Relationships between the North Atlantic Oscillation and isentropic water vapor transport into the lower stratosphere

Five winters of Microwave Limb Sounder (MLS) upper tropospheric water vapor observations are used to examine the relationship between the North Atlantic Oscillation (NAO) and isentropic water vapor transport into the lower stratosphere over the North Atlantic. Daily maps of MLS water vapor at 215 hPa show the frequent redistribution of dry stratospheric/moist tropospheric air across the tropopause associated with wave breaking events. Relationships between the NAO and wave breaking have been shown by several recent studies, suggesting that the NAO may also influence the amount and orientation of cross-tropopause water vapor transport in the region. A statistically significant correlation is observed between positive phases of the NAO and water vapor transport into the lower stratosphere. Although large water vapor transport into the stratosphere is consistently associated with a positive phase NAO index, the converse is not necessarily true: positive NAO events are not always accompanied by water vapor transport. The relationship is viable only during northern hemisphere winter, when wave breaking is strong.