Allison Brannan-June 28

African Easterly Wave Tracks as Simulated by CMIP5 Models

African Easterly Waves (AEWs) impact Sahel rainfall and serve as trigger disturbances for tropical cyclones (TCs) in the North Atlantic Ocean. Currently, there is no agreement regarding the projected TC trends for the future. So far, there has been very little investigation into the changes in intensity and location of AEWs from year to year and how these changes would impact the TC activity of the Atlantic Basin and rainfall across the Sahel. Gaining a more detailed understanding of AEW behavior within global climate models will reduce the uncertainty surrounding these TC precursors, potentially yielding higher confidence in future Sahel rainfall and TC trends.

This study uses a Hovmöller tracking technique to identify and track AEWs at 850 hPa. The magnitude, frequency, location, and speed of the AEWs is compared between reanalysis products (e.g. JRA-55, CFSR, ERA-Interim) and the coupled historical simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) to establish historical biases. Results show the resolution of the models is more important in reducing biases in the northern track AEWs compared to the southern. The tracking technique is then applied to future simulations (Representative Concentration Pathway 8.5) within the CMIP5 experiments. It is found that the CMIP5 models project an average increase of 1.5 AEWs per year in the northern track, including an increase in higher intensity waves. The signals from the southern track AEWs in the future simulations are more varied compared to the northern track models, consistently resulting in a disagreement regarding future changes. However, there is indication of a shift in seasonality of southern track AEWs towards the later summer months, which may impact the timing of the Atlantic TC season. A 3D tracking technique will be included in future work in order to definitively determine whether the CMIP5 models are able to simulate two distinct AEW tracks or whether they are merged into one.