(1) Hydroclimate data compilation:
For precipitation, we investigated gridded datasets and recently-digitized meteorological station measurements from the German Marine Observatory (Deutsche Seewarte) from former German colonies in Togo and Cameroon recorded from 1888–1914. We assembled long-term discharge records of major low-latitude-Atlantic-flowing rivers, reconstructed from different global and regional data sources back to the late-19th and early-20th centuries: the Amazon, Congo, Orinoco, Paraná, Niger, and Senegal; along with the Nile and its major tributaries. Furthermore, we examined a near-annual gridded compilation of sea surface salinity (SSS), which can be used as an indirect “ocean rain gauge” to indicate surface freshwater flux, covering most of the tropical and subtropical Atlantic north of 20°S beginning in the late-19th century.
(2) Leading variability:
We conducted empirical orthogonal function (EOF) analysis to investigate interannual-to-multidecadal variability in the compiled hydroclimate records. Three significant leading patterns of variability were found in annual maximum and peak seasonal discharge across the different river basins. The first leading pattern projects positively onto the tropical South American rivers (Orinoco and Amazon) and is significantly associated with the eastern Pacific sea surface temperature (SST) anomaly and the Pacific Walker circulation strength. The second leading pattern projects onto significant positive discharge anomalies in the Niger, Senegal, and Congo rivers, and significant negative anomalies in the Paraná River. This cross-Atlantic mode projects onto the Atlantic interhemispheric SST and sea level pressure (SLP) contrasts, and is associated with the latitude of the Atlantic intertropical convergence zone (ITCZ) and the ascending branch of the Hadley circulation. It underwent a dramatic reversal around 1970, coincident with the pronounced rainfall decrease in the Sahel region of North Africa. A third leading pattern features positive discharge anomalies in the Amazon and Congo rivers, and negative anomalies in the Orinoco River. It projects onto the equatorial Atlantic SST anomaly and has predominant variability on interannual timescales.
(3) External forcings:
We examined the late-19th and early-20th century Atlantic hydroclimate response to external forcings using the combined instrumental data and ModE-Sim, a medium-sized ensemble of the ECHAM6 climate model with PMIP4 radiative forcings and observed SST boundary conditions. We find that the 1912 high-latitude Katmai/Novarupta eruption induced pronounced hydrological cycle impacts around the tropical Atlantic: a drastic rainfall decrease across the Sahel; record low discharge the Niger, Senegal, Congo, and Nile rivers (stemming from the Blue Nile and Atbara tributaries which drain the Ethiopian Highlands); and a positive northern tropical Atlantic SSS anomaly. This combined rainfall-discharge-SSS fingerprint indicates a major disruption of the northern African Monsoon and southward displacement of the Atlantic ITCZ in the years following the eruption. Examination of the associated dynamic and thermodynamic processes revealed how hemispherically-asymmetric aerosols from the high-latitude eruption generate an interhemispheric Atlantic SST contrast, displacing the ascending branch of the Hadley circulation southward through cross-equatorial energy transport compensation.
Our results have been presented in international conferences and are being reported in three scientific publications (to be submitted). We also organized an interdisciplinary research workshop which connected researchers working across different aspects of tropical Atlantic hydroclimate using historical instrumental data.
