Abstract
Abstract. The El Niño–Southern Oscillation (ENSO) influences the most extensive tropospheric circulation cells on our planet, known as Hadley and Walker circulations. Previous studies have largely focused on the effect of ENSO on the strength of these cells. However, what has remained uncertain is whether interannual sea surface temperature anomalies can also cause synchronized spatial shifts of these circulations. Here, by examining the spatiotemporal relationship between Hadley and Walker cells in observations and climate model experiments, we demonstrate that the seasonally evolving warm-pool sea surface temperature (SST) anomalies in the decay phase of an El Niño event generate a meridionally asymmetric Walker circulation response, which couples the zonal and meridional atmospheric overturning circulations. This process, which can be characterized as a phase-synchronized spatial shift in Walker and Hadley cells, is accompanied by cross-equatorial northwesterly low-level flow that diverges from an area of anomalous drying in the western North Pacific and converges towards a region with anomalous moistening in the southern central Pacific. Our results show that the SST-induced concurrent spatial shifts of the two circulations are climatically relevant as they can further amplify extratropical precipitation variability on interannual timescales.
Highlights
Changes in the zonal equatorial Walker cell (WC; tropicalmean zonal cell) and the meridional Hadley Cell (HC; zonalmean meridional cell) are known to cause major climate disruptions across our planet
For fair comparison between observations and simulations, the mass stream function (MSF) fields in every single model were projected onto the empirical orthogonal function (EOF) pattern derived from the observations to generate principal component (PC) time series for the simulated WC and HC variability
Our results suggest that the spring SSTgrad anomalies that occur after the El Niño winter peak phase (Stuecker et al, 2015; Zhang et al, 2016) play an important role for the phase synchronization of WC and HC shift modes
Summary
Changes in the zonal equatorial Walker cell (WC; tropicalmean zonal cell) and the meridional Hadley Cell (HC; zonalmean meridional cell) are known to cause major climate disruptions across our planet. Observational analyses suggest more complicated relationships, which cannot be fully explained by peak ENSO dynamics alone (Clarke and Lebedev, 1996; Mitas and Clement, 2005; Tanaka et al, 2005; Ma and Zhou, 2016) These two large-scale circulations can change independently from each, either in terms of their strength or their geographical position, but during strong El Niño and La Niña winters these circulations can change in unison. Yun et al.: Synchronized spatial shifts of Hadley and Walker circulations during peak El Niño events in boreal winter, the WC weakens and the rising branch shifts eastward This is accompanied by a strengthening of the HC and an enhanced upward motion in the equatorial region (Ma and Li, 2008; Bayr et al, 2014; Guo and Tan, 2018; Minobe, 2004; Klein et al, 1999) ( see Fig. S1 and Table S1 in the Supplement). We will focus on 2 important degrees of freedom that characterize variations in these circulations: their strength and spatial position
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