Decadal Variability And Trends Of Oceanic Barrier Layers In Tropical Pacific And Assessment Of CMIP5 Models

Decadal variability and trends of the isothermal layer depth(ILD),mixed layer depth(MLD),and barrier layer thickness(BLT)were analyzed for the tropical Pacific during 1979-2015 based on the newly developed monthly temperature and salinity data derived from the Met Office Hadley Center Observations dataset EN4.2.0.The decadal variability of ILD,MLD and BLT shows a close connection with the Pacific Decadal Oscillation(PDO).During the PDO positive phase,the eastward shift of precipitation and weakened trade winds result in thinner BLT in the western Pacific and thicker BLT in the central and eastern Pacific.The situation is reversed during the PDO negative phase.The differences in BLT can be up to 9-15 m.The spatial distributions of decadal trends of ILD and MLD are complex,but a thickening of BLT in the western tropical Pacific is clearly present.The raw trends of ILD,MLD and BLT averaged in the tropical Pacific(30°N-30°S,120°E-75°W)are 1.62 m,1.20 m and 0.51 m per decade,respectively.PDO can only explain about 25%of total increasing trends of ILD,MLD and BLT.Global warming and/or variability at longer time scales are responsible for the remaining increasing trends.The BLT change is related to the warming and freshening of the western Pacific Warm Pool in recent decades.The ocean-atmosphere interaction is discussed about trade wind,precipitation/evaporation,ocean circulation,sea water temperature and salinity.The evaluation of 14 coupled atmosphere-ocean general circulation models(AOGCMs)of the fifth Coupled Model Intercomparison Project(CMIP5)about their performance on simulating historical ILD,MLD and BLT from 1979 to 2005 is conducted.The interannual to decadal variability of BLT,the responses of BLT to ENSO and its trend as well as the sources of simulation deviations are analyzed to provide some references for future model simulation.The majority of the 14 CMIP5 models can simulate the spatial distributions of the climatology of ILD,MLD and BLT well.The biases are mainly due to the simulation biases of vertical structures of temperature and salinity in the upper ocean.The EOF analysis indicates that the ability of modes simulating the temporal and spatial variations of ILD,MLD and BLT is limited.Most of the models can simulate the positive correlations between BLT and Nino3.4 index in the eastern and central Pacific but can not simulate the negative correlations between BLT and Nino3.4 index in the western Pacific.The increasing trend of BLT in the tropical Pacific from 1979 to 2005 observed in EN4.2.0 is 0.677 m per decade while the increasing BLT trend of Ishii observation is 1.061 m per decade.Among the 14 CMIP5 models,most of the models simulated a decreasing trned of BLT in the whole tropical Pacific,except GISS-E2-R,HadGEM2-CC,and MPI-ESM-MR.In general,there are still many deficiencies in CMIP5 climate models,especially in the ocean models,where the upper ocean is too diffusing.