Dynamic changes of subtropical western boundary currents under global warming

hu.yang [ at ] awi.de


The subtropical western boundary currents (WBCs) carry a large amount of heat from the low latitudes to the mid- and high-latitudes, which contribute to the Earth's energy balance and have a broad impact on the weather and climate over the adjacent mainland. Based on the analysis of ocean surface turbulent heat fluxes (THF) trends, we recognize a prominent increase of ocean heat loss over all the mid-latitude expansions of WBCs during the past half century, suggesting significant dynamic changes of these currents. To understand the background mechanism, several coupled parameters are analyzed, including the SST, net surface heat flux, ocean velocity, near-surface wind and sea level pressure. These data are collected from three types of independent data resources, i.e., reanalysis products, satellite-blended observations and climate model outputs from the third and fifth phase of the Climate Model Intercomparison Project (CMIP3/CMIP5). Based on these broad ranges of data sets, we find that the WBCs (except the Gulf Stream) are intensifying and shifting toward the poles as long-term effects of global warming. An intensification and poleward shift of near-surface ocean winds are proposed to be the forcing of such dynamic changes over both hemispheres. In contrast to the other WBCs, the Gulf Stream is expected to be weaker under global warming, which is most likely related to a weakening of the Atlantic Meridional Overturning Circulation (AMOC). The systemic intensification and poleward shift of ocean surface wind are proposed to be attributed to the positive trends of annular modes. Over the Northern Hemisphere, strong natural variations of Northern Annular Mode (NAM) are observed, which conceal the long-term effect of global warming. While both observations and climate models record a robust positive trend of Southern Annular Mode (SAM) under global warming. The mechanism for this phenomenon is still under debate. Here, we find that the major feature of global warming induced SST pattern over the Southern Hemisphere is characterized by a stronger equator-to-pole gradient. Model sensitivity study demonstrates that such meridional gradient in SST is potential to drive a stronger Southern Annular Mode in a warming climate.

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Thesis (PhD)
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Yang, H. (2017): Dynamic changes of subtropical western boundary currents under global warming , PhD thesis, Universität Bremen.

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