scholarly journals Tropospheric QBO–ENSO Interactions and Differences between the Atlantic and Pacific

2016 ◽  
Vol 29 (4) ◽  
pp. 1353-1368 ◽  
Author(s):  
Felicitas Hansen ◽  
Katja Matthes ◽  
Sebastian Wahl
Keyword(s):  
North Atlantic ◽  
El Niño ◽  
North Pacific ◽  
El Nino ◽  
La Niña ◽  
Quasi Biennial Oscillation ◽  
The North ◽  
La Nina ◽  
The North Atlantic ◽  
The North Pacific

Abstract This study investigates the interaction of the quasi-biennial oscillation (QBO) and the El Niño–Southern Oscillation (ENSO) in the troposphere separately for the North Pacific and North Atlantic region. Three 145-yr model simulations with NCAR’s Community Earth System Model Whole Atmosphere Community Climate Model (CESM-WACCM) are analyzed where only natural (no anthropogenic) forcings are considered. These long simulations allow the authors to obtain statistically reliable results from an exceptional large number of cases for each combination of the QBO (westerly and easterly) and ENSO phases (El Niño and La Niña). Two different analysis methods were applied to investigate where nonlinearity might play a role in QBO–ENSO interactions. The analyses reveal that the stratospheric equatorial QBO anomalies extend down to the troposphere over the North Pacific during Northern Hemisphere winter only during La Niña and not during El Niño events. The Aleutian low is deepened during QBO westerly (QBOW) as compared to QBO easterly (QBOE) conditions, and the North Pacific subtropical jet is shifted northward during La Niña. In the North Atlantic, the interaction of QBOW with La Niña conditions (QBOE with El Niño) results in a positive (negative) North Atlantic Oscillation (NAO) pattern. For both regions, nonlinear interactions between the QBO and ENSO might play a role. The results provide the potential to enhance the skill of tropospheric seasonal predictions in the North Atlantic and North Pacific region.

scholarly journals The Nonlinear Transient Atmospheric Response to Tropical Forcing

2007 ◽  
Vol 20 (22) ◽  
pp. 5642-5665 ◽  
Author(s):  
Hai Lin ◽  
Jacques Derome ◽  
Gilbert Brunet
Keyword(s):  
North Atlantic ◽  
El Niño ◽  
North Pacific ◽  
El Nino ◽  
La Niña ◽  
Mean Flow ◽  
The North ◽  
La Nina ◽  
The North Atlantic ◽  
The North Pacific

Abstract Ensemble integrations using a primitive-equation dry atmospheric model were performed to investigate the atmospheric transient response to tropical thermal forcings that resemble El Niño and La Niña. The response develops in the North Pacific within 1 week after the integration. The signal in the North Atlantic and Europe is established by the end of the second week. Significant asymmetry was found between the responses in El Niño and La Niña that is similar to the observations, that is, one feature is that the 550-hPa positive height response in the North Pacific of the La Niña run is located about 30° west of the negative response of the El Niño run; another feature is that the responses in the North Atlantic and Europe for the La Niña and El Niño cases have similar patterns with the same polarity. The first feature is established within 2 weeks of the integration, while the second feature develops starting from the end of the second week. Several factors contribute to this nonlinearity of the response. In the Tropics, the shape of the Rossby wave response and the zonal extent of the Kelvin wave are not symmetric between El Niño and La Niña, which seems to be associated with the dependence of the wave property on the modified zonal mean flow. This is especially important in the equatorial region to the west of the forcing, which is likely responsible for the phase shift of the major extratropical response in the North Pacific. The transient eddy activity in the extratropics feeds back to the response and helps to maintain the nonlinearity.

scholarly journals A Nonstationary ENSO–NAO Relationship Due to AMO Modulation

2018 ◽  
Vol 32 (1) ◽  
pp. 33-43 ◽  
Author(s):  
Wenjun Zhang ◽  
Xuebin Mei ◽  
Xin Geng ◽  
Andrew G. Turner ◽  
Fei-Fei Jin
Keyword(s):  
North Atlantic ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Circulation Model ◽  
La Niña ◽  
Late Winter ◽  
The North ◽  
La Nina ◽  
The North Atlantic

Abstract Many previous studies have demonstrated a high uncertainty in the relationship between El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). In the present work, decadal modulation by the Atlantic multidecadal oscillation (AMO) is investigated as a possible cause of the nonstationary ENSO–NAO relationship based on observed and reanalysis data. It is found that the negative ENSO–NAO correlation in late winter is significant only when ENSO and the AMO are in phase (AMO+/El Niño and AMO−/La Niña). However, no significant ENSO-driven atmospheric anomalies can be observed over the North Atlantic when ENSO and the AMO are out of phase (AMO−/El Niño and AMO+/La Niña). Further analysis indicates that the sea surface temperature anomaly (SSTA) in the tropical North Atlantic (TNA) plays an essential role in this modulating effect. Because of broadly analogous TNA SSTA responses to both ENSO and the AMO during late winter, a warm SSTA in the TNA is evident when El Niño occurs during a positive AMO phase, resulting in a significantly weakened NAO, and vice versa when La Niña occurs during a negative AMO phase. In contrast, neither the TNA SSTA nor the NAO shows a prominent change under out-of-phase combinations of ENSO and AMO. The AMO modulation and the associated effect of the TNA SSTA are shown to be well reproduced by historical simulations of the HadCM3 coupled model and further verified by forced experiments using an atmospheric circulation model. These offer hope that similar models will be able to make predictions for the NAO when appropriately initialized.

Model Biases in the Simulation of the Springtime North Pacific ENSO Teleconnection

2020 ◽  
Vol 33 (23) ◽  
pp. 9985-10002
Author(s):  
Ruyan Chen ◽  
Isla R. Simpson ◽  
Clara Deser ◽  
Bin Wang
Keyword(s):  
El Niño ◽  
North Pacific ◽  
Climate Models ◽  
El Nino ◽  
Coupled Model ◽  
La Niña ◽  
Late Winter ◽  
The North ◽  
La Nina ◽  
The North Pacific

AbstractThe wintertime ENSO teleconnection over the North Pacific region consists of an intensified (weakened) low pressure center during El Niño (La Niña) events both in observations and in climate models. Here, it is demonstrated that this teleconnection persists too strongly into late winter and spring in the Community Earth System Model (CESM). This discrepancy arises in both fully coupled and atmosphere-only configurations, when observed SSTs are specified, and is shown to be robust when accounting for the sampling uncertainty due to internal variability. Furthermore, a similar problem is found in many other models from piControl simulations of the Coupled Model Intercomparison Project (23 out of 43 in phase 5 and 11 out of 20 in phase 6). The implications of this bias for the simulation of surface climate anomalies over North America are assessed. The overall effect on the ENSO composite field (El Niño minus La Niña) resembles an overly prolonged influence of ENSO into the spring with anomalously high temperatures over Alaska and western Canada, and wet (dry) biases over California (southwest Canada). Further studies are still needed to disentangle the relative roles played by diabatic heating, background flow, and other possible contributions in determining the overly strong springtime ENSO teleconnection intensity over the North Pacific.

scholarly journals Nonlinearity in the Tropospheric Pathway of ENSO to the North Atlantic

2020 ◽  
Author(s):  
Bernat Jiménez-Esteve ◽  
Daniela I. V. Domeisen
Keyword(s):  
North Atlantic ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Atmospheric Model ◽  
La Niña ◽  
Winter Climate ◽  
The North ◽  
La Nina ◽  
The North Atlantic

Abstract. El Niño Southern Oscillation (ENSO) can exert a remote impact on North Atlantic and European (NAE) winter climate. This teleconnection is driven by the superposition and interaction of different influences, which are generally grouped into two main pathways, namely the tropospheric and stratospheric pathways. In this study, we focus on the tropospheric pathway through the North Pacific and across the North American continent. Due to the possible non-stationary behavior and the limited time period covered by reanalysis data sets, the potential nonlinearity of this pathway remains unclear. In order to address this question, we use a simplified physics atmospheric model forced with seasonally varying prescribed sea surface temperatures (SST) following the evolution of different ENSO phases with linearly varying strength at a fixed location. To isolate the tropospheric pathway the zonal mean stratospheric winds are nudged towards the model climatology. The model experiments indicate that the tropospheric pathway of ENSO to the North Atlantic exhibits significant nonlinearity with respect to the tropical SST forcing, both in the location and amplitude of the impacts. For example, strong El Niño leads to a significantly stronger impact over the North Atlantic Oscillation (NAO) than a La Niña forcing of the same amplitude. For La Niña forcings, there is a saturation in the response, with no further increase in the NAO impact even when doubling the SSTforcing, while this is not the case for El Niño. These findings may have important consequences for long-range predictions of the North Atlantic and Europe.

scholarly journals The Tropospheric Pathway of the ENSO–North Atlantic Teleconnection

2018 ◽  
Vol 31 (11) ◽  
pp. 4563-4584 ◽  
Author(s):  
Bernat Jiménez-Esteve ◽  
Daniela I. V. Domeisen
Keyword(s):  
North Atlantic ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
La Niña ◽  
Stationary Waves ◽  
The North ◽  
La Nina ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract El Niño–Southern Oscillation (ENSO) exerts an influence on the North Atlantic–European (NAE) region. However, this teleconnection is nonlinear and nonstationary owing to the superposition and interaction of a multitude of influences on this region. The stratosphere is one of the major players in terms of the influence of the ENSO signal on this sector. Nevertheless, there are tropospheric dynamical links between the North Pacific and the North Atlantic that are clearly influenced by ENSO. This tropospheric pathway of ENSO to the NAE has received less attention. In view of this, the present study revisits the tropospheric pathway of ENSO to the North Atlantic using ECMWF reanalysis products. Anomalous propagation of transient and quasi-stationary waves across North America is analyzed with respect to their sensitivity to ENSO. Transient (quasi-stationary zonal waves 1–3) wave activity flux (WAF) from the Pacific to the Atlantic increases during El Niño (La Niña) conditions leading to a negative (positive) phase of the North Atlantic Oscillation (NAO). This response is observed from January to March for El Niño and only visible during February for La Niña events. However, the stratosphere strongly modulates this response. For El Niño (La Niña) conditions a weaker (stronger) stratospheric vortex tends to reinforce the negative (positive) NAO with the stratosphere and troposphere working in tandem, contributing to a stronger and more persistent tropospheric circulation response. These findings may have consequences for the prediction of the NAO during times with an inactive stratosphere.

Contributions of Downstream Eddy Development to the Teleconnection between ENSO and the Atmospheric Circulation over the North Atlantic

2012 ◽  
Vol 25 (14) ◽  
pp. 4993-5010 ◽  
Author(s):  
Ying Li ◽  
Ngar-Cheung Lau
Keyword(s):  
North America ◽  
Atmospheric Circulation ◽  
North Atlantic ◽  
El Niño ◽  
North Pacific ◽  
El Nino ◽  
Storm Track ◽  
The North ◽  
The North Atlantic ◽  
The North Pacific

Abstract The spatiotemporal evolution of various meteorological phenomena associated with El Niño–Southern Oscillation (ENSO) in the North Pacific–North American–North Atlantic sector is examined using both NCEP–NCAR reanalyses and output from a 2000-yr integration of a global coupled climate model. Particular attention is devoted to the implications of downstream eddy developments on the relationship between ENSO and the atmospheric circulation over the North Atlantic. The El Niño–related persistent events are characterized by a strengthened Pacific subtropical jet stream and an equatorward-shifted storm track over the North Pacific. The wave packets that populate the storm tracks travel eastward through downstream development. The barotropic forcing of the embedded synoptic-scale eddies is conducive to the formation of a flow that resembles the negative phase of the North Atlantic Oscillation (NAO). The more frequent and higher persistence of those episodes during El Niño winters contribute to the prevalence of negative NAO conditions. The above processes are further delineated by conducting a case study for the 2009/10 winter season, in which both El Niño and negative NAO conditions prevailed. It is illustrated that the frequent and intense surface cyclone development over North America and the western Atlantic throughout that winter are associated with upper-level troughs propagating across North America, which in turn are linked to downstream evolution of wave packets originating from the Pacific storm track.

scholarly journals Can ENSO-Like Convection Force an ENSO-Like Extratropical Response on Subseasonal Time Scales?

2018 ◽  
Vol 31 (20) ◽  
pp. 8339-8349 ◽  
Author(s):  
Michael Goss ◽  
Sukyoung Lee ◽  
Steven B. Feldstein ◽  
Noah S. Diffenbaugh
Keyword(s):  
North America ◽  
El Niño ◽  
North Pacific ◽  
El Nino ◽  
La Niña ◽  
Convective Heating ◽  
Western North America ◽  
The North ◽  
La Nina ◽  
The North Pacific

A daily El Niño–Southern Oscillation (ENSO) index is developed based on precipitation rate and is used to investigate subseasonal time-scale extratropical circulation anomalies associated with ENSO-like convective heating. The index, referred to as the El Niño precipitation index (ENPI), is anomalously positive when there is El Niño–like convection. Conversely, the ENPI is anomalously negative when there is La Niña–like convection. It is found that when precipitation becomes El Niño–like (La Niña–like) on subseasonal time scales, the 300-hPa geopotential height field over the North Pacific and western North America becomes El Niño–like (La Niña–like) within 5–10 days. The composites show a small association with the MJO. These results are supported by previous modeling studies, which show that the response over the North Pacific and western North America to an equatorial Pacific heating anomaly occurs within about one week. This suggests that the mean seasonal extratropical response to El Niño (La Niña) may in effect simply be the average of the subseasonal response to subseasonally varying El Niño–like (La Niña–like) convective heating. Implications for subseasonal to seasonal forecasting are discussed.

scholarly journals Effect of the North Pacific Tropospheric Waveguide on the Fidelity of Model El Niño Teleconnections

2020 ◽  
Vol 33 (12) ◽  
pp. 5223-5237 ◽  
Author(s):  
Ronald K. K. Li ◽  
Tim Woollings ◽  
Christopher O’Reilly ◽  
Adam A. Scaife
Keyword(s):  
Rossby Wave ◽  
North Atlantic ◽  
El Niño ◽  
North Pacific ◽  
El Nino ◽  
Wave Source ◽  
The North ◽  
The Caribbean ◽  
The North Atlantic ◽  
The North Pacific

AbstractIn a free-running climate model, DJF tropical–extratropical teleconnections are assessed and compared to observed teleconnections in reanalysis data. From reanalysis, the leading mode of covariability between tropical outgoing longwave radiation (OLR) and Northern Hemisphere extratropical geopotential height (Z500) is identified using maximum covariance analysis (MCA). This mode relates closely to the El Niño pattern. The GCM captures the tropical OLR well but the associated extratropical Z500 less well. The GCM climatology has an equatorward shifted North Pacific jet bias. We examine whether the difference in the teleconnection pattern is related to the GCM’s jet bias. In both a ray-tracing analysis and a barotropic model, this jet bias is shown to affect the Rossby wave propagation from the tropical Pacific into the North Pacific. These idealized model results suggest qualitatively that the MCA difference is largely consistent with linear Rossby wave dynamics. While the basic state has a larger effect on the North Pacific MCA, a Rossby wave source (RWS) bias in the Caribbean has a larger effect on the North Atlantic MCA. The North Pacific jet bias is also proposed to affect the downstream propagation of waves from North America into the Caribbean, where it affects tropical RWS and the triggering of secondary waves into the North Atlantic. We propose that climatological biases in the tropics are one underlying cause of the jet bias. Our study may also help understand the results of other climate models with similar jet biases.

The Role of Ekman Ocean Heat Transport in the Northern Hemisphere Response to ENSO

2008 ◽  
Vol 21 (21) ◽  
pp. 5688-5707 ◽  
Author(s):  
Michael A. Alexander ◽  
James D. Scott
Keyword(s):  
Mixed Layer ◽  
North Atlantic ◽  
El Niño ◽  
North Pacific ◽  
El Nino ◽  
La Niña ◽  
Ekman Transport ◽  
Layer Model ◽  
The North ◽  
La Nina

Abstract The influence of oceanic Ekman heat transport (Qek) on air–sea variability associated with ENSO teleconnections is examined via a pair of atmospheric general circulation model (AGCM) experiments. In the mixed layer model (MLM) experiment, observed sea surface temperatures (SSTs) for the years 1950–99 are specified over the tropical Pacific, while a grid of mixed layer models is coupled to the AGCM elsewhere over the global oceans. The same experimental design was used in the Ekman transport/mixed layer model (EKM) experiment with the addition of Qek in the mixed layer ocean temperature equation. The ENSO signal was evaluated using differences between composites of El Niño and La Niña events averaged over the 16 ensemble members in each experiment. In both experiments the Aleutian low deepened and the resulting surface heat fluxes cooled the central North Pacific and warmed the northeast Pacific during boreal winter in El Niño relative to La Niña events. Including Qek amplified the ENSO-related SSTs by ∼⅓ in the central and northeast North Pacific, producing anomalies comparable to those in nature. Differences between the ENSO-induced atmospheric circulation anomalies in the EKM and MLM experiments were not significant over the North Pacific. The sea level pressure (SLP) and SST response to ENSO over the Atlantic strongly projects on the North Atlantic Oscillation (NAO) and the SST tripole pattern in observations and both model experiments. The La Niña anomalies, which are stronger than during El Niño, include high pressure and positive SSTs in the central North Atlantic. Including Ekman transport enhanced the Atlantic SST anomalies, which in contrast to the Pacific, appeared to strengthen the overlying atmospheric circulation.

The Impact of Strong El Niño and La Niña Events on the North Atlantic

2019 ◽  
Vol 46 (5) ◽  
pp. 2874-2883 ◽  
Author(s):  
S. C. Hardiman ◽  
N. J. Dunstone ◽  
A. A. Scaife ◽  
D. M. Smith ◽  
S. Ineson ◽  
...  
Keyword(s):  
North Atlantic ◽  
El Niño ◽  
El Nino ◽  
La Niña ◽  
The North ◽  
La Nina ◽  
The North Atlantic ◽  
The Impact
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