past present and back to the future of antarctic precipitation - essay

8/3/2019 Past Present and Back to the Future of Antarctic Precipitation - Essay

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Introduction

Made up of ice and rock, a great southern land exists at the bottom of the earth called Antarctica.

This region contains some of the harshest environmental conditions for life on the Planet. Some

have referred to it in the past as 'Terra Australis Incognita' which translates from Latin to English

as 'the unknown land of the south'. In more modern times it is somewhat affectionately known as

‘the ice’. However one of the more interesting nicknames used for this mass of land and ice is

‘the white desert’. This is perhaps due to the dry air and consequently relatively low precipitation

rates. So how could this place, of which a large proportion of its surface is made up of the water

molecule, possibly be the driest place on the planet? The simple answer is it is cold. It is so cold

in Antarctica that in appropriately named Dry Valleys of East Antarctica the only form of life,

other than the occasional enthusiastic scientist, is found on the inside of rocks (McKnight et al.

1999). When it is this cold water molecules huddle together, preferring to exist in what is known

as its solid state. But not all of Antarctica is this cold and dry. Furthermore, it is not this cold and

dry throughout the entire year. It is even suggested that Antarctica was once very warm and wet.

Several questions arise when attempting to understand Antarctic precipitation such as: What are

the variables? How do these variables affect Antarctica today, and what happens to these

variables if, as predicted, temperatures rise due to anthropogenic forcing? Unravelling this

mystery begins with understanding how the atmospheric systems operates.

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Atmospheric dynamics – How does the system work?

A key driver in the Earth system and especially in the atmospheric system is the sun. More

correctly, the solar radiation the sun emits. This solar radiation provides energy into the Earth

system in the form of short wave radiation, but this energy is not evenly distributed (Figure 1).

Depending on surface type, much of the solar radiation can be reflected due to the albedo effect

(Figure 2). Therefore, polar regions tend to be in deficite of energy and the tropics in excess of

energy. The second law of thermodynamics states that all things within a closed system trend

towards entropy, therefore, mixing of the warm and cold regions occurs. This is what

fundamentally drives atmospheric circulation (Sturman & Tapper 2006).

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Figure 1: Variance of solar radiation that reaches the Earth's surface (Sturman & Tapper 2006)



However this is overly simplified. The Earth is three dimensional, and so we must consider

vertical and horizontal motion. Cold air is denser than warm air and so when the met, cold air

will undercut warm air. This results in the formation of cold fronts and cloud

formation(Zemansky & Dittman 1981) (Figure 3).

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Figure 2: Albedo effect (Sturman & Tapper 2006)

Figure 3: Formation of cold fronts (adapted from Sturman and Tapper 2001)



Vertical motion is a key ingredient in the process of precipitation. This vertical motion allows an

air mass to reach saturation. This is because as the air mass rises, the moisture within it will cool

and condense. As this occurs a phase change happens between water in the vapour state to water

in the liquid or solid state. Similarly, changes in pressure can result in phase changes (Figure 4).

These changes in phase are a fundamental component of what drives the water cycle (Figure 5).

So we can see that to get precipitation there needs to be some sort of evaporation from the

surface to get moisture into the air. The air then needs to be cooled to increase the relative

humidity and to trigger the change in phase from a gas to a liquid or solid.(Musy & Higy 2011)

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Figure 4: Phase diagram of water representing the roles temperature and pressure play in the

state of water (Zemansky & Dittman 1981)



Antarctic Precipitation in the modern world

Precipitation over Antarctica is slightly more complicated than a simple evaporation to

precipitation cycle. The extent of sea ice around Antarctica restricts evaporation rates,

consequently restricting the amount of moisture in the air (Tietäväinen & Vihma 2008). This

means Antarctica relies on the transportation of moist maritime air from synoptic circulation

around the continent .

Synoptic circulation around Antarctica occurs as a result of the thermal energy gradient between

the equator and the polar region. This results in areas of high pressures and areas of low

pressures as sub tropical and polar air masses interact. In areas where thermal advection is high,

fronts can form (Figure 6).

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Figure 5: Basic water cycle (adapted from Musy & Higy 2011)



Figure 6 explains why there is significant spatial variability of Antarctic precipitation. As the the

troposphere circulates the continent in a west-east direction, moist air which has come from afar

interacts with the Antarctic topography, particularly the Antarctic Peninsula. This causes

orographic lifting of the air and as a result precipitation occurs on the western side of the

Antarctic Peninsula in greater rates than those on the eastern side due to the Foehn wind affect

(Figure 7). These winds cannot penetrate to the centre of the continent, which leads to

precipitation levels dropping significantly towards the centre of the continent. As a result of a

temperature inversion clear sky precipitation occurs over the centre of the continent (Figure 8).

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Figure 6: Typical synoptic situation map of the Antarctic region (adapted from

Australian Meteorology Bureau, 2011)



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Figure 7: Interaction between synoptic circulation and the Antarctic Peninsula (map adapted

from University of Wisconsin-something, 2011)

Figure 8: Spatial variability and the different types of Antarctic

precipitation (adpated from (Bromwich et al. 2004)



Variation of atmospheric circulation and Antarctic precipitation

It is evident that Antarctic precipitation is heavily reliant on atmospheric circulation. However,

atmospheric circulation itself is quite variable. The Antarctic Oscillation (AAO) also known as

Southern Annular Mode (SAM) relates to changes in sea level pressure between middle and

higher latitudes of the Southern Hemisphere is positively correlated with an increase of intensity

of the polar vortex (van den Broeke & van Lipzig 2004). As a consequence increased wind

anomalies occur, and an increase in precipitation occurs typically along the western side of the

Antarctic peninsula due to the orographic forcing mechanism described earlier (Figure 9). This

leads to SAM to be the key component of inter-annular variation in Antarctic precipitation

(Tietäväinen & Vihma 2008).

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Figure 9: Increases and decreases in a) temperature and b) precipitation as a result of AAO. Note: A decrease in temperature does not mean a decrease in precipitation (adapted

from (van den Broeke & van Lipzig 2004)



El Nino Southern Oscillation (ENSO) has been used in many global cases as a source of climate

variability (Guo et al. 2004a). The most likely location which would be directly affected by

ENSO is the West Antarctic. The link between ENSO and Antarctic precipitation is not as well

understood as the link with SAM. Some studies have claimed to found a significant signal in

Antarctic precipitation (Bromwich et al. 2000), whilst others challenged the correlation between

the two (Genthon & Krinner 1998). However it appears the general consensus is that there is a

connection, but the effect ENSO has had on Antarctic precipitation has varied across recent

decades, explaining the conflicting results (Genthon & Cosme 2003). For example, in the early

1980s the relationship between Antarctic precipitation and ENSO was insignificant and changed

to become quite strong in the 1990s (Genthon & Cosme 2003). Some evidence suggests that

quasi-stationary eddies during the 1990s are responsible for this variation by altering flow

patterns. However, due to the variety of precipitation estimates during the early 1980s this

explanation is not as certain (Guo et al. 2004b).

The constraints of measuring Antarctic precipitation and gaps of knowledge

It is very difficult to measure precipitation rates quantitatively in Antarctica. This is because the

standard ways to measure precipitation cannot compete with the extremes of the Antarctic

weather, in particular the katabatic winds (fig) One reason it is difficult to measure using this

equipment is that it gets damaged by the high winds which flow across Antarctica. Not only do

these high winds damage the precipitation gauges, but they also blow snow from one location to

another. This means that it may appear that a region has a higher precipitation rate than it really

does.

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In addition, there is an observational bias towards the coastal regions of Antarctica and much of

the reliable data only goes back to the International Geophysical Year (1957), because many of

the Antarctic meteorological stations were not operational before this time.

The paleoenvironment of Antarctica – Past variances of precipitation

Global climate was once very different to what is witnessed in the modern Earth. The Antarctic

Ice Sheets did once not exist, and have fluctuated throughout time (Figure 10). A fluctuating ice

sheet suggests a dynamic freshwater flux, and therefore dynamic precipitation rates. Adding to

this the palaeontological evidence which suggests a land which was once occupied with

abundant life forms (Francis & Poole 2002; Wardle 2001). For this to occur, temperature and

therefore precipitation must have been much higher than it is today.

10

Figure 10: Global climatic, tectonic, and carbon cycle changes in the last 65

million years (adapted from Zachos et al. 2001)



Back to the future?

So the question remains that could Antarctica return to something which resembles the past?

Temperatures in some parts of Antarctica are on the rise (Chapman & Walsh 2007). These

increases in temperature have the ability to alter the(Chapman & Walsh 2007)atmospheric energy

budget by decreasing sea ice extent and therefore the albedo affect. As a consequence, changes in

the occurrence of sea ice, which has been shown to affect precipitation in Antarctica , along with

changes in Atmospheric circulation could result in an unpredictable precipitation change over the

continent.

Most models working on the Intergovernmental Panel on Climate Change (IPCC) have

predicted overall increases in precipitation over the coming century (Genthon et al. 2009).

Although there is some variability in the exact values, the general trend of increases along the

coastal regions and the Antarctic Peninsula are in agreement.

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Figure 11: Predicted a) relative and b) absolute precipitation change over the next 100 years

(taken from (Genthon et al. 2009)



As a consequence of this change, it would remove the two greatest abiotic constraints to

vegetation increases on Antarctica. In fact some studies have already identified vegetation on

areas of the Antarctic peninsula previously unknown to support vegetation (Fretwell et al. 2011;

Holden 1994). These changes in surface cover could potentially change the surface albedo of the

region, allowing for more absorption of solar radiation, as a consequence creating a positive

feedback loop.

Conclusion

Changes in precipitation rates can have an impact on ocean salinity which is a vital component in

sea ice formation and ocean circulation. Therefore importance in understanding their interactions

is essential for understanding how predicted temperature increases might affect the Antarctic

system. The links between sea ice reductions and predicted temperature increases may increase

evaporation, which as a consequence could increase precipitation rates, leading to a positive

feedback loop.

Significant knowledge gaps exist in how precipitation might change as a result of

predicted temperature increases. This is because changes in precipitation rates link heavier

towards changes in Atmospheric circulation and wind patterns rather than temperature itself.

Therefore investigations into how SAM and SOI might change under increased global

temperatures is vital to understanding how precipitation might vary in the future. With a severe

lack of direct measurements of precipitation, there is some degree of uncertainty within the

models, resulting in sometimes conflicting results. Re-analysis of the results of studies into how

ENSO effects Antarctic precipitation is particularly needed in the coming years, as there still

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seems to be an element of conflict between studies. There appears to be a consensus that

precipitation will likely increase as temperatures increase in the coming century. However, the

validity of the models predicting how the Antarctic precipitation might change in the future rely

heavily on the data used in the present, therefore it is essential that the knowledge gaps as

described here are filled to better predict future variability and change. Because alarmingly, a

significant change in precipitation rates may not just have consequences for the Antarctic system,

but it may indeed have an important influence on global climate change.

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References

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and Temporal Variability', Journal of Climate, vol. 17, no. 3, pp. 427--47.

Bromwich, DH, Rogers, AN, Kållberg, P, Cullather, RI, White, JWC & Kreutz, KJ 2000,'ECMWF Analyses and Reanalyses Depiction of ENSO Signal in Antarctic

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Chapman, WL & Walsh, JE 2007, 'A Synthesis of Antarctic Temperatures', Journal of Climate,

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Francis, JE & Poole, I 2002, 'Cretaceous and early Tertiary climates of Antarctica: evidence from

fossil wood', Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 182, no. 1-2, pp.

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Fretwell, P, Convey, P, Fleming, A, Peat, H & Hughes, K 2011, 'Detecting and mappingvegetation distribution on the Antarctic Peninsula from remote sensing data', Polar Biology, vol. 34, no. 2, pp. 273-81.

Genthon, C & Cosme, E 2003, 'Intermittent signature of ENSO in west-Antarctic precipitation',Geophys. Res. Lett., vol. 30, no. 21, p. 2081.

Genthon, C & Krinner, G 1998, 'Convergence and Disposal of Energy and Moisture on the

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Genthon, C, Krinner, G & Castebrunet, H 2009, 'Antarctic precipitation and climate-change predictions: horizontal resolution and margin vs plateau issues', Annals of Glaciology,

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Guo, Z, Bromwich, DH & Hines, KM 2004a, 'Modeled Antarctic Precipitation. Part II: ENSO

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Guo, Z, Bromwich, DH & Hines, KM 2004b, 'Modeled Antarctic Precipitation. Part II: ENSOModulation over West Antarctica*', Journal of Climate, vol. 17, no. 3, pp. 448--57,59-65.

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past present and back to the future of antarctic precipitation - essay

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