Influence of the southwards shift of North American continent on North American monsoon

摘要

The North American continent, with its large terrain and being surrounded by oceans, does not provide the setup for a large-scale monsoon as we see in Asia. Here we examine the role of the southwards shift of the North American continent (140 degrees-50 degrees W, 20 degrees-80 degrees N) in determining North American monsoon (NAM) regions using the Community Atmosphere Model version 5.1. Results show that the meridional position of the North American continent plays a fundamental role in the existence of the NAM. When the North American continent is moved southwards to a certain latitude, most of the North American non-monsoon regions turn into monsoon regions, as strong and long-lasting surface sensible heating occurs from spring to summer, together with enhanced zonal land-sea thermal contrast. Thus, the low-level southerly wind and wind convergence are strengthened over the eastern side of the North American continent, resulting in a strong ascending motion in this region. On the other hand, the enhanced southerly wind and northwards equatorial airflow act to transport more moisture from the low-latitude oceans to the North American continent. As a result, the NAM circulation is strengthened, which is further maintained and developed via the "thermal adaptation" feedback.