The role of air–sea fluxes for the water vapour isotope signals in the cold and warm sectors of extratropical cyclones over the Southern Ocean

Abstract

Abstract. Meridional atmospheric transport is an important process in the climate system and has implications for the availability of heat and moisture at high latitudes. Near-surface cold and warm temperature advection over the ocean in the context of extratropical cyclones additionally leads to important air–sea exchange. In this paper, we investigate the impact of these air–sea fluxes on the stable water isotope (SWI) composition of water vapour in the Southern Ocean's atmospheric boundary layer. SWIs serve as a tool to trace phase change processes involved in the atmospheric water cycle and, thus, provide important insight into moist atmospheric processes associated with extratropical cyclones. Here we combine a 3-month ship-based SWI measurement data set around Antarctica with a series of regional high-resolution numerical model simulations from the isotope-enabled numerical weather prediction model COSMOiso. We objectively identify atmospheric cold and warm temperature advection associated with the cold and warm sector of extratropical cyclones, respectively, based on the air–sea temperature difference applied to the measurement and the simulation data sets. A Lagrangian composite analysis of temperature advection based on the COSMOiso simulation data is compiled to identify the main processes affecting the observed variability of the isotopic signal in marine boundary layer water vapour in the region from 35 to 70∘ S. This analysis shows that the cold and warm sectors of extratropical cyclones are associated with contrasting SWI signals. Specifically, the measurements show that the median values of δ18O and δ2H in the atmospheric water vapour are 3.8 ‰ and 27.9 ‰ higher during warm than during cold advection. The median value of the second-order isotope variable deuterium excess d, which can be used as a measure of non-equilibrium processes during phase changes, is 6.4 ‰ lower during warm than during cold advection. These characteristic isotope signals during cold and warm advection reflect the opposite air–sea fluxes associated with these large-scale transport events. The trajectory-based analysis reveals that the SWI signals in the cold sector are mainly shaped by ocean evaporation. In the warm sector, the air masses experience a net loss of moisture due to dew deposition as they are advected over the relatively colder ocean, which leads to the observed low d. We show that additionally the formation of clouds and precipitation in moist adiabatically ascending warm air parcels can decrease d in boundary layer water vapour. These findings illustrate the highly variable isotopic composition in water vapour due to contrasting air–sea interactions during cold and warm advection, respectively, induced by the circulation associated with extratropical cyclones. SWIs can thus potentially be useful as tracers for meridional air advection and other characteristics associated with the dynamics of the storm tracks over interannual timescales.

Stable water isotope signals in tropical ice clouds in the West African monsoon simulated with a regional convection-permitting model

“…Signals of d-excess in precipitation in these three regions (Fig. 2f,i) are generally consistent with the same processes as above: (1) decreasing d-excess along air parcels moving land inward due to the weak but non-negligible impact of the condensation temperature on the d-excess (Thurnherr et al, 2021, their Appendix A), (2) d-excess tends to be lower when the share of transpiration (non-fractionating) in evapotranspiration is relatively large (Risi et al, 2010a;Aemisegger et al, 2014), which can reasonably be assumed over central Africa, and (3) very low d-excess along the northern fringe of the tropical rain belt due to strong non-equilibrium fractionation during below cloud evaporation in dry air (Araguas-Araguas et al, 2000;Risi et al, 2008a;Graf et al, 2019).…”

“…These heavy isotopes undergo identical processes as the common water isotope H2 16 O except during phase transitions when isotopic fractionation occurs. COSMOiso has been used to study a variety of atmospheric processes in the hydrological cycle in idealized studies (Dütsch et al, 2016), detailed case studies (Pfahl et al, 2012;Aemisegger et al, 2015;Lee et al, 2019), and (semi-)climatological analyses (Dütsch et al, 2018;Christner et al, 2018;Dahinden et al, 2021;Diekmann et al, 2021;Thurnherr et al, 2021).…”

Estimation of water origins within an explosive cyclone over the Sea of Japan using an isotopic regional spectral model

“…In addition, as compared to the cold front, condensation in the west coast of the mainland of Japan showed relatively low d 2 H and high d-excess values (Figs.10b,e). This was caused by moisture along the DI that evaporated from the Sea of Japan with low d 2 H and high d-excess values under a cold and dry continental air mass during cold-air outbreaks (Figs.11b,e), which was consistent with previous studies(Aemisegger and Sjolte 2018;Thurnherr et al 2021).…”

“…It is worth noting the moisture uptake time and the time lag between evaporation from the ocean surface and condensation in the atmosphere. The average uptake time for whole moisture of air parcels was suggested by Thurnherr et al (2021) to be 36 (49) h in the cold (warm) sector for Southern Ocean cyclones. However, in the northwest Pacific region, Hirata et al (2015Hirata et al ( , 2016 showed that about 50% of the moisture (5 g kg 21 ) in air parcels along the CCB was taken up from the underlying ocean within 12 h before condensation, which indicates that the average time lag for moisture transported by the CCB from the underlying ocean to condensation was around 6 h.…”

“…In addition to data on moisture transport processes, information on moisture sources and their corresponding temperature, humidity, and isotopic composition are also important for improving our understanding of the mechanisms of cyclone intensification. Stable water isotopes have been widely used as natural tracers to investigate water origins in hydrologic and meteorologic studies based on onsite observations (Dansgaard 1964;Gat 2000;Fudeyasu et al 2008;Li et al 2017) and have been implemented in general and regional circulation models (Yoshimura et al 2003;Pfahl et al 2012;Aemisegger et al 2015;Thurnherr et al 2021); the isotopes can be used to validate models' performance by comparing simulated and observed isotopic results. Generally, the isotopic composition of water is controlled by environmental variables of temperature and humidity during the water's phase transitions in evaporation and condensation processes, whereas the isotopic composition of water is conserved during the moisture transport process.…”