ENSO teleconnections have been the primary basis for seasonal to interannual climate prediction. Droughts in Brazil and Indonesia and cooler and wetter conditions in the Southeast United States are likely when El Niño is present; the reverse pattern is expected in these areas during a La Niña episode (Diaz et al. 2001, Glantz 2000). It should be noted that the strength of the ENSO event can impact these global teleconnections nonlinearly. In addition, the location of the event (EPW versus CPW) can drastically change the climatological teleconnections. For example, Kim et al. (2009) found that the tracks of tropical storms in the North Atlantic Basin make landfall more often during CPW than during EPW. The number of named storms in the Atlantic hurricane season also rises during cold events and declines during EPW events.
The most prominent feature associated with the warm phase of ENSO is a dry anomaly wrapping around the area of warming in the eastern Pacific extending to the north- and southeast from the Western Equatorial Pacific. This anomalous "horseshoe" pattern is produced by the convective branch of the Walker cell being displaced eastward from Indonesia to the open ocean. There is anomalous precipitation over the unusually warm equatorial waters. This pattern can be seen in Fig. 1.3. Another striking feature is the rain in coastal Peru where, normally, there is little rain. During the cold phase, the convective branch of the Walker circulation strengthens and the "horseshoe" pattern is anomalously wet. Fig. 1.4 shows robust temperature ENSO teleconnections for both warm and cold events.
Fig. 1.3 Schematic representation of the principal ENSO precipitation teleconnections. From Ropelewki and Halpert (1987)
North American winters are heavily influenced by the warm and cold phases of ENSO. The Southeast United States experiences a cooler and wetter winter during the warm phase and a warmer and drier winter during the cold phase. Likewise, the northwestern portion of the continent has a milder winter during the warm phase and a colder winter during the cold phase. The ENSO-related shift of the Polar Jet over the Pacific and the North American continent is responsible for these changes (Glantz 2000).
Fig. 1.4 Schematic representation of the principal ENSO temperature teleconnections for (a) cold (b) warm. From Halpert and Ropelewki (1992).
The situation in North America during the summer is much different. During boreal summer, the North American continental climate has a much lower correlation to the ENSO index. Yet, a significant effect is felt from the changes in the seasonal tropical cyclone (TC) activity in the Atlantic basin (Gray 1984). During a warm phase there is less TC activity due to increased vertical shear in the TC development region that inhibits the strengthening of storms. A cold ENSO phase acts to reduce the amount of shear and more TC's are observed in cold years. Tang and Neelin (2004) have shown that the onset period of a warm phase can also alter TC activity. If the nonneutral phase begins at the start of a hurricane season, then the adjustment from neutral to warm (cold) causes the troposphere over the TC development region to be more (less) stable than it is during a neutral year, thus acting to diminish (promote) TC activity for several months. In a more recent paper, Kim et al. (2009) show that anomalous climatological global circulation associated with EPW and CPW causes a large difference in the number of landfalling TCs. In the case where warming occurs in the Eastern Pacific, the effect is a reduction in storms. If the warming is located in the Central Pacific, then the TC tracks in that hurricane season will more likely make landfall over the continental United States.
Whereas the conclusions about teleconnections discussed thus far have been based on observations, Hoerling and Kumar (2002) used model output to construct a more thorough outline of the teleconnections. The results were categorized as (a) weak, moderate, and strong events and (b) West, Central, and East Pacific events to illustrate the nonlinear relationships between ENSO forcing and regional climates.
In accordance with the North American teleconnections described above, modeled conditions in the northwest are drier during a weaker warm ENSO phase (Hoerling and Kumar 2002). It might be assumed that the stronger the warm anomaly, the drier conditions in this region would be. However, it was found that for moderate and strong warm phases, the Pacific storm track takes a different route and causes more precipitation in California rather than less. In the model, the cool-wet relationship in the Southeast United States does prove to have a more linear relationship with increasingly stronger warm events. The location of the warm event (either West, Central, and East Pacific) does not have a linear relationship with the climate of the South East United States. Contrary to the precipitation relationship implied by the TC tracks discussed in Kim et al. (2009), the Southern United States is drier during Central Pacific warm events in the model. This shows that improvements to the models are still needed to make seasonal predictions confidently.