The Southern Annular Mode (SAM) is a naturally occurring mode of atmospheric variability located in the Southern Hemisphere (Pohl & Fauchereau 2011). It is also referred to as the Antarctic Oscillation or High Latitude Mode (Arblaster & Meehl 2006) and is said to be unpredictable. The SAM is generally described as the north-south movement of westerly winds that circle Antarctica, generally dominating the middle to higher latitudes within the Southern Hemisphere (Bureau of Meteorology 2019). When the mid-latitude westerly winds shift poleward, the SAM is characterised as being in the positive phase and when shifted equatorwards, the SAM is characterised as being in the negative phase (Marshall et al. 2018). The SAM has effects on both Australian weather and climate and is an important driver of rainfall and temperature variability (Bureau of Meteorology 2019). It appears that the SAM is sensitive to the increasing greenhouse gas emissions, which may affect the intensity and occurrence of the phases (Arblaster and Meehl 2006). In Australia, the SAM is stronger in the months of December, January and February (Summer) compared to other months (Marshall et al. 2012). How the SAM works and its effect on Australian climate and weather will be discussed in this essay.
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How does SAM work?
The Southern Annular Mode occurs on inter-annual timescales, typically lasting 2 weeks (Hendon, Thompson & Wheeler 2007). The SAM is described as being zonally symmetric and is approximately centred at 50S (Ho, Kiem & Verdon-Kidd 2012). Cai et al. (2011) identifies that in the mid-latitudes, if the SAM contains high mean sea level pressure (MSLP), there will be a low MSLP in the high latitudes and vice versa. The circulation of the SAM is fundamentally driven by the uneven heating of the Earth and the energy transport and atmospheric circulations that occur as a result (Ho, Kiem & Verdon-Kidd 2012). Near the equator, the built-up energy results in rising air along the Inter-Tropical Convergence Zone as a consequence of surface convergence (Sturman and Tapper 2006). This rising air moves poleward and begins to sink. In the Southern Hemisphere, the descending air forms high pressure systems at the surface. This southward moving air intersects the air moving north from the South Pole resulting in rising air, forming the circumpolar trough (a region of relatively low-pressure surrounding Antarctica) (Ho, Kiem and Verdon-Kidd 2012). Therefore, the regions of high and low pressure characterised by the Southern Annular Mode can fundamentally be explained by the uneven heating of the Earth and its resulting circulations.
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The two phases of the SAM are generally opposites. The positive phase, also known as the high index phase and high polarity phase (Marshall et al. 2012) can be defined as lower than normal pressures in the polar regions and high pressure in the mid-latitudes (Ho, Kiem & Verdon-Kidd 2012), enhancing the westerly winds 55- 60 latitude (Risbey et al. 2009). This phase is an indicator of the strengthening of the circumpolar vortex and is associated with the storm track shifting towards the South Pole (Marshall 2003). Conversely, the negative phase, is defined as the increased westerly winds expanding towards the equator and consequently a stronger low-pressure system over southern Australia (Bureau of Meteorology 2019). This results in the storm track shifting towards southern Australia (Marshall et al. 2018). These phases can be seen in figure 1.
Figure 1. The positive and negative phases of the SAM (Agriculture Victoria 2017)
How SAM effects weather in Australia
When the SAM is prevalent, associated weather patterns occur, which are different for each phase, location and season. Affected weather events can include rainfall, extreme heat events, storms, extra-tropical cyclones, cold fronts and westerly and easterly winds (Meneghini, Simmonds & Smith 2007). According to Gillett, Kell & Jones (2006), during the positive phase, the weather is generally warmer and drier than normal between 40-60S, significantly over Tasmania and south-eastern Australia. This is due to the location of the descending air, as well as the positive sea level pressure anomalies occurring during this phase. The warming is more noticeable during the Australian summer, which can be explained by the larger amounts of solar radiation being received at this time as well as clearer skies. However, north of 40S, there is evidence of significant cooling in Australia during the positive phase, which has been explained to be due to enhanced cloud cover and weak irregular ascent in the region (Gillett, Kell & Jones 2006).
Similarly, Hendon, Thompson & Wheeler (2007) also mention this cooling and explains that its effect is largest during the Australian spring and summer. The opposite is said to occur during the negative phase (cooler temperatures in Tasmania and parts of southern Australia, and warmer temperatures north of 40S (Gillett, Kell & Jones 2006). These maximum and minimum temperature differences between each phase are seen in figure 2 and figure 3 for each season, highlighting affected areas.
Figure 2. Composite daily maximum temperature differences between phases of the SAM (Hendon, Thompson & Wheeler 2007)
Figure 3. Composite daily minimum temperature differences between phases of the SAM (Hendon, Thompson & Wheeler 2007)
Furthermore, the likelihood of extreme heat events is reduced during the positive phase and enhanced during the negative phase in most of Australia (Marshall et al. 2014).
Gillett, Kell & Jones (2006) also discuss the rainfall patterns associated with each phase. During the positive phase, the occurrence of precipitation decreases over Tasmania, the extreme southeast and southwest and anomalously increases over the rest of Australia (Pui et al. 2012). The opposite occurs during the negative phase. These irregular weather patterns can have implications for weather forecasting, as the SAM is not very predictable (Marshall et al. 2012). The increased rainfall patterns over eastern Australia that are associated with the positive phase can be explained by the abnormal easterly winds, enhancing moisture advection from the ocean and hence increasing rainfall (Hendon, Thompson & Wheeler 2007). Using daily recordings over a 30-year period, Hendon, Thompson & Wheeler (2007) found that 10-15% of the weekly rainfall variance in Australia is explained by variations in the SAM. In general, during the SAM, regions that have a decrease in maximum temperature, also experience an increase in precipitation and vice versa. Raut, Jakob & Reeder (2014) identified that the positive phase of the SAM can produce up to 5 times the rainfall in some situations. Pui et al. (2012) further found that the largest increase in the occurrence of rainfall during the positive phase occurs along the east coast, which can also be explained by the irregular easterly winds also mentioned by Hendon, Thompson & Wheeler (2007). Both studies also found the same pattern during the positive phase in the southern part of Australia, where there was a reduction in precipitation, especially during the period of June-August and is attributed to the reduction of the westerly winds towards the poles. Overall it can be seen that as the SAM only lasts for short periods of time, weather is affected in the form of temperature and precipitation variations.
How SAM effects climate in Australia
Similar to how the SAM affects Australian weather, the effect on the climate varies between each phase and the time of year. The main point mentioned throughout most of the literature suggests that the SAM has a trend towards its positive phase (Pohl & Fauchereau 2011, Arblaster & Meehl 2006, Hendon, Thompson & Wheeler 2007). Pohl and Fauchereau (2011) found that this trend began in the 1960s and that the SAM has a seasonal peak in December. Arblaster and Meehl (2006) found that the reason for this positive trend is related to the changes in the ozone and increase in greenhouse gases and suggested that this trend has been observed in the second half of the twentieth century, consistent with Pohl and Fauchereau (2011). This positive trend has an effect on Australia’s climate due to its peak during summer and its persistent (reoccurring) positive phase each year. This pattern means there is a cooling of most of Australia and warming of southern Australia, including Tasmania in the summer months. Hendon, Thompson & Wheeler (2007) however found that since about 1980, the SAM has caused an overall cooling trend of maximum temperatures by 0.2C per year in parts of eastern Australia. Similarly, Cai et al. (2011) found that the maximum temperatures in eastern Australia have levelled relative to the lasting positive trend, supporting the findings of Hendon, Thompson & Wheeler (2007). Along with a change in temperature climate comes a change in Australia’s precipitation climate.
Meneghini, Simmonds & Smith (2007) found that summer rainfall is above average in south-west western Australia. Gillett, Kell & Jones (2006) and Cai & Cowan (2006) both found that the positive phase is also associated with decreased winter rainfall. This trend has been found to be linked to the recent (winter) droughts in south-west Western Australia (SWWA) and south-east Australia (Ho, Kiem & Verdon-Kidd 2012). Cai & Cowan (2006) concluded that the winter rainfall reduction in SWWA and its relationship with the SAM explains 67% of the long-term decline. However, this contradicts the findings of Meneghini, Simmonds & Smith (2007), who concluded that these long-term reductions were unlikely to be due to the patterns in the SAM. Similarly, Feng et al. (2010) questioned if there was a link between the SAM and the SWWA trend in rainfall. They identified that this relationship was only important when there was an extremely wet year coupled with a negative SAM (the year of 1964). The difference between these findings could be due to the use of numerous indices to measure the SAM, limiting reliable data.
Cai et al. (2011) found a relationship between below average winter rainfall in SWWA in 2010, which occurred along with the highest recorded positive SAM value. Raut, Jakob & Reeder (2014) found that winter rainfall in south-west Australia decreased by 10-20% since the 1970s and summer precipitation in inland Australia has increased by 40-50%. Hendon, Thompson and Wheeler (2007) found the same trends as Meneghini, Simmonds & Smith (2007) during the summer periods from 1979-2005. Specifically including increased rainfall in eastern Tasmania and south-east Australia and a decrease over western Tasmania during the summer. Meneghini et al. (2007) found that generally the SAM has been linked to wetter than average summers in south-east Australia and eastern Tasmania and drier summers in western Tasmania, constant with the findings of Hendon, Thompson and Wheeler (2007). Figure 4 highlights how the SAM affects rainfall patterns for each season. It can be seen that the largest rainfall anomalies occur during winter (decrease) and summer (increase).
Figure 4. Composite daily rainfall and 850 hPa winds for the difference between the phases of the SAM for each season (Hendon, Thompson & Wheeler 2007)
From these findings, we can conclude that the SAM does affect most of Australia’s climate in conjunction with the upward trend in the positive phase. These effects are largest in the summer and winter months., especially in the southern parts of Australia. During the summer months, southern Australia’s climate (including eastern Tasmania) has become wetter and warmer and in winter, the climate in these regions is drier, contributing to long-term droughts in some regions (upon debate).
Conclusion
Over daily/weekly timescales it can be seen that generally the positive (negative) phase is associated with increased (decreased) precipitation and decreased (increased) maximum temperatures in most of Australia and a decrease (increase) in precipitation and an increase (decrease) in maximum temperatures in southern extremities of Australia. Due to the upward trend in the positive phase of the SAM, there has been significant effects on Australia’s climate including a wetter and warmer summer and a drier winter in southern parts of Australia. However, these affects are still uncertain due to the differences in numerous SAM indices. Therefore, from these studies it can be concluded that the SAM affects weather frequently, depending on its phase and over longer time scales can affect Australia’s climate due to its upward trend associated with global warming.
References
Agriculture Victoria 2017, SAM, online image, viewed 4 May 2019, http://agriculture.vic.gov.au/agriculture/weather-and-climate/understanding-weather-and-climate/climatedogs/sam
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