Tropical Atlantic Vertical Wind Shear Variability in a Future Climate

Abstract

Simulations from a suite of 21 fully-coupled global climate models (GCMs) collected for the International Panel on Climate Change’s Fourth Assessment Report (IPCC-AR4) provide a unique opportunity to explore the effects of climate change on tropical cyclone (TC) activity. Vertical wind shear is a key environmental variable that has a detrimental effect on the genesis and intensification of TCs. Variability of shear in the Atlantic is influenced by changes in the large-scale background circulation, forced by teleconnections such as the El Nino-Southern Oscillation and the West African monsoon system.

Spatio-temporal variability of ENSO and West African (Sahel) rainfall in the 20th century is examined for the suite of GCMs. This serves as a basis to determine which models have the most reasonable simulations of the 20th century so that they can be used to make predictions about changes to Atlantic vertical wind shear in the 21st century under global warming conditions. Model simulations of the 20th century are compared to observations gathered from various datasets. The models exhibit a wide range of skill in simulating the various features that modulate shear in the tropical Atlantic. Several models have deficient simulations of ENSO and Sahel rainfall in their 20th century simulations. Five models are determined to have accurate simulations of the 20th century climate and will be most useful for making predictions about shear changes in the 21st century.

Long-term trends of July-September Sahel rainfall and tropical Atlantic shear under 21st century global warming conditions simulated by the GCMs are examined. There is a strong disagreement across the full suite of models as to the changes in shear and Sahel rainfall in the 21st century. However, four out of the five models determined to have the most accurate simulations of the 20th century climate predict a significant increase in shear in the tropical Atlantic. A statistical approach is used to investigate whether the dichotomy in shear trends in the tropical Atlantic is related to a similar split in the model projections for future rainfall trends in the Sahel. It is suggested that the spread in projections of future Sahel rainfall variability contributes significantly to the uncertainty in tropical Atlantic shear predictions.

Atlantic shear and Sahel rainfall are well-correlated and vary together on interannual timescales. We can deduce that Sahel rainfall will continue to be a useful predictor of seasonal TC activity into the 21st century. It appears that the 21st century shear trend is at least partially explained by changes in Sahel rainfall, especially in the eastern tropical Atlantic, closest to the monsoonal forcing in West Africa. However, the degree of association is unclear. It is speculated that other teleconnections, such as ENSO, are becoming more dominant in influencing the multidecadal variability of shear in the tropical Atlantic.