Trend Variability In Air-Sea Fluxes And Their Relationship With SST

Air-sea surface heat fluxes are of great interest, because they play an integral role in understanding and modeling the energy budget and the fresh water cycle between the atmosphere and the ocean. Recently, most studies of air-sea surface heat fluxes focus on their characteristics and roles in the air-sea interaction processes on the synoptic, intraseasonal, seasonal and interannual time scales; in contrast, there has been the limited number of studies on the long-term (or decadal-scale) trends of air-sea surface heat fluxes. Here, using various datasets and analysis methods, we explore the trends in the air-sea surface heat fluxes and their relationship with the SST and the large-scale atmosphere circulation. Also, we introduce a new method called "trend-SVD" designed for systematically extracting coupled trend modes, albeit small, by performing an eigenanalysis of the inverse-rank covariance matrix between two fields. The main conclusions are presented as follows:The surface latent heat flux (LHF) has a large-scale positive trend pattern during1977-2006over the tropical and subtropical Pacific and it is closely linked to both the sea surface temperature (SST) warming and the surface wind speed (SWS) increasing. The SST warming is the primary direct/local cause of the LHF trend, while the SWS strengthening is an important indirect/nonlocal factor of the surface LHF trend, due to its contribution to the observed SST trend pattern. Also, the present work suggests that the coherent upward trends in surface LHF, surface wind speed, and SST should be in essence closely associated with the global warming forcing. The present results, together with those from some recent climate model simulations, suggest that the global warming forcing may have caused an intensification of SWS in the tropical Pacific Ocean by inducing the so-called La Nina like SST trend pattern due to the ocean dynamics. Meanwhile, the strengthening in the tropical Pacific Ocean surface trade winds may also feedback to enhance the La Nina like SST trend pattern under the positive wind-upwelling dynamic feedback mechanism.Under the global warming forcing, the large negative net heat flux (NHF) trends occurred over the western boundary currents (WBCs), due to the local SST warming, indicating the ocean forcing upon overlying atmosphere. In contrast, the pronounced positive NHF trends existed over in the central basins of Northern Subtropical Oceans (CNSOs), ascribed to the near-surface air specific humidity (qa) increasing, indicative of an oceanic response to the overlying atmospheric forcing. A new method called "trend-SVD" was introduced for systematically extracting coupled trend modes, albeit small, by performing an eigenanalysis of the inverse-rank covariance matrix between two fields. Applications to simple time series models and annual mean LHF and SST data for1958-2006show that trend-SVD can capture different coherent trends into different leading modes. The first trend-SVD mode between the global LHF and SST anomalies generally represents a large-scale increasing LHF trend induced by a warming SST trend; while the second trend-SVD mode is mainly associated with the Pacific Decadal Oscillation (PDO). Thus, trend-SVD analysis can cast the (global) long-term and (Pacific) decadal trends into the leading two modes, respectively.The spread is considerable with the tropical SST climatology among models in CMIP3, with the magnitude equal to or even beyond the natural variability of SST. EOF analysis of the inter-model variability of the tropical SST climatology suggests that some models have much higher SST, while some have much lower SST. Also, we find that the models with the less (more) total clound fraction allow the more (less) shortwave flux into the ocean and thus have the higher (lower) SST; on the other hand, the higher (lower) SST would cause the more (less) upward longwave flux and LHF which could blance the shortwave flux.