Mixed Layer Heat Variations in the South China Sea Observed by Argo Float and Reanalysis Data during 2012–2015

Liang Liang,Wang Wang,Zeng Zeng,Xing Xing

doi:10.3390/su11195429

Abstract

The atmospheric and oceanic causes of mixed layer heat variations in the South China Sea (SCS) are examined using data from six long-lived Array for Real-time Geostrophic Oceanography (Argo) floats. The mixed layer heat budget along each float trajectory is evaluated based on direct measurements, satellite and reanalysis datasets. Our results suggest that the mixed layer heat balance in the SCS has distinct spatial and seasonal variations. The amplitude of all terms in the mixed layer heat budget equation is significantly larger in the northern SCS than in the southern SCS, especially in winter. In the northern SCS, the mixed layer heat budget is controlled by the local surface heat flux and horizontal advection terms in winter, and the net heat flux term in summer. In the western and southeastern SCS, the mixed layer heat budget is dominated by the net surface heat flux in both winter and summer. Further analysis shows that in the SCS, surface shortwave radiation and geostrophic heat advection are major contributors to net heat flux and horizontal advection, respectively. Unlike the net heat flux and horizontal advection, the vertical entrainment is a sink term in general. The rate of mixed layer deepening is the most important factor in the entrainment rate, and a barrier layer may decrease the temperature difference between the bottom of the mixed layer and the water beneath. Residual analysis suggests that the residual term in the equation is due to the inexact calculation of heat geostrophic advection, other missing terms, and unresolved physical ocean dynamic processes.

Highlights

The South China Sea (SCS) is the largest marginal sea in East Asian waters and connects the western Pacific Ocean and eastern Indian Ocean by the South China Sea throughflow [1] through the deep Luzon Strait, which impacts significantly on the heat [2] and salinity [3,4,5,6] balance in the SCS with the intrusion of Kuroshio
We focus on mixed layer heat variations in different areas of the SCS and evaluate the contributions of different terms to the mixed layer heat budget
It is difficult to close the heat budget based on observational data [1,32], the tendency of the monthly mean sum of terms is consistent with the rate of mixed layer heat storage

Summary

Introduction

The South China Sea (SCS) is the largest marginal sea in East Asian waters and connects the western Pacific Ocean and eastern Indian Ocean by the South China Sea throughflow [1] through the deep Luzon Strait, which impacts significantly on the heat [2] and salinity [3,4,5,6] balance in the SCS with the intrusion of Kuroshio. The variability of sea surface temperatures (SSTs) in the SCS has a strong seasonal cycle signal, which is maximum in summer and minimum in winter. SSTs over the SCS display a pattern of cold waters in the western basin while comparatively warm water in the eastern basin. Currents in the SCS have a strong seasonal characteristic, which is related closely to monsoon. The monsoon over the SCS is southwesterly in summer but northeasterly in winter [9] This leads to a basin-scale upper ocean circulation in the SCS [10,11]: In summer a double gyre with the cyclonic circulation in the northern SCS and the anticyclonic circulation in the southern basin; in winter a cyclonic gyre over the basin with the strong Western Boundary Currents (WBC)

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Results

Discussion

Conclusion