The flow separations in the Taiwan Strait and ocean responses to the "hiatus" of the global mean surface temperature
Date
2017
Authors
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Publisher
University of Delaware
Abstract
Coastlines are fundamental to humans for habitation, commerce, and natural resources. Many coastal ecosystem disasters, caused by extreme sea surface temperature (SST), were reported when the global climate shifted from global warming to global surface warming hiatus between 1998 and 2013. The 2008 cold event in the Taiwan Strait (northwestern Pacific) is one of these disasters. In studying the behind dynamical mechanism of the cold event, four topics were raised in sequence and consisted of my thesis. Two off-shore flows are identified and investigated through observed and model (ROMS) results. In order to better understand this cold event within global context, the global coastal responses and heat redistribution in the Indian Ocean are then studied using satellite data and a global climate model (CESM). ☐ In winter, an off-shore flow of the coastal current can be inferred from satellite and in-situ data over the western Taiwan Bank. The dynamics related to this off-shore flow are examined here using observations as well as analytical and numerical models. The currents can be classified into three regimes. The downwind (southward) coastal current remains attached to the coast when the wind stress is stronger than a critical value depending on the upwind (northward) large-scale pressure gradient force, and an upwind current appears over the Taiwan Bank under a weak wind stress. The downwind coastal current and upwind current converge and the coastal current deflects onto the bank during a moderate wind. Analysis of the vorticity balance shows that the off-shore transport is a result of negative bottom stress curl that is triggered by the positive vorticity of the off-shore flow. The negative bottom stress curl is reinforced by the gentle slope over the bank, which enhances the off-shore current. Composite analyses using satellite observations show cool waters with high chlorophyll in the off-shore current under the moderate wind, which support the model findings and may explain the high productivity over the western bank in winter. ☐ The other off-shore flow in the northern strait often makes a U-turn to join the northward-flowing Taiwan wam current. In early 2008 of anomalously strong winter monsoon, cold water in the Taiwan Strait was moved sequentially by a cross-strait flow and a southward flow to the Penghu Island, causing a cold-related fish kill disaster. Except for the local wind forcing, the Coastal-Kelvin Waves (CKWs), intermittently propagating toward the TWS from north in winter, are an additional factor that could impact the flow patterns by changing cross-strait sea level gradient during the disaster. In the first stage (January 28-February 7), the reach of a large CKW trough induced an additional northward flow, which formed a cyclone after turning around the Zhangyun Ridge. Then, the cyclone led to an additional cross flow, which enhanced an eastward (offshore) movement of cold water. In the second stage (February 7-14), the arrival of a large CKW crest triggered an additional southward flow, which intensified a southward movement of the cold water. Due to the additional eastward and southward movements caused by the CKWs, the cold water could reach Penghu Island inducing a cold disaster. ☐ The study on the global coastal SST from 1982 to 2013 revealed a significant cooling trend in the low and mid latitudes (31.4% of the global coastlines) after 1998, while 17.9% of the global coastlines changed from a cooling trend to a warming trend concurrently. The trend reversals in the Northern Pacific and Atlantic coincided with the phase shift of Pacific Decadal Oscillation and North Atlantic Oscillation, respectively. These coastal SST changes are larger than the changes of the global mean and open ocean, resulting in a fast increase of extremely hot/cold days, and thus extremely hot/cold events. Meanwhile, a continuous increase of SST was detected for a considerable portion of coastlines (46.7%) with a strengthened warming along the coastlines in the high northern latitudes. This suggests the warming still continued and strengthened in some regions after 1998, but with a weaker pattern in the low and mid latitudes. ☐ The study of heat transport in the Indian Ocean discloses a different pathway that the anomalous heat moves southward instead of westward caused by a strengthened southward transport and a weakened south equatorial current. This induces a striking heat build-up in the middle latitude of South Indian Ocean, contributes to the Southern Ocean warming, and intensifies heat hemisphere asymmetry. The heat increase has important climate impacts such as changes to rainfall over the western coast of Australia and increased coral bleaching. The new path discovered here may be an essential route linking the tropical Indo-Pacific Ocean and the Southern Ocean during the surface warming hiatus period.