Seasonal Propagation of Sea Level along the Equator in the Atlantic

Lucia Bunge Department of Oceanography, The Florida State University, Tallahassee, Florida

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Allan J. Clarke Department of Oceanography, The Florida State University, Tallahassee, Florida

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Abstract

In the equatorial Atlantic the sea surface height (SSH) anomaly field is dominated by an annual signal propagating eastward. This signal has been previously interpreted in terms of propagating waves. In this article it is argued that this propagating signal is not a free equatorial Kelvin wave because the phase velocity observed is too small compared to first, second, or third baroclinic mode Kelvin waves, and is not the result of an equatorial forced wave because the zonal wind stress does not show a similar propagation. Rather, it is suggested that the eastward propagation in SSH is due to the sum of two independent modes of variability: one mainly driven by the wind stress curl off the equator, and the other driven by the zonal wind stress along the equator. These two modes are uncorrelated in time and space and therefore can be conveniently separated by an empirical orthogonal function analysis of the equatorial Atlantic sea surface height. The first mode explains 74% of the variance, is one-signed in longitude, and is interpreted as the variability of the warm water volume above the thermocline. The second mode explains 24% of the variance, consists of an east–west tilt along the equator, and is driven by variations of the zonal equatorial wind stress.

Corresponding author address: Allan J. Clarke, Dept. of Oceanography, The Florida State University, Tallahassee, FL 32306-4320. Email: clarke@ocean.fsu.edu

Abstract

In the equatorial Atlantic the sea surface height (SSH) anomaly field is dominated by an annual signal propagating eastward. This signal has been previously interpreted in terms of propagating waves. In this article it is argued that this propagating signal is not a free equatorial Kelvin wave because the phase velocity observed is too small compared to first, second, or third baroclinic mode Kelvin waves, and is not the result of an equatorial forced wave because the zonal wind stress does not show a similar propagation. Rather, it is suggested that the eastward propagation in SSH is due to the sum of two independent modes of variability: one mainly driven by the wind stress curl off the equator, and the other driven by the zonal wind stress along the equator. These two modes are uncorrelated in time and space and therefore can be conveniently separated by an empirical orthogonal function analysis of the equatorial Atlantic sea surface height. The first mode explains 74% of the variance, is one-signed in longitude, and is interpreted as the variability of the warm water volume above the thermocline. The second mode explains 24% of the variance, consists of an east–west tilt along the equator, and is driven by variations of the zonal equatorial wind stress.

Corresponding author address: Allan J. Clarke, Dept. of Oceanography, The Florida State University, Tallahassee, FL 32306-4320. Email: clarke@ocean.fsu.edu

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