Abstract
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 . It is also supported by a dynamical diagnostic applied to the ERA interim data-set over the wintertime period (1979–2012). 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.
Highlights
The meridional overturning circulation and the horizontal gyres of the North Atlantic ocean leave sharp features in its sea surface temperature (SST) distribution
To investigate the differences in the vertical motion field between the CNTL and SMTH experiments, it proved useful to compute back-trajectories starting from the core of the ascent at t = 24 h until some of them reach near the sea surface at t = 0 h
The second family is associated with air parcels travelling first at low levels over the Gulf Stream before ascending into the free troposphere, and it is this latter set of trajectories which is the focus of our study. (It turns out that these two sets of trajectories could be separated depending on whether or not the longitude and latitude of air parcels at t = 0 were over the Atlantic ocean – all trajectories over the ocean at t = 0 were found to be at low levels, none of them exceeding an altitude of 1800 m above the sea)
Summary
The meridional overturning circulation and the horizontal gyres of the North Atlantic ocean leave sharp features in its sea surface temperature (SST) distribution. The impact of the warm tongue of the Gulf Stream on air–sea interactions has been highlighted in many studies, in particular its role in setting a region of large net surface heating of the atmosphere, especially in winter Gill, 1982; Shaman et al, 2010) At this time of year, extra-tropical cyclones travelling from North America to the Atlantic ocean bring cold dry air of continental origin over the Gulf Stream warm tongue, leading to very large (in excess of hundreds of Wm−2) sensible and latent heat fluxes at the air–sea interface. The impact of the warm tongue above the marine boundary layer has traditionally been understood as a response to these large fluxes and their associated shallow heating of the troposphere (Hoskins and Karoly, 1981; Kushnir et al, 2002)
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