Surface Turbulent Heat Fluxes Over the Gulfstream and the NAC and Cyclone Activity in the North Atlantic Extratropics

We analyse the origins of extreme surface heat fluxes over the midlatitude North Atlantic in the context of the interaction of the North Atlantic extratropical cyclones with the ocean in winter. Our focus is on the atmospheric conditions triggering extreme air-sea turbulent fluxes. Surface turbulent flux extremes were quantified from the very high percentiles of the sensible and latent flux probability distributions from the NCEP-CFSR reanalysis for the period 1979-onwards. These fluxes exceeding 95^th percentile of the PDF amount to 30 to 60% of the total winter heat loss in the midlatitudinal North Atlantic. For these cases we further developed composites of the atmospheric conditions by collocating SLP, surface temperature, wind speed and vorticity fields for different North Atlantic regions. We demonstrate that the presence of the high pressure system following the rare part of the propagating cyclone is critical for the formation of extreme surface ocean fluxes, provided by outbreaks associated with the cyclone-anticyclone interaction zone rather than with cyclone per se. This is a very robust feature of all composites built for the Gulfstream area, North Atlantic Current region, Eastern Atlantic and the Labrador Sea Region. Importantly, considerable fraction of extreme surface turbulent flux cases is associated with the initial phase of cyclone development close to the cyclone generation moment. Over the Gulf Stream more than 60% of cyclogenesis events were associated with extreme surface fluxes. Next, we put the analysis of synoptic air-sea interaction into the context of the cyclone characteristics derived from the numerical storm tracking of reanalysis data. Further we analyse the role of the Northern Hemisphere circulation modes (such as NAO and PNA) in the long-term variability of the cyclone controlled extreme air-sea exchanges.