A ‘warm path’ for Gulf Stream–troposphere interactions

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 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.

Effect of Mesoscale Oceanic Eddies on Extratropical Cyclogenesis: A Tracking Approach

“…Recent high‐resolution satellite observations and numerical models have confirmed the forcing effect of mesoscale SST anomalies associated with oceanic eddies on atmospheric boundary layer and deep troposphere variables including surface wind, turbulent heat fluxes, boundary layer height, cloud, and rainfall (Bryan et al, ; Chelton et al, ; Frenger et al, ; Minobe et al, ; Putrasahan et al, ). Emerging research also began to notice the storm track and large‐scale atmospheric circulation response to oceanic fronts and mesoscale oceanic eddies in the western boundary current regions (Kuwano‐Yoshida & Minobe, ; Piazza et al, ; Révelard et al, ; Sheldon et al, ; Smirnov et al, ). However, mechanisms connecting mesoscale oceanic eddies and extratropical storm track dynamics is still not fully understood.…”

Mesoscale Signature of the North Atlantic Oscillation and Its Interaction With the Ocean

“…In Figure , we show the shear production term at 4.5‐km height in the case study considered by Sheldon et al () when the SST was realistic (left panel) and when it was heavily smoothed, thereby removing the SST signature of the Gulf Stream (right panel). Comparison of the two panels shows that in this particular experiment (UK Met Office model at 12‐km horizontal resolution—see Sheldon et al, , for details), the presence of the Gulf Stream warm core sets a region of positive shear production of MKE downstream of it. It is tempting to interpret the localized southwest‐northeast dipole in Figure f as evidence in the climatology of the signal seen in a single storm in Sheldon et al ().…”

“…We have linked empirically periods when the Gulf Stream warm core is more extended northeastward to a similar extension of MKE and its production terms. The study by Sheldon et al (), as well as the difference in ERA‐Interim composites with SST resolution noted above give support for the view that this relationship is causal. In Figure , we show the shear production term at 4.5‐km height in the case study considered by Sheldon et al () when the SST was realistic (left panel) and when it was heavily smoothed, thereby removing the SST signature of the Gulf Stream (right panel).…”

“…The study by Sheldon et al (), as well as the difference in ERA‐Interim composites with SST resolution noted above give support for the view that this relationship is causal. In Figure , we show the shear production term at 4.5‐km height in the case study considered by Sheldon et al () when the SST was realistic (left panel) and when it was heavily smoothed, thereby removing the SST signature of the Gulf Stream (right panel). Comparison of the two panels shows that in this particular experiment (UK Met Office model at 12‐km horizontal resolution—see Sheldon et al, , for details), the presence of the Gulf Stream warm core sets a region of positive shear production of MKE downstream of it.…”

“…Comparison of the two panels shows that in this particular experiment (UK Met Office model at 12‐km horizontal resolution—see Sheldon et al, , for details), the presence of the Gulf Stream warm core sets a region of positive shear production of MKE downstream of it. It is tempting to interpret the localized southwest‐northeast dipole in Figure f as evidence in the climatology of the signal seen in a single storm in Sheldon et al (). More modeling work is clearly needed to test this hypothesis fully.…”