El Niño is currently intensifying. Should we worry about its impact on our monsoon?
The latest forecasts have bumped up the possibility of witnessing moderate El Niño by June and continuation of El Niño throughout the monsoon season. The authorities must remain more cautious while planning aspects related to this year's monsoon
Akshay Deoras 27 May 2019 6:32 AM GMT
El Niño is a Spanish word which means "Little Boy". The popularity of El Niño has increased in the past few years -- particularly after witnessing subdued monsoon rainfall during 2014, 2015 and 2018
It's that time of year when meteorologists across the world are closely monitoring the El Niño conditions in the Pacific Ocean. In India, people are interested in understanding whether this year's El Niño will impact the south-west monsoon season. It's beyond any doubt that the popularity of El Niño has increased in the past few years -- particularly after witnessing subdued monsoon rainfall during 2014, 2015 and 2018.
El Niño, a component of the El Niño-Southern Oscillation (ENSO) is a major climate phenomenon that occurs roughly after two to seven years. It's a Spanish word which means "Little Boy" as this phenomenon was first recognised by the Peruvian fishermen in the 17th century. During El Niño years, these fishermen used to witness a significant decrease in the number of fishes off the coast of South America around the Christmas period and that is how they started calling such events as the birth of the "boy child" or "Christ Child".
Under the normal conditions, winds along the equator blow from the South American coast towards Indonesia and transport the sea water towards Indonesia (Western Pacific Ocean). At the same time, this complex interaction between the winds and the sea off the coast of South America results in upwelling of cooler and nutrient rich water which is favourable for the fishes. However, this usual wind pattern weakens due to certain weather events and this results in spreading of West Pacific warm waters towards the central and Eastern Pacific Ocean. In such a situation, upwelling of cooler water off South America gets reduced and drives the fishes away. This is El Niño!
The other component tied to El Niño is Southern Oscillation which simply refers to a "see-saw" of the air pressure between Western and Central Pacific Ocean. Thus, El Niño is a coupled phenomenon in which happenings in the ocean affect the atmosphere and vice versa. But why should we care about something that is happening thousands of kilometres from India?
In spite of occurring on a regional level (Pacific Ocean), El Niño is so powerful that its presence leads to warmer and drier weather conditions in many areas of Australia, Indonesia, India, Africa and South America. El Niño brings changes in the global atmospheric circulation patterns which changes the weather conditions in these parts of the world. There is a very simple pattern in the tropics -- air in some areas rises up in the atmosphere, moisture in it condenses, clouds and rainfall occur under appropriate conditions, and the same air (now much dry) sinks in some other regions leading to cloud-free conditions. For witnessing good rainfall during the monsoon season, the Indian subcontinent needs to stay away from such a sinking zone, but this is not possible during El Niño events. In addition, El Niño related influences disturb the monsoon winds and this leads to suppressed rainfall in the country. Unfortunately, the timing of most of El Niño events coincides with the south-west monsoon season.
If El Niño dominates the monsoon to such an extent and it can be linked to droughts, why do we still struggle with the accuracy of long-range monsoon forecasts? This is because of the following reasons: Indian monsoon depends on more factors, there are limitations in the climate models and forecasting of El Niño, and we still struggle to forecast the happenings on sub-seasonal timescales (for eg. questions like is the first fortnight of July going to be wetter than the second are difficult to answer).
The models which are used in long-range forecasting of the monsoon rainfall have limitations. A simple statistical model will assume relationships between various parameters and the monsoon rainfall to remain the same every year. The current official long-range forecast issued by the India Meteorological Department (IMD) is based on five-six global oceanic and atmospheric predictors and El Niño is just one of them! Different parameters influence the monsoon rainfall in different ways - some boost the rainfall, whereas some suppress it. In addition, the degree to which they influence the monsoon rainfall keeps changing with time.
India has received below average, average and even above average monsoon rainfall during the El Niño events recorded between 1950 and 2018. Thus, the effect of some negative parameters could be overpowered by other positive parameters. What complicates this is climate change which can induce more parameters which are currently unknown. More complicated models (such as a climate model) struggle to properly simulate the monsoon rainfall over the land and are unable to precisely simulate various features of the monsoon. In addition, all models have intrinsic errors which reduce the forecasting accuracy.
The monsoon season witnesses active-break spells of rain which are strongly modulated by low pressure systems and other meteorological factors. Currently, precise forecasting of these factors is limited to a few days to at the most a few weeks. This turns out to be a major hurdle in long-range forecasting as the monsoon rainfall is not uniform. The 2018 monsoon season is a perfect example as rainfall in India in September 2018 was 23% below average and most of the areas in north-east India witnessed deficient rainfall throughout the season.
The IMD issues the first long-range forecast around mid-April, a period during which precise El Niño forecasting is challenging. Since El Niño events generally start forming around that time, climate models can foretell a wide range of El Niño scenarios. In April this year, some of these models projected weakening of El Niño after June and this triggered a possibility that El Niño conditions won't dampen this year's monsoon rainfall after July. However, things have dramatically changed since the last month. El Niño is currently rapidly intensifying and the latest forecasts have bumped up the possibility of witnessing moderate El Niño by June and continuation of El Niño throughout the monsoon season. Thus, the authorities in India must remain more cautious while planning aspects related to this year's monsoon.
Akshay Deoras is a meteorologist pursuing his doctoral study at the Department of Meteorology, University of Reading, UK.
(Views are personal)