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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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Katherine H. Straub and George N. Kiladis

Abstract

Interactions between the convection and circulation fields of the boreal summer intraseasonal oscillation (ISO) and two types of higher-frequency tropical wave activity are examined through a statistical analysis of 22 yr of data. During the convectively active phase of the ISO, westward-propagating mixed Rossby–gravity (MRG)–tropical depression (TD)-type wave activity is enhanced within the low-frequency ISO convective envelope, and is strongly correlated with low-frequency 850-hPa westerly anomalies. At the same time, eastward-propagating convectively coupled Kelvin wave activity is enhanced well to the east of the active ISO convection, in the central Pacific.

A case study of an ISO event during July–September 1987 illustrates these statistically derived relationships. The enhanced phase of the ISO is shown to consist primarily of westward-propagating higher-frequency variability, including seven named tropical cyclones in the western Pacific, two of which project onto MRG–TD-type modes as they propagate westward across Southeast Asia into the Bay of Bengal. Successive eastward development of three tropical storms is suggested to be associated with an eastward dispersion of energy in the MRG–TD mode. Several Kelvin waves propagate across the Pacific to the east of the active ISO convective envelope.

Based on the statistical results and the 1987 case study, it is suggested that the high-frequency, westward-propagating MRG–TD disturbances and tropical cyclones may compose a significant portion of the low-frequency ISO signal. Eastward-propagating Kelvin wave variability, on the other hand, is more active outside the ISO convective envelope, to its east.

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George N. Kiladis and Klaus M. Weickmann

Abstract

Statistical evidence is presented to support the notion that tropical convection in the eastern Pacific and Atlantic intertropical convergence zone (ITCZ) during northern winter can be forced by disturbances originating in the extratropics. The synoptic-scale transients in these regions are characterized at upper levels by strong positive tilts in the horizontal and appear to induce vertical motions ahead of troughs as in midlatitude baroclinic systems. Two case studies of such interactions are examined, one for the eastern North Pacific ITCZ and another somewhat different type of interaction for the South Pacific convergence zone (SPCZ) over the western South Pacific.

Both cases are associated with upper-level troughs, strong cold advection deep into the tropics, and the formation of a frontal boundary at low levels. The ITCZ case is characterized by the advection of anomalously high isentropic potential vorticity air southward, a strong poleward flux of heat and westerly momentum, and the development of a subtropical jet downstream of the disturbance. The SPCZ disturbance is not strongly tilted, but is still accompanied by a strong poleward flux of heat and momentum. Evidence for the occurrence of cross-equatorial wave dispersion in the eastern Pacific during northern winter is also presented. These observations are consistent with theory and modeling of Rossby waves in a westerly basic state extending from the midlatitudes into the tropics.

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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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George N. Kiladis and Klaus M. Weickmann

Abstract

The relationship between deep tropical convection and large-scale atmospheric circulation in the 6–30-day period range is examined. Regression relationships between filtered outgoing longwave radiation at various locations in the Tropics and 200- and 850-mb circulation are mapped for the standard seasons, and the spatial structure and seasonal dependence of the results are interpreted in view of the basic-state circulation.

In regions where the convection is embedded in upper-level easterlies, anomalous equatorial easterly flow is typically present at 200 mb within and to the west of the convective signal, along with patterns of meridional outflow into subtropical anticyclonic perturbations. Lagged relationships suggest that the convection is forcing the circulation in many of these cases. The outflow and subtropical circulations are strongest into the winter hemisphere during the solstitial seasons, with more symmetric signals about the equator seen in the equinoctial seasons. The longitudinal positioning of the subtropical features with respect to the convection varies but is generally located due poleward or just to the east of the convection. There tends to be a first baroclinic mode vertical structure to these circulations, such that equatorial westerlies are present at 850 mb within the convection, with closed circulations on either side of the equator resembling equatorial Rossby modes especially common over the Atlantic and Pacific sectors.

As a contrast, in regions located within upper-level westerlies or along the margin of influence of upper westerly disturbances, convection appears to be forced by upper-level wave energy propagating into the deep Tropics, with the heating located in the upward motion region ahead of upper-level troughs. This occurs over the Atlantic and eastern Pacific sectors during northern winter and spring, and over Australia, the South Pacific, and South America during southern summer, when upper westerlies are at relatively low latitudes where they can interact with deep tropical convection. The results confirm theoretical and modeling ideas that suggest that Rossby wave energy is able to propagate into the deep Tropics in regions where upper-level westerlies exist in the Tropics.

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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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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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