On the importance of midlatitude oceanic frontal zones for the mean state and dominant variability in the tropospheric circulation

Abstract

Observations indicate that midlatitude weather systems are organized into “storm tracks” near oceanic frontal zones with pronounced sea‐surface temperature (SST) gradients. A pair of atmospheric general circulation model experiments with zonally uniform SST profiles prescribed show that their observed collocation is not fortuitous. In one experiment, a storm track is anchored around a midlatitude SST front that maintains near‐surface thermal gradients and energizes eddies. Westerly momentum transport by eddies produces a well‐defined polar‐front jet along the front, even in winter when a subtropical jet stream intensifies. In the other experiment, removal of the SST front leads to a substantial weakening in eddy activity and the PFJ especially in winter. It also leads to a weakening of the annular mode —the dominant mode of westerly‐jet variability— and its notable structural distortion in winter. Though idealized, our experiments suggest the importance of midlatitude oceanic fronts for the tropospheric circulation and its variability.

A new mechanism for ocean–atmosphere coupling in midlatitudes

“…The importance of ocean currents and western boundarycurrent regions found here is reminiscent of the recent study by Nakamura et al (2008)-see also Booth et al (2010). They proposed that air-sea heat exchanges at oceanic fronts are responsible for restoring the baroclinicity of the atmosphere at low levels.…”

Storm track sensitivity to sea surface temperature resolution in a regional atmosphere model

“…The aim of this study is not to determine the effect that the presence of the Gulf Stream has on the atmosphere, but rather to determine the consequences of not resolving the Gulf Stream properly in numerical models. The atmospheric response in these experiments is therefore expected to be smaller than in experiments such as those of Nakamura et al (2008) where the ocean front is removed in its entirety.…”

“…It is often assumed that these strong SST gradients are important contributions to the overall baroclinicity and hence storm growth, and some recent studies have indeed supported this (e.g. Inatsu et al 2002;Nakamura et al 2004;Brayshaw et al 2008;Nakamura et al 2008). There has been much speculation that SST variations in the Gulf Stream region may lead to variations in baroclinicity which could affect storm growth and so influence the whole of the storm track (e.g.…”

“Hot Spots” in the climate system—new developments in the extratropical ocean–atmosphere interaction research: a short review and an introduction

“…It has been pointed out that variability of the subarctic frontal zone in the Kuroshio-Oyashio Extension (KOE), a prominent oceanic frontal zone in the North Pacific, generates pronounced decadal SST variability that can force basin-scale atmospheric anomalies, manifested as the anomalous surface Aleutian Low, by modulating stormtrack activity (Frankignoul et al 2011;Taguchi et al 2012). It has been also pointed out that an oceanic frontal zone is important for maintaining the annular-mode variability, which is manifested as variability of the eddy-driven jet (Nakamura et al 2008). Through these processes, the WBCs and associated oceanic frontal zones can influence basin-scale or even hemispheric-scale atmospheric circulation and climate.…”