Uncertainty in Future Projections of the North Pacific Subtropical High and its Implication for California Winter Precipitation Change
Several sources project that the contrast between wet and dry regions and between wet and dry seasons will likely increase in a warmer climate. Sometimes referred to as the “rich-get-richer” mechanism, it displays as more precipitation in the deep tropics and mid-latitudes but less in the subtropics.
This study points to a major source of uncertainty in the response of winter precipitation to global warming for the West Coast of North America: the atmosphere-ocean coupling in the North Pacific. As a result, jury is still out with regard to the future prospect of wintertime precipitation in California.
A team of researchers, including DOE scientists at Pacific Northwest National Laboratory, examined the future projections of sea level pressure change in the North Pacific and its impact on winter precipitation changes in California. The multi-model analysis, based on the Coupled Model Intercomparison Project phase 5 models (under the Representative Concentration Pathway 8.5 scenario), shows a robust sea-level pressure change in the late 21st century over the western North Pacific. In this analysis, both the Aleutian Low and the North Pacific subtropical high (NPSH) circulations shift poleward in concert with a widening of the Hadley cell. This change is partly explained by a systematic increase of static stability in the subtropics. In contrast, the projected NPSH changes over the eastern North Pacific exhibit a substantial inter-model spread, a reason for uncertain projections of precipitation changes in California. This inter-model spread in the eastern North Pacific is also associated with a Pacific Decadal Oscillation-like surface temperature change in the western North Pacific and the resulting meridional temperature gradient change.