The role of ocean dynamics in the interaction between the Atlantic meridional and equatorial modes

This study examines a mechanism of the interaction between the tropical Atlantic meridional and equatorial modes. To derive robust heat content (HC) variability, the ensemble mean HC anomalies of six state-of-the-art global ocean re-analyses for 1979-2007 are analyzed. Compared with previous studies, characteristic oceanic processes are distinguished through their dominant time scales. Using the ensemble empirical mode decomposition (EEMD) method, the HC fields are firstly decomposed into components with different time scales. Our analysis shows these components are associated with distinctive ocean dynamics. The high frequency (first three) components can be characterized as the equatorial modes, while the low frequency (the 5th and 6th) components are featured as the meridional modes. In between, the 4th component on the time scale of 3-4 years demonstrates “mixed” characteristics of the meridional and equatorial modes due to an active transition from the predominant meridional to zonal structures on this time scale. Physically, this transition process is initiated by the discharge of the off-equatorial HCA, first accumulated as a part of the meridional mode, into the equatorial waveguide, which is triggered by the breakdown of the equilibrium between the cross-equatorial HC contrast and the overlying wind forcing, and results in a major heat transport through the equatorial waveguide into the southeastern tropical Atlantic. It is also shown the remote forcing from El Niño-Southern Oscillation (ENSO) exerts important influence on the transition from the equatorial to meridional mode, and may partly dictate its time scale of 3-4 years. Therefore, our result demonstrates another mechanism of the equatorial Atlantic response to the ENSO forcing.