The physical environment: The atmosphere—climate and weather patterns

2016

The physical environment includes both the nonliving factors that characterise an ecosystem (e.g. weather patterns, ice coverage, the atmosphere) and the processes that drive them (e.g. weathering of rocks, anthropogenic emissions that deplete ozone in the atmosphere).

The atmosphere—climate and weather patterns

Antarctica is a major driver of global weather and climate, and influences the large-scale patterns of circulation at all layers in the atmosphere. Interactions between the atmosphere, ice and ocean in the Antarctic region set up patterns of variability across the Southern Hemisphere that, ultimately, influence the weather and climate of Australia. Physical processes in the Antarctic atmosphere form part of the engine that globally transports greenhouse gases, human-made chlorofluorocarbon gases, other pollutants and volcanic dust. Consequently, remote sources of human-made and natural pollution have pathways to Antarctica.

The climate of our polar regions and their dominant weather patterns are a result of the shape and rotation of the planet. Largely because of the spherical shape of Earth, the poles receive less solar energy than the equator. The interior of Antarctica—where the ice sheet is 2–4 kilometres thick and, hence, high above sea level—remains very cold throughout the year, because it is generally well shielded from the warmer air masses found at mid-latitudes. In contrast, the equatorial regions, where seasonal change is barely apparent, remain warm all year round. The latitudinal temperature difference between the equator and Antarctica creates a pressure gradient that Earth’s rotation acts on, to create a belt of cyclonic weather systems between 40°S and 70°S. The clockwise rotation of cyclones (as seen from satellite imagery) transports heat from the equator towards the Antarctic continent (Gitelman et al. 1997). The high altitude of the Antarctic ice sheet (averaging around 2200 metres) allows the air above the continent to cool significantly, becoming much denser than the air at the coast. This results in strong gravity-driven katabatic (downslope) winds in the coastal regions, where they are particularly prevalent during winter. The outflow from the katabatic winds influences the southward extent of the cyclonic weather systems that continually move across the Southern Ocean.

Although the earliest substantial Antarctic weather instrumental records extend back to the beginning of the 20th century (to 1904 for the South Orkney Islands), the bulk of records suitable for surface climate analysis started to become available in the late 1940s. During the 1960s, regular ‘upper air’ measurements—which profile conditions in the troposphere (Earth’s surface to 10 kilometres high) and lower stratosphere (10–30 kilometres) using radiosonde balloons—began at several sites in the Antarctic region. The advent of polar-orbiting weather satellites in the late 1970s greatly improved the spatial, temporal and vertical remote sensing of the Antarctic region.

The Australian Bureau of Meteorology gathers year-round detailed weather information at Australia’s Antarctic and subantarctic stations. The Australian sites contribute to the Antarctic Observing Network of the World Meteorological Organization (WMO 2016a), which—in November 2015—comprised 113 stations. In 2015, Australia’s Davis and Macquarie Island research stations became candidate members of the Global Climate Observing System Reference Upper Air Network, which is designed to maintain long-term climate records (WMO 2016b). In addition to the records gathered at staffed sites, weather data are collected by automatic weather stations at more than 100 locations across Antarctica (Lazzara et al. 2012), and by drifting buoys, balloons, and various satellite and ground-based remote-sensing techniques (AAD 2008).

Although there is significant interannual variability in Antarctic weather because of various large-scale atmospheric and oceanic processes associated with the global movement of heat, trends are apparent in the recent historical record. During the past half-century, the Antarctic surface has warmed. Work by Nicolas and Bromwich (2014) provides a best estimate of 0.11 ± 0.08 °C per decade averaged across the continent. This warming is most significant in the western and northern parts of the Antarctic Peninsula, and in parts of the West Antarctic Ice Sheet (Turner et al. 2009, 2014; Hartmann et al. 2013). At the Faraday–Vernadsky Station on the western side of the Antarctic Peninsula, an annual trend of +0.54 °C per decade from 1951 to 2011 has been observed, which is the most significant trend in the peninsula region (Turner et al. 2014). Winter temperatures at this site exhibited a stronger trend of +1.01 °C per decade from 1950 to 2011 (Turner et al. 2014). West Antarctica warmed on average by approximately +0.22 °C per decade from 1958 to 2012, with the winter and spring seasons showing the strongest warming trends (Steig et al. 2009, Nicolas & Bromwich 2014). Byrd Station in West Antarctica appears to show the most rapid warming of any site on Earth, with a warming of +2.4 ± 1.2 °C per decade from 1958 to 2010 (Bromwich et al. 2013). Coastal East Antarctica is generally warming, although the trend is weaker than in West Antarctica, and some regions and seasons show evidence of cooling (SCAR 2009, Steig et al. 2009). For example, a statistically significant cooling trend has been observed at the high plateau at the South Pole.

At Australia’s Casey, Davis and Mawson Antarctic research stations, the long-term annual maximum and minimum temperatures have varied regionally (Figure ANT3). Mawson and Davis, which are separated by approximately 640 kilometres, show similar trends, whereas Casey, some 1390 kilometres east of Davis, exhibited a warming during the 1970s while the other stations cooled. Part of the variability in the temperatures at these stations over interannual and longer timescales can be attributed to changes in the Southern Annular Mode, particularly in summer. In the case of Macquarie Island, the long-term trend is of warming, which is most significant in summer (Box ANT2).

Although greenhouse gas changes are likely contributing to the warming of Antarctica (Hartmann et al. 2013), regional influences have been attributed to changes in the Southern Annular Mode (Marshall et al. 2006Box ANT2), which is, in turn, influenced by stratospheric ozone depletion and greenhouse gas changes (Box ANT3). Additionally, regional climate variability in Antarctica is influenced by connections to the tropical oceans, particularly through the El Niño–Southern Oscillation, Indian Ocean Dipole and Interdecadal Pacific Oscillation climate modes, and from variability in the tropical North Atlantic (Li et al. 2014, Turner et al. 2014, Turney et al. 2015). At present, it is unclear how these tropical connections are responding to global climate change (Hartmann et al. 2013).

In the upper atmospheric layers, balloon and satellite measurements indicate that the Antarctic lower troposphere (surface to 5 kilometres) has warmed during the past 50 years (SCAR 2010), while the lower stratosphere (10–30 kilometres) has cooled (Randel et al. 2009, WMO 2014). These temperature changes are likely to be because of the effects of increased greenhouse gas concentrations in the atmosphere and, particularly in the case of the stratosphere, decreases in ozone concentrations (Turner et al. 2014, WMO 2014). In the Antarctic mesosphere (50–95 kilometres), temperatures are expected to decrease in response to increasing CO2 levels (which is opposite to the situation in the troposphere). Above Davis, temperatures in the upper mesosphere near 87 kilometres exhibited a long-term cooling of –1.2 ± 0.9 °C per decade from 1995 to 2010 (after accounting for influences from the solar activity cycle). The cooling is more pronounced in spring; this is possibly associated with changes in atmospheric structure associated with the ozone hole (French & Klekociuk 2011).

Box ANT2 Variability and trends in East Antarctic temperatures and the Southern Annular Mode

The Southern Annular Mode (SAM) is the principal driver of atmospheric variability at southern high and mid-latitudes on the timescale of weeks to months (Turner et al. 2014). The SAM manifests as a see-sawing of pressure levels between high and mid-latitudes, and operates on seasonal and interannual timescales around the entire Southern Hemisphere. It is defined to be in a positive state when Antarctic surface pressures are below average, which is accompanied by a southwards shift of the belt of strong westerly winds across the Southern Ocean. The negative state has above average pressures and a northwards wind shift.

Since the 1960s, the SAM has shown a statistically significant positive trend during austral summer (December–February) and autumn (March–May). This results in a general decrease in surface air pressure around the Antarctic coast, and an increase in pressure at southern mid-latitudes during these seasons (Marshall 2003). The trend has deepened the meteorological feature known as the Amundsen Sea Low, which has contributed to warming of the Antarctic Peninsula (and, to a lesser extent, West Antarctica) and to a reduction of sea ice in these regions. Additionally, changes in the SAM have led to fewer but more intense cyclonic weather systems in the Antarctic coastal region (60°S–70°S), except in the region of the Bellingshausen–Amundsen seas (SCAR 2009).

Broadscale changes in atmospheric circulation brought about by the readjustment of the vertical and horizontal temperature gradients in the atmosphere drive year-to-year changes in the strength of the SAM. The trends in the SAM during summer and autumn have been linked to depletion of stratospheric ozone because of the Antarctic ozone hole (which has promoted stratospheric cooling) (Marshall et al. 2004) and, to a lesser extent, increasing greenhouse gases (which promote warming of the troposphere and cooling of the stratosphere). Additional variability in the SAM has been linked to natural modes of variability in ocean temperatures (Marshall & Connolley 2006).

The observed trends in the SAM (towards lower Antarctic surface pressures) have promoted cooling of the Antarctic region by reducing the inflow of heat from lower latitudes (because of an associated strengthening of the circumpolar westerly winds) and reducing the downwards heat transport near the surface (Nicolas & Bromwich 2014). By accounting for the effect of the SAM on Antarctic surface temperatures, a residual warming that is attributed to greenhouse gas increases has been identified (Gillett et al. 2008).

As shown in Figure ANT4, the summer temperature anomaly at Macquarie Island based on the 1971–2000 average exhibits multiyear variability. Generally, since the early 1970s, each year has been been warmer than the preceeding year. At Mawson, the summer temperature anomaly has been generally cooler since the mid-1990s compared with the earlier record.

In the case of Mawson and several other sites in East Antarctica, a general pattern of anticorrelation has been observed between the strength of the SAM and near-surface temperatures (Marshall 2007). In comparison, the SAM–temperature relationship is much weaker at Macquarie Island. For Mawson, around 25 per cent of the variance in the summer and winter temperature anomaly is explained by the SAM–temperature correlation, at least in 20-year intervals up to 2004 (Marshall 2007). The strength of this relationship indicates that, although seasonal temperatures in a given year can be influenced by the SAM, other sources of variability can play a more important role. Recent work by Marshall et al. (2013) suggests that the SAM–temperature relationship in East Antarctica reversed during the 2000s as a result of emerging internal climate variability associated with ocean–atmosphere interactions. Overall, continued collection of long-term, high-quality climate data in the Antarctic region is required to more fully interpret current trends.

Box ANT3 The Antarctic ozone hole

The Antarctic ozone hole is an anomalous reduction in the amount of ozone in the lower stratosphere (12–20 kilometres in altitude) above Antarctica, which has taken place each spring since around 1980 (WMO 2014). This phenomenon has led to an increase in damaging ultraviolet radiation reaching the surface of Earth (Bais et al. 2015) and a cooling of the stratosphere (Solomon et al. 2015).

The ozone hole is caused by chemical processes involving human-made halon gases, particularly chlorofluorocarbons (CFCs). Such chemical processes are promoted by the extreme cold and special circulation conditions in the Antarctic stratosphere during winter. Antarctic ozone destruction begins in August, reaches a peak from late September to early October (when up to approximately 70 per cent of the ozone column is destroyed in a region covering about twice the surface area of Antarctica; Figure ANT5), and usually ends in late November. Similar chemical processes also cause significant ozone destruction over the Arctic between December and March of some years, and contribute to a small, but important, overall reduction in global stratospheric ozone levels.

In 1987, the Montreal Protocol was signed to improve the health of the ozone layer by restricting the use of CFCs and other ozone depleting substances. This treaty has since been ratified by 197 parties and has led to a gradual reduction in equivalent effective stratospheric chlorine (EESC; Figure ANT6), which is an estimate of the effective quantity of halogens in the atmosphere. This estimate is used to quantify the amount of stratospheric ozone loss that can be explained by the level of human-made halogens. The levels of CFCs peaked in the mid-1990s; since then, the use of these chemicals has been greatly reduced. Based on modelling of the expected evolution in EESC levels, the Antarctic ozone hole is expected to be repaired during the latter part of the 21st century (Butchart et al. 2010, WMO 2014). Recent observations provide evidence that stratospheric ozone levels are improving (Solomon et al. 2016).

The recent behaviour of ozone levels above the British Halley Research Station, which has the longest available Antarctic record, is shown in Figure ANT7. Ozone levels fluctuate significantly from year to year because of the influence of meteorological factors. Several measures of the severity of Antarctic ozone loss indicate that the ozone hole has not increased in size since 2006 (Klekociuk et al. 2015). After accounting for year-to-year variability, Antarctic ozone has increased by approximately 5 per cent since 2000. Although this increase is consistent with the expected decrease in EESC levels (Figure ANT6), it cannot be definitely attributed to the change in EESC levels (WMO 2014).

Stratospheric cooling associated with the ozone hole, and ozone depletion in general, has led to an overall polewards shift of the Antarctic jet stream (strong westerly winds 7–12 kilometres above Earth’s surface), primarily in summer, in turn influencing the route of storms in the high to mid-latitudes and the SAM (see Box ANT2). The wind changes have been linked to regional changes in precipitation (Kang et al. 2011), changes in sea ice around Antarctica (SCAR 2009), warming of the Southern Ocean (SCAR 2009), a local decrease in the ocean sink of carbon dioxide (WMO 2014) and influences on the circulation in the mesosphere (Smith et al. 2010).

Throughout the remainder of the 21st century, the changes on the Antarctic surface brought about by the ozone hole are expected to gradually decrease (WMO 2014). However, the stratospheric effects of greenhouse gas increases will tend to cancel out the effects of ozone recovery, and the jet stream may stay at its current latitude. The interactions of these two competing circumstances are not yet fully understood, and much will depend on the speed of the ozone recovery and the rate of increase of greenhouse gases (McLandress et al. 2011, Polvani et al. 2011, Previdi & Polvani 2014).

Klekociuk A, Wienecke B (2016). Antarctic environment: The physical environment: The atmosphere—climate and weather patterns. In: Australia state of the environment 2016, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/antarctic-environment/topic/2016/physical-environment-atmosphere-climate-and-weather-patterns, DOI 10.4226/94/58b65b2b307c0