The Seasonal And Intra-seasonal Modulation Of The Diurnal Cycle Of Sea Surface Temperature In The Eastern Tropical Indian Ocean

As the interface of ocean and atmosphere, sea surface temperature (SST) plays a critical role in the process of air-sea interaction. As well as the seasonal variation, the diurnal cycle is the basic cycle of the air-sea system under the forcing of sun, which has not been recognized or revealed adequately. However, the complicated diurnal cycle of SST (DSST) has been revealed by many observations recently. Scale interaction between diurnal cycle and other time scale may play a great role in weather and climate system on seasanal and intra-seasonal variation. This is particularly important for exploring the predictability of intra-seasonal oscillation in climate models. Hence, investing the mechanism of DSST is crucial to the understanding of air-sea interaction.Based on the RAMA buoy data, the seasonal and intra-seasonal variation of DSST in two representative aera of eastern tropical Indian Ocean were analyzed in this paper. Using a one-dimensional ocean mixed layer model (PWP) and a coupled general circulation model (CGCM), the cause of DSST variation on seasonal and intra-seasonal time scale were diagnosed quantitatively. Some new results are found as follows:1. Recent in situ buoy observations revealed interesting seasonal features of the DSST in the tropical eastern Indian Ocean. Composite analysis shows areas away from the equator exhibit stronger seasonal variations of DSST, while a weaker seasonal variation appears near the equator. The most interesting characteristic is the distinctive contrast of seasonal variations of DSST between the Bay of Bengal (BOB) and the south of equator (particular at 12°S). While BOB DSST has the weakest range in the boreal summer, DSST at 12°S has the largest range in its austral summer. Furthermore, BOB DSST exhibits two peaks during the monsoon transitions (March-April and October), but DSST south of the equator shows a single peak in its annual cycle. The model results demonstrated that in BOB, both the surface shortwave radiation and wind stress contribute to the March maximum, whereas the wind stress alone dominates the October maximum. In contrast, the seasonal variation of DSST south of the equator is primarily caused by the annual cycle of the wind stress. The difference of monsoon precipitation strength mainly caused the different seasonal variation of sea surface shortwave radiation between 12°N and 12°S. And the distinct monsoon in the eastern Indian Ocean makes the annual cycle of wind stress on each side of equator special.2. Modulated by the Madden-Julian Oscillation (MJO), the DSST peaks on the convection suppressed phase and drops to its lowest on the convection active phase. What’s more, the DSST on the developing phase of MJO is much larger than that on the decaying phase. The cause of the difference of DSST during the evolution of MJO is diagnosed quantitatively through PWP model. It is demonstrated that the intro-seasonal variation of sea surface shortwave radiation (40%) and wind stress (38%) are the main reason for the striking difference of diurnal range of SST between suppressed phase and active phase of MJO. The intra-seasonal variation of latent heat flux (14%) is also important. Due to the scale interaction between MJO perturbation and background circulation, the asymmetry of zonal wind stress determines the asymmetry of diurnal range of SST between developing and decaying phase of MJO.3. The DSST simulated in CGCM is very weak, which is only 30% of obeservation in the middle of Bay of Bengal. The sensitive experiments indicate that the sea surface flux correlates very well with MERRA in BOB. The weak DSST in BOB may be caused by the poor vertical resolution of the upper ocean in POEM, which is a common defect in CGCM. While in the southern tropical Indian Ocean, the surface fluxes from CGCM have large difference compared with observation, which directly cause the weak DSST. The poor vertical resolution in the upper ocean in this area should be reasonable for the weak DSST too.