxes, regional composites of the associated atmospheric conditions were built using reanalysis output. These composites clearly demonstrate a critical role of the cyclone-anticyclone interaction zone in forming very strong surface fluxes. The implied importance of cyclones followed by anticyclones in generation of surface air-sea heat flux extremes was demonstrated by the analysis of case studies. We further used the results of numerical cyclone tracking to identify extratropical cyclones associated with air-sea flux events of different intensities and to quantify the life cycle characteristics of these cyclones. Analysis of frequency distribution of surface heat fluxes has shown that extreme fluxes over the North Atlantic are associated with less than 30% of winter cyclones and that this association occurs mostly during the initial stage of their life cycle. Analysis of life cycle characteristics of these cyclones shows, in turn, that they are considerably more intense than most North Atlantic cyclones and are characterized by rapid deepening and slower propagation. We argue that variability of the North American high is a key factor controlling atmospheric conditions favorable for the occurrence of high turbulent air-sea heat fluxes in the North Atlantic mid- and subpolar latitudes.