Understanding the evolution asymmetry between El Niño and La Niña events is challenging. Unlike El Niño, most La Niña events are characterised by a double-dip cooling (a.k.a. multi-year La Niña). Herein, we examined how single-and multi-year La Niña events differ by analysing observational and climate-model data sets. Single-year La Niña events tend to develop narrowly within the tropics from a central Pacific-type El Niño (Niño-4 > Niño-3), whereas multi-year La Niña events tend to originate from an eastern Pacifictype El Niño (Niño-3 > Niño-4) and are well-connected to mid-latitudes through the Pacific meridional mode, which leads to a meridionally wider response of the off-equatorial low-level atmospheric anti-cyclonic circulation. As the anti-cyclonic circulation controls the amount of equatorial upper-ocean heat recharge through Sverdrup transport, for single-year La Niña, efficient ocean recharging due to a narrower anti-cyclonic circulation causes a fast transition to an El Niño or a fast termination of a La Niña. In contrast, for multiyear La Niña, a weaker recharging causes surface cooling to persist, leading to another La Niña in the following year.

No abstract

Numerous studies have demonstrated that the North Pacific Meridional Mode (NPMM) plays an important role in determining El Niño-Southern Oscillation (ENSO) events in the following winter season. However, little attention has been given to significant differences among its spatial patterns. Here we show that the NPMM exhibits a large diversity in spatial patterns, leading to distinct impacts on ENSO. Based on objective clustering analysis, two distinct spatial patterns of NPMM are detected. Cluster 1 (C1) NPMM exhibits a strong sea surface temperature dipole over the subtropical eastern Pacific and midlatitude central Pacific whereas Cluster 2 (C2) features a dipole over the subtropical eastern Pacific and equatorial cold tongue region. We find that the C1 NPMM is strongly linked to following ENSO events while the C2 NPMM has no statistically significant relation. This gives new implications for ENSO dynamics and predictions.

No abstract

The North Pacific Oscillation (NPO), a primary atmospheric mode over the North Pacific in boreal winter, is known to trigger the El Niño-Southern Oscillation (ENSO) in the following winter, the process of which is recognized as the seasonal footprinting mechanism (SFM). Based on the analysis of model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5), we found that the SFM acts differently among models, and the correlation between the NPO and subsequent ENSO events, called the SFM efficiency, depends on the background mean state of the model. That is, SFM efficiency becomes stronger as the climatological position of the Pacific Intertropical Convergence Zone (ITCZ) moves poleward, representing an intensification of the northern branch of the ITCZ. When the Pacific ITCZ is located poleward, the wind-evaporation-sea surface temperature (SST) feedback becomes stronger as the precipitation response to the SST anomaly is stronger in higher latitudes compared to that of lower latitudes. In addition, such active ocean-atmosphere interactions enhance NPO variability, favoring the SFM to operate efficiently and trigger an ENSO event. Consistent with the model results, the observed SFM efficiency increased during the decades in which the northern branch of the climatological ITCZ was intensified, supporting the importance of the tropical mean state of precipitation around the Pacific ITCZ.