Teleconnections and local response to tropical SST anomalies

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.