Southern Oscillation- Atmospheric component of ocean's El Niño. Oscillation in the distribution of high and low pressure systems across the equatorial Pacific, affect wind patterns, which affect surface ocean circulation. A. Walker Cell E-W atmospheric circulation cell in region of equatorial Pacific. Walker Cells normally initiated at the Indonesian Low. Low pressure in western equatorial Pacific near Indonesia/Australia. Air rises, transported to east aloft, then sinks in eastern equatorial Pacific (high pressure), transported with Trade winds. Cause of low pressure is western Pacific warm pool (WPWP) (warmest water in the surface ocean).

Walker compiled extensive observations throughout the region and identified and named the Southern Oscillation which we now recognize as a pattern of interannual variability in the tropical ocean and atmosphere that causes El Nino. The subsiding air of the Walker circulation occurs in the eastern Pacific, and feeds into the Trade winds. The eastward traveling air aloft subsides in the east, and this so-called Walker cell in the atmosphere maintains the east to west pressure gradient along the equator that drives the Trades … (that produce the WPWP, that drive convection, that sustains the Walker cell)

Seasonal cycle of the Trades The coupling of winds and SST is apparent if we consider the seasonal cycle at the equator. Weaker southeast Trades in the austral late summer cause February-April to be the equatorial warm season. upwelling is weaker and sea surface temperature warms upwelling is strongest at the end of the austral winter in Sept-Oct when the southeast Trades blow at a steady 6 m s-1 Some years, this seasonal cycle seems to get amplified and the warming of the eastern Pacific early in the year becomes dramatic and persistent: This is an El Nino, or an ENSO warm event.

These El Nino events are part of a cycle that yields opposite but equally extreme conditions during non-El Nino years Known as La Nina, Philander(1989), cycle on average 3-15 years between El Nino events. Upwelling of cold waters during La Nina leads to lower air temps in N. Hemisphere Moreover, El Nino/La Nina affects conclusion of atmospheric CO 2 from cold upwelled water is lower during El Nino However, production is higher- uptake of CO 2

Sea Surface

SST Anomalies

Upwelling, or the result of Ekman drift, in response to a north- blowing wind in the Southern Hemisphere.

El Nino Winter

SST Anomalies

La Nina Winter