Tropospheric Rossby wave breaking and the SAM

An objective analysis of tropospheric anticyclonic and cyclonic breaking Rossby waves is performed for the Southern Hemisphere in austral summer (December-February) of 1979-2009. The climatology of both anticyclonic and cyclonic Rossby wave breaking (RWB) frequency is presented. The time series of austral summer anticyclonic RWB occurrence has a trend that closely matches the trend in the SAM index over the three decades.

The frequency of anticyclonic RWB is highest in an extended region of the eastern hemisphere on the anticyclonic side of the jet, while that of cyclonic RWB is highest on the cyclonic side of the jet. A composite analysis shows how anticyclonic RWB contributes to a positive SAM index and cyclonic RWB contributes to a negative SAM index.

Regions of RWB that are significantly correlated with the SAM index are objectively determined. Even though several such regions are identified, only two regions (anticyclonic and cyclonic) covering 17% of the area of the hemisphere are required in a linear regression model of the SAM index. The anticyclonic RWB region is zonally extended at 45S and explains 78% of the variability of the summer-mean SAM index. The cyclonic region is located at high latitudes somewhat decoupled from the jet, in the longitudinal sector of the Indian Ocean. On synoptic time scales, transitions of the SAM index respond to RWB without time lag.

ENSO cycles present an interesting zonal asymmetry to the distribution of Southern Hemispheric RWB in the central Pacific. We find an increase in anticyclonic RWB in the tropical/subtropical central Pacific during La Nina compared to El Nino that is related to the strong local decrease in zonal wind. At the same time anticyclonic RWB outside the central Pacific is decreased in frequency equatorward of 42S and increased in frequency poleward of 42S, corresponding to a positive SAM index.