The Impacts of a Weakened Atlantic Meridional Overturning Circulation on ENSO in a Warmer Climate

Abstract

This study quantifies the impacts of the Atlantic Meridional Overturning Circulation (AMOC) on the El Niño–Southern Oscillation (ENSO) under anthropogenic warming by comparing climate change model simulations with declining and fixed strengths of the overturning. After the 1980s, a weakened AMOC is shown to reduce the strength of the annual cycle of sea surface temperature (SST) in the eastern equatorial Pacific and induce anomalous cross‐equatorial northerly winds there, which strengthens ENSO variability by about 11%. An analysis of the Bjerknes stability index reveals that this intensification of ENSO results mainly from enhanced Ekman upwelling feedback due to amplified atmospheric wind response to SST anomalies and oceanic upwelling response to equatorial wind stress anomalies. The weakened AMOC also promotes the occurrence of Central Pacific El Niño events and reduces ENSO skewness. These AMOC impacts on ENSO magnitude and complexity throughout the twenty‐first century are however smaller than ENSO variations expected from internal climate variability.

An Inconsistent ENSO Response to Northern Hemisphere Stadials Over the Last Deglaciation

“…Climate models suggest the zonal temperature gradient of the tropical Pacific and trade wind strength may weaken under greenhouse warming (Collins et al., 2010; Lee et al., 2022), and modern observations warn of a weakening AMOC in response to Greenland ice melt (Boers, 2021; Caesar et al., 2021; Rahmstorf et al., 2015; Yang et al., 2016). The combined influence of these background climate conditions during H1 might suggest enhanced ENSO variability in the near future (Cai et al., 2015; W. Liu et al., 2023). In this context, modeling experiments testing the influence of meltwater forcing under different mean state configurations are clearly needed to clarify ENSO dynamics both during the deglaciation and in the future.…”

The Effect of Indian Ocean Temperature on the Pacific Trade Winds and ENSO

“…Additional caveats to consider include the new Arctic sea ice state, long‐term changes in AMOC and ITF, and changes in other types of tropical variability, such as the Madden‐Julien Oscillation (Tan et al., 2020). For example, in the TIO – 2°C experiment, a weakened AMOC state could potentially promote increased ENSO variability though weaker cross‐equatorial winds and stronger positive feedbacks in the eastern equatorial Pacific (Liu et al., 2023). However, these effects appear to be weaker than the direct impact of zonal wind weakening (Zhao & Fedorov, 2020).…”

The Weakened Atlantic Meridional Overturning Circulation Diminishes Recent Arctic Sea Ice Loss

“…Based on the free‐AMOC simulation, we conduct a parallel sensitivity (fixed‐AMOC thereafter) experiment with five ensemble members. The fixed‐AMOC experiment is branched from the free‐AMOC simulation in year 1980 and driven by the same historical and RCP8.5 forcing agents as the free‐AMOC simulation onwards except with a small amount of freshwater gradually removed over the region covering the north of 50°N in the North Atlantic and the Labrador and GIN (Greenland, Iceland and Norwegian) Seas and then uniformly redistributed to the rest of global oceans (Liu et al., 2020, 2023; Ren & Liu, 2021). In the model, we modify surface freshwater flux (FW) in terms of virtual salt flux (FV): $$$\text{FV}=-{s}_{\text{ref}}\text{FW}$$$ where the reference salinity s ref equals to 34.7 psu (practical salinity units).…”