El Niño and La Niña are two phases of the El Niño-Southern Oscillation (ENSO) climate pattern, which is a natural climate phenomenon. They occur when the Pacific Ocean and the atmosphere above it change from their neutral ('normal') state for several seasons. 

El Niño occurs when sea surface temperatures in the central and eastern tropical Pacific Ocean become much warmer than average. As a result, a shift in atmospheric circulation occurs. Usually, equatorial trade winds blow from east to west across the Pacific Ocean. El Niño events often lead to a weakening, or even reversal, of the prevailing trade winds. When this happens, ocean temperatures become warmer in the central and eastern Pacific making these areas more favourable for cloud development and tropical rainfall. Consequently, the heavy rainfall that would normally occur in Australia, moves to the central and eastern parts of the Pacific. 

In contrast, La Niña is characterised by a cooling of sea surface temperatures in the central and eastern tropical Pacific Ocean. This occurs as a result of trade winds becoming stronger which draws cooler, deep water from below. This makes the central and eastern Pacific region less favourable for cloud and rain. These same strong trade winds help to pile up warm surface waters in the western Pacific and to the north of Australia. As a result, the western Pacific area becomes more favourable for rising air, cloud development and rainfall potentially causing heavy rainfall to occur over Australia. 

Typical effects of an El Niño event on Longneck Lagoon include:

• Dry vegetation becomes susceptible to ignition. This leads to an increased bushfire risk

• Drier and warmer conditions can lead to prolonged periods of drought. As a result the lagoon has the potential to dry out

• Dry periods can benefit the lagoon by providing the conditions for seed germination and seed establishment supporting the growth of plants such as rushes. Dry conditions can also improve soil productivity and provide opportunities for wading birds such as spoonbills and plovers to stalk the shallows for fish and insects.

Typical effects of a La Niña event on Longneck Lagoon include:

• Above average rainfall and more intense rainfall events which can lead to flooding 

• Excessive rainfall can lead to waterlogging and runoff, affecting the quality of water in the lagoon

• Increased rainfall can benefit the lagoon as it supports the growth of large trees and water plants which provide a habitat and food source for birds and fish. Furthermore, waterbirds such as ducks and black swans feed, nest and breed in a filled wetland.

East Coast Lows (ECL) are intense low pressure systems which occur, on average, several times each year off the eastern coast of Australia. ECLs are more common during autumn and winter but can occur all year round. ECLs can generate heavy widespread rainfall, gale or storm force winds along the coast and adjacent waters, and very rough seas and heavy swells. 

Longneck Lagoon is located within the Hawkesbury-Nepean Catchment. This catchment is one of the largest coastal basins in NSW with numerous tributaries that flow into the Hawkesbury River. The combination of the geography of the catchment and the heavy, prolonged rainfall that occurs during east coast lows can lead to widespread flooding of the Hawkesbury River which overflows into Longneck Lagoon.