Tasmanian Climate Drivers

2.2.2.1 Southern Annular Mode and the Southern Westerly Winds

The Southern Annular Mode (SAM), also known as the Antarctic Oscillation (AAO), is the most important mode of atmospheric variability in the mid to high latitudes of the Southern Hemisphere on synoptic to multi-decadal timescales (Lim et al., 2013; Pohl et al., 2010; Hill et al. 2009; Thompson and Wallace 2000). Gong and Wang (1999) define the zonal mean pressure difference between 40°S and 65°S as the Antarctic Oscilllation Index and it is this seesaw in atmospheric pressure between the Antarctic region and the mid latitudes of the Southern Hemisphere which is the foundation of SAM. SAM is forced by the strong gradient of meridional temperature between high and low latitudes and is persistent for most of the year becoming slightly weaker during the austral summer (Hill et al. 2009 and references therein). SAM most strongly affects rainfall in southwest Tasmania, with little correlation between SAM and rainfall characteristic of eastern and north-eastern Tasmania (Mariani and Fletcher, 2016). During positive SAM events, anomalously low pressure occurs over Antarctica whilst higher than normal pressure occurs in the southern mid latitudes. This is accompanied by a southward shift in the southern westerly wind belt (SWW) which is associated with west to east moving storm tracks and cold fronts. Hence positive SAM results in reduced rainfall, particularly over the west, due to less frontal and extratropical cyclone activity over Tasmania. During negative SAM events, the southern westerly wind belt moves north towards the

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equator resulting in lower pressure and more storm systems moving over southern Australia which, in Tasmania, usually leads to more rainfall over western Tasmania in combination with drier conditions in the east as a result of orographic effects. SAM exhibits a general pattern of being in its most positive/negative phase during the austral summer/winter (Hill et al. 2009 and references therein; Pohl et al. 2010; Bureau of Meteorology, 2018a).

Although monthly to seasonal variations in SAM occur due to variability of internal atmospheric dynamics, a strong teleconnection between SAM and the El Niño-Southern Oscillation (ENSO) has been observed during the austral summer season at the interannual time scale where El Niño tends to correspond with negative SAM events and La Niña with positive SAM events (Carvalho et al. 2005; Pohl et al. 2010; Lim et al. 2013).

2.2.2.2 The Southern Westerly Winds (SWW)

The southern westerly winds are responsible for the majority of Tasmania’s rainfall which, together with the strong orographic influence of high mountains in the west and relatively flat topography in the east, results in the large west-east gradient of mean rainfall and variability. Rainfall seasonality is stronger in western Tasmania which displays high winter rainfall, but although a gradient of high rainfall variability in the west towards weaker variability in the east exists, the dominant mode of rainfall variability is island-wide. (Hill, et al 2009)

The occurrence of blocking high events is important in all seasons. During a blocking high, western Tasmania in particular receives reduced rainfall as a result of weakened westerly flow due to the blocking effect of the event. During spring, ENSO and blocking highs account for over 50% of the variance in rainfall falling in the north and east of the state (Risbey et al., 2009)

2.2.2.3 El Niño-Southern Oscillation (ENSO)

Whereas SAM is the most important mode of atmospheric variability in the mid to high latitudes, the El Niño Southern Oscillation (ENSO) is the most important pattern of large scale variability in the tropics. The influence of ENSO has been found to decrease with latitude, however, extends along the extent of the East Coast of Australia as far as Tasmania (Nicholls and Wong, 1990).

The Southern Oscillation index (SOI) is one way in which ENSO is quantified and is based on the difference in sea level pressure between Darwin and Tahiti, where the fluctuation between positive (La Niña) and negative (El Niño) phases occurs with an irregular periodicity of around 3-7 years (Hill et al. 2009). ENSO is also thought to operate on longer timescales of variability from millennial to semi-precessional. The degree of variability on these timescales is poorly

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known, however, an intensification of ENSO relative to modern conditions from the mid Holocene (c. 7-4 ka) has been observed (Petherick et al. 2013 and references therein).

El Niño years are characterised by above average pressure in the Indonesian-North Australian region and anomalously warm temperatures in the central and eastern Pacific Ocean resulting in lower than average rainfall and sometimes major drought episodes in northern and eastern Australia as far south as Tasmania. The converse applies during La Niña years (Petherick et al. 2013 and references therein). The ENSO signal has been found to project island-wide in Tasmania, but with a stronger influence in the north due to the weakening latitudinal effects of ENSO mentioned earlier (Hill et al. 2009).

ENSO also displays a seasonal cycle associated with the seasonal drift of the tropical

convection zones resulting in a cold state during the austral winter and a warm state during the austral summer. McBride and Nicholls (1983) found that, overall, ENSO most strongly influences rainfall in Australia during the austral spring (Hill et al. 2009).

2.2.2.4 The Pacific-South American (PSA) modes

The Pacific-South American (PSA) modes include two modes of variability which link tropical and extratropical climates. PSA1 relates to decadal scale sea surface temperature anomalies over the central and eastern Pacific which are thought to be a response to ENSO variability on an interannual time scale. PSA2, on the other hand, is thought to be linked with the quasi- biennial component of ENSO, displaying the strongest connections during the austral spring with a periodicity of 22-28 months (Hill et al. 2009; Mo and Paegle 2001).

The PSA has a significant impact on Tasmanian climate and appears to have a similar influence on Tasmanian rainfall as the SAM, despite having a different origin, through its influence on the SWW combined with orographic effects. During positive PSA years, an island-wide increase in rainfall is observed which is strongest in the west. During negative PSA years rainfall is reduced and also shows the strongest anomalies in the west. The strongest correlation with rainfall is observed during the austral winter (Hill et al. 2009). It is thought that PSA and SAM interact with each other resulting in an enhancement (reduction) of the SWW during positive (negative) PSA phases (Hill et al. 2009).

2.2.2.5 Interactions between the key climate drivers: the SAM, ENSO and PSA

The Southern Annular Mode (SAM), El Niño - Southern Oscillation (ENSO) and Pacific-South American mode (PSA) each have been found to have clear influences on the interannual variability of rainfall over Tasmania, however, the interactions between these three climate modes and their seasonal variation is complex. The impact of ENSO tends to be concentrated in the north whereas that of both the PSA and SAM is more significant in the west. ENSO and

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SAM have been found to be in phase during the austral summer but it is during winter when ENSO influences moisture availability and the strength and position of the westerlies is modified by SAM that the most coherent impacts are observed. The PSA displays weaker seasonality but the same projection as SAM during winter (Hill et al. 2009)

Two modes have been observed which explain rainfall variability of Tasmania to different degrees. The dominant mode, explaining 70% of rainfall variability, is an island-wide mode and mostly attributed to the PSA and ENSO and a lesser extent to SAM. The second mode,

explaining 14% of rainfall variability, is the out of phase mode which produces conditions of high rainfall fluctuations in the west (and low variability in the east) caused by the passage of anomalous westerly winds over the western mountains producing orographic rainfall in the west. This mode is likened to the negative SAM phase and positive PSA phase (Hill et al. 2009).