scholarly journals Robust Wind and Precipitation Responses to the Mount Pinatubo Eruption, as Simulated in the CMIP5 Models

2016 ◽  
Vol 29 (13) ◽  
pp. 4763-4778 ◽  
Author(s):  
Elizabeth A. Barnes ◽  
Susan Solomon ◽  
Lorenzo M. Polvani
Keyword(s):  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Internal Variability ◽  
Southern Annular Mode ◽  
Atmospheric Temperature ◽  
Forced Response ◽  
Cmip5 Models ◽  
Mount Pinatubo ◽  
Annular Mode

Abstract The volcanic eruption of Mount Pinatubo in June 1991 is the largest terrestrial eruption since the beginning of the satellite era. Here, the monthly evolution of atmospheric temperature, zonal winds, and precipitation following the eruption in 14 CMIP5 models is analyzed and strong and robust stratospheric and tropospheric circulation responses are demonstrated in both hemispheres, with tropospheric anomalies maximizing in November 1991. The simulated Southern Hemisphere circulation response projects strongly onto the positive phase of the southern annular mode (SAM), while the Northern Hemisphere exhibits robust North Atlantic and North Pacific responses that differ significantly from that of the typical northern annular mode (NAM) pattern. In contrast, observations show a negative SAM following the eruption, and internal variability must be considered along with forced responses. Indeed, evidence is presented that the observed El Niño climate state during and after this eruption may oppose the eruption-forced positive SAM response, based on the El Niño–Southern Oscillation (ENSO) state and SAM response across the models. The results demonstrate that Pinatubo-like eruptions should be expected to force circulation anomalies across the globe and highlight that great care must be taken in diagnosing the forced response as it may not fall into typical seasonal averages or be guaranteed to project onto typical climate modes.

scholarly journals An Investigation of Extreme Cold Events at the South Pole

2021 ◽  
pp. 1-35
Keyword(s):  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Southern Annular Mode ◽  
El Nino Southern Oscillation ◽  
South Pole ◽  
The South ◽  
Annular Mode ◽  
Cold Events ◽  
Extreme Cold

Abstract From 5 July to 11 September 2012, the Amundsen-Scott South Pole station experienced an unprecedented 78 days in a row with a maximum temperature at or below -50°C. Aircraft and ground-based activity cannot function without risk below this temperature. Lengthy periods of extreme cold temperatures are characterized by a drop in pressure of around 15 hPa over four days, accompanied by winds from grid east. Periodic influxes of warm air from the Weddell Sea raise the temperature as the wind shifts to grid north. The end of the event occurs when the temperature increase is enough to move past the -50°C threshold. This study also examines the length of extreme cold periods. The number of days below -50°C in early winter has been decreasing since 1999, and this trend is statistically significant at the 5% level. Late winter shows an increase in the number of days below -50°C for the same period, but this trend is not statistically significant. Changes in the Southern Annular Mode, El Niño Southern Oscillation, and the Interdecadal Pacific Oscillation/Tripole Index are investigated in relation to the initiation of extreme cold events. None of the correlations are statistically significant. A positive Southern Annular Mode and a La Niña event or a central Pacific El Niño Southern Oscillation pattern would position the upper-level circulation to favor a strong, symmetrical polar vortex with strong westerlies over the Southern Ocean, leading to a cold pattern over the South Pole.

Monthly Modulations of ENSO Teleconnections: Implications for Potential Predictability in North America

2021 ◽  
pp. 1-71
Author(s):  
William E. Chapman ◽  
Aneesh C. Subramanian ◽  
Shang-Ping Xie ◽  
Michael D. Sierks ◽  
F. Martin Ralph ◽  
...  
Keyword(s):  
North America ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Internal Variability ◽  
Forced Response ◽  
Signal To Noise ◽  
Potential Predictability ◽  
Enso Teleconnections ◽  
Upper Level

AbstractUsing a high-resolution atmospheric general circulation model simulation of unprecedented ensemble size, we examine potential predictability of monthly anomalies under El Niño Southern Oscillation (ENSO) forcing and back-ground internal variability. This study reveals the pronounced month-to-month evolution of both the ENSO forcing signal and internal variability. Internal variance in upper-level geopotential height decreases (∼ 10%) over the North Pacific during El Niño as the westerly jet extends eastward, allowing forced signals to account for a greater fraction of the total variability, and leading to increased potential predictability. We identify February and March of El Niño years as the most predictable months using a signal-to-noise analysis. In contrast, December, a month typically included in teleconnection studies, shows little-to-no potential predictability. We show that the seasonal evolution of SST forcing and variability leads to significant signal-to-noise relationships that can be directly linked to both upper-level and surface variable predictability for a given month. The stark changes in forced response, internal variability, and thus signal-to-noise across an ENSO season indicate that subseasonal fields should be used to diagnose potential predictability over North America associated with ENSO teleconnections. Using surface air temperature and precipitation as examples, this study provides motivation to pursue ‘windows of forecast opportunity’, in which statistical skill can be developed, tested, and leveraged to determine times and regions in which this skill may be elevated.

The Impact of ENSO on Wave Breaking and Southern Annular Mode Events

2010 ◽  
Vol 67 (9) ◽  
pp. 2854-2870 ◽  
Author(s):  
Tingting Gong ◽  
Steven B. Feldstein ◽  
Dehai Luo
Keyword(s):  
El Niño ◽  
Wave Breaking ◽  
El Nino ◽  
Southern Annular Mode ◽  
La Niña ◽  
Background Flow ◽  
Austral Spring ◽  
Annular Mode ◽  
La Nina ◽  
The Impact

Abstract This study examines the relationship between intraseasonal southern annular mode (SAM) events and the El Niño–Southern Oscillation (ENSO) using daily 40-yr ECMWF Re-Analysis (ERA-40) data. The data coverage spans the years 1979–2002, for the austral spring and summer seasons. The focus of this study is on the question of why positive SAM events dominate during La Niña and negative SAM events during El Niño. A composite analysis is performed on the zonal-mean zonal wind, Eliassen–Palm fluxes, and two diagnostic variables: the meridional potential vorticity gradient, a zonal-mean quantity that is used to estimate the likelihood of wave breaking, and the wave breaking index (WBI), which is used to evaluate the strength of the wave breaking. The results of this investigation suggest that the background zonal-mean flow associated with La Niña (El Niño) is preconditioned for strong (weak) anticyclonic wave breaking on the equatorward side of the eddy-driven jet, the type of wave breaking that is found to drive positive (negative) SAM events. A probability density function analysis of the WBI, for both La Niña and El Niño, indicates that strong anticyclonic wave breaking takes place much more frequently during La Niña and weak anticyclonic wave breaking during El Niño. It is suggested that these wave breaking characteristics, and their dependency on the background flow, can explain the strong preference for SAM events of one phase during ENSO. The analysis also shows that austral spring SAM events that coincide with ENSO are preceded by strong stratospheric SAM anomalies and then are followed by a prolonged period of wave breaking that lasts for approximately 30 days. These findings suggest that the ENSO background flow also plays a role in the excitation of stratospheric SAM anomalies and that the presence of these stratospheric SAM anomalies in turn excites and then maintains the tropospheric SAM anomalies via a positive eddy feedback.

scholarly journals Indo-Western Pacific Climate Variability: ENSO Forcing and Internal Dynamics in a Tropical Pacific Pacemaker Simulation

2018 ◽  
Vol 31 (24) ◽  
pp. 10123-10139 ◽  
Author(s):  
Chuan-Yang Wang ◽  
Shang-Ping Xie ◽  
Yu Kosaka
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Southern Oscillation ◽  
Internal Variability ◽  
Tropical Pacific ◽  
Western Pacific ◽  
Ocean Atmosphere ◽  
Sst Anomalies

El Niño–Southern Oscillation (ENSO) peaks in boreal winter but its impact on Indo-western Pacific climate persists for another two seasons. Key ocean–atmosphere interaction processes for the ENSO effect are investigated using the Pacific Ocean–Global Atmosphere (POGA) experiment with a coupled general circulation model, where tropical Pacific sea surface temperature (SST) anomalies are restored to follow observations while the atmosphere and oceans are fully coupled elsewhere. The POGA shows skills in simulating the ENSO-forced warming of the tropical Indian Ocean and an anomalous anticyclonic circulation pattern over the northwestern tropical Pacific in the post–El Niño spring and summer. The 10-member POGA ensemble allows decomposing Indo-western Pacific variability into the ENSO forced and ENSO-unrelated (internal) components. Internal variability is comparable to the ENSO forcing in magnitude and independent of ENSO amplitude and phase. Random internal variability causes apparent decadal modulations of ENSO correlations over the Indo-western Pacific, which are high during epochs of high ENSO variance. This is broadly consistent with instrumental observations over the past 130 years as documented in recent studies. Internal variability features a sea level pressure pattern that extends into the north Indian Ocean and is associated with coherent SST anomalies from the Arabian Sea to the western Pacific, suggestive of ocean–atmosphere coupling.

scholarly journals Continuation of tropical Pacific Ocean temperature trend may weaken extreme El Niño and its linkage to the Southern Annular Mode

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Eun-Pa Lim ◽  
Harry H. Hendon ◽  
Pandora Hope ◽  
Christine Chung ◽  
Francois Delage ◽  
...  
Keyword(s):  
El Niño ◽  
El Nino ◽  
Southern Annular Mode ◽  
Climate Impacts ◽  
Ocean Temperature ◽  
Annular Mode ◽  
Temperature Trends ◽  
Surface Climate ◽  
Mean State ◽  
Extreme El Niño

AbstractObservational records show that occurrences of the negative polarity of the Southern Annular Mode (low SAM) is significantly linked to El Niño during austral spring and summer, potentially providing long-lead predictability of the SAM and its associated surface climate conditions. In this study, we explore how this linkage may change under a scenario of a continuation of the ocean temperature trends that have been observed over the past 60 years, which are plausibly forced by increasing greenhouse gas concentrations. We generated coupled model seasonal forecasts for three recent extreme El Niño events by initialising the forecasts with observed ocean anomalies of 1 September 1982, 1997 and 2015 added into (1) the current ocean mean state and into (2) the ocean mean state updated to include double the recent ocean temperature trends. We show that the strength of extreme El Niño is reduced with the warmer ocean mean state as a result of reduced thermocline feedback and weakened rainfall-wind-sea surface temperature coupling over the tropical eastern Pacific. The El Niño-low SAM relationship also weakens, implying the possibility of reduced long-lead predictability of the SAM and associated surface climate impacts in the future.

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