scholarly journals On the Seasonality of the El Niño Teleconnection to the Amundsen Sea Region

2019 ◽  
Vol 32 (15) ◽  
pp. 4829-4845 ◽  
Author(s):  
Yu Yeung Scott Yiu ◽  
Amanda C. Maycock
Keyword(s):  
Rossby Wave ◽  
El Niño ◽  
Rossby Waves ◽  
El Nino ◽  
Austral Summer ◽  
South Pacific ◽  
Austral Winter ◽  
Amundsen Sea ◽  
Wave Source ◽  
Rossby Wave Source

Abstract The Amundsen Sea low (ASL) is a quasi-stationary low pressure system that affects climate in West Antarctica. Previous studies have shown that El Niño–Southern Oscillation (ENSO) modulates the position and strength of the ASL with the strongest teleconnection found in austral winter despite the amplitude of ENSO events generally being largest in austral autumn/summer. This study investigates the mechanisms behind the seasonality of the El Niño teleconnection to the Amundsen Sea region (ASR) using experiments with the HadGEM3 climate model forced with an idealized fixed El Niño sea surface temperature anomaly present throughout the year. The seasonality of the El Niño–ASR teleconnection is found to originate from seasonal differences in the large-scale zonal winds in the South Pacific sector. In austral winter, the region of strong absolute vorticity near ~30°S associated with the subtropical jet, in combination with the changes to upper-tropospheric divergence due to the El Niño perturbation, acts as an anomalous Rossby wave source that is largely absent in austral summer. Furthermore, in austral summer the poleward propagation of tropically sourced Rossby waves into the ASR is inhibited by the strong polar front jet in the South Pacific sector, which leads to Rossby wave reflection away from the ASR. In austral winter, Rossby waves are able to propagate into the ASR, forming part of the Pacific South America pattern. The lack of the Rossby wave source in the tropical Pacific and the absence of favorable conditions for wave propagation explains the weaker El Niño–ASR teleconnection in austral summer compared to austral winter.

scholarly journals Rossby Waves Mediate Impacts of Tropical Oceans on West Antarctic Atmospheric Circulation in Austral Winter

2015 ◽  
Vol 28 (20) ◽  
pp. 8151-8164 ◽  
Author(s):  
Xichen Li ◽  
David M. Holland ◽  
Edwin P. Gerber ◽  
Changhyun Yoo
Keyword(s):  
Atmospheric Circulation ◽  
Rossby Wave ◽  
Rossby Waves ◽  
Ocean Warming ◽  
Austral Winter ◽  
West Antarctica ◽  
Tropical Ocean ◽  
Amundsen Sea ◽  
Tropical Oceans ◽  
Simulation Results

Abstract Recent studies link climate change around Antarctica to the sea surface temperature of tropical oceans, with teleconnections from the Pacific, Atlantic, and Indian Oceans making different contributions to Antarctic climate. In this study, the impacts of each ocean basin on the wintertime Southern Hemisphere circulation are identified by comparing simulation results using a comprehensive atmospheric model, an idealized dynamical core model, and a theoretical Rossby wave model. The results herein show that tropical Atlantic Ocean warming, Indian Ocean warming, and eastern Pacific cooling are all able to deepen the Amundsen Sea low located adjacent to West Antarctica, while western Pacific warming increases the pressure to the west of the international date line, encompassing the Ross Sea and regions south of the Tasman Sea. In austral winter, these tropical ocean basins work together linearly to modulate the atmospheric circulation around West Antarctica. Further analyses indicate that these teleconnections critically depend on stationary Rossby wave dynamics and are thus sensitive to the background flow, particularly the subtropical/midlatitude jet. Near these jets, wind shear is amplified, which strengthens the generation of Rossby waves. On the other hand, near the edges of the jets the meridional gradient of the absolute vorticity is also enhanced. As a consequence of the Rossby wave dispersion relationship, the jet edge may reflect stationary Rossby wave trains, serving as a waveguide. The simulation results not only identify the relative roles of each of the tropical ocean basins in the tropical–Antarctica teleconnection, but also suggest that a deeper understanding of teleconnections requires a better estimation of the atmospheric jet structures.

Teleconnections from Tropics to Northern Extratropics through a Southerly Conveyor

2005 ◽  
Vol 62 (11) ◽  
pp. 4057-4070 ◽  
Author(s):  
Zhuo Wang ◽  
C-P. Chang ◽  
Bin Wang ◽  
Fei-Fei Jin
Keyword(s):  
Wave Propagation ◽  
Rossby Wave ◽  
Rossby Waves ◽  
Propagation Theory ◽  
Wave Source ◽  
Tropical Forcing ◽  
Rossby Wave Source ◽  
Rossby Wave Response ◽  
Rossby Wave Propagation ◽  
The Tropics

Abstract Rossby wave propagation theory predicts that Rossby waves in a tropical easterly flow cannot escape from the Tropics to the extratropics. Here the authors show that a southerly flow component in the basic state (a southerly conveyor) may transfer a Rossby wave source northward; thus, a forcing embedded in the deep tropical easterlies may excite a Rossby wave response in the extratropical westerlies. It is shown that the southerly conveyor determines the location of the effective Rossby wave source and that the extratropical response is relatively insensitive to the location of the tropical forcing, provided that the tropical response can reach the southerly conveyor. A stronger southerly flow favors a stronger extratropical response, and the spatial structure of the extratropical response is determined by the extratropical westerly basic flows.

scholarly journals Southern Hemisphere Sensitivity to ENSO Patterns and Intensities: Impacts over Subtropical South America

Atmosphere ◽  
2020 ◽  
Vol 11 (1) ◽  
pp. 77 ◽  
Author(s):  
Verónica Martín-Gómez ◽  
Marcelo Barreiro ◽  
Elsa Mohino
Keyword(s):  
South America ◽  
Rossby Wave ◽  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Wave Train ◽  
Circulation Model ◽  
South Pacific ◽  
El Niño Modoki ◽  
Wave Trains

El Niño flavors influence Subtropical South American (SSA) rainfall through the generation of one or two quasi-stationary Rossby waves. However, it is not yet clear whether the induced wave trains depend on the El Niño pattern and/or its intensity. To investigate this, we performed different sensitivity experiments using an Atmospheric General Circulation Model (AGCM) which was forced considering separately the Canonical and the El Niño Modoki patterns with sea surface temperature (SST) maximum anomalies of 1 and 3 °C. Experiments with 3 °C show that the Canonical El Niño induces two Rossby wave trains, a large one emanating from the western subtropical Pacific and a shorter one initiated over the central-eastern subtropical South Pacific. Only the shorter wave plays a role in generating negative outgoing longwave radiation (OLR) anomalies over SSA. On the other hand, 3 °C El Niño Modoki experiments show the generation of a large Rossby wave train that emanates from the subtropical western south Pacific and reaches South America (SA), promoting the development of negative OLR anomalies over SSA. Experiments with 1 °C show no impacts on OLR anomalies over SSA associated with El Niño Modoki. However, for the Canonical El Niño case there is a statistically significant reduction of the OLR anomalies over SSA related to the intensification of the upper level jet stream over the region. Finally, our model results suggest that SSA is more sensitive to the Canonical El Niño, although this result may be model dependent.

scholarly journals Oceanic Rossby Waves over Eastern Tropical Pacific of Both Hemispheres Forced by Anomalous Surface Winds after Mature Phase of ENSO

2016 ◽  
Vol 46 (11) ◽  
pp. 3397-3414 ◽  
Author(s):  
Hiroto Abe ◽  
Youichi Tanimoto ◽  
Takuya Hasegawa ◽  
Naoto Ebuchi
Keyword(s):  
Rossby Wave ◽  
El Niño ◽  
Rossby Waves ◽  
El Nino ◽  
Tropical Pacific ◽  
Sea Surface ◽  
Eastern Coast ◽  
Eastern Tropical Pacific ◽  
Enso Events ◽  
Anomalous Surface

AbstractThe present study examined ENSO-related wind forcing contribution to off-equatorial Rossby wave formations in the eastern tropical regions of the North and South Pacific using satellite altimeter data and atmospheric reanalysis data during the period of 1993–2013. After mature phases of ENSO events, the sea surface height anomaly fields showed that off-equatorial Rossby waves propagated westward along 11°N and 8°S from the eastern Pacific. Starting longitudes of the westward propagation were distant from the eastern coast, especially for weak El Niño events in the 2000s, in contrast to the strong 1997/98 El Niño event in which the propagations started from the coast. Based on observational data, it was hypothesized that the Rossby waves could be formed by off-equatorial zonal belts of wind stress curl anomalies (WSCAs) in 135°–90°W rather than by wave emissions from the eastern coast. A numerical model forced only by WSCAs, that is, without wave emissions from the coast, successfully reproduced observed features of the Rossby waves in 180°–120°W, supporting the study’s hypothesis. During mature phases of El Niño events, equatorially symmetric negative sea level pressure anomalies (SLPAs) resulting from hydrostatic adjustment to the underlying warm sea surface temperature anomalies dominated over the eastern tropical Pacific. Anomalous surface easterlies blowing around the negative SLPA area as geostrophic winds were a major contributor in forming the anticyclonic WSCAs. The polarity of the anomalies is reversed during La Niña events. Therefore, spatial patterns of the SLPAs associated with the ENSO events are necessary to understand the Rossby wave formations.

El Niño Flavors and Their Simulated Impacts on Atmospheric Circulation in the High Southern Latitudes*

2014 ◽  
Vol 27 (23) ◽  
pp. 8934-8955 ◽  
Author(s):  
Aaron B. Wilson ◽  
David H. Bromwich ◽  
Keith M. Hines ◽  
Sheng-hung Wang
Keyword(s):  
Atmospheric Circulation ◽  
High Latitude ◽  
El Niño ◽  
El Nino ◽  
South Pacific ◽  
Tropical Pacific ◽  
The South ◽  
Wave Source ◽  
South Central ◽  
El Niño Flavors

Abstract Two El Niño flavors have been defined based on whether warm sea surface temperature (SST) anomalies are located in the central or eastern tropical Pacific (CP or EP). This study further characterizes the impacts on atmospheric circulation in the high latitudes of the Southern Hemisphere associated with these types of El Niño events though a series of numerical simulations using the National Center for Atmospheric Research Community Atmosphere Model (CAM). Comparing results with the Interim ECMWF Re-Analysis (ERA-Interim), CAM simulates well the known changes to blocking over Australia and a southward shift in the subtropical jet stream across the eastern Pacific basin during CP events. More importantly for the high southern latitudes, CAM simulates a westward shift in upper-level divergence in the tropical Pacific, which causes the Pacific–South American stationary wave pattern to shift toward the west across the entire South Pacific. These changes to the Rossby wave source region impact the South Pacific convergence zone and jet streams and weaken the high-latitude blocking that is typically present in the Amundsen-Bellingshausen Seas during EP events. Anticyclonic flow becomes established farther west in the south central Pacific, modifying high-latitude heat and momentum fluxes across the South Pacific and South Atlantic associated with the ENSO–Antarctic dipole.

scholarly journals Differences in Atmospheric Circulation between the Development of Weak and Strong Warm Events in the Southern Oscillation

2007 ◽  
Vol 20 (10) ◽  
pp. 2191-2209 ◽  
Author(s):  
David J. Stephens ◽  
Michael J. Meuleners ◽  
Harry van Loon ◽  
Malcolm H. Lamond ◽  
Nicola P. Telcik
Keyword(s):  
High Pressure ◽  
El Niño ◽  
Large Scale ◽  
Rossby Waves ◽  
El Nino ◽  
Southern Oscillation ◽  
Surface Wind ◽  
South Pacific ◽  
The Pacific ◽  
Sst Anomalies

Abstract In this study temporal and spatial aspects of El Niño (warm event) development are explored by comparing composite sequences of sea level pressure (SLP), surface wind, and sea surface temperature (SST) anomalies leading into strong and weak events. El Niño strength is found to be related to the magnitude and spatial extent of large-scale SLP anomalies that move in a low-frequency mode. In association with this, it is also intricately linked to the amplitude and wavelength of the Rossby waves in the southern midlatitudes. The primary signature of the Southern Oscillation is a more pronounced standing wave of pressure anomalies between southeastern Australia and the central South Pacific leading into stronger events. A strong reversal in the strength of the annual cycle between these two regions causes a stronger (weaker) SLP gradient that drives southwesterly (northwesterly) wind stress forcing toward (away from) the western equatorial Pacific in austral winter–spring of year 0 (−1). Thus, pressure variations in the southwest Pacific preconditions the equatorial environment to a particular phase of ENSO and establishes the setting for greater tropical–extratropical interactions to occur in stronger events. Maximum warming in the Niño-3 region occurs between April and July (0) when a strong South Pacific trough most influences the trade winds at both ends of the Pacific. Cool SST anomalies that form to the east of high pressure anomalies over Indo–Australia assist an eastward propogation of high pressure into the Pacific midlatitudes and the demise of El Niño. Strong events have a more pronounced eastward propogation of SST and SLP anomalies and a much more noticeable enhancement of winter hemisphere Rossby waves from May–July (−1) to November–January (+1). Weak events require an enhanced South Pacific trough to develop but have much less support from the North Pacific. They also appear more variable in their development and more difficult to predict with lead time.

scholarly journals The Consistency of MJO Teleconnection Patterns on Interannual Time Scales

2020 ◽  
Vol 33 (9) ◽  
pp. 3471-3486 ◽  
Author(s):  
Kai-Chih Tseng ◽  
Eric Maloney ◽  
Elizabeth A. Barnes
Keyword(s):  
Time Scales ◽  
Rossby Wave ◽  
El Niño ◽  
El Nino ◽  
Reanalysis Data ◽  
La Niña ◽  
Wave Source ◽  
Teleconnection Patterns ◽  
La Nina ◽  
Wave Ray

AbstractThe Madden–Julian oscillation (MJO) excites strong variations in extratropical geopotential heights that modulate extratropical weather, making the MJO an important predictability source on subseasonal to seasonal time scales (S2S). Previous research demonstrates a strong similarity of teleconnection patterns across MJO events for certain MJO phases (i.e., pattern consistency) and increased model ensemble agreement during these phases that is beneficial for extended numerical weather forecasts. However, the MJO’s ability to modulate extratropical weather varies greatly on interannual time scales, which brings extra uncertainty in leveraging the MJO for S2S prediction. Few studies have investigated the mechanisms responsible for variations in the consistency of MJO tropical–extratropical teleconnections on interannual time scales. This study uses reanalysis data, ensemble simulations of a linear baroclinic model, and a Rossby wave ray tracing algorithm to demonstrate that two mechanisms largely determine the interannual variability of MJO teleconnection consistency. First, the meridional shift of stationary Rossby wave ray paths indicates increases (decreases) in the MJO’s extratropical modulation during La Niña (El Niño) years. Second, a previous study proposed that the constructive interference of Rossby wave signals caused by a dipole Rossby wave source pattern across the subtropical jet during certain MJO phases produces a consistent MJO teleconnection. However, this dipole feature is less clear in both El Niño and La Niña years due to the extension and contraction of MJO convection, respectively, which would decrease the MJO’s influence in the extratropics. Hence, considering the joint influence of the basic state and MJO forcing, this study suggests a diminished potential to leverage the MJO for S2S prediction in El Niño years.

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 Interannual Sea Level Variations in the South Pacific from 5° to 28°S

2007 ◽  
Vol 37 (12) ◽  
pp. 2882-2894 ◽  
Author(s):  
Jianke Li ◽  
Allan J. Clarke
Keyword(s):  
Sea Level ◽  
Rossby Wave ◽  
El Niño ◽  
Rossby Waves ◽  
El Nino ◽  
La Niña ◽  
Shelf Edge ◽  
Sea Level Variability ◽  
Sea Level Anomalies ◽  
La Nina

Abstract Ocean Topography Experiment (TOPEX)/Poseidon/Jason-1 satellite altimeter observations for the 11-yr period from January 1993 to December 2003 show that in the South Pacific Ocean most of the interannual sea level variability in the region 5°–28°S is west of 160°W. This interannual variability is largest from about 5° to 15°S and from 155°E to 160°W, reaching a root-mean-square value of over 11 cm. Calculations show that this interannual sea level signal can be described by first and second baroclinic vertical mode Rossby waves forced by the curl of the interannual Ekman transport. This curl, which tends to be positive during El Niño and negative during La Niña, generates positive (negative) sea level anomalies during El Niño (La Niña) that increase westward in amplitude in accordance with Rossby wave dynamics. The sea level anomalies are not exactly in phase with the curl forcing because Sverdrup balance does not hold—vortex stretching also contributes to the response. East of 160°W is a large “quiet” region of low interannual sea level variability, especially south of about 15°S. This is surprising because there is no flow into the coast, so the interannual sea level amplitude of equatorial origin should be constant along the coast, resulting in a source of westward-propagating Rossby waves of considerable amplitude. The large low-variability region results because coastal sea level amplitude falls between 5° and 15°S, so the Rossby wave source south of 15°S is weak. During El Niño the sea level is higher than normal at the coast, so the southward fall in anomalous sea level implies, by geostrophy, that there is an anomalous onshore flow. This flow feeds an anomalous southward El Niño current of up to 20 cm s−1 above the 30–50-km-wide shelf edge. During La Niña the sea level is lower than normal at the coast and the flows reverse: a narrow anomalously northward shelf-edge flow feeding a broad offshore flow between 5° and 15°S. South of 16°S the coastal flow is much weaker.

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