Abstract
Abstract. We investigate the freshwater budget of the Atlantic and Arctic oceans in coupled climate change simulations with the Community Earth System Model and compare a strongly eddying setup with 0.1∘ ocean grid spacing to a non-eddying 1∘ configuration typical of Coupled Model Intercomparison Project phase 6 (CMIP6) models. Details of this budget are important to understand the evolution of the Atlantic Meridional Overturning Circulation (AMOC) under climate change. We find that the slowdown of the AMOC in the year 2100 under the increasing CO2 concentrations of the Representative Concentration Pathway 8.5 (RCP8.5) scenario is almost identical between both simulations. Also, the surface freshwater fluxes are similar in their mean and trend under climate change in both simulations. While the basin-scale total freshwater transport is similar between the simulations, significant local differences exist. The high-ocean-resolution simulation exhibits significantly reduced ocean state biases, notably in the salt distribution, due to an improved circulation. Mesoscale eddies contribute considerably to the freshwater and salt transport, in particular at the boundaries of the subtropical and subpolar gyres. Both simulations start in the single equilibrium AMOC regime according to a commonly used AMOC stability indicator and evolve towards the multiple equilibrium regime under climate change, but only the high-resolution simulation enters it due to the reduced biases in the freshwater budget.
Highlights
One of the important tipping elements in the climate system (Lenton et al, 2008) is the Atlantic Meridional Overturning Circulation (AMOC)
We investigate the effect of improving the ocean model resolution on the Atlantic freshwater budget and its sensitivity by analyzing presentday control and high-CO2 concentration pathway simulations in two configurations of the Community Earth System Model: one with an ocean model grid spacing of 0.1◦ and the other with 1◦
The HR-CESM latitudinal gradient in the weakening trend around the maxima at 2000 m is weaker so that the HRCESM AMOC weakening is stronger at 34◦ S but weaker in the Northern Hemisphere compared to the LR-CESM simulation
Summary
One of the important tipping elements in the climate system (Lenton et al, 2008) is the Atlantic Meridional Overturning Circulation (AMOC). We use the terminology “strongly eddying” for ocean grids with 0.1◦ horizontal grid spacing as these are neither just eddy-permitting (typically 0.25◦) nor fully mesoscale turbulence resolving (Moreton et al, 2020) These high-resolution ocean models constitute the only consistent method to estimate eddy contributions to ocean variability and the mean climate state and generally result in significantly reduced ocean biases (Kirtman et al, 2012; Small et al, 2014). Whether ocean model resolution affects the AMOC response to forcing systematically remains an open question (Gent, 2018), there is evidence from eddy-permitting models that the AMOC mean state, in particular the sites of deep water formation, controls the response (Jackson et al, 2020).
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