15 MARCH 2011 G A R C I´ A-SERRANO ET AL. 1613

Extratropical Atmospheric Response to the Atlantic Nin˜ o Decaying Phase

JAVIER GARCI´A-SERRANO,* TERESA LOSADA, AND BELE´ N RODRI´GUEZ-FONSECA Departamento de Geofisica y Meteorologia, Facultad de CC Fisicas, Universidad Complutense de Madrid, Madrid, Spain

(Manuscript received 28 January 2010, in final form 22 November 2010)

ABSTRACT

The Atlantic Nin˜ o or Atlantic Equatorial Mode (EM) is the dominant coupled variability phenomenon in the tropical Atlantic basin during boreal summer. From the 1970s, the mode has changed, evolving in time from east to west and without persisting until the following winter. In a previous observational work, the authors have studied the atmospheric response to the EM during the 1979–2005 period, proposing three main issues along the decaying phase of this mode: 1) the continuous confinement of the anomalous deep con- vection over northeastern Brazil following the thermal-forcing decay; 2) an increasing dipole-like precipi- tation anomaly with dry conditions in the Florida–Gulf of Mexico region; and 3) the excitation of Rossby waves forced by the remaining upper-tropospheric divergence that are trapped into the subtropical jet but do not show a robust impact on the European sector. In this work, a 10-member ensemble simulation for the recent EM with the University of California, Los Angeles AGCM model has been analyzed for assessing the evolution of the atmospheric response to the summer Atlantic Nin˜ o decay. Results from the sensitivity experiment support that the former and the latter findings can be interpreted in terms of the Atlantic thermal forcing; while the negative rainfall anomalies in the western subtropical basin require an external forcing outside the tropical Atlantic. Prior studies point at the peaking Pacific El Nin˜ o as a potential player. An important conclusion of this work is that the seasonal atmospheric response to the Atlantic Nin˜ o decaying phase is mainly determined by the climatological jet stream’s position and intensity. In this way, this response shows an arching pattern over the North Atlantic region during summer–autumn and a zonally oriented wave train during autumn–winter.

1. Introduction Several works have described the characteristics of this summer mode (Carton et al. 1996; Sutton et al. 2000; The Atlantic Nin˜ o, or ‘‘Atlantic Equatorial Mode’’ Ruı´z-Barradas et al. 2000; Frankignoul and Kestenare (EM), is the dominant tropical Atlantic SST variability 2005a) as well as its local (Vizy and Cook 2002; Losada mode during boreal summer (Xie and Carton 2004; et al. 2009a) and remote tropical impacts (Wang 2006; Huang and Shukla 2005). The Atlantic Nin˜ o is an ocean– Kucharski et al. 2007, 2008, 2009; Losada et al. 2009b; atmosphere coupled mode in which the Bjerknes feed- Rodrı´guez-Fonseca et al. 2009). back operates by means of relaxation/reinforcement of Nevertheless, just a few works have paid attention to the tropical , redistribution of warm waters extratropical of the Atlantic Nin˜ o, and in the equatorial belt, and changes in the equatorial ther- a systematic description of the forced atmospheric re- mocline slope and heat content zonal gradient (Zebiak sponse to a time-evolving EM has not yet been docu- 1993; Keenlyside and Latif 2007). mented. While some studies conclude that the influence of the summer–autumn equatorial Atlantic on the winter North Atlantic Oscillation (NAO) is artificial (Frankignoul ´ * Current affiliation: Institut Catala de Ciencies del Clima, Barce- and Kestenare 2005b; Garcıa-Serrano et al. 2008), others lona, Spain. point to a lagged effect of the EM on the NAO through the poleward propagation of Rossby waves either from the Amazon region (Dre´villon et al. 2003) or from the Corresponding author address: J. Garcı´a-Serrano, Facultad CC Fisicas, Universidad Complutense de Madrid, Ave. Se´neca 2, 28040 central–eastern part of the tropical Atlantic basin (Peng Madrid, Spain. et al. 2005). All of these discrepancies lead to the need E-mail: javigarcia@fis.ucm.es for an updated revision of the potential relationship

DOI: 10.1175/2010JCLI3640.1

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Unauthenticated | Downloaded 09/29/21 12:43 PM UTC 15 MARCH 2011 G A R C I´ A-SERRANO ET AL. 1615 between tropical Atlantic SSTs and the North Atlantic longitudes (always north of the equator), while the circulation. anomalous rainfall along the Guinean coast has nearly In this work, we explore this lagged connection, taking disappeared (Fig. 1b). Finally, in NDJF the positive SST into account the results by Polo et al. (2008) for the re- anomaly is localized offshore of eastern Brazil at south- cent climate (1979–2002), in which the summer-peaking ern latitudes and a negative precipitation anomaly arises Atlantic Nin˜o does not persist into winter and is not cor- over the Florida–Gulf of Mexico region, while over South related with the early winter Atlantic Nin˜o II (Okumura America a positive rainfall anomaly is reduced spatially and Xie 2006). Here we will focus on the atmospheric to latitudes south of the equator (08–108S, Fig. 1c). response to this recent mode described in Polo et al., According to this precipitation evolution during the which starts in the Angola–Benguela upwelling region Atlantic Nin˜o decaying phase, Garcı´a-Serrano et al. (2008) and is caused by the alongshore wind stress anomalies; it stated three main points: i) the evolution of the anom- propagates westward via Rossby oceanic waves during alous deep tropical convection following the thermal the transition phase (spring–summer), while the ending is forcing decay and the final confinement of upper-level forced by anomalous latent heat fluxes and Kelvin oce- divergence anomaly over eastern Brazil; ii) the anoma- anic waves propagation from an off-equatorial forcing lous enhancement of the local ITCZ-related Hadley (summer–autumn). circulation, resulting in dry conditions over the Florida– In a focal study, Haarsma and Hazeleger (2007) re- Gulf of Mexico region at the last stages of the Atlantic captured the lagged relationship between the Atlantic Nin˜ o decay; and iii) the change from a poleward-arching Nin˜o and the east Atlantic atmospheric pattern previ- response during summer, crossing the North Atlantic, to ously suggested by Frankignoul and Kestenare (2005b). a zonally oriented pattern during winter, which is trap- In their model work, the equatorial Atlantic SST anomaly ped in the North African–Asian jet. was, however, fixed throughout the year. Even so, keep- Related to the first issue, the westward displacement ing the amplitude constant provides information about of the anomalous Walker ascending branch from the peak the sensitivity of the atmospheric response to the sea- phase (May–June) to the mature phase (July–September) sonal cycle. They proposed that the early winter North has been assessed in Losada et al. (2009b) using atmo- Atlantic response is part of a circumglobal teleconnec- spheric general circulation models (AGCMs). The pres- tion initiated in the tropical Atlantic and enhanced at ent work will focus both on the subsequent tropical extratropical latitudes by transient-eddy feedback. convection temporal decline and the assessment of the Garcı´a-Serrano et al. (2008) described the tropical– other two observational results by analyzing the same extratropical interaction in the course of the equatorial dedicated AGCM experiment (Losada et al. 2009a; see SST decay by using observations for the recent climate section 2; Figs. 1d–f). (1979–2002). They showed how the anomalous ascend- The paper is organized as follows: some details of the ing branch of the Atlantic Walker cell, related to a warm simulation and the AGCM model used are presented in EM, moves from the tropical (offshore the next section; the outputs are analyzed in section 3; Brazil–Gulf of Guinea) in summer to the southeastern and the summary and conclusions are shown in section 4. part of Amazonia during winter. Here we have replotted three lagged in Figs. 1a–c showing the anoma- 2. Experimental setup lous rainfall and SST during late summer [July–October (JASO)], late autumn [September–December (SOND)], To study the atmospheric response to the Equatorial and winter [November–February (NDJF)]. In JASO, the Mode decaying phase, the University of California, Los rainfall equatorial band seems to split into two regions, Angeles (UCLA) AGCM was forced with the leading west and east of 208W (Fig. 1a). In SOND, when the time-varying anomalous tropical Atlantic SST pattern anomalous SST has evolved to reach the whole equatorial from the extended-maximum covariance analysis (EMCA) area in equal proportions, the anomalous tropical con- described in Polo et al. (2008). This AGCM design vection over South America weakens but reaches western was adopted by the African Multidisciplinary

FIG. 1. (left) Homogeneous and heterogeneous regression maps of SST (contours, ci 5 0.18C) and standardized precipitation (shaded) onto the observed EMCA SST expansion coefficient as in Garcı´a-Serrano et al. (2008) for JASO, SOND, and NDJF. (right) Boundary conditions (SST, ci 5 0.38C) of the time varying prescribed EMP AGCM simulation.

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Analyses (AMMA) community in an intercomparison (JJAS), Fig. 2a], depicting the subsequent rainfall decay protocol. In this project, four AGCMs were used in the during autumn (Figs. 2b–d), and providing the final con- exercise but only the UCLA model integration was ex- finement of precipitation over South America in winter- tended to winter–spring after the monsoonal . The time (Figs. 2e,f). Even so, it is important to mention that UCLA AGCM runs were performed from 15 April to the EMP experiment seems to overestimate the rainfall 15 March (of the following year), with a total length of anomalies. This feature could be explained by the mag- 11 months. The version of the model used in this work is nitude of the prescribed anomalous SST itself or by the 7.3 (Mechoso et al. 2000; Richter et al. 2008) in its high- overestimation of precipitation in the UCLA AGCM resolution configuration (28latitude 3 2.58longitude, 29 (Losada et al. 2009a). sigma levels in the vertical). The experiment consisted In JJAS and JASO, positive rainfall anomalies spread of the performance of a 10-member ensemble simula- out from offshore northeastern Brazil to the African tion, in which the atmospheric initial conditions of each equatorial coastline, while negative anomalies are local- integration were different and taken from a long-term ized over the northern South American continent. A Atmospheric Model Intercomparison Project–type run weakening in the amplitude of both anomalies is no- with conditions representative of the end of the twen- ticeable from summer (JJAS, Fig. 2a) to late summer tieth century. The anomalous SST pattern (Figs. 1d–f) (JASO, Fig. 2b), reflecting the first stage on the decay of was composed by choosing extreme events of the lead- the atmospheric response to the SST cooling trend. In the ing observed EMCA between the tropical Atlantic SST subsequent season, during autumn [August–November and West African precipitation, computed by Polo et al. (ASON, Fig. 2c)], inland dry conditions almost vanish (2008)—to amplify the signal, the difference between and the anomalous deep tropical rainfall strongly weakens the positive and negative composites has been computed. over the Gulf of Guinea off the coast of Africa. A Due to underestimation of the amplitude of the anoma- northward displacement of the latter is also appreciable, lies in the regression technique, the prescribed anoma- as well as positive precipitation anomalies over the Cen- lies are roughly three times the regressed ones (Fig. 1) tral America–Caribbean Sea that appear to be enhanced. but approximately double the observed anomalies for Later, in SOND (Fig. 2d) a general decrease in the pos- the positive phase of the mode. This anomalous pattern itive precipitation anomalies coincides with the Atlantic was added to the 1979–2005 climatological SSTs (Smith Nin˜ o SST decay (Fig. 1e). At this stage, it is worth and Reynolds 2003). We will refer to this simulation as pointing out how anomalous convection in the central equatorial mode positive (EMP); further details of the tropical Atlantic is north of the equator. During early characteristics and computation of the anomalous SST winter [October–January (ONDJ, Fig. 2e)] positive pre- fields used can be found in Losada et al. (2009a). cipitation anomalies over the southern Caribbean clearly The results obtained from the EMP simulation will be weaken, disappearing in November–February (NDJF, compared with a 10-yr-long control run performed us- Fig. 2f). Also, in ONDJ anomalous wet conditions appear ing monthly climatological SSTs averaged from 1979 to in southeastern tropical Brazil, covering the latitudinal 2005. Throughout the whole work we will refer to the window from the equator to 108S, which are enhanced Atlantic Nin˜ o as the anomalous model response to the during winter (NDJF). Related to the remaining anom- difference between the EMP and the control runs. Sig- alous deep convection, there is a reinforcement of the nificance of the response will be assessed by applying a local Hadley circulation, resulting in negative rainfall t test of equal means (von Storch and Zwiers 2001) at the anomalies in the subtropical region; although its exten- 95% confidence level. sion does not reach the Florida–Gulf of Mexico region. This result rules out the role of the Atlantic Nin˜ode- caying phase on that observational feature. 3. Results To further illustrate the impact of Atlantic forcing on Figure 2 represents the anomalous precipitation se- the seasonal tropical convection, overplotted in Fig. 2 quence derived from the ensemble mean of the EMP is the anomalous field in the divergent wind at 200 hPa. simulation. Throughout all lagged seasons, no clear According to the anomalous rainfall sequence, the model rainfall pattern appears outside the tropics in the Euro– results reflect the anomaly in the local ITCZ over its Atlantic region. As also shown in observations (Garcı´a- seasonal cycle. Serrano et al. 2008), the precipitation response to the The rotational atmospheric response to the Atlantic Atlantic Nin˜o decaying phase is confined over the tropi- Nin˜ o decaying phase is shown in Fig. 3 through the cal Atlantic basin. Indeed, the simulated deep convection streamfunction (contours) and the derived rotational tightly resembles the observed evolution—covering the wind (arrows) at 200 hPa. The amplitude of the anom- whole tropical band during summer [June–September alies through the whole sequence compare well to those

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21 21 FIG. 2. JJAS to NDJF ensemble mean of precipitation (mm day ) and divergent wind (m s ) at 200 hPa from the EMP simulation.

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6 2 21 21 FIG. 3. JJAS to NDJF ensemble mean of streamfunction (contours, ci 5 10 m s ) and rotational wind (arrows; m s ) at 200 hPa from the EMP simulation. related to a typical ENSO event (not shown). First, pole- from the forcing region. As proposed by Haarsma and ward propagation of a Rossby wave to central Europe Hazeleger (2007), and also suggested by Garcı´a-Serrano seems to occur during the mature phase of the Atlantic et al. (2008), the propagation of these SST-forced anom- Nin˜o (JJAS, Fig. 3a). During the first stages of the SST alies reflects a hemispheric pattern by means of the decay (JASO and ASON), the forced Rossby wave–like waveguide effect in the westerly jet streams. According response arches along the eastern North Atlantic basin to this circumglobal , energy is not dis- and finally is trapped into the North African–Asian jet persed regionally and is able to propagate farther be- propagating toward the western North Pacific (Figs. 3b,c). fore being dissipated (Branstator 2002). In SOND, an In the winter hemisphere (Southern Hemisphere), where anomalous anticyclonic circulation, which represents the is more intense, a well-defined a weakening of the Aleutian low (Fig. 3d), emerges arching Rossby wave train is present. over the northern North Pacific. This anomaly clearly Subsequently, from SOND to NDJF the simulated at- strengthens in ONDJ and NDJF (Figs. 3e,f). The cir- mospheric response remains, depicting a Rossby wave– cumglobal response reaches Europe through the North like propagation but showing a marked zonal orientation Atlantic jet stream (Hoskins and Ambrizzi 1993),

Unauthenticated | Downloaded 09/29/21 12:43 PM UTC 15 MARCH 2011 G A R C I´ A-SERRANO ET AL. 1619 although it is statistically significant only during early (NDJF, Fig. 4f) when the anomalous atmospheric cir- winter (ONDJ, (Fig. 3e). culation circumscribes the Northern Hemisphere mid- The analysis of the anomalous rotational circulation latitudes. As can be seen, the spiral shape of the jet also gives insight into other features. On one hand, the stream is apparent just in these three seasons, resulting Atlantic Nin˜ o decaying phase is associated with a con- in an entrance over northern Africa in SOND and over tinuous reduction in magnitude of the local Gill-type the West African coastline in ONDJ–NDJF. In conjunc- response, reflecting the weakening of the atmospheric tion with this feature, a progressive latitudinal shift of response with the thermal forcing decline. On the other the subtropical jet occurs from autumn to winter: 408N hand, a close relationship between a forced extratropical in ASON, 258NinSOND,208NinONDJ,and158Nin response and the latitudinal seasonal cycle seems to be NDJF. These two characteristics of the local background operating during the SST decay. Note that the Rossby flow explain how the Rossby wave train becomes trapped wave train, propagating into the North African–Asian in the North African–Asian jet (Figs. 3d–f), as the vor- jet, reveals a southward migration from late summer ticity source is close to its entrance. (JASO, Fig. 3b) to winter (NDJF, Fig. 3f). This feature Although our results may be model dependent and is especially noticeable at the waveguide–jet entrance, should be taken with caution, the coherence with previ- where the anomalous cyclonic circulation is displaced ous observational (Garcı´a-Serrano et al. 2008) and mod- from 408N (in the Mediterranean Basin) in JASO to eling (Haarsma and Hazeleger 2007) works gives us 158N (over western North Africa) in NDJF. confidence in them. A forward attempt is now made to To better illustrate seasonality of the atmospheric re- explore the reliability of the results within a theoretical sponse to the Atlantic Nin˜ o decaying phase, the model framework. climatological wind at 200 hPa (arrows) is shown in The extratropical propagation of Rossby waves is not Fig. 4. Overplotted in this figure is the anomalous non- solely determined by the tropical energy release but is divergent (or rotational) meridional component of the also influenced by the mean flow in which these waves flow at 200 hPa (contours), as this variable fairly cap- propagate. It is known that, where jet streams are weak, tures the structure of the anomalies embedded in the the Rossby wave propagation tends to present a poleward- waveguide (Branstator 2002). Low-frequency circulation arching pattern (meridional propagation) whereas in anomalies tend to be zonally confined to latitudes where regions where the jet streams are intense the Rossby the tropospheric jets are centered, while they appear to wavetrains tend to be meridionally confined (zonal prop- propagate along a more meridional route where influ- agation) (Hoskins and Karoly 1981; Branstator 1983, ence of the background jets is minimal (Branstator 1983, 2002; Hoskins and Ambrizzi 1993; Ambrizzi et al. 1995). 2002). For stationary conditions, the initial direction of propa- As is well known (Lau and Peng 1992; Ambrizzi et al. gation a is specified by means of the zonal wavenumber k, 1995), Northern Hemisphere jet streams are weaker and the meridional gradient of the planetary vorticity b,and more zonal in summertime (JJAS to ASON, Fig. 4a–c) the zonal mean flow U as than during the winter season (SOND to NDJF, Figs. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4d–f). In JJAS the climatological flow over the North U cosa 5 k . Atlantic is weakest, providing favorable conditions for b ›2U/›y2 a meridional propagation of the atmospheric response (Fig. 3a). Following the increase of North Pacific zonal Figures 5a and 6a show the initial angles of Rossby wave winds, the North Atlantic background flow strengthens propagation, computed for the model climatology. The in JASO and ASON (Figs. 4b,c), which would restrict source regions associated with the Atlantic Nin˜ o decay the meridional extension and force the zonal propaga- have been selected from Fig. 3: 128N, 558W for summer– tion toward central southern Europe (Figs. 3b,c). Note autumn [June–October (JJASO)] and 128N, 308W for the weakening of meridional wind anomalies at polar autumn–winter [October–February (ONDJF)]. In the latitudes during ASON in comparison with JJAS and first season (JJASO, Fig. 5a), this theoretical computa- JASO. tion suggests that the Rossby wave train would be trig- The absence of circulation anomalies over the polar gered at a 5 74.98, thereby, supporting the result of a cap is depicted even better in SOND. During this sea- tropical–extratropical connection via marked meridio- son, a complete circumglobal teleconnection response nal propagation. In the last stages of the forcing (ONDJF, can be identified, mainly based on well-organized wind Fig. 6a), the theoretical initial angle of a Rossby prop- anomalies over the North Pacific and North Atlantic agation would be a 5 42.28, which is entirely consistent basins (Fig. 4d). This hemispheric pattern is pretty ap- with the evidence shown above for the waveguided parent during early winter (ONDJ, Fig. 4e) and winter anomalies.

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FIG. 4. JJAS to NDJF ensemble mean of meridional component of rotational wind at 200 hPa (contours, ci 5 0.5 m s21) from the EMP simulation and climatological flow at 200 hPa (arrows, m s21).

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FIG. 5. Summer–autumn atmospheric circulation, June–October. Ensemble mean of geo-

potential height at (a) 200 hPa (Z200,ci5 10 m) and (b) 500 hPa (Z500,ci5 5 m) from the EMP simulation. Indicated with an arrow in (a) is the initial angle of Rossby propagation at 128N, 558W (see text for details). (c) Climatological zonal flow at 200 hPa (shaded, m s21) and

horizontal wave activity flux associated with the asymmetric part of Z200 in (a). Anomalies larger than 0.3 m2 s22 are shown and the diagnostic has been computed according to Karoly et al. (1989).

These representations are accompanied by the anom- signals at midlatitudes. Likewise, the figures represent alous fields of geopotential height at 200 (Figs. 5a and 6a) a summary picture of the main goal in this study: the and 500 hPa (Figs. 5b and 6b). These circulation anom- Atlantic Nin˜o decaying phase triggers a Rossby wave train alies are shown to help assessing the importance of the crossing the North Atlantic basin during summer–autumn

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FIG. 6. As in Fig. 5 but for autumn–winter (October–February): initial angle of Rossby wave propagation computed at 128N, 308W in (a).

(Fig. 5) and generates a zonal Rossby wave train that is recurrent circumglobal waveguide pattern presented trapped into the North African subtropical jet during by Branstator (2002). Notice how the wavenumber-5 autumn–winter (Fig. 6). As described above, the sum- structure is apparently much better at upper-levels mer–autumn arching pattern finally propagates along the (200 hPa, Figs. 3, 5a, and 6a) than midlevels (500 hPa, westerly jet streams (roughly from the Middle East), re- Figs. 5b and 6b), in both summer–autumn and autumn– sembling a circumglobal atmospheric response. In this winter and how the circulation at 500 hPa fails to reveal way, the result supports the evidence, shown by Ding the tropical disturbances, which lead to the extratropical and Wang (2005, 2007), of a regional way for exciting the propagations.

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To better illustrate the regional energy dispersion, to the Atlantic Nin˜ o, our results agree with those by the wave activity flux is shown in Figs. 5c and 6c for Haarsma and Hazeleger just in part. The EMP simula- summer–autumn and autumn–winter, respectively. This tion suggests that the autumn–winter circumglobal re- has been done by computing the Plumb (1985) diagnostic sponse is more a function of the zonal mean flow than directly from the zonally asymmetric part of the geo- a dependency on local tropical disturbances. The mod- potential height anomalies at 200 hPa, as in Karoly et al. eled atmospheric responses tend to be phase locked to (1989). In JJASO, the largest wave activity divergence the seasonal evolution of both the North African–Asian is reached where the Gill-type response takes place (off- jet entrance over the eastern subtropical Atlantic and shore of northern South America; Figs. 3a,b), leading to the North Atlantic jet exit over high latitudes. In this a Rossby propagation to midlatitudes. The northeastward way, the different Rossby wave responses to the Atlantic wave activity flux, away from the North Atlantic jet, and Nin˜o decay show a time evolution, from an arching pat- the subsequent return to the North African–Asian jet tern in summer, when the jet is weak, to a zonally trapped crossing the Euro–Atlantic sector support the idea of an propagation in winter, when the jet is strong and close arching wave train in response to the first stages of the to the anomalous vorticity source. This feature differs Atlantic Nin˜ o decaying phase (Figs. 5a,b). By contrast, from the Haarsma and Hazeleger (2007) conclusion, no wave activity flux appears outside of the westerly jets which stated that the ability of a SST anomaly in gener- in the North Atlantic basin during ONDJF (Fig. 6c); note ating sufficiently large upper-tropospheric divergence is the wave activity convergence along the North African the key player in the timing of the response. Even so, our jet axis, as the local response (vorticity source) is directly conclusion, using a state-of-the-art AGCM, goes along over the jet stream entrance (Figs. 3e,f). with the proposed mechanism by Haarsma and Hazeleger, who used an AGCM of intermediate complexity—that is, the wintertime response to the Atlantic Nin˜ o shows 4. Summary and discussion a waveguided atmospheric pattern. This result disagrees In this study, we have used an AGCM model (UCLA with the direct (tropical–extratropical) Rossby wave train AGCM) to analyze the atmospheric response to the emerging from the Caribbean, proposed by Dre´villon summer Atlantic Nin˜ o decaying phase during the last et al. (2003), as well as with the one triggered from the decades of the twentieth century. The time period con- central–eastern tropical Atlantic to the North Atlantic, sidered here (1979–2005) is of marked importance be- described in Peng et al. (2005). cause several recent climate trends, and various worldwide Additional theoretical diagnostics shown at the end of phenomena, have been described within it (e.g.; Fedorov section 3 are in close agreement with the evidence and and Philander 2000; Terray and Dominiak 2005; Baines discussion presented previously. Even so, dedicated and Folland 2007; Kucharski et al. 2007; Losada et al. modeling efforts are required to provide further support 2009b; Rodrı´guez-Fonseca et al. 2009). Particularly for the to our conclusions. First, a larger number of members in tropical Atlantic, Polo et al. (2008) and Garcı´a-Serrano the ensemble exercise than in the AMMA protocol (10 et al. (2008) have shown how the recent summer Atlantic members) would result in a more significant signal. Nin˜ o does not persist into the following winter and is Second, a coupled ocean–atmosphere GCM experiment not connected with the Atlantic Nin˜ o II (Okumura and with an initial-value approach of the forcing (as in Wu Xie 2006). et al. 2007) would correct the limited outputs from The simulated rainfall sequence and associated upper- AGCMs and allow one to track the evolution of SST and level divergence anomalies (Fig. 2) point out the pro- the associated atmospheric response. gressive weakening of the anomalous deep convection Finally, the model results have revealed that the in the eastern part of the basin (Gulf of Guinea) and its thermal-forcing evolution of the summer Atlantic Nin˜ o clear confinement in the western part (over the Amazon) does not play an important role in the anomalous dry at the end of the Atlantic Nin˜o forcing—confirming the conditions over the Florida–Gulf of Mexico region, as presence of anomalous convection over the whole se- was proposed from observations (Garcı´a-Serrano et al. quence despite the weakening of SST anomalies. 2008). As also shown here in Fig. 1, a developing La Nin˜a Our model results show that the Atlantic Nin˜odecay- episode in the tropical Pacific is apparent in conjunc- ing phase is able to trigger a hemispheric teleconnection tion with the Atlantic Nin˜ o decaying phase. This feature during late autumn and winter and that this anomaly points at the El Nin˜ o–Southern Oscillation (ENSO) becomes entrapped in the westerly jets, as suggested as a potential player in producing precipitation anom- by Garcı´a-Serrano et al. (2008) following the work of alies over the western subtropical basin. Indeed, sev- Haarsma and Hazeleger (2007). Nevertheless, regarding eral observational and modeling studies have shown the seasonal dependence of the atmospheric response how that region has a large winter response to ENSO

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(Ropelewski and Halpert 1987, 1989, 1996; Kiladis and ——, and ——, 2007: Intraseasonal teleconnection between the Diaz 1989; Trenberth et al. 1998; Giannini et al. 2000, summer Eurasian wave train and the Indian monsoon. J. Cli- 2004; Alexander et al. 2002). According to this, some mate, 20, 3751–3767. Dre´villon, M., C. Cassou, and L. Terray, 2003: Model study of the works present evidence of a strong correlation between North Atlantic region atmospheric response to autumn trop- the summer Atlantic Nin˜ o and subsequent wintertime ical Atlantic sea surface temperature anomalies. Quart. J. Roy. ENSO when focusing on the period of study used in this Meteor. Soc., 129, 2591–2611. work, pointing to a precursor of the tropical Atlantic on Fedorov, A., and S. G. Philander, 2000: Is El Nin˜ o changing? Sci- the tropical Pacific (Melice and Servain 2003; Keenlyside ence, 288, 1997–2002. Frankignoul, C., and E. Kestenare, 2005a: Air–sea interactions in and Latif 2007; Polo et al. 2008). More recently, some the Tropical Atlantic: A view based on lagged rotated maxi- modeling studies (Rodrı´guez-Fonseca et al. 2009; Losada mum covariance analysis. J. Climate, 18, 3874–3890. et al. 2009b) have also shown how, during the last de- ——, and ——, 2005b: Observed Atlantic SST anomaly impact on cades of the twentieth century, the Atlantic Nin˜ o is able the NAO: An update. J. Climate, 18, 4089–4094. to generate an atmospheric response over the tropical Garcı´a-Serrano, J., T. Losada, B. Rodrı´guez-Fonseca, and I. Polo, 2008: Tropical Atlantic variability modes (1979–2002). Part II: Pacific that would favor the development of an anomalous Time-evolving atmospheric circulation related to SST-forced SST pattern of opposite sign in the tropical Pacific, thus tropical convection. J. Climate, 21, 6476–6497. confirming the potential link between Atlantic and Pacific Giannini, A., Y. Kushnir, and M. A. Cane, 2000: Interannual var- equatorial SST anomalies. This Atlantic–Pacific connec- iability of Caribbean rainfall, ENSO, and the Atlantic Ocean. tion could have a modulating role on the extratropical J. Climate, 13, 297–311. ——, R. Saravanan, and P. Chang, 2004: The preconditioning role atmospheric response to the Atlantic Nin˜ o decaying of Tropical Atlantic Variability in the development of the phase, as well as on its influence on the European cli- ENSO teleconnection: Implications for the prediction of mate. This is a topic of research that requires further Nordeste rainfall. Climate Dyn., 22, 839–855. study and will be the focus of a following work. Haarsma, R. J., and W. Hazeleger, 2007: Extratropical atmospheric response to equatorial Atlantic cold tongue anomalies. J. Cli- Acknowledgments. We thank Dr. Reindert Haarsma mate, 20, 2076–2091. Hoskins, B. J., and D. J. Karoly, 1981: The steady linear response (KNMI, The Netherlands) for his useful discussions. of a spherical atmosphere to thermal and orographic forcing. We also acknowledge the comments and queries from J. Atmos. Sci., 38, 1179–1196. two anonymous reviewers, which have improved the ——, and T. Ambrizzi, 1993: Rossby wave propagation on a re- clarity and scope of the manuscript. This research was alistic longitudinally varying flow. J. Atmos. Sci., 50, 1661– supported by the European AMMA project from the 1671. Huang, B., and J. Shukla, 2005: Ocean–atmosphere interactions 6th Framework Research Programme, and the national in the tropical and subtropical Atlantic Ocean. J. Climate, 18, projects CGL2006-04471, CGL2009-10285, and MOVAC- 1652–1672. 200800050084028. Karoly, D. J., R. A. Plumb, and M. Ting, 1989: Examples of the horizontal propagation of quasi-stationary waves. J. Atmos. Sci., 46, 2802–2811. Keenlyside, N. S., and M. Latif, 2007: Understanding equatorial REFERENCES Atlantic interannual variability. J. Climate, 20, 131–142. Alexander, M. A., I. Blade´, M. Newman, J. R. Lanzante, N.-C. Lau, Kiladis, G. N., and H. F. Diaz, 1989: Global climatic anomalies and J. D. Scott, 2002: The atmospheric bridge: The influence of associated with extremes in the Southern Oscillation. J. Cli- ENSO teleconnections on air–sea interaction over the global mate, 2, 1069–1090. oceans. J. Climate, 15, 2205–2231. Kucharski, F., A. Bracco, J. H. Yoo, and F. Molteni, 2007: Low- Ambrizzi, T., B. J. Hoskins, and H.-H. Hsu, 1995: Rossby wave frequency variability of the Indian monsoon–ENSO relation- propagation and teleconnection patterns in the austral winter. ship and the tropical Atlantic: The ‘‘weakening’’ of the 1980s J. Atmos. Sci., 52, 3661–3672. and 1990s. J. Climate, 20, 4255–4266. Baines,P.G.,andC.K.Folland,2007:Evidenceforarapid ——, ——, ——, and ——, 2008: Atlantic forced component of the global climate shift across the late 1960s. J. Climate, 20, 2721– Indian monsoon interannual variability. Geophys. Res. Lett., 2744. 35, L04706, doi:10.1029/2007GL033037. Branstator, G., 1983: Horizontal energy propagation in a baro- ——, ——, ——, A. Tompkins, L. Feudale, P. Ruti, and tropic atmosphere with meridional and zonal structure. J. At- A. Dell’Aquila, 2009: A Gill–Matsuno-type mechanism ex- mos. Sci., 40, 1689–1708. plains the tropical Atlantic influence on African and Indian ——, 2002: Circumglobal teleconnections, the jet stream wave- monsoon rainfall. Quart. J. Roy. Meteor. Soc., 135, 569–579. guide, and the North Atlantic Oscillation. J. Climate, 15, 1893– Lau, K.-M., and L. Peng, 1992: Dynamics of atmospheric tele- 1910. connections during the Northern summer. J. Climate, 5, Carton, J. A., X. Cao, B. S. Giese, and A. M. Da Silva, 1996: De- 140–158. cadal and interannual SST variability in the tropical Atlantic Losada, T., B. Rodrı´guez-Fonseca, S. Janicot, S. Gervois, F. Chauvin, Ocean. J. Phys. Oceanogr., 26, 1165–1175. and P. Ruti, 2009a: A multimodel approach to the Atlantic Ding, Q., and B. Wang, 2005: Circumglobal teleconnection in the Equatorial mode. Impact on the West African monson. Climate Northern Hemisphere summer. J. Climate, 18, 3483–3505. Dyn., 35, 29–43.

Unauthenticated | Downloaded 09/29/21 12:43 PM UTC 15 MARCH 2011 G A R C I´ A-SERRANO ET AL. 1625

——, ——, I. Polo, S. Janicot, S. Gervois, F. Chauvin, and P. Ruti, ——, and ——, 1996: Quantifying Southern Oscillation–precipitation 2009b: Tropical response to the Atlantic Equatorial mode: relationships. J. Climate, 9, 1043–1059. AGCM multimodel approach. Climate Dyn., 35, 45–52. Ruı´z-Barradas, A., J. A. Carton, and S. Nigam, 2000: Structure Mechoso, C. R., J. Y. Yu, and A. Arakawa, 2000: A coupled of interannual-to-decadal climate variability in the Tropical GCM pilgrimage: From climate catastrophe to ENSO simu- Atlantic sector. J. Climate, 13, 3285–3297. lationsGeneral Circulation Model Development: Past, Pres- Smith, T. M., and R. W. Reynolds, 2003: Extended reconstruction ent and Future. Proceedings of a Symposium in Honor of of global sea surface temperatures based on COADS data Professor Akio Arakawa, D. A. Randall, Ed., Academic, (1854–1997). J. Climate, 16, 1495–1510. 539–575. Sutton, R. T., S. P. Jewson, and D. P. Rowell, 2000: The elements of Melice, J. L., and J. Servain, 2003: The tropical Atlantic meridional climate variability in the tropical Atlantic region. J. Climate, SST gradient index and its relationships with the SOI, NAO 13, 3261–3284. and Southern Ocean. Climate Dyn., 20, 447–464. Terray, P., and S. Dominiak, 2005: Indian Ocean sea surface tem- Okumura, Y., and S.-P. Xie, 2006: Some overlooked features of perature and El Nin˜ o–Southern Oscillation: A new perspec- tropical Atlantic climate leading to a new Nin˜o-like phenome- tive. J. Climate, 18, 1351–1368. non. J. Climate, 19, 5859–5874. Trenberth,K.E.,G.W.Branstator,D.Karoly,A.Kumar,N.-C.Lau, Peng, S., W. A. Robinson, S. Li, and M. P. Hoerling, 2005: Tropical and C. Ropelewski, 1998: Progress during TOGA in understand- Atlantic SST forcing of coupled North Atlantic seasonal re- ing and modeling global teleconnections associated with tropical sponses. J. Climate, 18, 480–496. seas surface temperatures. J. Geophys. Res., 103, 14 291–14 324. Plumb, R. A., 1985: On the three-dimensional propagation of sta- Vizy, E. K., and K. H. Cook, 2002: Development and application of tionary waves. J. Atmos. Sci., 42, 217–229. a mesoscale climate model for the tropics: Influence of sea Polo, I., B. Rodrı´guez-Fonseca, T. Losada, and J. Garcı´a-Serrano, surface temperature anomalies on the West African monsoon. 2008: Tropical Atlantic variability modes (1979–2002). Part I: J. Geophys. Res., 107, 4026, doi:10.1029/2001JD000686. Time-evolving SST modes related to West African rainfall. von Storch, H., and F. W. Zwiers, 2001: Statistical Analysis in Cli- J. Climate, 21, 6457–6475. mate Research. Cambridge University Press, 484 pp. Richter, I., C. R. Mechoso, and A. W. Robertson, 2008: What de- Wang, C., 2006: An overlooked feature of tropical climate: Inter- termines the position and intensity of the South Atlantic an- Pacific–Atlantic variability. Geophys. Res. Lett., 33, L12702, ticyclone in austral winter? J. Climate, 21, 214–229. doi:10.1029/2006GL026324. Rodrı´guez-Fonseca, B., I. Polo, J. Garcı´a-Serrano, T. Losada, Wu, L., F. He, Z. Liu, and C. Li, 2007: Atmospheric teleconnec- E. Mohino, C. R. Mechoso, and F. Kucharski, 2009: Are At- tions of tropical Atlantic variability: Interhemispheric, tropical– lantic Nin˜os enhancing Pacific ENSO events in recent decades? extratropical, and cross-basin interactions. J. Climate, 20, Geophys. Res. Lett., 36, L20705, doi:10.1029/2009GL040048. 856–870. Ropelewski, C. F., and M. S. Halpert, 1987: Global- and regional- Xie, S.-P., and J. A. Carton, 2004: Tropical Atlantic variability: scale patterns associated with the El Nin˜ o–Southern Oscilla- Patterns, mechanisms, and impacts. Earth Climate: The Ocean– tion. Mon. Wea. Rev., 115, 1606–1626. Atmosphere Interaction, C. Wang et al., Eds., Geophys. Mon- ——, and ——, 1989: Precipitation patterns associated with the ogr., Vol. 147, Amer. Geophys. Union, 121–142. high index phase of the Southern Oscillation. J. Climate, 2, Zebiak, S. E., 1993: Air–sea interaction in the equatorial Atlantic 268–284. region. J. Climate, 6, 1567–1586.

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