Influence of the Decadal Variability of the Kuroshio Extension on the Atmospheric Circulation in the Cold Season

Abstract

The atmospheric response to the Kuroshio Extension (KE) variability during 1979-2012 is investigated using a KE index derived from sea surface height measurements and an eddy-resolving ocean general circulation model hindcast. When the index is positive, the KE is in the stable state, strengthened and shifted northward, with lower eddy kinetic energy, and the Kuroshio-Oyashio Extension (KOE) region is anomalously warm. The reverse holds when the index is negative. Regression analysis shows that there is a coherent atmospheric response to the decadal KE fluctuations between October and January. The KOE warming generates an upward surface heat flux that leads to local ascending motions and a northeastward shift of the zones of maximum baroclinicity, eddy heat and moisture fluxes, and the storm track. The atmospheric response consists of an equivalent barotropic large-scale signal, with a downstream high and a low over the Arctic. The heating and transient eddy anomalies excite stationary Rossby waves that propagate the signal poleward and eastward. There is a warming typically exceeding 0.6 K at 900 hPa over eastern Asia and western United States, which reduces the snow cover by 4%-6%. One month later, in November-February, a high appears over northwestern Europe, and the hemispheric teleconnection bears some similarity with the Arctic Oscillation. Composite analysis shows that the atmospheric response primarily occurs during the stable state of the KE, while no evidence of a significant large-scale atmospheric response is found in the unstable state. Arguments are given to explain this strong asymmetry.

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

“…The significant positive u anomaly of 0.2-0.3 K extends up to 300 hPa and is accompanied by the ascending motion (Czaja and Blunt 2011;Smirnov et al 2015;Wills et al 2016), while the significant negative u anomaly of 0.2-0.3 K up to 600 hPa is coincident with the descending motion. The deeper extension of the diabatic heating than the cooling is consistent with the finding by Révelard et al (2016), who attributed the uneven vertical distribution of the heating and cooling to the primary cause of the nonlinearity in observed circulation response to the changes in the Kuroshio Extension front (Qiu et al 2014).…”

“…In NCEP-1, the tropical influence is removed via linear regression (Révelard et al 2016); however, a very similar result is obtained even with the use of the full field (not shown). This suggests that the nonlinearity in the circulation response itself does not owe its existence to the tropical influence but rather to the extratropical atmospheric dynamics, a notion that is supported by the present model analysis (Figs.…”

“…When the Z 250 and SLP responses are in phase, this highlights the equivalent barotropic nature of the forced response. In NCEP-1, the tropical influence is removed by linear regression on the leading principal component time series of the IndoPacific tropical SST (Révelard et al 2016), although similar leading patterns are obtained with tropical influence included (not shown). The observed leading mode of variability for the period 1970-2009 is the NAO.…”

“…On the other hand, the magnitude of the atmospheric response tends to be more proportional to the amplitude of SST anomaly associated with the southward GS shift (second column). The nonlinearity in the amplitude of the response might be particularly more prone to taking place over the positive SST anomalies because the deeper distribution of diabatic heating (Révelard et al 2016; also in Fig. 6) would induce more energetic circulation adjustment processes (section 3c).…”

Interannual Modulations of Oceanic Imprints on the Wintertime Atmospheric Boundary Layer under the Changing Dynamical Regimes of the Kuroshio Extension

“…Iizuka (2010) indicated through regional atmospheric model simulations that the warm SST anomaly can enhance winter precipitation, and its imprints can reach the free troposphere. Using their own measures of the KE regimes derived from SSH and/or SST fields, several studies have documented relationship between the KE variability and large-scale atmospheric circulation, including the strength of the Aleutian low and storm-track activity (Seo et al 2014;Qiu et al 2014;O'Reilly and Czaja 2015;Révelard et al 2016). …”