scholarly journals The Seasonal Effects of ENSO on Atmospheric Conditions Associated with European Precipitation: Model Simulations of Seasonal Teleconnections

2014 ◽  
Vol 27 (3) ◽  
pp. 1010-1028 ◽  
Author(s):  
Jeffrey Shaman
Keyword(s):  
Rossby Wave ◽  
North Atlantic ◽  
Wave Train ◽  
Equation Model ◽  
Vorticity Equation ◽  
Boreal Summer ◽  
Wave Source ◽  
The North ◽  
The North Atlantic ◽  
European Coast

Abstract The seasonal upper-tropospheric teleconnection between ENSO and the North Atlantic/European sector is explored through a series of model experiments. A barotropic vorticity equation model is linearized about climatological conditions for each season of the year, and divergence forcing is applied over the equatorial Pacific to mimic El Niño–related convective activity. During boreal fall, winter, and spring, this forcing similarly excites a northeastward-propagating stationary barotropic Rossby wave train that extends across the North Atlantic to the European coast. Strong anomalies develop over the British Isles in the vicinity of the North Atlantic jet exit. Solutions during boreal summer produce no clear wave train; however, evidence exists for a North Atlantic response because of both eastward- and westward-propagating signals. These direct responses over the Atlantic and Europe are qualitatively similar to observed ENSO-associated anomalies during boreal spring and fall, but differ structurally during summer and winter. Further experiments with the vorticity equation model using full Rossby wave source forcing, which included vorticity advection, increase the amplitude of the response over Europe during some seasons; however, structural differences persist. Finally, experiments with the Community Atmosphere Model (CAM), version 4, reveal that the basic northeastward-propagating response is modulated by downstream feedbacks. These changes are most profound during boreal winter and engender an arching wave train pattern that, matching observations, reflects off the jet over North America, propagates southeastward over the North Atlantic, and fails to reach the European coast. Overall, the simulations with CAM correctly depict observed seasonal changes in the magnitude of the ENSO–North Atlantic/European teleconnection by producing a strong fall and winter response but a weaker spring and summer response. The CAM experiments also indicate that the seasonal response is not dependent on antecedent conditions; however, CAM simulations fail to project the upper-tropospheric anomalies appropriately to the lower troposphere.

scholarly journals Dynamical Evolution of North Atlantic Ridges and Poleward Jet Stream Displacements

2011 ◽  
Vol 68 (5) ◽  
pp. 954-963 ◽  
Author(s):  
Tim Woollings ◽  
Joaquim G. Pinto ◽  
João A. Santos
Keyword(s):  
Rossby Wave ◽  
North Atlantic ◽  
Wave Breaking ◽  
Large Scale ◽  
Wave Train ◽  
Jet Stream ◽  
Rossby Wave Breaking ◽  
The North ◽  
The North Atlantic ◽  
Circulation Anomalies

Abstract The development of a particular wintertime atmospheric circulation regime over the North Atlantic, comprising a northward shift of the North Atlantic eddy-driven jet stream and an associated strong and persistent ridge in the subtropics, is investigated. Several different methods of analysis are combined to describe the temporal evolution of the events and relate it to shifts in the phase of the North Atlantic Oscillation and East Atlantic pattern. First, the authors identify a close relationship between northward shifts of the eddy-driven jet, the establishment and maintenance of strong and persistent ridges in the subtropics, and the occurrence of upper-tropospheric anticyclonic Rossby wave breaking over Iberia. Clear tropospheric precursors are evident prior to the development of the regime, suggesting a preconditioning of the Atlantic jet stream and an upstream influence via a large-scale Rossby wave train from the North Pacific. Transient (2–6 days) eddy forcing plays a dual role, contributing to both the initiation and then the maintenance of the circulation anomalies. During the regime there is enhanced occurrence of anticyclonic Rossby wave breaking, which may be described as low-latitude blocking-like events over the southeastern North Atlantic. A strong ridge is already established at the time of wave-breaking onset, suggesting that the role of wave-breaking events is to amplify the circulation anomalies rather than to initiate them. Wave breaking also seems to enhance the persistence, since it is unlikely that a persistent ridge event occurs without being also accompanied by wave breaking.

The AMV phase-dependence of the connection between February NAO and March surface air temperature over the Tibetan Plateau

2021 ◽  
Author(s):  
Jingyi Li ◽  
Fei Li ◽  
Shengping He ◽  
Huijun Wang ◽  
Yvan J Orsolini
Keyword(s):  
Tibetan Plateau ◽  
Air Temperature ◽  
Rossby Wave ◽  
North Atlantic ◽  
Wave Train ◽  
Surface Air Temperature ◽  
The Tibetan Plateau ◽  
The North ◽  
Rossby Wave Train ◽  
The North Atlantic

<p>The Tibetan Plateau (TP), referred to as the “Asian water tower”, contains one of the largest land ice masses on Earth. The local glacier shrinkage and frozen-water storage are strongly affected by variations in surface air temperature over the TP (TPSAT), especially in springtime. This study reveals a distinct out-of-phase connection between the February North Atlantic Oscillation (NAO) and March TPSAT, which is non-stationary and regulated by the warm phase of the Atlantic Multidecadal Variability (AMV+). The results show that during the AMV+, the negative phase of the NAO persists from February to March, and is accompanied by a quasi-stationary Rossby wave train trapped along a northward-shifted subtropical westerly jet stream across Eurasia, inducing an anomalous adiabatic descent that warms the TP. However, during the cold phase of the AMV, the negative NAO does not persist into March. The Rossby wave train propagates along the well-separated polar and subtropical westerly jets, and the NAO−TPSAT connection is broken. Further investigation suggests that the enhanced synoptic eddy and low-frequency flow (SELF) interaction over the North Atlantic in February and March during the AMV+, caused by the enhanced and southward-shifted storm track, help maintain the NAO anomaly pattern via positive eddy feedback. This study provides a new detailed perspective on the decadal variability of the North Atlantic−TP connections in late winter−early spring.</p>

Extratropical Atmospheric Response to Equatorial Atlantic Cold Tongue Anomalies

2007 ◽  
Vol 20 (10) ◽  
pp. 2076-2091 ◽  
Author(s):  
Reindert J. Haarsma ◽  
Wilco Hazeleger
Keyword(s):  
Rossby Wave ◽  
North Atlantic ◽  
Maximum Amplitude ◽  
Vertical Shear ◽  
Boreal Summer ◽  
Atmospheric Response ◽  
Cold Tongue ◽  
The North ◽  
Subtropical Jet ◽  
The North Atlantic

Abstract The extratropical atmospheric response to the equatorial cold tongue mode in the Atlantic Ocean has been investigated with the coupled ocean–atmosphere model, Speedy Ocean (SPEEDO). Similar to the observations, the model simulates a lagged covariability between the equatorial cold tongue mode during late boreal summer and the east Atlantic pattern a few months later in early winter. The equatorial cold tongue mode attains its maximum amplitude during late boreal summer. However, only a few months later, when the ITCZ has moved southward, it is able to induce a significant upper-tropospheric divergence that is able to force a Rossby wave response. The lagged covariability is therefore the result of the persistence of the cold tongue anomaly and a favorable tropical atmospheric circulation a few months later. The Rossby wave energy is trapped in the South Asian subtropical jet and propagates circumglobally before it reaches the North Atlantic. Due to the local increase of the Hadley circulation, forced by the cold tongue anomaly, the subtropical jet over the North Atlantic is enhanced. The resulting increase in the vertical shear of the zonal wind increases the baroclinicity over the North Atlantic. This causes the nonlinear growth of the anomalies due to transient eddy feedbacks to be largest over the North Atlantic, resulting in an enhanced response over that region.

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.

scholarly journals Multi-model assessment of the late-winter tropospheric response to El Niño and La Niña

2021 ◽  
Author(s):  
Bianca Mezzina ◽  
Javier García-Serrano ◽  
Ileana Bladé ◽  
Froila M. Palmeiro ◽  
Lauriane Batté ◽  
...  
Keyword(s):  
Rossby Wave ◽  
North Atlantic ◽  
Large Scale ◽  
Southern Oscillation ◽  
Late Winter ◽  
Model Assessment ◽  
Wave Source ◽  
Enso Events ◽  
The North ◽  
The North Atlantic

<p>El Niño-Southern Oscillation (ENSO) is known to affect the Northern Hemisphere tropospheric circulation in late-winter (January–March), but whether El Niño and La Niña lead to symmetric impacts and with the same underlying dynamics remains unclear, particularly in the North Atlantic. Three state-of-the-art atmospheric models forced by symmetric anomalous sea surface temperature (SST) patterns, mimicking strong ENSO events, are used to robustly diagnose symmetries and asymmetries in the extra-tropical ENSO response. Asymmetries arise in the sea-level pressure (SLP) response over the North Pacific and North Atlantic, as the response to La Niña tends to be weaker and shifted westward with respect to that of El Niño. The difference in amplitude can be traced back to the distinct energy available for the two ENSO phases associated with the non-linear diabatic heating response to the total SST field. The longitudinal shift is embedded into the large-scale Rossby wave train triggered from the tropical Pacific, as its anomalies in the upper troposphere show a similar westward displacement in La Niña compared to El Niño. To fully explain this shift, the response in tropical convection and the related anomalous upper-level divergence have to be considered together with the climatological vorticity gradient of the subtropical jet, i.e. diagnosing the tropical Rossby wave source. In the North Atlantic, the ENSO-forced SLP signal is a well-known dipole between middle and high latitudes, different from the North Atlantic Oscillation, whose asymmetry is not indicative of distinct mechanisms driving the teleconnection for El Niño and La Niña.</p><p>The multi-model assessment, with 50 members for each experiment, contributes to the ERA4CS-funded MEDSCOPE project and includes: EC-EARTH/IFS (L91, 0.01hPa), CNRM/ARPEGE (L91, 0.01hPa), CMCC/CAM (L46, 0.3hPa).</p>

scholarly journals The MJO Cycle Forcing of the North Atlantic Circulation: Intervention Experiments with the Community Earth System Model

2015 ◽  
Vol 72 (2) ◽  
pp. 660-681 ◽  
Author(s):  
David M. Straus ◽  
Erik Swenson ◽  
Cara-Lyn Lappen
Keyword(s):  
Rossby Wave ◽  
North Atlantic ◽  
System Model ◽  
Earth System ◽  
Wave Source ◽  
The North ◽  
Rossby Wave Source ◽  
Community Earth System Model ◽  
The North Atlantic ◽  
Vorticity Flux

Abstract A three-dimensional evolution of Madden–Julian oscillation (MJO) diabatic heating for October–March from satellite data is constructed: the heating propagates eastward for three cycles, modulated by the likelihood for a given MJO phase to occur on a given calendar day. This heating is added to the temperature tendencies of each member of an ensemble of 48 (1 October–31 March) simulations with the Community Earth System Model. The leading two most predictable modes of the planetary wave vertically integrated total (added plus model generated) heating capture 81% of the ensemble-mean variance and form an eastward-propagating oscillation with very high signal-to-noise ratio. The two most predictable modes of the extratropical Northern Hemisphere 200-hPa height form an oscillation, as do those of the 300-hPa height tendency due to synoptic vorticity flux convergence, the 200-hPa Rossby wave source, and the envelope transient kinetic energy. The North Atlantic Oscillation (NAO+) occurs 15–25 days after the MJO convection crosses the 90°E meridian, supported by synoptic vorticity flux convergence and a distinct pattern of Rossby wave source. The daily North Atlantic circulation anomalies are categorized into four circulation regimes with a cluster analysis. The NAO+ and NAO− are equally likely in the control model runs, but the NAO+ is 10% more likely in the model runs with heating, compared to a difference of 14% in reanalyses. The daily occurrence of the NAO+ regime in the heating ensemble shows maxima at times when the leading two optimal modes of height also indicate NAO+ but also shows maxima at other times.

scholarly journals The Atlantic Multidecadal Variability phase-dependence of teleconnection between the North Atlantic Oscillation in February and the Tibetan Plateau in March

2021 ◽  
pp. 1-40
Author(s):  
Jingyi Li ◽  
Fei Li ◽  
Shengping He ◽  
Huijun Wang ◽  
Yvan J Orsolini
Keyword(s):  
Tibetan Plateau ◽  
Rossby Wave ◽  
North Atlantic ◽  
Wave Train ◽  
The Tibetan Plateau ◽  
Atlantic Multidecadal Variability ◽  
Multidecadal Variability ◽  
The North ◽  
Rossby Wave Train ◽  
The North Atlantic

AbstractThe Tibetan Plateau (TP), referred to as the “Asian water tower”, contains one of the largest land ice masses on Earth. The local glacier shrinkage and frozen-water storage are strongly affected by variations in surface air temperature over the TP (TPSAT), especially in springtime. This study reveals that the relationship between the February North Atlantic Oscillation (NAO) and March TPSAT is unstable with time and regulated by the phase of the Atlantic Multidecadal Variability (AMV). The significant out-of-phase connection occurs only during the warm phase of AMV (AMV+). The results show that during the AMV+, the negative phase of the NAO persists from February to March, and is accompanied by a quasi-stationary Rossby wave train trapped along a northward-shifted subtropical westerly jet stream across Eurasia, inducing an anomalous adiabatic descent that warms the TP. However, during the cold phase of the AMV, the negative NAO can not persist into March. The Rossby wave train propagates along the well-separated polar and subtropical westerly jets, and the NAO−TPSAT connection is broken. Further investigation suggests that the enhanced synoptic eddy and low frequency flow (SELF) interaction over the North Atlantic in February and March during the AMV+, caused by the enhanced and southward-shifted storm track, help maintain the NAO anomaly pattern via positive eddy feedback. This study provides a new detailed perspective on the decadal variability of the North Atlantic−TP connection in late winter−early spring.

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