Seo, H., R. Murtugudde, M. Jochum and A. J. Miller, 2008:
Modeling of mesoscale coupled ocean-atmosphere interaction and its feedback to ocean in the western Arabian Sea.
Ocean Modelling, 25, 120-131.
Observations of the western Arabian Sea over the last decade have revealed
a rich filamentary eddy structure, with large horizontal
SST gradients in the ocean, developing in response to the southwest
monsoon winds. This summertime oceanic condition triggers an intense mesoscale coupled interaction,
whose overall influence on the longer-term properties
of this ocean remains uncertain. In this study, a high-resolution
regional coupled model is employed to explore this feedback effect on
the long-term dynamical and thermodynamical structure of the ocean.
The observed relationship between the near-surface winds and mesoscale SSTs
generate Ekman pumping velocities at the scale of the cold filaments, whose magnitude
is the order of 1 m/day in both the model and observations. This additional Ekman-driven velocity,
induced by the wind-eddy interaction, accounts for approximately 10-20% of oceanic
vertical velocity of the cold filaments. This implies that Ekman pumping arising
from the mesoscale coupled feedback makes a nontrivial contribution to the vertical structure
of the upper ocean and the evolution of mesoscale eddies, with obvious implications for
marine ecosystem and biogeochemical variability.
Furthermore, SST features associated with cold filaments substantially reduce the latent heat
loss. The long-term latent heat flux change due to eddies in the model is
approximately 10-15 W/m2 over the cold filaments, which is consistent with
previous estimates based on short-term in situ measurements. Given the shallow
mixed layer, this additional surface heat flux warms the cold filament at the
rate of 0.3-0.4C/month over a season with strong eddy activity, and 0.1-0.2C/month over
the 12-year mean, rendering overall low-frequency modulation of SST feasible.
This long-term mixed layer heating by the surface flux is approximately +-10%
of the lateral heat flux by the eddies, yet it can be comparable to
the vertical heat flux. Potential dynamic and thermodynamic impacts of this observed
air-sea interaction on the monsoons and regional climate are yet to
be quantified given the strong correlation between the Somalia upwelling SST and the Indian summer monsoons.