Abstract
Vertical mixing is important in the ocean for maintaining its stratification, redistributing temperature and salinity, distributing nutrients and pollutants, and the energy cascade. It plays a key role in ocean energy transport, climate change, and marine ecosystems. Getting the mixing right in ocean circulation and climate models is critical in reproducing ocean and climate physics. Ocean models, like the Regional Ocean Modeling System (Rutgers ROMS 3.4), provide several options for determining vertical mixing through the vertical mixing parameterization schemes. To evaluate which of these methods best reproduces realistic vertical mixing by internal tides, simulations of baroclinic tides generated by a seamount were performed using seven different vertical mixing parameterizations: Mellor-Yamada 2.5 (MY), Large-McWilliams-Doney’s Kpp (LMD), Nakanishi-Niino’s modification of Mellor-Yamada (NN), and four versions of Generic Length Scale (GLS). The GLS versions in ROMS 3.4 severely overmixed the water column within a day and were not considered realistic. We suspect that a coding error has been introduced for it. We focused on the performance of the MY, LMD, and NN vertical mixing parameterizations. LMD was found to overmix the water column. The performance of MY and NN were nearly equivalent and both well reproduced the observed velocity and diffusivity fields. NN performed slightly better by having a lower rms for M2 and K1, less benthic mixing, more mid-water column mixing, less overmixing, and fewer extremely high diffusivities (> 1 m2 s−1).
Highlights
Vertical ocean mixing is an important physical process in the ocean
Lett. (2019) 6:15 this is estimated to result from mixing by internal tides, generated through interactions of the barotropic tide with topography
With the 6 and 4 h periods, the wavelengths are appreciably shorter than for the major tidal constituents. Internal tides at both the diurnal (K1 and O 1) and semidiurnal (M2 and S 2) frequencies were present in the upper 1000 m over the guyot as seen in the major axes of the tidal ellipses for the baroclinic anomalies, which is the difference of the velocity with the vertical mean velocity (Fig. 2)
Summary
Vertical ocean mixing is an important physical process in the ocean. It maintains the temperature and salinity stratification, drives the global overturning circulation (Munk and Wunsch 1998), redistributes nutrients and larvae for biological productivity and fisheries, distributes heat and salt, and influences climate dynamics. Vertical mixing is generally caused by turbulence, which Feynmann considered one of the greatest challenges of physics and one Around 20 processes are considered to be significant contributors to vertical mixing in the ocean (Garrett 2003); a few sources dominate. These forces are depth dependent, with the wind being the most important in the upper ocean. Interactions of geostrophic currents or eddies with topography can generate lee waves, which are estimated to contribute an additional 0.2 TW (Nikurashin and Ferrari 2011) Because of these topographic interactions with tides and/or currents, mixing has been found to occur in specific “hot” spots near topographic features (Garrett 2003). Mixing varies in time, e.g., tidal mixing follows the spring-neap and daily tidal cycles and wind mixing increases during storms
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