Transcript El Nino - La Nina

EL NINO- “Christ-child”
•Coined by local fisherman over a century ago to
describe warm water that emerged around Christmas
time in coastal regions off Peru, Ecuador, Chile, off
the Humboldt current, which is typically cold.
•Reduced upwelling effects, drop in nutrients,
phytoplankton and anchovy fisheries
•Especially evident in 1982-84, socioeconomic10-20 billion dollars worldwide.
• 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 CO2 from cold upwelled water is lower
during El Nino
•However, production is higher- uptake of CO2
Sea Surface
SST Anomalies
Upwelling, or the result of Ekman drift, in response to a northblowing wind in the Southern Hemisphere.
El Nino Winter
SST
Anomalies
La Nina Winter