Impact of the ocean mixed layer diurnal variations on the intraseasonal variability of Sea Surface Temperatures in the Atlantic Ocean

Abstract : This study investigates the non linear processes by which the ocean diurnal variations can affect the intraseasonal sea surface temperature (SST) variability in the Atlantic Ocean. The CNRMOM1D 1-dimensional ocean model is forced with ERA40 (ECMWF Re-Analysis) surface fluxes over the 1959–2001 period with a 1-hour frequency in solar heat flux in a first simulation and with a daily forcing frequency in a second simulation. This model has a vertical resolution of 1m near the surface. The comparison between both experiments shows that the daily mean surface temperature is modified by about 0.3°C to 0.5°C if the ocean diurnal variations are represented and this correction can persist for 15 to 40 days in the midlatitudes and more than 60 days in the Tropics. The so-called ”rectification” mechanism, by which the ocean diurnal warming enhances the intraseasonal SST variability by 20% to 40%, is found to be robust in the Tropics. In contrast, in the mid-latitudes, diurnal variations in wind stress and non solar heat flux are shown to affect the daily mean SST. For example, an intense wind stress or non solar heat flux toward the atmosphere during the first half of the day followed by weak fluxes during the second half result in a shallow mixed layer. The following day, this preconditioning results in heat being trapped near the surface and the daily mean surface temperature is higher than if these diurnal variations in surface forcings were not resolved.