Excessive 21^st century North Pacific solar absorption attributed to poleward storm track shift, sea surface warming, and aerosol decreases

Abstract. This study analyzes the trend in Absorbed Shortwave Radiation (ASR) in the North Pacific, where an increasing ASR trend of 3.31 W/m²/dec is observed, constituting the dominant regional contributor to the global ASR increase.  Multiple regression analysis shows that 1.57 W/m²/dec (47.5%) of that trend is explained by changes in cloud fraction, 1.08 W/m²/dec (32.7%) by changes in cloud optical thickness, and 0.61 W/m²/dec (18.3%) by changes in aerosol optical depth indicative of the aerosol direct effect. The pattern of cloud fraction change shows two distinct regimes: one dominated by decreases in high and middle clouds and a second dominated by decreases in low clouds. The cloud fraction contribution to the ASR trend can be broken then into a 0.88 W/m²/dec contribution from high and middle cloud fraction changes and a 0.73 W/m²/dec contribution from low cloud fraction changes. Analysis incorporating cloud controlling factors shows that the high/middle cloud fraction changes are driven by atmospheric dynamics and thermodynamics, while the low cloud fraction and optical thickness changes are driven by sea surface warming and aerosol indirect effects. High/middle cloud fraction decreases correlate with a poleward shift in the N. Pacific midlatitude storm track, pointing to a potential positive cloud feedback loop between Sea Surface Temperature increase and a poleward shift of the midlatitude storm track, mediated by the reduction in high/middle cloud fraction.