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Air–Sea Interaction in the Equatorial Atlantic Region

Stephen E. ZebiakLamont–Doherty Earth Observatory of Columbia University, Palisades, New York

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Abstract

Using a dynamically motivated analysis of observations, and an intermediate-level coupled model, the interannual variability within the equatorial Atlantic is studied. It is found that a significant part of the observed variability can be described by an equatorial coupled mode akin to ENSO (El Niño–Southern Oscillation). The Atlantic mode signature is even more tightly focused on the equator and is situated proportionally farther to the west within the basin than its Pacific counterpart.

Model simulations capture the equatorial coupled mode in relatively pure form and, for what are thought to be the most realistic parameter choices, show interannual oscillations favoring a 4-year period, which are not self-sustaining. The simulated spatial patterns agree well with those extracted from observations, including those features that distinguish the Atlantic from the Pacific.

Sensitivity experiments show that the Atlantic coupled-mode signal is less robust than the corresponding Pacific ENSO signal but is still well-defined qualitatively, within reasonable parameter ranges. The results demonstrate that the primary mechanisms of oscillation for the Atlantic and Pacific are the same but that differences in the zonal structure and strength of air–sea coupling and mean ocean stratification offset the large differences in basin size, allowing similar oscillation periods for the two basin modes. An explanation for the distinct spatial patterns of simulated Atlantic and Pacific anomalies is found in the differences in climatological mean fields and ocean basin configurations.

Together, the observational and model results present a picture of equatorial Atlantic variability in which coupled equatorial dynamics play an important but not exclusive role. It appears that the coupling is sufficiently strong to leave its imprint on the total variability but too weak to dictate it entirely, even at the equator.

Abstract

Using a dynamically motivated analysis of observations, and an intermediate-level coupled model, the interannual variability within the equatorial Atlantic is studied. It is found that a significant part of the observed variability can be described by an equatorial coupled mode akin to ENSO (El Niño–Southern Oscillation). The Atlantic mode signature is even more tightly focused on the equator and is situated proportionally farther to the west within the basin than its Pacific counterpart.

Model simulations capture the equatorial coupled mode in relatively pure form and, for what are thought to be the most realistic parameter choices, show interannual oscillations favoring a 4-year period, which are not self-sustaining. The simulated spatial patterns agree well with those extracted from observations, including those features that distinguish the Atlantic from the Pacific.

Sensitivity experiments show that the Atlantic coupled-mode signal is less robust than the corresponding Pacific ENSO signal but is still well-defined qualitatively, within reasonable parameter ranges. The results demonstrate that the primary mechanisms of oscillation for the Atlantic and Pacific are the same but that differences in the zonal structure and strength of air–sea coupling and mean ocean stratification offset the large differences in basin size, allowing similar oscillation periods for the two basin modes. An explanation for the distinct spatial patterns of simulated Atlantic and Pacific anomalies is found in the differences in climatological mean fields and ocean basin configurations.

Together, the observational and model results present a picture of equatorial Atlantic variability in which coupled equatorial dynamics play an important but not exclusive role. It appears that the coupling is sufficiently strong to leave its imprint on the total variability but too weak to dictate it entirely, even at the equator.

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