Inter-hemispheric decadal variations in SST, surface wind, heat flux and cloud cover over the Atlantic Ocean


Youichi Tanimoto

Graduate School of Environmental Earth Science, Hokkaido University, Japan

Shang-Ping Xie

International Pacific Research Center and Dept. of Meteorology, Univ. of Hawaii, USA

J. Meteor. Soc. Japan, in press


Abstract.  Atlantic decadal climate variations are studied using marine meteorological observations. To remove artificial interhemispheric correlation, we perform empirical orthogonal function (EOF) analysis of sea surface temperature (SST) variability separately for the North and South Atlantic. The first EOF for the North (South) Atlantic in the decadal (8-16 years) band features a meridional tripole (dipole). In the tropics, the northern and southern leading EOFs form a meridional dipole with a center of action at 15o on either side of the equator. The leading sea level pressure (SLP) EOFs for the North and South Atlantic each feature a center of action that is displaced poleward of the tropical SST extreme, at 30o latitude. The SLP center of action in the North Atlantic has a barotropic structure and contributes significantly to surface wind variability in the tropics. Despite being derived from statistically independent data samples, the principle components for the leading SST and SLP EOFs (four in total) are significantly correlated with one another, indicative of the existence of an interhemispheric mode spanning the entire Atlantic Ocean.  The same analysis for a longer SST record suggests that this pan-Atlantic decadal variability exists throughout the 20th century.

            In the North Atlantic, composite analysis of wind velocity and heat fluxes based on the PCs of the leading SST modes indicate that wind-induced latent heat flux is the major forcing for decadal SST variability. In the South Atlantic, by contrast, wind anomalies are neither organized in space nor in geostrophic balance with SLP, a problem likely due to poor sampling there.

            Spatially coherent anomalies of low-level cloud cover are found to be associated with the tropical Atlantic dipole, with increased (decreased) cloudiness over the cold (warm) lobe. These low-level cloud anomalies do not appear to be associated with significant surface wind convergence, unlike the deep convective clouds near the equator. By shielding solar radiation, these low-level cloud anomalies act to reinforce the underlying SST anomalies, reducing the Newtonian cooling rate for SST by as much as 30%.