Surface wave effects on water temperature in the Baltic Sea: simulations with the coupled NEMO-WAM model

Abstract

Coupled circulation (NEMO) and wave model (WAM) system was used to study the effects of surface ocean waves on water temperature distribution and heat exchange at regional scale (the Baltic Sea). Four scenarios-including Stokes-Coriolis force, sea-state dependent energy flux (additional turbulent kinetic energy due to breaking waves), seastate dependent momentum flux and the combination these forcings-were simulated to test the impact of different terms on simulated temperature distribution. The scenario simulations were compared to a control simulation, which included a constant wave-breaking coefficient, but otherwise was without any wave effects. The results indicate a pronounced effect of waves on surface temperature, on the distribution of vertical temperature and on upwelling's. Overall, when all three wave effects were accounted for, did the estimates of temperature improve compared to control simulation. During the summer, the wave-induced water temperature changes were up to 1°C. In northern parts of the Baltic Sea, a warming of the surface layer occurs in the wave included simulations in summer months. This in turn reduces the cold bias between simulated and measured data, e.g. the control simulation was too cold compared to measurements. The warming is related to seastate dependent energy flux. This implies that a spatiotemporally varying wave-breaking coefficient is necessary, because it depends on actual sea state. Wave-induced cooling is mostly observed in near-coastal areas and is the result of intensified upwelling in the scenario, when Stokes-Coriolis forcing is accounted for. Accounting for sea-state dependent momentum flux results in modified heat exchange at the water-air boundary which consequently leads to warming of surface water compared to control simulation.

Surface Stokes drift in the Baltic Sea based on modelled wave spectra

“…Locations of the buoys shown in Fig. 1 percentiles were of the same magnitude reported by Alari et al (2016) in moderate storms. The maximum speed of the surface Stokes drift had large spatial variability (not shown).…”

“…Previously, Alari et al (2016) have evaluated the importance of surface wave effects in the sea surface layer and identified that even in moderate storms, the surface Stokes drift can reach up to 0.35 ms −1 . Here, we calculated the surface Stokes drift for the entire Baltic Sea with the wave model WAM and gave mean values and percentiles of the Stokes drift speed from a 10-year model simulation (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015).…”

OpenDrift v1.0: a generic framework for trajectory modeling

“…Surface Stokes drift is normally obtained from a wave model (or by any Reader), and its decline with depth is calculated as described in Breivik et al (2016).…”

“…All objects are assumed to be small enough that direct wave scattering forces are insignificant. Furthermore, the Stokes drift (Kenyon, 1969;Breivik et al, 2014Breivik et al, , 2016) is inherently part of the leeway obtained from observations. As wind-generated waves have a mean direction closely aligned with the local wind direction it is neither practical nor desirable to disentangle the Stokes drift from the wind drag for leeway simulations.…”

The 14th international workshop on wave hindcasting and forecasting and the 5th coastal hazards symposium

“…The additional terms accounting for wave-current interaction that are considered in this study are the Stokes-Coriolis force (Breivik et al 2014) the sea-state-dependent energy and wave-modulated momentum fluxes. The same regional model setup with forcing from a WAM wave model has previously been described by Alari et al (2016) where the focus was the influence of wave effects on surface water temperature. The individual and collective role of these processes is quantified and the results are compared with a control run without wave effects as well as against current and water-level measurements from coastal stations.…”