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Meridional Heat Advection due to Mixed Rossby Gravity Waves in the Equatorial Indian Ocean

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  • 1 Japan Agency for Marine-Earth Science and Technology/Research Institute for Global Change, Kanagawa, Japan
  • | 2 Japan Agency for Marine-Earth Science and Technology/Research Institute for Global Change, Kanagawa, and The University of Tokyo/Graduate School of Science, Tokyo, Japan
  • | 3 Japan Agency for Marine-Earth Science and Technology/Research Institute for Global Change, Kanagawa, Japan
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Abstract

This study examines heat advection due to mixed Rossby gravity waves in the equatorial Indian Ocean using moored buoy observations at (0°, 80.5°E) and (0°, 90°E) and an ocean general circulation model (OGCM) output. Variability associated with mixed Rossby gravity waves is defined as that at periods of 10–30 days, where both observations and the OGCM results show high energy in meridional velocity and meridional gradient of temperature. The 10–30-day variability in meridional velocity causes convergence of heat flux onto the equator, the net effect of which amounts to 2.5°C month−1 warming at the depth of the thermocline. Detailed analysis shows that the wave structure manifested in temperature and velocity is tilted in the xz plane, which causes the phase lag between meridional velocity and meridional temperature gradient to be a half cycle on the equator and results in sizable thermocline warming. An experiment with a linear continuously stratified model shows that the contributions of many baroclinic modes, and the right zonal wavelength of wind forcing, are essential in generating the correct wave structure. It is also shown that contributions of mixed Rossby gravity waves to cross-equatorial heat transport are negligible, as temperature variability associated with this wave mode has a node on the equator.

Corresponding author address: Motoki Nagura, Japan Agency for Marine-Earth Science and Technology/Research Institute for Global Change, 2-15 Natsushima-cho, Yokosuka-city, Kanagawa 237-0061, Japan. E-mail: nagura@jamstec.go.jp

Abstract

This study examines heat advection due to mixed Rossby gravity waves in the equatorial Indian Ocean using moored buoy observations at (0°, 80.5°E) and (0°, 90°E) and an ocean general circulation model (OGCM) output. Variability associated with mixed Rossby gravity waves is defined as that at periods of 10–30 days, where both observations and the OGCM results show high energy in meridional velocity and meridional gradient of temperature. The 10–30-day variability in meridional velocity causes convergence of heat flux onto the equator, the net effect of which amounts to 2.5°C month−1 warming at the depth of the thermocline. Detailed analysis shows that the wave structure manifested in temperature and velocity is tilted in the xz plane, which causes the phase lag between meridional velocity and meridional temperature gradient to be a half cycle on the equator and results in sizable thermocline warming. An experiment with a linear continuously stratified model shows that the contributions of many baroclinic modes, and the right zonal wavelength of wind forcing, are essential in generating the correct wave structure. It is also shown that contributions of mixed Rossby gravity waves to cross-equatorial heat transport are negligible, as temperature variability associated with this wave mode has a node on the equator.

Corresponding author address: Motoki Nagura, Japan Agency for Marine-Earth Science and Technology/Research Institute for Global Change, 2-15 Natsushima-cho, Yokosuka-city, Kanagawa 237-0061, Japan. E-mail: nagura@jamstec.go.jp
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