Which Extratropical Cyclones Contribute Most to the Transport of Moisture in the Southern Hemisphere?

• Published in: Journal of geophysical research. Atmospheres Vol. 124; no. 5; pp. 2525 - 2545
• Main Authors: Sinclair, V. A., Dacre, H. F.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 16-03-2019
• Subjects: Algorithms; Antarctic ice sheet; Antarctica; Atmospheric precipitations; Control; Cyclone tracks; Cyclones; Dipoles; Downstream effects; Extratropical cyclones; Frontal waves; Geophysics; Glaciation; Global sea level; Ice; Latitude; meridional moisture transport; Moisture content; Moisture flux; Orientation; Precipitation; Propagation; Sea level; Snow; Southern Hemisphere; Tracking; Transport; Vorticity; Water content; Wave propagation; Weather; Southern Hemisphere; Antarctica
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2018JD028766

Predicted changes in Southern Hemisphere (SH) precipitation and Antarctic ice mass correspond to variations in the meridional moisture flux (MMF). Thirty‐five years of ERA‐Interim reanalysis data are combined with an extratropical cyclone (ETC) identification and tracking algorithm to investigate factors controlling SH MMF variability in the midlatitudes and near Antarctica. ETC characteristics which exert the strongest control on ETC MMF are determined thus identifying which ETCs contribute most to SH moisture transport. ETC poleward propagation speed exerts the strongest control on the ETC MMF across the Antarctic coastline. In SH winter, ETCs with the largest poleward propagation speeds transport 2.5 times more moisture than an average ETC. In the midlatitudes, ETC genesis latitude and poleward propagation speed have a similar influence on ETC MMF. Surprisingly, ETC maximum vorticity has little control on ETC MMF. Cyclone compositing is used to determine the reasons for these statistical relationships. ETCs generally exhibit a dipole of poleward and equatorward MMF downstream and upstream of the cyclone center, respectively. However, ETCs with the largest poleward propagation speeds resemble open frontal waves with strong poleward moisture transport downstream of the cyclone center only and thus result in the largest MMF. These results suggest that inhomogeneous trends and predicted changes in precipitation over Antarctica may be due to changes in cyclone track orientation, associated with changes to the large‐scale background flow, in addition to changes in cyclone number or intensity.
Plain Language Summary
We investigate how large‐scale weather systems transport moisture from low latitudes (which are very moist) to high latitudes (which are much drier) in the Southern Hemisphere. Of special interest is how these weather systems transport moisture to Antarctica. For snow (and rain) to occur, there needs to be moisture present in the atmosphere. If the amount of moisture transported from low latitudes to high latitudes changes, the amount of snow falling over Antarctica will also change. This changes the amount of water stored as ice and snow in the Antarctic ice sheet. Changes to the Antarctic ice sheet could have drastic impacts on global sea levels. We study how the amount of moisture transported depends on the type, or characteristics, of weather systems. Weather systems which move the fastest from low latitude or midlatitude toward Antarctic can move 2.5 times more moisture than an average weather system. Surprisingly, the strongest weather systems do not move much more moisture than the average weather system. These results tell us that we should consider how the track of weather systems will change in the future in addition to how the strength or number of these large‐scale weather systems will change.
Key Points
Moisture transport by extratropical cyclones (ETC) in the Southern Hemisphere is investigated using 35 years of ERA‐Interim reanalysis data
The amount of moisture transported poleward by an ETC depends strongly on the ETC poleward propagation speed and weakly on ETC intensity
Changes to storm track latitude and tilt may explain observed trends in SH precipitation better than changes in ETC intensity