Spatial And Temporal Variations And Causes Of Coral Bleaching Events Under Global Warming

Global warming has exacerbated the frequency and intensity of ocean heat redistribution,leading to higher sea surface temperature(SST)and a major threat to the health of tropical and subtropical coral ecosystems.Coral bleaching is an important indicator of the health of coral ecosystems.Under the influence of global warming,the frequency and severity of coral bleaching events continue to increase,and corals are facing an increasing risk of bleaching or death.Therefore,in-depth research on the temporal and spatial dynamic differentiation of global coral bleaching is of great significance for further understanding the ocean circulation pattern and heat redistribution mechanism under global change,as well as the distribution pattern,process and impact mechanism of coral bleaching events.This research is based on multi-source data(global coral bleaching rate,coral coverage,algae coverage,sea surface temperature anomaly(SSTA),heat stress anomaly,coral exposure and ENSO(El Ni(?)o–Southern Oscillation));Using model simulation methods such as Empirical Orthogonal Function Analysis(EOF)and Technique for Order Preference by Similarity to an Ideal Solution(TOPSIS),the basic pattern of temporal and spatial differentiation of coral bleaching from 1993 to 2020 was quantitatively analyzed,and the influence and driving mechanism of ocean heat redistribution on coral bleaching under the influence of extreme climate events,ocean currents and topography was discussed.Through the above research,the following main results are obtained:Firstly,the health status of the global coral ecosystem is closely related to mass coral bleaching events,and presents a fluctuating trend.Global mass coral bleaching events in1997-1998,2010 and 2014-2017 resulted in a decline in coral coverage from 32.5% to 28.8%.Coral coverage and the number of coral bleaching events show a significant negative correlation,that is,when coral coverage declines globally(1997-2002,2010-2018),bleaching events occur frequently and with high intensity.Global warming has exacerbated the intensity and frequency of ENSO,leading to a more severe coral bleaching situation.Among them,the SSTA during the La Ni(?)a period in 2017-2018(0.73°C)was higher than that in 1997-1998(0.40°C)and 2010(0.52°C)El Ni(?)o periods.Secondly,the adaptability of global coral ecosystems to ocean surface water warming events in different regions shows obvious differences.The global sea surface temperature that causes coral bleaching increased from 27.3°C(1997-2007)to 28.1°C(2008-2019),and the corresponding SSTA increased from 0.18°C to 0.35°C.Corals in the Caribbean and Brazil reefs have the strongest ability to adapt to heat,and the temperature causing bleaching increased by 0.42℃ and0.64℃ respectively,while the coral in the Pacific reef area had the weakest adaptability,and the SSTA causing bleaching was only 0.27℃.Coral ecosystems in different sea areas can recover quickly to a healthy state within 10 years after severe bleaching events.For example,from 1997 to2004,the coral coverage rate in the South Asian reef area decreased from 40.2% to 12.0%,and from 2004 to 2011,the coral coverage rate recovered rapidly,and the value increased from 12.0%to 38.3%.Thirdly,based on the in-depth analysis of the ocean heat change data and the global ocean current movement process,the northern hemisphere warm current-dominated model and the southern hemisphere warm current-dominated model are proposed,combined with the basic laws of ocean current movement and wind direction characteristics on the east coast of the continent,the process by which ocean currents and landforms affect heat accumulation and coral bleaching in coral reef regions(Caribbean,Western Pacific,Northern waters of Australia and Western Indian Ocean)is explained.Among them,the Caribbean Sea is blocked by the terrain surrounded by sea on three sides,which gathers the heat of the Atlantic equatorial current;The western Indian Ocean is affected by the heat of the Mozambique current and the Southern equatorial current,and the heat gathers in the sea area of 45°E-90°E and 15°S-5°S;The heat in the Western Pacific Ocean is concentrated in the sea area of 130°E-160°E and 0°-25°N,forming a heat transfer pattern dominated by the equatorial warm current and the Kuroshio Current.The heat in Oceania is concentrated in the sea area of 130°E-180°E and 15°S-30°S,forming a heat transfer pattern dominated by equatorial warm current.This study analyzed the distribution pattern and change process of coral bleaching events under global change,explored the impact mechanism and aggregation model of coral bleaching in different regions,and provided a scientific basis for promoting the protection of global coral habitat and speciesin the future.