Lecture 5: Climate variability (ENSO)

ENSO

El Nino Southern Oscillation, an irregularly periodical climate changed caused by variations in SST's over the tropical eastern Pacific Ocean.

What is El Nino, how does it evolve, and what are its impacts (at different spatial scales)?

El Nino refers to the warm phase of the El Nino Southern Oscillation (ENSO), and involves a band of warm ocean water that develops in the East equatorial Pacific, affecting the tropics and sub-tropics. It is and irregular large-scale atmosphere-ocean phenomenon that is sometimes known as 'the heartbeat of the world'. Normal Conditions: Trade winds blowing East-West push equatorial warm ocean surface water to the West Pacific, which is piled up around Australia/Indonesia. This warm pool of water, of continental size is warm due to equatorial warming. The warm sea surface temperatures in the West Pacific means the air is moist, thus higher rainfall events. The East Pacific around North America is influenced by the Peruvian Current which helps to unwell deeper, cold water that is nutrient-rich. Pressure gradient from a high pressure system in Eastern Pacific ocean, and low pressure over Indonesia. (Walker Circulation). El Nino: An El Nino occurs when the trade winds slacken or the Walker Circulation weakens. The warm surface water (of continental size) makes its way back across the Pacific. This vast river of ocean moves Eastward, taking all of its energy. The Peruvian current is weakened as a result. The Western Pacific develops a high pressure system, and experiences a decrease in sea level; the Eastern Pacific an increase in sea level. Eventually, the heat is dissipated, and sea-level returns to normal. Trade winds build up warm pool again, and returns read for next cycle. Globally, El Nino impacts temperature and precipitation. The prospect of an El Nino is somewhat a mixed blessing for California. On one hand, it may break drought. On the other, it may bring torrential rain causing flooding, or severe storms that cause death. Also, torrential rain is not much use as most water goes down storm drains. Australia and Indonesia may experience very dry, windy dust storms. The movement of the warm sea surface means decreased air moisture, thus decreased rainfall. This may be detrimental to agriculture. Famine and death. El Nino also impacts the genesis of cyclones and their tracks. During El Nino, as warm water moves east, so does the potential for cyclones to form over these areas, which increases. (Pushes cyclones eastward, decrease potential for cyclones around North of Aus). South America is negatively affected; the build-up of warm, nutrient-depleted surface water, and also the weakening of the Peruvian current, detrimental to fisheries. North America experiences warmer and wetter conditions. When El Nino occurs, warm surface water moves, and so does unsettled weather, affecting rainfall patterns across equatorial pacific as well as large scale wind patterns. Change in winds has knock on effect, changing rainfall and temperature in locations around world. Main impacts around topics: increased in risk of floods in Peru, and droughts in India and other parts of brazil. Extra heat at surface of pacific releases vast amounts of energy into the atmosphere which can temporarily push up global temperatures. This is why El Nino years warmest on record. El Nino changes the frequency and strength of the wind coming towards us. Effects topography; how much precipitation we get (depending on what side of mountains you are). Palmerston North experiences drier winters. Tauranga, effects agriculture and drying out in spring. Bottom of South Island: exposed area to increasing cold South-Westerly winds, wet and cold conditions. Impacts water supply system in Auckland; lack of ability of water reservoirs to recharge.

Teleconnection: 'distant' connection.

○ Although El Nino occurs in the Pacific, has a global-scale impact! ○ Effect on temperature: § Relatively cool conditions in NZ from El Nino. This is because pressure over Indonesia-Aus increases, resulting more Southerly Westerly winds which are the prevailing winds in NZ. Conversely during La Nina, other way around. ○ Effect on precipitation: § Associated with famine. India very dry from El Nino. Significant drought and famine. Normally, wetter.

La Nina

Cooling phase, associated with low air surface pressure in the tropical western Pacific. Counterpart of El Nino. Results from strengthened trade winds. Upwelling of cold nutrient-rich water enhanced as more cool water travels West from coast of South America.

Southern Oscillation

Is the atmospheric component of El Nino. This component is an oscillation in surface air pressure between the tropical eastern and the western Pacific ocean waters. El nino =negative, La Nina=positive.

Warm pool

Region of warm SST.

Walker circulation leading to el nino and la nina

The Walker circulation is caused by the pressure gradient force that results from a high pressure system over the eastern Pacific ocean, and a low pressure system over Indonesia (west). Temperature difference across Pacific, warmer water in Western, cooler in East. Warmer water adds extra heat to air causing it to rise with more vigour, and this rising air creates more unsettled weather with more cloudy and rainfall. This rising air sets up atmospheric circulation with warm moist air rising on west, and cooler drier air descending in west.

El Nino

The warm phase of the El Nino Southern Oscillation, and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific, affecting much of the tropics and subtropics. Accompanied with high air surface pressure in the tropical western Pacific. Causes big changes in SST, is a large-scale atmosphere-ocean phenomenon.

Normal Conditions

Trade winds blowing East-West push equatorial warm ocean surface water to the West Pacific, which is piled up around Australia/Indonesia. This warm pool of water, of continental size is warm due to equatorial warming. The warm sea surface temperatures in the West Pacific means the air is moist, thus higher rainfall events. The East Pacific around North America is influenced by the Peruvian Current which helps to unwell deeper, cold water that is nutrient-rich. Pressure gradient from a high pressure system in Eastern Pacific ocean, and low pressure over Indonesia. (Walker Circulation).