ENSO Teleconnection Patterns Modulate Australian Extratropical Cyclones

Abstract. While the effects of the El Niño-Southern Oscillation (ENSO) on Australian rainfall are well known, its modulation of the main rain-producing weather system—extratropical cyclones—is still not well understood. In particular, there is a lack of direct evidence for ENSO-related changes in cyclone day frequency around Australia because high internal variability makes it hard to detect the signal when calculating cyclone frequency under conventional seasonal windows. Given that ENSO influences the extratropics primarily through atmospheric teleconnections, its impact on extratropical cyclones should be detectable on days when teleconnections are well established. Focusing on the austral spring, when the strongest response to ENSO in the southern extratropics is exhibited, we identify persistent teleconnection pattern periods by projecting daily low-frequency circulation fields onto ENSO-composited teleconnection patterns. We then use these days as a sample space to calculate cyclone day frequency. Under this conditioning, we reveal that ENSO teleconnection patterns modulate Southern Hemisphere cyclone activity, with particular signals emerging over the Tasman Sea, where cyclone frequency increases (decreases) up to about 10 % during La Niña (El Niño). Tracking statistics further indicate enhanced cyclogenesis over the Tasman Sea during La Niña, and reduced genesis during El Niño, especially for the Central Pacific type. Cyclone residence time over this region also tends to increase during La Niña, consistent with the associated favourable dynamical and moisture environment. Decomposing rainfall anomalies into Tasman Sea cyclone and non-cyclone components further highlights the role of cyclone activity changes in eastern Australian rainfall anomalies during ENSO events. These results suggest the importance of low-frequency circulation regimes in mediating tropical forcing on extratropical weather and provide new insights into the ENSO-weather systems linkage study.