Atmospheric Circulation
The global atmospheric circulation is the movement of the air masses above the earth’s atmosphere due to the variation in temperature resulting from the temperature difference across the globe. The atmospheric circulation is closely related to the other geophysical components of the atmosphere such as the Coriolis Effect, the oceanic circulation and the oscillation or shifting of these air masses across latitudes. The factors that manipulate these air masses to flow are not just restricted to difference in temperature, pressure or density, but are largely influenced by the oceanic circulation, the convection of the Earth at large and surely the revolution of the Earth, which refers to seasonal changes that is experienced over the year. The report will dive into the global atmospheric circulation systems, identify the major trends and evaluate the effect of season with respect to the Australian province of New South Wales and Queensland which comprises the eastern region of Australia.
The Earths energy system revolves around the thermal energy received from the sun. The sun heats up the Earth’s atmosphere in varying degrees depending on the equinoxes and the solstices reflected through latitudinal variations. The amount of heat received by the atmosphere creates anomalies in the nature of the air masses by creating cool and warm air masses which tend to move vertically along the atmospheric column (Shepherd, 2014). The global circulation of air masses generally follows a common pattern with minor variations in the local circulations due to certain variable factors. The general process of atmospheric circulation will be analysed over the next few sections and the effect of seasons on atmospheric circulation will be evaluated.
The major factors affecting the atmospheric circulation in general are the following:
- Differential heating resulting in variation of temperature.
- Variance of atmospheric pressure
- Coriolis effect
- The effect of ocean temperature or commonly known as SST or sea surface temperature.
Among the above mentioned factors the aspect of differential heating is the primary influential factor that generates movement of air masses in the atmosphere. The differential heating of the atmosphere takes place due to the following reasons
- Difference in the latitudinal extent due to axial tilt of the Earth.
- Distance of the Earth from the Sun across the year, which refers to seasonal variations.
The Earth’s atmosphere receives heat in varying degrees being greater at the equator and gradually decreasing towards the poles. The equatorial regions receive greater amount of heat from the sun thereby increasing the temperature in the atmosphere. This increase in temperature leads to warming of the air mass and creating low atmospheric pressure. The Polar Regions have lower temperatures creating high pressure zones. This variation in pressure due to variation in temperature leads to the formation of pressure belts which forms different “cells of circulation”. These cells are identified with their discoverers and named as the Hadley cell, the Ferrel’s cell and the polar cell (Feng & Li, 2013). These are three circulatory systems prevalent in the atmosphere create the global atmospheric circulation system. The global atmospheric system is divided along latitudinal extent and atmospheric pressure belts are created.
Factors Affecting Atmospheric Circulation
The seasons are the product of the movement of the tilted Earth over the period of its revolution. The apparent movement of the sun, from the equator towards the pole due the changing inclination of the earth on its axis creates seasonal variation. During these movement of the earth variation in air pressure develops from two different processes as follows:
- Thermal variation: variation due to temperature
- Dynamic variation: variation due to earth rotation.
The cyclical change of seasons occurs due to the revolution of the earth, which creates variation in the amount of incoming heat in the two hemispheres. The Northern and the Southern hemispheres experiences different temperatures and seasons in the annual period. When the northern hemispheres experiences summers the southern hemispheres are experiencing completely opposite seasons and have colder weathers. The brief glance at calendar months will give an insight into the seasons of Australia.
Australia experiences four seasons over the calendar year which include Spring season from the month of September, October and November. They are followed by the Summers in the months of December, January and February. The next season is the obvious Autumn which includes the months of March, April and May. The coldest months are followed next by June, July and August. This variation in heating and cooling occurs with the apparent southward and northward movement of the sun across the equator.
The effect of varying temperature over the Australian landmass develops pressure belts over the landmass. The varying temperature as discussed above occurs primarily due to the apparent movement of the sun which creates equinoxes and solstices that mark the seasons. Due to the differential heating, the atmospheric circulation over Australian continent experiences shifting in the air cells. The air cells are masses of air that categorised according to their activity and latitudinal extent. The major cells include the Hadley cell circulations which affect the seasonal rainfall and drought conditions over Australia. The major impact is created by the following atmospheric circulation as depicted in the major beside which include the IOD or the Indian Ocean Dipole, the ENSO or the El Nino Southern Oscillation, the MJO or the Madden Julian oscillation and the Southern annular mode or the SAM. The map above gives us a brief overview of the pressure circulation over the Australian landmass. These pressure circulations manipulate the rainfall patterns over Australia and create drought or flood conditions over the year (Climatechangeinaustralia.gov.au (2018).
The Eastern part of the continent experiences the easterly troughs, the trade winds coming in from the higher latitudes to the lower ones, the effect of the Ferrel’s circulation, the east coast lows and the blocking highs are dominant over the eastern coast (About Australian Climate, 2018). The El Nino and the La Nina is dominant over the north eastern coast and has the most dominant effect over the Australian landmass. Among all the atmospheric circulations the El Nino and La Nina create the most dominant effect on the landmass. The following section will explore the effects of the EL Nino and La Nina on the Eastern part of Australia (Raut, Jakob & Reeder, 2014).
The Effect of Atmospheric Circulations
The Indian Ocean Dipole is an oscillation that happens irregularly with three phases of positive, negative and neutral. The warmer temperatures in the eastern coasts of the ocean it relates to negative phases and is responsible for creating rainfall over southern Australia during the warmer seasons. This might add to reasons behind wetter than average spring and flood conditions. The reverse scenario of colder temperatures to the eastern coasts of the Indian Ocean creates Positive phases of the IOD and results in drier seasons. The positive phase creates lesser than average rainfall and results in drought conditions during winter. The neutral phases also lead to drought conditions in the southern Australia (Bom.gov.au 2018).
The El Nino southern Oscillation or the ENSO and the La Nina are atmospheric circulation that keep shifting over the east and the west Pacific ocean and thereby affecting the west coast of Peru and the east coast of Australia. The southern Oscillation index or the SOI gives a summary of the effect of these atmospheric circulations and predicts the probable climatic condition that will prevail during the year the seasons as well (Agriculture.vic.gov.au. 2018). The SOI index retrieved from the Bureau of meteorology, Australian government will help us to evaluate the climatic situation of Australian climate during ENSO or La Nina episodes. The SOI positive indicates La Nina conditions which results in wetter Springs over the Australian continent. The SOI negative reflects La Nina situations which are known to have drier experiences over Australian continent (Bom.gov.au. (2018).
The Madden Julian oscillation is an atmospheric circulation related to the monsoons in Australia. The MJO generally extends with monsoon and is a result of the eastward moving clouds in a 30 to 60 day cycle and generally occurs in the tropical zone. This affects the monsoon in Australia and is associated with tropical cyclones in the region. This mostly affects the northern region but some affects are also observed in the southern part. This enhances the rainfall conditions in the northern parts and more than average rainfall is experienced during the active MJO phase. Inactive MJO can result in drier conditions in the southern part during the monsoon (Agriculture.vic.gov.au. 2018).
The southern Annular Mode or SAM also referred as Antarctic Oscillation affect rainfall in Australia to a great extent. The SAM is governed by north and south movement of strong westerly winds. The SAM creates fonts during its positive and negative event depending upon its latitudinal shift. The frontal activity picks up moisture from the Southern Ocean and results in winter rainfalls in southern Australia. SAM creates Low Pressure Belts during its phases. The positive SAM event reduces the chances of rainfall during winter during pole ward shift. The negative SAM event is experienced by shifting to the upper latitudes resulting in above average rainfall during winter (Agriculture.vic.gov.au. 2018).
Conclusion
The effect of seasons in the atmospheric circulation is un evadable and will continue to manipulate the atmospheric circulations around the globe. The Australian landmass experiences the various atmospheric circulations throughout the year which influence the rainfall patterns in the region. The positive and the negative phases of these circulations results in wetter than average conditions and drier than average conditions simultaneously. These affect the flood and drought conditions in the several parts of the continent. The Southern part of the Australian continent is affected by several atmospheric circulations and is greatly affected by fluctuations in these circulation systems.
References
About Australian Climate. (2018). Retrieved from https://www.bom.gov.au/climate/about/australian-climate-influences.shtml?bookmark=enso
Agriculture.vic.gov.au. (2018). The Climatedogs | Understanding weather and climate | Weather and climate | Agriculture | Agriculture Victoria. Retrieved from https://agriculture.vic.gov.au/agriculture/weather-and-climate/understanding-weather-and-climate/climatedogs
Bom.gov.au (2018). Indian Ocean climate influences. [online] Bom.gov.au. Available at: https://www.bom.gov.au/climate/iod/#tabs=Indian-Ocean-climate-drivers [Accessed 29 Oct. 2018].
Bom.gov.au. (2018). Southern Oscillation Index (SOI) history. [online] Available at: https://www.bom.gov.au/climate/current/soi2.shtml [Accessed 1 Oct. 2018].
Chou, C., Chiang, J. C., Lan, C. W., Chung, C. H., Liao, Y. C., & Lee, C. J. (2013). Increase in the range between wet and dry season precipitation. Nature Geoscience, 6(4), 263.
Climatechangeinaustralia.gov.au (2018). Australian climate influences. [online] Climatechangeinaustralia.gov.au. Available at: https://www.climatechangeinaustralia.gov.au/en/climate-campus/climate-system/australian-climate-influences/ [Accessed 29 Oct. 2018].
England, M. H., McGregor, S., Spence, P., Meehl, G. A., Timmermann, A., Cai, W., … & Santoso, A. (2014). Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus. Nature Climate Change, 4(3), 222.
Feng, J., & Li, J. (2013). Contrasting impacts of two types of ENSO on the boreal spring Hadley circulation. Journal of Climate, 26(13), 4773-4789.
McGregor, S., Timmermann, A., Stuecker, M. F., England, M. H., Merrifield, M., Jin, F. F., & Chikamoto, Y. (2014). Recent Walker circulation strengthening and Pacific cooling amplified by Atlantic warming. Nature Climate Change, 4(10), 888.
Raut, B. A., Jakob, C., & Reeder, M. J. (2014). Rainfall changes over southwestern Australia and their relationship to the Southern Annular Mode and ENSO. Journal of Climate, 27(15), 5801-5814.
Wang, S., Huang, J., He, Y., & Guan, Y. (2014). Combined effects of the Pacific decadal oscillation and El Nino-southern oscillation on global land dry–wet changes. Scientific reports, 4, srep06651.
Weather Maps. (2018). Retrieved from https://www.bom.gov.au/australia/charts/?ref=ftr
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