MANILA, Philippines — After nearly a decade’s absence, La Niña is back in the central and eastern equatorial Pacific Ocean as weather models have forecast above-average rainfall for Southeast Asia, according to the World Meteorological Organization.
The WMO said La Niña has developed and is expected to last into next year, affecting temperatures, precipitation and storm patterns in many parts of the world.
The United Nations weather agency noted that the global declaration of a La Niña event is used by governments to mobilize planning in climate-sensitive sectors like agriculture, health, water resources and disaster management.
“The WMO is now stepping up its support and advice for international humanitarian agencies to try to reduce the impacts among the most
vulnerable at a time when coping capacities are stretched by the COVID-19 pandemic,” it said.
This year’s La Niña is expected to be moderate to strong. The last time there was a strong event was in 2010-2011, followed by a moderate event in 2011-2012.
La Niña refers to the large-scale cooling of the ocean surface temperatures in the central and eastern equatorial Pacific Ocean, coupled with changes in the tropical atmospheric circulation, namely winds, pressure and rainfall. It usually has the opposite impacts on weather and climate as those of El Niño, which is the warm phase of the so-called El Niño Southern Oscillation (ENSO).
“El Niño and La Niña are major, naturally occurring drivers of the Earth’s climate system. But all naturally occurring climate events now take place against a background of human-induced climate change which is exacerbating extreme weather and affecting the water cycle,” WMO Secretary-General professor Petteri Taalas said.
“La Niña typically has a cooling effect on global temperatures, but this is more than offset by the heat trapped in our atmosphere by greenhouse gases. Therefore, 2020 remains on track to be one of the warmest years on record and 2016-2020 is expected to be the warmest five-year period on record,” Taalas added, noting that La Niña years now are warmer even than years with strong El Niño events of the past.
La Niña is expected to result in sea surface temperatures between two and three degrees Celsius cooler than average, according to Dr. Maxx Dilley, deputy director in charge of the Climate Services Department at WMO.
“These coolings of these large ocean areas have a significant effect on the circulation of the atmosphere that’s flowing over them. And the changes in the atmosphere in turn affect precipitation patterns around the world,” Dilley said.
The WMO’s new ENSO Update stated that there is a high likelihood (90 percent) of tropical Pacific sea surface temperatures remaining at La Niña levels through the end of 2020, and maybe through the first quarter of 2021 (55 percent). This follows more than a year of neutral ENSO conditions – neither El Niño nor La Niña. The update is based on forecasts from WMO Global Producing Centers of Long-Range Forecasts and expert interpretation.
The WMO warned of uneven effects of La Niña around the globe.
“Elsewhere, WMO’s weather models forecast above-average rainfall for Southeast Asia, some Pacific Islands and the northern region of South America,” it said.
The UN agency also warned that East Africa is forecast to see drier-than-usual conditions, which, together with the existing impacts of the desert locust invasion, might add to regional food insecurity.
The WMO said there is a 90-percent chance of tropical Pacific sea surface temperatures remaining at La Niña levels for the remainder of the year, and a 55-percent chance that this would continue through March next year.
It added that this is important because La Niña contributes to temperatures, rainfall and storm patterns in many parts of the world.
The WMO also noted that El Niño and La Niña are not the only factors that drive global and regional climate patterns.
“No two La Niña or El Niño events are the same, and their effects on regional climates can vary depending on the time of year and other factors. Therefore, decision makers should always monitor latest seasonal forecasts for the most up to date information,” it said.
For this reason, the weather agency said it is now adding to the existing portfolio of seasonal information provided through the National and Regional Climate Outlook Forums and has increased the frequency of the Global Seasonal Climate Update (GSCU) from quarterly to monthly.
In addition to El Niño and La Niña, the GSCU incorporates influences of other climate drivers, such as the North Atlantic Oscillation, the Indian Ocean Dipole, to assess their likely effects on regional surface temperature and precipitation patterns and as such used to underpin much of the seasonal discussions with the UN and other partners.