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A. P. Megann, A. L. New, A. T. Blaker, and B. Sinha


The control climates of two coupled climate models are intercompared. The first is the third climate configuration of the Met Office Unified Model (HadCM3), while the second, the Coupled Hadley–Isopycnic Model Experiment (CHIME), is identical to the first except for the replacement of its ocean component by the Hybrid-Coordinate Ocean Model (HYCOM). Both models possess realistic and similar ocean heat transports and overturning circulation. However, substantial differences in the vertical structure of the two ocean components are observed, some of which are directly attributed to their different vertical coordinate systems. In particular, the sea surface temperature (SST) in CHIME is biased warm almost everywhere, particularly in the North Atlantic subpolar gyre, in contrast to HadCM3, which is biased cold except in the Southern Ocean. Whereas the HadCM3 ocean warms from just below the surface down to 1000-m depth, a similar warming in CHIME is more pronounced but shallower and confined to the upper 400 m, with cooling below this. This is particularly apparent in the subtropical thermoclines, which become more diffuse in HadCM3, but sharper in CHIME. This is interpreted as resulting from a more rigorously controlled diapycnal mixing in the interior isopycnic ocean in CHIME. Lower interior mixing is also apparent in the better representation and maintenance of key water masses in CHIME, such as Subantarctic Mode Water, Antarctic Intermediate Water, and North Atlantic Deep Water. Finally, the North Pacific SST cold error in HadCM3 is absent in CHIME, and may be related to a difference in the separation position of the Kuroshio. Disadvantages of CHIME include a nonconservation of heat equivalent to 0.5 W m−2 globally, and a warming and salinification of the northwestern Atlantic.

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B. I. Moat, B. Sinha, S. A. Josey, J. Robson, P. Ortega, F. Sévellec, N. P. Holliday, G. D. McCarthy, A. L. New, and J. J.-M. Hirschi


An ocean mixed layer heat budget methodology is used to investigate the physical processes determining subpolar North Atlantic (SPNA) sea surface temperature (SST) and ocean heat content (OHC) variability on decadal to multidecadal time scales using the state-of-the-art climate model HadGEM3-GC2. New elements include development of an equation for evolution of anomalous SST for interannual and longer time scales in a form analogous to that for OHC, parameterization of the diffusive heat flux at the base of the mixed layer, and analysis of a composite Atlantic meridional overturning circulation (AMOC) event. Contributions to OHC and SST variability from two sources are evaluated: 1) net ocean–atmosphere heat flux and 2) all other processes, including advection, diffusion, and entrainment for SST. Anomalies in OHC tendency propagate anticlockwise around the SPNA on multidecadal time scales with a clear relationship to the phase of the AMOC. AMOC anomalies lead SST tendencies, which in turn lead OHC tendencies in both the eastern and western SPNA. OHC and SST variations in the SPNA on decadal time scales are dominated by AMOC variability because it controls variability of advection, which is shown to be the dominant term in the OHC budget. Lags between OHC and SST are traced to differences between the advection term for OHC and the advection–entrainment term for SST. The new results have implications for interpretation of variations in Atlantic heat uptake in the CMIP6 climate model assessment.

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Jon Robson, Matthew B. Menary, Rowan T. Sutton, Jenny Mecking, Jonathan M. Gregory, Colin Jones, Bablu Sinha, David P. Stevens, and Laura J. Wilcox


Previous work has shown that anthropogenic aerosol (AA) forcing drives a strengthening in the Atlantic Meridional Overturning Circulation (AMOC) in CMIP6 historical simulations over 1850–1985, but the mechanisms have not been fully understood. Across CMIP6 models, it is shown that there is a strong correlation between surface heat loss over the subpolar North Atlantic (SPNA) and the forced strengthening of the AMOC. Despite the link to AA forcing, the AMOC response is not strongly related to the contribution of anomalous downwelling surface shortwave radiation to SPNA heat loss. Rather, the spread in AMOC response is primarily due to the spread in turbulent heat loss. We hypothesize that turbulent heat loss is larger in models with strong AA forcing because the air advected over the ocean is colder and drier, in turn because of greater AA forced cooling over the continents upwind, especially North America. The strengthening of the AMOC also feeds back on itself positively in two distinct ways: by raising the sea surface temperature and hence further increasing turbulent heat loss in the SPNA, and by increasing the sea surface density across the SPNA due to increased northward transport of saline water. A comparison of key indices suggests that the AMOC response in models with strong AA forcing is not likely to be consistent with observations.

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