Quantifying the Role of the Eddy Transfer Coefficient in Simulating the Response of the Southern Ocean Meridional Overturning Circulation to Enhanced Westerlies in a Coarse-resolution Model

This study assesses the capability of a coarse-resolution ocean model to replicate the response of the Southern Ocean Meridional Overturning Circulation(MOC) to intensified westerlies,focusing on the role of the eddy transfer coefficient(κ).κ is a parameter commonly used to represent the velocities induced by unresolved eddies.Our findings reveal that a stratification-dependent κ,incorporating spatiotemporal variability,leads to the most robust eddy-induced MOC response,capturing 82% of the reference eddy-resolving simulation.Decomposing the eddy-induced velocity into its vertical variation(VV) and spatial structure(SS) components unveils that the enhanced eddy compensation response primarily stems from an augmented SS term,while the introduced VV term weakens the response.Furthermore,the temporal variability of the stratification-dependent κ emerges as a key factor in enhancing the eddy compensation response to intensified westerlies.The experiment with stratification-dependent κ exhibits a more potent eddy compensation response compared to the constant κ,attributed to the structure of κ and the vertical variation of the density slope.These results underscore the critical role of accurately representing κ in capturing the response of the Southern Ocean MOC and emphasize the significance of the isopycnal slope in modulating the eddy compensation mechanism.

Spatial Inhomogeneity of Atmospheric CO_(2) Concentration and Its Uncertainty in CMIP6 Earth System Models

This paper provides a systematic evaluation of the ability of 12 Earth System Models(ESMs)participating in the Coupled Model Intercomparison Project Phase 6(CMIP6)to simulate the spatial inhomogeneity of the atmospheric carbon dioxide(CO_(2))concentration.The multi-model ensemble mean(MME)can reasonably simulate the increasing trend of CO_(2) concentration from 1850 to 2014,compared with the observation data from the Scripps CO_(2) Program and CMIP6 prescribed data,and improves upon the CMIP5 MME CO_(2) concentration(which is overestimated after 1950).The growth rate of CO_(2) concentration in the northern hemisphere(NH)is higher than that in the southern hemisphere(SH),with the highest growth rate in the mid-latitudes of the NH.The MME can also reasonably simulate the seasonal amplitude of CO_(2) concentration,which is larger in the NH than in the SH and grows in amplitude after the 1950s(especially in the NH).Although the results of the MME are reasonable,there is a large spread among ESMs,and the difference between the ESMs increases with time.The MME results show that regions with relatively large CO_(2) concentrations(such as northern Russia,eastern China,Southeast Asia,the eastern United States,northern South America,and southern Africa)have greater seasonal variability and also exhibit a larger inter-model spread.Compared with CMIP5,the CMIP6 MME simulates an average spatial distribution of CO_(2) concentration that is much closer to the site observations,but the CMIP6-inter-model spread is larger.The inter-model differences of the annual means and seasonal cycles of atmospheric CO_(2) concentration are both attributed to the differences in natural sources and sinks of CO_(2) between the simulations.

Upper-Ocean Lateral Heat Transports in the Nino3.4 Region and Their Connection with ENSO

In the Nino3.4 region(tropical Pacific,5°S-5°N,170°-120°W),sea surface temperature(SST)changes are highly correlated with temperature variations in the upper 40-m layer.This study explores the upper-ocean heat budget in the Nino3.4 region using Ocean Reanalysis System 5(ORAS5)monthly data from 1979 to 2018,with a focus on ocean heat transports at lateral boundaries in the top 40-m layer and their correlation with temperature variations.In the region,there is a well-defined structure of opposite meridional circulation in the upper and lower parts of the thermocline,characterized by divergence in the upper layer above 40 m and convergence in the lower layer.The change of mean temperature in the upper layer is determined by the sum of zonal,meridional,and vertical heat transports,which,however,tend to largely compensate for each other.In general,part of the surface heat flux from the atmosphere to the ocean and the heat transport from the subsurface ocean are transported out of the domain by meridional and zonal currents,leaving only a tiny part to warm or cool the upper ocean.The amplitude of the net surface heat flux effective for the entire 40-m layer of the ocean is weaker than the lateral heat transport.On an interannual timescale,variations of heat transports in both zonal and meridional are positively correlated with temperature anomalies,while the vertical heat transport from the subsurface ocean is negatively correlated.Composite analyses for five El Nino events and five La Nina events also revealed that there is a positive contribution of horizontal transport convergence to temperature anomalies during the evolution of El Nino(warming)and La Nina(cooling),while vertical transport acts against temperature variations.