Rapid foundation SST Warming Captured by Alamo Floats during the Suppressed Phase of the MJO

Centre of Southern Hemisphere Oceans Research (CSHOR) conducted a field experiment in the tropical warm pool around the end of November 2018, to explore the importance of strong diurnal SST variations to the onset of MJO events. Six Alamo floats with high vertical resolution ≤ 1 m in the upper 50-m ocean were deployed, and measured strong diurnal SST variation (DV SST) > 2 °C from Dec 2^nd to 4^th 2018, about 10 days before the active phase of one MJO event. Interestingly, the foundation SST (SST_fnd) at three floats increases rapidly from 26.4 °C to more than 27.6 °C after the presence of strong DV SST, much faster than that at the other float (9205), even the floats are < 50 km away. The thick diurnal warm layer (DWL) with strong temperature stratification acting as a barrier layer, may reduce the SST cooling due to the nighttime convective mixing and facilitate the restratification in the surface mixed layer. The restratified upper ocean may sustain the high SST thereby the SST_fnd warming to enhance the daily mean air-sea heat fluxes until the onset of MJOs. The absence of rapid SST_fnd warming at the float 9205 may be due to the salt-finger instability for eroding the restratification. The KPP mixing in the 1-D ROMS is used for simulating the SST variations. Increasing the vertical resolution to < 1 m in the upper 20 m improves the simulated magnitude of DV SST, but fails to predict the rapid SST_fnd warming, presumably due to inappropriate diffusivity for simulating the DWL structure. Because the predictions on the SST variations can largely affect the air-sea heat fluxes to the MJOs thereby the monsoon onset, future field measurements on the diffusivity during diurnal warming are required for improving the global model forecast.