Extratropical Atmospheric Response to the Atlantic Niño Decaying Phase

Abstract

The Atlantic Niño or Atlantic Equatorial Mode (EM) is the dominant coupled variability phenomenon in the tropical Atlantic basin during boreal summer. From the 1970s, the mode has changed, evolving in time from east to west and without persisting until the following winter. In a previous observational work, the authors have studied the atmospheric response to the EM during the 1979?2005 period, proposing three main issues along the decaying phase of this mode: 1) the continuous confinement of the anomalous deep convection over northeastern Brazil following the thermal-forcing decay; 2) an increasing dipole-like precipitation anomaly with dry conditions in the Florida?Gulf of Mexico region; and 3) the excitation of Rossby waves forced by the remaining upper-tropospheric divergence that are trapped into the subtropical jet but do not show a robust impact on the European sector. In this work, a 10-member ensemble simulation for the recent EM with the University of California, Los Angeles AGCM model has been analyzed for assessing the evolution of the atmospheric response to the summer Atlantic Niño decay. Results from the sensitivity experiment support that the former and the latter findings can be interpreted in terms of the Atlantic thermal forcing; while the negative rainfall anomalies in the western subtropical basin require an external forcing outside the tropical Atlantic. Prior studies point at the peaking Pacific El Niño as a potential player. An important conclusion of this work is that the seasonal atmospheric response to the Atlantic Niño decaying phase is mainly determined by the climatological jet stream?s position and intensity. In this way, this response shows an arching pattern over the North Atlantic region during summer?autumn and a zonally oriented wave train during autumn?winter.