The South Indian Convergence Zone and Interannual Rainfall Variability over Southern Africa

Cook, Kerry H.

doi:10.1175/1520-0442(2000)013<3789:tsicza>2.0.co;2

Cited by 186 publications

(166 citation statements)

References 24 publications

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“…9, but for the composites of (a, c, e, g) four La Niña and (b, d, f, h) four El Niño during the period of 1982. Here, 1984/1985, 1988/1989/2000(1982/1983, 1986/1987, 1991/1992/1998) are defined as La Niña (El Niño) years following Ratnam et al (2013a). …”

Section: Discussionmentioning

confidence: 99%

“…In La Niña years, cloud bands related to the South Indian Convergence Zone tend to be preferentially located over southern Africa, resulting in higher precipitation. On the other hand, the cloud bands tend to move northeastward to Madagascar in El Niño years, leading to dry conditions in southern Africa (e.g., Cook 2000;Hart et al 2010Hart et al , 2012. However, ENSO influences are neither simple nor exclusive.…”

Section: Introductionmentioning

confidence: 99%

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Dynamical seasonal prediction of Southern African summer precipitation

et al. 2013

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Prediction skills for southern African (16˚-33˚E, 22˚-35˚S) summer precipitation in the Scale Interaction Experiment-Frontier coupled model is assessed for the period of 1982-2008. Using three different observation datasets, deterministic forecasts are evaluated by anomaly correlation coefficients, whereas scores of relative operating characteristic and relative operating level are used to evaluate probabilistic forecasts. It is shown that these scores for forecasts of December-February precipitation initialized on October 1 st are significant at 95%confidence level. On a local scale, the prediction skills in the northwestern and central parts of southern Africa are higher than those in northeastern South Africa. El Niño/Southern Oscillation (ENSO) provides the major source of predictability, but the relationship with ENSO is over-confident in the model. Also, the Benguela Niño, the basin mode in the tropical Indian Ocean, the subtropical dipole modes in the South Atlantic and the southern IndianOceans and ENSO Modoki may provide additional sources of predictability. When prediction skills are evaluated for the whole wet season from October to the following April, it is found that precipitation anomalies in December-February are most predictable. The present study presents promising results for seasonal prediction of precipitation anomalies in the extratropics, where seasonal forecast are considered a difficult task.3

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Dynamical seasonal prediction of Southern African summer precipitation

et al. 2013

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“…These mechanisms are also likely to be relevant to the weaker diagonal convergence zones found in the other ocean basins: the South Atlantic Convergence Zone (SACZ: Pohl et al, 2009;Hart et al, 2010) and the South Indian Convergence Zone (SICZ: Cook, 2000).…”

Section: Discussionmentioning

confidence: 99%

A multiscale framework for the origin and variability of the South Pacific Convergence Zone

Matthews

2011

Quart J Royal Meteoro Soc

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The diagonal South Pacific Convergence Zone (SPCZ) is the major climatological precipitation feature over the Pacific region during the Northern Hemisphere winter. However, the basic mechanisms that control its structure and variability are only partly understood. Here, an analysis of the SPCZ is carried out in a multiscale framework. This identifies two modes that dominate: a (westward) shifted SPCZ and an enhanced SPCZ, which occur independently of each other. Within both modes, the primary mechanism for the initiation of precipitation is a transient synoptic wave propagating along the subtropical jet, which is then refracted by the basic state toward the westerly duct over the central equatorial Pacific. Individual vorticity centres in the wave become elongated, with a diagonal (northwest-southeast) tilt. Convection then occurs in a diagonal band in the poleward flow ahead of the cyclonic vorticity anomaly in the wave. However, latent heat release in the convection leads to upper-tropospheric divergence and anticyclonic vorticity forcing, which dissipates the wave, shutting off the convective forcing and stopping the precipitation. Hence, each individual wave or event only lasts a few days and contributes a discrete pulse of diagonally oriented precipitation to the region. The sum of these events leads to the diagonal climatological SPCZ. Event occurrence is a stochastic process, the probability of which is modified by lower-frequency variability of the basic state, including the Madden-Julian Oscillation (MJO) and El Niño-Southern Oscillation (ENSO). For example, during periods of enhanced convection over the eastern Indian Ocean to western Pacific (MJO phases 3-6 and La Niña) the westerly duct expands westwards, allowing synoptic waves to refract equatorwards earlier and increasing the probability of westward-shifted SPCZ events. Hence, both the existence and variability of the SPCZ depend fundamentally on scale interactions between dynamical processes on time-scales ranging from daily to interannual.

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“…Surprisingly, even though 2000/2001 was a La Nin˜a year, near drought conditions were observed over South Africa. Though the performance of GDE scheme is poor for most of the year, this scheme could simulate the dry condition over South Africa for the year 2000/2001. Similarly, the year 1997/1998/2003, 2004/2010 were associated with El Nin˜o conditions but South Africa experienced normal rainfall.…”

Section: Enso Responsementioning

confidence: 99%

Performance assessment of three convective parameterization schemes in WRF for downscaling summer rainfall over South Africa

et al. 2013

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Austral summer rainfall over the period 1991/1992 to 2010/2011 was dynamically downscaled by the weather research and forecasting (WRF) model at 9 km resolution for South Africa. Lateral boundary conditions for WRF were provided from the European Centre for medium-range weather (ECMWF) reanalysis (ERA) interim data. The model biases for the rainfall were eval-uated over the South Africa as a whole and its nine prov-inces separately by employing three different convective parameterization schemes, namely the (1) Kain-Fritsch (KF), (2) Betts-Miller-Janjic (BMJ) and (3) Grell-Devenyi ensemble (GDE) schemes. All three schemes have gener-ated positive rainfall biases over South Africa, with the KF scheme producing the largest biases and mean absolute errors. Only the BMJ scheme could reproduce the intensity of rainfall anomalies, and also exhibited the highest cor-relation with observed interannual summer rainfall variability. In the KF scheme, a significantly high amount of moisture was transported from the tropics into South Africa. The vertical thermodynamic profiles show that the KF scheme has caused low level moisture convergence, due to the highly unstable atmosphere, and hence con-tributed to the widespread positive biases of rainfall. The negative bias in moisture, along with a stable atmosphere and negative biases of vertical velocity simulated by the GDE scheme resulted in negative rainfall biases, especially over the Limpopo Province. In terms of rain rate, the KF scheme generated the lowest number of low rain rates and the maximum number of moderate to high rain rates associated with more convective unstable environment. KF and GDE schemes overestimated the convective rain and underestimated the stratiform rain. However, the simulated convective and stratiform rain with BMJ scheme is in more agreement with the observations. This study also docu-ments the performance of regional model in downscaling the large scale climate mode such as El Nin˜o Southern Oscillation (ENSO) and subtropical dipole modes. The correlations between the simulated area averaged rainfalls over South Africa and Nino3.4 index were -0.66, -0.69 and -0.49 with KF, BMJ and GDE scheme respectively as compared to the observed correlation of -0.57. The model could reproduce the observed ENSO-South Africa rainfall relationship and could successfully simulate three wet (dry) years that are associated with La Nin˜a (El Ni˜no) and the BMJ scheme is closest to the observed variability. Also, the model showed good skill in simulating the excess rainfall over South Africa that is associated with positive sub-tropical Indian Ocean Dipole for the DJF season 2005/2006.

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The South Indian Convergence Zone and Interannual Rainfall Variability over Southern Africa

Cited by 186 publications

References 24 publications

Dynamical seasonal prediction of Southern African summer precipitation

Dynamical seasonal prediction of Southern African summer precipitation

A multiscale framework for the origin and variability of the South Pacific Convergence Zone

Performance assessment of three convective parameterization schemes in WRF for downscaling summer rainfall over South Africa

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