Structure and variability of the Antarctic coastal easterly winds - PowerPoint Presentation

1. Structure and variability of the Antarctic coastal easterly windsJohn King and Thomas BracegirdleBritish Antarctic SurveyCambridge, UKjcki@bas.ac.ukStart presentation

2. Introduction to the Antarctic coastal easterlies Climatology of the Antarctic coastal easterlies Variability – seasonal to interannual timescales Variability – synoptic timescales Conclusions and synthesis ReferencesPlan of presentationClick on the text boxes above to go to the start of a section or use the buttons to move through the presentation. will always bring you back hereThree paradigms for the coastal easterlies

3. DataUnless otherwise noted, all figures in this presentation are derived from ECMWF Interim Reanalysis (ERA-Interim) data (Dee et al, 2011).Thanks to ECMWF for making the data available, and to Tony Phillips (BAS) for his help with accessing and visualising these data.

4. Introduction to the Antarctic Coastal EasterliesThe Antarctic continent is encircled by the circumpolar trough (CPT) of low mean sea level pressure (MSLP)To the north of the trough lies the belt of strong westerly winds that extends into mid-latitudesThe Antarctic coastal easterlies are found to the south of the CPT. These winds are relatively weak over the coastal ocean, but strengthen over the coastal slopes of Antarctica.The coastal easterlies drive a westward-flowing current and a westward transport of sea ice around the Antarctic coast.They also play a key role in controlling ice shelf-ocean interaction (e.g. Dutrieux et al, 2014)It is therefore crucial that near-coastal winds are correctly represented in Earth System Models that couple atmosphere, ocean and cryosphere, BUT the drivers of Antarctic coastal winds and their variability are not well understood.

5. Introduction to the Antarctic Coastal EasterliesAnnual mean MSLP and 10m winds Strong westerlies north of the circumpolar troughWeak easterlies over the ocean south of the circumpolar troughStrong easterlies over the steep coastal slopes of AntarcticaCircumpolar trough

6. Three paradigms for understanding the Coastal Easterlies 1. Synoptic forcing: The easterlies are driven by the gradient in MSLP to the south of the circumpolar trough.This is clearly “true” at some level but doesn’t explain why the trough exists or why the easterlies are strongest over the coastal slopes of Antarctica.Contours – MSLP, shading – zonal 10 m wind

7. Three paradigms for understanding the Coastal Easterlies 2. Katabatic forcing: The easterlies are driven by the action of the Coriolis force on katabatic winds blowing down from the high plateau of AntarcticaThe katabatic winds are strongest over the slopes where we see the strongest easterlies, BUT:The easterlies are 1 – 2 km deep, while the katabatic flow is only a few hundred metres deep at most.Katabatic forcing is strongest in winter but the easterlies show little seasonal variation (Parish and Cassano, 2003).Katabatic flow rarely extends beyond the coast (Renfrew and Anderson, 2002).(After Mather & Miller, 1967)

8. Three paradigms for understanding the Coastal Easterlies 3. A “holistic” paradigm: The easterlies are a result of the modification of the large-scale baroclinicity of the atmosphere by the orography of Antarctica (Kottmeier and Stuckenberg, 1986; Fulton et al, 2017)In what follows, we look at the climatology of the Antarctic coastal easterlies within the context of this paradigm.Idealised model of Fulton et al, JGR Atmos., 2017. The Antarctic is modelled as an isentropic surface. The resulting baroclinicity drives strong easterlies over the steep coastal slopes and weaker easterlies over the near-coastal ocean.

9. Climatology of the Antarctic Coastal Easterlies DMLEAWAAntarctica is not perfectly zonally- symmetric so we examine the climatology of the coastal easterlies by constructing zonally-averaged latitude-height cross sections using ERA-Interim data within three longitude sectors:Dronning Maud Land (DML), 0-30°EEast Antarctica (EA), 90-135°EWest Antarctica (WA), 105-135°WWithin each of these sectors, the Antarctic coastline approximately follows a line of latitude.

10. Climatology of the Antarctic Coastal Easterlies “East Antarctic” cross-section, winter (JJA), 2000-2009Colours – zonal wind component (m/s)Contours – potential temperature (K)Westerly jetBaroclinic zoneEasterlies forced by enhanced baroclinicity over coastal slopesSurface of continent is approximately isentropic

11. Climatology of the Antarctic Coastal Easterlies DMLEAWASimilar structure in all three sectors, particularly DML and EA.Easterlies are shallower and weaker in WA sector. Here, the polar plateau has a lower elevation and the core of the westerly jet is further south (but also weaker)In all three sectors, the coastal easterlies are driven by the enhanced baroclinicity over the coastal slopes, which is a consequence of the continental surface being quasi-isentropic. The katabatic flow plays a role in maintaining this condition.

12. Variability – seasonal to interannual Winter (JJA) Summer (DJF) East Antarctic sector – seasonal cycleWeaker westerly jetStronger baroclinicityStronger easterliesStronger westerly jetWeaker baroclinicityWeaker easterlies

13. Variability – seasonal to interannual East Antarctic sector – seasonal cycle of 10m windu-component, coastal oceanu-component, slopesv-component, slopesThe easterly wind over the near-coastal ocean has a weak semi-annual cycle (connected to the semi-annual oscillation of the circumpolar trough?)Over the slopes, both components exhibit an annual cycle, which would be expected from the seasonal cycle of katabatic forcing but the amplitude of the cycle is much smaller than would be expected if katabatic forcing were the only driver (Parish and Cassano, 2003)

14. Variability – seasonal to interannual Monthly means, East Antarctic sector, JJA, 1980-2018 Mean for 1980-2018Composite mean, surface easterlies in upper decile in region marked by boxStrengthened coastal easterlies are associated with:Westerly jet weaker/further northReduced baroclinicity between the jet core and the coastEnhanced baroclinicity in the coastal regionStronger/deeper easterlies over the continental slopeDeeper convective mixed layer over the ocean

15. Variability – synoptic timescales We have used synoptic observations from coastal stations to select days (2000-2009) when the easterly component of the 10 m wind was in the upper decile. Composite anomaly maps and cross-sections were produced for these days, and for lead and lag times of up to three days.The results are shown in the following three slides. It is clear that extreme easterly events are associated with a cyclone (with an anticyclonic anomaly to the east) deepening as it moves south towards the Antarctic coast. Following the extreme event, the cyclone drifts eastward and fills rapidly.

16. Variability – synoptic timescales SyowaMawsonDumont d’UrvilleBear Peninsula AWSComposite anomalies of MSLP and 10 m wind vector for days when the easterly component of the 10 m wind at selected coastal stations (red dots) was in the upper decile.Click on each map to view a larger version

17. Lagged composite anomalies for Syowa T-72T-48T-24TT+24T+48Click here to view sequence as an animation

18. Variability – synoptic timescales Cross-sections through Syowa Mean Strong E’ly composite Strengthened coastal easterlies are associated with:Westerly jet stronger/further southReduced baroclinicity between the jet core and the coastEnhanced baroclinicity in the coastal regionStronger/deeper easterlies over the continental slope

19. Conclusions and Synthesis The Antarctic easterlies are most usefully viewed as the result of the baroclinicity of the large-scale flow, which is strongly modified by the steep topography of the continent.The katabatic flow from the continent plays an indirect role in forcing the easterlies by maintaining approximately isentropic conditions over the surface of the Antarctic continent.On monthly and longer timescales, strong easterlies are associated with a westerly jet that is weaker than average, and further north, together with slightly enhanced baroclinicity in the coastal region.On synoptic timescales, strong easterlies are associated with southward movement of a deepening cyclone, along with enhanced baroclinicity in the coastal region.

20. Thanks for viewing our presentation!Questions and feedback to John Kingjcki@bas.ac.uk

21. References Dee, D. P., et al. (2011). The ERA-Interim reanalysis: configuration and performance of the data assimilation system. QJRMS., 10.1002/qj.828Dutrieux, P., et al. (2014). Strong Sensitivity of Pine Island Ice-Shelf Melting to Climatic Variability, Science, 10.1126/science.1244341Fulton et al (2017) A Dynamical Explanation of the Topographically Bound Easterly Low-Level Jet Surrounding Antarctica, JGR Atmos., 10.1002/2017jd027192Kottmeier, C. and H.-U. Stuckenberg (1986). A quasi-geostrophic flow solution for the circulation over Antarctica. Beitrage zur Physik der Atmosphare (Contributions to Atmospheric Physics) 59 Mather, K. B. and Miller, G. S. (1967). Notes on topographic factors affecting the surface wind in Antarctica, with special reference to katabatic winds; and bibliography. University of Alaska, Fairbanks.Parish, T. R. and J. J. Cassano (2003). The Role of Katabatic Winds on the Antarctic Surface Wind Regime, Mon. Wea. Rev., 10.1175/1520-0493(2003)131<0317:Trokwo>2.0.Co;2 Renfrew, I. A. and P. S. Anderson (2002). The surface climatology of an ordinary katabatic wind regime in Coats Land, Antarctica. Tellus A, 10.3402/tellusa.v54i5.12162

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23. Syowa

24. Mawson

25. Bear Peninsula AWS

26. Dumont d’Urville