Spatial And Temporal Variation Of Ocean Mixed Layer Depth Based On FIO-ESM And Its Interaction With Atmospheric Boundary Layer

The global ocean mixed layer depth (MLD) is calculated using the First Institute of Oceanography-Earth System Model (FIO-ESM). Comparing with Argo observational data, the seasonal variation of the global MLD and the performance of the FIO-ESM simulation is shown in the present study. Next, the main attention has been paid to the spatial and temporal variation of the MLD in the best simulated basin-the South Atlantic Ocean. Additionally, the factors affecting MLD and the interaction between MLD and the atmospheric boundary layer in the South Atlantic Ocean are analyzed. The main conclusions are shown as follows:1) The global ocean MLD has a strong seasonal variation with deep MLD in winter and shallow MLD in summer,while the spring and fall seasons act as transitional periods. The typical monsoon climate makes the North Indian Ocean unique. The MLD is deep in both summer and winter, which exhibits a remarkable semi-annual feature. The FIO-ESM simulation well captures the seasonal variation in MLD in most areas, which makes a better performance during summer and fall than during winter and spring. The simulated MLD in the Southern Hemisphere is closer to the observation than that in the Northern Hemisphere. In general, the simulated MLD over the South Atlantic Ocean matches the observation best among the six areas. This is probably due to the fact that underestimation of the MLD over the southern hemisphere oceans in other climate models has been modified after the ocean surface waves are introduced in FIO-ESM.2) The MLD in the South Atlantic Ocean has obvious interannual variability.Through the empirical orthogonal function(EOF) analysis, it is found that the strongest interannual signal basins of the MLD are located in the mid-west of the South Atlantic Ocean(east of the La Plata River), southwest of the South Atlantic Ocean(north of the Drake Strait) and south of the Cape of Good Hope in Africa. The first two modes of EOF reflect the effects of wind stress, surface heat flux and non-breaking surface wave on MLD interannual variability. The third mode generally reflects the current induced mixing in the upper layer.3) MLD interacts with the atmospheric boundary layer. Wind stress and surface heat flux are crucial influencing factors for MLD. The wind stress has a greater effect on the warmer and shallower MLD, while the heat flux exchange in the upper ocean should be the key for the formation of the mixing layer over the equator and the 30°S latitude band controlled by subtropical high. The difference between the MLD from FIO-ESM and bulk model reflects the effect of non-breaking surface wave on MLD.There was a positive correlation between wind stress,non-breaking surface wave and MLD, the correlation was strong in spring and summer, while weak autumn and winter. There was a negative correlation between surface heat flux and MLD in most areas, the positive correlation in subtropical westerly and a few individual oceanic areas is mainly due to the role of surface wave. MLD has hysteresis effect on wind stress,sea surface heat flux and non-breaking surface wave. The deeper MLD has stronger inertia and more obvious lag.4) The atmospheric boundary layer height (ABLH) responds to the variation of the MLD. There was mainly positive correlation between MLD and ABLH,especially in spring and summer. The MLD can also influence the interannual variation and the longer time scale periodic oscillation of ABLH to a certain extent.