Assessment of Southern Ocean water mass circulation and characteristics in CMIP5 models: Historical bias and forcing response

Abstract

[1] The ability of the models contributing to the fifth Coupled Models Intercomparison Project (CMIP5) to represent the Southern Ocean hydrological properties and its overturning is investigated in a water mass framework. Models have a consistent warm and light bias spread over the entire water column. The greatest bias occurs in the ventilated layers, which are volumetrically dominated by mode and intermediate layers.The ventilated layers have been observed to have a strong fingerprint of climate change and to impact climate by sequestrating a significant amount of heat and carbon dioxide. The mode water layer is poorly represented in the models and both mode and intermediate water have a significant fresh bias. Under increased radiative forcing, models simulate a warming and lightening of the entire water column, which is again greatest in the ventilated layers, highlighting the importance of these layers for propagating the climate signal into the deep ocean. While the intensity of the water mass overturning is relatively consistent between models, when compared to observation-based reconstructions, they exhibit a slightly larger rate of overturning at shallow to intermediate depths, and a slower rate of overturning deeper in the water column. Under increased radiative forcing, atmospheric fluxes increase the rate of simulated upper cell overturning, but this increase is counterbalanced by diapycnal fluxes, including mixed-layer horizontal mixing, and mostly vanishes.

Development and evaluation of CNRM Earth system model – CNRM-ESM1

“…Consequently, the flow of deep water masses in CNRM-ESM1 has been improved with regards to that of CNRM-CM5, which ranges between 3.4 and 6.2 Sv over the same period Voldoire et al, 2013). As detailed in several intercomparison studies (de Lavergne et al, 2014;Heuzé et al, 2013;Sallée et al, 2013;Séférian et al, 2013), CNRM-CM5 substantially underestimated the flow of AABW leading to an erroneous distribution of hydrodynamical and biogeochemical fields at depth. Here, although stronger than the observation-based estimates, the flow of NADW and AABW improves the deep ocean ventilation as well as the distribution of tracers at depth (Sect.…”

The Southern Ocean as a constraint to reduce uncertainty in future ocean carbon sinks

“…The unique SO region benefits from the strong link between deep and surface ocean through the southern upwelling (Sallée et al, 2013a). Earlier studies analyzing the previous generation of ESMs also demonstrated that this region will be an important sink of future atmospheric CO 2 although the efficiency of the sink may decrease .…”

Role of Mesoscale Eddies in Cross-Frontal Transport of Heat and Biogeochemical Tracers in the Southern Ocean

“…Where the fronts flow around major topographic obstacles, hence developing large-scale meanders, departures from parallel flow conditions break the prevalent regime where cross-frontal transport is inhibited (Naveira Garabato et al 2011). Such ''hotspots'' of transport, or ''leaky jet'' segments, show enhanced effective eddy diffusivity, thus allowing more tracer transport across the fronts (Naveira Garabato et al 2011;Thompson and Sallée 2012;Sallée et al 2008b).…”

“…Error bars show the minimum and maximum of the observation-based datasets (on top of blue bars) and the CMIP5 multimodel standard deviation (on top of red bars) for each water mass class. The compilation of estimates from observations and the CMIP5 model ensemble, and the method used to find water mass boundaries, are described in Sallée et al (2013). A similar method was applied to the last 20 yr of CM2.6-miniBLING output.…”