Clim Dyn
DOI 10.1007/s00382-009-0624-6
Tropical response to the Atlantic Equatorial mode: AGCM
multimodel approach
T. Losada Æ B. Rodrı́guez-Fonseca Æ I. Polo Æ
S. Janicot Æ S. Gervois Æ F. Chauvin Æ P. Ruti
Received: 23 December 2008 / Accepted: 25 June 2009
Ó Springer-Verlag 2009
Abstract On the frame of the AMMA-EU project, sensitivity experiments for an Atlantic Equatorial mode
(AEM) which origin, development and damping resembles
the observed one during the last decades of the 20th century, has been analysed in order to investigate the influence
on the anomalous summer West African rainfall. Recent
studies raise the matter of the AEM influence on the next
Pacific ENSO episodes and also on the Indian Monsoon.
This paper evaluates the response of four different atmospheric global circulation models, using the above-mentioned AEM sensitivity experiments, to study the tropical
forcing associated with the Atlantic Niño mode. The results
show a remote signal in both the Pacific and Indian basins.
For a warm phase of the AEM the associated southward
location of the ITCZ, with rising motions over the Equatorial Atlantic, leads to a global subsidence over the rest of
the tropics, weakening the Asian Monsoon and favouring
This paper is a contribution to the special issue on West African
Climate, consisting of papers from the African Multidisciplinary
Monsoon Analysis (AMMA) and West African Monsoon Modeling
and Evaluation (WAMME) projects, and coordinated by Y. Xue and
P. M. Ruti.
T. Losada (&) B. Rodrı́guez-Fonseca I. Polo
Universidad Complutense de Madrid, Madrid, Spain
e-mail: [email protected]
S. Janicot S. Gervois
LOCEAN/IPSL, CNRS, Université Pierre et Marie Curie,
Paris, France
F. Chauvin
GAME/CNRM, Météo-France/CNRS, Toulouse, France
P. Ruti
Progetto Speciale Clima Globale, Ente Nazionale per le
NuoveTecnologie, l’Energia e l’Ambiente, Rome, Italy
the La Niña conditions in the central Pacific. Although
ocean–atmosphere coupled experiments are required to test
the latter hypothesis, the present studies shows how the
AEM is able to influence the rest of the tropics, a result
with important implications on ENSO seasonal
predictability.
1 Introduction
The question of how the tropical oceans interact with each
other, and what are the associated dynamical mechanisms
explaining these teleconnections is, nowadays, a debated
topic for the scientific community with a large amount of
recent references about it (Chang et al. 2006; Wang 2006;
Kug and Kang 2006; Kucharski et al. 2007, 2008, 2009;
Keenlyside and Latif 2007; Polo et al. 2008; Jansen et al.
2009).
In particular, recent observational and model studies
have emphasized the potential teleconnections between
the tropical Atlantic and the adjacent Tropical Indian
and Pacific Oceans (Wang 2006; Kucharski et al. 2007;
Keenlyside and Latif 2007; Kucharski et al. 2008, 2009;
Polo et al. 2008).
It is known how the atmospheric response to a heating in
the tropical Atlantic shows a Gill-type response (Vizy and
Cook 2001; Kucharski et al. 2008) that leads to an increase
in precipitation near the Gulf of Guinea (GG) and a
decrease to the east. This Matsuno-Gill response to the
Equatorial mode, would produce the propagation of a
Kelvin wave pattern trapped in the tropics as a response to
the tropical heating, which would influence the Indian
ocean basin (Kucharski et al. 2007, 2008, 2009). The works
of Kucharski and co-authors have described how, through
this mechanisms, the Equatorial mode could be modifiying
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T. Losada et al.: Tropical response to the Atlantic Equatorial mode
the ENSO–Indian Monsoon (IM) relation, according to
which a drier than normal monsoon season usually precedes peak El Niño conditions, and viceversa during la
Niña phase. In this way, this modification of the ENSO–IM
could be realistically simulated during the period of 1950–
1999 only if changes in the Tropical Atlantic SSTs in
boreal summer were properly accounted for.
How the tropical Atlantic (TA) variability affects the
tropical Pacific is a task that has been widely overlooked by
the scientific community. The indexes related to the
Atlantic and Pacific Niños do not contemporaneously
correlate each other, although Wang (2006) has reported
the existence of an Inter-basin SST gradient which induces
a positive feedback in between the tropical oceanic basins.
ENSO influence on the Equatorial Atlantic is less clear,
and it has been recently suggested that this is due to ENSO
competing dynamical and thermodynamical influences
there (Chang et al. 2006). Observations show that the TA
can influence the tropical Pacific Ocean via the inter-basin
SST gradient variability that is associated with the Atlantic
Walker circulation. Rainfall responds to the inter-basin
SST gradient, regardless of which ocean is anomalously
warm or cold (Wang et al. 2009). In fact, a feedback from
the Atlantic on ENSO appears to exist, and slightly
improves the retrospective forecast skill of the conceptual
model (Jansen et al. 2009).
Several recent papers have point to an statistical relationship between the Atlantic and Pacific Niños from the
70s, in which the AEM would lead the Pacific ENSO
several months in advance (Jury et al. 2002; Melice and
Servain 2003; Latif and Keenlyside 2008; RodriguezFonseca et al. 2008; Polo et al. 2008), pointing to the
possibility of increasing ENSO prediction due to the occurrence of a feedback from the Atlantic. However, physical
explanation about the dynamical mechanisms that could
explain the statistical link that the Equatorial mode, and its
associated atmospheric response, has on the adjacent Pacific
basin during recent decades, has not been account yet.
The present study focuses on the tropical atmospheric
response to an AEM calculated for the 1979–2005 period,
after the Pacific ‘‘climate shift’’. Climate shift refers to the
alteration during 1976–1977 in the central North Pacific
SST (Nitta and Yamada 1989; Trenberth 1990; Graham
1994; Miller et al. 1994). After this period, the propagation
characteristics of ENSO changed (Wang 1995; An and
Wang 2000; Fedorov and Philander 2000; Wang and An
2002; Latif and Keenlyside 2008) and also the onset phase,
being characterized by the appearance of SST anomalies in
the Central Pacific that propagate eastward (Terray and
Dominiak 2005); in contrast with the canonical evolution
of an ENSO event, in which the SST anomalies first appear
in the South American coast and then propagate to the west
(Rasmusson and Carpenter 1982). Besides, and it has been
123
indicated above, the ENSO–IM relationship also suffered a
weakening from the 70s which could be linked to the
Atlantic variability (Kucharski et al. 2008).
The main purpose of this work is to elucidate how the
interaction between the AEM and the West African convection influences the atmospheric circulation over the rest
of the tropical basins. To achieve this goal, we evaluate the
response of four different atmospheric general circulation
models (AGCMs) that are running in the frame of the
AMMA-EU project; to the tropical forcing associated with
the Atlantic Niño mode. Thus, the analysis of the multimodel sensitivity experiment will provide new insights on
the atmospheric global teleconnections associated with an
isolated Atlantic Niño pattern.
The AEM has been calculated taking into account its
seasonal evolution and, maximizing its covariability with
the West African precipitation. The reason of the latter is
that in the tropics, where horizontal differences in temperature and density are small, vertical motion, and thus
convection, is the main process by which tropical heating is
balanced (Hagos and Cook 2005).
This study is divided into four sections. In ‘‘Sect. 2’’ we
describe the experiments and models used, as well as the
observational data. The results of the study are presented in
‘‘Sect. 3’’. The discussion and conclusions are displayed in
‘‘Sect. 4’’.
2 Methodology and data
The models and simulations used in this work are ARPEGE (Déqué et al. 1994), ECHAM-4 (Roeckner et al. 1996),
LMDZ (Hourdin et al. 2006) and UCLA (Richter et al.
2008). The main characteristics of their configuration have
been described in Losada et al. (2009).
The methodology followed in this paper consists in the
realization of different ensemble simulations, each composed by ten members.
The control simulation was run with climatological
SSTs (Smith and Reynolds 2004) averaged from 1979 to
2005. The EMP simulation was run with a positive SST
anomaly in the tropical Atlantic added to the climatology
(Fig. 1).
The calculation of the Equatorial mode pattern added to
the climatology in the sensitivity experiments starts with
the computation of the extended multiple covariance
analysis (EMCA; Polo et al. 2008) between Atlantic SST
and West Africa precipitation with observational data.
Then different years are selected to define a composite of
years in which the covariance between the tropical Atlantic
SST and the rainfall over West Africa is maximized. The
years selected are 1984, 1987, 1988, 1989, 1998, 1999 for
positive SST anomalies; and 1982, 1983, 1992, 1997 for
T. Losada et al.: Tropical response to the Atlantic Equatorial mode
Fig. 1 MJ (left) and JAS (right)
SST anomalies (°C) used as
boundary conditions
negative SST anomalies. Details of the method of definition of the anomalous SST pattern used are found in Losada et al. (2009).
The global tropical composite (Fig. 2, contours) for
these years indicates the concomitant coexistence of a
Pacific La Niña, according with recent statitistical studies
that point to an statistical relationship between the Atlantic
and Pacific Niños from the 70s (Jury et al. 2002; Melice
and Servain 2003; Latif and Keenlyside 2008; RodriguezFonseca et al. 2008; Polo et al. 2008).
The anomalous response to the Equatorial mode will be
computed by subtracting the control to the EMP. In order
Fig. 2 Composite maps for the precipitation anomalies (mm/day,
shaded) and SST (°C, contours, dashed lines denote negative values,
contour interval is 0.5°C) observed for the years used in the
generation of the boundary conditions (see ‘‘Sect. 2’’)
to assess the significance of the response, we apply a test t
of equal mean (von Storch and Zwiers 1999) at 95% of
confidence level.
3 Results: tropical teleconnections related
to the Equatorial mode
Recent studies have shown how the atmospheric response
to a heating in the Equatorial Atlantic is a Gill-Matsuno
type response with a Kelvin wave propagating to the Indian
region and an equatorial trapped Rossby wave propagating
westward to Central America and eastern Pacific. These
studies point, on the one hand, to a link between the
African and Indian Monsoons through variations in the
equatorial circulation (Kucharski et al. 2008, 2009); and,
on the other hand, to a link between the Equatorial mode
and the Pacific El Niño (Polo et al. 2008). This study
revises the influence of the Equatorial mode on the adjacent
tropical basins, trying to give a dynamical explanation to
the teleconnections found.
Figure 3 show the precipitation anomalies given by the
different models over the Equatorial Africa in May–June
(MJ) and July–August–September (JAS), with positive
values over the Gulf of Guinea and West African coast, and
negative values to the east. This pattern, which is weaker
for ECHAM-4 and LMDZ, is present for all the models and
months except for ARPEGE in MJ, and it is stronger after
the onset (JAS, Fig. 3). Vizy and Cook (2001) and
Kucharski et al. (2009) found a similar pattern in their
experiments that can be explained as a Gill response to an
equatorial heating (Gill 1980), which produces an anomalous zonal flow from the east into the heating source
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T. Losada et al.: Tropical response to the Atlantic Equatorial mode
(a Kelvin wave response), with anomalous divergence and
a decrease in the precipitation to the east of the heating.
The anomalous precipitation pattern in the Indian basin
is more or less significant depending of the model and
month (Fig. 3), but a decrease of precipitation over west
India that is significant from July is present in all the
models. This weakness of the Indian monsoon under conditions of warm SST anomalies in the Equatorial Atlantic is
consistent with the statistical relation found by Kucharski
et al. (2008). In order to characterize the response in
our models, we look at the 200 hPa streamfunction
and velocity potential anomalies (Figs. 4, 5), as indicative
of the upper rotational and divergence circulation,
respectively.
The 200 hPa streamfunction (Fig. 4) response to the
Equatorial mode has the structure of a typical Gill-Matsuno
response to equatorial heating, which is a quadrupole with
a pair of symmetric anticyclones at both sides of the
equator and to the west of the maximum heating, and a pair
of cyclones to the east of the source of heating (Jin and
Hoskins 1995). This response is baroclinic in the tropical
Atlantic and barotropic the extratropics (not shown). The
centres of the anomalies move to the west from May to
September, following the evolution of the SST anomalies,
and the amplitude and significance of the anomalies get
stronger in time.
The westward migration of the anomalies over the
Equatorial Atlantic from May to September also appears in
the 200-hPa velocity potential (VP) response (Fig. 5), also
following the evolution of the tropical convection (Fig. 3)
associated with the surface westward Equatorial Atlantic
warming (Fig. 1). All the models show a negative VP
anomaly located over the equator and around 0° in MJ
(except ECHAM-4, that presents the anomaly centred in
20°W), that spreads to the west in time. This anomaly,
which coincides with the maximum anomalous rainfall
(Fig. 3), is flanked with two positive anomalies to the east
and west. In MJ, the positive centres are located over the
West Indian Ocean and Central America. From July, the
negative lobe has already covered the Amazonian and
Caribbean region, and the positive centres have been displaced to the central Pacific (around 160°W) and the
eastern Indian basin. The significance of the anomalies is
larger for JAS than for previous months.
The possibility of an influence of the east AtlanticWest African sector onto the Caribbean-South American
area, through changes in the vertical motions has already
been reported by Hagos and Cook (2005), during boreal
winter (January). Our results show how, for May–September season, the anomalous upper divergence over the
GG region, is stronger before the monsoon onset and
leads to anomalous upper convergence over the Caribbean, weakening the mean Walker circulation over the
Atlantic in the months previous to the northward shift of
the ITCZ. After the onset, the maximum anomalous upper
troposphere divergence is located in the west of the
Atlantic and the divergence anomalies cover the Caribbean and Central America regions; the upward branch of
the mean Walker circulation, that is located in this area, is
being reinforced from July. At this stage, positive velocity
potential anomalies become significant in the central
Pacific for all the models except the ARPEGE (the significance in JAS is clearer for ECHAM-4), and they get
stronger in August and September. This anomalous pattern depicts a reinforcement of the Walker circulation cell
of the Amazon-Central Pacific, in which both the
ascending and subsiding branches of the cell are stronger
than in the climatological state, and the descending
branch has spread a bit to the west.
To better illustrate the mechanism by which the anomalous convection associated with the AEM surface heating
is able to alter the Walker circulation, increasing the
Fig. 3 JAS global precipitation anomalies (mm day-1) for a ARPEGE, b ECHAM-4, c LMDZ and d UCLA EMP simulation. Contours
delimites regions denote those areas where the anomalies exceed the 95% significance level
123
T. Losada et al.: Tropical response to the Atlantic Equatorial mode
Fig. 4 Same as Fig. 3 but for 200 hPa streamfunction (106 m2 s-2)
Fig. 5 Same as Fig. 3 but for 200 hPa velocity potential (106 m2s-2)
subsidence over the Indian and Pacific basins, Fig. 6 depicts
this anomalous zonal circulation given by the four analyzed
models (longitude by height anomalous vertical velocityshaded-and zonal wind-contours- for JAS, averaged between
4°S and 4°N). All the simulations show strong anomalous
upward motion over the Atlantic and Amazon region,
accompanied with strong anomalous subsidence at both
sides of the maximum convection: one over East Africa
(around 40°E), which explains the dryness over that region;
and the other one from 50°W to the west, over the eastern
basin of the Pacific ocean. The latter is maybe a local
response to the fact that, in the Amazon region, the maximum
convection has been displaced eastward, towards the west
part of the Atlantic basin (the region of anomalous warming),
producing anomalous subsidence around 80°W. Also, all the
models show upper level divergence over the region of
heating and upper level convergence, very clear in the
anomalous zonal wind response (Fig. 6, contours), and
anomalous subsiding motions over the central Pacific,
around the date line (significant for all the models except
ECHAM-4). This feature closely resembles the one obtained
by Jin and Hoskins (1995) for a resting stratified atmosphere
forced by tropical heating, and it is consistent with the Gill
(1980) response. In this way, the response to a warming in the
Equatorial Atlantic would produce a westward displacement
of the maximums upward motions over the central Pacific.
This displacement could explain the presence of anomalous
upward motions in the eastern Pacific (around *120°W) for
most of the models.
Also, all the models show a tendency of a reinforced
upward branch of the Walker cell over the Indian Ocean,
UCLA model response is noisy, but the rest of the
models show a pretty clear signal in this way, although
the ascendant motions are confined to the middle and
lower troposphere. These anomalous upward motions
could be part of an anomalous local Hadley-type
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T. Losada et al.: Tropical response to the Atlantic Equatorial mode
Fig. 6 JAS 4°S–4°N averaged vertical
velocity (shaded; 100 Pa s-1) and zonal wind
anomalies (m s-1, contour interval is
1.5 m s-1). Only 95% significant areas are
plotted
response to the upper level convergence over the Indian
peninsula.
In this way, the atmospheric response to a warming on
the tropical Atlantic would lead to an enhancement of the
upward branch of the Walker circulation in the Amazon
region, anomalous upper divergence in that area, together
with anomalous convergence and subsiding motions over
the central Equatorial Pacific and over the Indian peninsula. The anomalous subsidence over India would induce
anomalous upward motions over the Equatorial Indian
Ocean that, in turn, produces a local reinforcement of the
upward branch of the Walker circulation over the region.
These anomalous subsiding motions in the central
Pacific and anomalous upward motions over the Indian
Ocean would enhance the zonal pressure gradient between
this two points, leading to an increase in the trade winds
between them, surface divergence and a cooling of the
SSTs over the central Equatorial Pacific. This situation
would favour the development of an ENSO cold event
starting on the central Pacific, which would be in agreement with the description of recent ENSO events made by
123
Wang (1995) and Terray and Dominiak (2005) among
others.
The presence of warm SST anomalies in the Equatorial
Atlantic would then be able to produce a response in the
circulation of the Equatorial Pacific atmosphere that would
be consistent with the forcing of a surface cooling in the
central Equatorial Pacific ocean. The consistency of the
four models response gives confidence to the results.
Whether or not the Pacific atmospheric anomalies can
actually have an influence in the Pacific ocean SST is a
topic that cannot be assess by the kind of experiments
performed in this work. Additional experiments, to be
performed with ocean-atmosphere coupled models, should
be designed in order to test the present hypotheses.
4 Conclusions
In this study, we use four AGCM (ARPEGE, ECHAM-4,
LMDZ, UCLA) in order to analyze the impact of the AEM
on the rest of the tropical basins during the last decades of
the 20th century.
T. Losada et al.: Tropical response to the Atlantic Equatorial mode
As a response to the equatorial heating of the Atlantic
basin, the atmosphere shows a Gill-Matsuno type quadrupolar circulation with an equatorial Rossby wave propagating to the west and a Kelvin wave to the east. This
response is reflected in the west-east dipole of anomalous
precipitation found in the Equatorial Africa. The anomalous upward motions over the Atlantic Ocean are compensated with anomalous subsiding motions over the
central Pacific, leading to a westward displacement of the
maximum location of the downward branch of the Pacific
Walker cell. Also, anomalous upper level convergence
appears over the Indian peninsula, causing the air to descend and producing a inhibition of the IM convection and a
decrease of rainfall over India, supporting the findings of
Kucharski et al. (2007, 2008, 2009). The anomalous subsidence over India would induce an anomalous local
Hadley circulation that would produce a reinforcement of
the ascending motions over the Equatorial Indian Ocean,
leading to the reinforcement of the upward motions in the
lower troposphere. This feature can be also observed in the
presence of positive precipitation anomalies in the Equatorial Indian Ocean for all the simulations. The perturbation of the vertical motions in the Indo-Pacific basin would
produce a gradient of SLP that would strengthen the trades
between these two points, with the appearance of surface
divergence over the Equatorial central Pacific that would
favour the conditions for a La Niña event in the Pacific.
Also, our results show how this possible impact appears
from July, when the deep convection associated to the
Atlantic SST anomalies returns to the west and the maximum upper divergent flow is placed over the Amazon
region. The influence of the Atlantic Equatorial mode on
the Pacific would lead to a development of ENSO events
beginning in the central Pacific, which would be consistent
with the work as reported by Wang (1995) and Terray and
Dominiak (2005), that state that from the climate shift
the onset phase of an ENSO event is characterized by the
appearance of SST anomalies in the Central Pacific that
propagate eastward. According to Wang and An (2002), the
main factor affecting the observed changes in ENSO is
the change in the basic state Equatorial winds and in the
upwelling associated with it, pointing to the increased
importance of the atmospheric bridge in the ENSO
behaviour. Thus, previous results would support the idea of
an influence of the Atlantic variability in the Pacific tropical basin through changes in the atmospheric circulation.
The observed negative link between Pacific and Atlantic
Niños could have an important influence on the summer
Sahelian rainfall. Although it has been documented that the
response to an isolated warm Pacific forcing is the decrease
of the precipitation over Sahel (Rowell et al. 1995; Janicot
et al. 2001; Giannini et al. 2003), when focusing in recent
decades, the relationship varies and a warming in the
Pacific is related to a decrease of rainfall in the GG during
the monsoon season (Janicot and Rodriguez-Fonseca
2007). In fact, the composite of the observed precipitation
anomalies for the years used in the computation of the
Equatorial mode used in the experiments (Fig. 2), shows a
precipitation dipole associated with SST anomalies in the
Equatorial Atlantic during MJ, with an decrease of rainfall
over the Sahel, but it disappear for JAS, in which the
reported increase of precipitation over the Sahelian region
associated with a cooling of the Pacific is observed. We
believe that the added influences of the Atlantic and Pacific
anomalies in the WAM are the responsible of this precipitation pattern evolution and further analysis is being done
in this topic. We have raised here the idea of a possible
influence of the Atlantic variability on the tropical Pacific
atmospheric circulation. Nevertheless, the influence of this
atmospheric anomalies in the Pacific ocean cannot be stated with this kind of experiments, and coupled modelling
studies are needed to state the existence or not of interactions between basins, as well as the feedbacks between
Atlantic and Pacific basins in relation with the West
African monsoon. Also, additional AGCM sensitivity
experiments for a tropical mode including the Atlantic and
the Indo-Pacific basins should be done in order to study the
associated mechanism in relation to the summer WA
anomalous rainfall.
Acknowledgments This study was supported by the EU-AMMA
project and the Spanish MEC project CGL2006-04471. Based on
French initiative, AMMA was built by an international scientific
group and is currently funded by a large number of agencies, especially from France, UK, US and Africa. It has been the beneficiary of
a major financial contribution from the European Community’s Sixth
Framework Research Programme. Detailed information on scientific
coordination and funding is available on the AMMA International
website http://www.amma-international.org. The authors thank Javier
Garcı́a-Serrano and Elsa Mohino for his very useful comments on the
manuscript, and the two anonymous reviewers for their constructive
suggestions and comments.
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