Extratropical circulation indices in the Southern Hemisphere based on station data

✍ Scribed by Jones, P.D.; Salinger, M.J.; Mullan, A.B.

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 251 KB
• Volume: 19
• Category: Article
• ISSN: 0899-8418
• DOI: 10.1002/(sici)1097-0088(199910)19:12<1301::aid-joc425>3.0.co;2-p

✦ Synopsis

Zonal and meridional pressure gradient indices of the Southern Hemisphere (SH) circulation are analysed in the mid-to-high (35-65°S) latitude zone. The dearth of land regions, and hence long pressure records, means that these are restricted to the southern South American and New Zealand sectors.

The Trans Polar Index (TPI) is the only large-scale station pressure based extratropical SH index that has been proposed, and is based on the normalised pressure difference between Hobart, Tasmania and Stanley, Falklands. This index is compared with variants which involve stations in the vicinity of New Zealand and southern South America. The index shows considerable year-to-year and some decadal-scale variability and is a measure of wavenumber 1 of the SH pressure field. Significant correlations (r: -0.3 to -0.5) occur between the TPI and southern South American temperatures in the austral summer and autumn seasons. Similar size correlations of the opposite sign occur in New Zealand but only in the austral summer season.

In New Zealand and southern South America, temperature series are strongly affected by the strength of the local meridional circulation (r values: -0.4 to -0.7 over New Zealand depending on season and period and values of : -0.2 to -0.4 for southern South America). In both regions there is no concomitant increase in northerly flow or decrease in southerly flow to explain the long-term increase in temperatures. The relationships are mostly at the interannual rather than the decadal and longer timescales. The decadal temperature rise, therefore, reflects a general warming of the Southern Ocean, rather than decadal-scale variations in the circulation.