It is thought that this was due to melting glaciers altering the salinity and temperature of the sea, and thus the strength of the Atlantic Thermohaline Circulation (ATHC) (a global 'conveyor belt' of ocean currents). In the Gulf of Guinea the sea surface temperature (SST) drops from 28°C in May to 23°C in August as the season changes. This seasonal change in SST widens the difference in temperature between the land and the sea, and brings about the annual onset of the African Monsoon. The SST seasonal cycle, however, depends on the condition of the ATHC.
The researchers investigated whether melting polar ice-caps, a potential scenario of modern-day climate change, could weaken the ATHC again and lead to the same monsoon failures. They used a climate model to simulate the effect of increased fresh water from melting polar ice-caps in the north Atlantic over a sixty year period.
The model showed that the polar meltwater led to strong sea cooling of 4°C north of the equator and warming of 1°C in the south. These changes were attributed to a substantially weakened ATHC and consequently reduced winds and rainfall that form part of the African monsoon.
The cooling effect was estimated to reach the tropics in less than a decade. More drastic effects were predicted to take place 25-35 years after the onset of increased freshwater, resulting in a rapidly weakened ATHC. The model showed that when the ATHC is weakened by meltwater from the north, the subsurface North East Brazil current is reversed, disrupting the temperature differences between land and sea that favour the African monsoon.
The authors identified the north-eastern coast of South America, where the North East Brazil Current originates, as an important region for observing key changes in the interactions between the North East Brazil and ATHC currents. Monitoring this area will help increase understanding of the ATHC's effects on the climate in the tropical Atlantic.
The authors believe that the hydrological changes revealed by the model could include a reduction in seasonal variation of SST over the Gulf of Guinea, changes of up to 60 per cent in seasonal rainfall and conditions which could lead to a permanent Atlantic Niño-like state. Atlantic Niño is similar to El Niño (in the Pacific), which intermittently causes the SST to rise and leads to a more variable climate. These findings heighten the urgency to monitor the ATHC changes.