The El Niño-Southern Oscillation (ENSO) is a regular climatic fluctuation but each El Niño event observed over recent decades has had different characteristics. Since ENSO has enormous impacts on natural and human systems – including agriculture, forestry, public health, the hydrological cycle, the global carbon cycle, marine and terrestrial ecosystems, and fisheries – being able to predict future ENSO events is critical. In an article recently published in Reviews of Geophysics, Yeh et al.  considered the atmospheric impacts of recent El Niños and how these might change in future with a warming climate. The editor asked one of the authors to explain why and how El Niño events vary, and the challenges of predicting future events.
Are the patterns of ENSO similar and predictable each cycle?
It depends on which period of observational data is analyzed. For example, it has been observed that the center of ENSO has shifted to the west from 1979-1997 to 1998-2015. Furthermore, the amplitude of El Niño events weakened from before the late 1990s to the first 15 years of the 21st century. Therefore, the ENSO properties, including the spatial pattern and amplitude, change on the low-frequency timescales.
How does the diversity of each ENSO cycle effect the predictability of teleconnections?
ENSO teleconnections in response to ENSO diversity result in different remote impacts in precipitation and/or temperature around the globe.
Consequently, this diversity affects the predictability of ENSO teleconnections, as well as the characteristics of extreme weather events in response to ENSO.
Therefore, the predictability of teleconnections largely depends on the diversity of each ENSO cycle.
The reason of why the ENSO cycle affects the predictability of teleconnections is that ENSO teleconnections are so sensitive to the longitude where atmospheric deep convection is triggered.
These longitudes are determined by the diversity of each ENSO cycle.
What are some of the most notable atmospheric teleconnections caused by ENSO?
ENSO atmospheric teleconnections influence weather and climate conditions over other parts of the globe. For example, ENSO activity causes anomalous convection which, once it reaches the upper atmosphere, excites the Rossby waves which then move in a Pacific-North American (PNA) pattern and Pacific-South American (PSA) pattern in the Northern and Southern Hemispheres respectively.
Although ENSO is based in the Pacific Ocean, how does it affect other oceans?
The impacts on regions outside the tropical Pacific arise through atmospheric teleconnections and oceanic teleconnections. In the tropical Pacific, for example, ENSO alteration of the Walker circulation (i.e., the near-equatorial zonal atmospheric overturning circulation) induces changes in cloud cover and evaporation in the South China Sea, Indian Ocean, and tropical North Atlantic, with a time lag of three to six months.
How are ENSO teleconnections likely to alter under global warming caused by greenhouse gases?
In the CMIP3 climate models, there is an expected eastward shift of the Pacific-North American teleconnection pattern under global warming, resulting from a systematic eastward migration of convection centers associated with both El Niño and La Niña. In a warmer climate, precipitation anomalies are enhanced and they move eastward over the equatorial Pacific because the enhanced mean sea surface temperature warming in the eastern basin reduces the barrier to deep convection there. Such an eastward shift in tropical convective anomalies can cause ENSO-forced PNA teleconnections to shift eastward and to intensify under greenhouse warming. The dominant mode of global precipitation-evaporation variability on interannual timescales is mainly due to ENSO teleconnections on interannual timescales. Therefore, the increase in frequency of extreme ENSO events may cause more severe droughts and floods in a warmer climate, with largest increases in the tropical Pacific and polar regions.
How do models need to improve to better predict ENSO?
To better predict ENSO characteristics and impacts, it is crucial to realistically simulate changes in the mean state of the tropical Pacific Ocean and the changes in ENSO properties, including the spatial pattern and amplitude. These factors are closely associated with the changes in the intensity and location of tropical convective forcing, which ultimately determine the characteristics of ENSO teleconnections. In particular, the current CMIP-class climate models still suffer from the double Intertropical Convergence Zone (ITCZ) syndrome, which is characterized by a too symmetric mean precipitation pattern toward the equator associated with an overly zonal South Pacific Convergence Zone.
—Sang-Wook Yeh, Department of Marine Science and Convergent Technology, Hanyang University, Korea; email: [email protected]