Abstract
The atmosphere typically receives heat from the open tropical ocean; an exception is the western and central Arabian Sea during the summer monsoon season. This article analyzes in situ observations from buoys and ships and examines mechanisms responsible for the reverse sea-air interfacial heat flux that have yet to be addressed. Key factors are the horizontal advection of heat and the heat flux derived via mixing and entrainment of air from above the atmospheric mixed layer in a Richardson number regime where shear-driven mixing would not normally be expected. This study invites a fresh look at the physics of entrainment and mixing in the atmospheric boundary layer. The reverse heat flux observations offer an opportunity to test the efficacy of physics encapsulated in boundary layer parameterization schemes for coupled models.
Highlights
For most inhabitants of Southeast Asia, monsoon season means bountiful rainfall, for ancient Arab traders, it meant a seasonally reversing wind system over the North Indian Ocean that had remarkable bearing on sailing
At the Arabian Sea buoy location, sea surface temperature (SST) ≥ the air (Ta) (i.e., ΔT ≥ 0) until the first week of June (Figure 3a), and their difference is reduced thereafter, coinciding with the beginning of the summer monsoon season, which is characterized by strong winds
We examine if Rig≤1 occurs in the North Indian Ocean during the summer monsoon season by analyzing the surface and upper air data collected during Arabian Sea Monsoon Experiment (ARMEX) and Ocean Mixing and Monsoon (OMM) cruises (Figures 5 and 6). (Upper air data are not available at the Woods Hole Oceanographic Institution (WHOI) buoy location in the Arabian Sea.) Figure 5a shows that Ta > SST (i.e., ΔT < 0) prevailed the majority of the time from June 26 to July 6 in the Arabian Sea, while the opposite was the case in the Bay of Bengal (Figure 5b)
Summary
For most inhabitants of Southeast Asia, monsoon season means bountiful rainfall, for ancient Arab traders, it meant a seasonally reversing wind system over the North Indian Ocean that had remarkable bearing on sailing. Monsoonal winds collect water vapor from south of the equator in the Indian Ocean, from the Arabian Sea, and from the Bay of Bengal and transport it to Southeast Asia (Figure 1). The highest rates of evaporation over the tropical ocean during the peak monsoon months of July and August are observed between. Secondary peaks are observed over the central Arabian Sea and the southern Bay of Bengal, which happen to be regions with the highest evaporation rates in the Northern Hemisphere. Qs can be either negligible or even negative over the western and central Arabian Sea (Figure 1c).
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