Arabian Sea

Daniel L. Rudnick

The monsoons of the Arabian Sea are remarkably strong and steady in comparison to the storms that dominate midlatitude locations, making the Arabian Sea a nearly ideal laboratory for the study of steady wind-driven processes. An array of surface moorings was deployed from October 1994 through October 1995 yielding the first year-long time series of atmospheric and oceanic variables spanning both monsoons. The array, funded by the Office of Naval Research, included five moorings set in a 50-km square pattern centered at 1530'N, 6130'E. and was a collaboration among investigators from Scripps Institution of Oceanography, Woods Hole Oceanographic Institution, University of Washington, Lamont-Doherty Earth Observatory, and University of California, Santa Barbara.

Figure 1. Daily averages of wind speed and direction, sea surface and air temperature, and net downward heat flux as measured on the northern SIO mooring. Note the directional steadiness of the wind during the monsoons. The air-sea temperature difference changes sign during the year, with a notable effect on the heat flux.

Two monsoons occur each year: the weaker NE monsoon in winter and the stronger SW monsoon in summer (Figure 1). As the SW monsoon strengthens in summer, the wind changes direction in a counterclockwise direction. At its strongest in July, the wind is concentrated in a feature often called the Findlater Jet. The location of the moored array was chosen to be near the climatological axis of the Findlater Jet in July to measure the strongest winds in the Arabian Sea. Typical wind speeds were 5 m/s during the NE monsoon, while speeds were more than 10 m/s throughout the SW monsoon. The air was generally cooler than the sea surface, except during the summer when the SW monsoon brought warm air from lower latitudes. The net heat flux is composed of latent, sensible, shortwave, and longwave fluxes, which were either measured directly or estimated using bulk parameterizations. The longest period of negative heat flux occurred during the SW monsoon, primarily due to latent heat losses.


Figure 2. A time-depth section of daily-averaged temperature as measured on the southern SIO mooring. Above 50 m the vertical resolution is 10 m, while below 50 m the vertical resolution is 20 m. Note the strong semi-annual cycle in upper ocean temperature.

The processes determining the temperature and salinity of the upper ocean are advection, turbulent mixing, and air-sea heat flux. Each of these processes is apparent in the year-long time series of temperature (Figure 2). Two periods of mixed-layer deepening occurred, coincident with the beginning of the two monsoons in December and June (Figure 3). The deepest mixed layer of the year is about 100 m during the NE monsoon in January because of intense latent heat loss. In contrast, the much stronger SW monsoon produces a mixed layer of only 70 m. The spring inter-monsoon restratification isolates much of the winter's deep mixed layer from the atmosphere. This deep isothermal layer persists through summer after which it is apparently advected away.


Figure 3. A time-depth section of shear, a time series of mixed-layer depth and a velocity stick plot as measured on the southern SIO mooring. The 4-hour average shear magnitude is plotted as a color image, with a vertical resolution of 4 m. The mixed-layer depth (black and white line) is defined by a 0.1 C difference from the surface. Daily averaged velocity at 20 m depth is represented by sticks with upward being to the north. Note the region of high shear beneath the relatively unsheared mixed layer. The velocity has temporal variability unrelated to the local wind.

Measurements of water velocity and wind allow the examination of the momentum transfer to the ocean by the monsoons. If the ocean were driven only by local wind then the current would be nearly steady during the monsoons. However, the strongest variations in current (Figure 3) are apparently unrelated to local wind, and are likely geostrophic. The shear, as measured by self-contained Acoustic Doppler Current Profilers, reveal some interesting features: (1) the mixed layer is relatively unsheared during periods of deepening, as assumed in the simplest bulk mixed-layer models; (2) thin regions of high shear are located just beneath the mixed-layer base; (3) high shear well beneath the mixed layer coinciding with a change in current direction in early August is an indication of a geostrophic current. Analysis continues on the Arabian Sea data with a goal of quantifying the effect of the monsoons on the mass, momentum, and heat budgets of the ocean.

Acknowledgements. The Instrument Development Group was responsible for the success of the SIO moorings. Robert Weller, Charlie Eriksen, Tommy Dickey, John Marra, and Chris Langdon are valued collaborators on the Arabian Sea moored array.


References

Rudnick, D. L., R. A. Weller, C. C. Eriksen, T. D. Dickey, J. Marra, and C. Langdon, 1997: Moored instruments weather Arabian Sea monsoons, yield data. Eos, Trans. Amer. Geophys. Union, 78, 117, 120-121.


Air-sea interaction has been a theme of much of my work. Here are some additional references.

Weller, R. A., D. L. Rudnick, C. C. Eriksen, K. L. Polzin, N. S. Oakey, J. W. Toole, R. W. Schmitt, and R. T. Pollard, 1991: Forced ocean response during the Frontal Air-Sea Interaction Experiment. J. Geophys. Res., 96, 8611-8638.

Rudnick, D. L., and R. A. Weller, 1993: Observations of superinertial and near-inertial wind-driven flow. J. Phys. Oceanogr., 23, 2351-2359.

Lee, C. M., and D. L. Rudnick, 1996: The upper ocean response to surface heating. J. Phys. Oceanogr., 26, 466-480.

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