ENSO Complexity Induced by State Dependence of Westerly Wind Events

2017 ◽  
Vol 30 (9) ◽  
pp. 3401-3420 ◽  
Author(s):  
Michiya Hayashi ◽  
Masahiro Watanabe
Keyword(s):  
El Niño ◽  
El Nino ◽  
State Dependence ◽  
The State ◽  
Stochastic Noise ◽  
Westerly Wind ◽  
Central Pacific ◽  
State Dependent ◽  
Wind Events ◽  
Westerly Wind Events

Coupled dynamics between westerly wind events (WWEs) and the El Niño–Southern Oscillation (ENSO) is examined using an atmosphere–ocean coupled model with intermediate complexity. The model incorporates state-dependent stochastic noise that mimics observed WWEs, which occur at the edge of the Pacific warm pool when the Niño-4 sea surface temperature (SST) anomaly increases positively. The model parameter that controls the efficiency of the thermocline feedback, γ, is perturbed to elaborate the sensitivity of the results to the system’s stability. Without the noise (experiment NO), the model produces an ENSO-like regular oscillation with a 6-yr period, the variance of which increases with γ. When additive noise is introduced over the western Pacific (experiment AD), the oscillations become irregular with a dominant period of 4–6 years and the increase in the variance relative to the NO experiment depends on γ. When state-dependent noise is included (experiment SD), the oscillatory solution is also irregular, and its variance and asymmetry are increased irrespective of the value of γ. Both the additive and state-dependent noise contribute to the occurrence of two types of variability, corresponding to the eastern Pacific (EP) and central Pacific (CP) El Niños. In SD, the state dependence of the stochastic noise guarantees the existence of CP El Niño regardless of γ since the increased likelihood of WWE occurrence with Niño-4 SSTs results in a positive feedback in the central Pacific. The above results suggest that the state dependence of WWEs plays a crucial role in the asymmetry and diversity of ENSO.

Observed El Niño SSTA Development and the Effects of Easterly and Westerly Wind Events in 2014/15

2017 ◽  
Vol 30 (4) ◽  
pp. 1505-1519 ◽  
Author(s):  
Andrew M. Chiodi ◽  
D. E. Harrison
Keyword(s):  
Wind Stress ◽  
El Niño ◽  
El Nino ◽  
Ocean Model ◽  
Equatorial Pacific ◽  
Westerly Wind ◽  
Easterly Wind ◽  
Model Studies ◽  
Wind Events ◽  
Westerly Wind Events

Abstract The unexpected halt of warm sea surface temperature anomaly (SSTA) growth in 2014 and development of a major El Niño in 2015 has drawn attention to our ability to understand and predict El Niño development. Wind stress–forced ocean model studies have satisfactorily reproduced observed equatorial Pacific SSTAs during periods when data return from the TAO/TRITON buoy network was high. Unfortunately, TAO/TRITON data return in 2014 was poor. To study 2014 SSTA development, the observed wind gaps must be filled. The hypothesis that subseasonal wind events provided the dominant driver of observed waveguide SSTA development in 2014 and 2015 is used along with the available buoy winds to construct an oceanic waveguide-wide surface stress field of westerly wind events (WWEs) and easterly wind surges (EWSs). It is found that the observed Niño-3.4 SSTA development in 2014 and 2015 can thereby be reproduced satisfactorily. Previous 2014 studies used other wind fields and reached differing conclusions about the importance of WWEs and EWSs. Experiment results herein help explain these inconsistencies, and clarify the relative importance of WWEs and EWSs. It is found that the springtime surplus of WWEs and summertime balance between WWEs and EWSs (yielding small net wind stress anomaly) accounts for the early development and midyear reversal of El Niño–like SSTA development in 2014. A strong abundance of WWEs in 2015 accounts for the rapid SSTA warming observed then. Accurately forecasting equatorial Pacific SSTA in years like 2014 and 2015 may require learning to predict WWE and EWS occurrence characteristics.

scholarly journals About the role of Westerly Wind Events in the possible development of an El Niño in 2014

2014 ◽  
Vol 41 (18) ◽  
pp. 6476-6483 ◽  
Author(s):  
Christophe E. Menkes ◽  
Matthieu Lengaigne ◽  
Jérôme Vialard ◽  
Martin Puy ◽  
Patrick Marchesiello ◽  
...  
Keyword(s):  
El Niño ◽  
El Nino ◽  
Westerly Wind ◽  
Wind Events ◽  
Westerly Wind Events

Westerly Wind Events and the 1997/98 El Niño Event in the ECMWF Seasonal Forecasting System: A Case Study

2003 ◽  
Vol 16 (19) ◽  
pp. 3153-3170 ◽  
Author(s):  
Frédéric Vitart ◽  
Magdalena Alonso Balmaseda ◽  
Laura Ferranti ◽  
David Anderson
Keyword(s):  
El Niño ◽  
El Nino ◽  
Seasonal Forecasting ◽  
Westerly Wind ◽  
El Niño Event ◽  
System A ◽  
Forecasting System ◽  
Wind Events ◽  
Westerly Wind Events

Subseasonal Atmospheric Variability and El Niño Waveguide Warming: Observed Effects of the Madden–Julian Oscillation and Westerly Wind Events*

2014 ◽  
Vol 27 (10) ◽  
pp. 3619-3642 ◽  
Author(s):  
Andrew M. Chiodi ◽  
D. E. Harrison ◽  
Gabriel A. Vecchi
Keyword(s):  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Southern Oscillation ◽  
Equatorial Pacific ◽  
Madden Julian Oscillation ◽  
Westerly Wind ◽  
Near Surface ◽  
Wind Events ◽  
Westerly Wind Events

Abstract Westerly wind events (WWEs) have previously been shown to initiate equatorial Pacific waveguide warming. The relationship between WWEs and Madden–Julian oscillation (MJO) activity, as well as the role of MJO events in initiating waveguide warming, is reconsidered here over the 1986–2010 period. WWEs are identified in observations of near-surface zonal winds using an objective scheme. MJO events are defined using a widely used index, and 64 are identified that occur when the El Niño–Southern Oscillation (ENSO) is in its neutral state. Of these MJO events, 43 have one or more embedded WWEs and 21 do not. The evolution of sea surface temperature anomaly over the equatorial Pacific waveguide following the westerly surface wind phase of the MJO over the western equatorial Pacific is examined. Waveguide warming is found for the MJO with WWE events in similar magnitudes as following the WWEs not embedded in an MJO. There is very little statistically significant waveguide warming following MJO events that do not contain an embedded WWE. The observed SST anomaly changes are well reproduced in an ocean general circulation model forced with the respective composite wind stress anomalies. Further, it is found that the occurrence of an MJO event does not significantly affect the likelihood that a WWE will occur. These results extend and confirm the earlier results of Vecchi with a near doubling of the period of study. It is suggested that understanding the sources and predictability of tropical Pacific westerly wind events remains essential to improving predictions of the onset of El Niño events.

scholarly journals Role of non-linear oceanic processes in the response to westerly wind events: New implications for the 1997 El Niño onset

2001 ◽  
Vol 28 (8) ◽  
pp. 1603-1606 ◽  
Author(s):  
J.-P. Boulanger ◽  
E. Durand ◽  
J.-P. Duvel ◽  
C. Menkes ◽  
P. Delecluse ◽  
...  
Keyword(s):  
El Niño ◽  
El Nino ◽  
Westerly Wind ◽  
Non Linear ◽  
Wind Events ◽  
Westerly Wind Events

Influence of Westerly Wind Events stochasticity on El Niño amplitude: the case of 2014 vs. 2015

2017 ◽  
Vol 52 (12) ◽  
pp. 7435-7454 ◽  
Author(s):  
Martin Puy ◽  
Jérôme Vialard ◽  
Matthieu Lengaigne ◽  
Eric Guilyardi ◽  
Pedro N. DiNezio ◽  
...  
Keyword(s):  
El Niño ◽  
El Nino ◽  
Westerly Wind ◽  
Wind Events ◽  
Westerly Wind Events

scholarly journals Pre- and Post-1997/98 Westerly Wind Events and Equatorial Pacific Cold Tongue Warming*

2009 ◽  
Vol 22 (3) ◽  
pp. 568-581 ◽  
Author(s):  
D. E. Harrison ◽  
A. M. Chiodi
Keyword(s):  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Equatorial Pacific ◽  
Cold Tongue ◽  
Westerly Wind ◽  
Basin Scale ◽  
Wind Events ◽  
Westerly Wind Events

Abstract Westerly wind events (WWEs) in the western equatorial Pacific have previously been shown to cause significant warming of sea surface temperature (SST) in the eastern equatorial Pacific. Observational statistics compiled during and prior to the large El Niño event of 1997/98 link WWEs to substantial (up to 3°C) warming in the eastern Pacific cold tongue region. Since 1998, however, relatively little WWE-related cold tongue warming has been observed, and warm equatorial Pacific SST anomalies (SSTAs) have tended to be trapped near the date line rather than extending to the American coast as in a classical El Niño–Southern Oscillation (ENSO) composite. Here, the relationship between WWEs and cold tongue warming is revisited using in situ and operational forecast winds and in situ and satellite-based SST. Significant differences are found in the basin-scale zonal wind anomalies associated with WWEs that occurred before and after 1997/98. Although the post-1997/98 composite WWE westerly anomalies are very similar to their predecessors within the WWE regions, conditions east of the WWE regions are different; there are enhanced equatorial easterlies in the post-1997/98 cases. General ocean circulation model experiments are conducted to explore the extent to which the observed changes in the character of post-1997/98 WWEs can explain the recent behavior of cold tongue SSTAs. It is found that the wind differences can account for the changes in the average cold tongue warming associated with pre- and post-1997/98 WWEs.

scholarly journals Extreme Noise–Extreme El Niño: How State-Dependent Noise Forcing Creates El Niño–La Niña Asymmetry

2016 ◽  
Vol 29 (15) ◽  
pp. 5483-5499 ◽  
Author(s):  
Aaron Levine ◽  
Fei Fei Jin ◽  
Michael J. McPhaden
Keyword(s):  
El Niño ◽  
Climate Models ◽  
El Nino ◽  
Southern Oscillation ◽  
State Dependence ◽  
Westerly Wind ◽  
State Dependent ◽  
Westerly Wind Burst ◽  
Enso Asymmetry ◽  
Extreme El Niño

Abstract A major open question about El Niño–Southern Oscillation (ENSO) is what causes ENSO amplitude asymmetry, with strong El Niños generally larger than strong La Niñas. The authors examine a leading hypothesis—that the ENSO state modifies the fetch and/or wind speed of westerly wind bursts (WWBs) that create asymmetric forcing and an asymmetric ENSO response. Further, in El Niño forecasts, the number of WWBs expected increases in the month following a strong WWB when compared with the month preceding it. Using a conceptual model, a relationship is derived between the magnitude of the westerly wind burst state dependence on ENSO and ENSO asymmetry. It is found that this relationship between the magnitude of the state dependence and ENSO asymmetry holds in both the observations and 21 coupled climate models. Finally, it is found that because of state-dependent westerly wind burst forcing, extreme El Niño events tend to be of the eastern Pacific variety.

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