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

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.

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.

Wave 1 Wave 3

Zonal AsymmetryZonal Mean

500 hPa Geopotential Height Field

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.

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).

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.

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.

The pattern of covariation exhibits both positive and negative

correlations. The strongest positive correlations occur in

the centers of action defined by EOF3.

Covariation between the PC of EOF3 and SIC in AMJ

The primary source of sea ice is the freezing of ocean water.

Sea ice formation therefore depends on:

Ocean surface 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.

Surface air temperature

Negative net sensible heat flux

indicates flow of energy from the

atmosphere to the ocean and

vice versa.

Positive temperature differences

coincide with regions of negative

net sensible heat flux and

vice versa.

500mb pattern of flow

inferred from zonal wave

three

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.

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.

1978-1992 1993-2003

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.

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.

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.

THE END

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