Ocean circulation drifts in multi-millennial climate simulations: the role of salinity corrections and climate feedbacks

Abstract

Low-resolution, complex general circulation models (GCMs) are valuable tools for studying the Earth system on multimillennial timescales. However, slowly evolving salinity drifts can cause large shifts in climatic and oceanic regimes over thousands of years. We test two different schemes for neutralising unforced salinity drifts in the FAMOUS GCM: surface flux correction and volumetric flux correction. Although both methods successfully maintain a steady global mean salinity, local drifts and subsequent feedbacks promote cooling (≈ 4 °C over 6000 years) and freshening (≈ 2 psu over 6000 years) in the North Atlantic Ocean, and gradual warming (≈ 0.2 °C per millennium) and salinification (≈ 0.15 psu per millennium) in the North Pacific Ocean. Changes in the surface density in these regions affect the meridional overturning circulation (MOC), such that, after several millennia, the Atlantic MOC (AMOC) is in a collapsed state, and there is a strong, deep Pacific MOC (PMOC). Furthermore, the AMOC exhibits a period of metastability, which is only identifiable with run lengths in excess of 1500 years. We also compare simulations with two different land surface schemes, demonstrating that small biases in the surface climate may cause regional salinity drifts and significant shifts in the MOC (weakening of the AMOC and the initiation then invigoration of PMOC), even when the global hydrological cycle has been forcibly closed. Although there is no specific precursor to the simulated AMOC collapse, the northwest North Pacific and northeast North Atlantic are important areas that should be closely monitored for trends arising from such biases.

Simulating stable carbon isotopes in the ocean component of the FAMOUS general circulation model with MOSES1 (XOAVI)

“…pled; Doney, 1999;Doney et al, 2004;Najjar et al, 2007). Thus, simulating carbon isotopes in a more complex ecosystem model would not necessarily yield substantially better results.…”

Simulating oceanic radiocarbon with the FAMOUS GCM: implications for its use as a proxy for ventilation and carbon uptake

“…Antarctic Circumpolar Current in FAMOUS is weak compared to observations (Dentith et al, 2019), it is still strong enough to homogenize the water column.…”

“…In this study, we used the Met Office Surface Exchange Scheme (MOSES) version 1 (Cox et al, 1999) generation of the model. Although the MOSES2.2 generation of the model offers increased and more dynamic Earth System capabilities (Essery et al, 2001(Essery et al, , 2003Williams et al, 2013;Valdes et al, 2017), the published setup does not accurately simulate the Meridional Overturning Circulation in multi-millennial simulations with constant pre-industrial boundary conditions (Dentith et al, 2019). At present, FAMOUS-MOSES1 is therefore a more appropriate tool for studying oceanic tracers.…”

“…This is due to the aforementioned insufficient convective mixing, which results in a relatively shallow mixed layer and excessive pooling of 14 C in the upper ocean. The simulated Δ 14 C values also indicate that the abyssal Atlantic waters are too well ventilated as a result of over-deep North Atlantic Deep Water (NADW) formation and insufficient Atlantic-sector Antarctic Bottom Water (AABW) formation, which are known limitations of the FAMOUS GCM (Smith, 2012;Dentith et al, 2019). Across the whole ocean, the observed minimum ∆ 14 C value is -240 ‰, which occurs in the northeast North Pacific Ocean, at a depth of approximately 2500 m. In the model, there is weak (< 1 Sv) convection to around 3 km depth in the sub-polar North Pacific (Dentith et al, 2019), which prevents the accumulation of old, 12 C-enriched (low ∆ 14 C)…”

“…For example, the simulated ∆ 14 C gradient between the surface ocean and approximately 1000 m depth is shallower than observed because of insufficient convective mixing. This is visible in all regions, except the Northern Hemisphere deep water formation region (NH_DWF) and the North Pacific (NP), where convection in the model is deeper than it should be (Dentith et al, 2019), thereby actively mixing the bomb signal into the sub-surface waters. In the modern oceans, NADW is formed in the Labrador and Nordic Seas (Kuhlbrodt et al, 2007); therefore, the observed depth profiles in these regions (LS and NH_DWF, respectively) are very similar.…”