Role of the Kelvin and Rossby waves in the seasonal cycle of the equatorial Pacific Ocean circulation

Abstract

[1] The dynamics of the seasonal cycle of the equatorial Pacific Ocean circulation are investigated in a hindcast study generated by the Poseidon quasi-isopycnal general ocean circulation model forced by wind stress from a Special Sensor Microwave Imager product and heat flux from an atmospheric mixed layer model. The simulated annual and semiannual oscillations are validated by the Tropical Atmosphere-Ocean mooring measurements and the altimetry sea levels on the equator, based on which the roles of the equatorial Kelvin and Rossby waves in the seasonal cycle dynamics are investigated. The Kelvin and Rossby waves are extracted from the hindcast in such a way as to treat the model zonal nonlinear momentum terms and wind stress as forcing terms so that the validity of the linear theory on wave propagation and reflection can be examined. It is found that the reflections of the first baroclinic mode waves at the Pacific western and eastern boundaries are in good agreement with the linear theory. In comparison, the reflections of the second and higher baroclinic mode waves are significantly different from the linear theory, suggesting that the reflections are potentially nonlinear. The hindcast has simulated the westward propagation of the dominant annual oscillations of the seasonal sea level in the central basin and the eastward propagation of the dominant semi-annual oscillations of the seasonal sea level in the western and eastern basins successfully. The westward propagation of the annual oscillations is primarily associated with the annual Rossby waves forced by the annual winds in the eastern-to-central equatorial Pacific, the upwelling phase of which forces progressive springtime reversal of the surface zonal currents from the east to the west. However, the internal dynamics of the ocean alter the amplitudes and phases of the springtime reversal significantly. The results of this study have shown that the internal dynamics of the ocean are an important source of the semiannual oscillations in the equatorial Pacific Ocean. The semi-annual harmonics of the Rossby waves are boosted by the internal dynamics and eventually dominate the seasonal cycle in the far western Pacific. The reflections of the Rossby waves at the western boundary produce semi-annual dominant Kelvin waves, which force significant semi-annual oscillations of sea level in the eastern basin.

Roles of Equatorial Waves and Western Boundary Reflection in the Seasonal Circulation of the Equatorial Indian Ocean

“…However, recent analyses of the altimetry data show significant discrepancies from this theory both in the western Indian Ocean (Le Blanc and Boulanger 2001) and in the western Pacific Ocean (Boulanger and Fu 1996;Zang et al 2002). The wave reflectivity obtained by Yuan et al (2004) and Yuan (2005) from an ocean general circulation model (OGCM) of the equatorial Pacific Ocean is also significantly different from this theory. The study of Spall and Pedlosky (2005) even suggests that the reflection is nonlinear.…”

“…The total decomposed Kelvin waves are shown in the upper-right panel in Fig. 10, which are understood to be forced by the alongshore, off-equatorial, and equatorial winds, and affected by the nonlinearity of the oceanic system over the equatorial area (Yuan et al 2004). The effects of equatorial winds and nonlinearity can be estimated by subtracting the WB-forced Kelvin waves (upper-left panel in Fig.…”

“…The use of the model zonal currents ensures the complete nonlinear zonal momentum balance of the OGCM in the decomposed waves (Yuan et al 2004). The horizontal structures of the decomposed wave coefficients are not sensitive to the density profile used in the calculation (Boulanger and Menkes 1995).…”

“…The eigenfunctions and wave speeds of the baroclinic modes were first calculated based on the averaged density profile over 5°S-5°N. The three-dimensional dynamic height and zonal velocity in reference to 2500-m depth were then projected onto the eigenfunctions to extract the coefficients associated with each baroclinic mode, which were further decomposed into equatorial Kelvin and Rossby waves according to the procedure described by Yuan et al (2004). The procedure is essentially the same as in Minobe and Takeuchi (1995) and Boulanger and Menkes (1995), except that the OGCM FIG.…”

Long-Wave Dynamics of Sea Level Variations during Indian Ocean Dipole Events

“…The dynamics of the IOD events are investigated using decomposed equatorial waves from an ocean general circulation model (OGCM) simulation. We use three-dimensional data of both zonal velocity and pressure from the OGCM simulation to extract the equatorial wave coefficients, which treat the nonlinear terms of the OGCM as forcing terms and ensure a rigorous dynamic consistency, in the long-wave approximation, of the decomposed waves with the OGCM dynamics by making use of the orthonormal relation of the base-mode functions of the equatorial waves (Yuan et al 2004;Yuan 2005;Yuan and Han 2006). The wave coefficients in this paper have shown some difference in structure from that in Le Blanc and Boulanger (2001).…”