FORTE Home Page

FORTE (Sinha and Smith, 2002) is a medium to coarse resolution ocean-atmosphere coupled general circulation model capable of integrations over timescales ranging from days to thousands of years. see Robin Smith's FORTE page

It is also capable of a high degree of flexibility - the model has been designed with the intention of performing experiments using highly idealised configurations, so it can accept new continental geometry, land surface orography and ocean bottom topography easily; the resolution of the ocean (and to a lesser extent, the atmosphere) is also configurable.

Its principal components are ocean and atmosphere General Circulation Models (GCMs) originally designed for solo operation. The ocean model is MOMA (Webb,93) a derivative of the well-known Modular Ocean Model, a z-coordinate primitive equation model based on the GFDL code. The atmosphere is adapted from IGCM3, a spectral GCM developed at Reading University (Hoskins,75 Forster,00). They are coupled together using OASIS (Terray,00), a flexible coupler that allows the models to pass data between each other whilst running.

A major advantage of this model is that it can be run with relatively modest computing resources - a recent 100yr integration of the present day climate took ~50 hours using a desktop PC (AMD 1800XP) and a couple of nodes of a multi-processor Origin 2000, and initial trials indicate that a higher performance PC (AMD 1900XP) could equal this on its own. We have incorporated the periodic coupling method of Sausen and Voss (2000) to allow the model to spin-up very quickly. In periodic coupling mode the model runs approximately 4x quicker than without. However periodic coupling is not suitable for investigating all types of problem.

BASIC CONFIGURATION The basic configuration of the model consists of a representation of present-day continental geometry, sea-floor topography and land-surface orography. The resolution is 4x4 degrees in the ocean and there is a spectral trunction of T21 in the atmosphere (equivalent to roughly 5.5x5.5 degrees). At this resolution we have run a 100year control run and are presently running an increasing CO2 concentration scenario.

IDEALIZED RUNS in addition, various idealised configurations have also been run - WaterWorld, where the entire surface of the globe is water covered, and RidgeWorld, where a thin strip of land is introduced. These idealised runs enable us to understand the fundamental dynamics of the climate system without the complicated effects of land geometry etc.

PERTURBATION EXPERIMENTS By adding freshwater pulses to the model North Atlantic, we can temporarily shut down the model Thermohaline Circulation (or at least the North Atlantic branch) and investigate the process involved in it's recovery. Due to the speed of FORTE we are able to do a number of experiments to investigate the sensitivity of the system to different magnitudes of perturbation. This would be more difficult with the slower and more unwieldy models which currently define the state-of-the-art in climate prediction (e.g. the Hadley Centre climate model, HadCM3). We plan to investigate whether the Thermohaline Circulation becomes more vulnerable to shutdown when there is global warming due to increased greenhous gas emissions and whether the recovery period is affected.

HIGHER RESOLUTION The model has been run in 2x2 and 2x4 degree ocean resolution mode, and we are currently working on increasing the atmosphere resolution to T42. The combination of T42 atmosphere and 2x2 degree ocean resolution will result in a much more accurate, but still fast-running model.

PALEOCLIMATE SCENARIOS The ability to run for millennial timescales makes this model suitable for paleo-climate runs, and there are plans to apply it to geographies similar to those thought to be present during the cretaceous period in an attempt to provide information about the sort of climate states that are compatible with that configuration.

The model is currently being used by 3 groups (two here at the Southampton Oceanography Centre and one at Liverpool University) and the code is being made available as a tool for the modelling community at large. For more information, email Robin Smith or Bablu Sinha.

Large Scale Modelling is partly funded as a core activity of the James Rennell Division for Ocean Circulation and Climate. The James Rennell Division is one of the SOC Divisions funded by the Natural Environment Research Council and managed by the University of Southampton.

Last Updated: Bablu Sinha 25th June 2003

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