GEOS 513
ENSO: Past, Present, Future
Michael N. Evans
Laboratory of Tree-Ring Research/Geosciences/Atmospheric Sciences
course web pages: http://ic.ltrr.arizona.edu/ic/enso/
January 18, 2006:
References
M.A. Cane, Oceanographic events during El Nino, Science 222(4629): 1189-1195 (1983);
E.M. Rasmussen and J.M. Wallace, Meteorological aspects of the El Nino/Southern Oscillation,
Science 222(4629): 1195-1202 (1983).
1. The equatorial Pacific: mean state, annual cycle, and ENSO
2. How was the 1982-3 event similar to the canonical ENSO event?
3. How was the 1982-3 event unusual (is it still considered unusual?)
Coupled ocean-atmosphere system
(With apologies to the graphically-enabled)
Ekman
effects
(Pond and
Pickard,
1989)
Annual cycle in sea surface temperature
100E
180
(Cane,
1983)
100W
Annual cycle: outgoing longwave radiation (OLR)
Canonical ENSO warm phase event (Cane, 1983)
Kelvin wave theory (Holton, 1992)
Rossby
wave
theory
(Holton,
1992)
absolute
Canonical oceanographic ENSO (Cane, 1983)
Canonical
ENSO
teleconnection
pattern
(Rasmussen
and Wallace,
1983)
Coupled ocean-atmosphere system
(With apologies to the graphically-enabled)
Canonical ENSO oceanographic indicators
Canonical ENSO atmospheric indicators
How was the 1982-3 ENSO warm phase event typical?
What made it special?
1982-3 ENSO warm phase event Kelvin wave
1982-3 event character (Cane, 1983)
1982-3 ENSO
warm phase
event OLR
anomaly
(Rasmussen
and Wallace,
1983)
Canonical vs. 1982-3 ENSO warm phase
teleconnection anomaly
canonical
1982-3
(Rasmussen and Wallace, 1983)
How was the 1982-3 event similar to the canonical event?
●
●
●
Largely explained by equatorial (Kelvin) wave theory
(oceans)
Largely explained by midlatitude (Rossby) wave
theory (atmosphere)
A superposition of
–
Coastal, annual-cycle-like anomalies
–
Central equatorial Pacific-centered anomalies
How was the 1982-3 event different from the canonical
event?
●
Anomaly propagation: west to east across centraleastern equatorial Pacific
●
Reversal of trade winds in central equatorial Pacific
●
Disappearance of Equatorial Undercurrent
●
●
Reversed event timing (or else, near-simultaneous
coastal and open-ocean ENSO anomalies)
Large amplitude, long duration
Was the 1997-8 El Nino event the
second 'El Nino of the Century'?
Source: Climate Analysis Center SST anomaly product
(http://iridl.ldeo.columbia.edu/expert/SOURCES/.CAC/
Summary
●
●
●
●
ENSO events can be described as an amplification of
the annual cycle of the couplued tropical Pacific oceanatmosphere system.
The 1982-3 event was largely consistent with Kelvin
and Rossby wave theory
...But it was unusual in terms of
–
Timing
–
Amplitude
–
Anomaly propagation
–
Severity of teleconnections
Until the 1997-8 event appeared.