Mid-latitude leading double-dip La Nina

Abstract

Understanding the evolution asymmetry between El Nino and La Nina events is challenging. Unlike El Nino, most La Nina events are characterised by a double-dip cooling (a.k.a. multi-year La Nina). Herein, we examined how single- and multi-year La Nina events differ by analysing observational and climate-model data sets. Single-year La Nina events tend to develop narrowly within the tropics from a central Pacific-type El Nino (Nino-4 > Nino-3), whereas multi-year La Nina events tend to originate from an eastern Pacific-type El Nino (Nino-3 > Nino-4) and are well-connected to mid-latitudes through thePacific 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 Nina, efficient ocean recharging due to a narrower anti-cyclonic circulation causes a fast transition to an El Nino or a fast termination of a La Nina. In contrast, for multi-year La Nina, a weaker recharging causes surface cooling to persist, leading to another La Nina in the following year.