Ocean Evaporation on a diurnal scale

Significance 

The ocean mixed layer acts as a vital interface for the exchange of mass, momentum, heat, and freshwater between the atmosphere and the ocean. As our climate continues to change, understanding the dynamics, turbulence, and mixing processes within the mixed layer becomes increasingly crucial. Diurnal variations in surface fluxes play a significant role in shaping sea surface temperature, sea surface salinity, and the ocean’s total heat content. While previous research has focused on the response of the upper ocean to diurnal surface thermal and momentum forcing, the impact of evaporative fluxes on the variations in sea surface salinity and turbulent mixing in the mixed layer remains largely unexplored. In a new study published in the peer-reviewed Journal Frontiers in Marine Science, Devang Falor, Dr. Bishakhdatta Gayen and Prof. Debasis Sengupta from the Indian Institute of Science and University of Melbourne together with Prof. Gregory N. Ivey from the University of Western Australia investigated these factors and provided insights into their spatio-temporal characteristics and implications for climate modeling. The researchers conducted extensive large-eddy simulations to model the oceanic phenomena. These simulations incorporated temperature and salinity as active scalars and accounted for diurnal variations in forcing within the Bay of Bengal. A cuboidal domain with periodic boundary conditions in the horizontal directions and a flat air-sea interface was utilized. The boundary conditions were specified based on air-sea flux observations from the Bay of Bengal, including momentum, heat, and evaporative haline fluxes. The thin laminar diffusive layer at the top of the domain was accurately represented using a stretched grid resolution.

The research team discovered that diurnal variations in surface fluxes, such as heat, momentum, and evaporative haline fluxes, had a significant impact on the dynamics of the mixed layer. During the night, the negative net heat flux induced convective deepening of the mixed layer. In contrast, daytime heating led to increased thermal stratification and a shallower mixed layer. The simulations revealed the presence of a salinity inversion layer and salt-fingering instability during the day, particularly under weak breezes. Evaporation played a crucial role in regulating sea surface salinity and enhancing mixing in the surface layer on diurnal timescales. To comprehend turbulent mixing accurately, the authors stressed the importance of considering evaporation and its effect on variations in sea surface salinity. The strength of mixing varied both horizontally and vertically, with the highest turbulent kinetic energy values observed near the surface. Moreover, the simulations demonstrated that the diurnal variation in turbulent kinetic energy was more pronounced in the upper portion of the mixed layer, underscoring the necessity of incorporating this variation in models.

The authors’ findings have important implications for climate modeling and predicting the response of the upper ocean to changing environmental conditions. Incorporating diurnal variations, along with considering the role of evaporative fluxes and sea surface salinity changes, can improve the accuracy of models in capturing the complex interactions between surface fluxes, turbulence, and mixing in the mixed layer. By accounting for these factors, researchers can develop more robust parameterizations that accurately reflect the diurnal variability and its effects on the dynamics of the upper ocean.

The new study enhances our understanding of the influence of diurnal surface fluxes, evaporation, and sea surface salinity changes on turbulence and mixing processes within the ocean mixed layer. As our climate continues to evolve, studies like this contribute to a better understanding of the intricate interplay between the atmosphere and the ocean, ultimately aiding in predicting and adapting to future climate change. Moreover, it allows scientists to analyze how these variables vary across different regions and timescales, leading to a more comprehensive understanding of the global ocean dynamics. Furthermore, the new study sheds light on the role of evaporative fluxes and sea surface salinity variations in modulating turbulent mixing within the mixed layer. By recognizing the significance of these factors, it will be possible to refine our understanding of the complex mechanisms driving mixing processes, which, in turn, affect the distribution of temperature and salinity within the mixed layer. This knowledge is crucial for comprehending the overall heat and freshwater budget of the ocean and its response to climate variability.

In conclusion, as our climate continues to change, it is crucial to comprehend the dynamics of the ocean-atmosphere system. The study’s findings help us understand the impact of diurnal fluxes, evaporation, and sea surface salinity on the mixed layer, which plays a critical role in climate regulation. This knowledge contributes to improved climate change adaptation strategies by providing a deeper understanding of the processes governing heat and freshwater exchange between the atmosphere and the ocean.

Reference

Devang Falor, Bishakhdatta Gayen, Debasis Sengupta, Gregory N. Ivey. Evaporation induced convection enhances mixing in the upper ocean. Frontiers in Marine Science, Volume 10, May 2023, 1176226.

Go To Frontiers in Marine Science

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