Mesoscale SST–wind stress coupling in the Peru–Chile current system: Which mechanisms drive its seasonal variability?
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Satellite observations and a high-resolution regional ocean–atmosphere coupled model are used to study the air/sea interactions at the oceanic mesoscale in the Peru–Chile upwelling current system. Coupling between mesoscale sea surface temperature (SST) and wind stress (WS) intensity is evidenced and characterized by correlations and regression coefficients. Both the model and the observations display similar spatial and seasonal variability of the coupling characteristics that are stronger off Peru than off Northern Chile, in relation with stronger wind mean speed and steadiness. The coupling is also more intense during winter than during summer in both regions. It is shown that WS intensity anomalies due to SST anomalies are mainly forced by mixing coefficient anomalies and partially compensated by wind shear anomalies. A momentum balance analysis shows that wind speed anomalies are created by stress shear anomalies. Near-surface pressure gradient anomalies have a negligible contribution because of the back-pressure effect related to the air temperature inversion. As mixing coefficients are mainly unchanged between summer and winter, the stronger coupling in winter is due to the enhanced large-scale wind shear that enables a more efficient action of the turbulent stress perturbations. This mechanism is robust as it does not depend on the choice of planetary boundary layer parameterization.
KeywordsOcean–atmosphere interactions Mesoscale SST–wind stress coupling Regional coupled modeling Eastern Boundary Upwelling System
This work is part of V. Oerder’s PhD thesis, sponsored by the Ministère de l’Enseignement Supérieur et de la Recherche. It is also part of the ANR project “PULSATION-11-MONU-010” and the LEFE/GMMC project “NEMPECH”. Simulations were performed on the supercomputer Curie from the GENCI at the CEA (projects 2011040542, 2012061047 and 2014102286). The authors want to thank Francoise Pinsard and Eric Maisonnave for their help in setting-up the coupled model NEMO-OASIS-WRF and Guillaume Samson, Hervé Giordani and Patrick Marchesiello for useful discussions. F. Lemarié acknowledges the support of the French LEFE/GMMC program through project SIMBAD. QSCAT WS data were provided by the CERSAT and are available online at ftp://ftp.ifremer.fr/ifremer/cersat/products/gridded/mwf-quikscat/data/. Microwave OI SST data are produced by Remote Sensing Systems and sponsored by National Oceanographic Partnership Program (NOPP), the NASA Earth Science Physical Oceanography Program, and the NASA MEaSUREs DISCOVER Project. Data are available at www.remss.com. Shortwave radiation from the ISCCP are available in the Objectively Analyzed air–sea Fluxes data and can be downloaded at http://oaflux.whoi.edu/. VOCALS-REx wind data are available online at ftp://precip.meas.ncsu.edu/pub/vocals/. Numerical data were obtained by model experiments described in Sect. 2.
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