Warm Conveyor Belts in the ERA-Interim Dataset (1979–2010). Part II: Moisture Origin and Relevance for Precipitation

Abstract

he role of moisture for extratropical atmospheric dynamics is particularly pronounced within warm conveyor belts (WCBs), which are characterized by intense latent heat release and precipitation formation. Based on the WCB climatology for the period 1979?2010 presented in Part I, two important aspects of the WCB moisture cycle are investigated: the evaporative moisture sources and the relevance of WCBs for total and extreme precipitation. The most important WCB moisture source regions are the western North Atlantic and North Pacific in boreal winter and the South Pacific and western South Atlantic in boreal summer. The strongest continental moisture source is South America. During winter, source locations are mostly local and over the ocean, and the associated surface evaporation occurs primarily during 5 days prior to the start of the WCB ascent. Long-range transport and continental moisture recycling are much more important in summer, when a substantial fraction of the evaporation occurs more than 10 days before the ascent. In many extratropical regions, WCB moisture supply is related to anomalously strong surface evaporation, enforced by low relative humidity and high winds over the ocean. WCBs are highly relevant for total and extreme precipitation in many parts of the extratropics. For instance, the percentage of precipitation extremes directly associated with a WCB is higher than 70%?80% over southeastern North America, Japan, and large parts of southern South America. A proper representation of WCBs in weather forecast and climate models is thus essential for the correct prediction of extreme precipitation events.