La Nina is a climatic phenomenon that can impact the weather in many parts of the world, leading to increased rainfall in some regions and colder winters in others. Over North America, La Nina causes the Jet Stream to move northward and to weaken over the Eastern Pacific. During La Nina Winters, the south sees warmer and drier conditions than usual. The North US and Canada tend to be wetter and colder.
Jet Stream is circumglobal. It is distributed around the world within a range of latitude. La Nina is an oceanic phenomenon that can impact the Jet Stream and weather pattern in India too. During winter months, the Jet Stream blows across Iran, Afghanistan, Pakistan and India with an undulating pattern. Mostly, the core of Jet Stream remain confined between 30°-35°N, but it does drop in latitude to track across plains of North India covering Rajasthan, Punjab, Haryana, Delhi and Uttar Pradesh. Historically, the La Nina is considered to be closely associated with a strong southwest monsoon and above average rains and colder winters in the sub-continent. However, it may not be universally true and will much depend on the characteristics of La Nina episodes.
The Pacific Ocean has witnessed more number of La Nina events than El Nino episodes in the last 15 years. Since 2008, there were four El Nino events ( 2009,2015, 2019, 2023) and six La Nina episodes (2010, 2011, 2016, 2020, 2021, 2022). There were more number of occasions when La Nina conditions prevailed during fall of the year and winter months of Indian Sub-Continent (2008, 2009, 2011, 2012, 2018, 2021, 2022, 2023). The statistical records during these years vindicate that winter rains (Jan & Feb) during these episodes were deficit on more number of occasions than being surplus.
In the last four weeks, equatorial SSTs were above average in the western Pacific Ocean. Near to below average SSTs were evident in the east-central and eastern Pacific Ocean. Overall, oceanic and atmospheric conditions align with an ENSO-Neutral state. The IRI ENSO prediction plume forecasts ENSO-Neutral conditions for Aug-Oct, and Sep-Nov 2024. The likely La Nina episode this season will have a delayed start. Also, the event is expected to be ‘weak’ and ‘short’. Borderline La Nina conditions are forecast during Oct-Dec, and Nov-Jan but with very weakly elevated probabilities. ENSO-Neutral is the most dominant category to re-emerge during the boreal winters and spring of 2025.
ENSO: There is a bit of mismatch between the CPC and IRI ENSO forecasts for the upcoming season. The CPC ENSO Outlook predicts a clear preference for La Nina onset in Sep-Nov (66% chances) with continuation until Jan-Mar 2025 (57%chances) and then a return to ENSO-Neutral state during boreal spring of 2025. In contrast, the objective IRI ENSO forecasts show late, weak, and temporary La Nina conditions in Oct-Dec and Nov-Jan. The forecasts, then show a return to ENSO-Neutral conditions that remain supreme during the boreal winter and spring of 2025.
All the four Nino indices in the equatorial Pacific are hovering close to 0°C. The most important of all, Nino 3.4, the marker index for ONI has become marginally negative over the last three weeks. The average value of the index over the last 12 weeks was 0.04°C, The index has to travel a long way to breach the threshold mark of La Nina and thereafter, remain consistent to come out of ENSO-Neutral conditions.
IOD: The Indian Ocean Dipole is neutral. The IOD index for the week ending 15Sep 2024 was -0.07°C. Most models indicate that, the IOD is likely to remain neutral, but slightly negative, for the rest of the year. An IOD event is quite unlikely.
MJO: The Madden-Julian Oscillation is currently strong and positioned over the Western Pacific in Phase 6&7. It is slowly propagating eastward, to reach Western Hemisphere in Phase 8, at the start of next month. The MJO pulse remains favourable for typhoon activity in the South China Sea and the Philippines Sea, during the last week of September 2024. Bay of Bengal will also remain an active basin in the closing phase of monsoon 2024 and start of October 2024.
Southwest monsoon has performed reasonably well. The core monsoon months of July & August and the closing month of September have registered surplus rainfall. It is a rare feat. Month of August has done exceptionally well, in the backdrop of its vulnerability to break-in-monsoon conditions. The monsoon current has gone weak for the last about four days. It is likely to pick up again during the last week of September. Teleconnection between La Nina and wintry conditions of Indian Sub-Continent will have to be validated against the popular belief.