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

Abstract

The Kuroshio Extension (KE) fluctuates between its different dynamic regimes on (quasi) decadal time scales. In its stable (unstable) regime, the KE jet is strengthened (weakened) and less (more) meandering. The present study investigates wintertime mesoscale atmospheric structures modulated under the changing KE regimes, as revealed in high-resolution satellite data and data from a particular atmospheric reanalysis (ERAInterim). In the unstable KE regime, a positive anomaly in sea surface temperature (SST) to the north of the climatological KE jet accompanies positive anomalies in upward heat fluxes from the ocean, surface wind convergence, and cloudiness. As revealed in the atmospheric reanalysis, these positive anomalies coincide with local lowering of sea level pressure, weaker vertical wind shear, warming and thickening of the marine atmospheric boundary layer (MABL), anomalous ascent, and convective precipitation. In the stable KE regime, by contrast, the corresponding imprints of sharp SST gradients across the KE and Oyashio fronts on the wintertime MABL are separated more distinctly, and so are the surface baroclinic zones along those two SST fronts. In the ERA-Interim data, such mesoscale imprints of the KE variability as above are not well represented in a period during which the resolution of SST data prescribed is relatively low. The present study thus elucidates the importance of high-resolution SST data prescribed for atmospheric reanalysis in representing modulations of the MABL structure and air-sea fluxes by the variability of oceanic fronts and/or jets, including the modulations occurring with the changing KE regimes through the hydrostatic pressure adjustment and vertical mixing mechanisms.

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

“…Compared to the JRA-55C, representation of surface wind convergence is slightly improved in the main JRA-55 thanks to assimilated satellite observations (not shown), although the signature is still much weaker than in the JRA-55CHS. Furthermore, the features in the JRA-55CHS are consistent with ERA-Interim for 2002−2012 (Masunaga et al 2016) and the Climate Forecast System Reanalysis (CFSR; Saha et al 2010; not shown), and therefore the JRA-55CHS is likely to be more realistic than JRA-55C. These results add another suggestion for the importance of high-resolution SST for an atmospheric reanalysis.…”

“…6h) than in the unstable regime (Fig. 6g), which can be interpreted as results of enhanced vertical mixing of momentum and/or adjustment of sea-level pressure through hydrostatic balance (Masunaga et al 2016). Accordingly, a precipitation band is confined meridionally into the climatological KE in its stable regime (Fig.…”

“…However, those relatively low-resolution SST cannot adequately resolve steep SST gradients associated with WBCs that can influence the atmosphere. Indeed, through analysis of the ERA-Interim (Dee et al 2011), Masunaga et al (2015Masunaga et al ( , 2016 have pointed out that thermal structure and three-dimensional motions in the marine atmospheric boundary layer (MABL) are not well constrained by data assimilation and therefore they are sensitive to SST fields prescribed.…”

Modulations of aerosol impacts on cloud microphysics induced by the warm Kuroshio Current under the East Asian winter monsoon

“…The SST data compiled in the ERA‐Interim reanalysis data set were also used where National Centers for Environmental Prediction (NCEP) real‐time global SST (NCEP RTG) and Operational SST and sea‐ice analysis had been used before and after January 2009, respectively. The horizontal resolution of the SST data used for the ERA‐Interim reanalysis since 2002 is high enough to resolve mesoscale impacts of the fine‐scale SST distribution associated with the Kuroshio and its extension on the MBL [ Masunaga et al ., , ].…”

Meridional Shift of the Oyashio Extension Front in the Past 36 Years

“…Featured by its strong SST gradient, the meridional shift of OE front could generate enhanced SST anomaly (Pak et al, ) and affect the overlaying atmospheric circulation and storm activity (Frankignoul et al, ; Smirnov et al, ). Especially in the low frequency (interannual to decadal time scales), SST anomalies in the OE front are considered as the driven force of atmosphere variability (Masunaga et al, ; Revelard et al, ; Taguchi et al, ) and the North Pacific decadal climate change (Kwon et al, ; Nakamura et al, ; Nakamura & Yamagata, ; Qiu et al, ). The meridional shift of OE front has been in the past attributed to the basin‐scale wind forcing (Kwon & Deser, ; Nonaka et al, ; Schneider et al, ; Seager et al, ) via the oceanic Rossby wave adjustment that modulates the latitude and strength of the OE front (Kwon et al, ; Nonaka et al, ).…”