All naturally occurring climate events like El Nino or La Niña now take place under the shadow of more powerful human-induced climate change, say experts
Like in 2022, winter in India bid adieu to the country early this year, too. Temperatures across northwest and central India have been settling well above normal, paving way for hot weather. In fact, February 2023 was the hottest since 1901, when state-run IMD first started collecting data.
January began on a brighter note, with some good spells of snow and rainfall across Western Himalayas. This also led to spells of cold waves across the Indo-Gangetic plains. However, February followed the same route as December. The month of December saw no rain and snowfall, while winter chill kept evading the entire north western plains and adjoining parts of Central India.
According to scientists, the prevailing weather conditions are a result of increasing global warming. Continuous increase in the global mean temperatures have been impacting dynamics of the larger weather phenomena such as Western Disturbances (WDs) and ENSO (El Nino Southern Oscillation). Change in characteristics of these systems have largely affected the precipitation over India during the winter season.
But before we get started, let’s first understand what these phenomena are.
What are western disturbances?
WDs are primarily disturbances originating in the west and travelling in the upper atmosphere in the subtropical westerly jet and arriving in the Indian sub-continent during the wintertime. Their frequency peaks during December to February, with an average number of 4-5 per month. In the course of their travel and evolution, they pick up a lot of moisture and character from the Atlantic Mediterranean Sea.
When these disturbances travel through India, they merge with the mid tropospheric low which is existing over Indian western coast and get more intensified. With this, it gets more moisture from the Arabian Sea in addition to what it has picked up on its way. The interaction of WDs with the Himalayas have a huge role in orographic control (when moist air is lifted as it flows above a mountain range) in forming the precipitation mechanism.
What is La niña?
La Niña is defined as an oceanic-atmospheric phenomenon, El Niño-Southern Oscillation (ENSO), wherein water temperatures in the Equatorial Pacific Ocean become cooler than normal because of the upwelling of cold water from the bottom of the sea. La Niña has the capacity to alter direction as well as velocity of the trade winds, which trigger the winter season in India.
World witnessed prolonged stay of La Niña conditions, which is presently in its third year. The occurrence of three consecutive La Niña in the Northern Hemisphere is a relatively rare phenomena and is known as ‘triple dip’ La Niña.
La Niña has an association with winter rains over North India. Unlike El Niño, it may not have a strong correlation with seasonal features but remains consequential for the seasonal performance. Though there is no rule book for La Niña’s behavioural patterns, the performance for winter rains could be less than normal over North India, snowfall over Western Himalayas and winter temperature in the plains could be less than normal, along with a prolonged winter season over North India.
Changing climate and its impact
Increases in global mean temperature have changed the dynamics of these disturbances. The interplay of WDs with the topography of the western Himalayas determines the spatial and vertical distribution of precipitation. Hence, it becomes very important to read the role of climate change and its impact on WDs.
“Above normal temperatures across the Arctic region due to global warming are a major cause of concern as it directly impacts the circulations affecting the Asia region. Warming in the Arctic region pushes air upwards leading to formation of low-pressure area, which attracts the circulations in the subpolar region. This has been the case this season. The intensifying Arctic heatwave pulled up the weather systems including Western Disturbances northwards, making them travel in higher latitudes and thus, it did not affect the weather over India,” said Mahesh Palawat, vice president, meteorology and climate change, Skymet Weather.
According to research, since polar amplification means that the higher latitudes will warm faster than the Northern Hemisphere as a whole, the latitudinal temperature gradient will decrease, which should have an impact on total winter snowfall from WDs.
Polar amplification is the phenomenon that any change in the net radiation balance (for example greenhouse gas intensification) tends to produce a larger change in temperature near the poles than in the planetary average.
As per another study, It is clear that any significant changes to the frequency of WDs, intensity of WDs, or moisture flux associated with WDs will affect both the region’s precipitation climatology and extremes.
“In this changing world, WDs are showing distinct structural and dynamical changes. With global warming, WDs are getting lighter due to more convection and heat coming in. With this, they are tracking in the upper atmosphere. Besides, they have become more dynamic also. Off late, it is seen that all WDs do not precipitate. This means there are days when there is no precipitation happening during their passage and contradictorily there are non-WD days when good amounts of precipitation is seen over North India,” said Dr A P Dimri, director, Indian Institute of Geomagnetism.
Dr Dimri further added, “Southern proposition of Northern Atlantic Oscillation (NAO) pushes the subtropical westerly jet further south, increasing the likelihood of a greater number of WDs. If reverse happens, there would be a smaller number of WDs. In the warming scenario, there will be a more stable atmosphere and global teleconnection, which will lead to weakening of WDs.”
A Representative Concentration Pathway (RCP) is a greenhouse gas concentration (not emissions) trajectory adopted by the IPCC. The pathways describe different climate future scenarios, all of which are considered possible depending on the volume of greenhouse gases (GHG) emitted in the years to come. RCP 4.5 scenario is described as a moderate scenario in which emissions peak around 2040 and then decline. RCP 8.5 is the highest baseline emissions scenario in which emissions continue to rise throughout the twenty-first century.
The above figure shows that a robust decline in western disturbance frequency is to be expected across all future climate scenarios, with greatest impact in the winter months. All future experiments (RCP2.6, RCP4.5, RCP6.0, RCP8.5) show a decline in annual WD frequency, of increasing magnitude with stronger radiative forcing in the more extreme RCPs.
The figures show that winter precipitation in northern India and Pakistan is projected to decrease over the coming century due to the falling WD activity since they are responsible for the majority of winter precipitation in this region. In fact, mean winter precipitation over Pakistan and northern India is expected to decline (by around 10%–20%).
Global warming defying oceanic phenomenon of La Niña
Moving on to La Niña. This season was quite surprising as despite the persistence of La Niña, 2021 and 2022 were listed amongst record hot years. As per World Meteorological Organisation (WMO), the past eight years (2015-2022) were the warmest on record globally, fuelled by ever-rising greenhouse gas concentrations and accumulated heat. According to scientists, the La Niña led cooling impact was brief and that it was not able to reverse the long-term warming trend caused by record levels of heat-trapping greenhouse gases in our atmosphere.
“La Niña is a climate pattern marked by colder-than-normal sea surface temperatures in the equatorial Pacific Ocean that tend to suppress global temperatures. However, global warming and changing climatic conditions tend to play a major role, which can be temporarily governed by these ENSO conditions but cannot completely outplay the effects of climate change. Apparently, all the naturally occurring climate events like El Nino /La Niña / Indian Ocean Dipole (IOD) now take place under the shadow of more powerful human induced climate change. La Niña cooling of 2020-2022 did not suffice and the years still got listed among the hottest on record,” said G P Sharma, president, meteorology and climate change, Skymet Weather.