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Fresh Water Pulses and Meridional Overturning Circulation – Forward and Adjoint Studies. Chris Hill, Patrick Heimbach, Peter Winsor, Alan Condron. Background

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Background

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  1. Fresh Water Pulses and Meridional Overturning Circulation – Forward and Adjoint Studies Chris Hill, Patrick Heimbach, Peter Winsor, Alan Condron Background Paleo records suggest cooling coinciding with melt-water events (for example glacial Lake Agassiz and Ojibway melt), hypothesized as impact of buoyancy flux on meridional overturning. Tools High-res forward modeling of perturbations North Atlantic surface current speed @ 4km resolution. Guidance from adjoint sensitivity studies YD Sensitivity of global 16km cube-sphere temperature to temperature one year prior. Goal Understand stability of realistic Atlantic (turbulent, eddying) meridional overturning circulation (AMOC) to fresh-water perturbations on ~decadal time-scales – both in recent past and in current system.

  2. Relevance to Abrupt Climate Change activity PNAS Glacial Melt • Questions • Sources, locations and pathways of freshwater that can impact AMOC deep-water sites. • Impact of fresh water pulse magnitude and duration on AMOC response. • Difference between mass and heat transport response. • When does non-linear hysteresis kick in. Arctic fresh-water Lake Aggasiz Arctic fresh water exports and/or Greenland ice sheet melt water estimates @ > 0.1Sv over 1 – 10 years. Exact profile is unpredictable so we are examining a range of scenarios.

  3. Approach – part 1 Start from globally assimilated ~15km resolution model – ECCO2 (http://ecco2.org ) 1 - provides bc’s to 4km resolution limited area control + perturbation calcs covering Arctic + North Atlantic – using “elongated” cube.  isotropic pole mesh.

  4. Approach – part 2 Use adjoint from globally assimilated ~15km resolution model – ECCO2 (http://ecco2.org ) 2 – MOC mass and heat transport cost functions. Using 2 to search perturbation space, and guide perturbation experiments in 1. Using 1 to quantify full non-linear response to specific perturbations.

  5. Summary In past North Atlantic fresh-water fluxes appear to have induced rapid climate change. Several potential sources for North Atlantic fresh-water are at play today – e.g. glacial melt, Arctic sea-ice/FW anomaly. Prior studies (e.g. left) show sensitivity, but output v. input has wide range of possible values. Q – sensitivity of MOC, SST in eddying, observationally constrained solution with “synoptic” timescale (and space scale) forcing? Manabe & Stouffer, 1995. 1Sv x 10 years Rahmstorf, 1995. 0.05Sv x 1000 years

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