Study Of The Air-Sea Interaction On Interannual And Intraseasonal Timescales Over The Northwest Pacific

Climate variability has received much attention by atmospheric and oceanic scientific research as a global environmental issue. Ocean and atmosphere, being two important components of climate system, interact on different spatial and temporal scales. The study of air-sea interaction, which helps to understand the process of climate variability, provides theoretical basis of climate simulation and prediction. Based on the heat fluxes and related meteorological variables datasets from Objectively Analyzed Air-sea Fluxes (OAFlux) Project of Woods Hole Oceanographic Institution and the reanalysis datasets from National Centers for Enviromental Prediction (NCEP), the air-sea interaction on interannual and intraseasonal timescales over the northwest Pacific, including interannual variability of air-sea turbulent heat fluxes over the northwest Pacific, characteristics of intraseasonal oscillation intensity of air-sea turbulent heat fluxes over the northwest Pacific, local air-sea interaction over the tropical western Pacific warm pool on the interannual timescale, as well as intraseasonal variability of air-sea interaction during the South China Sea summer monsoon onset is studied by means of empirical orthogonal functions (EOF) analysis, perturbation method, linear regression and correlation analysis. The main conclusion are as follows.1. Interannual variability of air-sea turbulent heat fluxes over the northwest Pacific is the most prominent in winter among four seasons. East China Sea and its extension to mid-east Pacific are the key regions of interannual variability of air-sea turbulent heat fluxes over the northwest Pacific. The anomalous wind speed has the greatest influence on the interannual variability of latent heat fluxes in winter in the subtropical Pacific and Philippine Sea, while the anomalous specific humidity difference has the greatest impact in the high-latitude and low-latitude oceans, especially in the equatorial mid-Pacific. The interannual variability of sensible heat fluxes in winter depends primarily on the air-sea temperature difference anomalies in the whole northwest Pacific. Under the influence of large-scale atmospheric circulation, negative (positive) air-sea specific humidity difference and temperature difference anomalies tend to occur to the east (west) of an anomalous low, therefore negative (positive) latent heat fluxes and sensible heat fluxes anomalies are found to the east (west) of an anomalous low. 2. The distribution of low-frequency oscillation intensity of latent heat flux (LHF) over the northwest Pacific is mainly affected by that of low-frequency oscillation intensity of anomalous air-sea humidity gradient (Δq′) as well as mean air-sea humidity gradient (Δq), while the distribution of low-frequency oscillation intensity of sensible heat flux (SHF) is mainly affected by that of low-frequency oscillation intensity of anomalous air-sea temperature gradient (ΔT′). The low-frequency oscillation of turbulent heat fluxes over the northwest Pacific is the strongest in winter and the weakest in summer. And the seasonal transition of low-frequency oscillation intensity of LHF is jointly influenced by those of low-frequency oscillation intensity ofΔq′, low-frequency oscillation intensity of anomalous wind speed (U′),Δq and mean wind speed (U ), while the seasonal transition of low-frequency oscillation intensity of SHF is mainly influenced by those of low-frequency oscillation intensity ofΔT′and U . Over the tropical west Pacific and sea areas north of 20°N, the low-frequency oscillation of LHF (SHF) is mainly influenced by atmospheric variables qa′(Ta′) and U′, indicating an oceanic response to overlying atmospheric forcing. In contrast, over the tropical eastern and central Pacific south of 20°N, qs′(Ts′) also greatly influences the low-frequency oscillation of LHF (SHF).3. Oceanic forcing is dominant in March but atmospheric forcing is dominant in June. While the interannual variability of sea surface temperature anomaly (SSTA) is larger than that of anomalous SST tendency in the case that oceanic forcing is dominant, the opposite is true when atmospheric forcing is dominant. The magnitude of the oceanic forcing of atmosphere tends to decrease in March for the occurrence of ENSO, however ENSO has little influence on the atmospheric feedback to ocean in June. The local air-sea interaction is substantially the same before and after removing the effect of Indian Oceanic Dipole (IOD). The reduction of shortwave radiation fluxes into the western pacific warm pool, due to the enhanced convection overlying the western pacific warm pool in March associated with ENSO, leads to the declination of SST tendency that will weaken the oceanic forcing of atmosphere.4. An index of SCS summer monsoon onset (IVIMT) is defined according to the characteristics analysis of VIMT before and after the SCS summer monsoon onset, then the onset dates of the SCS summer monsoon from 1951-2000 are ascertained with the index IVIMT. By analysis it is found that the onset dates of the SCS summer monsoon can be reasonably defined with the index IVIMT. Biweekly variability of air-sea interaction during the South China Sea summer monsoon onset is analyzed according to the above onset dates. It shows that atmosphere may play a role on sea surface temperature by wind-evaporation and cloud-radiation while the ocean impacts on the overlying atmospheric convection by moisture convergence. The response time between atmosphere and ocean is four days. Therefore suppressed convection leads th positive sea surface temperatue by four days while positive sea surface temperature leads the enhanced convections by four days.