AMIP II Diagnostic Subproject
35
Seasonal-to-decadal variability in
the tropical Atlantic Ocean
Project coordinators:
James Carton, Sumant Nigam and Jiande Wang
Department of Meteorology, University of Maryland, College Park, MD
20742
Background
Objectives
Methodology
Data Requirements
References
Background
Recent studies have presented a confusing array of interpretations
of the existence and causes of climate variability in the tropical Atlantic
sector (e.g. Hastenrath and Druyan, 1993; Enfield and Mayer, 1997; Chang
et al., 1997; Tourre, et al., 1998; Mehta, 1998; Rajagopalan et al., 1998;
Xie and Tanimoto, 1998; Delworth and Mehta, 1998). We have examined
these issues ourselves through a diagnostic study of historical atmospheric
and oceanic reanalysis data sets as well as COADS (Ruiz-Barradas, Carton,
and Nigam, 1999) covering the period 1958-1993. In this study we carry
out a combined rotated principal component analysis of atmospheric and
oceanic variables in order to identify covarying phenomena. We conclude
that in addition to externally forced variability, associated with the
tropical Pacific and North Atlantic, there are indeed at least two modes
of variability intrinsic to the tropical Atlantic. We refer to these
as the Atlantic Nino and the interhemispheric modes.
The underlying causes of these two modes seem to be different and lead
us to the following scenarios. In the Atlantic Nino mode, a relaxation
of the equatorial trade winds is associated with a deepening of the equatorial
thermocline. In the boreal summer months of June through August the
presence of a deep thermocline leads to unusually warm surface temperatures.
In the interhemispheric mode surface temperature anomalies in the Northern
Hemisphere lead to cyclonic wind anomalies, enhanced cross-equatorial winds,
and a general relaxation of the Northeast trade winds. The relaxation
of the trade winds reduces latent heat loss, leading to an amplification
of the original anomaly. This effect is most prominent during boreal
spring when the Intertropical Convergence Zone is at its most southern
position.
These scenarios are consistent with aspects of previous studies, but
are still quite incomplete. We would like to understand the sensitivity
of these modes to a variety of parameters such as atmospheric heating distributions,
etc. Finally, we want to know more about the reasonableness of approximations
that might be used in constructing simplified models.
Objectives
This is a proposal to carry out a statistical examination of surface winds
and heating in the tropical Atlantic sector of the available AMIPII GCMs
to examine the causes of interannual to decadal variability. Improved
understanding of how the tropical atmosphere responds to SST variations
in the tropical Atlantic has become a critical issue for the CLIVAR-Atlantic
program. Much of this understanding can only come from modeling studies.
Here we propose to examine the available AMIPII runs to see the extent
to which their variability is consistent with the historical record, and
to learn what we can about the physics of climate variability in the tropical
Atlantic. This effort will represent part of a long-term effort by
the
PIs to address climate variability in the tropical Atlantic.
Methodology
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Methodology Our methodology will follow that of Ruiz-Barradas, Carton,
and Nigam (1999). We will carry out a rotated principal component
analysis of the monthly anomaly fields looking for covariability among
atmospheric and oceanic variables where the anomalies are defined relative
to their monthly climatology.
-
We will begin with a three variable analysis using all twelve months of
SST and surface wind components. We will also explore a five variable
analysis using 500mb diabatic heating and ocean heat content (available
separately).
-
We will examine the projection of the modes of low frequency variability
on 800mb diabatic heating, and surface heat flux components.
-
We will examine the seasonal dependence of these relationships by carrying
out separate analyses in boreal spring (March-May) and summer (June-August)
when we can expect different responses based on our previous work.
-
We will use our previous work as a baseline calculation and look at the
projection of our principal component time series determined from
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the reanalysis data sets on each of the model runs.
-
Finally, we will specifically examine 'extreme events' as determined from
the reanalysis data sets.
Validation As indicated in the background section, we have already carried
out a similar analysis of the NCAR and ECMWF reanalysis data sets, and
of the COADS surface observation data set. We will determine the
extent to which we see similar behavior, spatially and temporally, in the
AMIPII runs. But, we will also be interested to explore the sensitivity
of the models by looking for differences among them. We will attempt
to interpret the differences based on what is known about the differences
in the physics and dynamics of the models.
As indicated above, we would also like to address some basic physics
questions, among them: The patterns of diabatic heating. The relationship
between SST anomalies in the tropical Atlantic and diabatic heating anomalies.
The relationship between heating anomalies over the surrounding continental
areas and over the oceanic sector. The relationship between pressure
gradient forces and winds
Data requirements
This study will require the following six monthly averaged fields for
the full period, January, 1979- December, 1995 for the available AMIPII
runs (a total of 19 as of July, 1999). The variables represent a
subset of those examine by Ruiz-Barradas et al. (1999) and Chung, Carton
and Nigam (1999, manuscript in preparation) and are chosen to reflect the
driving function for the ocean (winds, surface heating) and measures
of the tropical boundary layer (surface air pressure) and midtroposphere
(500mb heating).
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10m winds (zonal and meridional)
-
surface air pressure
-
temperature tendency due to total diabatic heating, 500mb
-
surface latent heat flux
-
surface incident short wave radiation
We only require this information for the Atlantic sector (90W-10E,
30S-60N). However, it may be more convenient to obtain the global
data sets.
References
Chang, P., L. Ji and H. Li, 1997: A decadal climate variation in the tropical
Atlantic Ocean from thermodynamic air-sea interactions. Nature, 385, 516-518.
Delworth, T. L. and V. M. Mehta, 1998: Simulated interannual to decadal
variability in the tropical and sub-tropical North Atlantic. Geophys. Res.
Lett., 25, 2825-2828.
Enfield, D. B. and D. A. Mayer, 1997: Tropical Atlantic SST variability
and its relation to El Nono-Southern Oscillation. J. Geophys. Res., 102,
929-945.
Hastenrath, S. and L. Druyan, 1993: Circulation anomaly mechanisms
in the tropical Atlantic sector during the Northeast Brazil rainy season.
Results from the GISS general circulation model. J. Geophys. Res.,
98, 14917-14923.
Mehta, V. M., 1998: Variability of the tropical ocean surface temperatures
at decadalmultidecadal time sacles, part I: the Atlantic ocean. J. Climate,
9, 1750-1771.
Rajagopalan, B., Y. Kushnir and Y. M. Tourre, 1998: Observed midlatitude
and tropical Atlantic climate variability. Geophys. Res. Lett., in press.
Ruiz-Barradas, A., J.A. Carton, and S. Nigam, 1999: Structure of interannual-to-decadal
climate variability in the tropical Atlantic sector, J. Clim., accepted.
Tourre, Y. M., B. Rajagopalan and Y. Kushnir, 1998: Dominant patterns
of climate variability in the Atlantic ocean region during the last 136
years. J. Climate, in press.