The observed and modelled influence of the Madden-Julian Oscillation on East African rainfall

Abstract

ABSTRACT:The aim of this work is to determine to what extent precipitation modulated by the Madden-Julian Oscillation (MJO) over East Africa can be forecast by the operational global Met Office Unified Model (MetUM). Observed patterns of rainfall were analysed over Kenya, Tanzania and Uganda and used to validate MetUM forecasts made over the period [2005][2006][2007][2008][2009][2010][2011][2012]. It was found that there is a large seasonal dependence on the MJO for episodes of enhancement and suppression of rainfall over the inland highlands and the coastal lowlands, particularly from March to May and October to December, when the Intertropical Convergence Zone is located directly over the region. In phases 2-4 of the MJO lifecycle, there is an enhancement of precipitation over the highland regions and suppression over the coast. This dipole is reversed throughout phases 6-8. These findings corroborate previous studies undertaken over the region. The observed patterns were replicated well by the MetUM global model, even up to a forecast lead time of 5 days (T + 120 h), though some minor drift is apparent and convective and suppressed centres tend to stray from those of the observed rainfall. Model resolution is thought to be a key component of this difference. The systematic errors will likely improve with further plans for model resolution and physics upgrades, although the overall quality of the MetUM's ability to forecast the MJO over this region is sound.

The effect of westerlies on East African rainfall and the associated role of tropical cyclones and the Madden–Julian Oscillation

“…Compared to the average days in each phase (Figure e, grey dots), this number is particularly high. During these phases East Africa is known to experience enhanced rainfall conditions over the highlands (Pohl and Camberlin, ; Hogan et al ., ). Furthermore, analysis by Hogan et al .…”

“…Furthermore, analysis by Hogan et al . () gives an indication that phase 3 tends to be accompanied by anomalous low‐level westerlies across East Africa, consistent with the Congo influence presented here. In addition, our work shows there were more days in the 2018 long rains, than in an average year, with an active MJO, i.e., there were more days with amplitude greater than 1.…”

Climate and climatic variability of rainfall over eastern Africa

“…Wet conditions tend to occur in the first half of the MJO cycle along the coast and in the second half of the MJO cycle in the highlands [ Hogan et al ., ]. The contrasting rainfall conditions found in the two regions for the opposite MJO phases are strongly correlated with the pressure gradient between the Indian and Atlantic Oceans [ Pohl and Camberlin , ].…”

Drivers of interannual variability of the East African “Long Rains”

“…They are therefore not directly comparable to the MJO composites by Pohl and Camberlin () or Hogan et al . (). In years with average MJO activity (positive and negative ω anomalies of average amplitude crossing East Africa) the magnitude of wet anomalies is smaller than during strong MJO years.…”

“…dynamical constraints on cloud top outflow and upper‐level shear: Collimore et al, ) would be challenging for models that use parametrized convection. The MJO has been linked to intraseasonal spells of enhanced or suppressed rainfall in East Africa, depending on MJO phase (Pohl and Camberlin, ; Hogan et al, ). These studies have found that intraseasonal spells tend to have an east–west spatial structure in which rainfall anomalies over coastal and highland regions are out of phase.…”