Causes of the Millennium Drought - Coursework Example

Causes of the Millennium Drought This paper aims at evaluating various factors affecting on the climate of South Eastern Australia (SEA), which can be the causes of the Millennium drought (1995-2009). In order to attain this objective, the paper will analyze the effects of El Niño Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), Sub-tropical ridge and Southern Annular Mode on the rainfall formation in SEA. Climatologists suppose that these factors as the main causes of the drought conditions formation. The paper will further look into other processes affecting on the climate of Australia including greenhouse emission. Causes of the Millennium Drought Introduction The causes of Millennium drought are the climate processes that lead to the creation of the extreme decreasing rainfall conditions in the years 1995-2009. The main feature of the drought was significant rainfall decline in autumn months. This feature distinguish Big Dry from the Federation and WWII droughts (Verdon-Kidd, and Kiem, 2009). Southeastern Australia suffered from the drought conditions many times. However, the Big Dry was the most severe drought in the history of this region (Sheffield and Wood, 2011). During the Millennium Drought, the mean rainwater inflow was 512 mm for the year in the southeastern Australia. It was 12 percent below the 1900–2010 mean annual rainfall established at the level 582 mm. The difference is much greater than the difference for the previous driest 13-year period lasted 1937–1949, which was 46 mm. Another interesting feature was that the Millennium drought in southeastern Australia took place at a time when the rest of the continent was facing more than average rainwater inflow. The north-west part of the continent experienced high summer rainfall. The previous droughts in the southeastern Australia tended to occur when dry conditions were typical for the whole continent (South Eastern Australian Climate Initiative, 2012). Fig. 1 Rainfall distribution diring the Millenium Drought in Australia (South Eastern Australian Climate Initiative, 2012). Climatologists came to conclusion that one factor cannot affect solely on the formation of the extreme drought conditions in Southeastern Australia during the Big Dry. The main factors are the large-scale atmospheric circulations (fig. 1.), which include El Niño Southern Oscillation ENSO, Indian Ocean Dipole (IOD), Southern Annular Mode (SAM), and the intensity of the sub-tropical ridge (STR) (Verdon-Kidd, and Kiem, 2009). Fig. 2 Processes affecting the climate of Australia (South Eastern Australian Climate Initiative, 2010) El Niño Southern Oscillation ENSO is connected with the temperature difference in the western and eastern parts of the Pacific Ocean. Its main characteristics is the Southern Oscillation Index (SOI), which is a measure of the pressure difference between Darwin and Tahiti (Clarke, 2008). El Niño creates dry and warm conditions in the Southeastern Australia. The reverse process La Niña results in rainfall in SEA (Murphy and Timbal, 2008). For the most part of Australia, ENSO effects on the rainfall conditions. One of the two major modes of the rainfall variability in Australia is directly linked to sea surface temperatures (SST) in the equatorial part of Pacific Ocean. ENSO effects on the rainfall mostly in winter and spring. Mean annual rainfall in Australia is linearly connected with the positive value of the southern oscillation index. Negative value of SOI results in below average rainfall. However, the level of the SOI is not the indicator of the rainfall shortage. For instance, significant SST anomalies in the eastern part of Pacific Ocean in 1997-98 had no impact on SEA rainfall. On the other hand, 2002–2003 episode with less SST changes lead to the more severe drought conditions (Murphy and Timbal, 2008). It is possible that the region with the strongest positive SST anomalies defines the impact of the El Niño on the rain inflow Australian. It was found that warming was near the dateline in 2002–2003, and it moved far on the east in 1997–1998. The changes in the Walker circulation that occurred afterward in 2002–2003, lead to the decrease of rainwater inflow in Australia and affected significantly on the rain conditions comparing the weather conditions in 1997–1998 (Murphy and Timbal, 2008). Temperature of the most Australia’s territory depends on the state of ENSO. Lower rainfall leads to less cloudy conditions and daily maximal temperature increases. Some regions of Australia show correlations between SOI and temperature. However, there is no correlation between SOI and rainfall. Surface temperature displays the same relationship to the SOI (Murphy and Timbal, 2008). According to the statistical analysis of the meteorological data, ENSO index explained 44% of climate change in eastern Australia. In southern Australia, ENSO explained 9% of the detected climate variance. The ENSO can be the cause for the most of the rainfall decrease for eastern Australia in 2001–2009 (8.1 of 12.3%) and partially in southern Australia (3.2 of 13.3%). In northeast Australia, ENSO can be treated as the source of the small rainwater inflow but it was neutralized by the similar positive effect of STR (Potter, Petheram and Zhang, 2011). However, ENSO has a limited effect in autumn. It means that the drier climate in SEA may not be connected with the ENSO changes. It was demonstrated in the rainfall probabilities calculated in the so-called Australian Bureau of Meteorology’s operational seasonal prediction scheme (Murphy and Timbal, 2008). Indian Ocean Dipole Indian Ocean Dipole (IOD) is the development of cold sea surface temperatures in the eastern part of Indian Ocean and warm water temperature in the western part of Indian Ocean. The IOD conditions is the reason of rains in eastern Africa and droughts in Indonesia. It may also effect on the rainfall conditions in Australia (Chandrasekar, 2010). The researches results showed that rainfall deficiencies in March–November during the 6 years of drought occurred when the positive IOD events took place. Southern Australia (without the east coast) had below average rainwater inflow. SEA showed the maximum rainfall deficiencies that was significantly below average (Murphy and Timbal, 2008). Indian Ocean SSTs in the eastern tropics is evidently correlated with SEA rainfall. However, scientists cannot say whether these sea surface temperatures really causes the rainfall variances. The IOD is created in May when rainfall deficiencies are already seen. The IOD may be a consequence of the same processes in the atmosphere as the SEA rainfall decrease. It was found that anomalies of high-pressure in Australia result in low rainfall due to the creation of eastern winds in the eastern part of Indian Ocean. It forces upwelling and forming of cool SSTs. Thus, the mechanism of the atmosphere response in SSTs may be related to SEA rainwater inflow. Despite the presence of link between SSTs and SEA rainwater intake, it is not clear whether Australian rainfall conditions is a direct response on SST forcing. It concerns mostly the dry conditions in autumn (Murphy and Timbal, 2008). Sub-tropical ridge and Southern Annular Mode Sub-tropical ridge (STR) is supposed to be the factor influencing seasonal climate of Australia. The location and strength of the STR may change the track of mid-latitude storms. These systems can move over the continent. It brings rain to the southern part of Australia, or they occur further south that result in dry weather conditions (Prosser, 2011). After1975, the annual cycle of the STR has changed comparing the period before 1975. The STR was located more northward and it was stronger in all months excluding autumn, mainly April and May. It is typical for the STR to move northward in this season. The mid-latitude storms move to the north bringing more frontal systems and rainy weather to SEA. However, the shift to north in April and May occurs more slowly since 1975 period. Together with the strengthening of the STR it effects on the rainfall reduction over SEA in autumn (Murphy and Timbal, 2008). Fig. 3 The conditions that lead to dry and warm weather in SEA (Gleick, 2012). However, the correlation between Australian rainfall and the latitude of the STR tends to weaken. The other question is unanswered. The STR behavior has changed after 1975 and there were no rainfall decline before the 1990s. The level of the STR changes in April-May and their impact on the rainfall has to be estimated more precisely (Murphy and Timbal, 2008). The recent researches showed that Southern Annular Mode is linked to the SEA rainfall. The SAM is the main mode of changes of the extra tropical circulation in the southern hemisphere. The high phase of SAM creates lower rainwater inflow in the southeast and southwest of Australia in winter, and higher rainwater inflow in summer in the southeast. Other researches results showed that the monthly mean values of temperature and rainfall for SEA are below average in the SAM positive phase (Murphy and Timbal, 2008). There are enough evidences of change in the atmospheric circulation in SEA in autumn. It was found that average pressures were higher at the south of Australia in the recent period. The pressure anomaly can be the reason for the Australian rainfall anomaly. The trend of the SAM and a shift of the extra tropical storm tracks southward may be related. However, it cannot explain the low impact of the SAM on autumn rainwater inflow in SEA. It was found that the rainfall decrease in SEA is connected with the higher pressure and reduced water content in the atmosphere. These researches can support the idea that changes in circulation can be the reason for some of the rainfall decline in SEA (Murphy and Timbal, 2008). Other Factors The other factors may also play their role in creation of dry conditions in Australia. For instance, the Madden-Julian Oscillation (MJO or the 30–60-day wave) controls the intraseasonal timescale in the tropical atmosphere. The MJO is a moving eastward region of higher tropical convection that influences rainfall in Australia, especially the northern part (Barry and Hall-McKim, 2014). In the Australian tropics, rain have a tendency to increase when the phase of MJO is in the local region, and to decrease when it is more distant. There is a global rainfall response to the MJO phase. The major circulation anomalies go together with different phases of MJO that are compatible with the rain anomalies. Movement of the MJO from the western part of Indian Ocean to the eastern part of Pacific Ocean can create significant changes in the atmospheric circulation leading to the rainfall variations in Australia. The resulting changes in the circulation may be the same as the IOD impact. In contrast to the IOD effect, the impacts of the MJO on rainfall in SEA are not widely spread and only take place at some places and seasons (Murphy and Timbal, 2008). Inter-decadal Pacific Oscillation (IPO) is another process affecting the SEA’s climate. It is a low frequency pattern of variability in the tropical and extra-tropical part of Pacific Ocean. The IPO may effect on the potential and frequency of ENSO events. Its positive and negative phases are connected with the higher frequency of El Niño and block effectss of La Niña. ENSO and IPO are treated as the primary causes of Federation drought. However, IPO impact on the creation of dry conditions during Millennium Drought is not certainly established (Verdon-Kidd and Kiem, 2009). The research showed that there are changes of the Hadley Cell that lead to changes of the STR. One of the reasons is connected with global warming. The STR is more intensive when the global surface temperature increases. This leads to the rainfall decrease in south-eastern Australia. To test this, SEACI (South Eastern Australian Climate Initiative) researchers created the simulation model of the global climate for the recent period using a global climate model. In this model, change in the effect of the sun and in volcanoes’ aerosols for the last century, changes in sources greenhouse emissions and a combination of natural and anthropogenic factors initiated the climate changes. The largest growth was found for the anthropogenic impact combined with natural processes (South Eastern Australian Climate Initiative, 2010, 2012). The effect of global warming on the climate of SEA and its impact on the creation of the dry conditions is not clearly defined due to the short period of study (Murphy and Timbal, 2008). Conclusion It is clearly understood that Millennium drought was not caused be a single factor. Such known processes as ENSO, IOD, SAM and STR contributed in the creation of dry conditions during the Big Dry. However, all mentioned processes (except STR) had no significant effect on the formation of dry weather in autumn that was the distinguish feature of the drought in 1997-2009. Experts also take into account the possible effect of global warming that may lead to the changes of Headley Cell and STR. The further researches may help to understand how the combination of the main climate processes in Southeastern Australia may effect on its climate and to predict the scale and the impact of drought in this region. References Barry, R. and Hall-McKim, E. (2014). Essentials of the Earth’s Climate System. New York: Cambridge University Press Chandrasekar, A. (2010). Basics of Atmospheric Science. New Dehli: PHI Private Limited Clarke, A. (2008). Dynamics of El Niño & the Southern Oscillation. Oxford: Elsevier Inc. Gleick, P. (ed.) (2012). The Worlds Water: The Biennial Report on Freshwater Resource. Washington, DC: Island Press Murphy, B. and Timbal, B. (2008). A review of recent climate variability and climate change in southeastern Australia. International Journal of Climatology, 28, 859-879 Potter, N.J., Petheram C., and Zhang L. (2011). Sensitivity of streamflow to rainfall and temperature in south-eastern Australia during the Millennium drought. 19th International Congress on Modelling and Simulation, Perth, Australia, 3636-3642. Retrieved September 1, 2014 from: http://mssanz.org.au/modsim2011 Prosser, P. (2011). Water: Science and Solutions for Australia. Collingwood, VIC: CSIRO Publishing Sheffield, J. and Wood, E. (2011). Drought: Past Problems and Future Scenarios. New York: Taylor & Francis South Eastern Australian Climate Initiative. (2012). Climate and water availability in south-eastern Australia: A synthesis of findings from Phase 2 of the South Eastern Australian Climate Initiative (SEACI). Retrieved September 1, 2014 from: http://www.droughtmanagement.info/literature/SEACI_Climate_Water_South_Eastern_Australia_2012.pdf South Eastern Australian Climate Initiative. (2010). Climate variability and change in south-eastern Australia: A synthesis of findings from Phase 1 of the South Eastern Australian Climate Initiative (SEACI). Retrieved September 1, 2014 from: http://www.csiro.au/Portals/Publications/Research--Reports/SEACI-report.aspx Van Dijk, A. et al. (2013). The Millennium Drought in southeast Australia (2001–2009): Natural and human causes and implications for water resources, ecosystems, economy, and society. Water Resources Research, 49, 1040–1057, DOI:10.1002/wrcr.20123 Verdon-Kidd, D., and Kiem, A. (2009). Nature and causes of protracted droughts in southeast Australia: Comparison between the Federation, WWII, and Big Dry droughts. Geophysical Research Letters, 36, DOI: 10.1029/2009GL041067 Read More