1. Preferential Pathways for Southern Hemisphere Extreme Cold Events
Nicholas D. Metz, Heather M. Archambault, Alan F. Srock*, Thomas J. Galarneau, and Lance F. Bosart
Department of Earth and Atmospheric Sciences University at Albany/SUNY
Support provided by the NSF grant ATM
14th Cyclone Workshop
26 September 2008

2. Motivation
Southern Hemisphere (SH) extreme cold events (ECEs) east of the Andes Mountains are high impact and related to the high terrain
Severe frosts in coffee growing areas of Brazil can cause significant crop loss and worldwide price increases (e.g., Marengo et al. 1997)
High impacts necessitate examination of ECEs from SH perspective to determine other favored regions and any comparable signatures of terrain

3. Terrain Influence Cold air flows equatorward along mountains due to:
blocked, stable air (e.g., Bell and Bosart 1988)
mountain-parallel component of pressure gradient force
Coriolis force causing flow component towards mountains

4. Terrain Influence Cold air flows equatorward along mountains due to:
blocked, stable air (e.g., Bell and Bosart 1988)
mountain-parallel component of pressure gradient force
Coriolis force causing flow component towards mountains
Previous studies suggest mountain ranges affect flow to one Rossby radius of deformation (r=Nhf1) from terrain (e.g., Colle and Mass 1995)
From 20–25°S east of Andes, r~1000 km (Garreaud 1999)

5. Synoptic Signatures
SH cold surge research has mostly focused east of the Andes Mountains and shown that cold air:
moves equatorward with strong meridional flow at low levels (e.g., Garreaud 1999, Lupo et al. 2001)
accompanies surface anticyclogenesis (e.g., Lupo et al. 2001)

6. Pathways of Terrain-Influenced Phenomena
Primary Cold Pathways
Secondary Cold Pathways
Cyclone Pathways
Modified from Garreaud (2001) - Fig. 1

7. Annual Means
20-yr mean NOAA OI v.2 SST (°C), 25-yr mean ERA-40 MSLP (hPa), and ERA-40 surface elevation (m)

8. Annual Pathways
n=18,262
20-yr mean SST (°C), 25-yr mean MSLP (hPa), 25-yr frequency of 925-hPa temperature <16°C (%), and surface elevation (m)

9. Annual Pathways Andes Pathway Highlands Pathway
20-yr mean SST (°C), 25-yr mean MSLP (hPa), 25-yr frequency of 925-hPa temperature <16°C (%), surface elevation (m), and points

10. Goals
Examine how ECE pathway character varies based on regional physiographic features
Compare ECEs along Andes Pathway with the less known Highlands Pathway by highlighting:
monthly distributions
latitudinal variation of 925-hPa temperature, meridional wind, and 925–700-hPa lapse rate distributions
synoptic-scale composites of the above variables

11. Methodology
Constructed a 925-hPa temperature climatology from 25-yr (1977–2001) twice daily (0000/1200 UTC) 2.5° ERA-40 grids
Defined ECEs as coldest 1% of climatology along points within pathways

12. ECE Monthly Distributions

13. ECE Monthly Distribution
Andes

14. ECE Monthly Distribution
Andes
Highlands

15. July Pathways Andes Highlands
20-yr mean SST (°C), 25-yr mean MSLP (hPa), 25-yr frequency of 925-hPa temperature <16°C (%), surface elevation (m), and points

16. Latitudinal Variation of Low-level Temperature, Meridional Wind, and Lapse Rate Distributions

17. ECE 925-hPa Temperature Distribution
Andes

18. ECE 925-hPa Temperature Distribution
Highlands
Andes

19. ECE 925-hPa Meridional Wind Distribution
Andes

20. ECE 925-hPa Meridional Wind Distribution
Highlands
Andes

21. ECE 925–700-hPa Lapse Rate Distribution
Andes

22. ECE 925–700-hPa Lapse Rate Distribution
Highlands
Andes

23. ECE Synoptic Composites

24. ECE Synoptic Composites at 15°S
Andes - 15°S, 65°W
Highlands - 15°S, 40°E
n=183
n=183
925-hPa temperature (°C), 925-hPa winds (m s-1), and 925–700-hPa lapse rate (°C km-1)

25. ECE Departure from Weighted Climatology at 15°S
Andes - 15°S, 65°W
Highlands - 15°S, 40°E
n=183
n=183
925-hPa southerly meridional wind anom (m s-1), 925-hPa temperature anom (°C), and 925–700-hPa lapse rate anom (°C km-1)

26. Concluding Remarks ECE monthly distributions:
peak in July (July/August) along Andes (Highlands)
span April–Nov (May–Oct) along the Andes (Highlands)
Moving poleward, ECE frequency distributions show:
increasing departure from climo, especially along Andes
wider range of meridional wind values, with definitive southerly preference at all latitudes
wider range of lapse rates, especially along Andes

27. Concluding Remarks (2)
Synoptic composites show Andes ECE Pathway to be more distinct than Highlands, but both contain:
anomalously southerly low-level meridional wind
anomalously stable low levels
ECEs along Andes Pathway are likely more intense than those along Highlands Pathway due to:
higher, more “knife-like” mountains
land stretching farther poleward
lack of moderating influence of water