Weather – What Is A La Nina And What Are The Effects?
This ocean temperature map shows a typical La Nina weather pattern, with cooler sea surface temperatures in the eastern pacific and warmer temperatures in the western pacific.

La Nina – Weather Phenomenon and Systems

La Niña is described as a coupled ocean-atmosphere phenomenon that’s the colder counterpart of El Niño, as a part of the broader El Niño–Southern Oscillation (ENSO) climate pattern.

La Niña originates from Spanish, meaning “the little girl” and is the opposite of El Niño, meaning “the little boy.” Within the past, it had also been called an anti-El Niño and El Viejo (meaning “the old man”).

What is the La Nina Weather System?

La Ninas are opposed to El Ninos. La Ninas typically bring above-average rainfall, intense rain-bearing systems that can produce flooding, more intense & severe thunderstorms, lower daytime, and higher nighttime temperatures due to increased cloud cover caused by increased humidity and moisture, and more intense and numerous cyclonic development resulting in higher than average mainland strikes and earlier onset of the cyclone season.

An El Nino is quite the opposite in those regions experiencing an El Nino event experiencing dryer weather, increased droughts, increased bush fires, and other effects.

La Ninas and El Ninos are one of the most influential climate drivers and patterns affecting the planet. An extremely large shift in warm air and temperatures over a large area of the planet causing these conditions.

They also cause lower & higher sea surface temperatures in the affected regions increasing more intense cyclonic formation and activity, especially in areas in the western pacific ocean and the Atlantic Oceans and the other areas affected by the La Nina.

During a La Niña period, the ocean surface temperature across the eastern equatorial part of the central Pacific will be below normal by 3 to five °C (5.4 to 9 °F). An appearance or event of la Niña persists for a minimum of five months.

It’s extensive effects on the weather across the world, particularly in North America, even affecting the Atlantic and Pacific hurricane seasons, during which more tropical cyclones occur within the Atlantic basin thanks to low wind shear and warmer ocean surface temperatures while reducing tropical cyclogenesis within the Eastern pacific and also increasing cyclonic formation in the western pacific.

La Niña is an intricate weather pattern that happens every few years. Consequently, variations in ocean temperatures within the Pacific equatorial band occur as strong winds blow warm water at the ocean’s surface from South America across the Pacific towards Indonesia.

Weather – What Is A La Nina And What Are The Effects?

As this warm water moves in a westerly direction, cold water from the deep sea rises to South America’s surface. It’s considered the cold phase of the broader El Niño–Southern Oscillation (ENSO) weather phenomenon and the opposite of the El Niño weather pattern.

The movement of a massive amount of heat across 1 / 4 of the earth, particularly in temperatures at the ocean surface, can have a significant or significant effect on weather across the planet.

For many years, La Niña events have occurred regularly during the first parts of both the 17th and 19th centuries. Since the start of the 20th century, La Niña events are documented for the following list of event years:

La Nina Event Years Since 1900

1903–1904 1906–1907 1909–1911 1916–1918 1924–1925 1928–1930 1938–1939 1942–1943 1949–1951 1954–1957 1964–1965 1970–1972 1973–1976 1983–1985 1988–1989 1994–1995 1998–2001 2005–2006 2007–2008 2008–2009 2010–2012 2016 2017–2018 2020–2021

La Nina Impacts on the world climate

La Niña impacts the world climate and disrupts standard weather patterns, which, as a result, can cause intense storms in some places and droughts in others.

la Nina Regional impacts

Observations and studies of la Niña events since 1950 show that consequences related to La Niña events depend upon what season it is. However, while certain circumstances and effects are expected to occur during events, it’s not inevitable or guaranteed that they’re going to happen.

Africa

Between 50,000 & 100,000 people died during the 2011 East Africa drought.

La Niña leads to wetter-than-normal conditions in Southern Africa from December to February and drier-than-normal conditions over equatorial East Africa over an equivalent period.

Asia

During the La Niña years, the formation and genesis of tropical cyclones, in conjunction with the subtropical ridge position, shifts westward across the western Pacific, which increases the landfall threat in China.

In March 2008, La Niña lowered sea surface temperatures over Southeast Asia by 2 °C (3.6 °F). It also caused intense rainfall over Malaysia, the Philippines, and Indonesia.

Australia

Across most of the continent, El Niño and La Niña impact climate variability than all other factors. La Niña is characterised by increased rainfall and overcast weather due to increased moisture levels brought in by trade winds, especially across the east and north; snow cover is increased.

There is a robust correlation between the strength of la Niña and rainfall: The greater the ocean surface temperature and Southern Oscillation difference from normal, the larger the rainfall change.

There also are cooler daytime temperatures south of the tropics and fewer extreme highs and warmer overnight temperatures within the tropics. There’s less risk of frost but increased risk of widespread flooding, tropical cyclones, and monsoon season starts earlier.

Weather – What Is A La Nina And What Are The Effects?

North America

La Niña causes the opposite effects of El Niño, above-average precipitation across the northern Midwest, the northern Rockies, Northern California, & the Pacific Northwest’s southern and eastern regions. Meanwhile, rainfall within the southwestern and southeastern states, likewise as Southern California, is below average.

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This also allows for creating or developing the many stronger-than-average hurricanes within the Atlantic and fewer in the Eastern Pacific, near South America Pacific.

The synoptic condition for Tehuantepecer winds is related to the high-pressure system forming in Sierra Madre of Mexico within the wake of an advancing front, which causes winds to speed up through the Isthmus of Tehuantepec.

Tehuantepecers primarily occur during the cold or winter season months for the region in the wake of cold fronts between October & February, with a summer maximum in July caused by the Azores’ westward extension Bermuda high-pressure system.

Wind magnitude is weaker during La Niña years than El Niño years, thanks to the less frequent cold frontal incursions during La Niña winters and colder months, with its effects can last from a couple of hours to 6 days. Between the years 1942 and 1957, La Niña had an impact that caused isotope changes within the plants of Baja California.

In Canada, La Niña will, generally, cause a colder, snowier winter, like the near-record-breaking amounts of snow and ice recorded in La Niña winter of 2007–2008 in Eastern Canada.

South America

During the time of la Niña, drought can occur in the coastal regions of Peru and Chile. From December to February, northern Brazil has higher than average rainfall than usual. La Niña causes above normal rainfall within the central Andes, which successively causes intense to catastrophic flooding and rain on the Llanos de Mojos of Beni Department, Bolivia.

Such flooding is historical from the years of 1853, 1865, 1872, 1873, 1886, 1895, 1896, 1907, 1921, 1928, 1929, and 1931.

Weather – What Is A La Nina And What Are The Effects?

What is La Niña, and how does it impact Australia?

Australia‘s Many climate drivers influence Australia’s weather. El Niño & La Niña have the most substantial influence on year-to-year climate variability for much of the country. They’re a part of a natural cycle referred to as the El Niño-Southern Oscillation (ENSO). The ENSO cycle loosely operates over timescales from 1 to 8 years.

La Niña typically means:
  • Increased rainfall and rain-bearing systems across much of Australia
  • Cooler daytime temperatures (south of the tropics) due to cloud cover.
  • Warmer overnight temperatures (in the north) due to cloud cover.
  • A shift in temperature extremes
  • Less frost
  • Greater tropical cyclone numbers, strikes, and formations
  • Earlier monsoon onset than normal

What causes La Niña?

The La Niña develops when equatorial trade winds become more robust, changing ocean surface currents and drawing cooler, deeper water up from below. This leads to a cooling of the central and eastern tropical Pacific. The enhanced trade winds also help bring together warm surface waters within the western Pacific and north of Australia.

The warming of ocean temperatures within the western Pacific means the realm becomes more favourable for rising air, cloud development, and rainfall. As a result, heavy rain can occur in the north of Australia.

Conversely, over the eastern & central tropical Pacific, air falls over the cooler waters, meaning the region is less favourable for cloud and rain. The air rising within the west and falling within the east enhances an atmospheric circulation – called the Walker Circulation – which may lead to changes to the climate felt across the world.

Monitoring La Niña

The term La Niña describes a specific phase of the ENSO climate cycle. ENSO is a coupled atmosphere-ocean phenomenon, which implies that the transition between La Niña, El Niño, and neutral conditions (neither El Niño nor La Niña) is controlled by interactions between the earth’s atmosphere & ocean circulation.

In the earth’s atmosphere, ENSO is monitored and observed via the Southern Oscillation Index (SOI), a measure of atmospheric circulation that takes the difference of atmospheric or air pressure between Darwin and Tahiti.

ENSO is most typically monitored within the ocean through observed sea surface temperatures within a central and eastern tropical Pacific neighborhood, referred to as NINO3.4.

El Niño and La Niña aren’t turned on and off, such as a switch. Instead, El Niño and La Niña are a function of departures’ strength from average in NINO3.4 and the SOI. This suggests that if conditions are near to La Niña (El Niño) thresholds, one might expect to ascertain some La Niña-like effects on Australia and its regions.

La Niña developments are typically defined when SOI values are sustained above +7, and NINO3.4 temperatures are over −0.8 °C below average. Events that maintain index values near these thresholds are generally classified as moderate to weak, while La Nina events that greatly exceed them are named as strong.

Effects of La Nina

Increased rainfall

The increased rainfall and cloudiness within the western Pacific related to La Niña usually mean an above-average winter-spring months rainfall for Australia, particularly across the continent’s eastern and northern areas.

The six wettest winter-spring seasons on record for eastern Australia occurred during the La Niña years. Within the Murray–Darling Basin, winter-spring rainfall averaged over all 18 La Niña events (including multi-year events) since 1900 was 22% above the long-term average, with the severe floods of 1955, 1988, 1998, and 2010 all related to La Niña.

Unlike El Niño years, the effects of La Niña often continue into the dryer months. In eastern Australia, the typical December-March rainfall during La Niña years is 20% above the long-term average, with 8 of the ten wettest periods occurring during La Niña years.

This is often particularly notable for the East Coast, which tends to be less suffering from La Niña over the winter months but can experience intense flooding during La Niña summers and the warmer months.

Weather – What Is A La Nina And What Are The Effects?

The presence of La Niña heightens the prospect of widespread and regional flooding. Of the eighteen La Niña events since 1900 (including multi-year events), 12 have resulted in floods for a few parts of Australia, with the Eastern Coast of Australia experiencing twice as many severe or intense floods during La Niña years than El Niño years.

Typically, some northern Australia areas will experience flooding during La Niña due to the rise in tropical cyclone formations.

The key between La Niña strength and rainfall is closely linked. The greater the ocean surface temperature and SOI difference from normal, the more intense and above-average the rainfall that can be expected. Indeed, the wettest and most significant weather years on record for Australia occurred during the strong La Nina of 2010–2012 and 1974 La Niña events.

These included Cyclone Tracey, (arguably the worst tropical cyclone to hit Australia in terms of lives lost and damage) which impacted the capital of the Northern Territory, Darwin, and the catastrophic flooding event that affected SE Queensland and Brisbane from the intense tropical Cyclone Wanda(see Cyclone Wanda track map) that weakened into a robust tropical depression, otherwise known as the 1974 floods. Mt Glorious(a City in Southeast Queensland) recorded 1318 mm of rainfall in 5 days.

The 2010–12 La Niña event was particularly devastating in terms of widespread severe flooding across Australia. However, the influence or driver of the Indian Ocean likely also increased and enhanced the widespread rainfall effect.

Cooler temperatures

La Niña years tend to experience colder than the average maximum or daytime temperatures across most of Australia, to the south of the tropics, particularly during the last half of the year.

This is often thanks to increased cloudiness and rainfall. However, the increased cloudiness tends to act as an insulator and leads to warmer than average minimum temperatures and nighttime temperatures across northern and eastern Australia during La Niña.

The shift in temperature extremes

The colder than average daytime temperatures during La Niña years are usually related to a decreased frequency higher than average daily high temperatures.

In the warmer period of the year, southern coastal locations like Adelaide and Melbourne experience fewer individual daily heat extremes and maximums during La Niña years but an above-average frequency of prolonged warmer spells. Of the thirty-two Victorian heatwaves between 1989 and 2009, 17 occurred during La Niña years while only six occurred during El Niño 3.

Due to increased cloudiness and warmer nights, regions of southern New South Wales and northern Victoria can experience fewer frost days during La Niña than the historical average.

Increased tropical cyclone numbers, formations, and land strikes.

During La Niña, there are generally more tropical cyclones within the Australia region and surrounding areas, with twice as many making landfall than during El Niño years on the average. The first cyclone to cross the Australian coast also tends to occur earlier within the season.

The only years with multiple severe tropical cyclone landfalls and strikes in Queensland are La Niña years. This suggests an increased likelihood of significant damage and flooding associated with strong winds, high seas & storm tides, and intense rainfall from tropical cyclones.

Earlier monsoon onset

The monsoon onset in tropical Australia is usually a fortnight earlier during a La Niña event than in El Niño events. This suggests that rainfall within the northern tropics is usually above-average during the first part of the wet season for La Niña years but only slightly above average during the latter part of the wet season.

Australian rainfall patterns during La Niña

The composite picture for summer (December to February) is shown within the following map. This map shows above-average rainfall amounts over a La Nina event in the summer or warmer months.

Weather – What Is A La Nina And What Are The Effects?
Image source: http://www.bom.gov.au/climate/enso/ninacomp.shtml

In relative terms, La Niña’s impact on Australian summer rainfall is smaller than the winter-spring impact. This is often especially the case over Tasmania, SA, the NT, and WA, which have large deciles 5 and 6 within the image above. As within the June-November period, the impact from La Niña is more widespread and intense during summer than the corresponding effect from El Niño.

There are a contraction and weakening of the effect in western Queensland. Still, within the east of that state, the La Niña-induced tendency towards wetter than average conditions continues to be moderate to strong.

This is often significant as of December to February are, on average, three of the four wettest months in this region of the country. In eastern NSW, there’s an increased tendency for wet conditions as compared with the impact during the winter-spring period.

Like east Queensland, this is often significant in northern NSW because the region experiences a natural summer rainfall maximum.

Further reading

Weather Maps and Tools


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