Upper ocean responses to solar heating and rain
ten Doeschate, Anneke M. M.
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This thesis presents observations made in the ocean surface boundary layer (OSBL), with the Air-Sea Interaction Profiler (ASIP), in conjunction with measurements of meteorological variables. These are used to establish the relationship between atmospheric forcing, and the upper ocean response in terms of density stratification and turbulent mixing. Two regimes in the near-surface layer of the ocean are investigated: the diurnal cycle of heating as a result of solar irradiance, and the formation of sea surface salinity anomalies due to rain on the ocean. The ASIP measurements are used to analyse the upper ocean's response to both phenomena, from the perspective of changes in the vertical distribution of the physical variables temperature (T), salinity (S) and turbulent dissipation rate ($\epsilon$). It is found that the evolution of a warm layer induced by daytime heating is strongly dependent on the balance between the turbulent mixing and buoyancy. Although these two forces collaborate during nighttime, under calm atmospheric conditions it is observed that a warm layer can persist over-night, which contributes to the gradual restratification of the OSBL in spring. \\ %It is found that the evolution of both the diurnal warm layer and fresh lenses is a function of the competing forces %of turbulence and buoyancy. Diurnal warming is a regularly occurring phenomena that needs to be accounted for in coupled models of the ocean-atmosphere system and weather forecasting. The simulation of warm layer properties by the commonly used bulk-flux algorithm COARE 3.0 is compared to the observations of diurnal warm-layers from three oceanographic campaigns during which the ASIP was deployed. In this comparison it is found that the COARE model predicts the phase and amplitude of the diurnal sea surface temperature signal well. However, the deepening rate of the warm layer in the afternoon and evening is underestimated, due to the under-representation of convective mixing. A sensitivity study to the effect of varying the parameterisation of the solar radiation absorption in COARE is performed. The simulated sea surface temperature varies by several tenths of a degree between the different formulations tested, and the predicted warm layer depth by several meters. Four periods of rainfall of varying intensity were encountered during a campaign in a mid-latitude region of the Northern Atlantic. Whether a stable layer of reduced salinity forms at the ocean surface as a result of rain, is observed to be highly dependent on the atmospheric forcing conditions associated with the rain event. Only one of the rain events observed resulted in the formation of a fresh water lens. Direct observation of $\epsilon$ shows that the shallow rain-induced stratification impacts the turbulence over the depth of the mixed layer. Within the fresh layer $\epsilon$ was somewhat enhanced, whilst in the remnant mixed layer below it it was rapidly reduced by two orders of magnitude. Relationships between rain rate and surface salinity anomaly based on numerical models and satellite validation studies are verified for the four rain events encountered. It is found that there is a more significant causal relation between the total rain amount and the salinity anomaly at the surface than between the peak rain rate and the salinity response. Suggestions from literature that rain enhances the turbulent mixing at the surface are thought to explain the observed enhancement in the variance of the vertical salinity gradient derived from the microstructure C/T-sensors.