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George N. Kiladis

Abstract

Rossby wave activity propagating into the eastern tropical Pacific from the midlatitudes during northern winter is examined in some detail. These waves are associated with the intrusion of high potential vorticity air into low latitudes, and they modulate cloudiness, stability, and vertical motion in the vicinity of the ITCZ. In the upper troposphere and lower stratosphere the horizontal phase and group propagation of the wave activity are qualitatively like those of a nondivergent barotropic Rossby wave. As the waves move equatorward, they become more shallow and propagate upward into the stratosphere. The horizontal and vertical propagation is consistent with the tilts of the waves, the large-scale three-dimensional background flow, and with the signatures of momentum and heat fluxes associated with the wave activity.

In the lower troposphere, paired cyclonic anomalies on either side of the equator accompany the upper level wave activity to the west of the ITCZ cloudiness signal. These waves amplify following the peak in the ITCZ cloudiness and propagate westward along the equator. This suggests that the upper-level wave activity, and possibly the associated convective heating, can trigger the excitation of the equatorially trapped Rossby modes.

The transient wave activity appears to be a crucial component of the momentum balance of the eastern tropical Pacific circulation. There is substantial interannual variability in the wave activity, consistent with observed changes in the large-scale basic state associated with the Southern Oscillation.

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Juliana Dias and George N. Kiladis

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.

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Matthew Wheeler and George N. Kiladis

Abstract

A wavenumber-frequency spectrum analysis is performed for all longitudes in the domain 15°S–15°N using a long (∼18 years) twice-daily record of satellite-observed outgoing longwave radiation (OLR), a good proxy for deep tropical convection. The broad nature of the spectrum is red in both zonal wavenumber and frequency. By removing an estimated background spectrum, numerous statistically significant spectral peaks are isolated. Some of the peaks correspond quite well to the dispersion relations of the equatorially trapped wave modes of shallow water theory with implied equivalent depths in the range of 12–50 m. Cross-spectrum analysis with the satellite-based microwave sounding unit deep-layer temperature data shows that these spectral peaks in the OLR are “coupled” with this dynamical field. The equivalent depths of the convectively coupled waves are shallower than those typical of equatorial waves uncoupled with convection. Such a small equivalent depth is thought to be a result of the interaction between convection and the dynamics. The convectively coupled equatorial waves identified correspond to the Kelvin, n = 1 equatorial Rossby, mixed Rossby-gravity, n = 0 eastward inertio-gravity, n = 1 westward inertio-gravity (WIG), and n = 2 WIG waves. Additionally, the Madden–Julian oscillation and tropical depression-type disturbances are present in the OLR spectra. These latter two features are unlike the convectively coupled equatorial waves due to their location away from the equatorial wave dispersion curves in the wavenumber-frequency domain.

Extraction of the different convectively coupled disturbances in the time–longitude domain is performed by filtering the OLR dataset for very specific zonal wavenumbers and frequencies. The geographical distribution of the variance of these filtered data gives further evidence that some of the spectral peaks correspond to particular equatorial wave modes. The results have implications for the cumulus parameterization problem, for the excitation of equatorial waves in the lower stratosphere, and for extended-range forecasting in the Tropics.

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Stefan N. Tulich and George N. Kiladis

Abstract

The coupling between tropical convection and zonally propagating gravity waves is assessed through Fourier analysis of high-resolution (3-hourly, 0.5°) satellite rainfall data. Results show the familiar enhancement in power along the dispersion curves of equatorially trapped inertia–gravity waves with implied equivalent depths in the range 15–40 m (i.e., pure gravity wave speeds in the range 12–20 m s−1). Here, such wave signals are seen to extend all the way down to zonal wavelengths of around 500 km and periods of around 8 h, suggesting that convection–wave coupling may be important even in the context of mesoscale squall lines. This idea is supported by an objective wave-tracking algorithm, which shows that many previously studied squall lines, in addition to “2-day waves,” can be classified as convectively coupled inertia–gravity waves with the dispersion properties of shallow-water gravity waves. Most of these disturbances propagate westward at speeds faster than the background flow. To understand why, the Weather Research and Forecast (WRF) Model is used to perform some near-cloud-resolving simulations of convection on an equatorial beta plane. Results indicate that low-level easterly shear of the background zonal flow, as opposed to steering by any mean flow, is essential for explaining the observed westward-propagation bias.

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Adrian J. Matthews and George N. Kiladis

Abstract

Equatorward-propagating wave trains in the upper troposphere are observed to be associated with deep convection over the eastern tropical Pacific on the submonthly timescale during northern winter. The convection occurs in the regions of ascent and reduced static stability ahead of cyclonic anomalies in the wave train. In this study an atmospheric primitive equation model is used to examine the roles of the dry wave dynamics and the diabatic heating associated with the convection.

Many features of a dry integration initialized with a localized wave train in the African–Asian jet on a three-dimensional climatological basic state quantitatively agree with the observations, including the zonal wavenumber 6–7 scale of the waves, the time period of approximately 12 days, and the cross-equatorial Rossby wave propagation over the eastern Pacific. There is ascent and reduced static stability ahead of the cyclonic anomalies, consistent with the interpretation that the waves force the convection. The spatial scale of the waves appears to be set by the basic state; baroclinic growth upstream in the Asian jet favors waves with zonal wavenumber 6. On reaching the Pacific sector, lower-wavenumber components of the wave train are not refracted so strongly equatorward, while higher-wavenumber components are advected quickly along the Pacific jet before they can propagate equatorward. Once over the Pacific, the wave train approximately obeys barotropic Rossby wave dynamics.

The observed lower-tropospheric anomalies include an equatorial Rossby wave that propagates westward from the region of cross-equatorial wave propagation and tropical convection. However, this equatorial Rossby wave is not forced directly by the dry equatorward-propagating wave train but appears in a separate integration as a forced response to the observed diabatic heating associated with the tropical convection.

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Flore Mounier, Serge Janicot, and George N. Kiladis

Abstract

This paper presents an investigation of the mechanisms giving rise to the main intraseasonal mode of convection in the African monsoon during northern summer, here identified as the quasi-biweekly zonal dipole (QBZD). The QBZD is primarily characterized by a quasi-stationary zonal dipole of convection whose dimension is larger than the West African monsoon domain, with its two poles centered along the Guinean coast and between 30° and 60°W in the equatorial Atlantic. The QBZD dynamical processes within the Atlantic–Africa domain are examined in some detail. The QBZD has a dipole pattern associated with a Walker-type circulation in the near-equatorial zonal plane. It is controlled both by equatorial atmospheric dynamics through a Kelvin wave–like disturbance propagating eastward between its two poles and by land surface processes over Africa, inducing combined fluctuations in surface temperatures, surface pressure, and low-level zonal winds off the coast of West Africa. When convection is at a minimum over central and West Africa, a lack of cloud cover results in higher net shortwave flux at the surface, which increases surface temperatures and lowers surface pressures. This creates an east–west pressure gradient at the latitude of both the ITCZ (10°N) and the Saharan heat low (20°N), leading to an increase in eastward moisture advection inland. The arrival from the Atlantic of the positive pressure signal associated with a Kelvin wave pattern amplifies the low-level westerly wind component and the moisture advection inland, leading to an increase in convective activity over central and West Africa. Then the opposite phase of the dipole develops. Propagation of the QBZD convective envelope and of the associated 200 high-level velocity potential anomalies is detected from the eastern Pacific to the Indian Ocean. When the effect of the Kelvin wave propagation is removed by filtering, the stationary character of the QBZD is highlighted. The impact of the QBZD in combination with a Kelvin wave is illustrated by a case study of the monsoon onset in 1984.

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Katherine H. Straub and George N. Kiladis

Abstract

Observations of the horizontal and vertical structure of convectively coupled Kelvin waves are presented and are compared with the predicted structures of moist Kelvin (or gravity) waves in three simple models of coupled wave instability: wave–conditional instability of the second kind (CISK), wind-induced surface heat exchange (WISHE), and stratiform instability. The observations are based on a linear regression analysis of multiple years of ECMWF reanalysis and station radiosonde data. Results suggest that both the wave-CISK and stratiform instability theories successfully predict many important features of observed moist Kelvin waves, but that unrealistic aspects of these models limit their ability to provide comprehensive explanations for the dynamics of these waves. It is suggested that an essential component of any theory for moist Kelvin waves is the second baroclinic mode heat source associated with stratiform precipitation.

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George N. Kiladis and Harry van Loon

Abstract

Composite surface pressure, temperature, and precipitation anomalies are mapped over the Indian and Pacific sectors during the various stages of Warm and Cold Events in the Southern Oscillation. In the year before the development of positive sea surface temperature anomalies in the central and eastern equatorial Pacific (Year–1 of a Warm Event), a strong South Pacific High is associated with below normal surface pressure over Australia and the Indian Ocean. This occurs concurrently with a poleward displacement of the Pacific convergence zones, with above normal air temperature and precipitation over the subtropical Pacific, and opposite conditions along the equator. By the next year (Year 0) of the Warm Event, thew anomalies have the opposite sign. The sequence of anomalies during a Cold Event is inverse to that during a Warm Event but otherwise the anomaly patterns are remarkably similar.

It appears that enhanced convection and low surface pressure within the Pacific convergence zones contribute to the observed westerly wind anomalies in the western equatorial Pacific at the end of Year–1, which are in turn tied to the onset of above normal equatorial SST in the following year. The observed reversal in atmospheric anomalies over the Indian and Pacific oceans daring Warm Events is an extreme manifestation of a general biennial tendency in these anomalies, with Cold Events occupying the opposite extreme.

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Katherine H. Straub and George N. Kiladis

Abstract

A case study of a convectively coupled Kelvin wave in the eastern Pacific intertropical convergence zone (ITCZ) is presented, as observed during the 1997 Pan American Climate Studies (PACS) Tropical Eastern Pacific Process Study (TEPPS). The large-scale convective envelope associated with this disturbance, with a zonal scale of approximately 1000–2000 km, propagates eastward at 15 m s−1 along the mean convective axis of the ITCZ. This envelope consists of many smaller-scale, westward-moving convective elements, with zonal scales on the order of 100–500 km.

As the convectively coupled Kelvin wave disturbance propagates eastward, it exerts a strong control on local convection. Radar and vertical profiler data collected aboard the NOAA R/V Ronald H. Brown during the wave passage show that convection deepens rapidly as the Kelvin wave approaches from the west, progressing from isolated, shallow cumuli to organized deep convective features within just 12 h. Initially, rainfall in the vicinity of the ship consists of a significant deep convective fraction, but as the large-scale envelope departs to the east, stratiform precipitation becomes dominant.

Radiosonde data collected during the Kelvin wave passage reveal dynamical perturbations in the troposphere and lower stratosphere that are consistent with linear equatorial Kelvin wave theory. The TEPPS radiosonde data also compare remarkably well with the vertical structure of a typical eastern Pacific Kelvin wave disturbance in the ECMWF reanalysis dataset, based on a 15-yr linear regression analysis. When this analysis is expanded to include all global grid points, it is shown that Kelvin waves in the eastern Pacific ITCZ have a dynamical structure that is nearly symmetric with respect to the equator, as would be expected based on linear Kelvin wave theory. However, the convective signal associated with these symmetric dynamical perturbations is itself primarily asymmetric with respect to the equator. The deepest convection is located significantly to the north of the equator, in the region of warmest sea surface temperatures. These observations present a somewhat different perspective on the dynamics of convectively coupled Kelvin waves, in that the symmetric dynamical fields and asymmetric convection interact to sustain the simultaneous eastward propagation of both fields.

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Patrick T. Haertel and George N. Kiladis

Abstract

The dynamics of the 2-day wave, a type of convectively coupled disturbance that frequents the equatorial western Pacific, is examined using observations and a linear primitive equation model. A statistical composite of the wave's kinematic and thermodynamic structure is presented. It is shown that 1) the wave's wind and temperature perturbations can be modeled as linear responses to convective heating and cooling, and 2) the bulk of the wave's dynamical and convective structure can be represented with two vertical modes. The observations and model results suggest that the 2-day wave is an n = 1 westward-propagating inertio–gravity wave with a shallow equivalent depth (14 m) that results from the partial cancelation of adiabatic temperature changes due to vertical motion by convective heating and cooling.

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