Temporal Variability of Surface Fluxes and Mixed Layer Response at 15.5 °N, 61.5 °E over a Monsoonal Cycle
ALBERT S. FISCHER Massachusetts Institute of Technology
ROBERT A. WELLER Woods Hole Oceanographic Institute
High-quality coincident time series of the atmospheric forcing and upper ocean resonse at 15° 30' N, 61° 30' E in the Arabian Sea were obtained during the winter and summer monsoons. A surface mooring was deployed from 15 October 1994 through 20 October 1995, part of an array just south of the climatological wind maximum. The mooring had a full suite of meteorological instruments, and collected subsurface temperature, salinity, and current data. Drs. John Marra, Tom Dickey, Van Holliday, and Chris Langdon attached additional bio-optical instrumentation.
The mooring had two full sets of meteorological sensors, including wind speed and direction, sea surface temperature, air temperature, incoming short wave and long wave radiation, barometric pressure, relative humidity and precipitation. Data was transmitted to WHOI in real-time by satellite, and was recorded every 450 seconds. The buoy hull also supported six temperature loggers (Tpods) in the upper 2.5 m, each protected by a radiation shield. The mooring line carried a wide variety of instruments in the upper 300 m. These included vector-measuring current meters (VMCMs), which also measured temperature, conductivity and temperature sensors (SEACATs), 4 muti-variable moored system (MVMS) sensors measuring velocity, temperature, photosynthetically available radiation and transmissivity, and further temperature loggers (Tpods). Subsurface instruments recorded every 225-900 seconds.
Figure 1a. Mooring location.
Figure 1b. A photograph of the Arabian Sea surface mooring, an instrumented 3 m discus buoy.
Figure 1c. A mooring diagram showing the positions of the various instruments (Jayne Doucette/WHOI Graphics).
The surface meteorological record is dominated by the northeast and southwest monsoons. Surface fluxes of momentum and heat were computed using the bulk flux algorithm developed recently during the TOGA Coupled Ocean Atmosphere Response Experiment (Fairall et al. 95a). The total heat flux drops and becomes negative during the winter monsoon due to strong evaporative heat losses. For the rest of the year including the summer monsoon, the total heat flux is positive, although this is limited in the early southwest monsoon again due to strong evaporative heat losses. The result is an overall observed oceanic heat gain.
Figure 3. Calculated surface fluxes, starting with the wind stress (tau). The four components of the surface heat flux: latent heating (Qlat), sensible heating (Qsens), net longwave radiation (Qlw), and filtered net shortwave radiation (Qsw), with positive values denoting a flux of heat into the ocean. The lowpass filtered total heat flux (Qtotal).
The temperature record reveals two yearly cycles of mixed layer deepening and cooling followed by shoaling, one during each monsoon. The salinity record shows the surface layer becoming more saline through the end of May. The high currents observed in the fall of 1994 and late summer 1995 are not correlated to the wind stress.
Figure 6. The one-dimensional heat and salt budgets of the upper ocean. The upper panel shows the time integral of the surface heat flux (red), and the lowpass filtered relative heat content of the upper ocean, vertically integrated to various reference depths (blue). The lower panel shows the expected salinity due to evaporation and precipitation (red) assuming mixing to 100 m, and the lowpass filtered average salinity in the upper 100 m (blue).
To further assess the relative role of surface and lateral fluxes, the one-dimensional Price Weller Pinkel (PWP) mixed layer model was run for the observed time period. The model was initialized with observed profiles of temperature and salinity, and was forced with the observed fluxes of heat, salt, and momentum. The model mixed layer deepened and cooled during the winter monsoon, but to a lesser extent than observed. The surface warming trend begun before the summer monsoon continued after a second period of deepening and cooling of the mixed layer during the early summer monsoon. Salinity shows a similar pattern of behavior, with final surface temperatures and salinities higher than observed.
Price, J. F., R. A. Weller and R. Pinkel. Diurnal cycling: observations and models of the upper ocean response to diurnal heating, cooling, and wind mixing. J. Geophys. Res., 91, 8411-8427, 1986.