Understanding future changes in Northern Hemisphere winter meridional wind and regional hydroclimate

Abstract:

A critical aspect of human-induced climate change is how it will affect climatological mean and extreme precipitation around the world. To leading order, the increased ability of the atmosphere to hold moisture as it warms, intensifies moisture transports, making subtropical dry regions drier and tropical and mid- to high latitude wet regions wetter. But regionally, precipitation change will also depend on how the atmospheric circulation responds to warming.

During Northern Hemisphere winter, climate models predict a robust (in terms of structure) change in the stationary wave field that brings wetting southerlies to the west coast of North America and drying northerlies to interior southwest North America and the Mediterranean. Here, the underlying cause of this change is investigated. Stationary wave model experiments reaveal that the strengthening of the zonal wind in the sub-tropical upper troposphere is primarily responsible through it's influence on the character of intermediate scale stationary waves. A robust and easily understood model response to global warming is, therefore, the prime cause and it should be expected to happen to some degree.

However, there is considerable model diversity in the magnitude of the stationary wave response and almost 50% of the model spread over North America can be explained through this mechanistic understanding, allowing an improved estimate of where the real world will sit within the spectrum of model behavior. It is concluded that many models are likely overestimating the magnitude of this response and as a result, the wetting of the North American west coast and drying in the interior south west will likely be weaker than predicted by the majority of models and the multi-model mean. The implications of this for future predicted hydroclimate extremes will also be discussed.