Characteristics And Mechanism Of The Diurnal Cycle Of Sea Surface Temperature In The Tropical Indian Ocean

The sea surface temperature (SST) is one of the most important factors of theair-sea interaction, while the variation of the diurnal cycle of SST plays a great impacton the ocean, atmosphere and climate progress. Using Research Moored Array forAfrican-Asian-Australian Monsoon Analysis and Prediction (RAMA) dataset, theseasonal and intra-seasonal variation of diurnal SST in the tropical Indian Ocean arestudied in this paper. The main conclusions are as follows:The diurnal SST varies like a quasi-sinusoida curve, which maximizes at6-7o’clock in the morning and minimizes at2-4o’clock in the afternoon. The amplitudeof diurnal SST varies in different months, which maximizes in Feb-Apr, andminimizes in Jun-Aug.During the period of monsoon transition, the wind speed falls to the lowest,leading to the weakest vertical mixing of the upper ocean and a lowest mixed layerdepth (MLD). And considering the maximum shortwave radiation due to the mostoften sunny days, the amplitudes of diurnal SST are larger than that in monsoonperiod. That is the amplitude of diurnal SST reaches to the first maximum in March,then falls to the minimum in July, and climb up to the second maximum (smaller thanthe first) in October on average. This variation is more obvious out of the equator (outof4°) than the equator area. This may be caused by the weak wind speed in theequator area all year along, which could make the vertical mixing of the upper oceanvery weak.The diurnal SST variation is asymmetric, and a negative correlation is foundbetween the inter-annual variation of skewness of diurnal SST and that of diurnalSST.Based on a mixed layer heat budget calculation in site [1.5°S,90°E], we can findthe difference of heat flux and entrainment is the main reason for the difference ofdiurnal variation of SST between February and July. And the shortwave radiation is the main heating factor, while the entrainment is the main cooling factor.The buoy on [15°N,90°E] is in the central Bay of Bengal, where the seasonalvariation of diurnal SST is significantly affected by seasonal monsoon conversionimpact. Before the summer monsoon onset, the weak wind speed and the verticalmixing of the ocean can lead to a thin mixed layer and a minimum latent heat fluxrelease. With the most often sunny days, the sea surface shortwave can get very large.As a result, the amplitude of diurnal SST gets to the largest one of the year. After theonset of summer monsoon, deepening mixed layer due to large wind speed anddecreasing sea surface shortwave radiation lead to the decrease of the amplitude ofdiurnal SST, and minimizes in July. During the period before winter monsoon onset,the amplitude of diurnal SST will reach to the second largest value.On the impact of Madden-Julian Oscillation (MJO), the sea surface temperature(SST) shows intra-seasonal variation notable. Based on the hourly data from IndianOcean buoy observation, composite analysis is used to analyze the intra-seasonalvariation of SST. The result show that both the daily mean SST and the amplitude ofdiurnal SST is significantly influenced by MJO. For daily mean SST, the positive(negative) SST anomalies are nearly90°out of phase with the suppressed (enhanced)phase, which is consistent with previous studies. For the diurnal variation of SST, theamplitude has an anti-phase relationship with atmospheric convective activity. Thatmeans the minimum amplitude (standard deviation:0.06°C) appears on the strongestconvective phase, while the maximum amplitude (standard deviation:0.23°C)appears on the weakest convective phase.Based on a mixed layer heat budget calculation in site [1.5°S,90°E], we can findthe intra-seasonal variation of surface heat flux (mainly shortwave radiation) andMLD are the primary causation for the difference of of diurnal variation of SST indifferent phase of MJO. Using a one-dimension ocean mixed layer model, forced bythe air-sea fluxes of buoy observations, we can reproduce the intra-seasonal variationof diurnal cycle of SST. And the results prove that the intra-seasonal variation of seasurface heat flux and mixed layer depth are the main ways through which the MJOaffects the amplitude of diurnal SST.