Diurnal Sea Surface Temperature Variabiilty and Air–Sea Interaction

Ocean surface fluxes observed from continuous measurements during field experiments show strong variability on temporal scales that range from the diurnal cycle to the life cycle of storms, and on spatial scales as small as that of an individual convective cloud. Beginning with the seminal Webster, Clayson, and Curry 1996 paper, the role of diurnal SST variability on air-sea feedbacks, particularly in the tropics, has been studied for over 20 years. In this presentation I will discuss what has been learned about the effects of this diurnal variability on the upper ocean and lower atmosphere across a variety of time and space scales. Studies have demonstrated that using a 24-hour mean SST or a pre-dawn SST changes prediction capability of such ocean-atmosphere coupled phenomena as convection and the Madden Julian Oscillation (MJO). Recent modeling studies clearly demonstrate the need to resolve diurnal SST variability even for interannual variability such as ENSO.

A focus of the presentation will be on the variability observed between diurnal SST and the MJO. While recent studies continue to show the importance of atmospheric moist dynamics and convection as a driving factor in the development and propagation of MJO events, it is also known that the upper ocean mixed layer responds in concert and may itself contribute to the amplification and decay of MJO related variability. Preconditioning of the environment prior to the active phase of the MJO has been noted, but the balance of theorized mechanisms to accomplish this process remains unresolved. Further, not all developing organized convection transitions into a propagating MJO signal. This work will provide analyses of observations from a variety of satellite, reanalyses, single-column coupled ocean-atmosphere models, and in situ observations (buoys, Argo) to investigate atmospheric variability during the suppressed phase of the MJO. A particular focus of these analyses will be to characterize the systematic relationship between diurnal SST/upper-ocean variability, low-level stability, and observed cloud regimes. Additional emphasis will be placed on scrutinizing the interactions between the atmosphere and ocean boundary layers via surface heat flux exchanges of heat, moisture, and momentum.