Six ALAMO floats are deployed within the tropical warm pool of the
eastern Indian Ocean, to study the thermal stratification in the diurnal
warm layer (DWL) during strong diurnal SST variation (DV SST) prior to
the onset of Madden-Julian Oscillations (MJO). Strong DV SST of
> 2 °C is measured by four floats before the passage of a
MJO event (i.e., during the suppressed phase), when the peak insolation
> 1000 W m-2 and the wind speed
< 3 m s-1. Even after the occurrence of
daytime peak SST, the temperature gradient in the DWL can still extend
to > 10 m until the midnight, which may be driven by the
turbulent mixing at the base of DWL. Interestingly, the foundation SST
(SSTfnd) at three floats increases rapidly from 26.4 °C
to > 27.6 °C over two days, coincident with the shoaling of
surface mixed layer depth (MLD) by more than 20 m. The strongly
stratified near surface layer may sustain higher SSTs and enhance
air-sea heat fluxes until the onset of stronger winds. The KPP mixing
scheme used in a 1-D model can simulate the observed DV SST magnitude
reliably, but fail to predict the rapid increase of
SSTfnd. The magnitude of DV SST is affected by the near
surface stratification, but the SSTfnd is modulated by
the evolution of stratification above the MLD. Future field measurements
in the upper ocean during diurnal warming are proposed to help improve
air-sea flux simulations and the forecast of MJOs.