The influence of extra-tropical, atmospheric zonal wave

1. The influence of extra-tropical, atmospheric zonal wave three on the regional variation of Antarctic sea ice Marilyn Raphael UCLA Department of Geography raphael@geog.ucla.edu Research Funded by the National Science Foundation

2. RESEARCH AIM Antarctic sea ice cover experiences strong interannual and weaker decadal variation. It is expected to respond to global climate change although the manner of this response is not fully determined. While local surface processes in sea ice formation may dominate short term changes in sea ice, long term changes may be facilitated by large scale processes in the atmosphere and ocean. The aim of this research is to understand more about the interaction between Antarctic sea ice distribution and the large scale, atmospheric dynamical processes of the Southern Hemisphere. The current focus is on the quasi-stationary waves, in particular zonal wave three, that exist at the middle to high latitudes of the Southern Hemisphere atmospheric circulation.

3. Spatial Trends of Antarctic Sea Ice Concentrations 1979-2002 a. Trend before removing the influence of the AAO and of ENSO. b.Trend after removing the influence of the AAO and ENSO Liu et al 2004, GRL Large scale circulation processes e.g the AAO and ENSO have influenced Antarctic sea Ice concentrations. (e.g. Bromwich et al, 2000; Carleton, 2003; Kwok and Comiso, 2002.

4. 500 hPa Geopotential Height Field Zonal Mean Zonal Asymmetry Wave 3 Wave 1

5. Annual cycle of zonal wave three ZW3 has positive and negative phases. Positive phase indicates strong meridional flow, negative phase indicates strong zonal flow. Strongest positive phase occurs in early winter, strongest negative phase in spring.

6. Time series of zonal wave three index 1960-2004 (Raphael 2004) Note the strong interannual variation that is characteristic of zonal wave three. Note also the shift towards more positive values from around 1979. This increase in strength of ZW3 (movement towards more meridional flow) has also been noted in station data by van Loon et al (1993).

7. Influence of zonal wave three on the circulation • Preferred regions of equatorward and poleward flow. Equatorward flow would bring colder air and poleward flow, warmer. • Has the potential to influence the Antarctic sea-ice region by influencing the meridional transport of heat in the atmosphere and ocean.

8. EOF3 of SIC; 9% variance explained Three centers of action, “Ross and Weddell seas outflow and off the Amery ice shelf. Sea ice growth and expansion is largest in the three centers of action defined by EOF3.

9. Covariation between the PC of EOF3 and SIC in AMJ The pattern of covariation exhibits both positive and negative correlations. The strongest positive correlations occur in the centers of action defined by EOF3.

10. The primary source of sea ice is the freezing of ocean water. Sea ice formation therefore depends on: Ocean surface temperature Surface air temperature Together they determine the size and direction of the flux of energy between the ocean and the atmosphere and ultimately the maintenance of water at or below freezing temperatures.

11. Positive temperature differences coincide with regions of negative net sensible heat flux and vice versa. Negative net sensible heat flux indicates flow of energy from the atmosphere to the ocean and vice versa.

12. 500mb pattern of flow inferred from zonal wave three

13. The mean (ANN) trend is consistent with Zwally et al, 2002. The trends in AMJ strongly resemble the mean indicating that mean trend is dominated by what occurs in fall through winter.

14. PC3 (inverted) and ZW3 of AMJ vs time Note the strong (and expected) interannual variation. Note also the longer term variation suggesting a trend towards smaller values of the ZW3 index from 1978 to 1992 and the opposite from 1993 to 2003.

15. 1993-2003 1978-1992 Reduction of SIC in the B/A seas is consistent over the period. Off the Amery ice shelf and in the Weddell Sea outflow, SIC increases (decreases) when ZW3 strengthens (weakens). This is an initial indication of the ability of ZW3 to influence the regional variation in SIC during the period when ice is growing.

16. Summary and Conclusions 1. ZW3 influences the regional variation in sea ice concentration around Antarctica. This influence appears greatest in AMJ, the period during which much of the ice grows and spreads northward. 2. When ZW3 is in its positive phase, i.e. The flow is meridional, it forces a clear alternating pattern of warmer and colder air associated with poleward and equatorward flow respectively. More (less) sea ice is found when the transport is equatorward (poleward). The associated net SHF corresponds to regions of warmer/cooler air. 3. The index of ZW3 experienced an apparent shift to more positive phases of ZW3 since the mid to late 1970s. This has implications for the region extending from the western Weddell Sea across the Amundsen/Bellingshausen Seas where sea ice has been decreasing in recent years.

17. Further Work At the surface ZW3 must have an influence on sea ice motion. Poleward flow would keep ice from spreading north and equatorward flow would encourage the spread of ice northward. This mechanical effect on ice advection would augment the foregoing thermal effect on sea ice distribution around Antarctica. Future work would examine the interaction between ZW3 and ice motion around Antarctica.

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