February 7, 2020: Ocean Circulation and Climate Dynamics Colloquium

Friday, February 7,2020
12:00, Lecture Hall, Düsternbrooker Weg 20

S. Neske^1,2, S. McGregor^1,3, M. Zeller^1,2 and D. Dommenget^1,3
1School of Earth, Atmosphere and Environment, Monash University, Clayton, Australia
²ARC Centre of Excellence for Climate System Science, Monash University, Melbourne, Australia
³ARC Centre of Excellence for Climate Extremes, Monash University, Melbourne, Australia

The El Niño- Southern oscillation (ENSO) is the largest climate variability on interannual time scales on our planet with near-global dramatic climate impacts. Thus, it is important to enhance our understanding of details of ENSO dynamics and its predictability both of which remain a major challenge for scientists today.

To enhance our understanding of ENSO, a wind forced ocean model is used to decompose the equatorial Pacific warm water volume (WWV) precursor of ENSO between 1980-2016 into two components, the (i) adjusted wind response, which is consistent with the ENSO phase change driver of traditional ENSO theories; and (ii) instantaneous wind response, which are the instantaneous WWV changes due to Ekman transports dominated by wind-forced Kelvin waves. While the adjusted contribution dominates the WWV changes during the pre-2000 period, the instantaneous contribution dominates the WWV changes during the post-2000 period. This change in the relative importance of WWV drivers is shown to explain the shortening of the WWV/ENSO SST lead time from 2-3 seasons during the pre-2000 period to only 1 season during the post-2000 period and it is consistent with the reduction in post-2000 ENSO prediction skill. Further, the contrasting post-2000 44% decline of the adjusted WWV and 15% increase of instantaneous WVV demonstrate that the generalization of strong anomalous equatorial Pacific wind stress (i.e. strong instantaneous WWV responses) leading to strong adjusted WWV responses cannot be correct. We shed light onto this misconception by dividing the oceanic adjusted responses to the wind stress of the strongest instantaneous WWV changes into three categories to better understand the importance of spatially varying wind stress patterns for ENSO dynamics.

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