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Extratropical Atmospheric Response to the Atlantic Nin˜O Decaying Phase

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Extratropical Atmospheric Response to the Atlantic Nin˜O Decaying Phase 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 trade winds, redistribution of warm waters extratropical teleconnections 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 Ó 2011 American Meteorological Society Unauthenticated | Downloaded 09/29/21 12:43 PM UTC 1614 JOURNAL OF CLIMATE VOLUME 24 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 Atlantic ocean (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 seasons 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 Monsoon 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. Unauthenticated | Downloaded 09/29/21 12:43 PM UTC 1616 JOURNAL OF CLIMATE VOLUME 24 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 season. 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.
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