Enhanced Turbulent Diapycnal Mixing in the Northern Sargasso Sea Inferred From a Finescale Parameterization

The Sargasso Sea, accommodating marine species and mode water, is highly productive relative to other oligotrophic subtropical regions with intense air-sea carbon and heat exchanges. Turbulent diapycnal mixing regulates these processes and is generally thought to be weakened in a more stratified ocean under anthropogenic forcing. However, by applying a finescale parameterization to long-term hydrographic profiles collected during the Bermuda Atlantic Time-Series Study (BATS), we show that its intensity has become stronger from 1994 to 2019 in the permanent thermocline. The enhanced turbulent diapycnal mixing is mainly attributed to the increase of wind power on internal waves along the Gulf Stream extension. Numerical simulations suggest that stronger internal waves excited along the Gulf Stream extension radiate downward and equatorward, causing enhanced turbulent diapycnal mixing in the northern Sargasso Sea including the BATS station. The findings have important implications for understanding responses of mode water and ecosystem in the Sargasso Sea to anthropogenic forcing. Turbulent mixing contributes to the vertical transport of heat, carbon and nutrients in the ocean, exerting significant effects on the ocean circulations and climate. It is thus important to know how turbulent mixing in the ocean will change under anthropogenic forcing. There is a prevailing thought that turbulent mixing should be weakened under anthropogenic forcing, as the enhanced stratification due to the faster warming in the upper than deeper ocean would hinder the processes generating turbulence. Here, using observation data sampled during the Bermuda Atlantic Time-Series Study (BATS), we find that the turbulent mixing in the permanent thermocline there has increased by 42% over the past three decades. This positive trend is due to stronger wind energy input on internal waves that are gravity waves propagating within the ocean interior rather than on its surface. As these waves radiate downward and equatorward, they transfer their energy into smaller-scale internal waves and eventually break, leading to enhanced turbulent mixing in the northern Sargasso Sea. Thermocline diapycnal mixing inferred from a finescale parameterization in the northern Sargasso Sea is enhanced from 1994 to 2019Enhanced thermocline diapycnal mixing in the northern Sargasso Sea is due to stronger internal waves excited by intensified winds