Precipitation over semi-arid regions of North Hemisphere affected by Atlantic Multidecadal Oscillation

As a key variable in climate change over semi-arid regions, precipitation variation has received much attention from the public, but the local mechanisms involved are still not fully understood. The results presented in this study show as the Atlantic Multidecadal Oscillation (AMO) underwent a phase transition from the negative to a positive phase in the past 30 years, precipitation over the areas with annual precipitation below 400 mm decreased, while the areas with annual precipitation of more than 400 mm showed an increase of precipitation in the semi-arid regions. In the past two whole cycles of the AMO, during the positive phase, less precipitation was distributed in the semi-arid regions of North America, more precipitation in the semi-arid regions of North Africa, Iberian Peninsula, Asia Minor Peninsula, and India, while opposite change pattern was observed in the negative phase. Such precipitation patterns which depend on AMO are associated with the North Atlantic SST anomalies. In the positive phase of AMO, the elevated SST weakened the summer sea-level pressure over the Atlantic. Then, three cyclonic circulation anomalies appear over North Atlantic, North America, and eastern Pacific. The cyclonic circulation anomalies intersected over the semi-arid regions of North America and caused anomalous low-level divergence and the North Atlantic cyclonic circulation enhanced the African monsoon, even affecting the precipitation in the semi-arid regions around the Atlantic. In the negative phase, a roughly opposite flow field occurred. Besides, according to the prediction of the E3SM 1-1 model, the negative AMO will appear in the coming decade, the same AMO which contributed to the increased semi-arid precipitation over southern part of North America and the decreased semi-arid precipitation over North Africa.