Abstract
Abstract. The interaction between low-level tropical clouds and the large-scale circulation is a key feedback element in our climate system, but our understanding of it is still fragmentary. In this paper, the role of upper-level extratropical dynamics for the development of contrasting shallow cumulus cloud patterns in the western North Atlantic trade wind region is investigated. Stable water isotopes are used as tracers for the origin of air parcels arriving in the sub-cloud layer above Barbados, measured continuously in water vapour at the Barbados Cloud Observatory during a 24 d measurement campaign (isoTrades, 25 January to 17 February 2018). These data are combined with a detailed air parcel back-trajectory analysis using hourly ERA5 reanalyses of the European Centre for Medium-Range Weather Forecasts. A climatological investigation of the 10 d air parcel history for January and February in the recent decade shows that 55 % of the air parcels arriving in the sub-cloud layer have spent at least 1 d in the extratropics (north of 35∘ N) before arriving in the eastern Caribbean at about 13∘ N. In 2018, this share of air parcels with extratropical origin was anomalously large, with 88 %. In two detailed case studies during the campaign, two flow regimes with distinct isotope signatures transporting extratropical air into the Caribbean are investigated. In both regimes, the air parcels descend from the lower part of the midlatitude jet stream towards the Equator, at the eastern edge of subtropical anticyclones, in the context of Rossby wave breaking events. The zonal location of the wave breaking and the surface anticyclone determine the dominant transport regime. The first regime represents the “typical” trade wind situation, with easterly winds bringing moist air from the eastern North Atlantic into the Caribbean, in a deep layer from the surface up to ∼600 hPa. The moisture source of the sub-cloud layer water vapour is located on average 2000 km upstream of Barbados. In this regime, Rossby wave breaking and the descent of air from the extratropics occur in the eastern North Atlantic, at about 33∘ W. The second regime is associated with air parcels descending slantwise by on average 300 hPa (6 d)−1 directly from the north-east, i.e. at about 50∘ W. These originally dry airstreams experience a more rapid moistening than typical trade wind air parcels when interacting with the subtropical oceanic boundary layer, with moisture sources being located on average 1350 km upstream to the north-east of Barbados. The descent of dry air in the second regime can be steered towards the Caribbean by the interplay of a persistent upper-level cut-off low over the central North Atlantic (about 45∘ W) and the associated surface cyclone underneath. The zonal location of Rossby wave breaking and, consequently, the pathway of extratropical air towards the Caribbean are shown to be relevant for the sub-cloud layer humidity and shallow-cumulus-cloud-cover properties of the North Atlantic winter trades. Overall, this study highlights the importance of extratropical dynamical processes for the tropical water cycle and reveals that these processes lead to a substantial modulation of stable water isotope signals in the near-surface humidity.
Highlights
Understanding and correctly predicting the patterns of shallow cloudiness over the trade-wind-dominated tropical oceans is one of the current big challenges of climate research (Bony and Stevens, 2012)
We show that in winter, the dynamics of the large-scale descent in the subtropics is essential for the variability in the stable water isotopes, the sub-cloud layer humidity and the low-level cumulus cloud cover in the western North Atlantic trade wind region
The stable water isotope signals in vapour and precipitation from a 24 d measurement campaign in January and February 2018 at the Barbados Cloud Observatory (BCO) are used as tracers to study the properties of two transport regimes associated with extratropical dynamics: the extratropical trade wind flow (61 % occurrence frequency in 2018) and extratropical dry intrusions (32 %)
Summary
Understanding and correctly predicting the patterns of shallow cloudiness over the trade-wind-dominated tropical oceans is one of the current big challenges of climate research (Bony and Stevens, 2012). The interaction of these low-level clouds with their large-scale environment and their impact on Earth’s radiation budget are key feedback elements in our climate system (Bony et al, 2015). The balance between convective drying by mixing in free-tropospheric dry air and turbulent moistening of the boundary layer by ocean evaporation under different large-scale forcing situations is an important element in the process chain of shallow-cumulus-cloud formation. The three main components of the boundary layer moisture budget (Risi et al, 2019), namely (1) ocean evaporation, (2) convective drying and (3) moistening by hydrometeor evaporation, carry a distinct signature in their stable water isotope composition (Aemisegger et al, 2015; Benetti et al, 2015; Sodemann et al, 2017; Aemisegger and Sjolte, 2018; Graf et al, 2019)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
