资源环境科技发展态势分析平台

Numerous observational studies have found that the Hadley cells are expanding poleward in both the Northern and Southern Hemispheres, and model results suggest that such expansion is likely to continue throughout this century as a result of global warming. This has led to concerns that the subtropical dry zones may also shift poleward. However, precipitation changes associated with Hadley cell width are zonally asymmetric-especially in the Northern Hemisphere-suggesting that a more regional focus may give a clearer picture of these changes. In this study, we consider the influence of the Northern Hemisphere subtropical highs on summertime precipitation in North America and contrast this with the influence of Hadley cell expansion. Specifically, we consider the North Pacific and North Atlantic subtropical highs (NPSH and NASH, respectively) and define, for each of these, three indices representing longitude, latitude, and strength. We find that short-term (monthly) precipitation variability over North America has comparable sensitivities to all six indices. However, for projections of 21st century climate change, North American summertime precipitation trends are driven more strongly by the longitudes of the highs than by their latitudes or strengths. Since the Hadley cells are defined with zonal-mean quantities, much of the impact of these subtropical high shifts cannot be captured by Hadley cell metrics.