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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 power spectrum density (PSD) of PC2 time series corresponding to EOF2 spatial pattern shows a significant multidecadal peak ~50 years.
Real-time monitoring
In 2025, the cumulative ETM index remains near zero. This indicates neutral ETM conditions. ETM-related wind anomalies are not supporting the development of either El Niño or La Niña.
In contrast, the ETM was strongly positive during 2024. These conditions did not favor the development of La Niña.
ETM during the 21st century
ETM index for the last 25 years (2001-2025) using the ERA5 dataset.
Apart from a few years, ETM has been mostly negative during the 21st century.
This implies ETM is favoruing a more La-Nina like conditions in the equatorial Pacific Ocean.
Key Strengths:
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).
The observed pattern is directly linked to other multidecadal timescale climate variability, such as the Interhemispheric gradient in the sea-surface temperature anomalies.
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
Please refer to the following link to learn more about the ETM: ERL article
Link to the video abstract: Video abstract
Key Limitations:
The observation prior to the 1940s, in different reanalysis, is not very reliable.
Spatial patterns related to ETM are similar to those of the Southern Oscillation index.
Physical mechanisms leading to the formation of ETM are not well-known.
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