Interannual North Equatorial Countercurrent variability and its relation to tropical Atlantic climate modes

Abstract

A synthesis product of the surface geostrophic circulation is used to quantify the interannual variability of the wind-driven North Equatorial Countercurrent (NECC). The first mode of a complex empirical orthogonal function (CEOF) decomposition of zonal geostrophic velocity in the NECC region reveals pronounced latitudinal displacements in addition to variations in current strength. While north-south migrations of the NECC are mainly captured by the real pattern, the imaginary pattern accounts for variations in its strength. Associated with these spatial patterns is prevailing propagation toward the northwest that is most pronounced northward of the mean NECC position (∼6N). There is evidence that the zonal propagation characteristics are consistent with long Rossby waves forced in the northeastern tropical Atlantic. The relationship between interannual NECC variations and the tropical Atlantic climate modes is investigated through regression and composite analyses. Sea surface temperature and wind stress patterns resembling the meridional and zonal modes are found for the CEOF regression. Composite analysis further shows consistent patterns for warm phases of the meridional mode and cold phases of the zonal mode; the response of the NECC to a positive meridional and negative zonal mode event may be viewed as a northward shift of its core and a current strengthening, respectively. These results support a link between the two dominant tropical Atlantic climate modes and show that the relation between interannual NECC variability and the meridional and zonal modes can primarily be regarded as a response to changes in the wind field. Copyright 2012 by the American Geophysical Union.