La Nina – Weather Phenomenon and Systems
- 1.1 What is the La Nina Weather System?
- 1.2 La Nina Event Years Since 1900
- 1.3 La Nina Impacts on the world climate
- 1.4 la Nina Regional impacts
- 1.5 Africa
- 1.6 Asia
- 1.7 Australia
- 1.8 North America
- 1.9 South America
- 1.10 Such flooding is historical from the years of 1853, 1865, 1872, 1873, 1886, 1895, 1896, 1907, 1921, 1928, 1929, and 1931.
- 1.11 What is La Nina, and how does it impact Australia?
- 1.12 What causes La Nina?
- 1.13 Monitoring La Nina
- 1.14 The effects of La Nina
- 1.15 Australian rainfall patterns during La Nina
- 1.16 Further reading
- 1.17 Weather Maps and Tools
La Nina – Weather Phenomenon and Systems
La Nina can be described as a coupled ocean/atmosphere phenomenon, which is the colder counterpart to El Nino. It’s part of the El Nino-Southern Oscillation climate pattern.
La Nina is Spanish for “the little girl” and the opposite to El Nino, which means “the little boy”. It was also known as El Viejo and an anti-El Nino in the past.
What is the La Nina Weather System?
La Ninas are known for bringing more than average rainfall and intense rain-bearing system that can cause flooding. They also have higher daytime and nighttime temperatures because of increased cloud cover due to higher humidity and moisture. This results in more severe cyclonic development and earlier onset of the 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.
El Ninos and La Ninas are two of the most important climate drivers that affect the planet. These conditions are caused by a significant shift in the temperature and warm air over large areas of the earth.
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.
The ocean surface temperature in the eastern equatorial region of the central Pacific during a La Nina period will be below the average level by three to five degrees Celsius (5.4 to 9.3 degrees F). A la Nina event or appearance lasts at most for 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 Nina, a complex weather pattern that occurs every few years, is a rare phenomenon. As strong winds blow warm ocean water from South America to the Pacific towards Indonesia, there are variations in ocean temperature within the Pacific equatorial bands.
This warm water flows westerly, and cold water from the deep sea rises to South America’s surface. This is the cold phase of El Nino-Southern Oscillation’s (ENSO) weather phenomena and the opposite to El Nino.
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.
La Nina events have been occurring regularly over the first part of the 17th century and the 19th century for many years. The following events years have been recorded for La Nina events since the beginning of the 20th century:
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 Nina 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
Studies and observations of la Nina events from 1950 to now show that the consequences of these events are dependent on their season. While certain events are likely to have specific effects and circumstances, they are not guaranteed.
Between 50,000 & 100,000 people died during the 2011 East Africa drought.
La Nina 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.
The La Nina years are when the formation and genesis tropical cyclones are enhanced by the subtropical Ridge shift across the western Pacific. This increases the risk of landfall in China.
La Nina, a tropical storm that lowered the sea surface temperatures in Southeast Asia by 2 degrees Celsius (3.6 degrees F) in March 2008, resulted from La Nina. It also brought heavy rainfall to Indonesia, Malaysia, and the Philippines.
El Nino, La Nina, and other factors significantly impact climate variability across the continent than any other factor. La Nina is characterized by more rain and cloudy weather, particularly in the east and north. Trade winds bring more moisture to the area. Snow cover is also higher.
There is a robust correlation between the strength of la Nina and rainfall: The more significant the ocean surface temperature and Southern Oscillation difference from normal, the more influential the rainfall change.
There are fewer extreme highs in the region and temperatures overnight within the tropics. Also, daytime temperatures are generally cooler south of the Tropics. The risk of frost is lower, but there’s more risk of flooding and tropical cyclones. Also, the monsoon season begins earlier.
La Nina is the opposite of El Nino. It causes above-average precipitation in the northern Midwest, northern Rockies, Northern California and the Pacific Northwest’s eastern and southern regions. However, rainfall in the southwestern and southern states and Southern California are below average.
This also creates or develops 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. A summer maximum in July is caused by the Azores’ westward extension Bermuda high-pressure system.
La Nina years have a lower wind magnitude than El Nino years. This is due to less frequent cold frontal incursions in La Nina winters. The colder months and shorter durations of the cold frontal intrusions can cause the wind to weaken. Between 1942 and 1957, La Nina caused significant isotope shifts in Baja California’s plants.
In Canada, La Nina will, generally, cause a colder, snowier winter, like the near-record-breaking amounts of snow and ice recorded in La Nina winter of 2007-2008 in Eastern Canada.
Drought can sometimes occur along the coast of Chile and Peru during la Nina. Northern Brazil experiences are higher than normal rainfall from December through February. La Nina, which causes more than normal rainfall in the central Andes and subsequent flooding and rain at the Llanos de Mojos (Bolivia) Department, causes severe to catastrophic flooding.
Such flooding is historical from the years of 1853, 1865, 1872, 1873, 1886, 1895, 1896, 1907, 1921, 1928, 1929, and 1931.
What is La Nina, and how does it impact Australia?
Australia‘s climate drivers have a significant impact on Australia’s weather. The most significant influence on climate variability for large parts of Australia is El Nino and La Nina. They are part of the El Nino-Southern Oscillation, a natural cycle. The ENSO cycle operates loosely over a time period of 1-8 years.
La Nina 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
- More significant tropical cyclone numbers, strikes, and formations
- Earlier monsoon onset than normal
What causes La Nina?
When equatorial winds become more vigorous, changing ocean surface currents, and drawing deeper, cooler water up from the bottom, the La Nina is formed. This causes cooling in the eastern and central tropical Pacific. Stronger trade winds also bring together warm waters in the western Pacific and northern Australia.
Warmer ocean temperatures in the western Pacific make it more favourable for rain, cloud development and rising air. Heavy rains can sometimes occur in the northern part of Australia.
The opposite is true for the central and eastern tropical Pacific. Air falls over cooler waters which makes the area less favourable for rain and cloud. The air rising within the west and falling within the east enhances an atmospheric circulation – called the Walker Circulation is a phenomenon that may cause changes in the climate around the globe.
La Nina refers to a phase of the ENSO climate cycle. ENSO is a coupled ocean-atmosphere phenomenon. This means that the transition between La Nina and El Nino and neutral conditions (neither El Nino nor La Nina) are controlled by the interactions between the earth’s atmosphere and ocean circulation.
ENSO is most typically monitored within the ocean through observed sea surface temperatures within a central and eastern tropical Pacific neighbourhood, referred to as NINO3.4.
El Nino or La Nina can’t be turned on and off like a switch. Instead, El Nino or La Nina depend on the strength of deviations from the average in NINO3.4 or the SOI. This means that when conditions are close to El Nino thresholds, it is possible to expect some La Nina-like effects in Australia and its regions.
La Nina events are defined as those with SOI values above +7 and NINO3.4 temperatures greater than -0.8 degrees Celsius below the average. Events with index values close to these thresholds are usually classified as moderate or weak. La Nina events that exceed them are called strong.
The effects of La Nina
The increased rainfall and cloudiness within the western Pacific related to La Nina usually mean an above-average winter-spring months rainfall for Australia, particularly across the continent’s eastern and northern areas.
The La Nina years were the wettest in eastern Australia’s history. The average winter-spring rainfall in the Murray-Darling Basin was 22% higher than the long-term average. This includes the severe floods that occurred in 2005, 1998, 1998, 2010 and 1988.
La Nina’s effects often last into the dry months, unlike El Nino years. The average December-March rainfall in eastern Australia is 20% higher than the long-term average. Eight of the ten wettest periods occur during the La Nina year.
This is often particularly notable for the East Coast, which suffers less from La Nina over the winter months but can experience intense flooding during La Nina summers and the warmer months.
La Nina increases the likelihood of flooding in large areas and regions. Twelve of the 18 La Nina events (including multi-year ones) have caused floods in parts of Australia. The Eastern Coast of Australia has experienced twice the number of severe or intense floods during the La Nina year than in El Nino years.
Typically, some northern Australian areas will experience flooding during La Nina due to the rise in tropical cyclone formations.
La Nina strength is closely related to rainfall. The more extreme and above-average rainfall can be expected, the higher the ocean surface temperature and the SOI difference. The extreme La Nina events of 2010-2012 and 1974 La Nina events were Australia’s most significant and wettest years.
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.
In terms of severe flooding, the 2010-12 La Nina event caused widespread destruction across Australia. The Indian Ocean’s influence on the rainfall effects was likely to have increased or been amplified.
La Nina years are colder than average maximum or daytime temperatures in most of Australia, south of the Tropics, especially during the last half-year.
This can often be attributed to higher levels of cloudiness and rain. However, increased cloudiness can act as an insulation and results in warmer than average minimum temperatures and nights across northern and eastern Australia during La Nina.
Temperature extremes have changed.
The colder than average daytime temperatures during the La Nina years are usually related to a decreased frequency higher than average daily high temperatures.
The year’s warmer months are warmer. Southern coastal areas like Adelaide and Melbourne have fewer heat extremes and maximums per day during the La Nina years but a higher frequency of longer warmer spells. Between 1989 and 2009, 17 Victorian heatwaves occurred during the La Nina years. El Nino was only six.
Due to increased cloudiness and warmer nights, southern New South Wales and northern Victoria regions can experience fewer frost days during La Nina than the historical average.
Increased numbers and formations of tropical cyclones, as well as land strikes.
La Nina is characterized by more tropical cyclones in the Australian region and the surrounding areas. On average, twice as many make landfall during La Nina than El Nino years. The season also has a tendency to see the first cyclone cross the Australian coast earlier.
La Nina years are the only years that have had multiple severe tropical storm strikes and landfalls in Queensland. This indicates an increase in the likelihood of flooding and significant damage from strong winds, storm tides and high seas & storm waves, as well as intense tropical cyclone rainfall.
Monsoon onset earlier
In Australia, the Monsoon onset is typically a fortnight sooner during a La Nina event than in El Nino events. This means that the rainfall in the northern tropics is generally above-average during the first half of the dry season for La Nina year but slightly below average during the second half of the wet season.
Australian rainfall patterns during La Nina
Below is the map showing the composite picture of summer (December to February).
Relatively speaking, La Nina’s effect on Australian summer rainfall was smaller than its winter-spring counterpart. This is especially true in Tasmania, SA and the NT. Like El Nino’s effect in June-November, La Nina’s summer impact is stronger than El Nino’s.
The effect is weakening and contracting in western Queensland. However, in the east, the La Nina-induced tendency toward wetter conditions than average continues to be strong.
This is important because December to February are, on average, three of the four most wet months in the country.
There is a greater likelihood of wet conditions in eastern NSW than there is during the winter-spring season.
Like east Queensland, this is often significant in northern NSW because the region experiences a natural summer rainfall maximum.
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