Evaluation of the HYbrid Coordinate Ocean Model (HYCOM) Using Atlantic Ocean Simulations

George Halliwell, Rainer Bleck, Eric Chassignet, Linda Smith

This slide show describes several Atlantic Ocean simulations performed using HYCOM with different vertical mixing schemes and with both hybrid and isopycnic vertical coordinates. Properties of HYCOM are outlined first. Model performance is then evaluated by comparing simulated fields to each other, to observed fields (Levitus climatology), and to fields simulated by the Miami Isopycnic Coordinate Ocean Model (MICOM).

OUTLINE

Press button to jump forward to each section:

I. The Model

2. Model Simulations

3. Surface/Mixed Layer Thermodynamical Fields

4. Surface/Mixed Layer Dynamical Fields

5. Cross-Sections

6. Model Censuses

7. Meridional Overturning Circulation and Heat Flux

8. Regional Differences in Upper Ocean Variability

9. Regional Mixed Layer Profiles

10. Summary

11. Referencess

The Model (1)

History

Developed from MICOM version 2.8

See Bleck (1998) for a description of MICOM

Hybrid coordinate scheme used in HYCOM

Model equations written in generalized vertical coordinates

Originally used in the Bleck and Boudra (1981) quasi-isopycnic model

Numerical schemes could not handle zero thickness layers

Minimum layer thickness was enforced

The vertical coordinate became non-isopycnic in regions where the layers became too thin

A hybrid (isentropic-sigma) vertical coordinate was used in the atmospheric model of Bleck and Benjamin (1993)

The HYCOM isopycnic-level-sigma vertical coordinate scheme is an adaptation and extension of the Bleck/Benjamin algorithm

The Model (2)

Major changes from MICOM

New vertical coordinate scheme

Multiple mixed layer models, including non-slab models

Interior convection due to static instability (two layers exchange fluid when the upper one is more dense)

Hydrostatic and momentum equations modified to handle non-isopycnic densities and horizontal density gradients

Choice of temperature-salinity, temperature-density, or salinity-density advection

The Model (3)

Vertical Coordinates

Hybrid coordinates

Nearsurface: z-coordinates

Shallow water: sigma (terrain-following) coordinates

Coordinates are isopycnic in the bulk of the ocean interior

Isopycnic coordinates (MICOM mode)

Hybrid Vertical Coordinate Scheme (part 1)

In the open ocean, the coordinates are isopycnic except year the surface where minimum coordinate separation is enforced. The minimum separation differs for each layer, giving the user great flexibility to set the vertical coordinate structure in the z-coordinate domain.

A cushion function described in Bleck and Benjamin (1993) provides a smooth transition between the isopycnic and z-coordinate domains.

To activate the sigma coordinate domain, the user specifies the number of sigma coordinates n and the coordinate separation d. Vertical coordinates become terrain-following where bottom depth is less than n*d. In extremely shallow water, the coordinates revert to level coordinates.

The Model (4)

Hybrid Vertical Coordinate Scheme (part 2)

In the deep ocean, the isopycnic-level coordinate transition is performed as follows:

If the density of a given layer does not equal the isopycnic reference density, the interfaces bounding the layer are adjusted to return the density to its reference value

If the layer is too light, the interface below is moved downward so that the entrained denser water returns the density to its reference value

If the layer is too dense, the interface above is moved upward in the same manner

If minimum coordinate separation is violated near the ocean surface, the cushion function is used to re-calculate the vertical coordinate location, prohibiting the restoration of isopycnic conditions.

Two of the thermodynamical variables T, S, and density are mixed across the moving interfaces (user selectable), with the third calculated from the equation of state. If T and S are mixed, exact isopycnal density is not restored, but repeated application keeps the error small.

The Model (5)

Horizontal Advection

Two of the thermodynamical variables T, S, and density are advected (user selectable), with the third calculated from the equation of state.

If HYCOM is run in MICOM mode, advection is performed as in MICOM 2.8, with T and S advected in layer 1 (the mixed layer) and S only advected in the isopycnic layers.

The Model (6)

Mixing Schemes

K-Profile Parameterization

Kraus-Turner Mixed Layer Model

Two Kraus-Turner mixed layer models available for hybrid coordinates

A third Kraus-Turner model is available when HYCOM is run in MICOM mode

Handles detrainment of water into isopycnic layers

Same algorithm used in MICOM 2.8

With hybrid coordinates, choice of explicit (MICOM-like) or implicit (KPP-like) interior diapycnal mixing

The Model (7)

K-Profile Parameterization (KPP)

Developed by Large, Mc Williams, and Doney (1994)

Governs Vertical Mixing of Entire Water Column

Parameterizes Several Physical Processes

Surface boundary layer

Mechanical wind mixing

Buoyancy flux forcing

Convective overturning

Non-local (counter-gradient) fluxes

Diapycnal mixing in ocean interior

Instability due to resolved vertical shear

Background internal wave mixing

Double diffusion mixing (diffusive convection and salt fingering)

Can Run at Relatively Low Vertical Resolution

The Model (8)

KPP Procedure (part 1):

Apply surface thermodynamical and momentum flux forcing

Calculate K profiles for interior diapycnal mixing from surface to bottom

Diagnose turbulent boundary layer thickness

Minimum depth H where a bulk Richardson number exceeds critical value

Turbulent boundary layer eddies can penetrate to depth H where the fluid becomes stable relative to local buoyancy and velocity

The Model (9)

KPP Procedure (part 2)

Calculate surface boundary layer k profiles for T, S, and momentum

Vertical diffusivity for T is parameterized as

where is the nonlocal transport term

Diffusivity is parameterized as

where w is a turbulent velocity scale that is a function of the stability of the forcing, G is a 3^rd order polynomial shape function, and sigma is a scale depth varying from 0 to 1 over the depth range H

Choose coefficients of G to match the interior and boundary layer K profiles, producing a final K profile with a continuous first vertical derivative

Solve diffusion equation semi-implicitly with two temporal iterations

Diagnose mixed layer thickness along with T, S, u, and v

The Model (10)

Implementation of Kraus-Turner mixing with hybrid coordinates

Mixed layer base is not a vertical coordinate interface as in MICOM

Must keep track of T, S, and density jumps across the mixed layer base

The Model (11)

Two hybrid-coordinate K-T mixed layer models have been developed.

K-T 1: Full model

This version has a prognostic mixed layer base. At a given grid point, the base is contained within layer k and divides this layer into two sublayers (see diagram on previous slide). T and S are estimated within these sublayers by “unmixing” each variable, then the TKE balance is calculated as in the MICOM K-T mixed layer.

K-T 2: Simplified model

This version was developed by Rainer Bleck to avoid unmixing and the associated computational overhead. At each grid point, the mixed layer base always resides on a vertical coordinate interface.

When HYCOM is run in MICOM mode, the MICOM 2.8 Kraus-Turner mixed layer model is used. The mixed layer base coincides with vertical coordinate interface 2 (layer 1 is the slab mixed layer). Another major difference from the hybrid-coordinate K-T models is the detrainment algorithm, since the density of detrained water must match the isopycnic reference density of the layer accepting the water.

Model Simulations (1)

General Properties of Model Simulations

Domain

Atlantic Ocean basin, 20S to 62N

Resolution: 2 degrees horizontal, 22 layers vertical

Forcing

Climatological annual cycle forcing derived from COADS

Driven by vector wind stress, wind speed, air temperature and humidity, precipitation, longwave and shortwave surface radiation

Model runs and analysis

25-year spinup from zonally-averaged climatology [p(lat)] derived from Levitus climatology

One-year and five-year analysis runs with fields archived monthly

Analyze year 26, winter (Feb.) and summer (Aug.)

Analyze 5-year time series, years 26-30

Minimum layer thickness in the z-coordinate domain set to 10 m for all layers

Model Simulations (2)

Surface/Mixed Layer Thermodynamical Fields

Primary Comparisons:

Five model simulations compared to Levitus climatology

HYCOM KPP

HYCOM KPP (theta-S)

HYCOM K-T Implicit

HYCOM K-T MICOM Mode

HYCOM K-T Explicit

The primary comparisons evaluate the primary vertical mixing schemes plus the consequences of selecting theta-S advection (where T is not conserved) instead of T-S advection

Other Comparisons:

HYCOM K-T MICOM mode vs. MICOM 2.8

Demonstrates expected strong similarity between the models

HYCOM K-T 1 vs. HYCOM K-T 2

Compare performance of the two hybrid K-T mixed layer models

HYCOM KPP (Rlx. BC) vs. Levitus Climatology

Compare “most realistic” simulation to observations

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Slide 19

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Slide 22

Slide 23

Slide 24

Slide 25

Slide 26

Slide 27

Surface/Mixed Layer Dynamical Fields

Primary Comparisons:

Five model simulations compared to Levitus climatology

HYCOM KPP

HYCOM KPP (theta-S)

HYCOM K-T Implicit

HYCOM K-T MICOM Mode

HYCOM K-T Explicit

The primary comparisons evaluate the primary vertical mixing schemes plus the consequences of selecting theta-S advection (where T is not conserved) instead of T-S advection

Other Comparisons:

HYCOM K-T MICOM mode vs. MICOM 2.8

Demonstrates expected strong similarity between the models

HYCOM K-T 1 vs. HYCOM K-T 2

Compare performance of the two hybrid K-T mixed layer models

HYCOM KPP (Rlx. BC) vs. Levitus Climatology

Compare “most realistic” simulation to observations

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Slide 30

Slide 31

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Slide 33

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Slide 36

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Slide 38

Slide 39

Cross-Sections (1)

Primary Comparisons:

Five model simulations compared to Levitus climatology

HYCOM KPP

HYCOM KPP (theta-S)

HYCOM K-T Implicit

HYCOM K-T MICOM Mode

HYCOM K-T Explicit

The primary comparisons evaluate the primary vertical mixing schemes plus the consequences of selecting theta-S advection (where T is not conserved) instead of T-S advection

Other Comparisons:

HYCOM K-T MICOM mode vs. MICOM 2.8

Demonstrates expected strong similarity between the models

HYCOM K-T 1 vs. HYCOM K-T 2

Compare performance of the two hybrid K-T mixed layer models

HYCOM KPP (Rlx. BC) vs. Levitus Climatology

Compare “most realistic” simulation to observations

Cross-Sections (2)

The comparisons described in the previous slide will be presented for meridional cross-sections at 33W.

Other cross-sections are also presented

65W to focus on the subtropical mode water

Equator to focus on the equatorial current structure

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Slide 47

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Slide 49

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Slide 55

Model Censuses (1)

Two types of model censuses were conducted:

Layer thickness and total heat content evolution during model spinup

Documents climate drift during spinup

Volumetric T-S censuses

Illustrates water mass distribution

Model Censuses (2)

The following three slides illustrate layer thickness and total heat content evolution during the 25-year spinups of six of the HYCOM simulations.

Results are similar for all six cases.

Model adjustments in the upper ocean have been largely completed after the 25-year spinup

Significant adjustments of intermediate and deep water are still occurring after 25 years. These large adjustments occur in part because the p(lat) initialization was not accurate at these depths.

The subsequent four slides present the volumetric T-S censuses.

Slide 58

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Slide 61

Slide 62

Slide 63

Slide 64

Meridional Overturning Circulation and Heat Flux

In the following slide, the annual cycle of meridional heat flux is contoured as a function of month and latitude for five of the HYCOM simulations.

Slide 66

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Slide 70

Slide 71

Slide 72

Regional Differences in Upper Ocean Variability

Time series of upper-ocean variability are presented for the eight model grid points shown on the following slide. The points are:

NAC (North Atlantic Current)

STMW (Subtropical Mode Water formation region)

SARG (Sargasso Sea, interior western subtropical gyre)

ESTG (interior eastern subtropical gyre)

EBC (subtropical eastern boundary current)

CRBN (Caribbean Sea)

TRDW (North Atlantic Trade Winds)

EQTR (Equator)

Two sets of analysis are presented for temperature

One year time series

Five year time series

Three cases are compared

HYCOM KPP

HYCOM K-T Implicit

HYCOM MICOM Mode

Slide 74

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Slide 76

Slide 77

Slide 78

Slide 79

Slide 80

Slide 81

Slide 82

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Slide 85

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Slide 87

Slide 88

Slide 89

Slide 90

Regional Mixed Layer Profiles

Time series of upper-ocean variability are presented for the ten model grid points shown on the following slide. The points are:

LABS (Labrador Sea)

SLOP (Slope Water)

NAC (North Atlantic Current)

STMW (Subtropical Mode Water formation region)

SARG (Sargasso Sea, interior western subtropical gyre)

ESTG (interior eastern subtropical gyre)

EBC (subtropical eastern boundary current)

CRBN (Caribbean Sea)

TRDW (North Atlantic Trade Winds)

EQTR (Equator)

Two cases are compared

HYCOM KPP

HYCOM MICOM Mode

Slide 92

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Slide 94

Slide 95

Slide 96

Slide 97

Slide 98

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Slide 102

RESULTS

The use of hybrid vertical coordinates and improved vertical mixing algorithms improved the quality of model simulations.

Improved representation of subtropical mode water.

Improved horizontal mixed layer salinity distribution in the subtropical gyre.

Improved resolution of tropical upper-ocean flow.

Explicit resolution of upper-ocean wind driven flow

HYCOM run in MICOM mode produced results extremely close to MICOM 2.8, indicating that code changes in HYCOM did not degrade the quality of the solution.

Many observed shortcomings of these HYCOM simulations can be traced to the lack of an ice model, surface forcing errors, the simple initial conditions, and the lack of river runoff.

The choice of which two thermodynamical variables are advected and also mixed in the vertical coordinate adjustment algorithm did not have a noticeable influence on the solutions even though in the KPP (theta-S) experiment, temperature was not conserved.

REFERENCES

Bleck, R., 1998: Ocean modeling in isopycnic coordinates. Chapter 18 in Ocean Modeling and Parameterization, E. P. Chassignet and J. Verron, Eds., NATO Science Series C: Mathematical and Physical Sciences, Vol. 516, Kluwer Academic Publishers, 4223-448.

Bleck, R. and D. Boudra, 1981: Initial testing of a numerical ocean circulation model using a hybrid (quasi-isopycnic) vertical coordinate. J. Phys. Oceanogr., 11, 755-770.

Bleck, R. and S. Benjamin, 1993: Regional weather prediction with a model combining terrain-following and isentropic coordinates, Part 1: Model description. Mon. Wea. Rev., 121, 1770-1785.

Large, W. G., J. C. Mc Williams, and S. C. Doney, 1994: Oceanic vertical mixing: a review and a model with a nonlocal boundary layer parameterization. Rev. Geophys. 32, 363-403.



	This slide show describes several Atlantic Ocean simulations performed using HYCOM with different vertical mixing schemes and with both hybrid and isopycnic vertical coordinates. Properties of HYCOM are outlined first. Model performance is then evaluated by comparing simulated fields to each other, to observed fields (Levitus climatology), and to fields simulated by the Miami Isopycnic Coordinate Ocean Model (MICOM).


	Press button to jump forward to each section:

		I. The Model
		2. Model Simulations
		3. Surface/Mixed Layer Thermodynamical Fields
		4. Surface/Mixed Layer Dynamical Fields
		5. Cross-Sections
		6. Model Censuses
		7. Meridional Overturning Circulation and Heat Flux
		8. Regional Differences in Upper Ocean Variability
		9. Regional Mixed Layer Profiles
		10. Summary
		11. Referencess


History
	Developed from MICOM version 2.8
		See Bleck (1998) for a description of MICOM
	Hybrid coordinate scheme used in HYCOM
		Model equations written in generalized vertical coordinates
		Originally used in the Bleck and Boudra (1981) quasi-isopycnic model
			Numerical schemes could not handle zero thickness layers
			Minimum layer thickness was enforced
				The vertical coordinate became non-isopycnic in regions where the layers became too thin
		A hybrid (isentropic-sigma) vertical coordinate was used in the atmospheric model of Bleck and Benjamin (1993)
			The HYCOM isopycnic-level-sigma vertical coordinate scheme is an adaptation and extension of the Bleck/Benjamin algorithm


	Major changes from MICOM
		New vertical coordinate scheme
		Multiple mixed layer models, including non-slab models
		Interior convection due to static instability (two layers exchange fluid when the upper one is more dense)
		Hydrostatic and momentum equations modified to handle non-isopycnic densities and horizontal density gradients
		Choice of temperature-salinity, temperature-density, or salinity-density advection


Vertical Coordinates
	Hybrid coordinates
		Nearsurface: z-coordinates
		Shallow water: sigma (terrain-following) coordinates
		Coordinates are isopycnic in the bulk of the ocean interior
	Isopycnic coordinates (MICOM mode)
Hybrid Vertical Coordinate Scheme (part 1)
	In the open ocean, the coordinates are isopycnic except year the surface where minimum coordinate separation is enforced. The minimum separation differs for each layer, giving the user great flexibility to set the vertical coordinate structure in the z-coordinate domain.
	A cushion function described in Bleck and Benjamin (1993) provides a smooth transition between the isopycnic and z-coordinate domains.
	To activate the sigma coordinate domain, the user specifies the number of sigma coordinates n and the coordinate separation d. Vertical coordinates become terrain-following where bottom depth is less than n*d. In extremely shallow water, the coordinates revert to level coordinates.


Hybrid Vertical Coordinate Scheme (part 2)
	In the deep ocean, the isopycnic-level coordinate transition is performed as follows:
		If the density of a given layer does not equal the isopycnic reference density, the interfaces bounding the layer are adjusted to return the density to its reference value
			If the layer is too light, the interface below is moved downward so that the entrained denser water returns the density to its reference value
			If the layer is too dense, the interface above is moved upward in the same manner
		If minimum coordinate separation is violated near the ocean surface, the cushion function is used to re-calculate the vertical coordinate location, prohibiting the restoration of isopycnic conditions.
		Two of the thermodynamical variables T, S, and density are mixed across the moving interfaces (user selectable), with the third calculated from the equation of state. If T and S are mixed, exact isopycnal density is not restored, but repeated application keeps the error small.


	Horizontal Advection
		Two of the thermodynamical variables T, S, and density are advected (user selectable), with the third calculated from the equation of state.
		If HYCOM is run in MICOM mode, advection is performed as in MICOM 2.8, with T and S advected in layer 1 (the mixed layer) and S only advected in the isopycnic layers.


Mixing Schemes
	K-Profile Parameterization
	Kraus-Turner Mixed Layer Model
		Two Kraus-Turner mixed layer models available for hybrid coordinates
		A third Kraus-Turner model is available when HYCOM is run in MICOM mode
			Handles detrainment of water into isopycnic layers
			Same algorithm used in MICOM 2.8
		With hybrid coordinates, choice of explicit (MICOM-like) or implicit (KPP-like) interior diapycnal mixing


K-Profile Parameterization (KPP)
	Developed by Large, Mc Williams, and Doney (1994)
	Governs Vertical Mixing of Entire Water Column
	Parameterizes Several Physical Processes
		Surface boundary layer
			Mechanical wind mixing
			Buoyancy flux forcing
			Convective overturning
			Non-local (counter-gradient) fluxes
		Diapycnal mixing in ocean interior
			Instability due to resolved vertical shear
			Background internal wave mixing
			Double diffusion mixing (diffusive convection and salt fingering)
	Can Run at Relatively Low Vertical Resolution



KPP Procedure (part 1):
	Apply surface thermodynamical and momentum flux forcing
	Calculate K profiles for interior diapycnal mixing from surface to bottom
	Diagnose turbulent boundary layer thickness
		Minimum depth H where a bulk Richardson number exceeds critical value
		Turbulent boundary layer eddies can penetrate to depth H where the fluid becomes stable relative to local buoyancy and velocity


KPP Procedure (part 2)
	Calculate surface boundary layer k profiles for T, S, and momentum
		Vertical diffusivity for T is parameterized as

			where is the nonlocal transport term
		Diffusivity is parameterized as
			where w is a turbulent velocity scale that is a function of the stability of the forcing, G is a 3^rd order polynomial shape function, and sigma is a scale depth varying from 0 to 1 over the depth range H
		Choose coefficients of G to match the interior and boundary layer K profiles, producing a final K profile with a continuous first vertical derivative
	Solve diffusion equation semi-implicitly with two temporal iterations
	Diagnose mixed layer thickness along with T, S, u, and v


	Implementation of Kraus-Turner mixing with hybrid coordinates
		Mixed layer base is not a vertical coordinate interface as in MICOM
		Must keep track of T, S, and density jumps across the mixed layer base


Two hybrid-coordinate K-T mixed layer models have been developed.
	K-T 1: Full model
		This version has a prognostic mixed layer base. At a given grid point, the base is contained within layer k and divides this layer into two sublayers (see diagram on previous slide). T and S are estimated within these sublayers by “unmixing” each variable, then the TKE balance is calculated as in the MICOM K-T mixed layer.
	K-T 2: Simplified model
		This version was developed by Rainer Bleck to avoid unmixing and the associated computational overhead. At each grid point, the mixed layer base always resides on a vertical coordinate interface.
When HYCOM is run in MICOM mode, the MICOM 2.8 Kraus-Turner mixed layer model is used. The mixed layer base coincides with vertical coordinate interface 2 (layer 1 is the slab mixed layer). Another major difference from the hybrid-coordinate K-T models is the detrainment algorithm, since the density of detrained water must match the isopycnic reference density of the layer accepting the water.


General Properties of Model Simulations
	Domain
		Atlantic Ocean basin, 20S to 62N
		Resolution: 2 degrees horizontal, 22 layers vertical
	Forcing
		Climatological annual cycle forcing derived from COADS
		Driven by vector wind stress, wind speed, air temperature and humidity, precipitation, longwave and shortwave surface radiation
	Model runs and analysis
		25-year spinup from zonally-averaged climatology [p(lat)] derived from Levitus climatology
		One-year and five-year analysis runs with fields archived monthly
		Analyze year 26, winter (Feb.) and summer (Aug.)
		Analyze 5-year time series, years 26-30
	Minimum layer thickness in the z-coordinate domain set to 10 m for all layers


Primary Comparisons:
	Five model simulations compared to Levitus climatology
		HYCOM KPP
		HYCOM KPP (theta-S)
		HYCOM K-T Implicit
		HYCOM K-T MICOM Mode
		HYCOM K-T Explicit
	The primary comparisons evaluate the primary vertical mixing schemes plus the consequences of selecting theta-S advection (where T is not conserved) instead of T-S advection

Other Comparisons:
	HYCOM K-T MICOM mode vs. MICOM 2.8
		Demonstrates expected strong similarity between the models
	HYCOM K-T 1 vs. HYCOM K-T 2
		Compare performance of the two hybrid K-T mixed layer models
	HYCOM KPP (Rlx. BC) vs. Levitus Climatology
		Compare “most realistic” simulation to observations



	The comparisons described in the previous slide will be presented for meridional cross-sections at 33W.
	Other cross-sections are also presented
		65W to focus on the subtropical mode water
		Equator to focus on the equatorial current structure


Two types of model censuses were conducted:
	Layer thickness and total heat content evolution during model spinup
		Documents climate drift during spinup
	Volumetric T-S censuses
		Illustrates water mass distribution


	The following three slides illustrate layer thickness and total heat content evolution during the 25-year spinups of six of the HYCOM simulations.
		Results are similar for all six cases.
		Model adjustments in the upper ocean have been largely completed after the 25-year spinup
		Significant adjustments of intermediate and deep water are still occurring after 25 years. These large adjustments occur in part because the p(lat) initialization was not accurate at these depths.
	The subsequent four slides present the volumetric T-S censuses.


	In the following slide, the annual cycle of meridional heat flux is contoured as a function of month and latitude for five of the HYCOM simulations.


	Time series of upper-ocean variability are presented for the eight model grid points shown on the following slide. The points are:
		NAC (North Atlantic Current)
		STMW (Subtropical Mode Water formation region)
		SARG (Sargasso Sea, interior western subtropical gyre)
		ESTG (interior eastern subtropical gyre)
		EBC (subtropical eastern boundary current)
		CRBN (Caribbean Sea)
		TRDW (North Atlantic Trade Winds)
		EQTR (Equator)
	Two sets of analysis are presented for temperature
		One year time series
		Five year time series
	Three cases are compared
		HYCOM KPP
		HYCOM K-T Implicit
		HYCOM MICOM Mode


	Time series of upper-ocean variability are presented for the ten model grid points shown on the following slide. The points are:
		LABS (Labrador Sea)
		SLOP (Slope Water)
		NAC (North Atlantic Current)
		STMW (Subtropical Mode Water formation region)
		SARG (Sargasso Sea, interior western subtropical gyre)
		ESTG (interior eastern subtropical gyre)
		EBC (subtropical eastern boundary current)
		CRBN (Caribbean Sea)
		TRDW (North Atlantic Trade Winds)
		EQTR (Equator)
	Two cases are compared
		HYCOM KPP
		HYCOM MICOM Mode


	The use of hybrid vertical coordinates and improved vertical mixing algorithms improved the quality of model simulations.
		Improved representation of subtropical mode water.
		Improved horizontal mixed layer salinity distribution in the subtropical gyre.
		Improved resolution of tropical upper-ocean flow.
		Explicit resolution of upper-ocean wind driven flow
	HYCOM run in MICOM mode produced results extremely close to MICOM 2.8, indicating that code changes in HYCOM did not degrade the quality of the solution.
	Many observed shortcomings of these HYCOM simulations can be traced to the lack of an ice model, surface forcing errors, the simple initial conditions, and the lack of river runoff.
	The choice of which two thermodynamical variables are advected and also mixed in the vertical coordinate adjustment algorithm did not have a noticeable influence on the solutions even though in the KPP (theta-S) experiment, temperature was not conserved.


	Bleck, R., 1998: Ocean modeling in isopycnic coordinates. Chapter 18 in Ocean Modeling and Parameterization, E. P. Chassignet and J. Verron, Eds., NATO Science Series C: Mathematical and Physical Sciences, Vol. 516, Kluwer Academic Publishers, 4223-448.

	Bleck, R. and D. Boudra, 1981: Initial testing of a numerical ocean circulation model using a hybrid (quasi-isopycnic) vertical coordinate. J. Phys. Oceanogr., 11, 755-770.

	Bleck, R. and S. Benjamin, 1993: Regional weather prediction with a model combining terrain-following and isentropic coordinates, Part 1: Model description. Mon. Wea. Rev., 121, 1770-1785.

	Large, W. G., J. C. Mc Williams, and S. C. Doney, 1994: Oceanic vertical mixing: a review and a model with a nonlocal boundary layer parameterization. Rev. Geophys. 32, 363-403.