Air-Sea Fluxes Variabilities, Trends, And Implications On Climate:Observation And Air-Sea Coupled Model Simulation

Air-sea surface heat fluxes play an integral role in atmospheric and oceanic circu-lations and global hydrological cycle. It is of great importance for understanding and numerical modeling dynamic air-sea processes to study the air-sea surface fluxes. This PhD thesis investigates the changes of air-sea fluxes including ocean surface evapora-tion, latent heat flux, net surface heat flux, etc. in global oceans (specially the Pacific ocean and Indian ocean) in observations, reanalysis data, and output from general cir-culation models (GCM). Potential causes of these changes and related models biases are also discussed.Using OAFlux data, analysis on the 1999-2000 trend reversal in global mean and local linear trend patterns of evaporation and related variables is performed. As global warming slowdown, the sea surface temperature induces trend changes of sea level pressure and near-surface wind speed with highly correlated decreasing trends. The weakened near-surface wind impacts oceanic evaporation in direct and indirect ways. These changes finally reduce the sea-air humidity contrast, and then result decreasing evaporation. Implication for global rainfall is discussed. With the decreasing of oceanic evaporation, land evapotranspiration has increased a lot. The relationship of evapora-tion and precipitation seems to be more local both for ocean and land. Weakening of surface atmospheric circulation over ocean is considered to be the cause of the imbal-ance of ocean surface water budget.The climatological mean state, seasonal variation and long-term upward trend of 1979-2005 latent heat flux (LHF) in historical runs of 14 coupled general circulation models from CMIP5 are evaluated against OAFlux data. Inter-model diversity of these models in simulating the annual mean climatological LHF is discussed. Results show that the models can capture the climatological LHF fairly well, but the amplitudes are generally overestimated. Model-simulated seasonal variations of LHF match well with observations with overestimated amplitudes. The possible origins of these biases are wind speed biases in the CMIP5 models. Inter-model diversity analysis shows that the overall stronger or weaker LHF over the tropical and subtropical Pacific region, and the meridional variability of LHF, are the two most notable diversities of the CMIP5 models. Regression analysis indicates that the inter-model diversity may come from the diversity of simulated SST and near-surface atmospheric specific humidity. Comparing the observed long-term upward trend, the trends of LHF and wind speed are largely underestimated, while trends of SST and air specific humidity are grossly overestimated, which may be the origins of the model biases in reproducing the trend of LHF.Changes of net ocean surface heat flux (Qnet) into oceans simulated by two ver-sions of FGOALS from CMIP5 project are evaluated using two observed ocean surface flux products. Results show that both the two models present basin-wide underestima-tion in net surface heat flux, possibly resulting from the positive latent heat flux biases. In Indian Ocean, both two models share an Indian Ocean Dipole -like bias in the net sur-face heat flux which have been traced to errors in the South Asian summer monsoon. Area-averaged annual time series analyses on surface heat budget imply that FGOALS-s2 bias lies more in radiative imbalance, illustrating the need to improve cloud simula-tion, while FGOALS-g2 bias presents ocean surface turbulence flux as the key process, requiring improvement in the oceanic processes simulations. Neither FGOALS-g2 nor FGOALS-s2 can well capture the trend of Qnet. All observed and simulated datasets imply surface latent heat flux as the primary contributing component, indicating the simulation biases of models may mainly come from the biases on simulating LH.