A New Framework for Near-Surface Wind Convergence Over the Kuroshio Extension and Gulf Stream in Wintertime: The Role of Atmospheric Fronts

It is well known that the wintertime time-mean surface wind convergence patterns over the Kuroshio Extension and Gulf Stream show significant imprints of the underlying oceanic fronts. Previous studies have suggested that this collocation results from a time-mean response to sea level pressure forcing from sea surface temperature gradients. However, more recent work has illustrated this phenomenon is heavily influenced by extratropical cyclones, although exact mechanisms are still debated. The purpose of this study is to introduce a new framework that explicitly distinguishes between two separate components in their contribution to the time-mean surface wind convergence, that associated with and without atmospheric fronts. It is then argued that this distinction can help better explain the mechanisms driving the Kuroshio Extension and Gulf Stream influence on the atmosphere. Plain Language Summary This paper presents a new framework for understanding the mean wintertime atmospheric state over the Kuroshio Extension and Gulf Stream regions, in the context of advancing our understanding of how these strong ocean currents can impact on the seasonal atmosphere. In recent years, many studies have attributed the oceanic imprint on the atmosphere in these regions to mechanisms based upon the time-mean response. However, observational analysis here illustrates that the regional wintertime atmosphere is, in fact, dominated by continuous extratropical storm systems. It is suggested this contribution to the mean atmospheric state can be further decomposed into situations when atmospheric fronts embedded within these storms are and are not present. By studying each of these distinct atmospheric scenarios, it is then argued that the oceanic imprint on the mean wintertime atmospheric state should instead be considered as an accumulation of processes driven by mechanisms acting on a synoptic timescale, not on the timescale of the time-mean. This framework presents a new paradigm for understanding extratropical frontal-scale air-sea interactions and is expected to have an immediate impact on atmospheric, oceanic, and climate communities.