Climate models are severely deficient in capturing rainfall extremes over northwest India and Pakistan

Climate models for northwest India and neighbouring Pakistan have come a long way over the years, but deficiencies persist. Monsoon patterns remain unpredictable, with models struggling to replicate real-world trends.

Pakistan has been flooded each year since 2010 and this year is no exception. It is just that the floods happened in April this year and the monsoon season has resulted in a deficit over Northwest India and Pakistan thus far. We still have half of the season left, so we are keeping an eye out for potential extremes on the edges of the monsoon.

Evolution of monsoon 2024

The El Niño of 2023-24 was expected to turn into a La Niña, bringing abundant rain to India. The monsoon arrived on time, but June proved to be largely deficient across much of the West Coast and North India, with only Peninsular India receiving excess rain. The rain has moved further north in July, but the central-east and northwest continue to be dry. La Niña has yet to make an appearance.

Temperatures broke records in 2023 and 2024 as a result of the El Niño phenomenon combined with global warming. The warming was exacerbated by the water vapour emitted by the underwater volcano Hunga Tonga in 2022. India saw cooler temperatures in the north, but heatwaves still caused havoc.

Northwest India experienced a deficit prior to the monsoon season, which continues to this day. As usual, the monsoon has surprised us in terms of evolution, surplus and deficit distribution, and extremes. As I write this, a deadly landslide has devastated Kerala's Wayanad district. We can only hope for the best and prepare for the worst as we await the rest of the season.

Monsoon forecasts continue to improve, but issues remain

India’s massive investments in dynamical monsoon forecasting have yielded impressive results in terms of skillful forecasts at short (days 1-3), medium (days 3-10) and extended (weeks 2-4) ranges, as well as for seasonal outlooks. The India Meteorological Department has forecast a normal to above-average monsoon this year, which is largely dependent on the expected arrival of La Niña. The rest of the season will determine how well this forecast holds up.

One can always complain that forecasts are not as accurate as they should be at hyperlocal scales, but many efforts to downscale global forecasts to local scales are yielding useful results for sectors such as agriculture, water, floods, heatwaves and others.

Forecasts will never be perfect, but we must be aware of systematic issues such as the general ‘dry bias’ in forecasts. In other words, forecast models tend to put excess rain over the oceans and deficit rain over the subcontinent. The reasons continue to be investigated.

Monsoon projections are mostly unreliable

Several studies have already pointed out that the Intergovernmental Panel on Climate Change (IPCC) models used for future projections of climate and the monsoon are severely deficient in reproducing the observed monsoon trends. Modelling and predicting and projecting rainfall in the tropics are understandably difficult. Unlike temperatures, which have large scale patterns, rainfall tends to evolve very rapidly and convective cells jump around like popcorn in a kettle.

Models warm the land more than the ocean since land has a lower heat capacity. The real world, however, has seen a weaker warming or even a cooling over the Indian subcontinent. This is blamed on the so-called solar dimming or reduced sunlight reaching the surface due to pollution. But it is possible that the cooling is related to other factors, such as the shift in monsoon winds over the Arabian Sea.

As it turns out, the models fail to capture the shift in the winds and / or the resulting rainfall trends.

Spring warming over West Asia

I have been a part of the team that has been reporting on the warming over the Arabian Sea in recent decades and the northward shift in monsoon winds starting in spring itself. Causal mechanisms show that it is the spring warming over West Asia that pulls the winds northward and produces highly variable and extreme rain events over northwest India into Pakistan. We investigated this observed causal link in the models.

Some of the IPCC models do reproduce the spring warming reasonably well. The warming is around 0.6 degrees Celsius per decade. This surface warming should lead to a warming of the atmosphere above and produce a low-pressure centre over West Asia. However, the models barely produce a third of the observed pressure drop associated with the warming.

The resulting pressure gradient from the Arabian Sea to West Asia then tends to pull the winds northward during the pre-monsoon in the real world. This northward shift results in excess rain during the monsoon over northwest India and Pakistan. The heating associated with this rainfall anomaly pulls in more moisture across the equator.

Many models appear to fail in producing the winds shifts but some do succeed in a weak northward shift. Unfortunately, all models fail to produce the excess rain associated with the northward shift and the associated moisture transport across the equator. 

India needs its own forecasting strategy

Needless to say, the failure to produce winds shift is a serious concern considering the high climate vulnerabilities in the region. Continued extreme events in the arid and semi-arid regions of this corner of the world and the associated impacts on lives, livelihoods and displacements of people can quickly turn into sociopolitical perturbations. The tensions across the border can do well without such additional national security issues driven by chronic and acute climate stressors.

India must pay attention to these model flaws, plan its own predictions at multi-year timescales up to a decade and avoid relying on shaky projections out to 2100. The country has all the modelling infrastructure in place to focus on the critical, societally relevant timescale of a few years to a decade, instead of participating in the expensive beauty contest of longer-term projections under the IPCC umbrella.

For this, India can simply downscale the first two decades of the IPCC projections provided by the numerous models to better capture the regional variabilities, trends and extremes at the decadal timescale for its own food, water, energy and national securities. This can help India produce forecasts better suited to its unique needs.

Raghu Murtugudde, Centre for Climate Studies, IIT Bombay, Mumbai, India; University of Maryland, College Park, Maryland, US. 

Views expressed are the author’s own and don’t necessarily reflect those of Down To Earth.