Ocean-Atmosphere Variability over the Pan-Atlantic Basin
Youichi TANIMOTO (Climate Diagnosis Group, Frontier Research System for Global Change, Shibaura 1-2-1, Seavans N Bldg.7F, Minato-ku, Tokyo 105-6791, Japan)
email: tanimoto@frontier.esto.or.jp
Shang-Ping, XIE (Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan)
email: xie@ees.hokudai.ac.jp
ABSTRACT
Ocean and atmosphere variability is investigated over the whole Atlantic basin in recent five decades of sea surface temperature (SST) and sea level pressure (SLP). In the tropics, SSTs variations are separated into two time scales: decadal (8-16 years) and interannual (＜5 years) time scales. An equatorial anti-symmetric SST dipole mode dominates on decadal time scales with centers of action with opposite polarities in the tropics, while interannual variations are characterized by SST anomalies of an equatorial symmetric mode that presents a same polarity of anomalies throughout the tropics: monopole.
The Pan-Atlantic decadal variations of SST anomalies coincide well with the tropical dipolar oscillation. Regressed SLP onto this dipolar oscillation shows an atmospheric bridge between tropics and extratropics. In the Northern Hemisphere, the North Atlantic Oscillation (NAO) appears when tne tropical dipole is developed. These results suggest that an extratropical forcing could excite the tropical Atlantic variability.
1. Introduction
It is well-known that sea surface temperature (SST) anomalies with an equatorial antisymmetric dipole structure in the tropical Atlantic affects on precipitation changes in surrounding areas (Hasternrath and Heller 1977, Lamb 1978, Mehta 1998), which is associated with a meridional displacement of the Atlantic Inter-tropical Convergence Zone (ITCZ) (Nobre and Shukla 1996). An initial increase of SST north of equator (and/or a decrease south of equator) induces southerly cross-equatorial wind. Under the basic condition of easterly trade winds, anomalous southerlies change a heat release from the ocean: enhanced to the north and suppressed to the south of equator. This air-sea coupled variability involving the wind-evaporation feedback is well simulated by the simple coupled models (Chang et al 1997, Xie and Tanimoto 1998, also in this volume).
However tropical SST anomalies in either side of equator hardly show a negative correlation. Zonal mean tropical SST anomalies have nearly decorrelated scatters between 10-20 degree latitudes (Fig. 1). Therefore, it seems that there are, at least, two modes in the SST anomaly field of the tropical Atlantic.
