Document Type

Article

Date of Original Version

1998

Abstract

A two‐and‐a‐half‐layer model of the tropical Pacific Ocean is used to investigate the energy source for the intraseasonal dynamic‐height variability observed near 6°N. A simulation of equatorial circulation is produced by forcing the model with mean‐monthly wind‐stress climatology. Two westward‐propagating waves appear in the upper layer in the central and eastern portion of the model basin. These two waves are distinguished by period and meridional structure. An off‐equatorial wave with period of 30 days and wavelength of 1100 km has a meridional sea‐level maximum near 6°N similar to that of the 30–50 day intraseasonal wave observed in the ocean. The meridional velocity signal also is asymmetric with respect to the equator, with maximum near 4°N. The second wave with period of 15 days has a strong meridional velocity signal centered on the equator. The sea‐level and zonal velocity signals associated with this equatorial wave have maxima near 1.5°N and 1.5°S. The eddy‐energy budget reveals strong conversions from the mean‐flow to eddy field through baroclinic and upper‐layer barotropic conversion terms. Conversion terms north of the equator exhibit a bimodal structure: one maximum between the equator and 3°N is dominated by upper‐layer barotropic conversion spatially coincident with the cyclonic shear along the equatorward edge of the South Equatorial Current (SEC), and a second smaller maximum between 3°N and 5°N is a combination of upper‐layer barotropic conversion along the poleward edge of the SEC (anticyclonic shear) and baroclinic conversion near the core of the SEC. The two peaks in the conversion terms, combined with similar structure in the flux‐divergence terms in the model eddy‐energy budget, provide evidence that two wave processes are generated at the different source regions: one near the equator and a second between 2°N and 5°N.

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