Response of the Zonally Asymmetric Flow to Time-Dependent Tropical Heating

Chung-Kyu Park Joint Center for Earth System Science, Department of Meteorology, University of Maryland, College Park, Maryland

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Max J. Suarez Laboratory for Atmospheres, Goddard Space Flight Center/NASA, Greenbelt, Maryland

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Siegfried D. Schubert Laboratory for Atmospheres, Goddard Space Flight Center/NASA, Greenbelt, Maryland

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Abstract

An atmospheric general circulation model is used to study the impact of idealized zonally propagating tropical heating anomalies on the low-frequency variability in the North Pacific region. The propagating heating is designed to mimic the thermal forcing associated with the Madden–Julian oscillation (MJO). Results are examined by separating the forced response from other variability and by comparing with runs employing fixed-phase (stationary) heating anomalies.

For both the forced and free circulations, the main modes of variability consist of a zonal expansion and retraction of the East Asian jet. The effective Rossby wave forcing associated with the heating is dominated by the advection term and located in the subtropics in the regions of strong absolute vorticity gradients.

Compared with cases using stationary forcing, the response to the propagating forcing is weaker and of different phase, indicating that the 40-day period used for the propagating anomalies is too short to allow the development of the steady-state response in the extratropics.

The model's total low-frequency variability in the North Pacific sector is dominated by the free oscillations that are the result of local processes uncorrelated with tropical variability. The relatively small forced response appear to be partly the result of the simplicity of the propagating heating anomaly that propagates at a constant phase speed and the simplification introduced into the GCM that do not allow transient feedback in the diabatic heating.

It is suggested that the lack of a significant Rossby wave stretching term in the subtropics is a distinguishing feature of the east–west dipole heating anomalies of the MJO and may contribute to the weakness of the response compared to interannual tropical heating anomalies.

Abstract

An atmospheric general circulation model is used to study the impact of idealized zonally propagating tropical heating anomalies on the low-frequency variability in the North Pacific region. The propagating heating is designed to mimic the thermal forcing associated with the Madden–Julian oscillation (MJO). Results are examined by separating the forced response from other variability and by comparing with runs employing fixed-phase (stationary) heating anomalies.

For both the forced and free circulations, the main modes of variability consist of a zonal expansion and retraction of the East Asian jet. The effective Rossby wave forcing associated with the heating is dominated by the advection term and located in the subtropics in the regions of strong absolute vorticity gradients.

Compared with cases using stationary forcing, the response to the propagating forcing is weaker and of different phase, indicating that the 40-day period used for the propagating anomalies is too short to allow the development of the steady-state response in the extratropics.

The model's total low-frequency variability in the North Pacific sector is dominated by the free oscillations that are the result of local processes uncorrelated with tropical variability. The relatively small forced response appear to be partly the result of the simplicity of the propagating heating anomaly that propagates at a constant phase speed and the simplification introduced into the GCM that do not allow transient feedback in the diabatic heating.

It is suggested that the lack of a significant Rossby wave stretching term in the subtropics is a distinguishing feature of the east–west dipole heating anomalies of the MJO and may contribute to the weakness of the response compared to interannual tropical heating anomalies.

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