# Presentation: 223374

## Ocean Modeling Requirementsfor Decadal-to-Centennial Climate

- Robert Hallberg
- NOAA/GFDL

## Ocean Modeling Considerationsfor Decadal-Centennial Climate

- Dec-Cen Climate is Operational, but not in Real Time.
- Ocean models are fully global and must include sea-ice.
- Models run for O(1000-year) timescales.
- Wall-clock model speeds are 103-104 times real time.
- Ocean biases after a spinup of hundreds of years must be acceptably small.
- Data assimilation during the run or unphysical damping are unacceptable in Dec-Cen climate models.
- Precise initial values are often relatively unimportant.
- The long timescales mean that a wide range of physical processes must be represented as credibly as possible.

## Ocean Climate Models must be Conservative.

- Non-conservation leads to drift and uncertainty.
- Tolerances for non-conservation:
- 	Mass – small compared to sea-level rise.
- 		20th Century rise ~10 cm.
- 		Err << 10-3 m cen-1 ; MOM, GOLD ~10-7 m cen-1
- 	Heat – small compared to anthropogenic forcing
- 		Anomalous Radiative Forcing ~4 W m-2 at CO2 doubling
- 		Err << 0.1 W m-2 ; MOM, GOLD ~3x10-6 W m-2
- 	Total Salt – small compared to sea-level rise dilution?
- 		Dilution of Salinity ~ 10-3 PSU cen-1
- 		Err << 10-4 PSU cen-1 ; MOM, GOLD ~3x10-9 PSU cen-1
*With care*, these tolerances can be achieved in all classes of ocean  models. (MOM Z-coord., GOLD -coord.)

## Key Metrics of a Global Ocean Climate Model

- SST biases at equilibrium
- ENSO statistics (Amplitude & Frequency) at equilibrium
- Tropical ocean circulation and watermass structure at equilibrium.
- Stability and strength of meridional overturning circulation and gyre circulations.
- 	(Important for meridional heat transport & sea-ice distribution)
- Equilibrium watermass properties, rates and processes of formation & destruction.
- 	(Important for storage of heat, carbon, etc.)
- Spurious diapycnal mixing << physical – Kd = 10-5 to 10-6 m2 s-1
- Overflows & entraining gravity currents
- Mode-water formation processes

## 100-year-mean SST Biases in GFDL’s CM2.1 Coupled Climate Model

**100-year-mean SST Biases in GFDL’s CM2.1 Coupled Climate Model**

## Equatorial Pacific Velocities and Temperatures after 500 years in CORE simulations with 7 models

- Drifts accumulate over ~50 years.
- Short runs may not be indicative of equilibrium climate.

- (Figs. from Griffies et al., Ocean Mod., Sub.)

## Challenges in Global Ocean Climate Modeling

- Increasing resolution to admit mesoscale eddies

## Role of eddies in Southern Ocean Dynamics

- Eddies alter the sensitivity to forcing changes of the Southern Ocean overturning circulation and the resultant ventilation of the interior ocean.

**Hallberg & Gnanadesikan, *****JPO 2006***

## Challenges in Global Ocean Climate Modeling

- Increasing resolution to admit mesoscale eddies
- Dramatic increase in model cost, reduction in speed
- Changes in required parameterizations
- Numerical constraints (e.g. on spurious diapycnal mixing) become harder to satisfy.
- Regional climate impacts

## SST in a Prototype 1/8° Global Ocean Model

- California Current

- Upwelling zone

- ~1°×1° Box
- Contemporary Climate Model Resolution

## Challenges in Global Ocean Climate Modeling

- Increasing resolution to admit mesoscale eddies
- Dramatic increase in model cost, reduction in speed
- Changes in required parameterizations
- Numerical constraints (e.g. on spurious diapycnal mixing) become harder to satisfy.
- Regional climate impacts
- Dominant scales of many ecosystems much smaller than well-resolved by global physical climate models.
- Two-way nesting is probably needed.

## The Existing GFDL Suite of Ocean Models

*GFDL/Princeton has leading developers of each widely used class of large-scale ocean models.*
- MOM – B-grid Z-coordinate model
- MITgcm – C-grid Z-coordinate model with nonhydrostatic capabilities
- HIM – C-grid isopycnal coordinate model
- POM – Princeton AOS program’s C-grid sigma coordinate model
*The “Generalized Ocean Layer Dynamics” (GOLD) ocean model unites the GFDL efforts.*
- GOLD is an ocean modeling system that combines the capabilities of GFDL’s MOM and HIM and the MITgcm ocean models into a single flexible code-base.
- GOLD numerics will be suitable for studying climate, but with demonstrated proficiency for a variety of other applications (tides, nonhydrostatic mixing, etc.)
- GOLD will include significant nesting and data assimilation capabilities.
*GFDL ocean models focus on integrity for climate applications.*

## GOLD and HYCOM

*GOLD **is structurally compatible with HYCOM.*
- The GOLD dynamic core & overall structure are derived from the isopycnal coordinate model HIM.  Similar issues must be addressed as for HYCOM.
- GOLD has necessary qualities for long-term climate studies.
- Conservation-to-roundoff of heat, salt, mass, and tracers.
- Accurate with a fully nonlinear equation of state.
- Robust parameterizations of small-scale processes.
- Rotated diffusion tensor & minimized spurious diapycnal mixing.
- An NSF-funded project, including GOLD, HYCOM and ROMS, is exploring standardization across ocean models.
- Collaborations are welcomed, provided they do not disrupt GFDL’s primary mission in long-term climate studies.

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## Potential for “One NOAA Ocean Model” Addressing Real-time Operational & Global Dec-Cen Climate

- What is a “model”?
- A single model configuration.
- Unlikely, due to mismatch in timescales, model speed, emphasis on initial conditions vs. bias.
- A shared model code-base.
- Repository of best theories/techniques for both real-time operations and climate. Natural synergies could emerge.
- Will take considerable work and resources.
- Important not to disrupt the on-going operational requirements for real-time forecasts or climate.
- Establishing common model interfaces would be a natural first step.
- Some ocean forecasting applications (e.g., tsunami warnings) are so different that they are unlikely ever to use the same model.

## Slide 15

## High-Resolution Ocean Modeling &the Ocean Research Priorities Plan

- Numerical ocean models are recognized as crucial tools for a wide range of  questions (ORPP, p. 47).
- High resolution global ocean modeling is central to the ORPP’s Theme 4: “The Ocean’s Role in Climate” and the near-term priority “Assessing Meridional Overturning Circulation Variability”
- Resolving the critical processes is crucial for reducing the uncertainty in projections.
- The insight gained from the Global Ocean Observing System is maximized when the data is interpreted in the context of numerical models.
- High resolution global ocean simulations are valuable in support of the other 5 Themes.
- Provide consistent boundary conditions for ultrafine regional models.
- Provide estimates of variability in the conditions faced by ecosystems, marine operations, or processes affecting human health.
- NOAA/GFDL is taking the lead in addressing the ORPP call for a coherent, comprehensive global modeling capability, addressing Theme 4 in particular.

## Mean Salinity Drifts in 7 CORE Forced Candidate Ocean Climate models (Griffies et al., 2008)

- Notes: Sea-ice growth is the sole cause of salinity drift in some models.
-            Some models balance salinity restoring, others do not.

## Frontal Dynamics and Resolution

- Upwelling jets & fronts require higher resolutions than current ocean climate models.
- With steady forcing all variability is due to ocean dynamics.

**Hallberg & Gnanadesikan, *****JPO 2006***

## Strengths and Weaknesses of Terrain-following Coordinate Models

- (Issues for global climate application addressed in detail by G. Danabasoglu later.)
- Strengths:
- Topography is represented very simply and accurately
- Easy to enhance resolution near surface.
- Lots of experience with atmospheric modeling to draw upon.
- Traditional Weaknesses:
- Pressure gradient errors are a persistent problem.
- Errors are reduced with better numerics (e.g., Shchepetkin & McWilliams, 2003)
- Gentle slopes (smoothed topography) must be used for consistency
- Traditional requirement for stability (Beckman & Haidvogel, 1993):
- ROMS requirement (Shchepetkin, pers. comm):
- Spurious diapycnal mixing due to advection may be very large. (Same issue as Z-coord.)
- Diffusion tensors may be especially difficult to rotate into the neutral direction.
- Strongly slopes require larger vertical stencil for the isoneutral-diffusion operator.
- Myth: Near bottom resolution can be arbitrarily enhanced.
- Hydrostatic consistency imposes horizontal resolution-dependent constraints on near-bottom vertical resolution, with serious implications for the ability to represent overflows

## Resolution requirements for avoiding numerical entrainment in descending gravity currents.

**Z-coordinate:**
**	**Require that
- 		**AND**
- 	to avoid numerical entrainment.
*	**(Winton, et al., JPO 1998)*
- Many suggested solutions for Z-coordinate models:
- "Plumbing" parameterization of downslope flow:
- 	Beckman & Doscher (JPO 1997), Campin & Goose (Tellus 1999).
- Adding a separate, resolved, terrain-following boundary layer:
- 	Gnanadesikan (~1998), Killworth & Edwards (JPO 1999), Song & Chao (JAOT 2000).
- Add a nested high-resolution model in key locations?
**Sigma-coordinate:**** **Avoiding entrainment requires that**  **
**	****But** hydrostatic consistency requires
**Isopycnal-coordinate:**** **Numerical entrainment is not an issue - BUT
- If resolution is inadequate, no entrainment can occur. Need

## Slide 21

## Horizontal Resolution (in km) Required to Permit 50m Vertical Resolution at Bottom

## Horizontal Resolution (in km) Required to Permit 50m Vertical Resolution at Bottom

## Horizontal Resolution (in km) Required to Permit 50m Vertical Resolution at Bottom