A Coupled Decadal Prediction of the Dynamic State of the Kuroshio Extension System

Abstract

Being the extension of a wind-driven western boundary current, the Kuroshio Extension (KE) has long been recognized as a turbulent current system rich in large-amplitude meanders and energetic pinched-off eddies. An important feature emerging from recent satellite altimeter measurements and eddy-resolving ocean model simulations is that the KE system exhibits well-defined decadal modulations between a stable and an unstable dynamic state. Here the authors show that the decadally modulating KE dynamic state can be effectively defined by the sea surface height (SSH) anomalies in the 318-368N, 1408-1658E region. By utilizing the SSH-based KE index from 1977 to 2012, they demonstrate that the time-varying KE dynamic state can be predicted at lead times of up to ;6 yr. This long-term predictability rests on two dynamic processes: 1) the oceanic adjustment is via baroclinic Rossby waves that carry interior wind-forced anomalies westward into the KE region and 2) the low-frequency KE variability influences the extratropical storm tracks and surface wind stress curl field across the North Pacific basin. By shifting poleward (equatorward) the storm tracks and the large-scale wind stress curl pattern during its stable (unstable) dynamic state, the KE variability induces a delayed negative feedback that can enhance the predictable SSH variance on the decadal time scales.

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).…”

Long-term change and variation of salinity in the western North Pacific subtropical gyre revealed by 50-year long observations along 137°E

“…Such eddy activity, at the same time, is expected to transport more (less) fresher water north of the KE to the STMW formation region to decrease (increase) the mixed layer S. When we look at the relation to the KE index (Fig. 6d), which was defined by Qiu et al (2014) as the average of four KE-related parameters based on the satellite altimeter measurements, there is a connection that the winter mixed layer S at 144°E decreased (increased) during the unstable (stable) KE period represented by a negative (positive) KE index. In fact, the annual change of the winter mixed layer S during 1993-2015 is highly correlated with the annual-mean KE index (R = 0.63; significant at the 99% confidence level).…”

JRA-55CHS: An Atmospheric Reanalysis Produced with High-Resolution SST

“…1, 2, 3, and 4) well reflect their SST differences. Since the SST fronts undergo seasonal-to-decadal variations (e.g., Kelly et al 2010;Qiu et al 2014), the differences depicted in Figs. 1, 2, 3, and 4 as annual climatologies must be underestimated, and the corresponding differences should be even greater for a specific month or week.…”

“…In this subsection, we briefly examine how mesoscale atmospheric structures are modulated under the regime changes of the KE. On the basis of the KE regime index proposed by Qiu et al (2014), we identify 14 winters (1989−94, 2001−05, and 2010−12) for the stable KE regime and 10 winters (1995−2000 and 2006−09) for the unstable regime. Figure 6 shows wintertime (DJF-mean) composite maps of selected variables separately for the unstable (left column) and stable (middle column) regimes of the KE and their differences (right column).…”