scholarly journals Impact of the El Niño type and PDO on the Winter Sub-seasonal North American Zonal Temperature Dipole via the Variability of Positive PNA Events

2021 ◽  
Author(s):  
Yao Ge ◽  
Dehai Luo
Keyword(s):  
North America ◽  
North American ◽  
El Niño ◽  
El Nino ◽  
Hadley Cell ◽  
Height Anomaly ◽  
Anticyclonic Anomaly ◽  
The Pacific ◽  
Westerly Winds ◽  
Midlatitude Westerly

Abstract In recent years, the winter (from December to February, DJF) North American surface air temperature (SAT) anomaly in midlatitudes shows a “warm west/cold east” (WWCE) dipole pattern. To some extent, the winter WWCE dipole can be considered as being a result of the winter mean of sub-seasonal WWCE events. In this paper, the Pacific SST condition linked to the WWCE SAT dipole is investigated. It is found that while the sub-seasonal WWCE dipole is related to the positive Pacific North American (PNA+) pattern, the impact of the PNA+ on the WWCE dipole depends on the El Niño SST type and the phase of Pacific decadal Oscillation (PDO). For a central-Pacific (CP) type El Niño, the positive (negative) height anomaly center of PNA+ is located in the western (eastern) North America to result in an intensified WWCE dipole, though the positive PDO favors the WWCE dipole. In contrast, the WWCE dipole is suppressed under an Eastern-Pacific (EP) type El Niño because the PNA+ anticyclonic anomaly dominates the whole North America. Moreover, the physical cause of why the El Niño type influences PNA+ is further examined. It is found that the type of El Niño can significantly influence the location of PNA+ through changing North Pacific midlatitude westerly winds associated with the Pacific Hadley cell change. For the CP-type El Niño, the eastward migration of PNA+ is suppressed to favor its anticyclonic (cyclonic) anomaly appearing in the west (east) region of North American owing to reduced midlatitude westerly winds. But for the EP-type El Niño, midlatitude westerly wind is intensified to cause the appearance of PNA+ anticyclonic anomaly over the whole North America due to enhanced Hadley cell.

scholarly journals Impact of the El Niño type and PDO on the winter sub-seasonal North American zonal temperature dipole via the variability of positive PNA patterns

2021 ◽  
Author(s):  
Yao Ge ◽  
Dehai Luo
Keyword(s):  
North America ◽  
North American ◽  
El Niño ◽  
El Nino ◽  
Hadley Cell ◽  
Height Anomaly ◽  
Central Pacific ◽  
The West ◽  
Anticyclonic Anomaly ◽  
The Impact

Abstract In recent years, the winter (from December to February, DJF) North American surface air temperature (SAT) anomaly in midlatitudes shows a “warm west/cold east” (WWCE) dipole pattern. To some extent, the winter WWCE dipole can be considered as being a result of the winter mean of sub-seasonal WWCE events. In this paper, the Pacific SST condition linked to the sub-seasonal WWCE SAT dipole is investigated. It is found that while the sub-seasonal WWCE dipole is related to the positive Pacific North American (PNA+) pattern, the impact of the PNA+ on the WWCE dipole depends on the El Niño SST type and the phase of Pacific decadal Oscillation (PDO). For a central-Pacific (CP) type El Niño, the positive (negative) height anomaly center of PNA+ is located in the west (east) part of North America to result in an intensified WWCE dipole, though the positive PDO favors the WWCE dipole. In contrast, the WWCE dipole is suppressed under an Eastern-Pacific (EP) type El Niño because the PNA+ anticyclonic anomaly dominates the whole North America.Moreover, the physical cause of why the type of El Niño influences the PNA+ is further examined. It is found that the type of El Niño can significantly influence the location of PNA+ through changing North Pacific midlatitude westerly winds (NPWWs). For the CP-type El Niño, the eastward migration of PNA+ is suppressed to favor its anticyclonic (cyclonic) anomaly appearing in the west (east) region of North American owing to reduced NPWWs. But for the EP-type El Niño, NPWWs are intensified to cause the appearance of the PNA+ anticyclonic anomaly over the whole North America due to enhanced Hadley cell and Ferrell cell.

North American Warm-west/Cold-East air temperature pattern and its linkage to different El Nino types

2020 ◽  
Author(s):  
Yao Ge ◽  
Dehai Luo
Keyword(s):  
North America ◽  
North American ◽  
El Niño ◽  
North Pacific ◽  
El Nino ◽  
Anticyclonic Anomaly ◽  
The North ◽  
Westerly Winds ◽  
Dipole Pattern ◽  
Midlatitude Westerly

<p><strong> </strong></p><p>In recent years, the surface air temperature (SAT) anomalies in winter over North America show a “warm-West/cool-East” (WWCE) dipole pattern. The underlying mechanism of the North American WWCE dipole pattern has been an important research topic. This study examines the physical cause of the WWCE dipole generation.</p><p>It is found that the positive phase (PNA<sup>+</sup>) of the Pacific North American (PNA) pattern can lead to the generation of the WWCE SAT dipole. However, the impact of the PNA<sup>+ </sup>on the WWCE SAT dipole over North America depends on the type of the El Nino SST anomaly. When an Eastern-Pacific (EP) type El Nino occurs, the anticyclonic anomaly center of the PNA<sup>+ </sup>over the North American continent is displaced eastward near 100°W due to intensified midlatitude westerly winds over North Pacific so that its anticyclonic anomaly dominates the whole North America. In this case, the cyclonic anomaly of the PNA<sup>+</sup> almost disappears over the North America. Thus, the WWCE SAT dipole over the North America is weakened. In contrast, when a central-Pacific (CP) type El Nino appears, the anticyclonic anomaly center of the associated PNA<sup>+</sup> is located over the North America west coast due to reduced midlatitude westerly winds over North Pacific. As a result, the cyclonic anomaly of the PNA<sup>+</sup> can appear over the east United States to result in an intensified WWCE SAT dipole over the North America</p>

scholarly journals Toward Identifying Subseasonal Forecasts of Opportunity Using North American Weather Regimes

2020 ◽  
Vol 148 (5) ◽  
pp. 1861-1875
Author(s):  
Andrew W. Robertson ◽  
Nicolas Vigaud ◽  
Jing Yuan ◽  
Michael K. Tippett
Keyword(s):  
North American ◽  
El Niño ◽  
Geopotential Height ◽  
Large Scale ◽  
El Nino ◽  
Lead Times ◽  
Seasonal Forecasts ◽  
The North ◽  
The Pacific ◽  
Graphical Tool

Abstract Large-scale atmospheric circulation regime structures are used to diagnose subseasonal forecasts of wintertime geopotential height fields over the North American sector, from the NCEP CFSv2 model. Four large-scale daily circulation regimes derived from reanalysis 500-hPa geopotential height data using K-means clustering are used as a low-dimensional basis for diagnosing the model’s forecasts up to 45 days ahead. On average, hindcast skill in regime space is found to be limited to 10–15 days ahead, in terms of anomaly correlation of 5-day averages of regime counts, over the 1999–2010 period. However, skill up to 30 days ahead is identified in individual winters, and intraseasonal episodes of high skill are identified using a forecast-evolution graphical tool. A striking vacillation between the West Coast and Pacific ridge patterns during December–January 2008/09 is shown to be predicted 20–25 days in advance, illustrating the possibility to identify “forecasts of opportunity” when subseasonal forecast skill is much higher than the average. The forecast-evolution tool also provides insight into the poor seasonal forecasts of California precipitation by operational centers during the 2015/16 El Niño winter. The Pacific trough regime is shown to be greatly overpredicted beyond 1–2 weeks in advance during the 2015/16 winter, with weather-scale features dominating the forecast evolution at shorter lead times. A similar though less extreme situation took place during the weaker El Niño of 2009/10, with the Pacific trough overforecast at S2S lead times.

scholarly journals Meridional Extent and Interannual Variability of the Pacific Ocean Tropical–Subtropical Warm Water Exchange

2005 ◽  
Vol 35 (3) ◽  
pp. 323-335 ◽  
Author(s):  
Christopher S. Meinen
Keyword(s):  
Pacific Ocean ◽  
El Niño ◽  
El Nino ◽  
Warm Water ◽  
Height Anomaly ◽  
The South ◽  
El Niño Event ◽  
The Pacific ◽  
The Pacific Ocean ◽  
The Tropics

Abstract Altimetric observations of sea surface height anomaly (SSHA) from the TOPEX/Poseidon and ERS satellites, hydrography, and the ECMWF and Florida State University wind products are used to track warm water (≥20°C) as it is exchanged between the equatorial Pacific Ocean and the higher latitudes during 1993–2003. The large El Niño event of 1997–98 resulted in a significant discharge of warm water toward the higher latitudes within the interior of the Pacific Ocean. The exchange of anomalous warm water volume with the Northern Hemisphere appears to be blocked under the intertropical convergence zone, consistent with most current ideas on the time-mean tropical–subtropical exchange. Little of the warm water discharged northward across 5° and 8°N during the 1997–98 El Niño event could be traced as far as 10°N. To the south, however, these anomalous volumes of warm water were visible at least as far as 20°S, primarily in the longitudes around 130°–160°W. In both hemispheres most of the warm water appeared to flow westward before returning to the Tropics during the recharge phase of the El Niño–La Niña cycle. The buildup of warm water in the Tropics before the 1997–98 El Niño is shown to be fed primarily by warm water drawn from the region in the western Pacific within 5°S–15°N. The exchange cycle between the equatorial band and the higher latitudes north of the equator leads the cycle in the south by 6–8 months. These results are found in all three datasets used herein, hydrography, altimetric observations of SSHA, and Sverdrup transports calculated from multiple wind products, which demonstrates the robustness of the results.

Predictability of Indian Ocean Dipole (IOD)

2016 ◽  
Author(s):  
Jing-Jia Luo
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
El Nino ◽  
Tropical Indian Ocean ◽  
The West ◽  
The North ◽  
The Pacific ◽  
Westerly Winds ◽  
Ocean Atmosphere ◽  
The Indian Ocean

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Climate Science. Please check back later for the full article. The tropical Indian Ocean is unique in several aspects. Unlike the Pacific and the Atlantic Oceans, the Indian Ocean is bounded to the north by a large landmass, the Eurasian continent. The large thermal heat contrast between the ocean in the south and the land in the north induces the world’s strongest monsoon systems in South and East Asia, in response to the seasonal migration of solar radiation. The strong and seasonally reversing surface winds generate large seasonal variations in ocean currents and basin-wide meridional heat transport across the equator. In contrast to the tropical Pacific and the Atlantic, where easterly trade winds prevail throughout the year, westerly winds (albeit with a relatively weak magnitude) blow along the equatorial Indian Ocean, particularly during the boreal spring and autumn seasons, generating the semi-annual Yoshida-Wyrtki eastward equatorial ocean currents. As a consequence of the lack of equatorial upwelling, the tropical Indian Ocean occupies the largest portion of the warm water pool (with Sea Surface Temperature [SST] being greater than 28 °C) on Earth. The massive warm water provides a huge potential energy available for deep convections that significantly affect the weather-climate over the globe. It is therefore of vital importance to discover and understand climate variabilities in the Indian Ocean and to further develop a capability to correctly predict the seasonal departures of the warm waters and their global teleconnections. The Indian Ocean Dipole (IOD) is the one of the recently discovered climate variables in the tropical Indian Ocean. During the development of the super El Niño in 1997, the climatological zonal SST gradient along the equator was much reduced (with strong cold SST anomalies in the east and warm anomalies in the west). The surface westerly winds switched to easterlies, and the ocean thermocline became shallow in the east and deep in the west. These features are reminiscent of what are observed during El Niño years in the Pacific, representing a typical coupled process between the ocean and the atmosphere. The IOD event in 1997 contributed significantly to floods in eastern Africa and severe droughts and bushfires in Indonesia and southeastern Australia. Since the discovery of the 1997 IOD event, extensive efforts have been made to lead the rapid progress in understanding the air-sea coupled climate variabilities in the Indian Ocean; and many approaches, including simple statistical models and comprehensive ocean-atmosphere coupled models, have been developed to simulate and predict the Indian Ocean climate. Essential to the discussion are the ocean-atmosphere dynamics underpinning the seasonal predictability of the IOD, critical factors that limit the IOD predictability (inter-comparison with El Niño-Southern Oscillation [ENSO]), observations and initialization approaches that provide realistic initial conditions for IOD predictions, models and approaches that have been developed to simulate and predict the IOD, the influence of global warming on the IOD predictability, impacts of IOD-ENSO interactions on the IOD predictability, and the current status and perspectives of the IOD prediction at seasonal to multi-annual timescales.

scholarly journals Climatology and Variability of Warm and Cold Fronts over North America from 1979 to 2018

2020 ◽  
Vol 33 (15) ◽  
pp. 6531-6554
Author(s):  
Ryan Lagerquist ◽  
John T. Allen ◽  
Amy McGovern
Keyword(s):  
North America ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Specific Humidity ◽  
Regional Patterns ◽  
Cold Fronts ◽  
The Pacific ◽  
Cyclone Tracks

AbstractThis paper describes the development and analysis of an objective climatology of warm and cold fronts over North America from 1979 to 2018. Fronts are detected by a convolutional neural network (CNN), trained to emulate fronts drawn by human meteorologists. Predictors for the CNN are surface and 850-hPa fields of temperature, specific humidity, and vector wind from the ERA5 reanalysis. Gridded probabilities from the CNN are converted to 2D frontal regions, which are used to create the climatology. Overall, warm and cold fronts are most common in the Pacific and Atlantic cyclone tracks and the lee of the Rockies. In contrast with prior research, we find that the activity of warm and cold fronts is significantly modulated by the phase and intensity of El Niño–Southern Oscillation. The influence of El Niño is significant for winter warm fronts, winter cold fronts, and spring cold fronts, with activity decreasing over the continental United States and shifting northward with the Pacific and Atlantic cyclone tracks. Long-term trends are generally not significant, although we find a poleward shift in frontal activity during the winter and spring, consistent with prior research. We also identify a number of regional patterns, such as a significant long-term increase in warm fronts in the eastern tropical Pacific Ocean, which are characterized almost entirely by moisture gradients rather than temperature gradients.

scholarly journals Enhanced North American carbon uptake associated with El Niño

2019 ◽  
Vol 5 (6) ◽  
pp. eaaw0076 ◽  
Author(s):  
Lei Hu ◽  
Arlyn E. Andrews ◽  
Kirk W. Thoning ◽  
Colm Sweeney ◽  
John B. Miller ◽  
...  
Keyword(s):  
North America ◽  
North American ◽  
Water Availability ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Surface Air Temperature ◽  
Net Ecosystem Exchange ◽  
Carbon Uptake ◽  
Annual Nee

Long-term atmospheric CO2mole fraction and δ13CO2observations over North America document persistent responses to the El Niño–Southern Oscillation. We estimate these responses corresponded to 0.61 (0.45 to 0.79) PgC year−1more North American carbon uptake during El Niño than during La Niña between 2007 and 2015, partially offsetting increases of net tropical biosphere-to-atmosphere carbon flux around El Niño. Anomalies in derived North American net ecosystem exchange (NEE) display strong but opposite correlations with surface air temperature between seasons, while their correlation with water availability was more constant throughout the year, such that water availability is the dominant control on annual NEE variability over North America. These results suggest that increased water availability and favorable temperature conditions (warmer spring and cooler summer) caused enhanced carbon uptake over North America near and during El Niño.

scholarly journals Implications of the permanent El Niño teleconnection "blueprint" for past global and North American hydroclimatology

2011 ◽  
Vol 7 (3) ◽  
pp. 723-743 ◽  
Author(s):  
A. Goldner ◽  
M. Huber ◽  
N. Diffenbaugh ◽  
R. Caballero
Keyword(s):  
United States ◽  
North America ◽  
North American ◽  
El Niño ◽  
Climate Model ◽  
El Nino ◽  
Global Climate Model ◽  
Future Climate Change ◽  
Analogue Model ◽  
Change In Mean

Abstract. Substantial evidence exists for wetter-than-modern continental conditions in North America during the pre-Quaternary warm climate intervals. This is in apparent conflict with the robust global prediction for future climate change of a northward expansion of the subtropical dry zones that should drive aridification of many semiarid regions. Indeed, areas of expected future aridification include much of western North America, where extensive paleoenvironmental records are documented to have been much wetter before the onset of Quaternary ice ages. It has also been proposed that climates previous to the Quaternary may have been characterized as being in a state with warmer-than-modern eastern equatorial sea surface temperatures (SSTs). Because equatorial Pacific SSTs exert strong controls on midlatitude atmospheric circulation and the global hydrologic cycle, the teleconnected response from this permanent El Niño-like mean state has been proposed as a useful analogue model, or "blueprint", for understanding global climatological anomalies in the past. The present study quantitatively explores the implications of this blueprint for past climates with a specific focus on the Miocene and Pliocene, using a global climate model (CAM3.0) and a nested high-resolution climate model (RegCM3) to study the hydrologic impacts on global and North American climate of a change in mean SSTs resembling that which occurs during modern El Niño events. We find that the global circulation response to a permanent El Niño resembles a large, long El Niño event. This state also exhibits equatorial super-rotation, which would represent a fundamental change to the tropical circulations. We also find a southward shift in winter storm tracks in the Pacific and Atlantic, which affects precipitation and temperature over the mid-latitudes. In addition, summertime precipitation increases over the majority of the continental United States. These increases in precipitation are controlled by shifts in the subtropical jet and secondary atmospheric feedbacks. Based on these results and the data proxy comparison, we conclude that a permanent El Niño like state is one potential explanation of wetter-than-modern conditions observed in paleoclimate-proxy records, particularly over the western United States.

scholarly journals How MJO Teleconnections and ENSO Interference Impacts U.S. Precipitation

2020 ◽  
Vol 33 (11) ◽  
pp. 4621-4640 ◽  
Author(s):  
Marybeth C. Arcodia ◽  
Ben P. Kirtman ◽  
Leo S. P. Siqueira
Keyword(s):  
North American ◽  
El Niño ◽  
El Nino ◽  
The United States ◽  
La Niña ◽  
Height Anomaly ◽  
Destructive Interference ◽  
Spatial And Temporal Patterns ◽  
La Nina ◽  
Upper Level

AbstractA composite analysis reveals how the Madden–Julian oscillation (MJO) impacts North American rainfall through perturbations in both the upper-tropospheric flow and regional low-level moisture availability. Upper-level divergence associated with the MJO tropical convection drives a quasi-stationary Rossby wave response to the midlatitudes. This forces a midlatitude upper-level dipolar geopotential height anomaly that is accompanied by a westward retraction of the jet stream and reduced rainfall over the central-eastern North Pacific. A reverse effect is found as the MJO propagates eastward across the Maritime Continent. These large differences in the extratropical upper-level flow, combined with anomalies in the regional supply of water vapor, have a profound impact on southeastern U.S. rainfall. The low-frequency variability, including that associated with ENSO, can modify the seasonal background flow (e.g., El Niño and La Niña basic states) affecting the distribution, strength, and propagation of the intraseasonal oscillation and the extratropical teleconnection patterns. The combined effects of the ENSO and the MJO signals result in both spatial and temporal patterns of interference and modulation of North American rainfall. The results from this study show that during a particular phase of an active MJO, the extratropical response can considerably enhance or mask the interannual ENSO signal in the United States, potentially resulting in anomalies of the opposite sign than that expected during a specific ENSO phase. Analyses of specific MJO events during an El Niño or La Niña episode reveal significant contributions to extreme events via constructive and destructive interference of the MJO and ENSO signals.

scholarly journals The North American Spring Coldness Response to the Persistent Weak Stratospheric Vortex Induced by Extreme El Niño Events

2021 ◽  
Vol 9 ◽  
Author(s):  
Xin Zhou ◽  
Quanliang Chen ◽  
Yang Li ◽  
Yawei Yang ◽  
Shaobo Zhang ◽  
...  
Keyword(s):  
North America ◽  
North American ◽  
El Niño ◽  
Climate Model ◽  
El Nino ◽  
Southern Oscillation ◽  
Stationary Waves ◽  
El Niño Events ◽  
Extreme El Niño ◽  
El Nino Events

The stratospheric pathway is a major driver of El Niño–Southern Oscillation (ENSO) impacts on mid-latitude tropospheric circulation and winter weather. The weak vortex induced by El Niño conditions has been shown to increase the risk of cold spells, especially over Eurasia, but its role for North American winters is less clear. This study involved idealized experiments with the Whole Atmosphere Community Climate Model to examine how the weak winter vortex induced by extreme El Niño events is linked to North American coldness in spring. Contrary to the expected mid-latitude cooling associated with a weak vortex, extreme El Niño events do not lead to North American cooling overall, with daily cold extremes actually decreasing, especially in Canada. The expected cooling is absent in most of North America because of the advection of warmer air masses guided by an enhanced ridge over Canada and a trough over the Aleutian Peninsula. This pattern persists in spring as a result of the trapping of stationary waves from the polar stratosphere and troposphere, implying that the stratospheric influence on North America is sensitive to regional downward wave activities.

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