PNNL

The Science

Storm tracks in the Southern Hemisphere (SH) are characterized by a distinct ∼25-day periodicity in their variability. This study focuses on the Indo-western Pacific sector and depicts the regional storm track mode variability: envelopes of midlatitude waves arise in that region from the large baroclinic instability there and then grow to their full strength downstream. These waves travel eastward and reenter their birthplace in about 12.5 days after travelling around the globe. During their second reentry (i.e., ∼25 days later), these waves can get amplified by the warm phase of an ∼100-day oceanic oscillation that gives rise to the ∼25-day periodicity.

The Impact

The chaotic nature of the governing dynamics of midlatitude weather means they rarely exhibit periodic behavior. The periodic behavior in Indo-western Pacific sector storm activity revealed in this study can help researchers predict weather extremes downstream in southern Australia, the southern Australian seaboard, and Patagonia. This work also discovered an ~100-day oscillation in the SH midlatitude ocean temperature that warrants further theoretical explanation.

Summary

A turbulent eddying motion dominates the SH midlatitude ocean and atmosphere, making periodicity hard to come by. Researchers took a regional perspective to investigate the intriguing ∼25-day periodicity in the storm activity of the midlatitude Indo-western Pacific sector of the Southern Hemisphere and explored possible underlying mechanisms. Results show that the 25-day periodicity exists in zonal and regional averages of eddy activity as long as the selected domain is wide enough to accommodate a wave packet. The favorable domain of the ∼25 days periodicity is located downstream of the largest baroclinicity in midlatitudes. The wave activity developed there can circle the global several times with a typical group velocity of 25 m s−1 and reenter the domain every 12.5 days, setting the base periodicity. A slower oceanic oscillation in the same region, with a time scale of ∼100 days, appears to select and reinforce multiples of 12.5 days and gives rise to the pronounced ∼25-day periodicity in wave activity. In turn, the re-emergence of the wave activity 25 days later serves as the delayed-action mechanism for the 100-day periodicity in the oceanic oscillation. 

PNNL Contact

Ruby Leung, Pacific Northwest National Laboratory, Ruby.Leung@pnnl.gov 

Funding

All authors were supported by the U.S. Department of Energy’s Office of Science, Biological and Environmental Research program’s Regional and Global Model Analysis program area.