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The biogeochemical (BGC) and physical capabilities of 16 CMIP5 and 16 CMIP6 Earth System models are examined in relation to present day (1976–2005) observational data sets for a region of the southwest Pacific Ocean (SWPAC). A relative ranking scheme enables identification of a “best” model ensemble over SWPAC, with the CMIP6 models having generally better physical representations compared to the CMIP5 set, but the BGC improvements less marked. For the period 2081–2100 using RCP8.5 and SSP585 for CMIP5 and CMIP6, respectively, we find significant surface temperature increases and mixed layer depth reductions compared to present day over SWPAC, and reductions in the macronutrients but increases in surface iron. Surface chlorophyll and zooplankton and integrated primary production are all found to decrease by end of century over SWPAC except for the southernmost waters. Sea …