Abstract
Abstract The mechanism by which the Gulf Stream sea surface temperature (SST) front anchors a band of precipitation on its warm edge is still a matter of debate, and little is known about how synoptic activity contributes to the mean state. In the present study, the influence of the SST front on precipitation is investigated during the course of a single extratropical cyclone using a regional configuration of the Met Office Unified Model. The comparison of a control run with a simulation in which SST gradients were smoothed brought the following conclusions: a band of precipitation is reproduced for a single extratropical cyclone, and the response to the SST gradient is dominated by a change of convective precipitation in the cold sector of the storm. Several climatological features described by previous studies, such as surface wind convergence on the warm edge or a meridional circulation cell across the SST front, are also reproduced at synoptic time scales in the cold sector. Based on these results, a simple boundary layer model is proposed to explain the convective and dynamical response to the SST gradient in the cold sector. In this model, cold and dry air parcels acquire more buoyancy over a sharp SST gradient and become more convectively unstable. The convection sets a pressure anomaly over the entire depth of the boundary layer that drives wind convergence. This case study offers a new pathway by which the SST gradient can anchor a climatological band of precipitation.
Highlights
The long-term climatological impact of the sea surface temperature (SST) gradient on the atmosphere has been identified both in observations and numerical experiments
The present study reconciles the explanation given for several climatological features of the Gulf Stream region and demonstrates the close connection between air–sea interactions during cold air outbreaks and the climatological band of precipitation
To evaluate the impact of air–sea interactions in the cold sector, we followed the strategy of Minobe et al (2008) and forced a regional mesoscale model by two different sets of SSTs, either observed (CNTL) or with smoothed gradients (SMTH)
Summary
The long-term climatological impact of the SST gradient on the atmosphere has been identified both in observations and numerical experiments. Minobe et al (2008) proposed a climatological pathway by which the Gulf. Stream SST gradient affects the troposphere above the boundary layer. The pressure in the marine atmospheric boundary layer (MABL) adjusts to the SST gradient and generates wind convergence that anchors a band of precipitation on the warm flank of the Gulf Stream. These results were obtained using reanalysis data and sensitivity experiments to SST at 1/28 resolution; when the SST gradient was smoothed the rainband disappeared.
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