Sea Level Pressure Minimum along the Kuroshio and Its Extension*

Abstract

Atmospheric effects of sea surface temperature (SST) fronts along the Kuroshio and Kuroshio Extension (K-KE) are investigated by examining spatial characteristics of the climatological sea level pressure (SLP), surface winds and surface heat flux (Q) fields based on an in situ observation dataset. A hydrostatic effect of the SST front is observed during the northwesterly monsoon characterized by a westward-extending low-SLP wedge (trough) slightly south of the peak in Q along the K-KE. Ageostrophic surface westerlies crossing SLP isobars toward a trough center are found north of the low-SLP wedge apparently because of the eastward acceleration by the vertical mixing effect. This feature of the ageostrophic winds is less pronounced south of it, a north-south asymmetry arising as vertical mixing accelerates (decelerates) eastward ageostrophic winds north (south) of it. While the SLP trough near the SST front is found nearly year-round except for summer, its meridional location seasonally migrates probably due to the change in the mean surface flow.Regional atmospheric model experiments are conducted to examine the effect of the SST front on the overlying atmosphere. The observed features in winter are adequately simulated when high-resolution SST is prescribed as the boundary condition. The strong Q along the K-KE and resultant SLP trough and ageostrophic surface winds are absent when the SST boundary condition is smoothed in space to weaken the SST front. These results illustrate that the cross-frontal change in Q is of great importance to leave the SST imprint on the overlying atmosphere via hydrostatic and vertical mixing adjustments.

Diagnostics for Near-Surface Wind Response to the Gulf Stream in a Regional Atmospheric Model*

“…The low (high) pressure anomalies produce cyclonic (anticyclonic) circulation and induce surface wind convergence over warm SSTs and divergence over cold SSTs. The effect may be manifested in the Laplacian of SLP (Minobe et al 2008;Bryan et al 2010;Putrasahan et al 2013) and can also be extracted by a spatial high-pass filter that masks meridional background SLP gradients in the midlatitudes (Tokinaga et al 2009;Minobe et al 2010;Tanimoto et al 2011). When advection of temperature in the MABL is important, the SLP pattern is shifted downwind side relative to the SST pattern, as seen in the atmospheric response to tropical instability waves (Small et al 2003).…”

Oceanic fronts and jets around Japan: a review

“…A front can modify cross-frontal winds acting to yield surface convergence/divergence, whereas surface wind curl can be generated for winds along the frontal axis. The modulation of surface winds could be complicated under the strong low-level thermal winds (Tanimoto et al 2011). Differential heat release across an oceanic front also modifies thermal conditions in the PBL and thereby surface pressure via the hydrostatic effect (Lindzen and Nigam 1987), with frictional surface convergence over locally warm water (Tanimoto et al 2011).…”

“…The modulation of surface winds could be complicated under the strong low-level thermal winds (Tanimoto et al 2011). Differential heat release across an oceanic front also modifies thermal conditions in the PBL and thereby surface pressure via the hydrostatic effect (Lindzen and Nigam 1987), with frictional surface convergence over locally warm water (Tanimoto et al 2011). This hydrostatic effect has been shown to be operative ubiquitously along the major warm western boundary currents (Shimada and Minobe 2011).…”

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

“…A frontal SST gradient also yields surface convergence/divergence through enhanced (suppressed) heat release from the ocean on the warmer (cooler) side of the front that hydrostatically acts to lower (raise) surface pressure and resultant frictional convergence (divergence) (Shimada and Minobe 2011;Tanimoto et al 2011). Generated through this pressure adjustment mechanism or the vertical momentum mixing mechanism (Schneider and Qiu 2015), the surface convergence yields ascending motion at the top of the boundary layer, acting to enhance cloudiness and precipitation locally (Tokinaga et al 2009;Tanimoto et al 2011;Masunaga et al 2015). Through essentially the same mechanisms, meso-scale oceanic eddies can also leave meso-scale imprints on surface wind, cloudiness and precipitation patterns.…”