Global Atmospheric Circulation is the movement of air around the planet. It explains how thermal energy and storm systems move over the Earth's surface. There are three main cells that move air and heat around the world with global impacts. They are The Hadley cell, The Ferrel cell and Polar cell. 

Hadley cells impact Australia's climate the most. It moves air from the equator to between 30 and 40 degrees north and south of the equator. At the equator the extreme direct sunlight and heat from the sun makes moist air rise from the earth and as it moves southwards, it cools and descends. Watch the clip below "What is global circulation" to explain the Hadley Cell's functioning. 

As air from the Hadley Cell circulation descends to the surface, the Coriolis effect, caused by the rotation of the earth, makes the air flow east to west, resulting in regular northeasterly trade winds. The Trades Winds affect much of northern and parts of eastern Australia. They collect moisture as they move eastward over the tropical Pacific Ocean towards the east coast of Australia and are associated with enhanced rainfall to tropical and sub-tropical areas of the east coast (bom.gov.au). These trade winds are important for the El Nino and La Nina cycles too (go to next section).

As a result of these global circulation patterns and other factors, Sydney has a temperate climate zone. This mean it has warm to hot summers and no dry season. See the climate graph for Sydney provided. 

For more information about the three main atmospheric cells go to: https://www.noaa.gov/jetstream/global/global-atmospheric-circulations

Wind and air masses

The prevailing winds reflect the air masses and  pressure systems which dominate Sydney’s weather. The strength and direction of prevailing winds affect factors such as the rates of erosion and the circulation of water in Sydney Harbour.   

There are three dominant wind patterns affecting Sydney Harbour. The strongest winds are from the south, the most frequently observed wind direction is from the northeast and the westerlies usually occur in winter. Global warming is increasing the intensity of storms, eg, East Coast lows with associated increases in wind speed.

La Niña refers to the extensive cooling of the central and eastern tropical Pacific Ocean, often accompanied by warmer than normal sea surface temperatures (SSTs) in the western Pacific, and to the north of Australia. La Niña events are associated with increased probability of wetter conditions over much of Australia, particularly over eastern and northern areas. La Niña events have been correlated with higher numbers of tropical cyclones during the cyclone season (November to April). (bom.gov.au)

El Niño refers to the extensive warming of the central and eastern tropical Pacific that leads to a major shift in weather patterns across the Pacific. El Niño events are often accompanied by cooler than normal sea surface temperatures (SSTs) in the western Pacific, and to the north of Australia. Over much of Australia, but particularly eastern Australia, El Niño events are associated with an increased probability of drier conditions. (bom.gov.au)

As our plant continues to warm driven by increasing greenhouse gas emissions, El Niño and La Niña events are becoming more frequent and intense. 

East coast lows are intense low-pressure systems bring heavy rainfall and strong and gusty winds to parts of southeastern Australia including Sydney. 

They can bring large offshore waves which can lead to erosion of coastal areas including the Sydney Harbour foreshore. East Coast Lows can provide ecosystems with fresh water for soil and plant nourishment but too much rainfall can lead to increased soil runoff and expose plant root systems. Additionally, strong winds can destroy large branches in the canopy cover of harbour bushland which can change habitats for wildlife. 

For more information about ECL's visit: https://www.abc.net.au/news/2020-02-07/what-is-an-east-coast-low-wild-weekend-weather/11936190

Sydney has a temperate climate zone. This mean it has warm to hot summers and no dry season. Sydney's climate has no extreme seasonal differences as the weather is moderated by proximity to the ocean. The ocean warms and cools more slowly than the atmosphere, thus coastal weather tends to be more moderate than continental weather, with fewer hot and cold extremes. See the climate graph below which represents this temperate climate. 

Orographic rainfall refers to the precipitation which is caused by hills or mountain ranges deflecting the moisture-laden air masses upward, causing them to cool and precipitate their moisture (forecast.weather.gov). In Sydney, the Great Dividing Range stops the moist air masses from crossing the state resulting in rain falling in the Sydney basin. Due to the orographic precipitation occurring along the Sydney coast, the areas to the west experience a rain shadow effect as the warm, moisture laden air from the ocean cool, condense and form precipitation before reaching inland areas. Balls Head and Goulburn are only 100 kilometres apart, yet have very different patterns of rainfall, as seen in the following mean rainfall charts below.

Balls Head receives rainfall as a result of its proximity to the coast as well as and orographic rainfall.