A rare drought in the Amazon Basin culminated in 2005, leading to near record-low streamflows, small Amazon river plume, and greatly enhanced fire frequency. This episode was caused by a combination of 2002-03 El Nino and a dry spell in 2005 attributable to a warm subtropical North Atlantic Ocean. An analysis of 10 precipitation datasets for the period 1979-2005 reveals that Atlantic influence on Amazon rainfall is comparable to the better known Pacific linkage. While El Nino influence is typically locked to the wet season, the 2005 Atlantic impact concentrated in the Amazon dry season when the hydroecosystem was most vulnerable. Such mechanisms may have wide-ranging implications for the past and future changes in the world's largest tropical rainforest in response to natural and anthropogenic perturbations.