A westward‐propagating Rossby‐like wave signal is found to explain a large fraction of the intraseasonal variance in cloud brightness over the Western Hemisphere. A series of diagnostic criteria suggest that this wave is a moisture mode: its moisture anomalies dominate the distribution of moist static energy (MSE) and are in phase with the precipitation anomalies; and the thermodynamic equation obeys the weak temperature gradient approximation. The wave propagates westward due to zonal moisture advection by the mean flow and is maintained by radiative heating and meridional moisture advection. These properties compare favorably with the westward propagating Rossby mode in an equatorial beta‐plane model with prognostic moisture, mean meridional moisture gradient, and mean zonal wind. These results underscore the importance of water vapor in the dynamics of slowly evolving tropical systems, and the limitations of dry shallow water theory that rely on a “reduced equivalent depth” to represent moist dynamics.
- Award ID(s):
- 1936810
- NSF-PAR ID:
- 10319690
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- Volume:
- 78
- Issue:
- 10
- ISSN:
- 0022-4928
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Intraseasonal oscillations affect the weather not just in the tropics but all around the globe. The convectively coupled equatorial Rossby wave is observed as the westward‐moving intraseasonal oscillation. The fundamental physics of its coupling is still unknown; thus, many questions remain unanswered. How is its phase speed altered by convection? What makes it unstable? Why is it an intraseasonal oscillation? Using the Fuchs and Raymond model with linearized governing equations on an equatorial beta plane, first baroclinic mode vertical structure, and moisture and wind‐induced surface heat exchange (WISHE) convective parametrizations, this paper seeks a fundamental analytical theory that can explain the basic features of the convectively coupled equatorial Rossby wave. The WISHE‐moisture theory leads to a large‐scale, unstable westward propagating mode in the
n = 1 case, which we call the westward propagating WISHE‐moisture mode. We find that the westward propagating WISHE‐moisture mode is indeed the free equatorial Rossby wave in the absence of moisture closure and WISHE. It is propagating westward due to the beta effect, and it slows down when it is convectively coupled. Its phase speed decreases mainly due to WISHE and cloud‐radiation interactions. Thex ‐y structure of the pressure and horizontal winds is similar to the free and observed Rossby wave, with convergent net flow. The strongest easterlies are to the west of the precipitation maximum increasing the moisture in that area. The mode is unstable due to the interplay of surface fluxes and moisture, which increases as a function of zonal wavelength. -
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JAS-D-21-0071.1
