Western Boundary Currents and Frontal Air–Sea Interaction: Gulf Stream and Kuroshio Extension

Abstract

In the Northern Hemisphere midlatitude western boundary current (WBC) systems there is a complex interaction between dynamics and thermodynamics and between atmosphere and ocean. Their potential contribution to the climate system motivated major parallel field programs in both the North Pacific [Kuroshio Extension System Study (KESS)] and the North Atlantic [Climate Variability and Predictability (CLIVAR) Mode Water Dynamics Experiment (CLIMODE)], and preliminary observations and analyses from these programs highlight that complexity. The Gulf Stream (GS) in the North Atlantic and the Kuroshio Extension (KE) in the North Pacific have broad similarities, as subtropical gyre WBCs, but they also have significant differences, which affect the regional air–sea exchange processes and their larger-scale interactions. The 15-yr satellite altimeter data record, which provides a rich source of information, is combined here with the longer historical record from in situ data to describe and compare the current systems. While many important similarities have been noted on the dynamic and thermodynamic aspects of the time-varying GS and KE, some not-so-subtle differences exist in current variability, mode water properties, and recirculation gyre structure. This paper provides a comprehensive comparison of these two current systems from both dynamical and thermodynamical perspectives with the goal of developing and evaluating hypotheses about the physics underlying the observed differences, and exploring the WBC’s potential to influence midlatitude sea–air interaction. Differences between the GS and KE systems offer opportunities to compare the dominant processes and thereby to advance understanding of their role in the climate system.

Ocean-atmosphere dynamics changes associated with prominent ocean surface turbulent heat fluxes trends during 1958–2013

“…The WBCs play a vital role in the climate over the adjacent mainland (Minobe et al, 2008;Kelly et al, 2010). Previously, the strengthening WBCs has been documented individually, for example, the Gulf Stream (Curry and McCartney, 2001), Kuroshio Current (Qiu and Joyce, 1992;Deser et al, 1999;Sato et al, 2006;Sakamoto et al, 2005) and the Eastern Australia Current (Cai et al, 2005;Qiu and Chen, 2006;Ridgway, 2007;Roemmich et al, 2007;Ridgway et al, 2008).…”

On the Predominant Nonlinear Response of the Extratropical Atmosphere to Meridional Shifts of the Gulf Stream

“…Air-sea interaction over the western boundary currents is one of the fundamental processes of extratropical climate variability Kelly et al 2010). In the North Atlantic, the largest surface heat flux and its strongest interannual variability are found over the Gulf Stream (GS).…”

“…Because of the potential basin-scale importance, the position of the GS path has been monitored for many decades (Rossby and Gottlieb 1998;Kelly et al 1999;Taylor and Stephens 1998). To evaluate the GS influence on the large-scale circulation, this study adopts the subsurface temperature-based proxy for the GS position defined by Joyce et al (2000).…”

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

“…Rather, over each of the midlatitude ocean basins, especially in winter, synoptic-scale cyclone and anticyclones that recurrently develop and migrate along a stormtrack act to maintain eddies. The variability of the frontal zone as a response of the ocean gyre to wind stress anomalies can yield strong SST anomalies and act to exert thermodynamic forcing on the atmosphere by modifying heat and moisture release (Kelly et al 2010;Kwon et al 2010). These WBCs and associated frontal zones, which the authors call "hot spots", can, therefore, be significant in shaping the tropospheric circulation and forcing its variability.…”