The Gravity Current Entrainment Climate Process Team. Sonya Legg Princeton University, NOAA-GFDL. Hydraulic control. z. Shear instability, entrainment. y. x. Geostrophic eddies. Bottom friction. Downslope descent. detrainment. Physical processes in overflows.
Princeton University, NOAA-GFDL
Shear instability, entrainment
Physical processes in overflowsRepresenting overflows in climate models
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.
Simulations in idealized configurations
Global climate simulationsHow 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.
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)
Ulrike Riemenschneider (WHOI), Laura Jackson (GFDL), Yeon Chang (Miami), Wanli Wu (NCAR)
For more details see http://www.cpt-gce.org
All of these developments have involved input from observations, idealized and regional numerical simulations, in addition to GCMs.
With thick plumes both interfacial shear mixing and drag-induced near bottom mixing are needed. (Legg, Hallberg and Girton,2006).
Interior Ri# Mixing Only
Shear Ri# Param.
Interior Ri# + Drag Mixing
Well-mixed Bottom Boundary Layer
Mixing driven by bottom stresses
Resolved mixing (LES)
500m x 30m
Observed profiles from Red Sea plume from RedSOX
Mediterranean outflow salinity: comparison between 1 degree Hallberg Isopycnal Model and observed climatology.
Spurious bottom plume
New parameterization eliminates spurious bottom plume
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.
(Ent. density based on 12 grid points)
(Source density based on 12 grid points)
g’ = g (ρS-ρO)/ρref
QS = g’ (hS)2 / 2f
NCAR MSBC implementation for FBC
QP = QSFr2/3
QP = QS + QE
Heat and salt conservation
Salinity at 1100m depth: comparison between climatology and NCAR 3 degree simulations for Med outflow. MSBC leads to credible Med salt tongue.
Abbreviated version: for full version see
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)
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.
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
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.
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
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.