Pacific Coastal Niños : Drivers of Divergent Evolution and the Connection to Westward Propagating El Niño Events

Variability in the tropical Pacific is largely controlled by the El Niño–Southern Oscillation (ENSO). In addition to basin-wide El Niño events, Coastal Niños occur along the South American coast. An extreme example was the 2017 Coastal Niño, which caused severe rainfall and flooding in Peru during February–April. Its exceptional intensity is linked to its early timing, coinciding with the climatologically warmest coastal sea surface temperatures (SSTs). Only during this season can warm anomalies trigger deep convection and heavy rainfall; events peaking later, under cooler SSTs, remain weaker and dominated by stratiform clouds. Another key factor is the state of the central Pacific. In 2017, it remained neutral to weak La Niña, while in 2023, a similar coastal event evolved into a basin-wide El Niño. Model analyses (CESM-LE) indicate that large-scale background conditions, weaker trade winds, and westward-propagating Rossby waves enable the spread of coastal SST anomalies westward, linking regional and basin-wide events. The thesis also introduces a new metric distinguishing westward and eastward propagating SST anomalies. Westward propagation is typical and driven by the climatological surface currents; eastward propagation occurs when these currents reverse and Kelvin waves dominate. Such reversals are linked to the Pacific Decadal Oscillation (PDO): during its positive phase (late 1970s–2000), warmer central Pacific conditions and weaker mean currents favored current reversal, even during moderate events. Overall, the study identifies the 2017 Coastal Niño as a seasonal and climatic outlier, while the 2023 El Niño resembles pre-1980 events influenced by decadal variability and the timing of equatorial wave activity.

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