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Creators/Authors contains: "Rose, Brian E. J."

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  1. Abstract

    While most models agree that the Atlantic meridional overturning circulation (AMOC) becomes weaker under greenhouse gas emission and is likely to weaken over the twenty-first century, they disagree on the projected magnitudes of AMOC weakening. In this work, CMIP6 models with stronger climatological AMOC are shown to project stronger AMOC weakening in both 1% ramping CO2and abrupt CO2quadrupling simulations. A physical interpretation of this result is developed. For models with stronger mean state AMOC, stratification in the upper Labrador Sea is weaker, allowing for stronger mixing of the surface buoyancy flux. In response to CO2increase, surface warming is mixed to the deeper Labrador Sea in models with stronger upper-ocean mixing. This subsurface warming and corresponding density decrease drives AMOC weakening through advection from the Labrador Sea to the subtropics via the deep western boundary current. Time series analysis shows that most CMIP6 models agree that the decrease in subsurface Labrador Sea density leads AMOC weakening in the subtropics by several years. Also, idealized experiments conducted in an ocean-only model show that the subsurface warming over 500–1500 m in the Labrador Sea leads to stronger AMOC weakening several years later, while the warming that is too shallow (<500 m) or too deep (>1500 m) in the Labrador Sea causes little AMOC weakening. These results suggest that a better representation of mean state AMOC is necessary for narrowing the intermodel uncertainty of AMOC weakening to greenhouse gas emission and its corresponding impacts on future warming projections.

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  2. Abstract

    This study quantifies the contribution to Arctic winter surface warming from changes in the tropospheric energy transport (Ftrop) and the efficiency with whichFtropheats the surface in the RCP8.5 warming scenario of the Community Earth System Model Large Ensemble. A metric for this efficiency,Etrop, measures the fraction of anomalousFtropthat is balanced by an anomalous net surface flux (NSF). Drivers ofEtropare identified in synoptic‐scale events during whichFtropis the dominant driver of NSF.Etropis sensitive to the vertical structure ofFtropand pre‐existing Arctic lower‐tropospheric stability (LTS). In RCP8.5, winter‐meanFtropdecreases from 95.1 to 85.4 W m−2, whileEtropincreases by 5.7%, likely driven by decreased Arctic LTS, indicating an increased coupling betweenFtropand the surface energy budget. The net impact of decreasingFtropand increasing efficiency is a positive 0.7 W m−2contribution to winter‐season surface heating.

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