Abstract
Low-resolution, complex general circulation models (GCMs) are valuable tools for studying the Earth system on multi-millennial 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.
Highlights
State-of-the-art, high resolution, high complexity general circulation models (GCMs) provide a sophisticated representation of the main components of the Earth system: the ocean, atmosphere, biosphere, and sea ice
This study focuses on multi-millennial salinity drifts that primarily arise because of inaccuracies in the formulation of the hydrological budget, which lead to the non-conservation of salt or freshwater (Bryan 1998; Gupta et al 2012)
As with many GCMs, the surface hydrological budget in FAMOUS is not fully closed, with imbalanced hydrology over inland seas and insufficient snowmelt leading to small salinity drifts
Summary
State-of-the-art, high resolution, high complexity general circulation models (GCMs) provide a sophisticated representation of the main components of the Earth system: the ocean, atmosphere, biosphere, and sea ice. They are impractical for running the long simulations required to spin-up the components of the Earth system that evolve on millennial timescales, such as deep ocean circulation (England 1995) and ocean biogeochemical cycles (Falkowski et al 2000; Key et al 2004). GCMs with significantly faster operational speeds, as a consequence of reduced spatial resolution and/or longer timesteps, are important tools for Earth system modellers who run long integrations (e.g. to study palaeoclimate, the carbon cycle and ice sheet evolution). These models allow multimillennial climate simulations to be conducted whilst still allowing considerable detail in the complexity of the feedbacks between different Earth system processes. Examples include FAMOUS (Jones et al 2005; Smith et al 2008; Williams et al 2013), the CSIRO Mk3L climate system model (Phipps et al 2011), and low-resolution versions of CCSM3 (Yeager et al 2006) and the GFDL coupled climate model (Dixon et al 2003)
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