Challenges with interpreting the impact of Atlantic Multidecadal Variability using SST-restoring experiments

Abstract

Climate model simulations that restore SSTs in the North Atlantic have been used to explore the climate impacts of Atlantic Multidecadal Variability (AMV). However, despite simulations and observations exhibiting similar North Atlantic SST anomalies, experiments with active SST-restoring in the Tropical North Atlantic exhibit strong positive surface heat-fluxes out of the ocean with warm SST anomalies, which is not replicated in other simulations or observations. The upward surface heat-fluxes that are systematically driven by the active SST-restoring in the Tropical North Atlantic are found to be crucial for generating a strong local precipitation response and the associated remote impact on the Pacific Walker circulation; these are both absent in other simulations. The results of this study strongly suggest that experiments employing SST-restoring (or prescribed SSTs) in the Tropical North Atlantic exaggerate the influence of the Atlantic on patterns of global climate anomalies and its role in recent multidecadal SST trends.

Mechanisms of Tropical Pacific Decadal Variability

“…Also, the connection between the Atlantic and Pacific becomes less clear when partially-coupled numerical experiments become more realistic 157 . These uncertainties indicate possible limitations of currently used partially-coupled experiments 158 , suggesting the need for additional research.…”

“…Properly designed coupled model sensitivity experiments, where SSTs are prescribed in certain regions could be used to isolate the contribution of the different regional sources of wind anomalies. Since these experiments may be affected by model biases and delicate to conduct 158 , they should be complemented by analyses of multi-variate empirical models 159 , which are trained on observations and allow a cleaner decoupling of feedbacks among different variables and regions [160][161][162] . In addition, simple ocean models that capture Rossby wave dynamics 18,163 can help assess the role of different aspects of the winds, including location and spectral characteristics, in reproducing key features of TPDV.…”

Regional climate change: consensus, discrepancies, and ways forward

“…Another puzzle related to climate model fidelity is that pacemaker simulations, whereby coupled model SST anomalies are nudged to observations, suggest coupled models can capture regional climate change when SSTs are nudged but cannot capture the trends when free-running (O'Reilly et al, 2023). If this means coupled models are misrepresenting the forced trend, then what are they missing?…”

Two Competing Drivers of the Recent Walker Circulation Trend

“…However, it is becoming clear that the climate model simulations are systematically biased to generate a weakening of the zonal contrast in SST and SLP over the past decades (Wills et al., 2022), leading to suspicion that the models do not adequately represent a forced response to GHGs or aerosols due perhaps to errors in the coupled mean state in the Pacific (Seager et al., 2019, 2022). There are several mechanisms to explain the observed strengthening, for example, aerosol forcing (Smith et al., 2016; Takahashi & Watanabe, 2016), remote effect of the Southern Ocean cooling (Dong et al., 2022; England et al., 2020; Kang et al., 2023; Kim et al., 2022), and coupling with tropical Atlantic and Indian Oceans (Li et al., 2016; McGregor et al., 2014; O’Reilly et al., 2023), but they are based on the potentially biased coupled model simulations, hindering a robust attribution of the past PWC intensification.…”