NCAR CSM Pacific Basin Model, version 3.e -- User's Guide         <<     Table of Contents     >>

1   Introduction

The NCAR Pacific basin model is based on the upper equatorial ocean model of Gent and Cane (1989). It has been updated and improved by the addition of the K-profile parameterization (KPP) boundary layer scheme of Large et al. (1994). It is a primitive equation model based on the reduced gravity assumption, so that the constant density deep ocean is at rest below the active upper ocean. The depth of the active ocean varies in time and space, but the mean depth is independent of time and is usually chosen to be 400m. The model domain is the tropical Pacific Ocean from 30°S to 30°N. The model consists of a fixed depth upper layer and the remainder of the active ocean is divided into an arbitrary number of numerical layers by means of a sigma coordinate. Only temperature is predicted in the model, so that the ocean salinity is taken to be constant. Horizonal mixing is achieved by Shapiro filtering and the vertical viscosity and diffusivity are functions of the local vertical Richardson number. The time--stepping scheme is that due to Lorenz. Full details of the continuous equations and numerical algorithms can be found in Gent and Cane (1989) and Gent (1991). Details of the KPP scheme and its numerical implementation can be found in Large et al. (1994) and the NCAR Technical Note on the NCAR CSM ocean model.

The wind stress and heat flux to force the model are determined as follows. Input required to determine these forcings are the atmospheric surface wind velocity, u, and the fractional cloud cover, C. The model has mostly been forced using the monthly pseudostress climatology from Florida State University and the fractional cloud cover from the ISCCP C1 data set. The wind stress and heat flux are given by
tau = rho aa CDu|u| ,
Qtotal = Qsol - Qlat - Qsen - Qlgw ,
Qsol = (1-A) (1-aCLD * C + aALPHA* Theta) Qo ,
Qlat = rhoa LCE|u| [q(SST) - RH*q(Tatm)] ,
Qsen = rhoa Cp CH |u| (SST - Tatm) ,
Qlgw = epsilon sigma [SST4 [0.39 - 0.05 exp(½)] (1 - bCLD*C²) + 4 * SST³ (SST-Tatm)] ,
q(T) = 6.4 * 108 exp [ -5105 / T ] ,
Tatm(°C) = [5*SST (°C) + 22]/ 6.

Theta is the solar altitude, Qo is the clear sky flux, A is albedo and RH is relative humidity. The formulae for Qlat, Qlgw and q require the temperatures to be in degrees Kelvin. Default values for coefficients are A=0.06, aCLD=0.75, aALPHA=0.002, RH=0.75 and bCLD=0.6. The solar flux, Qsol, is distributed in the upper three layers of the model according to Jevlov water type Ib with 33% of the flux being absorbed exponentially with depth with an e-folding scale of 17m. Finally, a minimum wind speed, WNDmin, can be set in the formulae for Qlat and Qsen, and for monthly wind forcing this has mostly been set to 4m/s.

  • Gent, P. R., and M. A. Cane, 1989: A reduced gravity, primitive equation model of the upper equatorial ocean. . Comp. Phys., 81, 444-480.
  • Gent, P. R., 1991: The heat budget of the TOGA--COARE domain in an ocean model. J. Geophys. Res., 96, 3323-3330.
  • Large, W. G., J. C. McWilliams and S. C. Doney, 1994: Oceanic vertical mixing: A review and a model with a nonlocal boundary layer parameterization. Rev. of Geophys., 32, 363-403.