4 Discussion & Conclusions
Both CYGNSS and GISS-E2.1 indicate surface heat fluxes from the extratropics are greatest outside of times with ARs. Diminished surfaces fluxes from beneath the ARs imply that both heat and moisture fluxes must be sourced from further away. Here we have leveraged computationally efficient vapor source distribution tracers available in the GISS model to assess the moisture provenance of ~7500 ARs during the CYGNSS mission to provide a much broader picture of the movements of moisture and energy over the lifetime of these events. Although our assessment occurs over a short period of time to allow overlap with CYGNSS, the number of AR events examined here exceeds previous studies by two orders of magnitude (Hu and Dominguez, 2019), allowing us to examine events globally and across all relevant latitudes. We can confidently say that ARs source moisture and heat from further afield than climatology.
When partitioning surface heat fluxes by the presence of ARs, there is a clear transition at around 20° in the change in these fluxes with poleward extratropics having greater nonAR surface fluxes and the tropics having greater surface fluxes during ARs.
Here, moisture travels further and is sourced closer to the equator during ARs than during non-ARs, consistent with previous results (Nusbaumer & Noone, 2018; Sodemann & Stohl, 2013; Ramos et al., 2016), but with a broader, quantitative and global diagnosis. We show that this result holds in all of the ocean basins. ARs take moisture from ~500 km further away, and over land, they source moisture from up to 1000 km or more away. This moisture originates closer to the equator than non-AR moisture by about 10°, depending on the latitude and basin, with moisture traveling further meridionally closer to the polar regions, aligning with the Southern Hemisphere case study of Terpstra et al. (2021). Our results are also consistent with the Dacre et al. (2019) assessment that moisture is drawn in from downstream (eastward) of the cold front during genesis.
We used the Guan and Waliser (2019) algorithm because it tracks AR lifecycles, while other algorithms do not tag each event and thus first, mature, and last timesteps. ARTMIP (Shields et al 2018) covers that diversity. Smaller ARs and AR boundaries, which barely pass the threshold for some but not all of these automated methods, might be included/excluded depending on the identifying algorithm. Future work might include screening ARs to include only those that persist for a longer period of time or those that have geometries that are consistent with other detection methods. We suspect that winnowing down the 7500 events would serve to pull apart the AR transport distance from the non-AR transport distance further in Fig. 4 – that the ‘allAR’ line would look less like the ARgenesis since this flagging initial timestep is likely more sensitive to the exact definition of AR.
The key result, based on both the CYGNSS observational analysis and the modeling work, is that the oceans directly below ARs are not their primary source of moisture and energy. ARs reduce evaporation and surface heat fluxes out of the ocean, tapping moisture from further and more equatorward than during non-ARs. However, if ARs acted exactly as rivers, there would be no surface fluxes from ocean to atmosphere at all, that is not what we find.
Future work will seek to characterize whether a relationship exists between the intensity (rainfall, updrafts, winds, etc.) of ARs sourcing moisture over longer distances nearer the tropics are related and whether these characteristics and relationships are sensitive to climate change. Future study will also examine whether the transition zone at 20° from greater/lesser out-of-ocean LHF and moisture flux out of the ocean moves meridionally in the future. ARs do have a complex relationship with ETC, which hasn’t been addressed here. An obvious follow-on study, thus, would be to complete a similar genesis and mature evaluation for ETC, tag the ETC-AR’s that are related, and contrast to the AR moisture provenance results of this study. We hope to categorize and generalize both ETC and ARs to see if we can generalize the types of storms associated with larger surface heat fluxes and moisture transports, with the above study is the first step.