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Ángel F. Adames

Abstract

Column moisture and moist static energy (MSE) budgets have become common tools in the study of the processes responsible for the maintenance and evolution of the MJO. While many studies have shown that precipitation is spatially correlated with column moisture, these budgets do not directly describe the MJO-related precipitation anomalies. Other spatially varying fields may also play a role in determining the horizontal distribution of anomalous precipitation. In this study, an empirical precipitation anomaly field is derived that depends on three variables in addition to column moisture. These are the low-frequency distribution of precipitation, the low-frequency column saturation water vapor, and the sensitivity of precipitation to changes in column relative humidity. The addition of these fields improves upon moisture/MSE budgets by confining these anomalies to the climatologically rainy areas of the tropics, where MJO activity is strongest. The derived field adequately describes the MJO-related precipitation anomalies, comparing favorably with TRMM precipitation data.

Furthermore, a “precipitation budget” is presented that emphasizes moist processes over the regions where precipitation is most sensitive to free-tropospheric moisture. It is found that moistening from vertical moisture advection in association with regions of shallow ascent plays a central role in the propagation of the MJO. The overall contribution from this process is comparable to the contribution from horizontal moisture advection to propagation. Consistent with previous studies, it is found that vertical advection arising from longwave radiative heating maintains the intraseasonal precipitation anomalies against drying by horizontal moisture advection.

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Ángel F. Adames

Abstract

A linear two-layer model is used to elucidate the role of prognostic moisture on quasigeostrophic (QG) motions in the presence of a mean thermal wind (u¯T). Solutions to the basic equations reveal two instabilities that can explain the growth of moist QG systems. The well-documented baroclinic instability is characterized by growth at the synoptic scale (horizontal scale of ~1000 km) and systems that grow from this instability tilt against the shear. Moisture–vortex instability—an instability that occurs when moisture and lower-tropospheric vorticity exhibit an in-phase component—exists only when moisture is prognostic. The instability is also strongest at the synoptic scale, but systems that grow from it exhibit a vertically stacked structure. When moisture is prognostic and u¯T is easterly, baroclinic instability exhibits a pronounced weakening while moisture vortex instability is amplified. The strengthening of moisture–vortex instability at the expense of baroclinic instability is due to the baroclinic (u¯T) component of the lower-tropospheric flow. In westward-propagating systems, lower-tropospheric westerlies associated with an easterly u¯T advect anomalous moisture and the associated convection toward the low-level vortex. The advected convection causes the vertical structure of the wave to shift away from one that favors baroclinic instability to one that favors moisture–vortex instability. On the other hand, a westerly u¯T reinforces the phasing between moisture and vorticity necessary for baroclinic instability to occur. Based on these results, it is hypothesized that moisture–vortex instability is an important instability in humid regions of easterly u¯T such as the South Asian and West African monsoons.

Open access
Ángel F. Adames

Abstract

The weak temperature gradient (WTG) approximation is extended to the basic equations on a rotating plane. The circulation is decomposed into a diabatic component that satisfies WTG balance exactly and a deviation from this balance. Scale analysis of the decomposed basic equations reveals a spectrum of motions, including unbalanced inertio-gravity waves and several systems that are in approximate WTG balance. The balanced systems include equatorial moisture modes with features reminiscent of the MJO, off-equatorial moisture modes that resemble tropical depression disturbances, “mixed systems” in which temperature and moisture play comparable roles in their thermodynamics, and moist quasigeostrophic motions. In the balanced systems the deviation from WTG balance is quasi nondivergent, in nonlinear balance, and evolves in accordance to the vorticity equation. The evolution of the strictly balanced WTG circulation is in turn described by the divergence equation. WTG balance restricts the flow to evolve in the horizontal plane by making the isobars impermeable to vorticity and divergence, even in the presence of diabatically driven vertical motions. The vorticity and divergence equations form a closed system of equations when the irrotational circulation is in WTG balance and the nondivergent circulation is in nonlinear balance. The resulting “WTG equations” may elucidate how interactions between diabatic processes and the horizontal circulation shape slowly evolving tropical motions.

Significance Statement

Many gaps in our understanding of tropical weather systems still exist and there are still many opportunities to improve their forecasting. We seek to further our understanding of the tropics by extending a framework known as the “weak temperature gradient approximation” to all of the equations for atmospheric flow. Doing this reveals a variety of motions whose scales are similar to observed tropical weather systems. We also show that two equations describe the evolution of slow systems: one that describes tropical thunderstorms and one for the rotating horizontal winds. The two equations may help us understand the dynamics of slowly evolving tropical systems.

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Ángel F. Adames and Daehyun Kim

Abstract

A linear wave theory for the Madden–Julian oscillation (MJO), previously developed by Sobel and Maloney, is extended upon in this study. In this treatment, column moisture is the only prognostic variable and the horizontal wind is diagnosed as the forced Kelvin and Rossby wave responses to an equatorial heat source/sink. Unlike the original framework, the meridional and vertical structure of the basic equations is treated explicitly, and values of several key model parameters are adjusted, based on observations. A dispersion relation is derived that adequately describes the MJO’s signal in the wavenumber–frequency spectrum and defines the MJO as a dispersive equatorial moist wave with a westward group velocity. On the basis of linear regression analysis of satellite and reanalysis data, it is estimated that the MJO’s group velocity is ~40% as large as its phase speed. This dispersion is the result of the anomalous winds in the wave modulating the mean distribution of moisture such that the moisture anomaly propagates eastward while wave energy propagates westward. The moist wave grows through feedbacks involving moisture, clouds, and radiation and is damped by the advection of moisture associated with the Rossby wave. Additionally, a zonal wavenumber dependence is found in cloud–radiation feedbacks that cause growth to be strongest at planetary scales. These results suggest that this wavenumber dependence arises from the nonlocal nature of cloud–radiation feedbacks; that is, anomalous convection spreads upper-level clouds and reduces radiative cooling over an extensive area surrounding the anomalous precipitation.

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Ángel F. Adames and John M. Wallace

Abstract

The large-scale circulation features that determine the structure and evolution of MJO-related moisture and precipitation fields are examined using a linear analysis protocol based on daily 850- minus 150-hPa global velocity potential data. The analysis is augmented by a compositing procedure that emphasizes the structural features over the Indo-Pacific warm pool sector (60°E–180°) that give rise to the eastward propagation of the enhanced moisture and precipitation.

It is found that boundary layer (BL) convergence in the low-level easterlies to the east of the region of maximum ascent produces a deep but narrow plume of equatorial ascent that moistens the midtroposphere, while weakly diffluent flow above the BL spreads moisture away from the equator. Vertical advection of moisture from this plume of ascent accounts for the eastward propagation of the positive moisture anomalies across the Maritime Continent into the western Pacific. When the convection is first developing over the Indian Ocean, horizontal moisture advection contributes to both the eastward propagation and the amplification of the positive moisture anomalies along the equator to the east of the region of enhanced convection. Neither horizontal advection nor the net moistening from vertical advection and the apparent moisture sink exhibit significant westward tilt with height in the equatorial plane, but when they are superposed they explain the westward tilt of the moisture field. The strong spatial correlation between relative humidity and vertical velocity underscores the important role of equatorial wave dynamics in shaping the structure and evolution of the MJO.

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Ángel F. Adames and John M. Wallace

Abstract

The two leading principal components of the daily 850- minus 150-hPa global velocity potential in the Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) (1979–2011) data are used as time-varying Madden–Julian oscillation (MJO) indices. Regression maps and meridional cross sections based on these indices are used to document the structure and evolution of the zonal wind (u) and geopotential height (Z) anomalies in the MJO cycle. The data are daily, and they are not separated by season. At upper-tropospheric levels the MJO signature is dominated by eastward-propagating planetary wave packets consisting of equatorial Kelvin waves flanked by Rossby waves centered along 28°N/S, for which the westerly jet streams serve as waveguides. At lower-tropospheric levels the pattern more closely resembles the response to a pulsating heat source over the Maritime Continent, where the Andes block the eastward-propagating Kelvin wave pulse. The contrasting upper- and lower-tropospheric patterns are made up of the same building blocks: a deep, baroclinic modal structure with a node at the 400-hPa level, which dominates the tropical signature, and a barotropic residual field consisting mainly of extratropical wave trains oriented along great circles. The extratropical wave trains emanate from the flanking Rossby waves in the baroclinic modal structure. The strongest of them, which resembles the Pacific–North America (PNA) pattern, extracts kinetic energy from the climatological-mean flow in the jet exit region. At other longitudes the jet stream seems to act as a barrier to the poleward propagation of MJO-related wave activity.

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Ángel F. Adames and John M. Wallace

Abstract

The features in the planetary-scale wind field that shape the MJO-related vertical velocity field are examined using the linear analysis protocol based on the daily global velocity potential field described in a companion paper, augmented by a compositing procedure that yields a more robust and concise description of the prevalent patterns over the Indo-Pacific warm pool sector (60°E–180°). The analysis elucidates the structural elements of the planetary-scale wind field that give rise to the characteristic “swallowtail” shape of the region of enhanced rainfall and the “bottom up” evolution of the vertical velocity profile from one with a shallow peak on the eastern end of the region of enhanced rainfall to one with an elevated peak on the western end. These distinctive features of the vertical velocity field in the MJO reflect the juxtaposition of deep overturning circulation cells in the equatorial plane and much shallower frictionally driven cells in the meridional plane to the east and west of the regions of enhanced rainfall. The zonal overturning circulations determine the pattern of ∂u/∂x and the meridional overturning circulations determine the pattern of ∂υ/∂y in the divergence profiles. These features are at least qualitatively well represented by the Matsuno–Gill solution for the planetary wave response to a stationary equatorial heat source–sink dipole.

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Ángel F. Adames and John M. Wallace

Abstract

The linear atmospheric signature of ENSO, obtained by regressing fields of geopotential height Z, wind, vertical velocity, and rainfall upon the Niño-3.4 sea surface temperature (SST) index, is partitioned into zonally symmetric and eddy components. The zonally symmetric component is thermally forced by the narrowing and intensification of the zonally averaged equatorial rain belt during El Niño and mechanically forced by the weakening of the upper-tropospheric equatorial stationary waves and their associated flux of wave activity. The eddy component of the ENSO signature is decomposed into barotropic (BT) and baroclinic (BC) contributions, the latter into first and second modal structures BC1 and BC2, separable functions of space (x, y), and pressure p, using eigenvector analysis. BC1 exhibits a nearly equatorially symmetric planetary wave structure comprising three dumbbell-shaped features suggestive of equatorial Rossby waves, with out-of-phase wind and geopotential height perturbations in the upper and lower troposphere. BC1 and BT exhibit coincident centers of action. In regions of the tropics where the flow in the climatological-mean stationary waves is cyclonic, BT reinforces BC1, and vice versa, in accordance with vorticity balance considerations. BC1 and BT dominate the eddy ENSO signature in the free atmosphere. Most of the residual is captured by BC2, which exhibits a shallow, convergent boundary layer signature forced by the weakening of the equatorial cold tongue in SST. The anomalous boundary layer convergence drives a deep convection signature whose upper-tropospheric outflow is an integral part of the BC1 contribution to the ENSO signature.

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Victor C. Mayta and Ángel F. Adames

Abstract

The dynamical and thermodynamical features of Amazonian 2-day westward-propagating inertia–gravity (WIG) waves are examined. On the basis of a linear regression analysis of satellite brightness temperature and data from the 2014–15 Observations and Modeling of the Green Ocean Amazon (GoAmazon) field campaign, it is shown that Amazonian WIG waves exhibit structure and propagation characteristics consistent with the n = 1 WIG waves from shallow water theory. These WIG waves exhibit a pronounced seasonality, with peak activity occurring from March to May and a minimum occurring from June to September. Evidence is shown that mesoscale convective systems over the Amazon are frequently organized in 2-day WIG waves. Results suggest that many of the Amazonian WIG waves come from preexisting 2-day waves over the Atlantic, which slow down when coupled with the deeper, more intense convection over tropical South America. In contrast to WIG waves that occur over the ocean, Amazonian 2-day WIG waves exhibit a pronounced signature in surface temperature, moisture, and heat fluxes.

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Kuniaki Inoue, Ángel F. Adames, and Kazuaki Yasunaga

Abstract

A new diagnostic framework is developed and applied to ERA-Interim to quantitatively assess vertical velocity (omega) profiles in the wavenumber–frequency domain. Two quantities are defined with the first two EOF–PC pairs of omega profiles in the tropical ocean: a top-heaviness ratio and a tilt ratio. The top-heaviness and tilt ratios are defined, respectively, as the cospectrum and quadrature spectrum of PC1 and PC2 divided by the power spectrum of PC1. They represent how top-heavy an omega profile is at the convective maximum, and how much tilt omega profiles contain in the spatiotemporal evolution of a wave. The top-heaviness ratio reveals that omega profiles become more top-heavy as the time scale (spatial scale) becomes longer (larger). The MJO has the most top-heavy profile while the eastward inertio-gravity (EIG) and westward inertio-gravity (WIG) waves have the most bottom-heavy profiles. The tilt ratio reveals that the Kelvin, WIG, EIG, and mixed Rossby–gravity (MRG) waves, categorized as convectively coupled gravity waves, have significant tilt in the omega profiles, while the equatorial Rossby (ER) wave and MJO, categorized as slow-moving moisture modes, have less tilt. The gross moist stability (GMS), cloud–radiation feedback, and effective GMS were also computed for each wave. The MJO with the most top-heavy omega profile exhibits high GMS, but has negative effective GMS due to strong cloud–radiation feedbacks. Similarly, the ER wave also exhibits negative effective GMS with a top-heavy omega profile. These results may indicate that top-heavy omega profiles build up more moist static energy via strong cloud–radiation feedbacks, and as a result, are more preferable for the moisture mode instability.

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