Interannual variability simulated in the Tropical Pacific
porge Macias, tDavid Stephenson, tLaurent Terray, tSophie Belamari
tCERFACS 1
42, avo Coriolis
31057 Toulouse Cedex
France.
tCERFACS y Grupo de Analisis Matematico Aplicado.
Universidad de Malaga
Campus de Teatinos sin
29071 Malaga.
1 Introduction.
A number of different models of varying complexity have been developed and used in the last
two decades to simulate EI Nino/Southern Oscillation (ENSO) interannual climate variability in
the tropical Pacific (Neelin et o.t., 1992). These models range from simple conceptual systems,
involving only one equation and one variable, to Coupled General Circulation Models (CGCMs)
with a large number of degrees of freedom. All of these models have played a role in understanding ENSO. Conceptual low order models drastically reduce the spatial structure of the
atmosphere and ocean. Simple models also exist which represent the ocean and/or atmosphere
with simplified sets of partial differential equations. Both of these types of model are useful
for illustrating fundamental processes of air-sea interaction and have been developed as tools
to understand more complicated systems. Intermediate coupled models using reduce-gravity
ocean and atmosphere components and realistic thermodynamics are sophisticated enough to
produce realistic solutions, and are also simple enough to diagnose (Zebio.k and Cane, 1987).
High resolution CGCMs can be closely compared to observations, but the important processes
at work in them are not always easy to determine.
Another important category is that of hybrid coupled models (HCMs). They consist in a simplified atmospheric model, which represents the atmospheric response to oceanic boundary conditions, coupled to an ocean GCM (OGCM). This design invokes the hypothesis of Wyrtki (1985)
that the memory of the coupled ocean-atmosphere system is provided entirely by the dynamical
oceanic component, while the atmosphere represents the fast adjusted component. It is also
based on the assumption that the low-frequency near-surface wind stress is determined by SST.
Thus, the atmosphere effectively responds instantaneously in the planetary boundary layer to
ocean forcing only. Analysis of atmospheric GCM (AGCM) simulations forced by observed SST
suggest this is a reasonable assumption (Zebiak, 1986; Neelin, 1988). In these models, temporal
variability in the ocean-atmosphere system is due primarily to the coupling with the dynamical
ocean. This feature of this class of models make them a suitable tool for understanding the
inherent properties of the coupled system in the absence of atmospheric stochastic forcing.
HCMs have the advantage that they are much cheaper computationally than fully coupled
atmosphere and ocean GCMs. Neelin (1989, 1990) coupled the ocean GCM of Cox (1984) to
'Centre Europeen de Recherche et de Formation Avancee en Calcul Scientifique
NATO ASI Series. Vol. 148
The Mathematics of Models for Climatology
and Environment
Edited by Jesus lldefonso Diaz
© Springer-Verlag Berlin Heidelberg 1997
porge Macias, tDavid Stephenson, tLaurent Terray, tSophie Belamari
tCERFACS 1
42, avo Coriolis
31057 Toulouse Cedex
France.
tCERFACS y Grupo de Analisis Matematico Aplicado.
Universidad de Malaga
Campus de Teatinos sin
29071 Malaga.
1 Introduction.
A number of different models of varying complexity have been developed and used in the last
two decades to simulate EI Nino/Southern Oscillation (ENSO) interannual climate variability in
the tropical Pacific (Neelin et o.t., 1992). These models range from simple conceptual systems,
involving only one equation and one variable, to Coupled General Circulation Models (CGCMs)
with a large number of degrees of freedom. All of these models have played a role in understanding ENSO. Conceptual low order models drastically reduce the spatial structure of the
atmosphere and ocean. Simple models also exist which represent the ocean and/or atmosphere
with simplified sets of partial differential equations. Both of these types of model are useful
for illustrating fundamental processes of air-sea interaction and have been developed as tools
to understand more complicated systems. Intermediate coupled models using reduce-gravity
ocean and atmosphere components and realistic thermodynamics are sophisticated enough to
produce realistic solutions, and are also simple enough to diagnose (Zebio.k and Cane, 1987).
High resolution CGCMs can be closely compared to observations, but the important processes
at work in them are not always easy to determine.
Another important category is that of hybrid coupled models (HCMs). They consist in a simplified atmospheric model, which represents the atmospheric response to oceanic boundary conditions, coupled to an ocean GCM (OGCM). This design invokes the hypothesis of Wyrtki (1985)
that the memory of the coupled ocean-atmosphere system is provided entirely by the dynamical
oceanic component, while the atmosphere represents the fast adjusted component. It is also
based on the assumption that the low-frequency near-surface wind stress is determined by SST.
Thus, the atmosphere effectively responds instantaneously in the planetary boundary layer to
ocean forcing only. Analysis of atmospheric GCM (AGCM) simulations forced by observed SST
suggest this is a reasonable assumption (Zebiak, 1986; Neelin, 1988). In these models, temporal
variability in the ocean-atmosphere system is due primarily to the coupling with the dynamical
ocean. This feature of this class of models make them a suitable tool for understanding the
inherent properties of the coupled system in the absence of atmospheric stochastic forcing.
HCMs have the advantage that they are much cheaper computationally than fully coupled
atmosphere and ocean GCMs. Neelin (1989, 1990) coupled the ocean GCM of Cox (1984) to
'Centre Europeen de Recherche et de Formation Avancee en Calcul Scientifique
NATO ASI Series. Vol. 148
The Mathematics of Models for Climatology
and Environment
Edited by Jesus lldefonso Diaz
© Springer-Verlag Berlin Heidelberg 1997