Upper tropospheric water vapour variability at high latitudes -- Part 1: Influence of the annular modes.

Seasonal and monthly zonal medians of water vapour in the upper troposphere and lower stratosphere (UTLS) are calculated for both Atmospheric Chemistry Experiment (ACE) instruments for the northern and southern high-latitude regions (60-90 and 60-90° S). Chosen for the purpose of observing high-latitude processes, the ACE orbit provides sampling of both regions in eight of 12 months of the year, with coverage in all seasons. The ACE water vapour sensors, namely MAESTRO (Measurements of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) and the Fourier Transform Spectrometer (ACE-FTS) are currently the only satellite instruments that can probe from the lower stratosphere down to the mid-troposphere to study the vertical profile of the response of UTLS water vapour to the annular modes. The Arctic oscillation (AO), also known as the northern annular mode (NAM), explains 64% (r = -0.80) of the monthly variability in water vapour at northern highlatitudes observed by ACE-MAESTRO between 5 and 7km using only winter months (January to March 2004-2013). Using a seasonal timestep and all seasons, 45%of the variability is explained by the AO at 6.5 ± 0.5 km, similar to the 46% value obtained for southern high latitudes at 7.5 ± 0.5 km explained by the Antarctic oscillation or southern annular mode (SAM). A large negative AO event in March 2013 produced the largest relative water vapour anomaly at 5.5 km (+70 %) over the ACE record. A similarly large event in the 2010 boreal winter, which was the largest negative AO event in the record (1950-2015), led to > 50% increases in water vapour observed by MAESTRO and ACE-FTS at 7.5 km. [ABSTRACT FROM AUTHOR]