Evaluation of Subtropical North Atlantic Ocean Circulation in CMIP5 Models against the Observational Array at 26.5°N and Its Changes under Continued Warming

Abstract

Observationally based metrics derived from the Rapid Climate Change (RAPID) array are used to assess the large-scale ocean circulation in the subtropical North Atlantic simulated in a suite of fully coupled climate models that contributed to phase 5 of the Coupled Model Intercomparison Project (CMIP5). The modeled circulation at 26.5°N is decomposed into four components similar to those RAPID observes to estimate the Atlantic meridional overturning circulation (AMOC): the northward-flowing western boundary current (WBC), the southward transport in the upper midocean, the near-surface Ekman transport, and the southward deep ocean transport. The decadal-mean AMOC and the transports associated with its flow are captured well by CMIP5 models at the start of the twenty-first century. By the end of the century, under representative concentration pathway 8.5 (RCP8.5), averaged across models, the northward transport of waters in the upper WBC is projected to weaken by 7.6 Sv (1 Sv ≡ 106 m3 s−1; −21%). This reduced northward flow is a combined result of a reduction in the subtropical gyre return flow in the upper ocean (−2.9 Sv; −12%) and a weakened net southward transport in the deep ocean (−4.4 Sv; −28%) corresponding to the weakened AMOC. No consistent long-term changes of the Ekman transport are found across models. The reduced southward transport in the upper ocean is associated with a reduction in wind stress curl (WSC) across the North Atlantic subtropical gyre, largely through Sverdrup balance. This reduced WSC and the resulting decrease in the horizontal gyre transport is a robust feature found across the CMIP5 models under increased CO2 forcing.