Research brief: Natural variations shift rain-bearing winds south

CLEX researchers found the influence of climate change and the depletion in stratospheric ozone are the major drivers over the Atlantic Oceans that shift westerly winds further south. However, over the Pacific and Indian oceans natural variations induced by sea surface temperature changes in the tropical Pacific also play an important role.

Picture (above): Windy ocean waves. Credit: Verstappen Photography (Unsplash).

Climate change has caused the Southern Hemisphere’s summertime westerly winds to move south towards Antarctica. This has changed where rainfall bearing systems occur in the mid-latitudes.

While the influence of increasing greenhouse gases and depleting stratospheric ozone on this poleward movement is well-established, less is known about the role of natural variations within the climate system that can also cause shifts north or south.

CLEX researchers with colleagues from the National Center for Atmospheric Research used climate model simulations to isolate the influence of tropical sea surface temperatures on the position and strength of these westerly winds.

The study has found that the influence of climate change and the depletion in stratospheric ozone are the major drivers over the Atlantic Oceans that shift the westerly winds further south. However, over the Pacific and Indian oceans, natural variations induced by sea surface temperature changes in the tropical Pacific also play an important role in the southward shift and can intensify these winds in the southwest Pacific Ocean.

A major component of this natural variation is the Interdecadal Pacific Oscillation (IPO). This is a shift in the state of sea surface temperatures, air pressure and wind direction over decadal timescales. The study found that the state of the IPO in the early-21^st Century led to warming in the subtropics and mid-latitudes, which increased the generation of transient eddies at around 55°S, acting to strengthen and shift the winds south. Changes in the Southern Hemisphere westerly winds have profound impacts on Southern Hemisphere rainfall patterns, the Southern Ocean and Antarctica. Understanding the mechanisms that create these wind shifts has significant implications for prediction of Southern Hemisphere climate over the coming decades.

Paper: Yang, D., Arblaster, J. M., Meehl, G. A., England, M. H., Lim, E. P., Bates, S., & Rosenbloom, N. (2020). Role of tropical variability in driving decadal shifts in the Southern Hemisphere summertime eddy-driven jet. Journal of Climate, (2020). https://doi.org/10.1175/JCLI-D-19-0604.1