EL-NINO

1. Context 

2. Key points

• La Nina itself is going on for a record-breaking third consecutive year.
• Now, forecasts for the 2023 fall and winter are predicting that its companion phenomenon the El Nino will occur with more than a 50 per cent probability.

3. About El Nino and La Nino

• El Nino refers to a band of warmer water spreading from west to east in the equatorial Pacific Ocean.
• Years in which an El Nino occurs are simply called "El Nino years and global weather patterns in that year tend to be anomalous in certain ways.
• Similarly, a La Nina occurs when the band of water spreads east-west and is cooler.
• Both phenomena affect the weather worldwide and can have drastic effects on economies that depend on rainfall.
• Together, El Nino and La Nina make up a cyclical process called the El Nino Southern Oscillation (ENSO).

   

4. El Nino and La Nina forecasts

• The first thing to note is that El Nino forecasts before spring tend to be notoriously unreliable due to a so-called spring predictability barrier.
• The climate system is quite noisy in spring as the Sun transitions across the equator from one hemisphere to the other. This complicates El Nino's predictions before the spring.
• Second and perhaps more importantly, in a La Nina year, the tropical Pacific Ocean soaks up heat like a sponge and builds up its volume of warm water.
• During El Nino, this warm water spills from the western part of the Pacific Ocean to the eastern part. 
• But the earth has had three straight La Nina years, which means the Pacific's warm-water volume is fully loaded and is likely to birth an El Nino soon.
• An El Nino year creates a global-warming crisis in miniature, since the warm water spreading across the tropical Pacific releases a large amount of heat into the atmosphere.

• Again, it is not clear if this transient spike will produce anything more dramatic beyond the extremes we are already experiencing.
• An El Nino will of course bring its usual global perturbations, including cyclones and the monsoon.

5. Effects on the northern Indian Ocean

• A transition from a La Nina winter which we are in currently to an El Nino summer has historically tended to produce the largest deficit in the monsoon, on the order of 15 per cent.
• This means that pre-monsoon and monsoon circulations tend to be weaker in an El Nino Year.
• The vertical shear (change in the intensity of winds from the surface to the upper atmosphere) tends to be weaker as well.
• This in turn can favour enhanced cyclogenesis i.e. cyclone formation.
• Of course, the global climate system is not so simple. Intraseasonal or sub-seasonal timescale variability in sea-surface temperature and winds is also very important for cyclogenesis over the northern Indian Ocean.
• These timescales denote the durations for which certain temperature and wind characteristics persist in the pre and post-monsoon periods.
• This said the net effect is for cyclogenesis to be subdued in an El Nino year.
• Again, we will have to wait until spring to get a sense of how the cyclone season will play out this year.

6. Monsoons

• As for the monsoon itself: if an El Nino state does emerge by summer, we will likelier than not have a deficit monsoon in 2023.
• Some research has indicated that the Indian Ocean diploe seesawing of sea-surface temperature over the western Indian Ocean could compensate for the negative effects of an El Nino.
• But it is not yet clear whether there is a robust relation between the dipole, El Nino and the summer monsoon. We also do not know if the dipole will evolve the "right" way this year.
• As has been the case in recent decades, a monsoon deficit itself will be accompanied by a smorgasbord of both wet and dry extreme events.
• A weaker monsoon circulation will produce a severe deficit over much of India.
• And while the overall seasonal total could be deficient, there are likely to be isolated pockets of heavy or very heavy rainfall. It's complicated the monsoon version.
• Then there are the monsoon's vagaries themselves.  For example, pre-monsoon cyclones are susceptible to warming in the Arctic region and could in turn affect the onset of the summer monsoon.
• Indeed, the summer monsoon system is quite complicated: its various components are affected by a plethora of meteorological events both local and global. Even details at the intraseasonal scale can have an effect.
• For example, the Bay of Bengal has of late been receiving freshwater from heavy rains as well as anomalously high river runoffs.
• These waters tend to sneak into the Arabian Sea and produce surface warming and the build-up of subsurface heat.
• These changes together may create favourable conditions for the formation of bigger and worse cyclones, especially if the circulation and the vertical shear are weaker as well.
• In all, India will have to wait for the El Nino forecast to be updated in the coming weeks.
• It will also have to hope for the best and unavoidably, prepare for the worst.
• Apart from preparedness, an unfavourable prediction will also test the India Meteorological Department's suite of forecast products and efforts to translate its forecasts into usable advisories for fishing, farming, flood alerts, etc.

For Prelims & Mains