Ocean Currents and Climate

1 Global Energy Budget

Climate is the average weather conditions over a long period of time for a region. Without greenhouse gases, the Earth would not be liveable. These gases act like a blanket, trapping some of the radiation on its way through our atmosphere. Incoming solar radiation is: • Reflected back to the atmosphere by clouds and the Earth’s surface • Absorbed by the atmosphere, clouds, land and ocean Eventually this energy is re‐radiated back into space (some of this is temporarily trapped by greenhouse gases on the way out).

2 Latitude

A factor affecting climate is latitude. Places at lower latitudes (nearer the equator) are warmer than at high latitudes (polar regions). When compared with the equator, heat energy received from the Sun has had to pass through more atmosphere and, due to the curvature of the Earth, must cover a much larger area. For this reason, every square metre of land or sea in the polar region receives less solar energy than every square metre nearer the equator.

3 Elevation/topography

Temperatures usually decrease at a rate of 1degree Celsius for every 100 metres of height. This is largely due to the fact that the higher you go the less dense the air becomes and less able to retain heat.

4 Proximity to the ocean

The ocean heats up and cools down far more slowly than the land. For this reason, coastal regions tend to experience more temperate conditions and alower temperature range than inland regions of similar latitude and altitude.

5 Cold or warm ocean currents

Driven by prevailing winds, ocean currents move heat around the globe. Warm currents originating in tropical regions carry heat towards the polar regions. Cold currents flowing from the polar regions help to cool the lower latudes. Ocean currents have a large impact on climate. For example, Perth receives most of its rainfall in autumn/winter due to the prevalence of the warm Leeuwin Current bringing warm water down our coast. The prevailing westerly winds travel across the ocean picking up moist air, leading to precipitation on land. During spring/summer the cold West Australian current is more predominant so rainfall is reduced (less moisture above the cold current). Worldwide examples include: Northern Europe experiences a mild and wet winter due to the warm Gulf Stream Drift (pushing into the North Atlantic Current). Similar latitudes in North Canada experience a much harsher winter due to the cold Labrador Current running off its coast (more snow and hail). Albedo also bears mentioning here. Albedo is the reflectivity of asurface. Polar regions covered in ice and snow have a high albedo which means they reflect a large portion of the solar radiation reaching them, leading to further cooling. Desert areas have a low albedo so absorb much of the solar radiation reaching them.

6 ENSO • El Niño Southern Oscillation Index • Operates in the Pacific Ocean • We are impacted by the see‐sawing pressures over the ocean between South America and Australia

7 Normal Conditions Trade Winds

Trade winds blow across the Pacific Ocean pushing warmer surface waters west.

Between 30 degrees north and 30 degrees south of the equator, the Earth’s rotation causes air to slant toward the equator. This is called the Coriolis Effect and it causes the trade winds to move from east to west on both sides of the equator across this belt. Intense heat at the equator causes evaporation and a rising air mass. The air moves toward the poles and then sinks back to the surface, triggering the calm trade winds.

8 Normal Conditions

Humid and wet in Weather the north –normal conditions

Relatively dry – normal conditions

The western side –namely the east coast of Australia typically has warm, wet weather (particularly in the north). This is due to the moisture sitting above the warm oceanic waters which is readily collected by winds and can be precipitated on to land. This region often experiences thunderstorms. The eastern side –namely the west coast of South America is characterised by relatively dry conditions due to the loss of warm surface waters and its replacement by the cold Humboldt/Peru Current. Air above these cool surface waters holds very little moisture resulting in low rainfall in this region.

9 El Niño Trade Winds

Trade winds weaken or reverse. Warm water ‘slumps’ back towards the east.

Occurring about 3‐7 years apart an El Niño is a time when trade winds weaken or reverse in the Pacific Ocean. This state can last anywhere between nine months to two years. At this time the pressure differences (or SOI) are relatively small. This leads to a warming of the central and eastern tropical Pacific Ocean. (The SOI is computed from fluctuations in the surface air pressure difference between Tahiti and Darwin.)

10 El Niño Increased chance of Drier than average Weather floods and hurricane

Increased chance of drought and bushfire Wetter than average

Due to the lower than average sea surface temperatures the east coast of Australia (in the West Pacific) experiences drier than average conditions. This can lead to drought and bushfires. The east pacific –namely the west coast of South America experiences higher than average rainfall. This is due to the warmer surface waters providing moisture. At this time there is an increased chance of floods and Category 5 hurricanes.

11 La Niña Trade Winds

Trade winds strengthen pushing warm surface waters across the Pacific.

Traditionally La Niña’s occurred at a similar frequency to El Niño’s, current records show a shift towards El Nino events in recent years. At the time of aLaNiña the pressure differences between the west and east Pacific (or SOI) are heightened. Trade winds are strengthened pushing warm surface waters west. This leads to a warming of the western Pacific and cooling of the eastern Pacific.

12 La Niña Increased chance of drought and bushfire Weather Wetter than average More cyclones

Drier than average

On the west coast of South America, La Niña events are accompanied with a decrease in rainfall which may lead to drought and increase the chances of severe bushfires. (due to cool sea surface temperatures = less moisture = less rainfall) On the east coast of Australia wetter than average conditions are experienced. This can lead to flooding events, like the 2010 Queensland floods. The west coast of Australia is potentially impacted by an increase in cyclonic acvity. You may also want to view our animated video on this topic ‐ https://www.youtube.com/watch?v=ifGOjSTwbM8

13 References

• Australian & South American Outline Maps, About.com Geography, accessed at http://geography.about.com/library/blank/blxaustralia.htm on 12 October, 2012 • NOAA: What are the trade winds? Accessed at https://oceanservice.noaa.gov/facts/tradewinds.html on 19 March 2020 Unless otherwise stated, all information and graphics are from: • Tompkins, D.E. (Ed.), 2010, Exploring Earth and Environmental Science Stages 1, 2 and 3, Earth Science Western Australia or • ESWA photo/graphic library

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