Why 1986 El Niño and 2005 La Niña evolved different from a typical El Niño and La Niña

Abstract

1986 El Niño (EN) and 2005 La Niña (LN) experienced distinctive evolution features compared to the typical EN and LN events. The 1986 EN persisted for more than 2 years, whereas the 2005 LN transitioned into a warm episode in the following winter. The physical mechanisms that caused the distinctive evolution features are investigated through an oceanic mixed-layer heat budget (MLHB) analysis. For both cases, major differences with typical EN and LN lie in wind induced anomalous zonal advection and thermocline feedback terms. An anomalous cyclone appeared over the western North Pacific (WNPC) in late 1986, in contract to an anomalous anticyclone (WNPAC) during the mature winter of the typical EN. The eastward propagation of anomalous convective heating from tropical Indian Ocean in late 1986 holds a key for generating the anomalous WNPC in the mature winter of 1986/87 EN. The equatorial westerly anomaly south of the WNPC triggered downwelling Kelvin waves, prolonging the positive SSTA throughout 1987. The negative SSTA center of the 2005 LN shifted eastward by at least 20° in longitude compared to the typical LN. As a result, an anomalous WNPC, with pronounced westerly anomalies at the equator, developed in the 2005 LN mature phase. This led to a rapid decay of the 2005 LN, and by the following boreal summer a positive SSTA formed. The season-dependent air-sea feedback further strengthens the warming in northern fall, leading to a transition from LN to EN in the succeed winter.