Central and eastern equatorial Pacific Ocean temperatures (SSTs) continue to exceed El Nino thresholds. Models indicate further warming of the entire Nino region in the Pacific. El Nino conditions are expected to continue till early next year. The Indian Ocean Dipole is strengthening over the Indian Ocean. Usually, it peaks during Sep – Oct and the same is indicated by the majority of the numerical models.
ENSO: Oceanic indicators firmly support an El Nino state. The atmosphere is also responding to the pattern of SSTs in the tropical Pacific. The resultant coupling of ocean and atmosphere, as normally seen during typical El Nino events has begun, since early this month. Such a coupling sustains and strengthens the El Nino conditions for extended periods.
Nino 3.4 region, the principal indicator for assessing, monitoring and predicting El Nino is fairly warm. The positive anomaly of 1.6°C is retained for the 3rd successive week. The eastern half of the Nino region, hosting Nino 3 and Nino 1+2, is warmer by over 2°C. Such a state favours canonical rainfall impacts of El Nino, globally. The Southern Oscillation Index (SOI) continue to be negative. The negative phase of SOI conforms to large-scale fluctuations in the air pressure occurring between the western and eastern tropical Pacific Ocean.
IOD: The Indian Ocean Dipole index for the week ending 17Sep2023 was 1.25°C. This is the 5th week of +VE IOD, above the threshold value of +0.4°C. Continuation of such +VE values, as forecast by different models, indicate likely positive IOD event this year. Positive IOD and El Nino are two opposing ends of pendulum, trying to swing the monsoon their own way. Typical of positive IOD event, a clear gradient between western and eastern tropical Indian Ocean is developing. Cooling has expanded westward from Jawa and warming has strengthened over the western pole of IOD, close to the Horn of Africa.
MJO: Madden Julian Oscillation had a brief uptick, earlier this month. Later, it mostly remained incoherent and disorganized. It was largely maligned due to destructive interference with the ongoing El Nino and other tropical variability. Most dynamical models indicate subdued amplitude till month end and even during the first week of October. Thereafter, a consensus is building up for a more coherent eastward propagating MJO signal, reemerging across the global tropics. MJO’s proximity to the Indian Ocean, albeit with a minimal amplitude, may support the monsoon activity in the remaining days of September.
Recent trade winds strength is on the decline across the tropical Pacific. This testaments the presence of strong El Nino and its sustenance till the fall of the year. Monsoon 2023 has witnessed large intra-seasonal variability with contrasting rainfall patterns during the core monsoon months of July and August. Courtesy positive IOD, the southwest monsoon having derailed completely in August, has been brought back on track in September.
