Search Results

You are looking at 1 - 4 of 4 items for

  • Author or Editor: Ryusuke Masunaga x
  • Refine by Access: All Content x
Clear All Modify Search
Ryusuke Masunaga, Hisashi Nakamura, Bunmei Taguchi, and Takafumi Miyasaka

ABSTRACT

High-resolution satellite observations and numerical experiments have revealed local enhancement of time-mean surface wind convergence along the axes of warm western boundary currents and divergence slightly poleward. A recent study has suggested that frequent occurrence of persistent atmospheric fronts and sea level pressure (SLP) troughs along a sea surface temperature (SST) front are responsible for shaping the frontal-scale wind convergence and divergence contrast as seen in the wintertime climatology near the Kuroshio Extension (KE). These events tend to induce surface wind convergence with moderate magnitude. Through atmospheric reanalysis with high-resolution SST, the present study reveals that, as in the vicinity of the KE, surface wind convergence with moderate magnitude and divergence with moderate-to-extreme magnitude are found to play a primary role in shaping the climatological-mean wind convergence–divergence contrasts across the SST fronts near the Gulf Stream (GS) and Agulhas Return Current (ARC) in winter. In contrast, strong-to-extreme convergence events associated with synoptic-scale atmospheric disturbances are found to yield horizontally uniform time-mean wind convergence. Furthermore, cluster analysis and case studies suggest that persistent atmospheric fronts and SLP troughs are responsible for inducing moderate wind convergence also near the GS and ARC. Thus, these features are consistent with their counterpart near the KE, but the impacts of the ARC tend to be substantially weaker, probably due to its cooler SST among other potential factors.

Open access
Ryusuke Masunaga, Hisashi Nakamura, Bunmei Taguchi, and Takafumi Miyasaka

Abstract

High-resolution satellite observations and numerical simulations have revealed that climatological-mean surface wind convergence and precipitation are enhanced locally around the midlatitude warm western boundary currents (WBCs) with divergence slightly to their poleward side. While steep sea surface temperature (SST) fronts along the WBCs have been believed to play an important role in shaping those frontal-scale atmospheric structures, the mechanisms and processes involved are still under debate. The present study explores specific daily scale atmospheric processes that are essential for shaping the frontal-scale atmospheric structure around the Kuroshio Extension (KE) in winter, taking advantage of a new product of global atmospheric reanalysis. Cluster analysis and case studies reveal that a zonally extending narrow band of surface wind convergence frequently emerges along the KE, which is typically observed under the surface northerlies after the passage of a developed synoptic-scale cyclone. Unlike its counterpart around the cyclone center and associated cold front, the surface convergence tends to be in moderate strength and more persistent, contributing dominantly to the distinct time-mean convergence/divergence contrast across the SST front. Accompanying ascent and convective precipitation, the band of convergence is a manifestation of a weak stationary atmospheric front anchored along the SST front or generation of a weak meso-α-scale cyclone. By reinforcing the ascent and convergence, latent heating through convective processes induced by surface convergence plays an important role in shaping the frontal-scale atmospheric structure around the KE.

Open access
Ryusuke Masunaga, Hisashi Nakamura, Takafumi Miyasaka, Kazuaki Nishii, and Youichi Tanimoto

Abstract

Mesoscale structures of the wintertime marine atmospheric boundary layer (MABL) as climatological imprints of oceanic fronts within the Kuroshio–Oyashio Extension (KOE) region east of Japan are investigated by taking advantage of high horizontal resolution of the ERA-Interim global atmospheric reanalysis data, for which the resolution of sea surface temperature (SST) data has been improved. These imprints, including locally enhanced sensible and latent heat fluxes and local maxima in cloudiness and precipitation in association with locally strengthened surface-wind convergence in the vicinities of SST fronts along the warm Kuroshio Extension and cool Oyashio to its north, are also identified in high-resolution satellite data. In addition to these mesoscale MABL features, meridionally confined near-surface baroclinic zones and zonally oriented sea level pressure (SLP) minima associated with the dual SST fronts are represented in ERA-Interim only in the period of high-resolution SST, but those imprints of the Oyashio front are missing in the low-resolution SST period. In the presence of the prevailing monsoonal northerlies, latitudinal displacements of the SLP trough, baroclinic zone, and the peak meridional gradient of the turbulent heat fluxes from each of the corresponding SST fronts are also found to be sensitive to the frontal width that depends on the SST resolution. The analysis herein suggests that the converging surface northerlies into the SLP minima can contribute positively to the formation of a surface baroclinic zone along the Kuroshio Extension, while a stronger baroclinic zone along the Oyashio front is maintained primarily through the pronounced cross-frontal contrast in sensible heat release from the ocean.

Full access
Ryusuke Masunaga, Hisashi Nakamura, Takafumi Miyasaka, Kazuaki Nishii, and Bo Qiu

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 (ERA-Interim). 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.

Full access