Cold Surge Impacts on the Structure, Energy Budget, and Turbulence of the South China Sea Boundary Layer
Journal
Monthly Weather Review
Journal Volume
152
Journal Issue
9
Start Page
2129
End Page
2148
ISSN
0027-0644
1520-0493
Date Issued
2024-09
Author(s)
Abstract
Episodic cold surges in the East Asian winter monsoon can penetrate deep into the South China Sea (SCS), enhance consequent tropical rainfall, and further strengthen the East Asia meridional overturning circulation. These cold surges can promote strong surface fluxes and lead to a deeper marine boundary layer (MBL). However, there is a lack of boundary layer studies over the SCS, unlike many other well-studied regions such as the North Atlantic Ocean and the central-eastern Pacific Ocean. In this study, we use high-resolution radiosonde data of temperature and humidity profiles over Dongsha Island (20.708N, 116.698E) to identify the inversion layer, mixed layer, cloud base, cloud top, and factors controlling low cloud cover for the period of December–February from 2010 to 2020. We perform an energy budget analysis with ERA5 meteorological variables and surface fluxes. Here, we show a strong turbulent flux convergence of both heat and moisture within the SCS MBL during cold surges, which leads to a lifting of the mixed layer to;1.0 km and the inversion layer to;2.0 km and associated cloud development over Dongsha Island. The cold and dry horizontal advection is balanced by this vertical turbulent flux convergence in the energy budget. Overall, cold surges over the SCS enhance a lower branch of winter monsoon meridional overturning circulation with stronger inversion and higher low cloud covers. SIGNIFICANCE STATEMENT: Cold surges in the East Asian winter monsoon bring cold and dry air from Eurasian continent to the South China Sea where strong air–sea fluxes and pronounced shallow clouds are unique climatological features. The convective boundary layer (CBL) over the SCS and upstream northwest Pacific (NWP) is important in maintaining the East Asia (EA) meridional overturning circulation. However, the CBL over the SCS–NWP is poorly understood and the lack of understanding can lead to unrealistic boundary layer turbulence and energy transport such that the tropical convection and the overturning circulation are incorrectly represented. In this study, we use high-quality radiosonde data at Dongsha, reanalysis, and satellite cloud data to show the CBL structure and their evolution during the passage of cold surges in northern SCS. We anticipate that our study will motivate more atmosphere–ocean joint observation and PBL-related studies over the SCS–NWP.
Subjects
Atmosphere-ocean interaction
Boundary layer
Clouds
Cold air surges
SDGs
Publisher
American Meteorological Society
Type
journal article