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

Introduction and Objectives

Winds over the Arabian Sea are dominated by strong and steady monsoonal patterns with large spatial scales. During the summer the southwest monsoon produces an air flow known as the Findlater Jet, with high wind stress curl and upwelling to the north and west, and downwelling to the south and east. Late summer sea surface temperatures are observed to be cooler than those in the spring. In the winter northeast monsoon, the wind reverses direction and is weaker, but is still quite steady. The objectives of this observational program were to make quantitative assessments of the surface and lateral fluxes of heat, salt, and momentum, and to better ascertain their role in mixed layer dynamics.

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).

Surface Meteorology and Fluxes


Figure 2. Measured wind speed, direction, air temperature (AT), sea surface temperature (SST), barometric pressure (BP), relative humidity (RH), and accumulated rainfall. The April gap is due to mooring recovery and redeployment.

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.

Observations of the Mixed Layer


Figure 4. Contour of lowpass filtered subsurface temperature and salinity. The estimated mixed layer depth is plotted in white over the contours, and is based on a 0.5 °C temperature differential from the surface.


Figure 5. Two-day averaged subsurface velocities. Up is northward flow. Velocities at 300 m from UW-S mooring.

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.

1D Budget Calculations

To help assess the relative importance of surface and lateral fluxes of heat and salt, one-dimensional budgets were computed. While there is good agreement through the winter monsoon, advective fluxes seem to become important at the start of the summer monsoon.


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).

1D PWP Model Calculations


Figure 7. Contours of the modelled subsurface temperature and salinity, using the same colormap as Fig. 4. The model mixed layer depth is plotted in white, and is based on a strict 10^-6 density differential from the surface.


Figure 8. Filtered modelled velocity. Up is northward flow.

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.

Summary

A moored time series of surface meteorology and upper ocean response during the Arabian Sea winter and summer monsoons was obtained. The goal of the experiment was to quantify surface and lateral fluxes, and to better understand their role in controlling the mixed layer. Observations show two monsoonal cycles of mixed layer deepening and cooling, and evidence of advective fluxes of heat and salinity. From surface fluxes alone, we might expect a continuous increase in salinity, and a slight winter cooling followed by continuous heating (Fig. 6). The PWP model confirms this, showing the inadequacy of 1D dynamics in explaining the observed mixed layer behavior.

Acknowledgements

This research was funded by the Office of Naval Research, as a \ part of the Arabian Sea Mixed Layer Dynamics Experiment.

Selected References

Fairall, C. W., E. F. Bradley, D. P. Rogers, J. B. Edson and G. S. Young. Bulk parameterization of air-sea fluxes for TOGA COARE, J. Geophys. Res., accepted 1995.

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.

Contact Information

Albert S. Fischer, Department of Earth, Atmospheric, and Planetary Sciences, M.I.T. Room 54-1511A, Cambridge, MA 02139 USA. e-mail: afischer@mit.edu.

Robert A. Weller, Department of Physical Oceanography, MS 29, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 USA. .

This is the web version of a poster originally displayed at the 1996 AGU/ASLO Ocean Sciences conference (poster OS11A-13). Created 26 March 1996, A. Fischer.

Last updated: May 19, 2008