Structure of interannual-to-decadal climate variability in the tropical Atlantic sector

A search for coupled modes of atmosphere-ocean interaction in the tropical Atlantic sector is presented. Previous studies have provided conflicting in dications of the existence of coupled modes in this region. The subject is revisited through a rotated principal component analysis performed on datas ets spanning the 36-yr period 1958-93. The analysis includes four variables , sea surface temperature, oceanic heat content, wind stress, and atmospher ic diabatic heating. The authors find that the first rotated principal comp onent is associated with fluctuations in the subtropical wind system and co rrelates with the North Atlantic oscillation (NAO), while the second and th ird modes, which are the focus of interest, are related to tropical variabi lity. The second mode is the Atlantic Nino mode with anomalous sea surface temper ature and anomalous heat content in the eastern equatorial basin. Wind stre ss weakens to the west of anomalously warm water, while convection is shift ed south and eastward. Surface and upper-level wind anomalies of this mode resemble those of El Nino-Southern Oscillation (ENSO) events. When the anal ysis is limited to boreal summer, the season of maximum amplitude, the Atla ntic Nino mode explains 7.5% of the variance of the five variables. Thermod ynamic air-sea interactions do not seem to play a role for this mode. The third mode is associated with an interhemispheric gradient of anomalous sea surface temperature and a dipole pattern of atmospheric heating. In it s positive phase anomalous heating occurs over the warmer Northern Hemisphe re with divergence aloft shifting convection to the north and west of the e quator and intensifying the subtropical jet stream, while descending motion occurs on the western side of the Southern Hemisphere. Surface and subsurf ace structures in the ocean are controlled by surface winds. This interhemi spheric mode is strongest in boreal spring when it explains 9.1% of the com bined variance of the five variables. Thermodynamic air-sea interactions do seem to control the associated sea surface temperature anomalies, although equatorial dynamics may play a role as well. The authors also examine the connection of the tropical Atlantic to other b asins, ENSO events cause patterns of winds, heating, and sea surface temper atures resembling the interhemispheric mode described above. The lag betwee n changes in the Atlantic and Pacific is 4-5 months for the interhemispheri c mode. In contrast, no significant impact of ENSO is found on the Atlantic Nino mode. Likewise, no impact of the midlatitude North Atlantic (the NAG) is found on the Tropics, but some impact of the Tropics is found on the mi dlatitude North Atlantic.