Spatiotemporal Evolution Of Marine Heatwaves Under Climate Warming And Their Impact On Sea-air CO₂ Fluxe

By using an adaptive data analysis method,this study extracted the time-varying trends in sea surface temperatures（SSTs）and marine heatwaves（MHWs）based on the OISST high-resolution data covering the past four decades（1982–2021）.With time-varying trends extracted,we quantify how the MHW metrics evolve with the SST warming trend during the past four decades.Secondly,based on this quantitative analysis,we picks out three CMIP6 models that perform best in simulating the SSTs and MHWs to simulate the time-varying trends in future SSTs and to predict the future MHW evolution from 2022to 2100 under the three socioeconomic pathways.In addition,based on a reconstructed observation-based product of air-sea CO₂flux（the MPI-SOMFFN method）,this study analyzes the influence of MHWs on air-sea CO₂flux in the central-eastern tropical Pacific Ocean by using an adaptive data analysis.It is found that this oceanic CO₂source region suffers from a significant reduction in CO₂outgassing caused by MHWs.Finally,simulation skills of CMIP6 models are assessed in the context of the spatialtemporal evolutions of MHWs and air-sea CO₂flux as well as their relationships in the central-eastern tropical Pacific.Conclusions of this study are stated briefly blow.MHWs in response to the rising SSTs are quantified based on the observational datasets.Under the global warming,MHWs have increased in frequency,duration,intensity,and spatial coverage in the global ocean over the past four decades.With the aid of an adaptive data analysis method,time-varying trends in global SSTs and MHWs during the past four decades are clearly extracted.Although global SSTs have risen significantly over the past four decades,large spatial variations exist in SST warming trends.Except for the central-eastern tropical Pacific region,the significantly rising SST trends dominate more than 2/3 of the global ocean.In the region with significant SST warming,results for the quantitative analysis shows the globally averaged increasing rates of the annual MHW frequency,duration,and maximum intensity are approximately 3.7 events,7.5 days,and 2.2 degrees Celsius per degree Celsius of SST rise,respectively.The increasing rates for the annual MHW days,cumulative intensity,and the fraction of the spatial extents to the global ocean affected by MHWs are approximately 58.8 days,84.2degrees Celsius days,and 13.9%per degree Celsius of SST rise,respectively.Specifically,over the past four decades,annual MHW days,duration,and frequency in the northeast and the tropical Pacific Ocean,the Gulf Stream with its related extensions,the central/western South Indian Ocean,and portions of the Antarctic Circumpolar Current are most sensitive to the rising SSTs,while MHW maximum intensity in the mid-to-high latitudes is most sensitive to the rising SSTs.Based on the observational-based quantitative relationships between SSTs and MHWs and the projected SST warming from the selected models,the spatiotemporal patterns of the future MHW evolution are projected.Increases in MHWs are projected to accelerate further under continued global warming.Results for MHW projections show that the spatial distributions of changes in the annual MHW days,frequency,and cumulative intensity are projected to exhibit 6 to 8-fold increases under the SSP5-8.5,much higher than those under the SSP1-2.6（2-fold）and SSP2-4.5（4-fold）scenarios.Under SSP5-8.5,the globally averaged annual MHW days will increase to approximately 224.2±26.9days.Largest increases in MHWs are projected to occur in the whole tropical oceans,the western boundary currents and their extensions,the northwest Atlantic,the northeast Pacific,the Bering Sea,and the western South Indian Ocean,with approximately 14.8±5.7%of the global ocean reaching a permanent MHW state by the end of the twenty-first century under SSP5-8.5.In the central-eastern tropical Pacific Ocean where oceanic CO₂releases to the atmosphere,the influence of MHWs on air-sea CO₂flux are analyzed and used in reconstructing air-CO₂flux.The CO₂outgassing in the central-eastern tropical Pacific is observed to be attenuated by MHWs over 1982–2020.On the interannual,decadal,and long-term change timescales,there are significant inverse relationships between the MHW metrics and air-sea CO₂flux（positive into atmosphere）in this region by using an adaptive data analysis.Spatially,in the central tropical Pacific,MHWs affect air-sea CO₂flux by driving the oceanic stratification,while in the eastern tropical Pacific（including coastal upwelling regions）,MHWs may mainly regulate air-sea CO₂flux by altering the carbonate balance associated with biogeochemical processes.Meanwhile,based on the regressions between the MHW metrics and air-sea CO₂flux,the spatiotemporal patterns of air-sea CO₂flux in the central-eastern tropical Pacific can be effectively reconstructed,which is important for further prediction of air-sea CO₂flux.In the preliminary evaluations for the simulation skills of climate models,12 CMIP6models are qualitatively evaluated for their simulation skills in reproducing the observed spatialtemporal evolutions of MHWs and air-sea CO₂flux as well as their relationships from 1982 to 2014 in the central-eastern tropical Pacific.None of the assessed 12 CMIP6models could reproduce the observed increase of CO₂release during 1982–2014 in this study region.Among these assessed models,IPSL-CM6A-LR,Nor ESM2-MM,GFDL-ESM4,and Nor ESM2-LM are most skillful in capturing the features of MHW metrics and air-sea CO₂flux as well as their relationships.However,CESM2,CESM2-WACCM,MPI-ESM1-2-LR,and MPI-ESM1-2-HR are the poorest-skill models,which even show opposite relationships between MHWs and air-sea CO₂flux compared to observations.This is probably because these models could not reproduce the important processes driven by MHWs（e.g.,upwelling reduction weakening）that has significant influence to the oceanic CO₂outgassing.It is still a challenge for CMIP6 models to accurately reproduce the spatiotemporal evolution in air-sea CO₂flux in the central-eastern tropical Pacific.Given to the irreplaceable role of MHWs in modulating air-sea CO₂flux in this region,the improvement of CMIP6 models in reproducing MHW evolutions may achieve accurate simulations and predictions in air-sea CO₂flux in this region.