Modulation of equatorial Pacific westerly/easterly wind events by the Madden–Julian oscillation and convectively-coupled Rossby waves

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.

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Upper-Ocean Circulation Anomalies of the Western Equatorial Pacific Observed in 2016 Summer

10.5194/egusphere-egu2020-2333 ◽

2020 ◽

Author(s):

Yilong Lyu

Keyword(s):

Ocean Circulation ◽

Rossby Waves ◽

New Guinea ◽

Equatorial Pacific ◽

Eastern Pacific ◽

Boreal Summer ◽

Western Boundary ◽

Easterly Wind ◽

Western Equatorial Pacific ◽

Circulation Anomalies

<p>Mooring measurements at ~140&#176;E in the western equatorial Pacific documented greatly intensified eastward subsurface currents, which largely represents the nascent Equatorial Undercurrent (EUC), to ~67 cm s<sup>-1</sup> in boreal summer of 2016. The eastward currents occupied the entire upper 500 m, with the westward surface currents nearly diminished. Similar variations were also observed during previous El Ni&#241;o events, as suggested by historical in-situ data. Further analysis combining satellite and reanalysis data reveals that the eastward currents observed at ~140&#176;E are a component of an anomalous counterclockwise circulation straddling the equator, with westward current anomalies retroflecting near the western boundary and feeding southeastward current anomalies along New Guinea coast. A 1.5-layer reduced-gravity ocean (RGO) model is able to crudely reproduce these variations, and a hierarchy of sensitivity experiments are performed to understand the underlying dynamics. The observed circulation anomalies are largely the delayed ocean response to the strong equatorial wind anomalies over the central-to-eastern Pacific basin emerging in the mature stage of El Ni&#241;o (September-April). Downwelling equatorial Rossby waves are generated by the reflection of equatorial Kelvin waves and easterly wind anomalies in the eastern Pacific. Upon reaching western Pacific, the Southern Hemisphere lobe of Rossby waves encounter the slanted New Guinea island and deflects equatorward, establishing a local sea surface height maximum near the equator and leading to the detour of westward currents flowing from the Pacific interior. Additional experiments with edited western boundary geometry confirm the importance of topography in regulating the structure of this cross-equatorial anomalous circulation.</p>

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

Journal of Climate ◽

10.1175/jcli-d-13-00547.1 ◽

2014 ◽

Vol 27 (10) ◽

pp. 3619-3642 ◽

Cited By ~ 34

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.

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Energy Flow Diagnosis of ENSO from an Ocean Reanalysis

Journal of Climate ◽

10.1175/jcli-d-20-0704.1 ◽

2021 ◽

Vol 34 (10) ◽

pp. 4023-4042

Author(s):

Takahiro Toyoda ◽

Hideyuki Nakano ◽

Hidenori Aiki ◽

Tomomichi Ogata ◽

Yoshiki Fukutomi ◽

...

Keyword(s):

Data Assimilation ◽

Time Evolution ◽

Wave Energy ◽

El Niño ◽

El Nino ◽

Surface Wind ◽

Equatorial Pacific ◽

Ocean Reanalysis ◽

Easterly Wind ◽

Wind Events

AbstractA method is introduced for diagnosing the time evolution of wave energy associated with ENSO from an ocean reanalysis. In the diagnosis, time changes of kinetic and available potential energy are mainly represented by energy inputs caused by surface wind stress and horizontal energy fluxes for each vertically decomposed normal mode. The resulting time evolutions of the wave energy and vertical thermocline displacements in the 1997/98 and 2014–16 El Niño events are consistent with our previous knowledge of these events. Further, our result indicated that representation of several vertical modes is necessary to reproduce the broadly distributed downward thermocline displacements in the central to eastern equatorial Pacific, generated by a westerly wind event in the western equatorial Pacific (e.g., in March 1997), that are preconditioning for El Niño development. In addition, we investigated the wave energy budget, including the influence of data assimilation, on the complicated time evolution of equatorial thermocline displacements caused by repeated westerly and easterly wind events during the 2014–16 El Niño event. Our result suggests that noise from a momentum imbalance near the equator associated with data assimilation, which possibly affected the El Niño prediction failure in 2014, was much reduced by our developed ocean data assimilation system and reanalysis. This study, which provides a new connection between the theoretical works and reanalysis products that use sophisticated systems for synthesizing OGCMs and observations, should be useful for climate research and operational communities interested in ENSO.

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Dynamical Ocean Forcing of the Madden–Julian Oscillation at Lead Times of up to Five Months

Journal of Climate ◽

10.1175/jcli-d-11-00268.1 ◽

2012 ◽

Vol 25 (8) ◽

pp. 2824-2842 ◽

Cited By ~ 18

Author(s):

Benjamin G. M. Webber ◽

David P. Stevens ◽

Adrian J. Matthews ◽

Karen J. Heywood

Keyword(s):

Indian Ocean ◽

Rossby Waves ◽

Surface Wind ◽

Wind Forcing ◽

Global Ocean ◽

Dynamical Response ◽

Madden Julian Oscillation ◽

Easterly Wind ◽

Ocean Response ◽

Equatorial Rossby Waves

Abstract The authors show that a simple three-dimensional ocean model linearized about a resting basic state can accurately simulate the dynamical ocean response to wind forcing by the Madden–Julian oscillation (MJO). This includes the propagation of equatorial waves in the Indian Ocean, from the generation of oceanic equatorial Kelvin waves to the arrival of downwelling oceanic equatorial Rossby waves in the western Indian Ocean, where they have been shown to trigger MJO convective activity. Simulations with idealized wind forcing suggest that the latitudinal width of this forcing plays a crucial role in determining the potential for such feedbacks. Forcing the model with composite MJO winds accurately captures the global ocean response, demonstrating that the observed ocean dynamical response to the MJO can be interpreted as a linear response to surface wind forcing. The model is then applied to study “primary” Madden–Julian events, which are not immediately preceded by any MJO activity or by any apparent atmospheric triggers, but have been shown to coincide with the arrival of downwelling oceanic equatorial Rossby waves. Case study simulations show how this oceanic equatorial Rossby wave activity is partly forced by reflection of an oceanic equatorial Kelvin wave triggered by a westerly wind burst 140 days previously, and partly directly forced by easterly wind stress anomalies around 40 days prior to the event. This suggests predictability for primary Madden–Julian events on times scales of up to five months, following the reemergence of oceanic anomalies forced by winds almost half a year earlier.

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Asymmetry of Westerly and Easterly Wind Events: Observational Evidence

SOLA ◽

10.2151/sola.2016-009 ◽

2016 ◽

Vol 12 (0) ◽

pp. 42-45 ◽

Cited By ~ 1

Author(s):

Michiya Hayashi ◽

Masahiro Watanabe

Keyword(s):

Observational Evidence ◽

Easterly Wind ◽

Wind Events ◽

Easterly Wind Events

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Understanding the atmospheric circulations that lead to high particulate matter concentrations on the west coast of Namibia

Clean Air Journal ◽

10.17159/2410-972x/2017/v27n2a9 ◽

2017 ◽

Vol 27 (2) ◽

Cited By ~ 1

Author(s):

H Liebenberg-Enslin ◽

H Rauntenbach ◽

B Sibiya ◽

R Oosthuizen

Keyword(s):

Particulate Matter ◽

West Coast ◽

Climatic Factors ◽

Arid Zone ◽

Dust Storms ◽

The West ◽

Easterly Wind ◽

Atmospheric Circulations ◽

Wind Events ◽

Easterly Wind Events

Atmospheric circulations play a significant role in determining the extent and impact of local and regional air pollution. The Erongo Region, located in the western part of Namibia, falls within the west coast arid zone of southern Africa, and is characterised by low rainfall, extreme temperatures and unique climatic factors influencing the natural environment and biodiversity. Episodic dust storms, associated with easterly wind conditions, are common during austral autumn and winter months. During these events, dust is transported westwards over long distances across the Namibian continent towards the Atlantic Ocean. During 2017, such easterly wind conditions appeared to occur earlier and more frequently than in previous years. Of interest is that high PM10 concentrations (particulate matter with aerodynamic diameters of less than or equal to 10 micron) measured at the coastal towns of Swakopmund and Walvis Bay in the Erongo Region during 2017 were found to also coincide with south-westerly to north-westerly winds from the ocean during prevailing easterly wind events. In this study, the easterly wind events that occurred on 19 March 2017 and 6 July 2017 were assessed to investigate how local-scale coastal atmospheric circulation changes could have developed from the easterly wind conditions, and how such development could have contributed to wind direction deviations and the high PM10 concentrations measured at Swakopmund and Walvis Bay. It was found that in addition to the westward transport of PM10 from inland sources during easterly wind events, higher coastal concentrations of PM10 can also develop as a result of north-easterly / south-westerly wind conversion lines and the cyclonic circulation enhancement associated with easterly wind induced coastal troughs and coastal lows.

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Western Iberian winter wind indices based on significant wind events

Ocean Science Discussions ◽

10.5194/osd-2-105-2005 ◽

2005 ◽

Vol 2 (2) ◽

pp. 105-127 ◽

Cited By ~ 5

Author(s):

E. Mason ◽

A. M. P. Santos ◽

&Aacute; J. Peliz

Keyword(s):

Negative Impact ◽

Significant Negative Correlation ◽

Hybrid Index ◽

Easterly Wind ◽

Annual Variations ◽

Wind Speeds ◽

Wind Event ◽

Wind Events ◽

Environmental Prediction ◽

Offshore Transport

Abstract. Wind speed data obtained from the National Centers for Environmental Prediction (NCEP) Reanalysis project are used to construct winter (November–March) wind indices for the western Iberian Peninsula. The data used represent a 2.5&deg square area, centred at 41.0&deg N, 9.4&deg W, over the period 1948-2003. The NCEP data are well correlated with a time-series (1980–2001) of wind measurements from the Cape Carvoeiro lighthouse on the western Portuguese coast (39.4&deg N, 9.4&deg W). The new indices, of which there are four corresponding to northerlies, easterlies, southerlies and westerlies, constitute measures of numbers of significant wind event days, where a significant wind event is defined to be 4 or more consecutive days of wind speeds exceeding 4 m s-1. Results show both intra- and inter-annual variations in the numbers of significant wind event days, as well as clear decadal trends. A comparison between a hybrid index, composed of the numbers of significant northerly and easterly wind event days - both promote offshore transport, which is thought to have a negative impact on pelagic fish recruitment - and western Iberian sardine catch data, reveal an extensive period of significant negative correlation. The relationship over the most recent period, ~1999–2000, is unclear.

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Assessing the Relationship between MJO and Equatorial Pacific WWBs in Observations and CMIP5 Models

Journal of Climate ◽

10.1175/jcli-d-17-0526.1 ◽

2018 ◽

Vol 31 (16) ◽

pp. 6393-6410 ◽

Cited By ~ 6

Author(s):

Jie Feng ◽

Tao Lian

Keyword(s):

Temperature Anomaly ◽

Equatorial Pacific ◽

Cmip5 Models ◽

Madden Julian Oscillation ◽

Sea Surface Temperature Anomaly ◽

Anomalous Surface ◽

Favorable Conditions ◽

The Relationship ◽

Wind Bursts ◽

The Tropical Pacific

This study evaluates the relationship between the Madden–Julian oscillation (MJO) and the occurrence of equatorial Pacific westerly wind bursts (WWBs). During the convective MJO phase, anomalous surface westerlies prevail in and west of the convective MJO center, providing favorable conditions for WWBs. Compared with the probability of WWBs expected under a null hypothesis that WWBs occur randomly, the convective MJO phase almost doubles the probability of a WWB occurring. Nevertheless, only 34.46% of WWBs co-occur with the convective MJO, which is much less than that reported in previous studies. We show that when the MJO and WWBs are defined using the same field with overlapping frequencies, the percentage of WWBs co-occurring with the convective MJO shows a significant increase. However, the higher percentage is simply caused by the fact that the strong WWBs during a convective MJO are more likely to be identified than those during the suppressed and neutral MJO phases. A total of 45.80% of WWBs are found to occur in the full MJO phase (both the convective and suppressed MJO phases), which is slightly higher than that expected based on randomness. Although the full MJO has statistically significant impact on the likelihood of WWBs, the influence from the full MJO on the tropical Pacific sea surface temperature anomaly is much weaker as compared to that from the WWBs. The relationships between the MJO and WWBs simulated in CMIP5 models are also assessed, and the percentage of WWBs that co-occur with the MJO simulated in models is in general less than that in observations.

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A robust equatorial Pacific westerly response to tropical volcanism in multiple models

Climate Dynamics ◽

10.1007/s00382-020-05453-6 ◽

2020 ◽

Vol 55 (11-12) ◽

pp. 3413-3429

Author(s):

Jing Chai ◽

Fei Liu ◽

Chen Xing ◽

Bin Wang ◽

Chaochao Gao ◽

...

Keyword(s):

El Niño ◽

Land Surface ◽

Radiative Forcing ◽

El Nino ◽

West African Monsoon ◽

Equatorial Pacific ◽

West African ◽

Surface Cooling ◽

Easterly Wind ◽

African Monsoon

Abstract After each of the 1963 Agung, 1982 El Chichón, and 1991 Pinatubo eruptions, an El Niño was observed. The increased likelihood of an El Niño after a tropical eruption has also been found in long-term reconstructed proxy data. Through examining simulations over the last millennium by 11 different models, we show that a tropical volcano eruption can robustly excite a western-to-central equatorial Pacific (WCEP) westerly anomaly at 850 hPa in eight out of the 11 models; such a westerly anomaly is favorable for El Niño development. Under the volcanic forcing, there are significant extratropical continent surface cooling and tropical drying with negative precipitation anomalies over the South–South East Asia (SSEA), West African monsoon, and Intertropical Convergence Zone (ITCZ) regions. This common precipitation suppression response occurs in most of the models. Sensitivity experiments show that a WCEP westerly anomaly can be excited by the tropical land cooling, especially the SSEA cooling induced precipitation suppression rather than by the extratropical land surface cooling. Theoretical results show that a WCEP westerly anomaly is excited due to a Gill response to reduced precipitation over the SSEA and West African monsoon regions; and the SSEA contributes more than the West African monsoon does. The ITCZ weakening, however, excites an easterly wind anomaly. The models with more sensitive convective feedback tend to simulate an El Niño more easily, while a failed simulation of an El Niño after a robust westerly anomaly in some models calls for further studies on these models’ delayed responses to radiative forcing induced by volcano eruptions.

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