Images: Monsoon Winds and Carbon Cycles in the Arabian Sea
The surface winds and ocean mixed layer depths in the Arabian Sea in July (left) and December (right). The depth of the mixed layer, the upper layer that is well-stirred by surface forcing, is shown in meters. The strong surface winds of the Southwest Monsoon are evident in July, and the moderate winds typical of the Northwest Monsoon blow in December. The locations of the surface Air-Sea Interaction (ASI) and sediment trap moorings are marked by red and dark blue dots respectively.
An overview of the winds and directly forced ocean response associated with the Southwest Monsoon is provided by this schematic cross-section of the ocean taken perpendicular to the coast of Oman. The jet of the surface winds drives water to its right in the northern hemisphere as "Ekman transport" (red arrows). Along the coast because of the boundary this leads to cool water rising to the surface, or upwelling. Moving offshore, the varying strength of the wind leads to differences in the strength of the Ekman transport, causing a divergence of surface water and upwelling inshore of the wind maximum, and a convergence and downwelling offshore of the wind maximum (blue arrows).
The Air-Sea Intraction (ASI) mooring buoy is equipped with two sets of meteorological sensors to measure wind speed and direction, air and sea temperature, incoming shortwave and longwave radiation, barometric pressure, relative humidity, and rain. Current meters and other instruments are located along the mooring line beneath the ASI buoy.
Sea surface height, as measured by a satellite altimeter. The height of the sea surface reflects the geostrophic current field associated with large eddies. These eddies form off the Horn of Africa, near the island of Socotra, during the height of the Southwest Monsoon and later drift to the northeast. The strong currents with slowly varying directions seen in May and September in the figure opposite are associated with the passage of these large eddies through the array of moorings
Time series plots from the surface mooring and nearest sediment trap mooring. Peaks in the variability in primary production (top), based on the measurements on the surface mooring by Dickey and Marra correlate well with peaks in the export production (lower two) measured by the sediment traps at 2,229 and 3,478 meters. Note the correlation as well between the strong currents (second from top) measured by the surface mooring during export production peaks in May and September. These peaks in velocity were associated with the passing eddies.
Time series plots from the surface mooring and the 2,229 meter sediment trap. The sea surface and air temperature (upper plot) show the cooling that occurs during both the northeast monsoon and the southwest monsoon. The modest winds of the northeast monsoon together with loss of heat from the ocean to the atmosphere lead to deepening of the mixed layer (third plot down) in December and January. The deepening of the mixed layer in June and July during the southwest monsoon results from mixing driven by the very strong winds. The mass flux and carbon flux time series (fourth and fifth down) track the export production seen in the plots above these. Time series of the ratios of chemical species in the 2,229 meter trap, organic to inorganic carbon and silicon to calcium, and of the flux of inorganic and organic carbon (bottom) indicate that the chemical signatures vary with wind velocity (positive relationship) and mixed layer depth (negative relationship). The authors are intrigued by the variability and the possibility of linking it to the physics of the mixing that brings nutrients to the surface.