Abstract Linearized wave solutions on the equatorial beta plane are examined in the presence of a background meridional moisture gradient. Of interest is a slow, eastward-propagating n = 1 mode that is unstable at planetary scales and only exists for a small range of zonal wavenumbers ( ). The mode dispersion curve appears as an eastward extension of the westward-propagating equatorial Rossby wave solution. This mode is therefore termed the eastward-propagating equatorial Rossby wave (ERW). The zonal wavenumber-2 ERW horizontal structure consists of a low-level equatorial convergence center flanked by quadrupole off-equatorial gyres, and resembles the horizontal structure of the observed MJO. An analytic, leading-order dispersion relationship for the ERW shows that meridional moisture advection imparts eastward propagation, and that the smallness of a gross moist stability–like parameter contributes to the slow phase speed. The ERW is unstable near planetary scales when low-level easterlies moisten the column. This moistening could come from either zonal moisture advection or surface fluxes or a combination thereof. When westerlies instead moisten the column, the ERW is damped and the westward-propagating long Rossby wave is unstable. The ERW does not exist when the meridional moisture gradient is too weak. A moist static energy budget analysis shows that the ERW scale selection is partly due to finite-time-scale convective adjustment and less effective zonal wind–induced moistening at smaller scales. Similarities in the phase speed, preferred scale, and horizontal structure suggest that the ERW is a beta-plane analog of the MJO.
more »
« less
A Simple Model of Convectively Coupled Equatorial Rossby Waves
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
- NSF-PAR ID:
- 10460178
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Advances in Modeling Earth Systems
- Volume:
- 11
- Issue:
- 1
- ISSN:
- 1942-2466
- Page Range / eLocation ID:
- p. 173-184
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract 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.
-
more » « less
Abstract A recent analytical model of the Madden‐Julian Oscillation (MJO), the WISHE‐moisture mode theory, successfully models all the main characteristics of the MJO, namely, that it is a planetary, unstable, and eastward propagating tropical mode. The main assumption of the WISHE‐moisture mode theory is that an interplay between the global mean easterly zonal winds and moisture leads to the propagation and destabilization of the MJO. We investigate this theory by building a climatological MJO using reanalysis, OAFlux surface latent heat fluxes, and outgoing longwave radiation data and compare it to the WISHE‐moisture mode. The necessary condition for the WISHE‐moisture mode theory—easterly global mean zonal winds—is always present in the tropics. Comparing the zonal wind, moisture, and moisture tendency anomalies to the WISHE‐moisture mode, we find that the WISHE‐moisture mode theory is in agreement with the reanalysis‐derived climatological MJO. Reanalysis and OAFlux surface latent heat fluxes are in agreement with the WISHE‐moisture mode theory. Although meridional surface winds seem to contribute negatively to the surface latent heat flux anomaly, we show that their effect is secondary in the region of enhanced surface latent heat fluxes which moisten the boundary layer and lead to the propagation of the MJO.
-
more » « less
Abstract We study the adjustment of the tropical atmosphere to localized surface heating using a Lagrangian atmospheric model (LAM) that simulates a realistic Madden–Julian oscillation (MJO)—the dominant, eastward-propagating mode of tropical intraseasonal variability modulating atmospheric convection. Idealized warm sea surface temperature (SST) anomalies of different aspect ratios and magnitudes are imposed in the equatorial Indian Ocean during MJO-neutral conditions and then maintained for 15 days. The experiments then continue for several more months. Throughout these experiments, we observe a robust generation of an MJO event, evident in precipitation, velocity, temperature, and moisture fields, which becomes a key element of atmospheric adjustment along with the expected Kelvin and Rossby waves. The MJO circulation pattern gradually builds up during the first week, and then starts to propagate eastward at a speed of 5–7 m s−1. The upper-level quadrupole circulation characteristic of the MJO becomes evident around day 14, with two anticyclonic gyres generated by the Gill-type response to convective heating and two cyclonic gyres forced by the excited Kelvin waves and extratropical Rossby wave trains. A moisture budget analysis shows that the eastward propagation of the MJO is controlled largely by the anomalous advection of moisture and by the residual between anomalous moisture accumulation due to converging winds and precipitation. The initial MJO event is followed by successive secondary events, maintaining the MJO for several more cycles. Thus, this study highlights the fundamental role that the MJO can play in the adjustment of the moist equatorial atmosphere to localized surface heating.
-
more » « less
Abstract Azimuthally asymmetric perturbations are important to hurricanes because they can influence the track, structure, and intensity of a hurricane. In this work, we applied space‐time spectral analysis on both dynamic and thermodynamic fields of these perturbations and found two distinct power peaks in most of the fields. We obtained the structure of each mode by first filtering the fields through a frequency‐wavenumber spectral window selected for each mode and then regressing these fields on an index based on the filtered radar reflectivity. We found that the fast‐propagating wave is dominated by perturbations near the eyewall, and its structure is similar to that of the unstable mixed vortex Rossby inertia gravity wave. The other peak corresponds to a slow‐propagating wave that has comparable perturbations in and beyond the eyewall. The slow wave has a retrograde intrinsic propagating speed and has a vertical structure that resembles that of convectively coupled waves.
