ORIGINAL PAPER Changes in Hadley circulation: the Azores high and winter precipitation over tropical northeast Africa M. J. Iqbal 1 & S. U. Rehman 2 & Sultan Hameed 3 & M. A. Qureshi 1 Received: 10 November 2018 /Accepted: 2 January 2019 # Springer-Verlag GmbH Austria, part of Springer Nature 2019 Abstract We present a study of changes in winter precipitation over a region in tropical northeast Africa during 1951–2015. A persistent drying trend is present in this region in addition to interannual variability. We present evidence that the changes in precipitation are primarily influenced by a strengthening of Azores high (AH) pressure. The correlation of AH pressure to changes in precipitation in tropical northeast Africa is statistically more significant than correlations between precipitation and the Arctic Oscillation (AO), the Southern Oscillation (SO), or the North Atlantic Oscillation (NAO). It is also shown that the AH pressure in winter has steadily increased in recent decades which explains the drying trend over tropical northeast Africa over the same period. The AH represents the subsiding branch of Hadley circulation over the north Atlantic sector, i.e., increasing pressure of the AH is evidence of intensification of Hadley circulation in the north Atlantic region since the 1950s. 1 Introduction Several studies have discussed the annual march of precipita- tion and its variability over the African continent (e.g., Kidson 1977; Nicholson 1981; Nicholson 2000; Hsu and Wallace 1976; Giannini et al. 2008). As can be expected of a large continent, there are significant differences in the characteris- tics of precipitation over different subregions of Africa. Some studies have discussed the variability of climate over northern Africa due to NAO (e.g., Meehl and Van Loon 1979; Lamb and Peppler 1987; McHugh and Rogers 2001). Meehl and Van Loon (1979) suggested that air temperature fluctuation between northern Europe and Greenland was associated with latitudinal shifts in January rainfall around the southeast inter- tropical convergence zone (ITCZ). During the BGreenland Below^ Januaries rainfall was higher south of 15° S latitude than during Januaries dominated by the opposite seesaw phase. This suggests that in the northern winter season, the ITCZ over east Africa shifts southward when the NAO is in a positive phase, i.e., the westerlies are anomalously strong. Nicholson (1996) stated that westerly influx of air into equa- torial Africa brings rainfall. More recently, McHugh and Rogers (2001) also described that the relationship between precipitation variability over southeast Africa and NAO is significant. The purpose of this study is to show that the effect of NAO in tropical northeast Africa occurs through changes in the pressure system of Azores high (AH). The difference in pressure anomalies at a station in the tropics such as Azores island or Lisbon and a station in Iceland is defined as NAO (Hurrell 1995). These station measurements aim to represent changes in the semipermanent extended pressure systems, the AH and the Icelandic low (IL) respectively, which are prominently visible in monthly or seasonal maps of sea level pressure. It is not surprising that rainfall anom- alies in the tropical sections of Africa would be influenced by changes in the AH and not in the IL which is situated in temperate latitudes. It is therefore more direct to study the relationship of precipitation in this region to changes in the AH rather than to changes in the NAO index. The relation- ship of precipitation with the AH is interesting for the additional reason that the high represents the downward branch of Hadley circulation in the north Atlantic sector. At ocean’ s surface, the descending branch of Hadley cell (HC) is associated with changes in subtropical ridge (STR), a high pressure belt over the subtropics. The descending branch of HC is also responsible for subsidence in both hemispheres. * S. U. Rehman saqib.rehman@uok.edu.pk 1 Institute of Space and Planetary Astrophysics, University of Karachi, Karachi, Pakistan 2 Department of Mathematics, University of Karachi, Karachi, Pakistan 3 School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY, USA Theoretical and Applied Climatology https://doi.org/10.1007/s00704-019-02765-4