The North Atlantic Oscillation (NAO) is one of the most important atmospheric teleconnection patterns over the Northern Hemisphere. Its variability has a strong impact on the climate over North Atlantic, North America, Europe, and even downstream Asia (e.g., Walker and Bliss, 1932; van Loon and Rogers, 1978; Wallace and Gutzler, 1981; Barnston and Livezey, 1987; Hurrell, 1995; Chang et al., 2001; Li et al., 2003; Yang et al., 2004; Yu and Zhou, 2004). Previous studies have focused on the impacts of NAO in winter as it is strongest during that period.

Some recent studies have found that the summer NAO (SNAO) is also strong and can influence the Northern Hemispheric air temperature, precipitation, and even the extreme hot events (e.g., Sun et al., 2008; Yuan and Sun, 2009; Folland et al., 2009; del Rí o et al., 2011; Linderholm et al., 2011; Seo et al., 2012). In addition, variability of the SNAO experienced a significant decadal change around the late 1970s (Sun et al., 2008), following which its southern active center shifted eastward from the North Atlantic to the Mediterranean-Black Sea region. Such a change of the SNAO pattern significantly enhanced its impact on the North Hemispheric temperature, precipitation, and extreme hot events (Sun et al., 2008; Yuan and Sun, 2009; Sun and Wang, 2012; Sun, 2012).

Analyzing the observations and performing numerical experiments using an atmospheric general circulation model (AGCM), Sun and Yuan (2009) found that the sea surface temperature (SST) over the Mediterranean-Black Sea played an important role in the decadal change of the SNAO. In addition, Sun et al. (2009) revealed that around the late 1970s, SST over the tropical Atlantic warmed significantly, strengthening the local Hadley circulation over eastern North Atlantic-western Europe region; this in turn enhanced the variability of the circulation over the Mediterranean-Black Sea (eastern part of the SNAO southern center), resulting in an eastward shift of the SNAO southern center. However, these findings of Sun et al. (2009) were based only on the analysis of observations, and hence could not reflect well the cause-effect physical relationship between the tropical Atlantic SST and the circulation variability over the Mediterranean-Black Sea. In this study, we further investigated the impact of the tropical Atlantic SST on the circulation over the Mediterranean-Black Sea region by performing numerical experiments.

The SST dataset used here was the National Oceanic and Atmospheric Administration extended reconstructed SST version 3 (NOAA ERSST V3) (Smith et al., 2008), which was provided by the NOAA/Office of Oceanic and Atmospheric Research (OAR)/Earth System Research Laboratory, Physical Sciences Division (ESRL PSD), Boulder, Colorado, USA. The SST data were stored in a 2° latitude × 2° longitude grid cell. The NOAA ERSST V3 dataset covers a period of more than 100 years. In this study, data covering the period 1948-2011 were used.

The numerical model used in this study was an AGCM developed at the International Centre for Theoretical Physics (ICTP) (Molteni, 2003; Kucharski et al., 2013). The ICTP-AGCM is an intermediate complexity model, which has a horizontal resolution of T30 and eight vertical levels, as well as a fairly comprehensive set of realistic but simplified physical parameterizations. It has been used in research conducted on various subjects such as the NAO (Kucharski et al., 2006a), South Asian monsoon (Kucharski et al., 2006b), El Niño-Southern Oscillation (ENSO) (Bulic and Kucharski, 2011), Sahel rainfall variability (Kucharski et al., 2012), Mediterranean climate (Losada et al., 2011), etc.

SST over the tropical Atlantic has significant effects on climate and biogeochemistry (Carton et al., 1996; Hirst and Hastenrath, 1983; Ruiz-Barradas et al., 2000). Recently, Tokinaga and Xie (2011) have reported that the equatorial Atlantic cold tongue has been weakened over the past six decades, indicating that the tropical Atlantic SST has been warming over past several decades. Sun et al. (2009) found that the tropical Atlantic SST experienced a decadal change around the late 1970s. As shown in Fig. 1a, compared to the period before the late 1970s, SST over the tropical Atlantic has significantly warmed; its warming center is located over the tropical southern Atlantic, with a maximum value of 0.7° C. In order to investigate the impact of such SST warming, we conducted a sensitivity experiment using the ICTP-AGCM. In order to eliminate strong SST gradients over southern and northern flanks of the tropical Atlantic (Fig. 1a), the SST anomalies over the tropical Atlantic were linearly decreased poleward, as shown in Fig. 1b.

Figure 1

Figure 1 (a) Composite differences of the summer (June-September) SST (° C) over the tropical Atlantic between the two subperiods (1980-2011 minus 1948-1979) and (b) idealized SST anomalies (° C) for the sensitivity experiment (EXP).

We performed a 35-year run with the model’ s climatological SST and sea ice boundary conditions and defined the output for the last 30 years as the control run (CTL). A 35-year run was also performed for the sensitivity experiment (EXP), and output of the last 30 years was used for the analysis. The EXP was performed with a combined SST boundary condition: the tropical Atlantic summer warming, as shown in Fig. 1b, was superimposed on the model’ s climatological monthly SST. Differences between EXP and CTL reveal the influences of the imposed boundary forcing of SST after the late 1970s.

In general, a warmer SST can impact its overlying atmosphere by changing the rate of energy transfer to the atmosphere and the hydrological cycle. As observed during this study, the warmer SST over the tropical Atlantic produced a stronger convergence during the period after the late 1970s, resulting in a large zonal belt of enhanced convection activity along the equator (Fig. 2). In addition, convection was decreased over the Sahel, the core region of the western African summer monsoon. It was observed that when the summer precipitation was increased over the Sahel, summer rainfall over the Guinea Coast was generally decreased, and vice versa; this seesaw pattern of the convection over the Sahel and Guinea Coast is consistent with the findings of some previous studies (e.g., Ward, 1998).

Figure 2

Figure 2 Anomalies (EXP-CTL) of the outgoing longwave radiation (W m-2). Yellow (blue) shading shows areas where composite difference is positively (negatively) significant at the 0.05 level.

Anomalous convection activity can stimulate anomalous meridional atmospheric circulations, emanating from the tropical Atlantic poleward to the high latitudes. As shown in Fig. 3, a warmer SST over the tropical Atlantic leds to the enhancement of the local Hadley circulation over the region. The upward (downward) branch of the Hadley circulation was found to be strengthened over the tropical Atlantic (the Sahel region). Along with the enhanced Hadley circulation, the corresponding Ferrell circulation is also strengthened. The variability of the meridional circulation is generally concurrent with the zonal wind. Thus, a warmer SST over the tropical Atlantic can excite a meridional teleconnection pattern over the Eastern Atlantic-North Africa-Western Europe region, impacting the atmospheric circulation over the Extratropics.

Figure 3

Figure 3 Anomalies (EXP-CTL) of latitude-pressure cross-section of (a) meridional circulation (meridional wind: m s-1; vertical velocity: -200 Pa s-1) and (b) zonal wind (m s-1) averaged along 11.25° W-30° E. Yellow (blue) shading shows areas where composite difference is positively (negatively) significant at the 0.05 level.

Figure 4 shows the sea-level pressure (SLP) situation. A warmer SST over the tropical Atlantic led to the development of significant negative SLP anomalies over that region. In addition, significant negative SLP anomalies were also found to exist over the Mediterranean region, indicating that the tropical Atlantic SST could have activate the atmospheric variability over the region (eastern part of the SNAO southern center), consequently favoring the eastward shift of the SNAO southern center. On the decadal timescale, the SNAO has been in the negative phase during the period after the late 1970s as compared to before. Figure 4 shows the anomalous SLPs with a dipole pattern over the North Atlantic region. Such an anomalous distribution indicates that the tropical Atlantic SST has contributed to the decadal phase change of the SNAO. These simulation results are highly consistent with the observations made by Sun et al. (2009). Thus, the sensitivity experiment further confirmed the impact of the tropical Atlantic SST variation has impact on the decadal changes of the SNAO.

Figure 4

Figure 4 Anomalies (EXP-CTL) of the SLP (hPa). Yellow (blue) shading shows areas where composite difference is positively (negatively) significant at the 0.05 level.

Using the ICTP-AGCM, this study investigated how the tropical Atlantic SST decadal change impacted the decadal change of the SNAO pattern around the late 1970s. An analysis of the numerical simulation showed that warming of SST over the tropical Atlantic around the late 1970s could have significantly enhanced the convection over the tropical Atlantic, which further changed the local meridional circulation over the Eastern Atlantic-North Africa-Western Europe region. All these led to the formation of a meridional teleconnection that transferred the influence of the tropical Atlantic SST to the Extratropics. Via these processes, the tropical Atlantic might have activated the variability of the eastern part of the SNAO southern center, consequently leading to an eastward shift of the SNAO southern center around the late 1970s. Such physical processes are highly consistent with those revealed in the observations by Sun et al. (2009), consequently providing a further confirmation of the impact of the tropical Atlantic SST variation on the SNAO decadal changes.