The Gravity Current Entrainment Climate Process Team Sonya Legg Princeton University, NOAA-GFDL

Representing overflows in climate models Shear instability, entrainment detrainment Geostrophic eddies x z y Downslope descent Bottom friction Physical processes in overflows Hydraulic control Dense water masses formed in marginal seas enter open ocean through overflows, e.g. Denmark Straits, Faroe Bank Channel, Antarctic slope overflows. Mixing and transport in overflows determines properties of ocean bottom waters, e.g. NADW, AABW. Ocean models need to represent overflow processes correctly to get dense water properties right. Z-coordinate models have difficulties getting water downslope without excessive mixing at coarse climate resolution. Isopycnal models need to parameterize diapycnal mixing. Narrow straits are an issue for all coarse resolution models.

How do we improve ocean model overflow representation? The Gravity Current Entrainment Climate Process Team: a multi-institutional collaboration between those studying processes in detail and those building and running climate models. A US CLIVAR project funded by NSF and NOAA, 2003-2008. Observations Laboratory and numerical process studies Regional simulations Simulations in idealized configurations Global climate simulations Improved parameterizations compare calibrate evaluate

Postdocs/Researchers CPT-GCE participants: Models, Numerical and Lab process studies, Observations Core PIs Stephen Griffies (GFDL), Robert Hallberg (GFDL),William Large (NCAR), Gokhan Danabasoglu (NCAR), Peter Gent (NCAR), Jim Price (WHOI), Jiayan Yang (WHOI), Sonya Legg (WHOI/Princeton), Hartmut Peters (Miami), Eric Chassignet (Miami), Tamay Ozgokmen (Miami), Tal Ezer (Princeton), Arnold Gordon (Columbia), Paul Schopf (GMU) Postdocs/Researchers Ulrike Riemenschneider (WHOI), Laura Jackson (GFDL), Yeon Chang (Miami), Wanli Wu (NCAR) For more details see http://www.cpt-gce.org

CPT Activities, years 1-3 Annual workshops Table of observations Put observationalists, process study modelers and GCM developers in the same room Opportunity for team to provide input/feedback to results/plans of team members Opportunity to share results/plans with invited members of wider community Forum for discussion of joint activities/plans Starting point for continuing one-on-one collaborations Table of observations Compiled by team observationalists, lead by Arnold Gordon. Quick reference for: Parameters needed in GCM overflow representations Intercomparison of observed overflow characteristics Comparison with regional and climate simulations. Publications and presentations Numerous individual publications 2 group publications in USCLIVAR Variations 1 synthesis article in preparation for BAMS Numerous team posters/presentations

Summary of principal achievements of first 3 years of CPT Parameterization of frictional bottom boundary mixing in overflows: implementation in Hallberg Isopycnal Model has made a 1st order difference to its credibility as a climate ocean model at 1 degree. The Marginal Sea Boundary Condition has been implemented in climate models, NCAR POP and HYCOM: Med Sea Outflow can now be represented credibly at coarse resolution. New and improved shear mixing parameterizations have been developed. Partially open barrier method: identified as a promising method for representing narrow straits. Reduction in spurious mixing in z-coordinate models: several promising solutions are under investigation. All of these developments have involved input from observations, idealized and regional numerical simulations, in addition to GCMs.

Focus on Frictional bottom boundary mixing Legg et al., 2005 With thick plumes both interfacial shear mixing and drag-induced near bottom mixing are needed. (Legg, Hallberg and Girton,2006). Interior Ri# Mixing Only HIM 10 km Actively mixing Interfacial Layer Shear Ri# Param. appropriate here. Interior Ri# + Drag Mixing HIM 10 km Well-mixed Bottom Boundary Layer Mixing driven by bottom stresses Resolved mixing (LES) MITgcm 500m x 30m Observed profiles from Red Sea plume from RedSOX (Hartmut Peters)

Impact of frictional bottom boundary mixing on GCM results Mediterranean outflow salinity: comparison between 1 degree Hallberg Isopycnal Model and observed climatology. New parameterization eliminates spurious bottom plume Spurious bottom plume

Focus on the Marginal Sea Boundary Condition Parameterizes both narrow straits and entrainment, suitable for both isopycnal and z-coord models. Developed by Yang and Price, implemented in NCAR POP for Med Sea and Faroe Bank Channel, and in HYCOM for Med Sea. ARCTIC ATLANTIC Surface ρ0 (Ent. density based on 12 grid points) ρS (Source density based on 12 grid points) 686m QS g’ = g (ρS-ρO)/ρref QE -z 945m NCAR MSBC implementation for FBC QS = g’ (hS)2 / 2f QP = QSFr2/3 QP = QS + QE + Heat and salt conservation QP BOTTOM TOPOGRAPHY 2100m

Team interactions involved in MSBC implementation Parameterization originally developed by Yang and Price, who continue to consult on implementation issues. Numerous parameters must be specified: values are taken from Table of observations, and/or from regional simulations (Ulrike Riemenschneider, FBC). Whole team provides input on scenarios for examining impact in GCMs. Yang and Price continue to improve/update MSBC, with new developments to be ported to GCMs when ready.

Results: Impact of MSBC in GCM simulations Salinity at 1100m depth: comparison between climatology and NCAR 3 degree simulations for Med outflow. MSBC leads to credible Med salt tongue.

CPT plans for years 4 and 5 include: Complete implementation of MSBC in NCAR POP and HYCOM, for all climatologically important overflows. Complete new shear-driven mixing parameterizations, implement in HIM, MOM4 and other models. Complete implementation of partially open barriers in global models and compare with other methods of representing narrow straits. Focus on impact of parameterizations on Nordic overflows and Antarctic overflows. Examine impact of parameterizations on global climate simulations, including climate change scenarios, Greenland ice-melt scenarios.

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Results:Table of observations Purpose: Quick reference for: Parameters needed in GCM overflow representations Intercomparison of observed overflow characteristics Comparison with regional and climate simulations. Effort led by Arnold Gordon. Abbreviated version: for full version see http://www.cpt-gce.org/Table_of_observations.htm

Results: Improved parameterizations of shear-driven mixing. Calibration of entrainment parameterization by comparison with nonhydrostatic benchmark calculations (Miami CPT members) Calibrated parameterization is validated by comparison between regional simulations and observations. Example: Mediterranean outflow simulated in 0.08 degree HYCOM regional implementation (Xu et al, 2006). Downstream evolution of entrainment in HYCOM simulations with different entrainment parameterizations compared to nonhydrostatic (Nek) benchmark calculations. with E0= 0.2 and Ric=0.25 (Xu et al, 2006)

Results: Improved parameterizations of shear-driven mixing II Jackson and Hallberg (GFDL) are developing a new parameterization of shear-driven mixing for both isopycnal and z-coord models, with a parameterized diffusivity of the form: where S is the vertical shear of the resolved horizontal velocity is the buoyancy length scale (the scale of the overturns), N is the buoyancy frequency, and Q is the turbulent kinetic energy, found from an energy budget. F(Ri) is a function of shear Richardson number Ri such as Initial comparison with DNS of shear layers and jets looks promising. Validation with LES is continuing. DNS data ET parameterisation New parameterisation F0 = 0.14, cN = 0.41, cS = 0.10,  = 0.6 F0 = 0.11, cN = 0.20, cS = 0.10,  = 0.7 F0 = 0.12, cN = 1.87, cS = 0.10,  = 0.9

Results: Impact of shear-driven mixing parameterization on climate In coupled simulations using Hallberg Isopycnal model, with entrainment in Nordic overflows SSTs are warmer near entrainment site, and cooler to south, due to change in location of Gulf Stream induced by DWBC transport changes.

Results: Treatment of narrow straits Regional simulations of Red Sea (Chang et al, 2006), Mediterranean (Xu et al, 2006) and Faroe Bank Channel (Riemenschneider and Legg, 2006) show results are more sensitive to resolution of topography than to mixing parameterization. Representation of channels below grid scale by thin walls and partially open barriers is under development by Adcroft and Hallberg (GFDL). This technique improves Mediterranean outflow simulations in Hallberg Isopycnal model at 1 degree (110km) resolution by reducing Gibraltar width to 12km. Salinity in HIM global simulations and observations

Results: reducing spurious mixing in z-coordinate models Idealized simulations (Legg et al, 2006) and regional simulations (Riemenschneider and Legg, 2006) show that z-coordinate models produce too much numerical mixing, principally due to advection schemes. Efforts to reduce spurious mixing in MOM4 (Griffies, GFDL) include: New, less diffusive advection schemes Improved sigma-diffusion schemes (Beckmann and Doescher, 1997). Difference between CM2.1 simulations with and without sigma-diffusion at 100m in Med outflow region. Promising recent development includes non-local horizontal communication in sigma-diffusion.