Estimating the Gross Moist Stability of the Tropical Atmosphere

Jia-Yuh Yu, Chia Chou, and J. David Neelin
J. Atmos. Sci., 55, 1354-1372, 1998.

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© Copyright 1998 by the American Meteorological Society.

Abstract. Recent studies have shown that large-scale phenomena associated with deep convection in the tropics, such as the ITCZ (Intertropical Convergence Zone) and the MJO (Madden-Julian Oscillation), are closely related to the small but finite magnitude of the "gross moist stability" as well as the "gross moisture stratification." We note that both quantities can be internally defined in a continuously-stratified atmosphere under the CID (Convective Interaction with Dynamics) quasi-equilibrium constraints. In this paper, we use 10-year monthly mean analyses from ECMWF and NMC to estimate the horizontal distribution of these two quantities in the tropical atmosphere. The result shows that the domain of applicability is obvious such that the concept of using these two quantities in interpreting the large-scale characteristics applies best in the central and western Pacific areas. The result also indicates that the gross moist stability changes only modestly in the convergence zones that corresponds only to a phase speed change from 7 to 12 m/s. The sea surface temperature, however, affects its magnitude in a subtle way such that there is no single correspondence between them. The gross moisture stratification, on the other hand, exhibits much larger variations, from 200 J kg-1 to 900 J kg-1, in the domain of interest with large values marking the positions of convergence zones. Our analysis further indicates that a high degree of cancellation between the low-level moisture increase and the cloud- top effect contributes to the flatness of the gross moist stability in the deep convection regions. The behavior of these two quantities provides nice implications to the MJO and El Niño studies using simple models. The fact that phase speed of the MJ mode tends to be less sensitive to the SST implies that the Gill-like models may be able to qualitatively reproduce the atmospheric kinematical response to SST anomalies in the convergence zone. However, the precipitation (heating) response of the these models tends to be very different from the observations due to insufficient representation of the inhomogeneous moisture reservoir in the tropics.

Citation. Yu, J.-Y., C. Chou, and J. D. Neelin, 1998: Estimating the gross moist stability of the tropical atmosphere. J. Atmos. Sci., 1998, 55, 1354-1372.

Acknowledgements. This work was supported under NSF grant ATM-9521389 and NOAA grant NA46GP0244. The first author was also partially supported by the National Sciences Council of Taiwan under Grant NSC85-2111-M-034-002.

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