10/16/2012
El Niño and Maximum Temperature Extremes
Summary
A study led by a U.S. Department of Energy (DOE)-funded scientist examines the impact of the El Niño-Southern Oscillation (ENSO) on temperature extremes for both observations and coupled climate model simulations. The recently developed observed gridded dataset of climate extremes indices (HadEX2) shows marked contrasts in seasonal composites of the monthly maximum value of daily maximum temperatures during the cold and warm phases of ENSO. Extreme maximum temperatures are significantly cooler over Australia, southern Asia, Canada, and South Africa during strong La Niña events compared to El Niño events and significantly warmer over the contiguous United States and southern South America. Two versions of the DOE-National Science Foundation Community Climate System Model (CCSM3 and CCSM4) are contrasted for their ability to capture these relationships given their very different simulations of ENSO. The CCSM3 ENSO simulation has a strong biennial frequency that is more narrowly confined along the equator than observations, while the CCSM4 ENSO simulation is more realistic in both frequency and pattern. While both models capture some aspects of the observed regional changes across the globe, the fidelity of the ENSO simulation appears to be crucial for simulating the magnitude and sign of the extreme maximum temperature relationships. Over the United States in particular, the composite pattern of maximum temperature extremes with ENSO is weak and opposite in sign for CCSM4 compared to that observed for CCSM3. CCSM4 is much improved, capturing the observed increase in U.S. maximum temperature extremes during La Niña with realistic amplitude, pattern, and statistical significance. In a future emissions scenario of CCSM4, the contrast between maximum temperature extremes during El Niño and La Niña events strengthens over Australia whereas it weakens slightly over the United States. Further understanding of the mechanisms leading to these projected changes will enable better predictions of regional changes due to ENSO.
References
Arblaster, J. M., and L.V. Alexander. 2012. “The Impact of the El Niño-Southern Oscillation on Maximum Temperature Extremes,” Geophysical Research Letters 39(20), L20702. DOI: 10.1029/2012GL053409.