Aerosol-forced multidecadal variations across all ocean basins in models and observations since 1920

Abstract

Earth’s climate fluctuates considerably on decadal-multidecadal time scales, often causing large damages to our society and environment. These fluctuations usually result from internal dynamics, and many studies have linked them to internal climate modes in the North Atlantic and Pacific oceans. Here, we show that variations in volcanic and anthropogenic aerosols have caused in-phase, multidecadal SST variations since 1920 across all ocean basins. These forced variations resemble the Atlantic Multidecadal Oscillation (AMO) in time. Unlike the North Atlantic, where indirect and direct aerosol effects on surface solar radiation drive the multidecadal SST variations, over the tropical central and western Pacific atmospheric circulation response to aerosol forcing plays an important role, whereas aerosol-induced radiation change is small. Our new finding implies that AMO-like climate variations in Eurasia, North America, and other regions may be partly caused by the aerosol forcing, rather than being originated from the North Atlantic SST variations as previously thought.

Quantifying Contributions of Internal Variability and External Forcing to Atlantic Multidecadal Variability Since 1870

“…A Lanczos filter with 19 points and a 13‐year cutoff period was used in the low‐pass filtering. Statistical significance tests were based on a Student's t test with reduced degree of freedom following Qin et al (2020).…”

“…It also affects Sahelian and South American rainfall (Hua et al, 2019; Knight et al, 2006), Atlantic hurricane activity (Zhang & Delworth, 2006), Arctic sea ice extent (Miles et al, 2014), and even global‐mean temperature (Chylek et al, 2014; Dai et al, 2015). While AMV's profound influence on global climate is well documented, the fundamental physical mechanisms and processes underlying the AMV are not fully understood and remain a source of contention (Qin et al, 2020; Sutton et al, 2018; Vecchi et al, 2017).…”

“…An ongoing debate is whether external forcing has played a major role in NASST's multidecadal variations during the last 150 years or so. Many modeling studies (Bellomo et al, 2018; Bellucci et al, 2017; Booth et al, 2012; Hua et al, 2019; Murphy et al, 2017; Qin et al, 2020; Watanabe & Tatebe, 2019) suggest that volcanic aerosol (VA) and anthropogenic aerosol (AA) have contributed significantly to the recent AMV. This finding challenges the conventional view that the AMV is an internally generated mode of variability (Zhang et al, 2019).…”

“…A common method is to detrend NASST either linearly (Enfield et al, 2001) or using observed global‐mean SST (GMSST) (Trenberth & Shea, 2006) to remove the long‐term warming signal, but such detrending does not accurately estimate and remove the externally forced changes from the observed NASST. This is because the externally forced signal is nonlinear while GMSST contains both internally generated and externally forced variations (Dai et al, 2015; Qin et al, 2020). Thus, the resultant AMV in such detrended NASST is a mix of internally generated and externally forced signals, and it does not provide any attribution information.…”

Aerosol Forcing Masks and Delays the Formation of the North Atlantic Warming Hole by Three Decades

“…In addition, aerosol forcing was proposed to dominate the North Atlantic surface temperature variability (Booth et al, 2012; Qin et al, 2020), while other studies highlighted internal variability as dominant source (Terray, 2012; Zhang et al, 2013). Here, analyzing data from CMIP6 models (Phase 6 of the Coupled Model Intercomparison Project) and CESM‐LE (the Community Earth System Model Large Ensemble), we show that the forcing due to anthropogenic aerosol opposes the formation of the NAWH, consistent with previous studies demonstrating aerosol‐induced AMOC strengthening (Cai et al, 2006; Delworth & Dixon, 2006; Menary et al, 2020).…”