Impact of SLA assimilation in the Sicily Channel Regional Model: model skills and mesoscale features

Abstract

Abstract. The impact of the assimilation of MyOcean sea level anomalies along-track data on the analyses of the Sicily Channel Regional Model was studied. The numerical model has a resolution of 1/32 • degrees and is capable to reproduce mesoscale and sub-mesoscale features. The impact of the SLA assimilation is studied by comparing a simulation (SIM, which does not assimilate data) with an analysis (AN) assimilating SLA along-track multi-mission data produced in the framework of MyOcean project. The quality of the analysis was evaluated by computing RMSE of the misfits between analysis background and observations (sea level) before assimilation. A qualitative evaluation of the ability of the analyses to reproduce mesoscale structures is accomplished by comparing model results with ocean colour and SST satellite data, able to detect such features on the ocean surface. CTD profiles allowed to evaluate the impact of the SLA assimilation along the water column. We found a significant improvement for AN solution in terms of SLA RMSE with respect to SIM (the averaged RMSE of AN SLA misfits over 2 years is about 0.5 cm smaller than SIM). Comparison with CTD data shows a questionable improvement produced by the assimilation process in terms of vertical features: AN is better in temperature while for salinity it gets worse than SIM at the surface. This suggests that a better a-priori description of the vertical error covariances would be desirable. The qualitative comparison of simulation and analyses with synoptic satellite independent data proves that SLA assimilation allows to correctly reproduce some dynamical features (above all the circulation in the Ionian portion of the domain) and mesoscale structures otherwise misplaced or neglected by SIM. Such mesoscale changes also infer that the eddy momentum fluxes (i.e. Reynolds stresses) show major changes in the Ionian area. Changes in Reynolds stresses reflect a different pumping of eastward momentum from the eddy to the mean flow, in turn influencing transports through the channel.

Hydrodynamic modelling of coastal seas: the role of tidal dynamics in the Messina Strait, Western Mediterranean Sea

“…Surface boundary conditions are provided by ECMWF meteorological forecasting system. TSCRM assimilates AVISO data along track Sea Level Anomalies on a daily basis through a 3D Variational procedure (see OCEANVAR in Dobricic et al, 2008), with the same implementation described in Olita et al (2012) for the previous version of the TSCRM operational system (SCRM, Gabersek et al, 2007). The system predicts, on hourly frequency, water currents, temperature and salinity at different water depths and for a 5 days time lag.…”

Impact of currents on surface flux computations and their feedback on dynamics at regional scales

“…Considering that the model boundaries are provided by an assimilative model we think that for such a domain the information contained in the boundaries would propagate to the nested model without a substantial loss of information. On the contrary for larger off-line nested domains it was shown in literature (Vandenbulcke et al, 2006;Olita et al, 2012) that assimilation would be needed as information coming from boundaries quickly dissipates. Further, and maybe more important, in the present work we aim to observe changes generated by different parametrizations of surface physics: for this reason we should not hide this signal with any statistical correction.…”

“…In order to investigate the impact of the fluxes on the simulated dynamics, we separated the stable and the fluctuating part of the velocity field as already described, for example, in Olita et al (2013). The time-averaged term u = u + u represents the stable part of the flow, while u is its fluctuating part.…”

“…This is a valid generalization for large scales, while an insight of what happens at midlatitude and at regional and coastal scales was not provided yet. To address this issue we focused our attention in the seas around Sardinia (Western Mediterranean sea), which is a highly variable and dynamic area interested by several (sub-)mesoscale structures of different origin (Fuda et al, 2000;Puillat et al, 2002;Ribotti et al, 2004;Testor et al, 2005;Olita et al, 2013) and also affected by strong wind events characterized both by seasonality and high-frequency peaks.…”

“…Olita et al, 2014). In a recent paper (Olita et al, 2013) we suggested, through the analysis of the outputs of a 3-D assimilative model, that the upwelling occurring along the SW Sardinian coast was pre-conditioned by the presence of a quasipermanent southward current (Western Sardinian Current -WSC) whose origin was in part due to the approaching of anticyclonic eddies to the western Sardinia shelf. This was also supported by the findings of Pinardi et al (2013) where the same current (they called Southerly Sardinia Current -SSC) is described as permanent at low-frequency scales (decadal) bordering a northern branch of the Atlantic water flow in the Western Mediterranean.…”