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

Abstract

Abstract. 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 high-latitudes observed by ACE-MAESTRO between 5 and 7 km 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.

Water vapour variability in the high-latitude upper troposphere – Part 2: Impact of volcanic eruptions

“…This data set has been validated as discussed in the companion paper (Sioris et al, 2015). Over the microwindows used to retrieve water vapour from ACE-FTS spectra , absorption by this trace gas is completely uncorrelated with the spectrally smooth aerosol extinction signature.…”

“…MAESTRO is currently the only satellite instrument capable of simultaneously measuring vertical profiles of both water vapour and extinction by fine aerosols (Sioris et al, 2010b) down to the mid-troposphere. The MAESTRO water vapour retrieval relies on the 940 nm absorption band and is described by Sioris et al (2010a) and has been updated recently (Sioris et al, 2015). The water vapour profiles have ∼ 1 km vertical resolution (Sioris et al, 2010a).…”

“…The first step toward accurate trends is to improve our understanding of UTWV variability at high latitudes. The variability of upper tropospheric water vapour (UTWV) at high latitudes is dominated by dynamics (Sioris et al, 2015). In this companion paper, a second phenomenon is identified that contributes secondarily to the variability of UTWV: volcanic eruptions.…”

“…The ACE-Imager NIR data at northern high latitudes in May 2010 confirm an aerosol layer at 7.5 ± 0.5 km (not shown). The Arctic oscillation would be expected to increase water vapour by < 8 % at 8.5-9.5 km in May 2010 according to the regression using year-round monthly sampled data as determined in the companion paper (Sioris et al, 2015), and is thus insufficient to explain the increase. Also, although dehydrated and rehydrated layers were observed in the 2010 winter (Khaykin et al, 2013), water vapour in the upper troposphere and lower stratosphere (UTLS, 5-20 km) in the northern high-latitude region in March 2010 was normal according to both MAE-STRO and ACE-FTS.…”

“…To obtain a water vapour relative anomaly time series for the UTLS, the method is described by Sioris et al (2015). The monthly climatology, used to deseasonalize the time series, is generated by averaging the monthly medians over the populated years, with a minimum sample size of 20 observations per altitude bin in each individual month.…”