Model overview:Model overview:A A hierarchy hierarchy of ocean modelsof ocean models
Jay McCrearyJay McCreary
Summer School on:
Dynamics of the North Indian Ocean
National Institute of OceanographyDona Paula, Goa
June 17 – July 29, 2010
IntroductionIntroduction
1) General circulation models (GCMs)
2) Linear, continuously stratified (LCS) model: (barotropic and baroclinic modes)
3) Layer ocean models (LOMs)
4) Steady-state balances
General circulation models
Linear, continuously stratified (LCS) model
Equations: A useful set of simpler equations is a version of the GCM equations linearized about a stably stratified background state of no motion. (See the HIG Notes for a discussion of the approximations involved.) The resulting equations are
where Nb2 = –gbz/ is assumed to be a function only of z. Vertical
mixing is retained in the interior ocean. To model the mixed layer, wind stress enters the ocean as a body force with structure Z(z). To expand into vertical normal modes, the structure of vertical mixing of density is modified to (κρ)zz.
Now, assume that the vertical mixing coefficients have the special form: ν = κ = A/Nb
2(z). In that case, the last three equations can be rewritten in terms of the operator, (∂zNb
–2∂z), as follows
Since the z operators all have the same form, under suitable conditions (noted next) we can obtain solutions as expansions in the eigenfunctions of the operator.
Vertical modes: Assuming further that the bottom is flat and with boundary conditions consistent with those below, solutions can be represented as expansions in the vertical normal (barotropic and baroclinic) modes, ψn(z). They satisfy,
subject to boundary conditions and normalization
Integrating (1) over the water column gives
(1)
Constraint (2) can be satisfied in two ways. Either c0 = in which case ψn(z) = 1 (barotropic mode) or cn is finite so that the integral of ψn vanishes (baroclinic modes).
(2)
The solutions for the u, v, and p fields can then be expressed as
where the expansion coefficients are functions of only x, y, and t. The resulting equations for un, vn, and pn are
Thus, the ocean’s response can be separated into a superposition of independent responses associated with each mode. They differ only in the value of cn, the Kelvin-wave speed for the mode.
Steady-state balances
It is useful to extend the concepts of Ekman and Sverdrup balance to apply to individual baroclinic modes. The complete equations are
Sverdrup balance
A mode in which the time-derivative terms and all mixing terms are not important is defined to be in a state of Sverdrup balance.
It is useful to extend the concepts of Ekman and Sverdrup balance to apply to individual baroclinic modes. The complete equations are
Ekman balance
A mode in which the time-derivative terms, horizontal mixing terms, and pressure gradients are not important is defined to be in a state of Ekman balance.
An equatorial balance related to Ekman balance is the 2d, Yoshida balance, in which x-derivatives are negligible. The equations are.
Yoshida (2-dimensional) balance
In this balance, damping is so strong that it eliminates wave radiation. High-order modes in the McCreary (1981) model of the EUC are in Yoshida balance.
The meridional structure Y(y) gradually weakens to zero away from the equator.
McCreary (1981) used the LCS model to study the dynamics of the Pacific Equatorial Undercurrent (EUC), forcing it by a steady patch of easterly wind of the separable form
X(x)
Equatorial Undercurrent
When the LCS model includes diffusion (A ≠ 0), realistic steady flows can be produced near the equator.
The linear model reproduces the GCM solution very well! The color contours show v and the vectors (v, w).
Comparison of LCS and GCM solutions
Layer models
If a particular phenomenon is surface trapped, it is often useful to study it with a model that focuses on the surface flow. Such a model is the 1½-layer, reduced-gravity model. Its equations are
where the pressure is
1½-layer model
The model allows water to transfer into and out of the layer by means of an across-interface velocity, w1.
In a linear version of the model, h1 is replaced by H1, and the model response behaves like a baroclinic mode of the LCS model, and w1 is then analogous to mixing on density.
If a phenomenon involves two layers of circulation in the upper ocean (e.g., a surface coastal current and its undercurrent), then a 2½-layer model may be useful. Its equations can be summarized as
where i = 1,2 is a layer index, and the pressure gradients in each layer are
2½-layer model
Note that when water entrains into layer 1 (w1 > 0), layer 2 loses the same amount of water, so that mass is conserved.
In this case, when hi is replaced by Hi the model response separates into two baroclinic modes, similar to the LCS model.
Variable-temperature, 2½-layer model
If a phenomenon involves upwelling and downwelling by w1, it is useful to allow temperature (density) to vary within each layer. Equations of motion of are
where the terms
ensure that heat and momentum are conserved when w1 causes water parcels to transfer between layers.
Meridional section from a solution to a 4½-layer model of the Pacific Ocean, illustrating its layer structure across the central basin.
Water can transfer between layers with across-interface velocities wi.
AAIW
NPIWSPLTW
thermocline
4½-layer model
4½-layer model
mixed layer
diurnal thermocline
seasonal thermocline
main thermocline
Schematic diagram of the structure of a 4½-layer model used to study biophysical interactions in the Arabian Sea.
for layer 2, and the pressure gradients are
2-layer model
for layer 1,
If the circulation extends to the ocean bottom, a 2-layer model is useful. Its equations are
Variable-temperature, 2½-layer model
where the density terms are given by
Because Ti varies horizontally, the pressure gradient depends on z [i.e., pz = –gρ (p)z = –gρ], within each layer. So, the equations use the depth-averaged pressure gradients in each layer,
