Representation of the Mean Climate and Interannual Variability of Kiremt Rains over the Ethiopian Highlands within ICON AMIP Simulation

Abstract

While it is well known that the interannual variability of Kiremt (boreal summer) Rains in Ethiopia is forced by Sea Surface Temperature (SST) in the Pacific Ocean, the mechanisms for ENSO-Kiremt Rains teleconnections and the role of other oceans are not fully understood. In this study, the Ethiopian Kiremt Rains interannual variability was analyzed using observational data and higher-resolution SST-forced ICON experiments for the period 1981–2017. Such fine-grid global and two-way nests over the Greater Horn of Africa (GHA) were carried out here for the first time. The physical mechanisms that link ENSO influence on the Kiremt Rains in the model and ERA5 reanalysis are also investigated. It is found that the model reasonably simulates the main features of the JJAS rainfall climatology over GHA and also reproduces horizontal wind intensity and patterns at (150, 600, 850, and 925- hPa) levels over Africa. It is shown that there is a substantial skill in reproducing the leading modes of Kiremt Rains interannual variability (r = 0.64), given the SSTs are known. The results suggest that the majority (> 50%) of Kiremt Rains anomalies are driven by Equatorial Pacific SST variability, while the SST effects from other regions counteracted ENSO impact in the model. Consistent with previous studies, it is found that the El Niño phase of the ENSO drives a corresponding large-scale circulation anomaly, which weakens the monsoon trough over the Arabian Peninsula, and descending motion and upper-level convergence right over Ethiopia. The subsidence over the GHA region induces upper (lower) level westerly (easterly) wind anomalies over North Africa, weakening Tropical Easterly Jet, Somali Low-Level Jet, and reducing the moist westerly from Atlantic and Congo basin, and thus a reduction of Kiremt Rains over Ethiopia. The opposite pattern is considered under La Niña events and enhanced surface westerlies leading to wetter Kiremt Rains. This mechanism represents an anomalous Walker-type circulation for the ENSO-Kiremt Rains teleconnection. The results will have ramifications for climate model improvement and seasonal forecast improvement in Ethiopia and GHA.