In response to global warming, climate model simulations suggest a weakening of the Walker Circulation (WC), which is supported by long-term sea level pressure observations over the 20th century. Here, we show the observations and multiple reanalyses yield the opposite trend from 1979 to present−a WC intensification. Atmosphere-only simulations driven by the real-world evolution of sea surface temperatures (SSTs) simulate this observed intensification, whereas coupled ocean atmosphere simulations do not. Thus, the recent WC intensification is related to real-world SST evolution. Assuming the multi-model mean SSTs from 20th century coupled climate model simulations accurately represent the externally forced response, the observed SSTs can be decomposed into a forced and an unforced component. Idealized Community Atmosphere Model version 5 (CAM5) simulations driven by the unforced component of SSTs yield significant WC strengthening, whereas negligible WC changes occur when driven by the forced component of SSTs. Although coupled climate models may be deficient in their tropical response to anthropogenic warming, our results suggest natural SST variability, and in particular a La Niña-like SST pattern, is primarily responsible for the strengthening of the WC since 1979.

Pristine aerosols are important drivers of poor air quality, particularly near dust source regions, impacting up to 1 billion people. 10• Air quality guidelines' spatial invariance may obscure the real impact of modern 11 emissions in regions with high levels of pristine aerosol. 12• Air quality policies targeting anthropogenic emission reductions may never achieve 13 "clean air" in pristine "polluted" regions.

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.

The tropical belt has widened during the last several decades, and both internal variability and anthropogenic forcings have contributed. Although greenhouse gases and stratospheric ozone depletion have been implicated as primary anthropogenic drivers of tropical expansion, the possible role of other drivers remains uncertain. Here, we analyze the tropical belt width response to idealized perturbations in multiple models. Our results show that absorbing black carbon (BC) aerosol drives tropical expansion, and scattering sulfate aerosol drives contraction. BC, especially from Asia, is more efficient per unit radiative forcing than greenhouse gases in driving tropical expansion, particularly in the Northern Hemisphere. Tropical belt expansion (contraction) is associated with an increase (decrease) in extratropical static stability induced by absorbing (scattering) aerosol. Although a formal attribution is difficult, scaling the normalized expansion rates to the historical time period suggests that BC is the largest driver of the Northern Hemisphere tropical widening but with relatively large uncertainty. Plain Language SummaryThe tropical belt has widened over the past several decades, and this is associated with poleward movement of the descending branches of the Hadley Cell and the subtropical dry zones. Internal climate variability and anthropogenic forcers-including greenhouse gases and stratospheric ozone depletion-are important contributors. Leveraging idealized single-forcing experiments, we show that anthropogenic aerosols, including black carbon and sulfate, drive significant tropical expansion and contraction, respectively. Aerosols, particularly those emitted from Asia, are more efficient than greenhouse gases in perturbing tropical belt width. Although relatively large uncertainty exists, linearized scaling suggests that black carbon is the dominant driver of the Northern Hemisphere tropical widening over the historical time period.

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