Sensitivity of tropical cyclones in the Southwest Indian Ocean to the topography of Madagascar

Christensen, Zachary

Sensitivity of tropical cyclones in the Southwest Indian Ocean to the topography of Madagascar

Thesis / Dissertation

2023

Abstract

In this study, simulations from the Weather Research and Forecasting (WRF) model are used to investigate the sensitivity of tropical cyclone (TC) characteristics in the Southwest Indian Ocean (SWIO) to the presence of Madagascar topography. This is achieved through running two sensitivity experiments where the topography is i) flattened to a uniform plateau of 300 m height (FLAT run) or ii) ocean grid points completely replace Madagascar with sea surface temperatures (SST) interpolated from the neighboring South Indian Ocean (SEA). These are then compared to the control run (CNTRL), where the topography is unchanged. Each run receives the same boundary conditions at the surface and lateral boundaries from the Climate Forecast System run compares reasonably well with the modern European Centre for Medium-Range Weather Forecasts reanalysis data (ERA5). The model outputs are then run through the TRACK-1.5.2 program to identify and track TCs. It is found that the TC characteristics in CNTRL are reasonable when compared to those found in observations, which are provided by the International Best Track Archive for Climate Stewardship. Based on WRF being able to adequately represent the track and regional atmospheric circulation, the analysis of TC genesis and tracks in the three WRF runs indicates that these weather systems are sensitive to the Madagascan mountains. Each sensitivity run showed cyclonic high-pressure anomalies across southern Madagascar and enhanced low-level easterly winds, possibly from an enhanced South Indian Ocean high pressure system, causing TCs to track further westward into Mozambique when the mountains are either flattened or removed completely. Additionally, the SEA shows considerably more TCs generated in the far west of the basin, which are stronger than CNTRL and tend to follow a more zonal track towards and onto the mainland. These differences relate to Madagascar being replaced by warm SST with increased latent heat release. The results suggest that the island of Madagascar plays an important role in the tracks, intensity, and numbers of TCs found in the real world in the Mozambique Channel (MC) region.

Keywords

Oceanography

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