Wind and wave variability in re-analysis models and satellite measurements in the North Atlantic
University of Southampton, (May 2010)Abstract
Winds and ocean surface gravity waves play a major role in many engineering and environmental issues both in the open ocean and in coastal zones. It is therefore essential to improve our knowledge on the spatial and temporal variability of the winds and the wave climate. This study aims at investigating this variability in the North Atlantic (66oN-10oS, 100oW-10oE) for the period 1992-2009, from ECMWF re-analysis models, ERA-40, the recently released ERA-Interim and new long-term calibrated gridded satellite altimeter datasets developed at NOCS. ERA-Interim gives larger maximum values of wind speed compared to ERA-40 for the time period September 1992 – August 2002, but shows similar results for the four first modes of the Empirical Orthogonal Functions (EOFs). Maximum significant wave height (swh) is also increased in ERA-Interim but still underestimated in comparison to TOPEX data (January 1993 – August 2002). The first EOF from ERA-Interim explains 30\% of the variance whereas the first EOF from the TOPEX data explains 22\% of the variability. The principal components are correlated against the climatic indices of the North Atlantic Oscillation (NAO) and the East Atlantic Pattern (EAP). The satellite data of ERS-2 + Envisat dataset (May 1995 – December 2008) gives the strongest correlations and showed the first EOF is related to the EAP and the second EOF to the NAO. The NAO has much less of an influence on the first EOF in comparison to the shorter TOPEX data, suggesting an increase in the EAP's influence on the dominant mode of variability or a decrease in the NAO's influence in recent years. Large differences are observed between the two models and the satellite data for wave period. The spatial patterns are different between the two models, with the first EOF for ERA-40 explaining 47\% of the variance and the first EOF for ERA-Interim accounting for only 27\% of the variance. This is reduced to 14\% with the TOPEX data and gives a different spatial pattern. The TOPEX data also showed the first EOF to be explained by the EAP and the second EOF by the NAO. The TOPEX + Jason-1 4 dataset (May 1995 – December 2008) shows a decrease of the NAO's influence on the first EOF in recent times. The wind speed in the North Atlantic in observed to show teleconnections from the tropical Pacific by correlating the anomalies against the Multivariate ENSO Index anomalies, as an area of positive correlation with r = 0.3, which is statistically significant at the 95\% level, in the western tropical North Atlantic with a lag of 4-6 months occurs. These teleconnections are also observed in swh and wave period.