scholarly journals The role of the South Pacific in modulating Tropical Pacific variability

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Christine T. Y. Chung ◽  
Scott B. Power ◽  
Arnold Sullivan ◽  
François Delage
Keyword(s):  
El Niño ◽  
Decadal Variability ◽  
El Nino ◽  
South Pacific ◽  
Tropical Pacific ◽  
Equatorial Pacific ◽  
La Niña ◽  
The South ◽  
La Nina ◽  
Sst Anomalies

AbstractTropical Pacific variability (TPV) heavily influences global climate, but much is still unknown about its drivers. We examine the impact of South Pacific variability on the modes of TPV: the El Niño-Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO). We conduct idealised coupled experiments in which we suppress temperature and salinity variability at all oceanic levels in the South Pacific. This reduces decadal variability in the equatorial Pacific by ~30% and distorts the spatial pattern of the IPO. There is little change to overall interannual variability, however there is a decrease in the magnitude of the largest 5% of both El Niño and La Niña sea-surface temperature (SST) anomalies. Possible reasons for this include: (i) reduced decadal variability means that interannual SST variability is superposed onto a ‘flatter’ background signal, (ii) suppressing South Pacific variability leads to the alteration of coupled processes linking the South and equatorial Pacific. A small but significant mean state change arising from the imposed suppression may also contribute to the weakened extreme ENSO SST anomalies. The magnitude of both extreme El Niño and La Niña SST anomalies are reduced, and the associated spatial patterns of change of upper ocean heat content and wind stress anomalies are markedly different for both types of events.

How Many ENSO Flavors Can We Distinguish?*

2013 ◽  
Vol 26 (13) ◽  
pp. 4816-4827 ◽  
Author(s):  
Nathaniel C. Johnson
Keyword(s):  
El Niño ◽  
El Nino ◽  
Warm Pool ◽  
Tropical Pacific ◽  
La Niña ◽  
Western Pacific ◽  
Western Pacific Warm Pool ◽  
La Nina ◽  
Pacific Warm Pool ◽  
Sst Anomalies

Abstract It is now widely recognized that El Niño–Southern Oscillation (ENSO) occurs in more than one form, with the canonical eastern Pacific (EP) and more recently recognized central Pacific (CP) ENSO types receiving the most focus. Given that these various ENSO “flavors” may contribute to climate variability and long-term trends in unique ways, and that ENSO variability is not limited to these two types, this study presents a framework that treats ENSO as a continuum but determines a finite maximum number of statistically distinguishable representative ENSO patterns. A neural network–based cluster analysis called self-organizing map (SOM) analysis paired with a statistical distinguishability test determines nine unique patterns that characterize the September–February tropical Pacific SST anomaly fields for the period from 1950 through 2011. These nine patterns represent the flavors of ENSO, which include EP, CP, and mixed ENSO patterns. Over the 1950–2011 period, the most significant trends reflect changes in La Niña patterns, with a shift in dominance of La Niña–like patterns with weak or negative western Pacific warm pool SST anomalies until the mid-1970s, followed by a dominance of La Niña–like patterns with positive western Pacific warm pool SST anomalies, particularly after the mid-1990s. Both an EP and especially a CP El Niño pattern experienced positive frequency trends, but these trends are indistinguishable from natural variability. Overall, changes in frequency within the ENSO continuum contributed to the pattern of tropical Pacific warming, particularly in the equatorial eastern Pacific and especially in relation to changes of La Niña–like rather than El Niño–like patterns.

scholarly journals Asymmetric Modulation of El Niño and La Niña and the Linkage to Tropical Pacific Decadal Variability

2017 ◽  
Vol 30 (12) ◽  
pp. 4705-4733 ◽  
Author(s):  
Yuko M. Okumura ◽  
Tianyi Sun ◽  
Xian Wu
Keyword(s):  
El Niño ◽  
Decadal Variability ◽  
El Nino ◽  
Southern Oscillation ◽  
Tropical Pacific ◽  
La Niña ◽  
Climate System Model ◽  
Pacific Decadal Variability ◽  
La Nina ◽  
Leading Mode

El Niño–Southern Oscillation (ENSO) in a 1300-yr preindustrial control simulation of the Community Climate System Model, version 4 (CCSM4), exhibits distinct modulation in association with tropical Pacific decadal variability (TPDV). The frequency and duration of El Niño events modulate with changes in the interbasin sea surface temperature (SST) gradient related to the leading mode of TPDV, which resembles the interdecadal Pacific oscillation (IPO). La Niña shows similar changes with the IPO but is also controlled by changes in El Niño that often precedes La Niña, and these effects tend to cancel each other. The amplitude of ENSO, on the other hand, is closely related to the second leading mode of TPDV that affects the zonal and meridional contrast of tropical Pacific climate. Significant changes in the pattern and seasonal evolution related to this TPDV mode are found mainly for El Niño because of the nonlinear relation between the atmospheric deep convection and SSTs. The resultant changes in the amplitude of El Niño, in turn, affect the amplitude and duration of the following La Niña, as well as the asymmetry in their patterns and duration. The decadal ENSO modulation associated with both TPDV modes is not symmetrical between El Niño and La Niña and thus is not likely to occur solely as a result of random variability. The patterns of TPDV in CCSM4 have resemblance to those simulated by its atmospheric component coupled to a slab ocean model, suggesting that TPDV induced by stochastic atmospheric variability interacts with the ENSO dynamics.

Asymmetry in the Duration of El Niño and La Niña

2010 ◽  
Vol 23 (21) ◽  
pp. 5826-5843 ◽  
Author(s):  
Yuko M. Okumura ◽  
Clara Deser
Keyword(s):  
El Niño ◽  
El Nino ◽  
Deep Convection ◽  
Mirror Image ◽  
Equatorial Pacific ◽  
La Niña ◽  
Past Century ◽  
Cold Tongue ◽  
La Nina ◽  
Sst Anomalies

Abstract El Niño and La Niña are not a simple mirror image, but exhibit significant differences in their spatial structure and seasonal evolution. In particular, sea surface temperature (SST) anomalies over the equatorial Pacific cold tongue are larger in magnitude during El Niño compared to La Niña, resulting in positive skewness of interannual SST variations. The associated atmospheric deep convection anomalies are displaced eastward during El Niño compared to La Niña because of the nonlinear atmospheric response to SST. In addition to these well-known features, an analysis of observational data for the past century shows that there is a robust asymmetry in the duration of El Niño and La Niña. Most El Niños and La Niñas develop in late boreal spring/summer, when the climatological cold tongue is intensifying, and they peak near the end of the calendar year. After the mature phase, El Niños tend to decay rapidly by next summer, but many La Niñas persist through the following year and often reintensify in the subsequent winter. Throughout the analysis period, this asymmetric feature is evident for strong events in which Niño-3.4 SST anomalies exceed one standard deviation in December. Seasonally stratified composite analysis suggests that the eastward displacement of atmospheric deep convection anomalies during El Niño enables surface winds in the western equatorial Pacific to be more affected by remote forcing from the Indian Ocean, which acts to terminate the Pacific events.

scholarly journals Contribution of Synoptic Transients to the Potential Predictability of PNA Circulation Anomalies: El Niño versus La Niña

2015 ◽  
Vol 28 (21) ◽  
pp. 8347-8362 ◽  
Author(s):  
Muhammad Adnan Abid ◽  
In-Sik Kang ◽  
Mansour Almazroui ◽  
Fred Kucharski
Keyword(s):  
El Niño ◽  
El Nino ◽  
Signal To Noise Ratio ◽  
Tropical Pacific ◽  
La Niña ◽  
Potential Predictability ◽  
La Nina ◽  
The Pacific ◽  
Circulation Anomalies ◽  
Sst Anomalies

Abstract The potential predictability (PP) of seasonal-mean 200-hPa geopotential height (Z200) anomalies in the Pacific–North American (PNA) region is examined for El Niño and La Niña separately by using 50 ensemble members of twentieth-century AGCM simulations. Observed sea surface temperature (SST) is prescribed for the period 1870–2009, and 14 El Niño and La Niña years after 1900 are selected for the present study. The domain-averaged value of PP for Z200 in the PNA region, as measured by the signal-to-noise ratio, for El Niño is about 60% larger than that of La Niña. Such a large PP is mainly due to a larger signal and partly to less noise during El Niño compared to that during La Niña . The transient eddy feedback to the PNA circulation anomalies is stronger during El Niño events (about 50%) than that during La Niña, and this difference in the transients contributes significantly to the different Z200 signals in the PNA region. The noise variance of the transients during El Niño is about 17% smaller than during La Niña, and thus transients play an important role in the reduction of Z200 noise during El Niño. Idealized experiments with the same spatial pattern but different signs of SST anomalies confirm the results mentioned above. Moreover, these experiments with several different amplitudes of positive and negative phases of tropical Pacific SST anomalies show that signals of Z200 and transients are proportional to precipitation anomalies in the tropical Pacific, and noises of Z200 for El Niño cases are somewhat smaller than the corresponding values of La Niña.

scholarly journals Changes in Spread and Predictability Associated with ENSO in an Ensemble Coupled GCM

2006 ◽  
Vol 19 (17) ◽  
pp. 4378-4396 ◽  
Author(s):  
Renguang Wu ◽  
Ben P. Kirtman
Keyword(s):  
Surface Pressure ◽  
El Niño ◽  
El Nino ◽  
Tropical Pacific ◽  
La Niña ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Central Pacific ◽  
La Nina ◽  
Signal Change

Abstract The present study documents the influence of El Niño and La Niña events on the spread and predictability of rainfall, surface pressure, and 500-hPa geopotential height, and contrasts the relative contribution of signal and noise changes to the predictability change based on a long-term integration of an interactive ensemble coupled general circulation model. It is found that the pattern of the El Niño–Southern Oscillation (ENSO)-induced noise change for rainfall follows closely that of the corresponding signal change in most of the tropical regions. The noise for tropical Pacific surface pressure is larger (smaller) in regions of lower (higher) mean pressure. The ENSO-induced noise change for 500-hPa height displays smaller spatial scales compared to and has no systematic relationship with the signal change. The predictability for tropical rainfall and surface pressure displays obvious contrasts between the summer and winter over the Bay of Bengal, the western North Pacific, and the tropical southwestern Indian Ocean. The predictability for tropical 500-hPa height is higher in boreal summer than in boreal winter. In the equatorial central Pacific, the predictability for rainfall is much higher in La Niña years than in El Niño years. This occurs because of a larger percent reduction in the amplitude of noise compared to the percent decrease in the magnitude of signal from El Niño to La Niña years. A consistent change is seen in the predictability for surface pressure near the date line. In the western North and South Pacific, the predictability for boreal winter rainfall is higher in El Niño years than in La Niña years. This is mainly due to a stronger signal in El Niño years compared to La Niña years. The predictability for 500-hPa height increases over most of the Tropics in El Niño years. Over western tropical Pacific–Australia and East Asia, the predictability for boreal winter surface pressure and 500-hPa height is higher in El Niño years than in La Niña years. The predictability change for 500-hPa height is primarily due to the signal change.

scholarly journals Role of the Indo-Pacific Interbasin Coupling in Predicting Asymmetric ENSO Transition and Duration

2012 ◽  
Vol 25 (9) ◽  
pp. 3321-3335 ◽  
Author(s):  
Masamichi Ohba ◽  
Masahiro Watanabe
Keyword(s):  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Southern Oscillation ◽  
La Niña ◽  
La Nina ◽  
The Pacific ◽  
Enso Asymmetry ◽  
Sst Anomalies

Warm and cold phases of El Niño–Southern Oscillation (ENSO) exhibit a significant asymmetry in their transition/duration such that El Niño tends to shift rapidly to La Niña after the mature phase, whereas La Niña tends to persist for up to 2 yr. The possible role of sea surface temperature (SST) anomalies in the Indian Ocean (IO) in this ENSO asymmetry is investigated using a coupled general circulation model (CGCM). Decoupled-IO experiments are conducted to assess asymmetric IO feedbacks to the ongoing ENSO evolution in the Pacific. Identical-twin forecast experiments show that a coupling of the IO extends the skillful prediction of the ENSO warm phase by about one year, which was about 8 months in the absence of the IO coupling, in which a significant drop of the prediction skill around the boreal spring (known as the spring prediction barrier) is found. The effect of IO coupling on the predictability of the Pacific SST is significantly weaker in the decay phase of La Niña. Warm IO SST anomalies associated with El Niño enhance surface easterlies over the equatorial western Pacific and hence facilitate the El Niño decay. However, this mechanism cannot be applied to cold IO SST anomalies during La Niña. The result of these CGCM experiments estimates that approximately one-half of the ENSO asymmetry arises from the phase-dependent nature of the Indo-Pacific interbasin coupling.

scholarly journals Indo-Pacific Variability on Seasonal to Multidecadal Time Scales. Part II: Multiscale Atmosphere–Ocean Linkages

2018 ◽  
Vol 31 (2) ◽  
pp. 693-725 ◽  
Author(s):  
Dimitrios Giannakis ◽  
Joanna Slawinska
Keyword(s):  
Time Scales ◽  
El Niño ◽  
El Nino ◽  
Walker Circulation ◽  
La Niña ◽  
Central Pacific ◽  
Australian Monsoon ◽  
La Nina ◽  
Combination Modes ◽  
Sst Anomalies

The coupled atmosphere–ocean variability of the Indo-Pacific domain on seasonal to multidecadal time scales is investigated in CCSM4 and in observations through nonlinear Laplacian spectral analysis (NLSA). It is found that ENSO modes and combination modes of ENSO with the annual cycle exhibit a seasonally synchronized southward shift of equatorial surface zonal winds and thermocline adjustment consistent with terminating El Niño and La Niña events. The surface winds associated with these modes also generate teleconnections between the Pacific and Indian Oceans, leading to SST anomalies characteristic of the Indian Ocean dipole. The family of NLSA ENSO modes is used to study El Niño–La Niña asymmetries, and it is found that a group of secondary ENSO modes with more rapidly decorrelating temporal patterns contributes significantly to positively skewed SST and zonal wind statistics. Besides ENSO, fundamental and combination modes representing the tropospheric biennial oscillation (TBO) are found to be consistent with mechanisms for seasonally synchronized biennial variability of the Asian–Australian monsoon and Walker circulation. On longer time scales, a multidecadal pattern referred to as the west Pacific multidecadal mode (WPMM) is established to significantly modulate ENSO and TBO activity, with periods of negative SST anomalies in the western tropical Pacific favoring stronger ENSO and TBO variability. This behavior is attributed to the fact that cold WPMM phases feature anomalous decadal westerlies in the tropical central Pacific, as well as an anomalously flat zonal thermocline profile in the equatorial Pacific. Moreover, the WPMM is found to correlate significantly with decadal precipitation over Australia.

Dependence of global monsoon response to volcanic eruptions on the background oceanic states

2021 ◽  
pp. 1-53
Author(s):  
Meng Zuo ◽  
Wenmin Man ◽  
Tianjun Zhou
Keyword(s):  
El Niño ◽  
El Nino ◽  
Volcanic Eruptions ◽  
La Niña ◽  
Initial Condition ◽  
Monsoon Precipitation ◽  
Global Monsoon ◽  
La Nina ◽  
Precipitation Changes ◽  
Sst Anomalies

AbstractBoth proxy data and climate modeling show divergent responses of global monsoon precipitation to volcanic eruptions. The reason is however unknown. Here, based on analysis of the CESM Last Millennium Ensemble simulation, we show evidences that the divergent responses are dominated by the pre-eruption background oceanic states. We found that under El Niño-Southern Oscillation (ENSO) neutral and warm phases initial conditions, the Pacific favors an El Niño-like anomaly after volcanic eruptions, while La Niña-like SST anomalies tend to occur following eruptions under ENSO cold phase initial condition, especially after southern eruptions. The cold initial condition is associated with stronger upper ocean temperature stratification and shallower thermocline over the eastern Pacific than normal. The easterly anomalies triggered by surface cooling over the tropical South America continent can generate changes in SST through anomalous advection and the ocean subsurface upwelling more efficiently, causing La Niña-like SST anomalies. Whereas under warm initial condition, the easterly anomalies fail to develop and the westerly anomalies still play a dominant role, thus forms an El Niño-like SST anomaly. Such SST response further regulates the monsoon precipitation changes through atmospheric teleconnection. The contribution of direct radiative forcing and indirect SST response to precipitation changes show regional differences, which will further affect the intensity and sign of precipitation response in submonsoon regions. Our results imply that attention should be paid to the background oceanic state when predicting the global monsoon precipitation responses to volcanic eruptions.

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