Tropical origins for recent and future Northern Hemisphere climate change

Abstract

During the past decades, the North Atlantic Oscillation (NAO) exhibited a long, positive trend, characterised by a strengthening of the wintertime westerly flow over the North Atlantic region, a strong warming trend over the Eurasian continent and falling temperatures over northeastern North America. At the same time, the global mean temperature increased. This global warming has been attributed to the increased concentrations of greenhouse gases in the atmosphere. But does the NAO trend also find its origin in the enhanced greenhouse effect ?

Results from a 62-member ensemble of coupled climate model simulations over the period 1940-2080 will be discussed which suggest that the observed NAO trend is not due to the enhanced greenhouse effect but is largely an expression of a random, internal climate variation driven by increased precipitation over the tropical Indian Ocean. Instead, the enhanced greenhouse effect drives a change in the extra-tropical winter circulation through intensified precipitation over the tropical West Pacific. This change is characterized by a wave train encompassing the whole Northern hemisphere, a pattern known as the Circumglobal Waveguide Pattern.