Simple climatic indices for the tropical Atlantic Ocean and some applications

“Similarly, using an Atlantic wind stress and SST product (Servain 1991) 1964– 88 gives the same leading Atlantic pattern.”

“Traders and missionaries visiting Greenland were aware of this “teleconnection” between Danish and Greenland climate, although they did not have the necessary measurements for further scientific investigation.”

“Early principal component analyses of SST variability that followed [Weare, 1977; Servain, 1991] seemed to confirm the presence of a pattern of variability in SST that was geographically stationary, with decadal time-scales.”

“…displacement of the Inter Tropical Convergence Zone ITCZ, as demonstrated by various studies (Moura and Shukla 1981, Hastenrath 1990, 2012, Servain 1991, Andreoli et al. 2012, Nobre and Shukla 1996, Uvo et al. 1998, Marengo et al. 2013, 2016b, Rodrigues and McPhaden 2014, Rodrigues et al…”

“…and positive rainfall anomalies along the Guinean coast, to the south, was found to result from a warming in the south equatorial Atlantic and/or cooling in the North Atlantic (Lamb, 1978a,b; Hastenrath, 1984; Lough, 1986; Servain, 1991; Janicot, 1992; Fontaine and Janicot, 1996; Ward, 1998).”

“One of these presumed modes is the ‘Tropical Atlantic Dipole’ (e.g. Servain, 1991) characterized by a north—south interhemispheric gradient in sea surface temperature (SST).”

“A similar calculation was also done by Servain (1991), who used a longer time series and showed distinct decadal variation of SST difference between the north and the south tropical Atlantic.”

“…temperature dipole, a system somewhat analogous to El Niño in the tropical Pacific, indicate that positive feedback between winds and ocean temperatures can occasionally create west to east wind anomalies veering toward the Guinea coast (Servain 1991; Enfield et al. 1999; Jury et al. 2002).”

“…its projection into the tropical Atlantic (e.g., Enfield and Mayer 1997), the tropical Atlantic meridional SST gradient (e.g., Moura and Shukla 1981; Servain 1991; Chang et al. 1997; Xie 1999; Enfield et al. 1999), the equatorial Atlantic El Niño (e.g., Zebiak 1993; Carton and Huang 1994; Latif…”

“On decadal timescales throughout the instrumental period, small, but persistent, SST anomalies develop in the northern and southern tropical Atlantic (Servain, 1991).”

“Some researchers have claimed that this behavior is a characteristic, antisymmetric “dipole” mode (e.g., Weare 1977; Hastenrath 1978; Moura and Shukla 1981; Servain 1991; Nobre and Shukla 1996; Chang et al. 1997; Xie 1999), while studies such as Houghton and Tourre (1992), Enfield and Mayer (1997), Mehta (1998), Enfie…”

“…to decadal time scales it is common to describe tropical Atlantic climate variability in terms of two coupled ocean‐atmosphere modes [e.g., Servain, 1991; Ruiz‐Barradas et al., 2000], namely a meridional or gradient mode which represents an interhemispheric, cross‐equatorial SST…”

“…dipole pattern that is most pronounced on decadal time scales, and is known for its influence on the Atlantic Intertropical Convergence Zone (ITCZ) [Servain, 1991; Nobre and Shukla, 1996] and on rainfall over the Sahel [Folland et al., 1986] and northeast Brazil [Carton et al., 1996; Mehta, 1998].”

“…(Atlantic) and remote (Pacific) origin (Hastenrath and Heller 1977; Moura and Shukla 1981; Ropelewski and Halpert 1987, 1996; Kiladis and Diaz 1989; Servain 1991; Hastenrath and Greischar 1993; Nobre and Shukla 1996; Uvo et al. 1998; Wallace et al. 1998; Saravanan and Chang 2000; Chiang et al.…”

“The definition of these regions follows Servain (1991), who used these indices to form the much-discussed “dipole” of Atlantic SST variability.”

“…event shows many similarities to the Atlantic meridional mode (AMM), which is characterized by changes in SST and winds north and south of the ITCZ causing meridional movement of the ITCZ and consequently possibly impacting Sahel rainfall (Servain 1991; Carton et al. 1996; Chiang and Vimont 2004).”

“Some emphasize the more general inter-hemispheric temperature contrast (Folland et al., 1986, 1991; Servain, 1991).”

“15a) is often portrayed as a dipole (Servain 1991; Nobre and Shukla 1996) but the actual SST variations in the two trade regions do not display an instantaneous nega-”

“Atlantic Meridional Mode [9] The Atlantic Meridional Mode (AMM) is also known historically as the Atlantic Dipole or Interhemispheric Mode [Servain, 1991; Xie and Philander, 1994; Carton et al., 1996] or the tropical Atlantic gradient mode [Chiang et al., 2002].”

“Servain 1991 ) could possibly contribute to the development of the warm bias.”

“, 1999], which is known to influence West African climate [Servain, 1991].”

“This mode has long been linked to regional climate variabilities, for example, precipitation anomalies in the Sahel and the Brazilian Nordeste [ Servain , 1991].”

“3a it is clear that there is a large interannual variability in the number of EWDs, especially with ENSO episodes, such as during La Niña (El Niño) in the years 1989, 1996, 1999, 2000, 2001, 2006 and 2008 (1991, 1992, 1993, 2003 and 2005) where they are generally higher (lower) in number.”

“The southern limit of TS is the one of our data set (20� S). The northern limit of TN is fixed at 28� N, rather than 30� N, in order to preserve an equilibrium between the annual thermal changes of both hemispheric basins ( Servain 1991 ).”

“…variability of the ITCZ position is apparent during the first half of the time series, in agreement with a simultaneous decadal variability of the meridional SST gradient (Servain 1991; Wainer et al. 2008), while the latter half of the record is mainly characterized by a long-term stability.”

“Similarly, the GRADIENT index provides an objective measure of the phases of the Atlantic interhemispheric SST gradient mode (Servain, 1991).”

“…climate in the tropical Atlantic at interannual and decadal timescales are an interhemispheric SST mode often referred to as the Atlantic dipole [Weare, 1977; Servain, 1991; Nobre and Shukla, 1996; Chang et al., 1997] and a zonal equatorial mode [Merle et al., 1980; Hisard, 1980; Philander, 1986].”

“The cool SST in the tropical South Atlantic in conjunction with relatively warm SST conditions in the tropical North Atlantic give rise to a positive meridional SST gradient (Servain 1991; Nobre and Shukla 1996).”

“This mode has been referred to as a SST dipole mode [ Servain, 1991 ].”

“However, although a dipole-like structure appears in a leading EOF of tropical Atlantic SST (e.g., Weare 1977; Hastenrath 1978; Lough 1986; Servain 1991; PM98), such an analysis represents a statistical compromise among regions of high variability, whether or not those regions are dynamically…”

“The dipolar NDVI pattern seems associated with the occurrence of the “tropical Atlantic dipole” (Servain 1991), showing a colder south tropical Atlantic and a warmer north tropical Atlantic when the photosynthetic activity over the Guinean coast (the central Sahel) is low (high).”

“The AMM has also been referred to historically as the Atlantic Dipole or Inter-hemispheric Mode (Servain 1991; Xie and Philander 1994; Carton et al. 1996) or the tropical Atlantic gradient mode (Chiang et al. 2002).”

“The AMM was originally identified by Servain (1991) using a principal component analysis.”

“A strong connection between the Nordeste rainfall and the tropical Atlantic SSTA dipole during rain seasons was confirmed in previous studies (e.g., Servain 1991).”

“Servain, 1991; Uvo et al., 1998), which is in part controlled by ENSO variability but also has a local Atlantic origin (Saravanan and Chang, 2000; Pezzi and Cavalcanti, 2001; Giannini et al., 2004).”

“El Ni˜ no (La Ni˜ na) remotely induces below (above) normal SLP anomalies in the TNA, which reach their maximum values during MAM (Covey and Hastenrath, 1978; Saravanan and Chang, 2000 ).”

“Others have shown a relation between the so-called Atlantic Meridional Mode (Servain 1991)—a phenomenon correlated with AMV—and interannual time scale variability in Atlantic tropical cyclone activity (Vimont and Kossin 2007; Kossin and Vimont 2007; Kossin et al. 2010).”

“The Atlantic meridional mode is characterized by the cross-equatorial meridional gradient of SST anomaly in the tropical Atlantic during boreal spring (Servain 1991; Xie and Carton 2004 for a recent review).”

“Such an SST pattern agrees with the positive impact of the AMM (cold in the North, warm in the South), which is widely discussed in the literature, on the NNEB rainfall variability [1, 39].”

“On the other hand, Latif and Grötzner (2000) found that observed fluctuation favors the quasi-biennial time band (Servain 1991; Ruiz-Barradas et al. 2000), which is more consistent with the delayed oscillator mechanism (Schopf and Suarez 1988) operating in a smaller zonal basin than that of the Pacific.”

“…and Ramankutty, 1994; Chang et al., 1997; Delworth and Mann, 2000;Andreoli and Kayano, 2003;Knight et al., 2005; Rayner et al., 2006; Chylek et al., 2011] and the tropical Atlantic dipole [Servain, 1991; Enfield et al., 1999] and their interactions with the Hadley and Walker circulations.”

“The main mode of decadal SST variability in the tropical Atlantic has been described as a dipole across the equator (Servain 1991).”

“Studies suggest that this mode of variability is associated with the rainfall variability over the Northern South America and the African Sahel [Moura & Shukla, 1981; Servain, 1991; Nobre & Shukla, 1996; Parker et al, 1988].”

“Forecasts over longer periods, extending perhaps up to 6 months, have commonly used statistical procedures that relate streamflow and/or rainfall to explanatory variables such as sea surface temperatures (SST) [Servain, 1991; Robertson and Mechoso, 1998; Diaz et al., 1998; Uvo and Graham, 1998; Hamlet and Lettenmaier,…”

“1d over the same region (Servain 1991; Zebiak 1993).”

“…associated with the so-called Atlantic meridional gradient mode, characterized by the fluctuation of the SST meridional gradient near the equator from interannual to decadal time scales, largely driven by off-equatorial SSTAs (e.g., Servain 1991; Nobre and Shukla 1996; Enfield and Mayer 1997).”

“Following Servain (1991), the TB index is the averaged SSTA from 20°S to 30°N, the NB index represents the averaged SSTA in the north (5°–28°N), the SB index expresses the averaged SSTA in the south (20°S–5°N), and the dipole index is defined as NB–SB.”

“The Atlantic meridional mode is known historically as the Atlantic dipole or interhemispheric mode (Servain 1991; Xie and Philander 1994; Carton et al. 1996) or the tropical Atlantic gradient mode (Chiang et al. 2002).”

“In the tropical Atlantic, a positive Atlantic dipole (Lamb, 1978; Parker et al., 1988; Servain, 1991) is present with a cold anomaly (down to −1 ◦C) along the Equator, which also affects the Atlantic cold tongue in the eastern equatorial basin.”

“…decadal-timescale Atlantic meridional mode, which is characterized by asymmetric SST anomalies about the equator within the tropical Atlantic (e.g., Servain 1991; Servain et al. 1999; Nobre and Shukla 1996; Chang et al. 1997; Penland and Matrosova 1998; Tanimoto and Xie 2002; Chiang Corresponding…”

“0 index (Zebiak, 1993), and the TNA-TSA index (Servain, 1991) to characterize their respective anomalies over the period covered of 1979–2007.”

“Through an analysis of the mixed layer heat balance, we have shown that variations of mixed layer heat storage in the tropical North and South Atlantic Ocean are driven primarily by latent heat loss and shortwave radiation.”

“0 index (Zebiak 1993), and the TNA-TSA index (Servain 1991) to represent their respective anomalies.”

“However, the interannual to decadal variability of the tropical Atlantic Ocean is a strong signal [e.g., Servain, 1991] that provides strong motivation for studying the physics of subtropicaltropical connections within this basin.”

“originally identified by Servain (1991) using a principal component analysis.”

“WAM precipitation has been found relatively well correlated with the Atlantic Meridional Mode (AMM; Servain, 1991; Kossin and Vimont, 2007).”

“The latitudinal annual migration of the ITCZ varies as a function of the surface temperature gradient between the Tropical North Atlantic and Tropical South Atlantic, referred to in literature as the Atlantic Dipole (AD) [17,76,77].”

“1; Servain 1991).”