The Pivotal Role of Subsurface Cold Water in the Secondary Cooling Phase in Multi‐Year La Niña Events

Abstract Composite analysis for single‐year La Niña (SL) events and multi‐year La Niña (ML) events of the GODAS data set since 1980s are conducted in this study. Compared to SL events, the progress of sea surface temperature in ML events shows the most significant differences in the second half of the persistence year (year 01). Analysis of surface and subsurface temperature anomalies reveals that subsurface exhibits strong negative temperature anomalies in ML events. Vertical profiles of temperature indicate that this colder water primarily originates from the Southern Hemisphere. An oblique section further clarifies the subsurface intrusion process from the southeast Pacific. An adjusted flux method is used to quantitatively analyze the heat transport during the secondary cooling phase, ML events exhibit negative net heat transport to the key ocean domain, in stark contrast to the positive net heat transport observed in SL events during the same period. Among all these boundaries, the eastern and bottom boundaries contribute the most, accounting for 87.1% of the total cooling transport, which aligns well with the subsurface cold water intrusion from the southeast Pacific. Further analysis reveals that, the primary contributor to the cold transport difference is the advection of mean zonal temperature difference by anomalous zonal currents at the eastern interface. As for the bottom boundary, both the advection of anomalous temperature difference by mean flow and the advection of mean temperature difference by anomalous flow exert crucial effects.