An assessment of the CMIP5 models in simulating the Argo geostrophic meridional transport in the North Pacific Ocean
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An assessment of the CMIP5 models in simulating the Argo geostrophic meridional transport in the North Pacific Ocean* LI Xiang1, 2, YUAN Dongliang1, 2, 3, ** 1
Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, Chinese Academy of Sciences, and Pilot National
2
Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China
3
University of Chinese Academy of Sciences, Beijing 100049, China
Laboratory for Marine Science and Technology (Qingdao), Qingdao 266071, China
Received Jan. 3, 2020; accepted in principle Apr. 14, 2020; accepted for publication Jul. 22, 2020 © Chinese Society for Oceanology and Limnology, Science Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Eleven climate system models that participate in the Coupled Model Intercomparison Project phase 5 (CMIP5) were evaluated based on an assessment of their simulated meridional transports in comparison with the Sverdrup transports. The analyses show that the simulated North Pacific Ocean circulation is essentially in Sverdrup balance in most of the 11 models while the Argo geostrophic meridional transports indicate significant non-Sverdrup gyre circulation in the tropical North Pacific Ocean. The climate models overestimated the observed tropical and subtropical volume transports significantly. The non-Sverdrup gyre circulation leads to non-Sverdrup heat and salt transports, the absence of which in the CMIP5 simulations suggests deficiencies of the CMIP5 model dynamics in simulating the realistic meridional volume, heat, and salt transports of the ocean. Keyword: Coupled Model Intercomparison Project phase 5 (CMIP5) models; Sverdrup balance; meridional transport; Argo geostrophic currents
1 INTRODUCTION The latest Coupled Model Intercomparison Project phase 5 (CMIP5) provides hindcast and forecast simulations for the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (Stocker et al., 2014). As an essential component of the global climate system, ocean general circulation plays an important role in redistributing heat, salt, and passive tracers in the world oceans. So far, the simulated transports of the ocean circulation in these coupled models have not been evaluated with observations at the basin scale. The general ocean circulation theory suggests that the currents of the world oceans are determined by the vertical velocity, like the Ekman pumping, which forces the meridional movement of the water. The vertical velocity in the ocean is very small and difficult to measure directly. In contrast, the meridional velocity can be measured directly and can be used to assess model simulations. The meridional transport can also be used to calculate the zonal transport from
the continuity equation. In practice, the large-scale meridional transport is estimated based on geostrophy from temperature and salinity profiles observed in the ocean, since direct current measurements are scarce. The geostrophic currents are estimated based on the thermal wind relation, which usually
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