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Geoffrey Gebbie
and
Eli Tziperman

1. Introduction Westerly wind bursts (WWBs) have occurred in the tropical Pacific during the onset and development of every major El Niño event of the last 25 years ( Kerr 1999 ; McPhaden 2004 ). Recent observational evidence shows that the timing and characteristics of these seemingly random wind bursts are, in fact, modulated by the large-scale SST and, in particular, by the phase of ENSO ( Yu et al. 2003 ; Batstone and Hendon 2005 ; Tziperman and Yu 2007 , hereafter TY ). Recent modeling

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Minmin Fu
and
Eli Tziperman

1. Introduction Westerly wind bursts (WWBs) are episodic, anomalous westerly perturbations of the zonal equatorial winds which can result in local reversals of the trade winds. Previous studies have defined them to have a minimum anomalous wind strength ranging from 2 to 5 m s −1 , with a zonal extent of 5°–40° and duration of 2–30 days, and found their frequency to be around 5 to 15 times per year ( Hartten 1996 ; Harrison and Vecchi 1997 ; Yu et al. 2003 ; Seiki and Takayabu 2007a ). WWBs

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Ayako Seiki
and
Yukari N. Takayabu

– 882 . Bjerknes , J. , 1969 : Atmospheric teleconnections from the equatorial Pacific. Mon. Wea. Rev. , 97 , 163 – 172 . Chu , P-S. , 1988 : Extratropical forcing and the burst of equatorial westerlies in the western Pacific: A synoptic study. J. Meteor. Soc. Japan , 66 , 549 – 564 . Eisenman , I. , L. Yu , and E. Tziperman , 2005 : Westerly wind bursts: ENSO’s tail rather than the dog? J. Climate , 18 , 5224 – 5238 . Fedorov , A. V. , 2002 : The response of the

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Minmin Fu
and
Eli Tziperman

1. Introduction Westerly wind bursts (WWBs) are episodic, anomalous equatorial westerly wind anomalies, which can result in local reversals of the trade winds. Previous studies have defined them as events with an anomalous wind strength exceeding 5–7 m s −1 over a zonal extent of at least 10°, and meeting the above criteria for at least 2–5 days. Following the definitions above, they occur about 5–15 times per year ( Hartten 1996 ; Harrison and Vecchi 1997 ; Yu et al. 2003 ; Seiki and

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Eli Tziperman
and
Lisan Yu

1. Introduction Westerly wind bursts in the equatorial Pacific may be defined as wind events with speed larger than, say, 4 m s −1 and lasting at least a few days. These events are known to play an important role in ENSO’s dynamics, and in particular during ENSO’s onset (e.g., Lau and Chan 1988 ; Lengaigne et al. 2004 ; McPhaden 1999 ). These events seem to result from various mechanisms, from tropical cyclones ( Keen 1982 ) to cold surges from midlatitudes ( Chu 1988 ), the Madden

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Geoffrey Gebbie
,
Ian Eisenman
,
Andrew Wittenberg
, and
Eli Tziperman

1. Introduction Episodes of strong westerly winds frequently occur over the tropical Pacific ( Harrison and Giese 1988 ; Verbickas 1998 ; Delcroix et al. 1993 ). These wind events, known as westerly wind bursts (WWBs), last for 5–40 days, and have no easterly wind analog. Although different definitions have been proposed to diagnose WWBs from observations (e.g., Harrison and Vecchi 1997 ; Yu et al. 2003 ), it is clear that every significant El Niño of the past 25 years has been accompanied

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Ayako Seiki
,
Yukari N. Takayabu
,
Takuya Hasegawa
, and
Kunio Yoneyama

forcing along the equator called westerly wind bursts (WWBs). WWBs are synoptic-scale westerly winds that blow over the surface near the equator. These winds can force strong eastward currents that advect warm surface water from the western Pacific to the central Pacific. In addition, WWBs can excite oceanic downwelling Kelvin waves along the equator that propagate eastward and depress the thermocline, resulting in positive sea surface temperature (SST) anomalies in the eastern Pacific where the

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Sungduk Yu
and
Alexey V. Fedorov

1. Introduction Episodic zonal wind anomalies, commonly known as westerly wind bursts (WWBs; also called westerly wind events) are frequently observed in the equatorial Pacific during El Niño events, in particular during extreme events such as those in 1982, 1997, and 2015 ( Fig. 1 ). The occurrence of WWBs depends on a variety of phenomena whose time scales are shorter than El Niño–Southern Oscillation (ENSO), including the Madden–Julian oscillation ( Yu and Rienecker 1998 ; Lau et al

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Chaopeng Ji
,
Mu Mu
,
Xianghui Fang
, and
Lingjiang Tao

Rienecker 1998 ; Vecchi et al. 2006 ; Mu et al. 2007 ; Xu et al. 2021 ). A widely accepted theoretical hypothesis is that El Niño is maintained by external atmospheric stochastic forcing (e.g., Fedorov 2002 ; McPhaden et al. 2006 ; Chen et al. 2022 ). Uncertainties in stochastic forcing thus allow the forecasting of El Niño amplitude to inevitably contain uncertainties ( Lian and Chen 2021 ; Fang and Chen 2023 ). Among the stochastic forcings, westerly wind bursts (WWBs) are the most relevant

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Ayako Seiki
and
Yukari N. Takayabu

1. Introduction Westerly wind bursts (WWBs) are synoptic-scale disturbances represented by strong westerly winds near the equator and have been known to trigger or enhance El Niño development ( Harrison and Schopf 1984 ; McPhaden et al. 1988 ; McPhaden et al. 1992 ; McPhaden 1999 ; Lengaigne et al. 2002 ; McPhaden 2004 ). In Seiki and Takayabu (2007 , hereafter Part I ) of this study, significant lag correlations were found between Niño-3 region sea surface temperature (SST) anomalies

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