REGIONAL CLIMATIC CONTROLS
Several major oceanographic and atmospheric controls affect the climate of the southwest Pacific. The northern part is influenced by variations in the position of the Inter-Tropical Convergence Zone (ITCZ), which migrates southwards into the extreme northern part of Australia during the austral summer and is a major control on local moisture supply.
The northern part of Australia also lies within the direct influence of Western Pacific Warm Pool (WPWP), which is a major source of global latent heat release. The size of the WPWP is sensitive to changes in the El Niño-Southern Oscillation (ENSO), and hence the strength of atmospheric circulation across the Pacific. During ‘warm’ ENSO (El Niño) events, the WPWP contracts towards the equator, the South Pacific Convergence Zone (SPCZ) migrates into the ITCZ and, in the mid-latitudes, a high-pressure anomaly develops over southern Australia (Hooker and Fitzharris, 1999), resulting in enhanced southwesterly flow over New Zealand. The degree to which the teleconnection is strengthened across the region appears to be modulated by the Interdecadal Pacific Oscillation (Salinger et al., 2001).
Ocean circulation also plays a potentially important role in the transmission of ENSO signals to high southern latitudes via the Antarctic Circumpolar Wave (ACW) (White and Peterson, 1996) through the transmission of sea surface temperature (SST) anomalies. The SST anomalies are transmitted via Rossby Waves, where they are subsequently propagated eastwards into the Indian and south Atlantic Oceans. At present, however, the long-term stability of the ACW and IPO are presently unknown.
Independent of ENSO-generated anomalies, the oceans transport heat southward via the East Australian Current (EAC) which has a significant influence on coastal regions of eastern Australia, with approximately half of the EAC moving eastward at the Tasman Front (around 34°S) while the remainder continues south (Roughan and Middleton, 2004). The warm, low-salinity tropical Leeuwin Current (LC) flows polewards and then eastwards along the coasts of western and southern Australia (Okada and Wells, 1997). In the mid-latitudes, westerly airflow centred over New Zealand roughly tracks the flow of subantarctic waters, migrating northwards during the austral winter to influence southern Australia (Sturman and Tapper, 1996).
Figure: Principal atmospheric circulation during the austral summer and annual mean location of ocean masses. ‘ITCZ’ denotes Inter-Tropical Convergence Zone, ‘EAC’ denotes East Australian Current, ‘LC’ denotes the Leeuwin Current, ‘TF’ denotes Tasman Front, ‘STF’ denotes Sub-Tropical Front, and ‘SAF’ denotes Sub-Antarctic Front.
While it is frequently speculated that late Pleistocene climate variations in the southwest Pacific probably reflect latitudinal variations in the positions of the ITCZ, the westerlies and the East Australian Current (e.g. Kershaw, 1995; Barrows and Juggins, 2005) few local terrestrial records are available that enable the frequency, timing and latitudinal span or even direction of these migrations to be reconstructed.
Yet understanding the climate history of the southwest Pacific has potentially global implications. For example, changes in WPWP sea surface temperatures have recently been interpreted to have led Northern Hemisphere ice melting (Visser et al., 2003). Also, ENSO cyclicity is now considered to have operated continuously during the last cold stage at the interannual, millennial and semi-precessional timescales (Tudhope et al., 2001; Turney et al., 2004), potentially exerting a global influence on climate.
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