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.
Highlights
Introduction analysesof observationsand numerical modelsindicateThe parallelshearflowsthat characterizethe equatorial circulation are potential sitesfor instabilities
The seasonallymodulated synoptic signal in the upper-equatorial Pacific Ocean may be more complex than a singleinstability wave;perhapsmore than oneseatof instability exists,assuggestedby Luther and eddy viscosity; V horizontal gradient operator; g accelerationdue to gravity; f unit vectordirected
The sea-level spectrum plot (Figure 4, bottom panel) showsvery little powernear 15 days;most of the spectralenergyis confined to periods near 30 days
Summary
The parallelshearflowsthat characterizethe equatorial circulation are potential sitesfor instabilities. Analysisof upperoceancurrentand densityfieldsby Lutherand Johnson[1990]suggesttshreedifferentinstabilities take place, each at a different phaseof the seasonalcycle: one instability results from shear betweenEquatorialUndercurrent(EUC) andSouthEquatorial Current (SEC) during boreal summerand fall, bilities within the wind-driven near-surfacecurrents, play an important role in the heat, momentum, and energy balancesof the tropical Pacific. The symbolsare definedas follows: lated to the sea-levelanomaliesviewed by the altimeter The presenceof this vortex indicatessynoptic-scale variability near 5øN may be distinct from that closerto the equator,but McPhaden[1996]arguedthat meridional advection creates the "red shift" seen in the offhi(x, y, t) vi(x, y, t) - (ui, vi) ith layerthickness; ith layerhorizontal velocity; equatorial temperature spectrum. The seasonallymodulated synoptic signal in the upper-equatorial Pacific Ocean may be more complex than a singleinstability wave;perhapsmore than oneseatof instability exists,assuggestedby Luther and eddy viscosity; V horizontal gradient operator; g accelerationdue to gravity; f unit vectordirected.
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