Observed air-sea interaction in the tropical and North Atlantic

The dominant air-sea interactions that are at play in the Atlantic Ocean are revisited, using a lagged rotated Maximum Covariance Analysis (MCA) of monthly anomaly data taken from the 1958-2002 NCEP reanalysis. The method is first used to better separate the relative influence of tropical and extratropical sea surface temperature (SST) anomalies on the North Atlantic Oscillation in early winter, confirming the predominant role of the horseshoe SST anomaly pattern. The main air-sea feedbacks in the tropical Atlantic are then discussed. The dominant mode of variability is the ENSO-like zonal equatorial SST mode, which is a strongly coupled ocean-atmosphere mode sustained by a positive feedback between wind and SST. The turbulent heat flux feedback is negative, except west of 25�W where it is positive, but countered by a negative radiative feedback associated with the meridional displacement of the ITCZ. The second climate mode involves a SST anomaly in the North Tropical Atlantic which is primarily generated by the surface heat flux and, in boreal winter, wind changes off the coast of Africa. After it has been generated, the SST anomaly is sustained in the deep tropics by the positive wind-evaporation-SST feedback linked to the wind response to the SST. However, north of 10�N where the SST anomaly is largest, the wind response is weak and the heat flux feedback is negative, thus damping the SST anomaly. As the in-phase Maximum Covariance Patterns primarily reflect the atmospheric forcing of the SST, simultaneous correlations cannot be used to describe the atmospheric response to the SST anomaly, except in the deep tropics.