Abstract
Subtropical Mode Water (STMW) is a distinctive upper ocean structure in the western part of subtropical gyres in the world ocean. This paper proposes a common criterion of STMW to quantify and compare spatial structures of STMWs in different basins. STMW can be defined as a thermostad (a layer weakly stratified in temperature) by applying a criterion of averaged core layer temperature (CLT)±1oC with its layer thickness greater than 100 m. Two features are highlighted when comparing the STMWs in different basins. Firstly, the North Atlantic hosts the thickest STMW in the world ocean. Secondly, the South Atlantic STMW has an unique vertical structure of density compensating temperature and salinity stratification. By comparing the thickness of STMW against the strength of winter cooling and the volume transport of associated western boundary current (WBC) in different basins, it is shown that thicker STMW tends to be accompanied with stronger WBC. From a view point of vorticity dynamics, we suggest that the North Atlantic may have the most efficient condition to host the thickest STMW and the strongest recirculation gyre in the world ocean.
Highlights
The structure of the subtropical thermocline has fascinated theoretical and observational physical oceanographers alike for many years, because it is perhaps one of the most prominent aspects of the ocean
We propose that any subtropical mode waters (STMW) could be defined with uniform criterion of core layer temperature (CLT) ± 1◦C with a single temperature profile once we know CLT
Based on the averaged CLT of each STMW calculated above, STMW is defined as a thermostad applying a criterion of averaged CLT ± 1◦C with its layer thickness greater than 100 m
Summary
The structure of the subtropical thermocline has fascinated theoretical and observational physical oceanographers alike for many years, because it is perhaps one of the most prominent aspects of the ocean. One effective approach is to focus on water masses that are associated with distinctive ocean structures. We can consider physical mechanisms that form ocean structure by examining/investigating the formation, advection and dissipation of those water masses. Hanawa and Talley (2001) categorizes mode waters into three categories according to their distribution areas. Type 2 mode waters are located eastern part of subtropical gyre. Type 3 mode waters are mainly located in the subpolar area. Mode waters prevail in the upper layer of the world ocean with a large horizontal distribution of over several hundred thousand square kilometers and several hundred meter thickness (Hanawa and Talley, 2001)
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