scholarly journals Observed Relationships between Oceanic Kelvin Waves and Atmospheric Forcing

2006 ◽  
Vol 19 (20) ◽  
pp. 5253-5272 ◽  
Author(s):  
Paul E. Roundy ◽  
George N. Kiladis
Keyword(s):  
Wind Stress ◽  
El Niño ◽  
El Nino ◽  
Phase Speed ◽  
Kelvin Waves ◽  
Apparent Difference ◽  
Kelvin Wave ◽  
Central Pacific ◽  
Basin Scale ◽  
The Waves

Abstract The Madden–Julian oscillation (MJO) has been implicated as a major source of the wind stress variability that generates basin-scale Kelvin waves in the equatorial Pacific. One source of debate concerning this relationship is the apparent difference in the frequencies of the two processes. This work utilizes data from the Tropical Atmosphere Ocean (TAO) array of moored buoys along with outgoing longwave radiation data to show by means of a multiple linear regression model and case studies that the frequency discrepancy is due to a systematic decrease in the phase speeds of the Kelvin waves and an increase in the period of the waves toward the east as conditions adjust toward El Niño. Among the potential contributing factors to this phase speed decrease is an apparent air–sea interaction that enhances the wind forcing of some of the Kelvin waves, allowing them to continue to amplify because the propagating wind stress anomaly decelerates to the speed of the developing Kelvin wave instead of the significantly faster speed of the typical MJO. Kelvin waves appear to be most effectively amplified during periods when the temperature gradient above the thermocline across the equatorial central Pacific is strong, the thermocline shoals steeply toward the east in the central Pacific, and/or when the phase speed of the propagating wind stress forcing is closest to that of the Kelvin wave. These conditions tend to occur as the ocean adjusts toward El Niño. Since Kelvin waves are instrumental to the development of El Niño events, isolating the detailed relationship between the waves and the MJO will lead to a better understanding of interannual ocean–atmosphere interactions.

scholarly journals The role of tropical volcanic eruptions in exacerbating Indian droughts

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Suvarna Fadnavis ◽  
Rolf Müller ◽  
Tanusri Chakraborty ◽  
T. P. Sabin ◽  
Anton Laakso ◽  
...  
Keyword(s):  
El Niño ◽  
Climate Model ◽  
El Nino ◽  
Volcanic Eruptions ◽  
Summer Monsoon Rainfall ◽  
Indian Region ◽  
Volcanic Plume ◽  
Kelvin Wave ◽  
Central Pacific ◽  
Climate Model Simulations

AbstractThe Indian summer monsoon rainfall (ISMR) is vital for the livelihood of millions of people in the Indian region; droughts caused by monsoon failures often resulted in famines. Large volcanic eruptions have been linked with reductions in ISMR, but the responsible mechanisms remain unclear. Here, using 145-year (1871–2016) records of volcanic eruptions and ISMR, we show that ISMR deficits prevail for two years after moderate and large (VEI > 3) tropical volcanic eruptions; this is not the case for extra-tropical eruptions. Moreover, tropical volcanic eruptions strengthen El Niño and weaken La Niña conditions, further enhancing Indian droughts. Using climate-model simulations of the 2011 Nabro volcanic eruption, we show that eruption induced an El Niño like warming in the central Pacific for two consecutive years due to Kelvin wave dissipation triggered by the eruption. This El Niño like warming in the central Pacific led to a precipitation reduction in the Indian region. In addition, solar dimming caused by the volcanic plume in 2011 reduced Indian rainfall.

scholarly journals Intraseasonal Tropical Atmospheric Variability Associated with the Two Flavors of El Niño

2012 ◽  
Vol 140 (11) ◽  
pp. 3669-3681 ◽  
Author(s):  
Daria Gushchina ◽  
Boris Dewitte
Keyword(s):  
El Niño ◽  
El Nino ◽  
Wave Activity ◽  
Kelvin Wave ◽  
Atmospheric Variability ◽  
Central Pacific ◽  
Cp El Niño ◽  
Ocean Data Assimilation ◽  
Double Space ◽  
Ep El Niño

ABSTRACT The characteristics of intraseasonal tropical variability (ITV) associated with the two flavors of El Niño [i.e., the canonical or eastern Pacific (EP) El Niño and the Modoki or central Pacific (CP) El Niño] are documented using composite and regression analysis. Double space–time Fourier analysis is applied to the NCEP–NCAR zonal wind at 850 hPa (U850) to separate the different components of the ITV in the tropical troposphere, which is then used to define indices of wave activity, and document the spatial pattern of the waves. It is shown that the ITV characteristics are altered during CP El Niño compared to the typical seasonal dependence of the ITV–ENSO relationship. In particular, while EP El Niño is characterized by enhanced MJO and equatorial Rossby (ER) wave activity during spring–summer prior to the ENSO peak, during CP El Niño, the ITV activity is increased during the mature and decaying phases. It is suggested that ITV is more propitious to the triggering of the EP event; while during the CP event, it contributes mostly to the persistence of positive SST anomalies. The oceanic response of these ITV anomalous patterns is further investigated in the Simple Ocean Data Assimilation (SODA) reanalysis by documenting the seasonal evolution of the intraseasonal equatorial oceanic Kelvin wave (IEKW) activity during the two flavors of El Niño. It is shown that anomalous westerlies associated with ITV may generate the corresponding response in the ocean in the form of anomalous IEKW activity.

Factors Controlling the Diversities of MJO Propagation and Intensity

2021 ◽  
pp. 1-41
Author(s):  
Tianyi Wang ◽  
Tim Li
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
El Nino ◽  
Phase Speed ◽  
Radiative Heating ◽  
Intensity Difference ◽  
Growth Speed ◽  
Maritime Continent ◽  
Central Pacific ◽  
Moisture Condition

AbstractThe diversity of the Madden-Julian Oscillation (MJO) in terms of its maximum intensity, zonal extent and phase speed was explored using a cluster analysis method. The zonal extent is found to be significantly correlated to the phase speed. A longer zonal extent is often associated with a faster phase speed.The diversities of zonal extent and speed are connected with distinctive interannual sea surface temperature anomaly (SSTA) distributions and associated moisture and circulation patterns over the equatorial Pacific. An El Niño–like background SSTA leads to enhanced precipitation over the central Pacific, allowing a stronger vertically overturning circulation to the east of the MJO. This promotes both a larger east-west asymmetry of column-integrated moist static energy (MSE) tendency and a greater boundary-layer moisture leading, serving as potential causes of the faster phase speed. The El Niño–like SSTA also favors the MJOs intruding further into the Pacific, causing a larger zonal extent.The intensity diversity is associated with the interannual SSTA over the Maritime Continent and background moisture condition over the tropical Indian Ocean. An observed warm SSTA over the Maritime Continent excites a local Walker cell with a subsidence over the Indian Ocean, which could decrease the background moisture, weakening the MJO intensity. The intensity difference between strong and weak events would be amplified due to distinct intensity growth speed. The faster intensity growth of a strong MJO is attributed to a greater longwave radiative heating and a greater surface latent heat flux, as both of which contribute to a greater total time change rate of the column-integrated MSE.

scholarly journals A review of ENSO theories

2018 ◽  
Vol 5 (6) ◽  
pp. 813-825 ◽  
Author(s):  
Chunzai Wang
Keyword(s):  
El Niño ◽  
El Nino ◽  
Enso Event ◽  
Tropical Pacific ◽  
Kelvin Waves ◽  
Western Pacific ◽  
Central Pacific ◽  
Central Pacific El Niño ◽  
Negative Feedbacks ◽  
The Tropical Pacific

Abstract The El Niño and the Southern Oscillation (ENSO) occurrence can be usually explained by two views of (i) a self-sustained oscillatory mode and (ii) a stable mode interacting with high-frequency forcing such as westerly wind bursts and Madden-Julian Oscillation events. The positive ocean–atmosphere feedback in the tropical Pacific hypothesized by Bjerknes leads the ENSO event to a mature phase. After ENSO event matures, negative feedbacks are needed to cease the ENSO anomaly growth. Four negative feedbacks have been proposed: (i) reflected Kelvin waves at the ocean western boundary, (ii) a discharge process due to Sverdrup transport, (iii) western-Pacific wind-forced Kelvin waves and (iv) anomalous zonal advections and wave reflection at the ocean eastern boundary. These four ENSO mechanisms are respectively called the delayed oscillator, the recharge–discharge oscillator, the western-Pacific oscillator and the advective–reflective oscillator. The unified oscillator is developed by including all ENSO mechanisms, i.e. all four ENSO oscillators are special cases of the unified oscillator. The tropical Pacific Ocean and atmosphere interaction can also induce coupled slow westward- and eastward-propagating modes. An advantage of the coupled slow modes is that they can be used to explain the propagating property of interannual anomalies, whereas the oscillatory modes produce a standing oscillation. The research community has recently paid attention to different types of ENSO events by focusing on the central-Pacific El Niño. All of the ENSO mechanisms may work for the central-Pacific El Niño events, with an addition that the central-Pacific El Niño may be related to forcing or processes in the extra-tropical Pacific.

scholarly journals A Coupled GCM Analysis of MJO Activity at the Onset of El Niño

2009 ◽  
Vol 66 (4) ◽  
pp. 966-983 ◽  
Author(s):  
A. G. Marshall ◽  
O. Alves ◽  
H. H. Hendon
Keyword(s):  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Heat Content ◽  
Ocean Dynamics ◽  
Western Pacific ◽  
Eastern Pacific ◽  
Kelvin Wave ◽  
Central Pacific ◽  
The Western Pacific

Abstract The ocean dynamics of the Madden–Julian oscillation (MJO) and its interaction with El Niño–Southern Oscillation (ENSO) are assessed using a flux-corrected coupled model experiment from the Australian Bureau of Meteorology. The model demonstrates the correct oceanic Kelvin wave response to the MJO-related westerly winds in the western Pacific. Although there may be a role for the MJO in influencing the strength of El Niño, its impact is difficult to separate from that of strong heat content preconditioning of ENSO. Hence, the MJO–ENSO relationship is assessed starting from a background state of low heat content anomalies in the western Pacific that are also characteristic of recent observed El Niño events. The model shows a strong relationship between ENSO and the MJO near the peak of El Niño. At this time, the sea surface temperature (SST) anomaly is largest in the central Pacific, and it is difficult to separate cause and effect. Near the onset of El Niño, however, when Pacific Ocean SST anomalies are near zero, an increase in MJO activity is associated with Kelvin wave activity and stronger subsequent ENSO warming. A significant increase in the number of MJO events, rather than the strength of individual MJO events, leads to stronger eastern Pacific warming; the MJO appears not to be responsible for the occurrence of El Niño itself, but, rather, is important for influencing its development thus. This research supports a role for downwelling oceanic Kelvin waves and subsequent deepening of the thermocline in contributing to eastern Pacific warming during the onset of El Niño.

scholarly journals Contrasting Impacts of ENSO on the Interannual Variations of Summer Runoff between the Upper and Mid-Lower Reaches of the Yangtze River

Atmosphere ◽  
2018 ◽  
Vol 9 (12) ◽  
pp. 478 ◽  
Author(s):  
Xiaochen Ye ◽  
Zhiwei Wu
Keyword(s):  
El Niño ◽  
Yangtze River ◽  
El Nino ◽  
Enso Cycle ◽  
Apparent Difference ◽  
The Yangtze River ◽  
Central Pacific ◽  
Large Variability ◽  
Lower Yangtze ◽  
Floods And Droughts

The Yangtze River Basin is an El Niño–Southern Oscillation (ENSO)-sensitive region, prone to floods and droughts. Hydrological records were collected to examine the temporal and spatial distribution of runoff in this drainage basin. An apparent difference in runoff variations between the upper and mid-lower Yangtze reaches was detected in response to ENSO. The upper basin usually experiences floods or droughts during the summer of ENSO developing years, while the mid-lower runoff variations tend to coincide with ENSO decaying phases. Composite analysis is employed to investigate the underlying mechanism for the teleconnection between the specific phases of the ENSO cycle and Yangtze runoff variation. Results show that the Western Pacific Subtropical High (WPSH) exhibits large variability on its western side in summer with different ENSO phases, thus resulting in a contrasting influence between the upper and mid-lower Yangtze floods and droughts. During the central Pacific-La Niña developing summers, the WPSH is significantly enhanced with its westward extension over the Yangtze upper basin. Anomalous water vapor converges in its northwest edge thus favoring upper-basin flooding. Meanwhile, the mid-lower reaches are controlled by the WPSH, and the local rainfall is not obvious. In addition, when the El Niño decaying phases occur, the WPSH denotes a westward extending trend and the position of its ridge line shifts to the mid-lower Yangtze reaches. The southwest moisture cannot extend to the upper basin but converges in the mid-lower basin. Accompanied by the anomalous 100 hPa South Asia High and lower-tropospheric Philippines anticyclone movements, this upper–middle–lower configuration acts as a key bridge linking ENSO and Yangtze floods and droughts.

scholarly journals On the Interpretation of EOF Analysis of ENSO, Atmospheric Kelvin Waves, and the MJO

2015 ◽  
Vol 28 (3) ◽  
pp. 1148-1165 ◽  
Author(s):  
Paul E. Roundy
Keyword(s):  
Skin Temperature ◽  
El Niño ◽  
El Nino ◽  
Spatial Scales ◽  
Longwave Radiation ◽  
Kelvin Waves ◽  
Eof Analysis ◽  
State Variables ◽  
Kelvin Wave ◽  
Full Characterization

Abstract Empirical orthogonal function (EOF) analysis is frequently applied to derive patterns and indexes used to identify and track weather and climate modes as expressed in state variables or proxies of convection. Individual EOFs or pairs of EOFs are often taken to be a complete description of the phenomenon they are intended to index. At the same time, in the absence of projection of the phenomenon onto multiple EOFs yielding multiple similar eigenvalues, each EOF is often assumed to represent a physically independent phenomenon. This project analyzed the leading EOFs of the earth’s skin temperature on the equator and outgoing longwave radiation (OLR) anomalies filtered for atmospheric equatorial Kelvin waves. Results show that the leading two EOFs of the skin temperature data—including east Pacific El Niño and El Niño Modoki—frequently evolve as a quadrature pair during El Niño events, even though the first EOF explains roughly 6 times as much variance as the second. They together diagnose the longitude of the SST anomaly maximum, and their linear combination frequently shows eastward or westward propagation. Analysis of the filtered OLR anomalies shows that the first six EOFs each represent Kelvin wave signals, with the first, second, and third pairs representing Kelvin waves characterized by zonal wavenumbers 2, 3, and 4, respectively. This result demonstrates that if a phenomenon occurs across a range of spatial scales, it is described by multiple EOFs at different scales. A similar analysis demonstrates that the Madden–Julian oscillation probably exhibits spread across a range of spatial scales that would also require multiple EOFs for full characterization.

Dynamic Causes of ENSO Decay and its Asymmetry

2021 ◽  
pp. 1-56
Author(s):  
Xiaomeng Song ◽  
Renhe Zhang ◽  
Xinyao Rong
Keyword(s):  
Wind Stress ◽  
El Niño ◽  
El Nino ◽  
Heat Budget ◽  
La Niña ◽  
Decay Phase ◽  
Small Contribution ◽  
Central Pacific ◽  
La Nina ◽  
Advection Term

AbstractEl Niño and La Niña exhibit asymmetric evolution characteristics during their decay phases. The decay speed of El Niño is significantly greater than that of La Niña. This study systematically and quantitatively investigates the relative contributions of the equatorial western Pacific (WP) and central-eastern Pacific (CEP) wind stress anomalies to ENSO decay and its asymmetry through data analysis, numerical experiments, and dynamic and thermodynamic diagnoses. It is demonstrated that the sea surface temperature anomalies (SSTAs) forced by the wind stress anomalies in the equatorial CEP play a dominant role in ENSO decay and contribute to ENSO decay asymmetry, while the forcing by the equatorial WP wind stress anomalies has a small contribution. Diagnoses of the oceanic mixed layer heat budget indicate that anomalous zonal advection term and vertical advection term forced by the wind stress anomalies in the equatorial CEP are the most important dynamic terms contributed to ENSO decay. Both terms in El Niño decay phase are much larger than in La Niña decay phase, resulting in a larger decay speed in El Niño than in La Niña. The contributions of these two terms do not depend on the equatorial WP wind field, confirming that the equatorial WP wind stress anomalies do not act as a pivotal part in ENSO asymmetric decay. Moreover, it is demonstrated that within the equatorial CEP, dominant contribution comes from the wind stress anomalies in the equatorial central Pacific, in which those in the equatorial southern central Pacific play a major role.

scholarly journals The OMZ and nutrient features as a signature of interannual and low-frequency variability in the Peruvian upwelling system

2017 ◽  
Vol 14 (20) ◽  
pp. 4601-4617 ◽  
Author(s):  
Michelle I. Graco ◽  
Sara Purca ◽  
Boris Dewitte ◽  
Carmen G. Castro ◽  
Octavio Morón ◽  
...  
Keyword(s):  
El Niño ◽  
El Nino ◽  
Kelvin Wave ◽  
Central Pacific ◽  
Data Set ◽  
Central Pacific El Niño ◽  
Humboldt Current ◽  
Upwelling System ◽  
Anammox Activity ◽  
Weak Regime

Abstract. Over the last decades, the Humboldt Current upwelling ecosystem, particularly the northern component off the coast of Peru, has drawn the interest of the scientific community because of its unique characteristics: it is the upwelling system with the biggest catch productivity despite the fact it is embedded in a shallow and intense oxygen minimum zone (OMZ). It is also an area of intense nitrogen loss and anammox activity and experiences large interannual variability associated with the equatorial remote forcing. In this context, we examined the oceanographic and biogeochemical variability associated with the OMZ off central Peru from a monthly time series (1996–2011) recorded off the coast of Callao (12° 02′ S, 77° 29′ W). The data reveal a rich spectrum of variability in the OMZ that includes frequencies ranging from seasonal to interannual scales. Due to the efficient oceanic teleconnection off Peru, the observed variability is interpreted in the light of an estimate of the equatorial Kelvin wave contribution to sea level anomalies considering the peculiarities of its vertical structure (i.e., the first two baroclinic modes). The span of the data set allows us to contrast two OMZ regimes. The strong regime is associated with the strong 1997–1998 equatorial Pacific El Niño, during which the OMZ adjusted to Kelvin-wave-induced downwelling conditions that switched off the upwelling and drastically reduced nutrient availability. The weak regime corresponds to the post-2000 period associated with the occurrence of moderate central Pacific El Niño events and enhanced equatorial Kelvin wave activity, in which mean upwelling conditions are maintained. It is shown that the characteristics of the coupling between physics and biogeochemistry is distinct between the two regimes with the weak regime being associated with a larger explained variance in biogeochemical properties not linearly related to the ENSO oceanic teleconnection. The data also reveal a long-term trend from 1999 corresponding to a deepening of the oxygen-deficient waters and warming. The implications of our results for understanding the OMZ dynamics off Peru are discussed.

scholarly journals The asymmetric influence of ocean heat content on ENSO predictability in the CNRM-CM5 coupled general circulation model

2021 ◽  
pp. 1-57
Author(s):  
Yann Y. Planton ◽  
Jérôme Vialard ◽  
Eric Guilyardi ◽  
Mathieu Lengaigne ◽  
Michael J. McPhaden
Keyword(s):  
Wind Stress ◽  
El Niño ◽  
El Nino ◽  
Heat Content ◽  
La Niña ◽  
Western Pacific ◽  
Westerly Wind ◽  
Central Pacific ◽  
Enso Predictability ◽  
La Nina

AbstractUnusually high western Pacific oceanic heat content often leads to El Niño about 1 year later, while unusually low heat content leads to La Niña. Here, we investigate if El Niño Southern Oscillation (ENSO) predictability also depends on the initial state recharge, and discuss the underlying mechanisms. To that end, we use the CNRM-CM5 model, which has a reasonable representation of the main observed ENSO characteristics, asymmetries and feedbacks. Observations and a 1007-years long CNRM-CM5 simulation indicate that discharged states evolve more systematically into La Niña events than recharged states into neutral states or El Niño events. We ran 70-members ensemble experiments in a perfect-model setting, initialized in boreal fall from either recharged or discharged western Pacific heat content, sampling the full range of corresponding ENSO phases. Predictability measures based both on spread and signal-to-noise ratio confirm that discharged states yield a more predictable ENSO outcome one year later than recharged states. As expected from recharge oscillator theory, recharged states evolve into positive central Pacific sea surface temperature anomalies in boreal spring, inducing stronger and more variable Westerly Wind Event activity and a fast growth of the ensemble spread during summer and fall. This also enhances the positive wind stress feedback in fall, but the effect is offset by changes in thermocline and heat flux feedbacks. The state-dependent component of westerly wind events is thus the most likely cause for the predictability asymmetry in CNRM-CM5, although changes in the low-frequency wind stress feedback may also contribute.

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