Thursday, 13 January 2000: 8:30 AM
During El Niño years, there is a cold sea surface temperature (SST) 'horseshoe'
pattern in the western Pacific, in addition to the large warm SST anomaly in the
eastern and central Pacific. The precipitation anomalies in the Pacific include
enhanced precipitation in the eastern and central Pacific, but this is
surrounded by reduced rainfall to the west and to the north and south. While
these also occur in a 'horseshoe pattern', it is not obvious whether they are
related to the cold or warm SST anomalies. At the same time, anomalies in both
SST and rainfall often occur in other basins that may or may not be related to
those in the Pacific. We use an intermediate atmosphere model (QTCM), combined
with a slab mixed-layer ocean and/or specified SST anomalies to investigate some
of the possible teleconnection pathways. The 1997-1998 El Niño event is used as
an example. One set of experiments aims at separating the effects of warm SST
anomalies over the main El Niño region from the effects of cold western Pacific
SSTs. The preliminary model results suggest that the reduced precipitation is
mostly caused by subsidence induced by the enhanced convection over central and
eastern Pacific. In experiments with the mixed layer active over all of the
tropics except the main El Niño region, atmospheric teleconnections actually
tend to produce small warm anomalies in the descent region, tending to spread
the warm anomaly. This suggests that the observed cold 'horseshoe' SST
anomalies are produced by ocean dynamical effects, not by surface fluxes. When
the observed cold anomalies are specified in the Pacific, they contribute to the
local negative precipitation anomaly, but with smaller magnitude. The positive
SST anomaly over eastern and central Pacific is clearly connected to the drought
over the northern South America. It produces a weak positive SST anomaly over
the western tropical Atlantic, but has little effect on Indian ocean SST in the
mixed-layer model. Teleconnection pathways are more evident in the surface wind
fields, which tend to have longer zonal scales than the subsidence anomalies.
![](images/flag.png)
![](images/blue_ribbon.png)