THE ANTARCTIC DIPOLE
A Standing Wave in the Western Hemisphere of the
What is the Antarctic Dipole (ADP)?
The Antarctic Dipole in Sea Ice Edge anomaly
The Antarctic Dipole best presents in
EOF mode of sea ice edge anomalies (37% of total
variance). The first
eigenvector (a) has a peak in the western Amundsen Sea
and a peak with opposite
sign in the Weddell Sea. The reconstructed sea ice
edge anomalies from the first
mode (b) emphasizes the dipole's nature as a standing
The sea ice edge (30% of ice concentration) data were derived from monthly sea
generated by the bootstrap algorithm from NASA
micorwave imagers from 10/1978
to 12/1999. The seasonal means were subtracted to
yield anomaly field.
The anomaly data were low-pass filtered to remove
before the EOF analysis.
The Antarctic Dipole in Surface Air Temperature
The Antarctic Dipole best presents in
the first two
EOF modes of surface air temperature anomalies along
65S (53% of total variance).
The combined eigenvector (c) and reconstracted
temperature anomaly from these
two modes (d) reveal the same dipole pattern.
Surface air temperature was taken from NCEP/NCAR
reanalysis air temperature at
the 1000mb surface from 1/1975 to 12/1999 (plotted in
the same period as sea
ice data). The similar data processes had beed done as
to the sea ice data.
The ADP and ACW
Much of the interannual variability in the Southern Ocean has been
described in the context of an Antarctic Circumpolar Wave (ACW) --
a wavenumber two phenomenon propagating in ice, pressure, wind and
temperature fields around the Antarctic (White and Peterson, 1996).
Here we compare the variance of the ADP (represented by the first two modes
of sea ice and surface air temperature, and the third mode of sea level pressure,
plotted in blue) and ACW (bandpass filtered between 3 and 7 years, plotted in red) with the total variance of the fields (plotted in green). The surface temperature
and pressure data were taken from NCEP/NCAR reanalysis along 65S. Clearly,
the ADP dominates variability in ice and temperature fields. The ADP and ACW are
comparable in the pressure fields.
Since the ACW and ADP have approximate the same wavelength and
similar period, they are likely related. Likely relationships include:
(1) The ACW propages through the area and excites the standing wave, and (2)
the ADP is excited by extra-polar teleconnections and its anomaly is advected out
of the dipole area by the Antarctic Circumpolar Current and/or air-sea-ice
coupling processes. We favor the latter relationship for the following three
reasons: (1) The eastern Pacific and Weddell Sea regions are very sensitive to
extra-polar climate (Yuan and Martinson, 2000), (2) the ADP in sea ice can be
predicted by extra-polar climate under certain conditions, and (3) the magnitude
of ADP variability is much larger than the
Dipole's relationship with ENSO
The Teleconnection Pattern
The leading mode of EOF analysis conducted on the
surface air temperature anomaly
in the Pacific and Atlantic south of 20N reveals a
linking the tropical Pacific and the dipole region in
the Southern Ocean.
The eigenvector shows a characteristic ENSO pattern
with maximun amplitude in
the central tropical Pacific. Associated with this
ENSO pattern is a
circumpolar pattern that resembles the Antarctic
Dipole in the ice edge
field, showing a pole in the near 60S in the South
Pacific (of like-phase
to the tropical signal) and another pole of opposite
phase in the Weddell Sea.
Responses of Sea Ice Edge to ENSO
This figure shows September (the maximum ice coverage
in the Southern Hemisphere)
sea ice edge anomaly as function of longitude following five El Nino events in
1980, 1983, 1988, 1992 and 1997 (a) and four La Nina events in 1985, 1989, 1996
and 1999 (b). In the same way, September sea ice edge anomaly containing only
the first EOF mode in the same El Nino years (c) and La Nina year (d). The ice
edge anomaly in the dipole region responds to the
tropical conditions much more regularly than the ice
edge in other regions. Also apparent is the fact that
the ice edge anomaly in the dipole regions responds more
consistently to La Nina conditions than to El Nino conditions. The leading EOF
mode strikingly domenstrates the later phenomenon.
The Spatial Scale of ENSO Influences
The ENSO impact is generated by subtracting mean ice
concetration at each month
following La Nina events (above mentioned 4 years)
from the mean ice
concentration at each month following El Nino events
(above mentioned 5 years).
The white (black) line indicates the mean ice adge
following El Nino events
(La Nina events). The figure shows the ENSO impact in
following September. The
impact is stronger in the dipole regions than in other
regions. Moreover, the
impact is more or less limited near the ice edge. The
ENSO influence on ice
deep into the basins is rather small.
- See the seasonal cycle
of the ENSO Impact on ice
pack in movies
- See the paper
This page is maintained by Xiaojun Yuan.
The last updated was made on May 10, 2001