Submesoscale structures related to upwelling events in the Gulf of Finland, Baltic Sea (numerical experiments) Germo Väli a, ⁎, Victor Zhurbas a,b , Urmas Lips a , Jaan Laanemets a a Marine Systems Institute at Tallinn University of Technology, Akadeemia tee 15A, 12618 Tallinn, Estonia b Shirshov Institute of Oceanology, 36 Nakhimovsky Prospect, 117851 Moscow, Russia abstract article info Article history: Received 8 April 2016 Received in revised form 6 June 2016 Accepted 20 June 2016 Available online xxxx The appearance of submesoscale structures in the Gulf of Finland was investigated using model simulations for series of coastal upwelling events in July–September 2006. We applied the Princeton Ocean Model. The horizon- tal step of the model grid was refined to 0.5, 0.25 and 0.125 nautical miles in the gulf and reached 4 times the res- olution in the rest of the Baltic Sea; there were 60 σ-levels in the vertical direction for all simulations. The contribution of salinity to the strength of baroclinic front of upwelling along the northern and southern coasts and thereby to the submesoscale dynamics of the gulf's surface layer was analyzed. Model results with refine- ment of the grid size to 0.125 nautical miles revealed different forms of submesoscale structures in the gulf's sur- face layer such as the high Rossby number (Ro) threads (elongated spots of Ro N 1 with typical width and length of 2–3 km and 10–50 km, respectively), cyclonic vortices with Ro N 1 core of 4–6 km diameter, and spiral cyclonic eddies (spirally wrapped high Rossby number threads) of 10–15 km diameter. The high potential vorticity threads presumably formed during the development phase, while the cyclonic vortices and spiral cyclonic eddies during the relaxation phase of upwelling. One of the simulated submesoscale cyclonic eddies, at the beginning with the Ro N 1 core extension as deep as 31–66 m was traced for the period of 33 days. The power spectral den- sity of temperature and velocity fluctuations in the surface layer pointed at some increase of spectral levels and shallowing of spectral slopes towards -2 on the shorter (submesoscale) wavelengths with the refinement of model grid. © 2016 Elsevier B.V. All rights reserved. Keywords: Upwelling Submesoscale structures Rossby number Baltic Sea Gulf of Finland 1. Introduction High-resolution satellite and field observations and model simula- tions showed that considerable spatial variability of water mass proper- ties exists in submesoscale (10–1 km) and processes at these scales significantly contribute to the horizontal and vertical transport in the ocean (e.g. Thomas et al., 2008; Bouffard et al., 2012; Schroeder et al., 2012; Gula et al., 2014; Martin et al., 2015; Shcherbina et al., 2015; Swart et al., 2015). Submesoscale flows can be defined based on their dynamics, as those where the bulk Rossby number, Ro b =U/fL and the bulk Richardson number,Ri b =N 2 H 2 /U 2 , are both of the order of unity (Thomas et al., 2008). Here U, H, and L are the characteristic speed, ver- tical length scale, and horizontal length scale of the velocity field, re- spectively, f is the Coriolis parameter, N 2 =b z is the square of the buoyancy frequency (Brunt-Väisälä frequency), b = - gρ/ρ 0 is the buoyancy, ρ is the density, g is the gravity acceleration, and ρ 0 = 1000 kg m -3 is the reference density. Submesoscale processes with horizontal length scale of the order of 1 km are particularly dominant in the upper ocean layer including the upper mixed layer and upper thermocline where they are frequently displayed in the form of elon- gated regions, typically related with filaments or outcropping isopycnals, within which the relative vertical vorticity ς =v x -u y being positive (cyclonic) equals or exceeds the planetary vorticity f. Moreover, submesoscale processes are characterized by a conspicuous asymmetry of the relative vertical vorticity and vertical velocity distributions with an enhancement of cyclonic (positive) vorticity and downwelling. Submesoscale processes are supposed to be vital for the transport of vorticity, buoyancy, momentum, matter and biogeochemical properties throughout the upper mixed layer and upper thermocline (e.g. Capet et al., 2008). The Gulf of Finland, an elongated basin (about 400 km long and 48–135 km width) lies in the north-eastern part of the Baltic Sea (Fig. 1). The maximum depth at cross-gulf sections decreases from 80 to 110 m at its entrance to 20–30 m in the eastern part. The freshwater runoff, mainly the River Neva (an average 77.6 km 3 yr -1 , Bergström et al., 2001) in the eastern part and saltier northern Baltic Proper water intrusion from west cause surface layer salinity decrease from 6 to 7 g kg -1 at the entrance to about 1 g kg -1 in the Neva estuary. The surface layer salinity decreases across the gulf towards the north and typically is between 4.5 and 5.5 g kg -1 in the central part of the gulf. The bottom layer salinity varies between 8 and 11 g kg -1 at the Journal of Marine Systems xxx (2016) xxx–xxx ⁎ Corresponding author. E-mail address: germo.vali@msi.ttu.ee (G. Väli). MARSYS-02842; No of Pages 12 http://dx.doi.org/10.1016/j.jmarsys.2016.06.010 0924-7963/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys Please cite this article as: Väli, G., et al., Submesoscale structures related to upwelling events in the Gulf of Finland, Baltic Sea (numerical experiments), J. Mar. Syst. (2016), http://dx.doi.org/10.1016/j.jmarsys.2016.06.010