Warm advection by the Gulf Stream creates a characteristic 'tongue' of warm water leaving a strong imprint on the sea surface temperature (SST) distribution in the western North Atlantic. This study aims at quantifying the climatological impact of this feature on cyclones travelling across this region in winter using a combination of reanalysis data and numerical experiments. It is suggested that the Gulf Stream 'warm tongue' is conducive to enhanced upward motion in cyclones because (i) it helps maintain a high equivalent potential temperature of air parcels at low levels which favors deep ascent in the warm conveyor belt of cyclones and (ii) because the large SST gradients to the north of the warm tongue drive a thermally direct circulation reinforcing and, possibly, destabilizing, the transverse circulation embedded in cyclones. This hypothesis is confirmed by comparing simulations at 12 km resolution from the Met Office Unified Model forced with realistic SST distribution to simulations with an SST distribution from which the Gulf Stream warm tongue was artificially removed or made colder by 3 • C. It is also supported by a dynamical diagnostic applied to the ERA interim data-set over the wintertime period . The mechanism of oceanic forcing highlighted in this study is associated with near thermal equilibration of low level air masses with SST in the warm sector of cyclones passing over the Gulf Stream warm tongue, which is in sharp contrast to what occurs in their cold sector. It is suggested that this 'warm path' for the climatic impact of the Gulf Stream on the North Atlantic storm-track is not currently represented in climate models because of their coarse horizontal resolution.

The upper-ocean heat content of the North Atlantic has undergone significant changes over the last 50 years but the underlying physical mechanisms are not yet well understood. In the present study, the authors examine the North Atlantic ocean heat content change in the upper 700 m between the 1955-70 and 1980-95 periods. Consistent with previous studies, the large-scale pattern consists of warming of the tropics and subtropics and cooling of the subpolar ocean. However, this study finds that the most significant heat content change in the North Atlantic during these two time periods is the warming of the Gulf Stream region. Numerical experiments strongly suggest that this warming in the Gulf Stream region is largely driven by changes of the large-scale wind forcing. Furthermore, the increased ocean heat content in the Gulf Stream region appears to feedback on to the atmosphere, resulting in warmer surface air temperature and enhanced precipitation there.

The seasonal and interannual variability of an index measuring the potential for deep (surface-to-tropopause) convection over the extratropical oceans is studied using reanalysis data. It is found that most of the conditional instability is concentrated over the world's western boundary currents during winter, but shifts equatorward of the currents in summer. Conditional instability is only detected over the Gulf Stream and the East Australian Current in their respective summer season.The coupled ocean-atmosphere mechanisms controlling the variability of the convective index are then studied. It is found that the convective index displays a large interannual variability, which is primarily controlled by the erratic displacements of the storm tracks. Only weak negative feedback from the oceans is singled out on short (intraseasonal) time-scales, reflecting the stabilization of the troposphere through the development of cold sea-surface temperature anomalies. A larger role for warm oceanic advection in destabilizing the troposphere is, however, suggested on longer (interannual and decadal) time-scales. Citation: L. Sheldon, A. Czaja. 2014. Seasonal and interannual variability of an index of deep atmospheric convection over western boundary currents. Q.

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