El Nino and La Nina: Asymmetry, Impact and Decadal Variability
Yuko Okumura (UTIG)
Fri, November 17, 10:30am - 11:30am
Host: Pedro Di Nezio
Abstract: The equatorial Pacific Ocean undergoes episodic warming (El Nino and cooling (La Nina) at intervals of 3-8 years, accompanied by changes in surface winds. This so-called El Nino-Southern Oscillation (ENSO) phenomenon arises from dynamic and thermodynamic feedbacks between the tropical ocean and atmosphere, and affects weather patterns around the world through atmospheric teleconnections.
El Nino ad La Nina are not a simple mirror image and exhibit significant differences in their spatial patterns and temporal evolution, due to nonlinearities in the tropical ocean-atmosphere system. In particular, most strong El Nino events terminate after one year whereas La Nina events that follow tend to last 2 years or longer. Observational analysis shows that these multi-year La Nina events have persistent influences on atmospheric circulation and drought conditions across the southern tier of the United States. La Nina impact becomes stronger and more extensive in the second year, compared to the first year, despite weakening in the equatorial Pacific cooling. This paradoxical strengthening of La Nina impact in the second year is related to subtle changes in the pattern of tropical Pacific cooling.
The ENSO is thus a key source of climate variability on interannual timescales. The amplitude of ENSO, however, is known to modulate on decadal timescales. Analysis of a long climate model simulation shows that not only the amplitude but also other properties of ENSO vary systematically in association with decadal changes in the background state of tropical Pacific climate. For example, when the eastern tropical Pacific becomes warmer relative to the western tropical Pacific, the ENSO amplitude increases and El Nino and La Nina become more asymmetric, with increased duration of La Nina hen the tropical Pacific warms across the basin, on the other hand, El Nino becomes more frequent and lasts longer. A comparison of hierarchical climate model simulations suggests positive feedbacks between decadal changes in the background state and the ENSO.
