The ACPI Project, Element 1: Initializing a Coupled Climate Model from Observed Conditions
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Authors
Pierce, David W.
Barnett, Tim P.
Tokmakian, Robin
Semtner, Albert
Maltrud, Mathew
Lysne, Joanne
Craig, Anthony
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Date of Issue
2004
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Abstract
A problem for climate change studies with coupled ocean-atmosphere models has been
how to incorporate observed initial conditions into the ocean, which holds most of the ‘memory’
of anthropogenic forcing effects. The first difficulty is the lack of comprehensive three-dimensional
observations of the current ocean temperature (T) and salinity (S) fields to initialize to. The second
problem is that directly imposing observed T and S fields into the model results in rapid drift back
to the model climatology, with the corresponding loss of the observed information. Anthropogenic
forcing scenarios therefore typically initialize future runs by starting with pre-industrial conditions.
However, if the future climate depends on the details of the present climate, then initializing the
model to observations may provide more accurate forecasts. Also, this ∼130 yr spin up imposes
substantial overhead if only a few decades of predictions are desired. A new technique to address
these problems is presented. In lieu of observed T and S, assimilated ocean data were used. To
reduce model drift, an anomaly coupling scheme was devised. This consists of letting the model’s
climatological (pre-industrial) oceanic and atmospheric heat contents and transports balance each
other, while adding on the (much smaller) changes in heat content since the pre-industrial era as
anomalies. The result is model drift of no more than 0.2 K over 50 years, significantly smaller
than the forced response of 1.0 K. An ensemble of runs with these assimilated initial conditions
is then compared to a set spun up from pre-industrial conditions. No systematic differences were
found, i.e., the model simulation of the ocean temperature structure in the late 1990s is statistically
indistinguishable from the assimilated observations. However, a model with a worse representation
of the late 20th century climate might show significant differences if initialized in this way.
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Oceanography
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This work was supported by the Department of Energy under grant DE-FG03– 98ER62505.
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Climatic Change, Volume 62, pp. 13-28, 2004.
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This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.