Antarctic Oscillation (AAO) / Southern Annual Mode (SAM) Changes During Nuclear Winter

1. EOF Loading Pattern of the AAO

The Arctic and Antarctic Oscillations are the dominant modes of wintertime variability in each respective hemisphere. The positive mode of each oscillation index strengthens the polar vortex and the negative mode indicates a weakening. The negative mode is associated with poleward winds, which can lead to warming of the waters near Antarctica. The injection of black carbon into the upper troposphere and lower stratosphere heavily influences the phase and amplitude of these metrics which define the surface circulation. Theoretically, the presence of aerosols near the tropopause should create a differential heating gradient in the upper atmosphere during the winter, strengthening the circulation around the pole. This occurs in the Northern Hemisphere, as a +AO mode occurs during the winter and a -AO mode occurs during the summer. In the Southern Hemisphere, a negative mode of the AAO occurs during the summer months (DJF) but there is no strong positive mode of the AAO during the winter months (JJA). CESM-WACCM does a decent job at representing the circulation around the Southern Ocean and Antarctica. The loading pattern of 700 mb geopotential heights in CESM-WACCM4 (Fig 1a.), compared to the climatological loading pattern (Fig 1b.):

2. AAO Changes During 150 Tg US-Russia Nuclear War
Monthly evolution of the AAO (Fig 2a) and AO (Fig 2b) are shown during the 150 Tg US-Russia case. Key differences are that the AAO is mostly negative (higher pressure at the South Pole compared to the Southern Ocean) while the AO fluctuates wildly between positive during DJF and negative during JJA. During JJA, the AAO is more negative than during DJF, but it does not reverse phase like in the AO.

3. -AAO Effect on Wind in the Southern Ocean
A negative AAO is correlated with weaker surface winds over much of the Southern Ocean, with an increase in surface winds closer to the shores of Antarctica. The plot below plots show a composite of the surface wind anomaly when the AAO is below -2.0 during the control run (left) and the wind stress anomaly during the first DJF of the 150 Tg soot injection (right). Discrepancies include: increased wind stress is not observed near New Zealand and to the west of Australia during the nuclear war case. There is an increase in the surface winds closer to the shore of Antarctica in the nuclear war case, while there is a remarkable reduction for all places to the north.

Surface wind anomaly during first four years of the 150 Tg US-Russia nuclear war:
DJF Year 1, JJA Year 1

DJF Year 2, JJA Year 2

DJF Year 3, JJA Year 3

DJF Year 4, JJA Year 4
DJF Year 5, JJA Year 5
DJF Year 6, JJA Year 6
DJF Year 7, JJA Year 7
DJF Year 8, JJA Year 8
DJF Year 9, JJA Year 9
DJF Year 10, JJA Year 10

Effect of -AAO on surface winds: weakening over Southern Ocean, strengthening along the New Zealand latitude band.
Effect of +AAO on surface winds: strengthening over the Southern Ocean, opposite of -AAO response.

Effect of -AAO on Antarctic surface temperatures: more meridional flow, warming over West Antarctica.
Effect of +AAO on Antarctic surface temperatures: cooling over West Antarctica.