Asymmetry of Westerly and Easterly Wind Events: Observational Evidence

Abstract

Westerly (WWEs) and easterly (EWEs) wind events, shortlived anomalous westerly and easterly surface winds, are known to occur with unusual large magnitude over the equatorial Pacific. As their relative frequency of occurrence and dependence on background conditions are yet to be fully clarified, we analyzed daily surface winds for 1982−2013 from which WWEs and EWEs are detected. Both types of events appear over the Pacific warm pool, where sea surface temperature (SST) is sufficiently high for active deep convection, and favorably occur with increasing Niño4 SST. However, the frequency of occurrence of EWEs is less than that of WWEs, resulting in asymmetry in wind amplitude. Local and remote anomalous convections are equally important in exciting these events, but different local development processes cause the asymmetry in the frequency of occurrence. These results can also be seen in wind stress anomalies, albeit obscured due to nonlinearity therein.(Citation: Hayashi, M., and M. Watanabe, 2016: Asymmetry of westerly and easterly wind events: Observational evidence. SOLA, 12, 42−45,

Importance of background seasonality over the eastern equatorial Pacific in a coupled atmosphere-ocean response to westerly wind events

“…In reality, the timing and location of WWEs are state dependent, that is, partly controlled by background SST, sea level pressure, and low-level zonal wind (Lengaigne et al 2003;Yu et al 2003;Eisenman et al 2005;Seiki and Takayabu 2007a, b;Sooraj et al 2009;Miyama and Hasegawa 2014;Hayashi and Watanabe 2016). Generally, WWEs frequently appear over the western Pacific warm pool, and their timing is partly determined by atmospheric intraseasonal phenomena such as the MJO (Puy et al 2015), although the statistical relationship between the WWEs and MJO is not very significant in previous studies (Seiki and Takayabu 2007a;Chiodi et al 2014).…”

“…Although the ITCZ cannot interact with the warmer SSTA induced by the WWE in cold cases, there is the interaction between them in moderate warm cases, similar to the neutral cases. Because, in nature, the strength of zonal wind is skewed positively over the western equatorial Pacific (e.g., Hayashi and Watanabe 2016), the WWE is a candidate for the phenomena that generate the asymmetry of the ENSO.…”

“…Various atmospheric phenomena such as the Madden-Julian oscillation (MJO), tropical cyclones, and cold surges are thought to cause WWEs stochastically (Luther et al 1983;Hartten 1996;Harrison and Vecchi 1997;Vecchi and Harrison 2000) or state dependently (Eisenman et al 2005;Seiki and Takayabu 2007a, b;Puy et al 2015;Hayashi and Watanabe 2016). Despite their short duration, WWEs are important elements to the tropical climate variability as they sometimes trigger El Niño events (Wyrtki 1975;McPhaden 1999;McPhaden and Yu 1999).…”

Effective ENSO Amplitude Forecasts Based on Oceanic and Atmospheric Preconditions

“…Similarly, the eastward counterpart to the WWEs, known as the easterly wind surges (EWSs), were claimed to be important for the onset and development of La Niña (Chiodi and Harrison 2015). Observational evidence shows that the WWEs and EWSs exhibit several asymmetric features, such as zonal extension (Puy et al 2016) and frequency of occurrence (Hayashi and Watanabe 2016). The basic characteristics of WWEs and EWSs and their relationship with ENSO has been extensively studied, although the root causes of these wind bursts are still debated (Chang et al 1979;Keen 1982;Love 1985;Nitta 1989;Lander 1990;Chiodi et al 2014;Puy et al 2016).…”

Impact of El Niño Modoki on Indian summer monsoon rainfall: Role of western north Pacific circulation in observations and CMIP5 models

“…Many observational studies in the recent period have shown that the El Niño Modoki has intensified in the past 3 decades (Lee and McPhaden, 2010). Numerous studies (e.g., Weng et al, 2007;Kim et al, 2009;Taschetto and England, 2009;Lee and McPhaden, 2010;Mo, 2010;Yu et al, 2010;Feng et al, 2011;Yu and Kim, 2011;Kim et al, 2012;Yu et al, 2012;Yuan and Yang, 2012;Wang, 2013, 2014;Liu et al, 2014;Capotondi et al, 2015;Hayashi and Watanabe, 2016;Tan et al, 2016) have demonstrated different impacts of these two types of El Niños on many regional phenomena like the summer climate of the Pacific rim countries, East Asian summer climate, Atlantic hurricane frequency, warming in the south Pacific ocean, and winter surface temperature of the United States etc. Using a simple baroclinic model Feng et al (2010) have studied the impacts of conventional ENSO and El Niño Modoki on wintertime Southeast Asian rainfall and related mechanisms.…”