How Rossby wave breaking modulates the water cycle in the North Atlantic trade wind region

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.

A climatology of trade-wind cumulus cold pools and their link to mesoscale cloud organization

“…The occurrence of the Fish pattern is strongly connected to trailing cold fronts of extratropical origins (Aemisegger et al, 2021;Schulz et al, 2021). The more front-like character of the Fish cold pools with steady showers and downdrafts is clearly evident in the example timeseries.…”

Lagrangian formation pathways of moist anomalies in the trade-wind region during the dry season: two case studies from EUREC<sup>4</sup>A

“…S1.1 and S1.2 in Supplement 1 for other time steps) to get an impression of the pathway variability across the vertical profile and during the EUREC 4 A time period. The choice of four days is inspired by the study of Aemisegger et al (2021), who showed that the transport during this pre-arrival time window is essential for understanding moisture anomalies induced by the large-scale circulation in the trades. Typically, multiple airstreams (coherent bundles of trajectories with similar characteristics) arrive in each of the two layers during 24 h. For each airstream, defined in Sect.…”

“…An EDI day is identified if at least 5% of the trajectories arriving from 00 to 23 UTC between 1000 and 650 hPa over the BCO descended 400 hPa (48 h) −1 within the four days prior to arrival. The considered time span of four days ensures that only EDIs subsiding over the North Atlantic (corresponding to the extratropical dry intrusion regime in Aemisegger et al (2021), their Fig. 3a) and not over Europe/Africa (corresponding to the extratropical trade-wind regime in Aemisegger et al (2021), their Fig.…”

“…The considered time span of four days ensures that only EDIs subsiding over the North Atlantic (corresponding to the extratropical dry intrusion regime in Aemisegger et al (2021), their Fig. 3a) and not over Europe/Africa (corresponding to the extratropical trade-wind regime in Aemisegger et al (2021), their Fig. 3b) are selected.…”

“…The Lagrangian perspective is useful to investigate such disturbances as it allows us to identify the origin and transport history of individual air parcels eventually arriving in the trade-wind region. This approach has been adopted previously in several studies, e.g., to investigate the origin of dry and humid layers in the (sub)tropics (Yoneyama and Parsons, 1999;Waugh, 2005 et al, 2005, 2007) or to link the cloudiness in Barbados to the large-scale transport (e.g., Aemisegger et al, 2021;Schulz et al, 2021). Here, we use it to study the formation of moist anomalies above Barbados and, more specifically, to identify air parcels that (1) rapidly descend from the upper-level extratropics towards Barbados, so-called dry intrusions (DI, e.g., Browning and Roberts, 1994;Browning and Golding, 1995;Wernli, 1997, we use here the abbreviation EDI to emphasize the extratropical origin); or (2) flow out of deep and mid-level convective systems over tropical South America and spread over Barbados at mid (e.g., Johnson et al, 1996Johnson et al, , 1999Stevens et al, 2017) or upper levels.…”