Nino indices are measured over a large area of 1.2million sq km over the Equatorial Pacific Ocean. Prominently four Nino indices are measured through a range of data collection platforms including the space-based sensors. Nino-4 is the farthest in the Western Pacific and Nino 1+2 is the closest to the American continent. In between is the Nino 3.4 region which is designated for El Nino monitoring, tracking and predicting the event. The Oceanic Nino Index (ONI), which is the running mean of the Sea Surface Temperature (SST) anomaly in the Nino 3.4 region is the principal measure and indicator for ascertaining the presence or absence of El Nino.
Beginning mid - September 2019, above-average SSTs expanded from the Dateline to the Eastern Pacific Ocean. Currently, the equatorial SSTs are near-to above average across the Pacific Ocean, however, ENSO-neutral conditions are present. Unlike rest meteorological agencies across the world which consider 0.5 degrees as the threshold value, the Bureau of Meteorology (BoM) follows this cut off at 0.8 degrees.
The spring season has a unique relationship with El Nino. In the world of Meteorology, we call this season 'Spring Barrier'. Normally the results derived by the weather model is better and best when closest to the period of forecast. However, this is not true for the ENSO forecast during the spring. This is the time when the El Nino readings are considered biased and therefore may not be the true representative of the ocean state. The month of March and part of April is dubious for reliable forecasts from the ocean models. The last three weeks, the Nino indices across the Pacific ocean are:
Though these values currently appear to be morphing and are indicative of the warmer state of the ocean but are not exorbitantly high. The western half and central part of the ocean is slightly hot. But they are projected to stay cool during the monsoon and therefore may not corrupt the seasonal rains. But we keep our fingers crossed as El Nino is quite notorious for throwing surprises. Another couple of weeks. the confidence in the forecast is going to rise high and reliable too.