A Process Study of Surface Heat Flux Anomaly Introduced to an
Atmosphere-Ocean Coupled Model in the South Atlantic
Carlos A. D. Lentini1, Reindert J. Haarsma2, Edmo J. D. Campos1,
and Rainer Bleck3
ABSTRACT: A process study is performed with an atmospheric-oceanic
coupled system to examine whether and how imposed surface anomalies
interact with the subtropical mixed layer and water masses beneath it on
interannual time scales in the South Atlantic region. The coupled model,
which covers the area between 20oN-45oS and 65oW-20oE, is forced with
anomalous surface heat fluxes for one year after the coupled system is spun
up. After this period, the system is free to evolve. Ensemble means of forced
and non-forced experiments are the basis of our results. Preliminary results
suggest that advection by the mean ocean flow and Rossby waves are the
two main ways of propagating the imposed surface anomalies away from its
origin toward the South American east coast. Westward propagating oceanic
Rossby waves not only introduce circulation anomalies to the mean flow,
but they also seem to strongly influence the evolution of the intensity and
shape of subsurface anomalies. Furthermore, these results indicate the
importance of using a more “realistic” ocean model, which has its dynamics
and its thermodynamics, if one attempts to address climate variability
issues.
(1)Instituto Oceanográfico da Universidade de São Paulo
Praça do Oceanográfico, 191
05508-900 São Paulo, SP, Brazil
Phone: (55-11) 3091-6584 Fax: (55-11) 3091-6597
E-mail: lentini@io.usp.br
(2)Royal Netherlands Meteorological Institute
Wilhelminalaan 10
3730 Ad De Bilt, The Netherlands
(3)Los Alamos National Laboratory
Los Alamos, New Mexico, USA
RESUMO: Um estudo de caso é executado com um sistema acoplado
atmosférico-oceânico para examinar de que forma e como anomalias
superficiais impostas interagem com a camada mistura subtropical e massas
de água abaixo dela em escalas inter-anuais na região do Atlântico Sul. O
modelo acoplado, que cobre a área entre 20oN-45oS e 65oW-20oE, é forçado
com fluxos anômalos de calor na superfície durante um ano depois que o
sistema acoplado atinge seu equilíbrio. Após este período o sistema é livre
para evoluir. Um conjunto de experimentos com forçante e sem forçante é a
base de nossos resultados. Os resultados preliminares sugerem que a
advecção pelo fluxo oceânico médio e ondas de Rossby são os dois modos
principais de propagar as anomalias geradas na superfície, impostas longe
de sua origem para a costa leste Sul-Americana. Ondas de Rossby oceânicas
propagando para oeste não só apresentam anomalias na circulação do fluxo
médio, mas elas também parecem influenciar a evolução da intensidade e
forma das anomalias de sub-superfície fortemente. Além disso, estes
resultados indicam a importância de usar um modelo oceânico mais
“realístico”, que tem sua dinâmica e termodinâmica presentes, tem que ser
empregado se assuntos ligados à variabilidade do clima forem enfocados.
Keywords: South Atlantic Dipole, SPEEDY, MICOM, coupled system,
Rossby waves, advection, subtropical ocean gyre.
INTRODUCTION
Air-sea coupling in mid-latitudes has received more attention lately
due to global climate variability issues. Particularly, a lot of scientific effort
has been put on practice to understand the physical mechanisms behind the
ocean-atmosphere coupling and the link between sub-tropical and tropical
regions (e.g., Gu and Philander, 1997; Lysne et al., 1997; Murtugudde et al.,
1996; Stramma and England, 1999).
Barnett et al. (1999) identified two main unresolved questions on the
ocean-atmosphere coupling in mid-latitudes: (i) what is the direct impact of
mid-latitude upper ocean anomalies on atmospheric variability? (ii) is there
a feedback between the ocean and atmosphere that allows the existence of
low-frequency oscillatory modes? If the low-frequency climate change
results from a two-way interactive process between these two fluids that
involve a delay due to the high inertia of the ocean (Latif and Barnett,
1996), then the process is potentially predictable if its hydrodynamics is
understood. On the other hand, if the ocean integrates random atmospheric
forcing (Barsugli and Battisti, 1998) without significant feedback to the
atmosphere, the benefit of climate modeling for long-lead forecasts is
marginal at best.
To answer the question of how mid-latitude ocean anomalies impact
the atmospheric variability and vice-versa, modeling studies are often used.
The motivation for numerical simulations can be explained by the difficulty
of separating “cause and effect” from observational data alone, since the
ocean-atmosphere system is intrinsically coupled. The way in which ocean
surface anomalies can be imposed in a process study is via prescribed sea
surface temperatures or heat flux anomalies. However, the intensity, shape,
and duration of such imposed forcing is extremely important as these
anomalies may affect the coupled system differently. Therefore, an
isopycnal coordinate ocean model coupled to an atmosphere model has been
used to investigate the response of this coupled system forced with
anomalous heat fluxes in the form of the South Atlantic Dipole (Sterl and
Hazeleger, 2003) on interannual time scales. The main focus of this study is
to understand how the ocean responds to such a forcing on these time scales.
This paper is outlined as follows: a short introduction of the
atmospheric and oceanic models used in the coupled system simulations; the
model spin-up and the forcing set-up description; the results of the ocean
component are presented and discussed followed then by concluding
remarks.
THE COUPLED SYSTEM
Atmospheric model
The atmosphere global model used in this study is SPEEDY (Molteni,
2003). It is an intermediate complexity model based on a spectral primitiveequation core and a set of simplified parameterization schemes. The
parameterization package has been especially designed to work in models
with just a few vertical levels, and is based on the same physical principles
adopted in the schemes of state-of-the-art AGCMs. The parameterized
processes include large-scale condensation, convection, clouds, short and
long wave radiation, turbulent surface fluxes and vertical diffusion. The
horizontal resolution is T30 and it has 7 vertical levels. It is at least an order
of magnitude faster than a state-of-the-art AGCM. The quality of the
simulated climate compares well with that of more complex AGCMs. Some
aspects of the systematic errors of SPEEDY are in fact typical of many
AGCMs, although the error amplitude is higher than in those models.
Ocean model
The ocean model used in this study is the Miami Isopycnal Coordinate
Ocean Model (MICOM) (Bleck et al., 1992) in a regional configuration.
The basin is confined to the South and Tropical Atlantic from 45oS to 20oN.
Outside this basin the boundary condition for the atmospheric model is a
passive mixed layer which will be briefly described below. Apart from the
reduction in computing time an advantage of a basin configuration is that
the mechanism of air-sea interaction over the Atlantic can be isolated from
other processes like the influence of Pacific SST anomalies on the
atmospheric circulation over the Atlantic. The resolution is 1 degree in the
horizontal direction and 16 vertical layers. Outside the oceanic domain of
interest, the boundary condition for the atmospheric model is a “slab ocean”.
In this model the ocean is represented by a passive mixed layer for which
the temperature of the mixed layer “T” is given by,
T
Q

 Fm
t h w c p
where: Q is the net surface heat flux leaving the ocean, h is the mixed layer
depth, w the sea water density, cp the specific heat capacity of sea water,
and Fm represents the induced heat transport by the ocean and all other
processes neglected in this balance. To ensure that the climatology of the
mixed layer model stays close to the observed climatology, Fm is diagnosed
from the above equation in a run with SPEEDY in which “T” is prescribed
as,
Fm 
Tc lim Qdiag

t
h w c p
where: Tclim is the daily mean observed climatological SST computed by
linear interpolation from the climatological monthly mean SST of Da Silva
et al. (1994), and Qdiag is the daily diagnosed net surface heat flux from the
model. Qdiag represents the ocean heat transport which can not be simulated
in the slab model. Fm computed by the above equation varies in space and
goes through an annual cycle.
MODEL SPIN-UP AND FORCING SET-UP
The surface heat flux anomaly experiment
The coupled model is integrated for 40 years. SPEEDY-MICOM is
spun up from the Levitus climatology for the first 20 years before the
atmospheric anomalous forcing is introduced. Haarsma et al (2003) show
that the main heat source for generating SST anomalies in the area of
interest is the latent heat flux (LHF), whereas the sensible heat flux (SHF)
and radiation terms are less important. Therefore, the ocean model is forced
by LHF anomalies imposed by the atmosphere which have the same spatial
pattern of the South Atlantic dipole (Sterl and Hazeleger, 2003). This
dipole-like structure is obtained from a SVD analysis of MSLP and SST in
the SPEEDY-MICOM coupled model without any forcing during a 60-yr
integration (Haarsma et al, 2003) which is very similar to the one obtained
by Sterl and Hazeleger (2003) from the NCEP-NCAR Reanalysis data
(Figure 1). The choice of forcing the ocean model with this particular
pattern is two-fold: (i) it is a consistent feature in the coupled system and
has been observed both on data and model output analyses, and (ii) LHF
anomalies is one of the main processes leading to the observed SST
variability in the South Atlantic (Sterl and Hazeleger, 2003; Haarsma et al,
2003).
In order to have robust results and to decrease the model's internal
variability, a set of identical experiments was done with slight different
initial conditions. Therefore, two sets of experiments were done: with and
without forcing. The coupled system is forced then by this dipole for one
year as the focus of this research is the interannual variability response of
imposed surface anomalies (Figure 2). After this 1 year, no forcing is added
and the coupled system is free to evolve for the following 19 years.
Ensemble mean of the non-forced experiment (hereafter, Control) is
compared to an ensemble mean of the forced experiment (hereafter, Forced).
Seasonal mean outputs are the basis of all subsequent analyses.
Figure 1: First SVD mode between sea-level pressure (SLP) and sea surface temperatures
(SST) computed from SPEEDY-MICOM (Haarsma et al., 2003, top panel) and from
NCEP-REANALYSIS (Sterl and Hazeleger, 2003, bottom panel) which each one explains
33% and 38% of the total variability, respectively.
Figure 2: The anomalous surface latent heat fluxes introduced to the coupled system after
the 20 years of spin up. The coupled system is forced during one year. After that, the
system is free to evolve.
RESULTS
The stream-function of the barotropic velocity (BT-Vel, contours) of
the control experiment superimposed on the sea surface height (SSH, colors)
averaged over the last 20 years after the spin-up captures well the mean
general circulation of the South Atlantic ocean (Figure 3).
The asymmetric dipole-like surface latent heat flux anomalies
imposed during year 21 generate cool SST anomalies (SSTAs) in the
northern part of the dipole and warm SSTAs in the southern part,
respectively (Figure 4). Intensification of negative SSTAs takes place from
austral summer (JFM) to austral fall (AMJ) reaching values of –1.2oC, and
then decrease during the following season. However, it is during austral
spring (OND) that negative SSTAs expand from 30oS toward the equator,
increase their magnitude, and reach a minimum of -1.2 to -1.5oC roughly
centered at 20oS and 10oW. Warm SSTAs observed in the southern part of
the domain show a more seasonal dependence, reaching positive values
during summer and spring seasons. Both warm and cool SSTAs basically
disappear in the following year (not shown) after the atmospheric surface
forcing is turned off and the coupled model is free to evolve.
Beneath the ocean’s surface, the results show a relatively significant
increase of the intensity of the anomalies with time and their rapid and large
vertical extension. During year 21 the northern part of the dipole, which is
associated with cool SSTAs, subducts trough Ekman pumping and, at the
end of the forced year, the resulting subsurface anomaly reaches its
maximum value of + 0.9°C and extends from 200-m to 600-m deep (Figure
4). Although the occurrence of positive subsurface temperature anomalies
may sound counter-intuitive at this point, one has to notice that these
subducted
anomalies
have
slightly
higher
temperatures
than
its
surroundings; therefore they appear as positive anomalies instead of cool
anomalies. After year 21, horizontal maps of layer thickness anomaly
referenced to the 600-m depth level for the following first 8 years show that
the subducted perturbation moves toward the South American east coast
with a mean velocity of ~4cm/s, a value very close to the intensity of the
mean
South
Atlantic
subtropical
gyre
circulation.
However,
the
displacement of the perturbation indicates that there is an undulatory
phenomenon at play. First, the increase in intensity of the subducted
anomaly core resembles an amplitude increase of a wave signal through
interaction between different vertical modes (Liu, 1999). Second, the rapid
and considerable vertical extension of this anomaly as well as its extension
to a deeper isopycnal layer is suggestive of downward wave propagation
indeed. One can clearly observe the subducted perturbation of the mean
flow propagating westward from its initial location in the interior of the
ocean basin around the 25o of latitude which is interpreted as a first mode
baroclinic Rossby wave. Velocity estimates are in agreement with
observational computations of phase speed of oceanic Rossby waves around
this latitude for the South Atlantic (Paulo S. Polito, 2004, pers. comm.).
From years 26 to 28, the anomaly core follows the Brazilian coast and
moves poleward.
Figure 3: MICOM control output averaged over the last 20 years after the spin-up time in
the coupled system for the area of study.
Figure 4: Vertical profile of the temperature difference between Forced and Control runs
for the four seasons when the forcing is introduced to the coupled system. The green lines
represent the layer thickness of the control run.
Figure 5: 8-year time evolution of the layer thickness anomaly (Forced – Control runs, in
meters) referenced to the 600-m level. Positive layer thickness anomaly values are reddish,
whereas negative values are bluish.
CONCLUDING REMARKS
A process study is performed with an atmosphere-ocean coupled
model to examine how imposed surface heat flux anomalies interact with
the subtropical mixed layer and water masses beneath it on interannual time
scales in the South Atlantic region. The trajectory and speed of the
subducted perturbation is in agreement with the mean flow to a first
approximation, a result consistent with previous observational studies in the
Atlantic (Stramma and England, 1999). Our results indicate that a
combination of mean subtropical flow advection and Rossby wave
dynamics are the two main ways of propagating the imposed anomalies
across the ocean basin. Moreover, a relatively significant increase of the
intensity of the subducted anomalies with time and their rapid and large
vertical extension as a consequence of Ekman pumping and air-sea
interaction is observed. For instance, cooling by latent heat flux anomalies
creates warm subsurface temperature anomalies due to an increase in
salinity anomaly at the surface. The westward propagation of the induced
perturbation is associated with a salinity-compensation phenomenon that
strongly balances the temperature-related density perturbation. The
propagation of this perturbation does not seem to be related to air-sea
interaction but to oceanic adjustment in the perturbed subsurface density
field.
Acknowledgments: This work has been funded by IAI (SACC-CRN 060),
FAPESP (grants 01/10965-6 and 04/01849-0).
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