The length of the sea ice season summarizes in one number the ice cove
rage conditions for an individual location for an entire year. It beco
mes a particularly valuable variable when mapped spatially over a larg
e area and examined for regional and interannual differences, as is do
ne here for the Southern Ocean over the years 1979-1986, using the sat
ellite passive microwave data of the Nimbus 7 scanning multichannel mi
crowave radiometer. Three prominent geographic anomalies in ice season
lengths occur consistently in each year of the data set, countering t
he general tendency toward shorter ice seasons from south to north: (1
) In the Weddell Sea the tendency is toward shorter ice seasons from s
outhwest to northeast, reflective of the cyclonic ice/atmosphere/ocean
circulations in the Weddell Sea region. (2) Directly north of the Ros
s Ice Shelf anomalously short ice seasons occur, lasting only 245-270
days, in contrast to the perennial ice coverage at comparable latitude
s in the southern Bellingshausen and Amundsen Seas and in the western
Weddell Sea. The short ice season off the Ross Ice Shelf reflects the
consistently early opening of the ice cover each spring, under the inf
luence of upwelling along the continental slope and shelf and atmosphe
ric forcing from winds blowing off the Antarctic continent. (3) In the
southern Amundsen Sea, anomalously short ice seasons occur adjacent t
o the coast, owing to the frequent existence of coastal polynyas off t
he many small ice shelves bordering the sea. Least squares trends in t
he ice season lengths over the 1979-1986 period are highly coherent sp
atially, with overall trends toward shorter ice seasons in the norther
n Weddell and Bellingshausen seas and toward longer ice seasons in the
Ross Sea, around much of East Antarctica, and in a portion of the sou
th central Weddell Sea.