Abstract

The dynamics of convectively coupled gravity waves traveling over a precipitating region are analyzed in an idealized model for the large-scale atmospheric circulation. The model is composed of a shallow water system coupled to an advection equation for moisture through the convection term, utilizing a quasi-equilibrium relaxation to moisture closure. Here the authors investigate the model in the strict quasi-equilibrium (SQE) of infinitely short relaxation time. This framework is applied to study the behavior of a disturbance propagating along a narrow precipitation band, similar to the intertropical convergence zone (ITCZ). For an ITCZ width on the order of the equatorial Rossby radius, Kelvin waves propagate at the moist gravity wave speed (about 15 m s⁻¹), whereas for a narrow ITCZ, the propagation speed is comparable to the dry gravity wave (about 50 m s⁻¹). It is also shown that a Kelvin wave propagating along a narrow precipitation region exhibits a meridional circulation that modulates the precipitation rate and affects the propagation speed of the wave.

Abstract

This paper presents a theoretical study of the effects of moist convection on geostrophic adjustment in an infinite channel. The governing equations correspond to a linearized shallow water system of equations for the atmosphere first vertical baroclinic mode, which is coupled to a vertically averaged moisture equation. The coupling is through a parameterization that represents precipitation. The transient behavior and final state of the flow initially at rest with active precipitation limited to half of the channel is investigated, both numerically and analytically. It is shown that an initial imbalance resulting from precipitation induces a circulation that dries out the nonprecipitating region and further enhances precipitation. This interaction between precipitation and dynamics leads to a sharper temperature gradient and stronger jet in the final state, when compared to the dry adjustment. Unlike in the dry case, the moist geostrophic adjustment cannot be entirely determined from the initial unbalanced flow, since it depends on the time scale for convection. Analytic approximations are derived in limits of both fast and slow convective adjustment time.

Abstract

A number of studies suggest a two-way feedback between convectively coupled Kelvin waves (CCKWs) and the intertropical convergence zone (ITCZ). Viewed here as a proxy for deep convection, analysis of brightness temperature data reveals several aspects of these interdependencies. A wavenumber–frequency spectral analysis is applied to the satellite data in order to filter CCKWs. The ITCZ is characterized by a region of low brightness temperature and a proxy for both the ITCZ location and width are defined. The phase speed of CCKW data is determined using the Radon transform method. Linear regression techniques and probability density analysis are consistent with previous theoretical predictions and observational results. In particular, the fastest waves are found when the ITCZ is the farthest from the equator and the narrowest. Conversely, the slowest waves coincide with broad ITCZs that are located near the equator.

Abstract

This study examines the diurnal cycle of rainfall and cloudiness associated with the Madden–Julian oscillation (MJO) using TRMM rainfall rate and ISCCP multilevel cloud fraction data. There are statistically significant differences in diurnal cycle amplitude and phase between suppressed and enhanced envelopes of MJO convection. The amplitude of the diurnal rainfall rate and middle–deep cloudiness increases within enhanced MJO convection, especially over the ocean. However, the differences in diurnal cycle amplitude between enhanced and suppressed MJO are generally smaller than the differences in daily mean values, so that its relative contribution to total rainfall or cloudiness variance within enhanced MJO convection becomes smaller. Near the coastlines of islands within the Maritime Continent, the diurnal cycle amplitude tends to increase 5–10 days prior to the arrival of the peak enhanced MJO convection, but this relationship is weaker over the interior areas of larger islands where the climatological diurnal amplitude is already large. Within enhanced MJO convection, the diurnal rainfall peak is frequently delayed by about 3 h and cloud height decays at slower rate compared to suppressed conditions. More stratiform rainfall occurs following the peak convective rainfall within enhanced MJO convection, delaying the total rainfall peak by a few hours as a result of its greater horizontal extent. The results of this study suggest that the MJO modulates both the amplitude and phase of the diurnal cycle of tropical rainfall and cloudiness by influencing cloud type population distribution and associated rainfall rates.

Abstract

Precipitation variability over the Maritime Continent is predominantly explained by its diurnal cycle and large-scale disturbances such as the Madden–Julian oscillation (MJO) and convectively coupled equatorial waves (CCEWs). To advance our understanding of their interactions and physical processes, this study uses satellite data to examine changes in the diurnal cycle of rainfall associated with the MJO and CCEWs over the Maritime Continent. We find that diurnal cycle modulations associated with the passage of any type of large-scale disturbance are closely tied to changes in rain types and land–sea diurnal propagation of rainfall. When the amplitude of the diurnal cycle increases over the islands, the phase of the diurnal cycle is delayed by a few hours as clouds are more organized and rainfall from stratiform-anvil clouds increases. Enhanced amplitude of the diurnal cycle can alter the speed of land–sea diurnal propagation of rainfall, which then influences the timing of diurnal rainfall over coastal regions. These changes in the diurnal cycle occur asymmetrically across the island terrain associated with the MJO and equatorial Rossby waves, while such asymmetric modulations are not observed for other waves. Geographical and wave dependencies of the diurnal cycle are linked to differences in large-scale lower tropospheric wind, vertical motion, and moisture profile perturbations, which are in turn tied to differences in cloud population evolution. The results of this study highlight the importance of further improving our understanding of the sensitivity of cloud populations to varying large-scale phenomena.

Abstract

Space–time spectral analysis of tropical cloudiness data shows strong evidence that convectively coupled n = 0 mixed Rossby–gravity waves (MRGs) and eastward inertio-gravity waves (EIGs) occur primarily within the western/central Pacific Ocean. Spectral filtering also shows that MRG and EIG cloudiness patterns are antisymmetric with respect to the equator, and they propagate coherently toward the west and east, respectively, with periods between 3 and 5 days, in agreement with Matsuno’s linear shallow-water theory. In contrast to the spectral approach, in a companion paper it has been shown that empirical orthogonal functions (EOFs) of 2–6-day-filtered cloudiness data within the tropical Pacific Ocean also suggest an antisymmetric pattern, but with the leading EOFs implying a zonally standing but poleward-propagating oscillation, along with the associated tropospheric flow moving to the west. In the present paper, these two views are reconciled by applying an independent approach based on a tracking method to assess tropical convection organization. It is shown that, on average, two-thirds of MRG and EIG events develop independently of one another, and one-third of the events overlap in space and time. This analysis also verifies that MRG and EIG cloudiness fields tend to propagate meridionally away from the equator. It is demonstrated that the lack of zonal propagation implied from the EOF analysis is likely due to the interference between eastward- and westward-propagating disturbances. In addition, it is shown that the westward-propagating circulation associated with the leading EOF is consistent with the expected theoretical behavior of an interference between MRGs and EIGs.

Abstract

The dynamics of convectively coupled equatorial waves (CCEWs) is analyzed in an idealized model of the large-scale atmospheric circulation. The model is composed of a linear rotating shallow-water system with a variable equivalent height, or equivalent gravity wave speed, which varies in space. This model is based on the hypothesis that moist convection acts to remove convective instability, therefore modulating the equivalent height of a shallow-water system. Asymptotic solutions are derived in the case of a small perturbation around a constant coefficient, which is assumed to be a mean moist equivalent height derived from satellite observations. The first-order solutions correspond to the free normal modes of the linear shallow-water system and the second-order flow is derived solving a perturbation eigenvalue problem. The asymptotic solutions are documented in the case of a zonally varying equivalent height and for wavenumbers and frequencies that are consistent with observations of CCEWs. This analysis shows that the dynamics of the secondary divergence and its impact on the full divergence varies mode by mode. For instance, for a negative equivalent height anomaly, which is interpreted as a moister background, the secondary divergence is nearly in phase with the primary divergence in the case of Kelvin waves—in contrast to mixed Rossby–gravity waves where the secondary divergence acts to attenuate the primary divergence. While highly idealized, the modeled waves share some features with observations, providing a mechanism for the relationship between CCEWs phase speed, amplitude, and horizontal structure.

Abstract

The relationship between n = 0 mixed Rossby–gravity waves (MRGs) and eastward inertio-gravity waves (EIGs) from Matsuno’s shallow-water theory on an equatorial beta plane is studied using statistics of satellite brightness temperature T _b and dynamical fields from ERA-Interim data. Unlike other observed convectively coupled equatorial waves, which have spectral signals well separated into eastward and westward modes, there is a continuum of MRG–EIG power standing above the background that peaks near wavenumber 0. This continuum is also present in the signals of dry stratospheric MRGs. While hundreds of papers have been written on MRGs, very little work on EIGs has appeared in the literature to date. The authors attribute this to the fact that EIG circulations are much weaker than those of MRGs for a given amount of divergence, making them more difficult to observe even though they strongly modulate convection.

Empirical orthogonal function (EOF) and cross-spectral analysis of 2–6-day-filtered T _b isolate zonally standing modes of synoptic-scale convection originally identified by Wallace in 1971. These display antisymmetric T _b signals about the equator that propagate poleward with a period of around 4 days, along with westward-propagating MRG-like circulations that move through the T _b patterns. Further analysis here and in Part II shows that these signatures are not artifacts of the EOF approach but result from a mixture of MRG or EIG modes occurring either in isolation or at the same time.

Abstract

The organization of tropical convection is assessed through an object-based analysis of satellite brightness temperature data T_b , a proxy for convective activity. The analysis involves the detection of contiguous cloud regions (CCRs) in the three-dimensional space of latitude, longitude, and time where T_b falls below a given threshold. A range of thresholds is considered and only CCRs that satisfy a minimum size constraint are retained in the analysis. Various statistical properties of CCRs are documented including their zonal speed of propagation, which is estimated using a Radon transformation technique. Consistent with previous studies, a majority of CCRs are found to propagate westward, typically at speeds of around 15 m s⁻¹, regardless of underlying T_b threshold. Most of these zonally propagating CCRs have lifetimes less than 2 days and zonal widths less than 800 km, implying aggregation of just a few individual mesoscale convective systems. This object-based perspective is somewhat different than that obtained in previous Fourier-based analyses, which primarily emphasize the organization of convection on synoptic and planetary scales via wave–convection coupling. To reconcile these contrasting views, an object-based data reconstruction is developed that objectively demonstrates how the spectral peaks of synoptic- to planetary-scale waves can be attributed to the organization of CCRs into larger-scale wave envelopes. A novel method based on the randomization of CCRs in physical space leads to an empirical background spectrum for organized tropical convection that does not rely on any smoothing in spectral space. Normalization by this background reveals spectral peaks associated with synoptic- and planetary-scale waves that are consistent with previous studies.

Abstract

Observational evidence of two extratropical pathways to forcing tropical convective disturbances is documented through a statistical analysis of satellite-derived OLR and ERA5 reanalysis. The forcing mechanism and the resulting disturbances are found to strongly depend on the structure of the background zonal wind. Although Rossby wave propagation is prohibited in easterlies, modeling studies have shown that extratropical forcing can still excite equatorial waves through resonance between the tropics and extratropics. Here this “remote” forcing pathway is investigated for the first time in the context of convectively coupled Kelvin waves over the tropical Pacific during northern summer. The extratropical forcing is manifested by eddy momentum flux convergence that arises when extratropical eddies propagate into the subtropics and encounter their critical line. This nonlinear forcing has similar wavenumbers and frequencies with Kelvin waves and excites them by projecting onto their meridional eigenstructure in zonal wind, as a form of resonance. This resonance is also evidenced by a momentum budget analysis, which reveals the nonlinear forcing term is essential for maintenance of the waves, while the remaining linear terms are essential for propagation. In contrast, the “local” pathway of extratropical forcing entails the presence of a westerly duct during northern winter that permits Rossby waves to propagate into the equatorial east Pacific, while precluding any sort of resonance with Kelvin waves due to Doppler shifting effects. The intruding disturbances primarily excite tropical “cloud plumes” through quasigeostrophic forcing, while maintaining their extratropical nature. This study demonstrates the multiple roles of the extratropics in forcing in tropical circulations and illuminates how tropical–extratropical interactions and extratropical basic states can provide be a source of predictability at the S2S time scale.