The ENSO transition Mode (ETM) is defined as the second leading mode (EOF2) of March-April-May averaged sea-level pressure (SLP) in the Southern Indo-Pacific region (10°N-90°S, 45°E-290°E). It has maximum spatial loading in the South Pacific. The ETM pattern consists of high-pressure anomalies equatorward to 60°S and from the western edge of the box up to the dateline. In contrast, a belt of low-pressure anomalies extends along 60°S. It intrudes into the subtropical southeastern Pacific Ocean owing to strong high-pressure anomalies centred at around 60°S and 250°E. Overall, ETM consists of a zonal dipole in SLP in the Southern Pacific Ocean during the boreal spring. This zonal dipole modulates the zonal winds in the equatorial Pacific and influences boreal winter-to-summer seasonal transitions of ENSO.

The PC2 time series corresponding to the EOF2 spatial pattern and its power spectrum density (PSD) shows a significant multidecadal peak ~50 years.

Real time monitoring of ETM

For the current year, the cumulative index (shown in dark red curve) for ETM during boreal spring (MAM) shows a positive value. This implies that ETM-related SLP supports anomalous westerlies in the equatorial Pacific and is not conducive to La Nina development.

ETM during the 21st century

• An important aspect of multidecadal climate variability from the Southern Pacific Ocean, which impacts equatorial zonal winds, and has a direct impact on ENSO's seasonal transitions.
• The ETM pattern is robust across different datasets (like 20CRv3, NCEP-I, ERA5 etc).
• Since EOF analysis is sensitive to the time period used for the calculations, we employ an alternative way to calculate ETM's year-year temporal variability in different reanalysis products.
• ETM's year-to-year temporal evolution can be represented by the difference in the SLP over the western (100°E-180°E, 20°S-45°S) and eastern (200°E-280°E, 20°S-45°S) boxes in the Southern Pacific Ocean: ΔSLP = SLPW - SLPE