Very high-resolution modelling of submesoscale turbulent patterns and processes in the Baltic Sea

Abstract

Abstract. In order to simulate submesoscale turbulent patterns and processes (STPPs) and to analyse their properties and dynamics, the Regional Ocean Modeling System (ROMS) was run for June 2016 in a subregion of the Baltic Sea. To create a realistic mesoscale environment, ROMS with 500 m horizontal resolution (referred to as R500) is one-way nested into an existing operational model, and STPPs with horizontal scales < 1 km are resolved with a second nest of 100 m resolution (R100). Both nests use 10 terrain-following layers in the vertical. The comparison of the R500 results with a satellite image shows fair agreement. While R500 is driven by realistic air–sea fluxes, the atmospheric forcing is turned off in R100 because it prevents the generation of STPPs and blurs submesoscale structures. Therefore, R100 provides deep insight into ageostrophic processes and associated quantities under quasi-adiabatic conditions that are approximately met in no-wind or light-wind situations. The validity of the results is furthermore limited to the selected region and the time of the year. STPPs evolve rapidly within a about a day. They are characterized by vertical speeds of 𝒪(10) m d−1 and relative vorticities and divergences reaching multiples of the Coriolis parameter. Typical elements of the secondary circulation of two-dimensional strain-induced frontogenesis are identified at an exemplary front in shallow water, and details of the ageostrophic flow field are revealed. The conditions for inertial and symmetric instability are evaluated for the whole domain, and the components of the tendency equation are computed in a subregion. While anticyclonic eddies are generated solely along coasts, cyclonic eddies are rolled-up streamers and found in the entire domain. A special feature of the cyclones is their ability to absorb internal waves and to sustain patches of continuous upwelling for several days, favouring plankton growth. The kinematic properties show good agreement with observations, while some observed details within a small cyclonic eddy are only partly reproduced, most likely due to a lack of horizontal resolution or nonhydrostatic effects.

High‐Resolution Simulations of Submesoscale Processes in the Baltic Sea: The Role of Storm Events

“…It also (approximately) separates the active surface mixing layer from the non-turbulent deeper region generated by the restratification process (see Figure 12). Note that this threshold is much smaller than the typical density contrast required to trace the top of the seasonal thermocline of the Baltic Sea (Lips et al, 2011;Onken et al, 2020).…”

“…Despite the ubiquity of SML fronts and submesoscale features in the Baltic Sea, and their wellknown implications for SML energetics and biogeochemistry in other marine systems, their properties and dynamics are still largely unexplored for the case of the Baltic Sea. Notable exceptions are recent studies focusing mainly on the Gulf of Finland (Väli et al, 2017;Vankevich et al, 2016) and the southern part of the Baltic Sea (Onken et al, 2020;Zhurbas et al, 2019).…”

Properties and evolution of a submesoscale cyclonic spiral

“…2). The first nest with 500-m horizontal resolution (R500) is nested into the operational HIROMB-BOOS model (Berg, 2012, for details see Onken et al (2020a)) and creates the mesoscale background for June 2016.…”

“…Submesoscale structures develop rapidly in the second nest with 100-m resolution (R100), amongst others several cyclonic spirals with diameters around 1 km (Onken et al, 2020a). One of these spirals will be investigated in detail in this article by means of a third nest with 33.3-m resolution, referred to as R33.…”

“…the fluxes of momentum, heat, and fresh water are set to zero. The reasons for this setup are described in detail in Onken et al (2020a) and in Section 5.1 below.…”