Daily scale wintertime sea surface temperature and IPC-Navidad variability in the southern Bay of Biscay from 1981 to 2010

The combination of remotely sensed gappy Sea surface temperature (SST) images with the missing data filling DINEOF (data interpolating empirical orthogonal functions) technique, followed by a principal component analysis of the reconstructed data, has been used to identify the time evolution and the daily scale variability of the wintertime surface signal of the Iberian Poleward Current (IPC), or Navidad, during the 1981-2010 period. An exhaustive comparison with the existing bibliography, and the vertical temperature and salinity profiles related to its extremes over the Bay of Biscay area, show that the obtained time series accurately reflect the IPC-Navidad variability. Once a time series for the evolution of the SST signal of the current over the last decades is well established, this time series is used to propose a physical mechanism in relation to the variability of the IPC-Navidad, involving both atmospheric and oceanic variables. According to the proposed mechanism, an atmospheric circulation anomaly observed in both the 500 hPa and the surface levels generates atmospheric surface level pressure, wind-stress and heat-flux anomalies. In turn, those surface level atmospheric anomalies induce mutually coherent SST and sea level anomalies over the North Atlantic area, and locally, in the Bay of Biscay area. These anomalies, both locally over the Bay of Biscay area and over the North Atlantic, are in agreement with several mechanisms that have separately been related to the variability of the IPC-Navidad, i.e. the south-westerly winds, the joint effect of baroclinicity and relief (JEBAR) effect, the topographic beta effect and a weakened North Atlantic gyre.ERA-Interim data were obtained from the ECMWF data server (http://data.ecmwf.int/data). Pathfinder v5.2 data were provided by GHRSST and the US National Oceanographic Data Center. AMSR-E data are produced by Remote Sensing Systems and sponsored by the NASA Earth Science MEaSUREs DISCOVER Project and the AMSR-E Science Team. Data are available at www.remss.com. The altimeter products were produced by Ssalto/Duacs and distributed by Aviso, with support from Cnes (http://www.aviso.oceanobs.com/duacs/). ICOADS data for this study have been retrieved from the Research Data Archive (RDA) which is maintained by the Computational and Information Systems Laboratory (CISL) at the National Center for Atmospheric Research (NCAR). Word Ocean Database data were obtained from http://www.nodc.noaa.gov/OC5/WOD/pr_wod.html. DINEOF source code is freely available at http://modb.oce.ulg.ac.be/mediawiki/index.php/DINEOF. G. Esnaola is supported by a research grant (Interaccion Atmosfera-Oceano en el Golfo de Bizkaia) from Fundacion Centros Tecnologicos Inaki Goenaga. Jon Saenz would like to thank the financial support from project CGL2008-03321 (Spanish National R+D+I Programme). He also thanks funding provided by the University of the Basque Country (UFI 11/55, PPH12/01 and GIU 11/01). The Basque Government (Department of Agriculture, Fisheries and Food) has funded the project VARIACIONES. The authors want to express their gratitude to A. Caballero from AZTI-Tecnalia for her help with the Altimetry data and to B. Le Cann from UBO-CNRS-IRD-IFREMER for his useful comments on an early draft of this document. This is contribution number 649 of the Marine Research Division of AZTI-Tecnalia