Anthropogenic aerosol impacts on Pacific Coast precipitation in CMIP6 models

Abstract

Studies show anthropogenic aerosols (AAs) can perturb regional precipitation, including the tropical rain belt and monsoons of the Northern Hemisphere (NH). In the NH mid-latitudes, however, the impact of AAs on regional climate and precipitation remains uncertain. This work investigates the influence of AAs on wintertime precipitation along the North American Pacific Coast using models from the Coupled Model Intercomparison Project phase 6 (CMIP6). Over the early to mid-20th century, when U.S. and European AA and precursor gas emissions rapidly increased, a robust wintertime precipitation dipole pattern exists in CMIP6 all-forcing and AA-only forcing simulations, with wetting of the southern Pacific Coast (southward of 40N) and drying to the north. A corresponding dynamical dipole pattern also occurs--including strengthening of the east Pacific jet southward of 40N and weakening to the north--which is related to a Rossby wave teleconnection that emanates out of the tropical Pacific. Over the 21st century, when AAs are projected to decrease, an opposite hydro-dynamic dipole pattern occurs, including drying southward of 40N (including California) and wetting to the north. Although Pacific Coast precipitation is dominated by natural variability, good multi-model agreement in the forced component of Pacific Coast precipitation change exists, with the AA pattern (north south dipole) dominating the greenhouse gas (uniform) pattern in the historical all-forcing simulations. A high level of agreement in individual model-realization trends also exists, particularly for the early part of the 20th century, suggesting a robustness to the human signature on Pacific Coast precipitation changes. Thus, historical precipitation responses along the Pacific Coast are likely to have been driven by a mixture of natural variability and forced changes. Natural variations appear to drive a large fraction of this change, but human influences (i.e.,~aerosols) are likely to have preconditioned the variability of the climate in this region.

Investigating Catchment‐Scale Daily Snow Depths of CMIP6 in Canada

“…C07 shows significant overestimation (271.84%), likely due to its complex terrain in this region, featuring mountains, ocean interactions with land, and persistent snow cover, presenting challenges in climate modeling (e.g., Allen & Zhao, 2022; Li et al., 2022; Masud et al., 2021; Zhang et al., 2022). These challenges are also common in the Yukon River catchment C11 (118.68% bias).…”

Anthropogenic Aerosols Contribute to the Recent Decline in Precipitation Over the U.S. Southwest

“…While these trends were often attributed to internal variability, we show evidence with CESM2 that they are, in part, a forced response to anthropogenic aerosols (AA; up to 42% of the observed precipitation trend). Specifically, the AA‐driven negative PDV‐like pattern originates from the post‐1980 shift of aerosol emissions from the Western to the Eastern Hemisphere, which has been suggested to be conducive to SWUS precipitation declines (Allen & Zhao, 2022; Allen et al., 2020).…”

“…There are two modes to describe the historical AA emissions (Kang et al., 2021; Shi et al., 2022, 2023): (a) the aerosol increase mode, a global aerosol increase in the mid‐20th century, inducing Northern Hemisphere cooling, and (b) the aerosol shift mode, an aerosol emission shift from the Western to the Eastern Hemisphere since 1980, inducing a negative PDV‐like SST pattern. Idealized experiments with prescribed strong radiative forcings from aerosols (e.g., 5–10 times the observed emissions; Allen et al., 2020; Dow et al., 2021) and transient historical aerosols forcing (Allen & Zhao, 2022) show that AA, especially black carbon emissions from Asia, can induce a negative PDV‐like pattern, weaken the AL, and influence precipitation over Western North America. Given the anticipated regional changes to aerosol emissions in the near future (Persad et al., 2022), understanding its influence on historical precipitation trends is key to develop more robust future projections.…”

Impact of industrial versus biomass burning aerosols on the Atlantic Meridional Overturning Circulation

“…Furthermore, several studies have found a connection between the NAO and the AMOC [48][49][50] , with a positive NAO leading to a stronger AMOC by extracting heat from the subpolar gyre. Similarly, several studies have found that anthropogenic aerosols, through heterogeneous heating/cooling and the generation of Rossby waves [51][52][53][54] , can trigger atmospheric teleconnection (e.g., remote SLP perturbations). Although we do not pursue this farther here, we note the bulk of the 1960-1990 BMB anthropogenic aerosol burden increase occurs over Canada (Fig.…”