Who is La Niña, and where does she come from?
The trade winds in the tropical Pacific Ocean are important drivers of La Niña. At the onset of a La Niña event, the trade winds blowing from east to west strengthen as a direct result of the pressure differences between two regions: Darwin, Australian in the western Pacific and Tahiti in the eastern Pacific.
The importance of the air pressure in these two regions was brought to light by research from Sir Gilbert Walker in the early 1900s. Walker was studying the tropical Pacific in the hopes of understanding the drivers of the Indian monsoon system when he discovered that when the air pressure is high in Tahiti, it is lower in Darwin. Air naturally moves from high pressure areas to low pressure ones, generating wind and this particular, consistent, airflow from high-pressure areas near Tahiti to low-pressure ones near Darwin accounts for the trade winds in the tropical Pacific Ocean. The difference in air pressure between Tahiti and Darwin is heightened during a La Niña event.
Warm waters in the western Pacific Ocean near Darwin provide ample energy for the atmosphere. Because warm air rises, cloud generation increases, as does convection through the formation of storms. This in turn creates regions of lower air pressure, while colder, sinking air in the eastern Pacific Ocean leads to high pressure at the ocean surface. The below figure shows the atmospheric and oceanic conditions during a La Niña event.
As the trade winds strengthen and the wind blows towards the west, surface ocean water is also pushed westward. In order to replace the water that was transferred to the west, cold, deep nutrient-rich water rises up along the west coast of South America. The process of cold, nutrient-rich water rising to the surface is referred to as upwelling. In the image above, the cooler blues and greens represent colder water upwelling from greater depths, causing warmer water to retreat towards the west of the western tropical Pacific.
The stronger the trade winds the more upwelling, which results in anomalously cold sea-surface temperatures (SST). SST are how meteorologists measure the relative strength of any given ENSO anomaly; as the anomaly increases, its impacts tend to become more pronounced. La Niña events are ones in which SST are anomalously cold, whereas in an El Niño event, SST are anomalously warm.
The high pressure and cold SST over the eastern tropical Pacific Ocean suppresses cloud formation, which leads to lower than normal precipitation in the area. Generally, the cold SST in the eastern tropical Pacific Ocean also leads to cooler than normal surface air temperature, specifically on the west coast of South America. On the other hand, the low pressure and warm SST in the western tropical Pacific aids in the formation of clouds and precipitation leading to warmer and wetter conditions in western Australia.
There is a portion of the equatorial Pacific Ocean (highlighted in the above image) between 5°S-5°N latitude and 170°-120°W longitude, commonly referred to as the Niño 3.4 region (which touches both the western and eastern tropical Pacific) that is often used as a barometer for ENSO.
Still, La Niña events can lead to temperature and precipitation changes not only in these Pacific region, but also throughout the rest of the world.
So what are the models saying for La Niña?
According to the International Research Institute for Climate and Society (IRI) and Climate Prediction Center (CPC), SST anomalies are expected to decrease into summer 2016 and by autumn 2016, these anomalies are expected to become negative. A helpful way to understand agreement in models is to analyze a plume, which plots various model predictions on one chart, enabling one to quickly understand the dispersion in model estimates. The below figure represents the IRI/CPC forecast for the Mid-April 2016 Plume of Model ENSO Predictions.
The image above shows the SST for overlapping three-month periods from various models: The SST are predicted for the Niño 3.4 region and are calculated as the average SST between 5°S-5°N latitude and 170°-120°W longitude.
According to the Mid-April 2016 IRI/CPC model-based ENSO forecast probabilities, there is a 60 percent chance of La Niña conditions developing by December 2016. In order for a La Niña event to be classified, there need to be five consecutive overlapping three-month periods in which SST anomalies are - 0.5°C or less.
La Niña events are categorized as weak, moderate, strong, or very strong. A weak La Niña event has SST anomalies between -0.5°C and -0.9°C; a moderate event has SST anomalies between -1.0°C and -1.4°C; a strong La Niña has SST anomalies between -1.5°C and -1.9°C ; and finally, very strong ones have SST anomalies at or greater than -2.0°C. These same anomaly bands apply to positive anomalies as well, which would indicate the relative strengths of a given El Niño event (as El Niño events are indicative of a warming in SST, while La Niña ones indicate a cooling of SST). The last La Niña was in 2011-2012, which was considered a “weak” event. In 2010-2011, there was a moderate La Niña which followed the moderate El Niño of 2009-2010.
While it does appear that La Niña conditions are likely to emerge in by the end of 2016, there is a great deal of uncertainty in the short-term. The May, June, July (MJJ) estimates range from nearly a +1.0°C El Niño anomaly to a -0.75°C La Niña anomaly. The majority of models indicate that SST will be between +0.5°C and -0.5°C during these three months, which is a neutral phase. A neutral phase indicates anomalies are not significant enough to meet either El Niño or La Niña criteria, and any ENSO-related impacts would be minimal. Models for June, July August (JJA) are even more chaotic, with anomaly ranges from +0.75°C to -1.4°C. Models indicate that La Niña conditions will likely arrive, but it is still too early to rely on the models to answer when this will occur.
Despite the uncertainty in its timing and severity, several news sources have already suggested that La Niña conditions would be in place by July and would therefore soon start affecting agricultural production. And the potential agricultural impacts of a La Niña are vast. In the United States, for example, La Niña typically ushers in above-average temperatures and below-average rainfall in the Southeast and Midwest, which is bad for a range of crops. Similarly, in southern Brazil, La Niña conditions from October through March can bring about below-average rainfall, which can seriously harm maize and soybean yields. But at the same time, a weak La Niña can be favorable for wheat and other winter cereals in southern Brazil, during winter months, as the drier spring reduces outbreaks of spike diseases and can improve the quality of the wheat. Additionally, a weak La Niña can be favorable for irrigated rice cultivation in southern Brazil, as more solar radiation is likely to due to clearer than average skies.
In parts of Southeast Asia, including Indonesia, La Niña can be detrimental to many farmers, including those who produce rice, cocoa, vegetables, and palm oil, as the event often extends the rainy season (November to March) by several months. The extended rainy season leads to more frequent tropical storms, heavy rains, and ultimately, flooded soils, which damage yields.
Some of the alarmist news reports warning of the soon-forthcoming and potentially destabilizing El Niño were issued as early as last November, when El Niño was just reaching its peak! It was premature to issue such warnings so early in the cycle.
Even today, claims that a La Niña event will arrive at any point within the summer season are highly prone to error. Meteorologists Edward Berry and David Gold of Weather Decision Technologies (WDT) advise caution that models are not only erratic in terms of their estimates for ENSO, but they are also attempting to model seasonal weather patterns in a non-stationary climate. In other words, climate change makes it difficult, and frequently inaccurate, to attempt to consider the transition period that has followed historically strong El Niños, such as those of 1982/3 or 1997/98, to subsequent La Niñas and apply that to the present. The meteorologists note that the warmer waters that extend from the Indian Ocean to the East Pacific could propagate northward, which would further delay the onset of La Niña. For now, farmers should note that La Niña conditions are likely but the timing is still highly uncertain. It may very well take until late autumn 2016 for conditions to meet La Niña criteria.
Above-average rainfall from Indian monsoon
Outside of the Niño 3.4 region in the tropical Pacific, ENSO can influence the development of the Indian Summer Monsoon (ISM). The ISM is determined by a seasonal shift in wind patterns between the Indian landmass and the Indian Ocean. During the summer monsoon, which typically occurs from June through September, a low pressure system persists over the Indian landmass while high pressure persists over the Indian Ocean. The combination of low pressure, rising air over India, and winds flowing from the Indian Ocean towards India enhances cloud development and therefore summertime precipitation. The ISM is critical for South Asia, as it accounts for 75 to as much as 90 percent of the region’s annual rainfall. The figure below demonstrates the wind flow during the ISM.
As the wind flows from the Indian Ocean towards India, the moist oceanic air is uplifted due to the presence of the Himalayan Mountains, further enhancing cloud formation and precipitation in India. Cloud formation and precipitation associated with the Indian summer monsoon can be more pronounced during a La Niña event due to the effect of the warm SST and enhanced convection in the western tropical Pacific.
While a La Niña event would enhance the ISM, it seems the 2016 monsoon may already have a boost, thanks to the warmer-than average water temperatures in the Indian Ocean. Warmer water provides more energy for the atmosphere, resulting in heavier and steadier rain bands. Should the temperature anomalies last through June, this year’s Indian summer monsoon may bring in above-average rainfall, which would be particularly beneficial in a country in which roughly 60 percent of agriculture is rain-fed. The success of this upcoming monsoon season will be particularly paramount, as many parts of India are currently experiencing their worst drought in nearly half a century, with some 330 million people affected.
Focusing on La Niña’s impact on coffee
In Brazil, a La Niña winter often brings drier-than-normal conditions to the southern state of Minas Gerais, the most important in terms of coffee production. The lack of rainfall can stress the region’s coffee crops, particularly if the drought conditions occur in the crop’s flowering stage, when rainfall is critical to berry development. Given that Brazil produces over 42 percent of global Arabica output, a strong drought could spell major trouble for the coffee market.
Indeed, the La Niña outlook for Arabica can be even worse when one considers its potential effect in Colombia, the world’s second largest producer of the coffee variety. During a La Niña, Colombia experiences higher than average precipitation. And this excessive rainfall can encourage the rotting of coffee tree roots and facilitate the spread of diseases such as Phanerochaete salmonicolor, also known as Pink Disease. La Niña’s impact on Colombian coffee can also therefore lead to an increase in Arabica prices, particularly Colombian milds, during and after La Niña events. One study found that strong La Niña events have on average reduced Colombian coffee output by 6 percent. At the same time, it is important to note that Colombia is currently experiencing drought conditions, and so La Niña’s potential impact may be subdued as a result. Still, it comes as little surprise that some agricultural economists believe the price of Arabica coffee tends to rise as a result of a La Niña event.
For Robusta coffee, the impact of La Niña can vary. A large portion of Robusta is produced in Southeast Asia, a region that experiences above average precipitation during La Niña events. Robusta coffee thrives in wet environments, but too much rain can diminish soil quality and lead to stunted yields. Depending on the strength and pattern of the rains caused by La Niña, this may or may not be the case: La Niña conditions can aid Robusta production in Southeast Asia in some years and impede it in others.
It is likely that we will see La Niña conditions develop by the end of 2016, and it is possible that this La Niña could affect multiple regions and commodities. Still, it is impossible to say definitively when La Niña will occur, or to predict how supply will be affected. Seasonal forecasting is dynamic, and even veteran meteorologists concede that forecasting more than three months into the future is highly speculative.
The agricultural community should absolutely pay close watch to ENSO models and discussions by scientific organizations, such as the National Oceanic and Atmospheric Administration and the International Research Institute for Climate and Society, for the latest predictions on La Niña’s timing and strength. Agricultural stakeholders should be made weary by brazen predictions of La Niña conditions arriving any time before autumn 2016; while this is possible, it is not guaranteed or even likely, and as such these forecasts warrant skepticism.