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Adrian J. Matthews

Abstract

The intraseasonal variability over Africa during northern summer was analyzed, using 25 years of NCEP– NCAR reanalysis and satellite data. The dominant pattern of variability was one of enhanced deep convection over the whole African monsoon region. It appeared to arise at least partly as a remote response to the intraseasonal (Madden–Julian) oscillation over the warm pool sector. Twenty days prior to the maximum in convection over Africa, there was no signal over Africa but convection was reduced over the equatorial warm pool. An equatorial Kelvin wave response to this change in warm pool convection propagated eastward and an equatorial Rossby wave response propagated westward and between them they completed a circuit of the equator and met up 20 days later over Africa, where the negative midtropospheric temperature anomalies in the Kelvin and Rossby waves favored deep convection. Over West Africa, the Kelvin wave component contained lower-tropospheric westerly anomalies that acted to increase the boundary layer monsoon flow and moisture supply. The westerly anomalies also increased the cyclonic shear on the equatorward flank of the African easterly jet, leading to enhanced African easterly wave and transient convective activity, which then contributed to the enhanced convection over Africa on the longer intraseasonal time scale. The implications of this intraseasonal mode for predictability over Africa are discussed.

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Adrian J. Matthews and Jason Lander

Abstract

“Nonlinear Kelvin wave-CISK modes” are critically assessed as a possible mechanism for the Madden–Julian Oscillation (MJO) with a global spectral transform model and a one-dimensional analog. Convection is parameterized using a simple “positive-only CISK” scheme, where tropospheric diabatic heating is proportional to the low-level convergence, and is set to zero in regions of low-level divergence. Although the modes have many properties that are consistent with the MJO, they also have a serious drawback. The growth rate of unstable modes depends crucially on the width of the heating region, which is resolution dependent. The “CISK catastrophe” has not been averted, and the heating region collapses to the smallest localized scale that the model can support. This scale is larger than the model resolution, as measured by both the gridpoint scale and the inverse wavenumber or half-zonal-wavelength of the highest wavenumber basis function, and is associated with the appearance of negative Gibbs fringes, which are then cut off by the positive-only CISK parameterization.

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Benjamin Pohl and Adrian J. Matthews

Abstract

The Madden–Julian oscillation (MJO) is analyzed using the reanalysis zonal wind– and satellite outgoing longwave radiation–based indices of Wheeler and Hendon for the 1974–2005 period. The average lifetime of the MJO events varies with season (36 days for events whose central date occurs in December, and 48 days for events in September). The lifetime of the MJO in the equinoctial seasons (March–May and October–December) is also dependent on the state of El Niño–Southern Oscillation (ENSO). During October–December it is only 32 days under El Niño conditions, increasing to 48 days under La Niña conditions, with similar values in northern spring. This difference is due to faster eastward propagation of the MJO convective anomalies through the Maritime Continent and western Pacific during El Niño, consistent with theoretical arguments concerning equatorial wave speeds.

The analysis is extended back to 1950 by using an alternative definition of the MJO based on just the zonal wind component of the Wheeler and Hendon indices. A rupture in the amplitude of the MJO is found in 1975, which is at the same time as the well-known rupture in the ENSO time series that has been associated with the Pacific decadal oscillation. The mean amplitude of the MJO is 16% larger in the postrupture (1976–2005) compared to the prerupture (1950–75) period. Before the 1975 rupture, the amplitude of the MJO is maximum (minimum) under El Niño (La Niña) conditions during northern winter, and minimum (maximum) under El Niño (La Niña) conditions during northern summer. After the rupture, this relationship disappears. When the MJO–ENSO relationship is analyzed using all-year-round data, or a shorter dataset (as in some previous studies), no relationship is found.

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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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Sally L. Lavender and Adrian J. Matthews

Abstract

Observations show that rainfall over West Africa is influenced by the Madden–Julian oscillation (MJO). A number of mechanisms have been suggested: 1) forcing by equatorial waves; 2) enhanced monsoon moisture supply; and 3) increased African easterly wave (AEW) activity. However, previous observational studies are not able to unambiguously distinguish between cause and effect. Carefully designed model experiments are used to assess these mechanisms. Intraseasonal convective anomalies over West Africa during the summer monsoon season are simulated in an atmosphere-only global circulation model as a response to imposed sea surface temperature (SST) anomalies associated with the MJO over the equatorial warm pool region. 1) Negative SST anomalies stabilize the atmosphere leading to locally reduced convection. The reduced convection leads to negative midtropospheric latent heating anomalies that force dry equatorial waves. These waves propagate eastward (Kelvin wave) and westward (Rossby wave), reaching Africa approximately 10 days later. The associated negative temperature anomalies act to destabilize the atmosphere, resulting in enhanced monsoon convection over West and central Africa. The Rossby waves are found to be the most important component, with associated westward-propagating convective anomalies over West Africa. The eastward-propagating equatorial Kelvin wave also efficiently triggers convection over the eastern Pacific and Central America, consistent with observations. 2) An increase in boundary layer moisture is found to occur as a result of the forced convective anomalies over West Africa rather than a cause. 3) Increased shear on the African easterly jet, leading to increased AEW activity, is also found to occur as a result of the forced convective anomalies in the model.

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Adrian J. Matthews and Roland A. Madden

Abstract

The structure of the 33-h Kelvin wave, a normal mode of the atmosphere, is examined in 6-hourly station and NCEP–NCAR reanalysis data. Cross-spectral analysis of 6 yr (1993–98) of tropical station pressure data shows a peak in coherence in a narrow frequency band centered near 0.74 cycles per day, corresponding to a period of approximately 33 h. The phase angles are consistent with an eastward-propagating zonal-wavenumber-1 structure, implying an equatorial phase speed of approximately 340 m s−1. The global structure of the mode is revealed by empirical orthogonal function and regression analysis of 31 yr (1968–98) of reanalysis data. The horizontal structure shows a zonal-wavenumber-1 equatorial Kelvin wave with an equatorial trapping scale of approximately 34° lat. The vertical structure has zero phase change. The amplitude of the wave is approximately constant in the troposphere with an equatorial geopotential height perturbation of 0.9 m, and then increases exponentially with height in the stratosphere. Cross-spectral analysis between the station and reanalysis data shows that the results from the two datasets are consistent. No evidence can be found for forcing of the wave by deep tropical convection, which is is examined using a twice-daily outgoing longwave radiation dataset.

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Adrian J. Matthews and George N. Kiladis

Abstract

The interannual variability of transient waves and convection over the central and eastern Pacific is examined using 30 northern winters of NCEP–NCAR reanalyses (1968/69–1997/98) and satellite outgoing longwave radiation data starting in 1974. There is a clear signal associated with the El Niño–Southern Oscillation, such that differences in the seasonal-mean basic state lead to statistically significant changes in the behavior of the transients and convection (with periods less than 30 days), which then feed back onto the basic state.

During a warm event (El Niño phase), the Northern Hemisphere subtropical jet is strengthened over the central Pacific; the region of upper-tropospheric mean easterlies over the tropical western Pacific expands eastward past the date line, and the upper-tropospheric mean “westerly duct” over the tropical eastern Pacific is weakened. The transients tend to propagate along the almost continuous waveguide of the subtropical jet; equatorward propagation into the westerly duct is reduced. The transient convective events over the ITCZ typically observed to be associated with these equatorward-propagating waves are subsequently reduced both in number and magnitude, leading to a seasonal-mean net negative diabatic heating anomaly over the central Pacific from 10° to 20°N, which then feeds back onto the basic state. During a cold event (La Niña phase), the situation is reversed. The different propagation characteristics of the transients in El Niño and La Niña basic states are well simulated in initial value experiments with a primitive equation model.

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Adrian J. Matthews and George N. Kiladis

Abstract

The interaction between high-frequency transient disturbances and convection, and the Madden–Julian Oscillation (MJO), is investigated using NCEP–NCAR reanalysis and satellite outgoing longwave radiation data for 15 northern winters. During the phase of the MJO with enhanced convection over the East Indian Ocean and Indonesia, and suppressed convection over the South Pacific convergence zone, both the Asian–Pacific jet and the region of upper-tropospheric tropical easterlies over the warm pool are displaced westward. These changes in the basic state lead to a weaker or “leakier” waveguide in the Asian–Pacific jet, with a westward-displaced “forbidden” region of tropical easterlies, such that high-frequency transient waves propagate equatorward into the deep Tropics over the central Pacific near the date line. As these waves induce convection in the region of ascent and reduced static stability ahead of the upper-level cyclonic disturbances, there is an enhancement of high-frequency convective variability over the central Pacific intertropical convergence zone during this phase of the MJO. This enhanced high-frequency convective variability appears to project back onto intraseasonal timescales and forms an integral part of the slowly varying diabatic heating field of the MJO. In the opposite phase of the MJO, the Asian–Pacific jet is extended eastward and there is an almost continuous waveguide across the Pacific. Together with the expanded forbidden region of tropical easterlies over the warm pool, this leads to a more zonal propagation of high-frequency transients along the waveguide with less equatorward propagation, and hence reduced high-frequency convective variability over the tropical central Pacific. There is also evidence of high-frequency waves propagating into the Indian Ocean region at the beginning of the MJO cycle, which may be important in the initiation of intraseasonal convective anomalies there.

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Jonty D. Hall, Adrian J. Matthews, and David J. Karoly

Abstract

The observed relationship between tropical cyclone activity in the Australian region and the Madden–Julian oscillation (MJO) has been examined using 20 yr of outgoing longwave radiation, NCEP–NCAR reanalysis, and best track tropical cyclone data. The MJO strongly modulates the climatological pattern of cyclogenesis in the Australian region, where significantly more (fewer) cyclones form in the active (inactive) phase of the MJO. This modulation is more pronounced to the northwest of Australia. The relationship between tropical cyclone activity and the MJO was strengthened during El Niño periods. Variations of the large-scale dynamical conditions necessary for cyclogenesis were explored, and it was found that MJO-induced perturbations of these parameters correspond with the observed variation in cyclone activity. In particular, 850-hPa relative vorticity anomalies attributable to the MJO were found to be an excellent diagnostic of the changes in the large-scale cyclogenesis patterns.

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Barnaby S. Love, Adrian J. Matthews, and Gareth J. Janacek

Abstract

A simple guide to the new technique of empirical mode decomposition (EMD) in a meteorological–climate forecasting context is presented. A single application of EMD to a time series essentially acts as a local high-pass filter. Hence, successive applications can be used to produce a bandpass filter that is highly efficient at extracting a broadband signal such as the Madden–Julian oscillation (MJO). The basic EMD method is adapted to minimize end effects, such that it is suitable for use in real time. The EMD process is then used to efficiently extract the MJO signal from gridded time series of outgoing longwave radiation (OLR) data.

A range of statistical models from the general class of vector autoregressive moving average (VARMA) models was then tested for their suitability in forecasting the MJO signal, as isolated by the EMD. A VARMA (5, 1) model was selected and its parameters determined by a maximum likelihood method using 17 yr of OLR data from 1980 to 1996. Forecasts were then made on the remaining independent data from 1998 to 2004. These were made in real time, as only data up to the date the forecast was made were used. The median skill of forecasts was accurate (defined as an anomaly correlation above 0.6) at lead times up to 25 days.

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