The return of La Nina could have implications on global temperatures and also on the upcoming winter season of 2021-2022.
La Nina conditions (cooler-than-normal water) developed early last year in the equatorial part of the Pacific Ocean and then intensified some during the past winter season. In the first several months of this year, water temperatures in the tropical Pacific Ocean have trended towards neutral, but there are signs of a resurgence in La Nina as we head towards autumn. In fact, there are signs right now that suggest the resurgence may already be underway in the central Pacific Ocean. The return of La Nina could have implications on global temperatures and also on the upcoming winter season of 2021-2022.
The Pacific Ocean and signs of a resurgence in La Nina
The Pacific Ocean is the largest on the planet, covers more than 30 percent of the Earth’s surface, and is bigger than the landmass of all the continents combined. The warm waters of the equatorial Pacific Ocean store a great amount of latent heat compared to cooler waters and breed a great deal of convection which impact downstream ridging and troughing in the atmosphere. As such, its sea surface temperature (SST) pattern has a tremendous influence on all weather and climate around the world and the more anomalous the sea surface temperatures, the more the impact can be on the atmosphere around the world. The El Nino-Southern Oscillation (ENSO) is a recurring climate pattern involving changes in the temperature of waters in the central and eastern tropical Pacific Ocean. El Nino warmer-than-normal water) and La Nina (colder-than-normal water) are the extreme phases of the ENSO cycle; between these two phases is a third phase called ENSO-neutral.
Several independently-made computer forecast models support the idea of a change in the central part of the tropical Pacific Ocean by the fall of 2021 from the current near neutral conditions to one featuring La Nina once again. The plume of El Nino Southern Oscillation (ENSO) model forecasts (see plot) generated in mid-June indicate a resurgence of La Nina conditions is likely to take place by later this summer and continue through autumn. Indeed, some models (e.g., NOAA’s CFS v2) are predicting La Nina will last right into the winter of 2021-2022 with sea surface temperatures as much as 1.0°C below-normal in the “Nino 3.4” region (i.e., central tropical Pacific Ocean) during December/January/February.
Teleconnection indices are suggesting a resurgence in La Nina conditions across the equatorial Pacific Ocean may already be underway. Specifically, these types of indices can be tracked to provide information on pressure patterns across the Atlantic and Pacific Oceans which, in turn, can directly impact winds and sea surface temperatures. Specifically, the magnitudes of these indices which can be in positive or negative territory can provide clues about upcoming wind changes and sea surface temperature patterns. Two closely-related teleconnection indices that provide clues about the North Atlantic Ocean are known as the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO).
For the Pacific Ocean, there is a metric known as the Southern Oscillation Index (SOI) which is directly correlated with the pressure differences between Tahiti and Darwin. This index can signal changes in wind flow and sea surface temperature patterns providing us with clues on the potential development and/or intensification of El Nino or La Nina. Sustained negative values of the SOI lower than −7 often indicate El Nino episodes are underway. These negative values are usually accompanied by a decrease in the strength of the Pacific Trade Winds and sustained warming of the central and eastern tropical Pacific Ocean. Sustained positive values of the SOI greater than +7 are typical of a La Nina with waters in the central and eastern tropical Pacific Ocean becoming cooler during this time. In recent days, the SOI has increased noticeably while in positive territory – perhaps a significant early sign of a resurgent La Nina.
Impact on global temperatures
What goes on in the Pacific Ocean in terms of sea surface temperatures (i.e., El Nino, La Nina) can indeed have an impact around the world with respect to global temperatures in the lower part of the atmosphere. In recent years, the number of El Nino episodes have surpassed the number of La Nina events and global temperatures have often reacted with noticeable spikes. For example, temperatures spiked in 2016/2017 following a strong El Nino event and they had a secondary peak in 2019 associated with a second El Nino episode in the tropical Pacific. Prior to that, strong El Nino events that centered on the years of 1997/1998, 2009/2010 and 2015/2016 were associated with sharp upticks in lower atmosphere global temperatures. In times of La Nina such as during 2007/2008 and 2010/2011, there have been noticeable downturns in global temperatures of the lower atmosphere.
Since the most recent El Nino incident of 2018/2019, La Nina has taken control and global temperatures have responded accordingly. In fact, there has been quite a noticeable drop in global temperatures over the past several months to the point that levels are nearly normal on a worldwide basis as of the latest reading in June 2021. If La Nina does indeed have a resurgence later this year, global temperatures could respond by holding at nearly normal levels or perhaps even falling to below-normal levels.
via The Global Warming Policy Forum
July 20, 2021