Atmospheric Boundary Layer Response to Mesoscale SST Anomalies in the Kuroshio Extension

Shunya Koseki Graduate School of Environmental Science, Hokkaido University, Sapporo, Japan

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Masahiro Watanabe Center for Climate System Research, University of Tokyo, Kashiwa, Japan

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Abstract

The atmospheric boundary layer (ABL) response to mesoscale eddies in sea surface temperature (SST) in the Kuroshio Extension was investigated using a high-resolution (T213L30) atmospheric general circulation model. A control run was performed first by integrating the model for 40 days, driven by the satellite-derived, eddy-resolving SST during January 2006. The spatial pattern of surface wind anomalies—that is, a deviation from large-scale winds—reveals a positive correlation with the spatial pattern of mesoscale SST anomalies. The momentum budget analysis of the anomalous zonal wind was performed to investigate the formation of the ABL response. The most dominant term was the pressure gradient force; the advection term was comparable but in the opposite sense. Vertical mixing acts to weaken the anomalous zonal wind near the surface; however, the downward (upward) vertical turbulent flux anomalies were dominant near the ABL top over the warm (cold) SST anomalies, suggesting that the vertical mixing mechanism is effective. The role of the vertical mixing was further examined by a sensitivity experiment in which the turbulent diffusion coefficient for momentum was spatially smoothed. While the pressure gradient force and the advection terms were almost unchanged in the momentum budgets, the deceleration due to turbulence was enhanced because of the absence of the momentum input from the free atmosphere. The result is a reduction in the amplitude of the surface zonal wind anomalies to approximately half in the sensitivity experiment.

* Current affiliation: Pan-Okhotsk Research Center, Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan

Corresponding author address: Shunya Koseki, Graduate School of Environmental Science, Hokkaido University, Nishi 5 Kita 10 Kita-ku, Sapporo 060-0810, Japan. Email: k-shunya@ees.hokudai.ac.jp

Abstract

The atmospheric boundary layer (ABL) response to mesoscale eddies in sea surface temperature (SST) in the Kuroshio Extension was investigated using a high-resolution (T213L30) atmospheric general circulation model. A control run was performed first by integrating the model for 40 days, driven by the satellite-derived, eddy-resolving SST during January 2006. The spatial pattern of surface wind anomalies—that is, a deviation from large-scale winds—reveals a positive correlation with the spatial pattern of mesoscale SST anomalies. The momentum budget analysis of the anomalous zonal wind was performed to investigate the formation of the ABL response. The most dominant term was the pressure gradient force; the advection term was comparable but in the opposite sense. Vertical mixing acts to weaken the anomalous zonal wind near the surface; however, the downward (upward) vertical turbulent flux anomalies were dominant near the ABL top over the warm (cold) SST anomalies, suggesting that the vertical mixing mechanism is effective. The role of the vertical mixing was further examined by a sensitivity experiment in which the turbulent diffusion coefficient for momentum was spatially smoothed. While the pressure gradient force and the advection terms were almost unchanged in the momentum budgets, the deceleration due to turbulence was enhanced because of the absence of the momentum input from the free atmosphere. The result is a reduction in the amplitude of the surface zonal wind anomalies to approximately half in the sensitivity experiment.

* Current affiliation: Pan-Okhotsk Research Center, Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan

Corresponding author address: Shunya Koseki, Graduate School of Environmental Science, Hokkaido University, Nishi 5 Kita 10 Kita-ku, Sapporo 060-0810, Japan. Email: k-shunya@ees.hokudai.ac.jp

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