Geoscience Reference
In-Depth Information
Once an ENSO event is triggered, the cycle of
climatic change operates over a minimum of two years.
In historical records, the longest ENSO event lasted
four years: from early 1911 to mid-1915. However,
recent events have dispelled the belief that ENSO
events are biennial phenomena. Following the
1982-1983 ENSO event, the warm water that traveled
northward along the western coast of North America
slowly crossed the north Pacific, deflecting the
Kuroshio Current a decade later. As a result, northern
Pacific sea surface temperatures increased abnormally,
affecting general circulation across the North American
continent. It is plausible that the drought on the Great
Plains of the United States in the summer of 1988, and
even the flooding of the Mississippi River Basin in the
summer of 1993 (attributable by most to the 1990-1995
ENSO event) were prolonged North American climatic
responses to the 1982-1983 El Niño. Warm water left
over from the 1982-1983 ENSO event persisted in the
north Pacific until the year 2000. The oceanographic
effects of a major El Niño thus can have large decadal
persistence outside the tropics.
The 1990-1995 ENSO event was even more
anomalous. This event appeared to wane twice, but
continued with warm central Pacific waters for five
consecutive years, finally terminating in July 1995 - an
unprecedented time span. Probability analysis indi-
cates that an ENSO event of five years' duration should
only recur once every 1500-3000 years. The changes in
climate globally over these five years have been as
dramatic as any observed in historical records. Two of
the worst cyclones ever recorded in the United States,
Hurricanes Andrew in Florida and Iniki in Hawaii,
both in August 1992, occurred during this event.
Hurricane Andrew was unusual because Atlantic hurri-
canes should be suppressed during ENSO events. The
Mississippi River system recorded its greatest flood
ever in 1993 (and then again in 1995), surpassing the
flood of 1973. Record floods devastated western
Europe in 1994-1995. Eastern Australia and Indonesia
suffered prolonged droughts that became the longest
on record. Cold temperatures afflicted eastern North
America in the winter of 1993-1994, together with
record snowfall, while the western half of the continent
registered its highest winter temperatures ever. Record
high temperatures and drought also occurred in Japan,
Pakistan, and Europe in the summer of 1994.
Not all of the climatic extremes of 1990-1995 were
consistent with that formulated for a composite ENSO
event. For example, the Indian monsoon operated
normally in 1994, while eastern Australia recorded its
worst drought. Even locally within Australia, while
most of the eastern half of the continent was in drought
in 1992-1993, a 1000 km 2 region south of Sydney
received its wettest summer on record. It is question-
able whether or not all of these climatic responses can
be attributed to the 1990-1995 ENSO, but they have
occurred without doubt in regions where ENSO
linkages or teleconnections to other climate phenom-
ena operate. Additionally, the unprecedented nature
of the 1982-1983 and 1990-1995 ENSO events may
be linked to significant volcanic eruptions. While the
Mexican El Chichon eruption of 1981 did not trigger
the 1982-1983 El Niño, it may have exacerbated its
intensity. Similarly, the 1990-1995 event corresponded
well with the eruption and subsequent global cooling
generated by significant volcanic eruptions in 1991 of
the Philippines' Mt Pinatubo and Chile's Mt Hudson
and, in 1992, of Mt Spurr in Alaska.
The events of the late twentieth century appear
exceptional. They are not. Historical and proxy records
are showing that mega-ENSO events occur every
400-500 years. For example, around 1100 AD, rivers in
the Moche Valley of Peru reached flood levels of 18 m,
destroying temples and irrigation canals built by the
Chimu civilization. In the same location, the worst
floods of the twentieth century (in 1926) reached
depths of only 8 m. The latter event resulted in massive
fires in the Rio Negro catchment of the Amazonian
Basin. However, similar if not more extensive fires have
occurred in 500, 1000, 1200, and 1500 AD. The 500
and 1100 AD events appear in palaeo-records from
Veracruz and Mexico. In Veracruz, the latter events
produced flooding and laid down sediments over a
metre thick. Compared to this, the 1995 event, which
has been viewed as extreme, deposited only 10-15 cm
of sediment. Other mega-ENSO events occurred
around 400, 1000, 1600, 2400, 4800, and 5600 BC in
Veracruz. As with the prolonged 1990-1995 ENSO
event, it is probable that more than one ENSO event
was involved at these times. The recent prolonged
1990-1995 ENSO cycle may not be unusual, but on
geological timescales represents the upper end of a
shifting climate hazard regime.
La Niña events
Exceptionally 'turned on' Walker circulation is now
being recognized as a phenomenon in its own right.
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