Impact Of Systematical Biases In CMIP5/6 On Tropical Pacific SST Change Simulation And Projection

The tropical Pacific has the strongest air-sea interaction over the world,figuring out the response of SST change over the tropical Pacific to global warming is thus an important step in evaluating and understanding the future changes of the global climate system.Previous studies usually projected the tropical Pacific SST change and its climatic effects based on the outputs provided by CMIP5/6 directly.However,CMIP5/6 models cannot correctly reproduce the observed SST trends in the tropical Pacific over the past,which deeply reduces the credibility of their future projections.By using the reanalysis datasets during 1901-2010 as the reference,here we find great systematical biases of CMIP5/6 in simulating 13 physical processes,and then,we investigate the total impacts of 13 systematical biases on simulating the tropical Pacific SST trend over the past century and projecting the tropical Pacific SST change in future.Based on the projected tropical Pacific SST change after removing the total impacts of systematical biases,we investigate the extreme El Ni?o frequency changes and the zonal shift of the northwestern tropical Pacific monsoon trough in the future projections.The main conclusions are as follows:(1)The total impacts of 13 systematical biases contribute to 80%-90% of the discrepancy between the observed and CMIP5/6 simulated tropical Pacific SST trend over the past century.Compared with the results in the reanalysis datasets during 1901-2010,the CMIP5/6 models display systematical biases in simulating all of the 13 processes,which cover the tropical Pacific long-term trends,the inter-basin interactions among three tropical oceans,the tropical SST mean-states,the positive and negative air-sea feedbacks in the tropical Pacific and the skewness of ENSO.The independent impact of each systematical bias on the tropical Pacific SST trend simulation is different,or even opposite one another.By using the multiple regression method,the total impacts of 13 systematical biases display a “La Ni?a-like” pattern,which could explain the differences between the observed and CMIP5/6 simulated tropical Pacific SST trend well.(2)The total impacts of 13 systematical biases may result in the east-minus-west tropical Pacific SST gradient change being over-projected by 0.52-0.53℃/1℃ of globally averaged SST warming.The “La Ni?a-like” SST change over the tropical Pacific is over-projected as “El Ni?o-like” SST change due to the total impacts of systematical biases.The weakening of eastminus-west tropical Pacific SST gradient leads to the slackening of Walker circulation,strengthening of ascent motion and increasing of precipitation over the central-eastern tropical Pacific in the original CMIP5/6 future projections.In sparking contrast,after removing the impacts of 13 systematical biases,the tropical Pacific SST change displays a "La Ni?a-like" pattern,associated with enhanced climatological descent motion and reduced precipitation over the central-eastern tropical Pacific.(3)The total impacts of systematical biases may result in the over-projected extreme El Ni?o frequency change in the original CMIP5/6 future projections.Due to the total impacts of systematical biases,the tropical Pacific is over-projected as “El Ni?o-like” SST change in the original CMIP5/6 future projections.The induced slackening of Walker circulation contributes to the strengthening of Ni?o3 ascent motion,which further leads to the 2-fold increase of extreme El Ni?o frequency.Note that the extreme El Ni?o here is defined as total Ni?o3 omega value averaged from 0 to 500 h Pa being negative in boreal winter.However,after removing the total impacts of 13 systematical biases,the climatological ascent motion would weaken in the Ni?o3 area,and thus the frequency of extreme El Ni?o would keep almost unchanged,or only insignificantly increase in future.(4)The total impacts of CMIP6 systematical biases may lead to the eastward extension of northwestern tropical Pacific monsoon trough under global warming.The interannual and interdecadal east-west shift of the northwestern tropical Pacific monsoon trough are mainly determined by the variations of equatorial central Pacific SST.Consistently,the zonal shift of the monsoon trough under global warming is also determined by the equatorial central Pacific SST change in the future projections.The original CMIP6 MME projects the northwestern tropical Pacific monsoon trough would eastward extend about 1 longitude during 2011-2098.However,based on the SST change projections after correction,the northwestern tropical Pacific monsoon trough is expected to westward retreat about 7 longitudes in future.