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Impact of Air–Sea Coupling on the Madden–Julian Oscillation in a General Circulation Model

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  • 1 NOAA CIRES Climate Diagnostics Center, Boulder, Colorado
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Abstract

The impact of air–sea coupling on the dynamics of the tropical Madden–Julian oscillation (MJO) is investigated with an atmospheric general circulation model (GCM) coupled to an ocean mixed layer model. In the uncoupled GCM, where climatological sea surface temperature (SST) is specified, realistic space–time spectra of near-equatorial zonal wind and precipitation are produced, with power concentrated at eastward wavenumbers 1–3 with periods of 35–90 days. However, the simulated MJO is roughly 50% stronger than observed, largely resulting from enormous activity during northern summer. Furthermore, during southern summer, when the observed MJO is most dominant across the Indian and western Pacific Oceans, intraseasonal variance in the uncoupled model is overly concentrated to the north and east of Australia with little activity extending into the equatorial Indian Ocean. Contrary to other recent modeling studies, coupling did not alleviate either of these problems nor did it have any other appreciable impact on the model’s MJO.

Feedback of the SST anomalies onto the MJO, both observed and diagnosed in other coupled models, appears to result from correlation of positive equatorial SST anomalies across the warm pool with surface low pressure to the east of the convective anomaly. This feedback is insignificant in the present coupled model because the SST anomalies, besides being too weak and not spatially coherent, do not systematically exhibit the requisite phasing with the surface pressure. The observed SST anomalies result from a combination of shortwave radiation and latent heat flux, whereby reduced shortwave radiation associated with enhanced convection slightly leads enhanced latent heat flux associated with increased surface westerlies. The model does produce realistic shortwave radiation anomalies, but its latent heat flux anomalies are too weak and do not constructively add with the shortwave radiation anomalies. It is concluded that coupling is not a panacea for problems of simulating the MJO in uncoupled GCMs and that coupling, if it is important, depends critically on the structure of the surface fluxes produced by the MJO.

Corresponding author address: Harry Hendon, Climate Diagnostics Center, Mail Code R/E/CD, 325 Broadway, Boulder, CO 80303-3328.

Email: hhh@cdc.noaa.gov

Abstract

The impact of air–sea coupling on the dynamics of the tropical Madden–Julian oscillation (MJO) is investigated with an atmospheric general circulation model (GCM) coupled to an ocean mixed layer model. In the uncoupled GCM, where climatological sea surface temperature (SST) is specified, realistic space–time spectra of near-equatorial zonal wind and precipitation are produced, with power concentrated at eastward wavenumbers 1–3 with periods of 35–90 days. However, the simulated MJO is roughly 50% stronger than observed, largely resulting from enormous activity during northern summer. Furthermore, during southern summer, when the observed MJO is most dominant across the Indian and western Pacific Oceans, intraseasonal variance in the uncoupled model is overly concentrated to the north and east of Australia with little activity extending into the equatorial Indian Ocean. Contrary to other recent modeling studies, coupling did not alleviate either of these problems nor did it have any other appreciable impact on the model’s MJO.

Feedback of the SST anomalies onto the MJO, both observed and diagnosed in other coupled models, appears to result from correlation of positive equatorial SST anomalies across the warm pool with surface low pressure to the east of the convective anomaly. This feedback is insignificant in the present coupled model because the SST anomalies, besides being too weak and not spatially coherent, do not systematically exhibit the requisite phasing with the surface pressure. The observed SST anomalies result from a combination of shortwave radiation and latent heat flux, whereby reduced shortwave radiation associated with enhanced convection slightly leads enhanced latent heat flux associated with increased surface westerlies. The model does produce realistic shortwave radiation anomalies, but its latent heat flux anomalies are too weak and do not constructively add with the shortwave radiation anomalies. It is concluded that coupling is not a panacea for problems of simulating the MJO in uncoupled GCMs and that coupling, if it is important, depends critically on the structure of the surface fluxes produced by the MJO.

Corresponding author address: Harry Hendon, Climate Diagnostics Center, Mail Code R/E/CD, 325 Broadway, Boulder, CO 80303-3328.

Email: hhh@cdc.noaa.gov

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