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Coupled Rossby Waves in the Indian Ocean on Interannual Timescales

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  • 1 Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California
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Abstract

Gridded fields of TOPEX/Poseidon sea level height (SLH) from 1993 to 1998 and National Centers for Environmental Prediction sea surface temperature (SST) and meridional surface wind (MSW) anomalies from 1970 to 1998 are used to examine coupled Rossby waves in the Indian Ocean from 10°S to 30°S. Time–longitude diagrams of monthly SLH, SST, and MSW anomalies yield significant peak spectral energy density in propagation wavenumber–frequency spectra for westward propagating waves of >2 yr period and >4000 km wavelength. Subsequent low-pass filtering of SLH, SST, and MSW anomalies for these interannual timescales >2 yr finds them propagating westward over the Indian Ocean in fixed phase with one another at speeds significantly less (0.04–0.07 m s−1) than first-mode baroclinic Rossby waves, taking 3 to 4 years to cross the basin. These coupled Rossby waves display weak beta refraction patterns in all three variables. Significant squared coherence between interannual SLH and SST (SST and MSW) anomalies yield phase differences ranging from 0° to 45° (150° to 180°). Warm SST anomalies overlie high SLH anomalies, suggesting that pycnocline depth anomalies associated with the Rossby waves modify vertical mixing processes to maintain SST anomalies against dissipation. Warm SST anomalies are associated with outgoing latent heat flux anomalies in the eastern and central ocean, indicating that the ocean is capable of forcing the overlying atmosphere. Poleward MSW anomalies occur directly over warm SST anomalies, suggesting that anomalous planetary vorticity advection balances anomalous low-level convergence in response to SST-induced midtroposphere convection. These inferred thermodynamic processes allow a simple analytical model of coupled Rossby waves to be constructed that yields much slower westward phase speeds than for free Rossby waves, as observed. Maintenance of wave amplitude against dissipation occurs for coupled waves that travel westward and poleward, as observed.

Corresponding author address: Warren B. White, Physical Oceanography Research Division, Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0230.

Email: wbwhite@ucsd.edu

Abstract

Gridded fields of TOPEX/Poseidon sea level height (SLH) from 1993 to 1998 and National Centers for Environmental Prediction sea surface temperature (SST) and meridional surface wind (MSW) anomalies from 1970 to 1998 are used to examine coupled Rossby waves in the Indian Ocean from 10°S to 30°S. Time–longitude diagrams of monthly SLH, SST, and MSW anomalies yield significant peak spectral energy density in propagation wavenumber–frequency spectra for westward propagating waves of >2 yr period and >4000 km wavelength. Subsequent low-pass filtering of SLH, SST, and MSW anomalies for these interannual timescales >2 yr finds them propagating westward over the Indian Ocean in fixed phase with one another at speeds significantly less (0.04–0.07 m s−1) than first-mode baroclinic Rossby waves, taking 3 to 4 years to cross the basin. These coupled Rossby waves display weak beta refraction patterns in all three variables. Significant squared coherence between interannual SLH and SST (SST and MSW) anomalies yield phase differences ranging from 0° to 45° (150° to 180°). Warm SST anomalies overlie high SLH anomalies, suggesting that pycnocline depth anomalies associated with the Rossby waves modify vertical mixing processes to maintain SST anomalies against dissipation. Warm SST anomalies are associated with outgoing latent heat flux anomalies in the eastern and central ocean, indicating that the ocean is capable of forcing the overlying atmosphere. Poleward MSW anomalies occur directly over warm SST anomalies, suggesting that anomalous planetary vorticity advection balances anomalous low-level convergence in response to SST-induced midtroposphere convection. These inferred thermodynamic processes allow a simple analytical model of coupled Rossby waves to be constructed that yields much slower westward phase speeds than for free Rossby waves, as observed. Maintenance of wave amplitude against dissipation occurs for coupled waves that travel westward and poleward, as observed.

Corresponding author address: Warren B. White, Physical Oceanography Research Division, Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0230.

Email: wbwhite@ucsd.edu

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