Anthropogenic and Natural Contributions to the Lengthening of the Southern Hemisphere Summer Season

2020 ◽  
Vol 33 (24) ◽  
pp. 10539-10553
Author(s):  
Evan Weller ◽  
Bo-Joung Park ◽  
Seung-Ki Min
Keyword(s):  
Greenhouse Gas ◽  
Southern Hemisphere ◽  
Dominant Role ◽  
Coupled Model ◽  
Internal Variability ◽  
Southern Annular Mode ◽  
Summer Season ◽  
External Forcings ◽  
Regional Trends

AbstractThis study provides the first quantitative assessment of observed long-term changes in summer-season timing and length in the Southern Hemisphere (SH) and its subregions over the past 60 years, enabling a global completeness by complementing such characteristics previously reported for the Northern Hemisphere (NH). Using an objective algorithm that is based on temperature indices, relative measures of summer onset, withdrawal, and duration are determined at each land location over the period 1953–2012. Significant widespread summer-season lengthening, due to earlier onset and delayed withdrawal, has occurred across the SH, a longer period for extreme heat-wave events and wildfires to potentially occur. The asymmetric magnitude (onset vs withdrawal) in summer-season lengthening is slightly less over the SH than over the NH. Contributions of anthropogenic and natural factors to the observed trends in summer-season characteristics were investigated using phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel simulations integrated with observed external forcings [anthropogenic plus natural (ALL)], greenhouse gas forcing only (GHG), and natural forcing only [solar and volcanic activities (NAT)]. Overall, consistent with the NH, increased greenhouse gases were the main cause of observed changes in the SH, with negligible contribution from other external forcings. ALL and GHG simulations also reproduced a slight tendency for earlier summer onset to contribute more to summer lengthening. Proportions of observed regional trends in summer-season indices attributable to trends in long-term internal variability in the SH, namely, the interdecadal Pacific oscillation (IPO) and southern annular mode (SAM), suggests such variability can only explain up to ~12%, supporting the dominant role of greenhouse gas forcing.

Anthropogenic and Natural Contributions to the Lengthening of the Summer Season in the Northern Hemisphere

2018 ◽  
Vol 31 (17) ◽  
pp. 6803-6819 ◽  
Author(s):  
Bo-Joung Park ◽  
Yeon-Hee Kim ◽  
Seung-Ki Min ◽  
Eun-Pa Lim
Keyword(s):  
Northern Hemisphere ◽  
Coupled Model ◽  
Atlantic Multidecadal Oscillation ◽  
Summer Season ◽  
Season Length ◽  
The Past ◽  
External Forcings ◽  
Regional Trends ◽  
Intercomparison Project

Observed long-term variations in summer season timing and length in the Northern Hemisphere (NH) continents and their subregions were analyzed using temperature-based indices. The climatological mean showed coastal–inland contrast; summer starts and ends earlier inland than in coastal areas because of differences in heat capacity. Observations for the past 60 years (1953–2012) show lengthening of the summer season with earlier summer onset and delayed summer withdrawal across the NH. The summer onset advance contributed more to the observed increase in summer season length in many regions than the delay of summer withdrawal. To understand anthropogenic and natural contributions to the observed change, summer season trends from phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel simulations forced with the observed external forcings [anthropogenic plus natural forcing (ALL), natural forcing only (NAT), and greenhouse gas forcing only (GHG)] were analyzed. ALL and GHG simulations were found to reproduce the overall observed global and regional lengthening trends, but NAT had negligible trends, which implies that increased greenhouse gases were the main cause of the observed changes. However, ALL runs tend to underestimate the observed trend of summer onset and overestimate that of withdrawal, the causes of which remain to be determined. Possible contributions of multidecadal variabilities, such as Pacific decadal oscillation and Atlantic multidecadal oscillation, to the observed regional trends in summer season length were also assessed. The results suggest that multidecadal variability can explain a moderate portion (about ±10%) of the observed trends in summer season length, mainly over the high latitudes.

scholarly journals Causes of Robust Seasonal Land Precipitation Changes*

2013 ◽  
Vol 26 (17) ◽  
pp. 6679-6697 ◽  
Author(s):  
Debbie Polson ◽  
Gabriele C. Hegerl ◽  
Xuebin Zhang ◽  
Timothy J. Osborn
Keyword(s):  
Greenhouse Gas ◽  
Coupled Model ◽  
Anthropogenic Aerosol ◽  
Detection And Attribution ◽  
Aerosol Forcing ◽  
External Forcings ◽  
Spatial Coverage ◽  
Intercomparison Project ◽  
Precipitation Changes

Abstract Historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5) archive are used to calculate the zonal-mean change in seasonal land precipitation for the second half of the twentieth century in response to a range of external forcings, including anthropogenic and natural forcings combined (ALL), greenhouse gas forcing, anthropogenic aerosol forcing, anthropogenic forcings combined, and natural forcing. These simulated patterns of change are used as fingerprints in a detection and attribution study applied to four different gridded observational datasets of global land precipitation from 1951 to 2005. There are large differences in the spatial and temporal coverage in the observational datasets. Yet despite these differences, the zonal-mean patterns of change are mostly consistent except at latitudes where spatial coverage is limited. The results show some differences between datasets, but the influence of external forcings is robustly detected in March–May, December–February, and for annual changes for the three datasets more suitable for studying changes. For June–August and September–November, external forcing is only detected for the dataset that includes only long-term stations. Fingerprints for combinations of forcings that include the effect of greenhouse gases are similarly detectable to those for ALL forcings, suggesting that greenhouse gas influence drives the detectable features of the ALL forcing fingerprint. Fingerprints of only natural or only anthropogenic aerosol forcing are not detected. This, together with two-fingerprint results, suggests that at least some of the detected change in zonal land precipitation can be attributed to human influences.

Forced modulations of Sahel rainfall at decadal timescale over the 20th Century.

2021 ◽  
Author(s):  
Cassien Diabe Ndiaye ◽  
Juliette Mignot ◽  
Elsa Mohino
Keyword(s):  
North Atlantic Ocean ◽  
Coupled Model ◽  
Internal Variability ◽  
Boreal Summer ◽  
Semiarid Region ◽  
Rainfall Regime ◽  
Anthropogenic Aerosols ◽  
Coupled Models ◽  
External Forcings ◽  
Sahel Precipitation

<p>The semiarid region of the Sahel was marked during the 20<sup>th</sup> Century by significant modulations of its rainfall regime. Part of these modulations has been associated with the internal variability of the climate system, mediated by changes in oceanic sea surface temperature (SST). We show here that the external forcings, and in particular anthropogenic aerosols, might have played a role more important than previously thought in setting these variations. The study is based on the recent simulations performed for CMIP6 with the IPSL-CM6A-LR coupled model. As in most coupled models, the maximum precipitation is limited to the southern Sahel during boreal summer in the IPSL-CM6A-LR model. A novel definition of the Sahel precipitation region is proposed in order to take this bias into account. Our results show that external forcings induce decadal modulations of Sahel precipitation that correlate significantly at 0.6 with the observed precipitations and that the anthropogenic aerosols explain more than 70% of these modulations. These results confirm recent results of CMIP6 highlighting an important role of aerosol forcing for the decadal climate in and around the North Atlantic ocean.</p>

Greenhouse-gas contribution to Arctic sea-ice loss

2021 ◽  
Author(s):  
Yeon-Hee Kim ◽  
Seung-Ki Min
Keyword(s):  
Sea Ice ◽  
Greenhouse Gas ◽  
Coupled Model ◽  
Arctic Sea Ice ◽  
The Arctic ◽  
Detection Analysis ◽  
External Forcings ◽  
Arctic Sea ◽  
Intercomparison Project ◽  
Arctic Sea Ice Loss

<p>Arctic sea-ice area (ASIA) has been declining rapidly throughout the year during recent decades, but a formal quantification of greenhouse gas (GHG) contribution remains limited. This study conducts an attribution analysis of the observed ASIA changes from 1979 to 2017 by comparing three satellite observations with the Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model simulations using an optimal fingerprint method. The observed ASIA exhibits overall decreasing trends across all months with stronger trends in warm seasons. CMIP6 anthropogenic plus natural forcing (ALL) simulations and GHG-only forcing simulations successfully capture the observed temporal trend patterns. Results from detection analysis show that ALL signals are detected robustly for all calendar months for three observations. It is found that GHG signals are detectable in the observed ASIA decrease throughout the year, explaining most of the ASIA reduction, with a much weaker contribution by other external forcings. We additionally find that the Arctic Ocean will occur ice-free in September around the 2040s regardless of the emission scenario.</p>

CMIP6 multi-model projection of summer season expansion over the Northern Hemisphere land areas

2021 ◽  
Author(s):  
Bo-Joung Park ◽  
Seung-Ki Min
Keyword(s):  
Northern Hemisphere ◽  
High Latitude ◽  
Coupled Model ◽  
Arctic Region ◽  
Summer Season ◽  
Significant Advance ◽  
Multiple Model ◽  
North Asia ◽  
Near Term

<p>Due to the ongoing robust global warming, summer season is expected to get warmer in future over the Northern Hemisphere (NH) land areas. This study examined how the summer season defined by local temperature-based thresholds would change during the 21st century under Shared Socioeconomic Pathway scenarios (SSP1-2.6, SSP2-4.5 and SSP5-8.5) using Coupled Model Intercomparison Project phase 6 (CMIP6) multiple model simulations. The projection results relative to the current climatology (1995-2014) indicate the significant advance of summer onset and delay of withdrawal over all NH land areas except high latitude locations, with longer than 10 days of summer expansion even in the weakest scenario (SSP1-2.6) in the near-term future (2021-2040). The advance and delay of summer season timing become stronger in the mid-term (2041-2060) and long-term (2081-2020) future periods, ranging from about 10 days to a month depending on SSP scenarios. Largest summer expansion is observed in the middle latitudes, including Europe in high latitude, while the weakest changes are seen over North Asia. Canadian Arctic region is characterized by an asymmetric change with a small advance of summer onset but a relatively large delay in summer ending. CMIP6 models exhibit large inter-model differences, which increase from near-term to long-term future periods. Western North Asia region display larger inter-model difference in summer onset projections while Europe has the largest inter-model spread of summer withdrawal changes. Physical mechanisms associated with these regional and timing-dependent changes in the future summer season lengthening will be further examined.</p>

scholarly journals Forced and Unforced Decadal Behavior of the Interhemispheric SST Contrast during the Instrumental Period (1881–2012): Contextualizing the Late 1960s–Early 1970s Shift

2020 ◽  
Vol 33 (9) ◽  
pp. 3487-3509 ◽  
Author(s):  
Andrew R. Friedman ◽  
Gabriele C. Hegerl ◽  
Andrew P. Schurer ◽  
Shih-Yu Lee ◽  
Wenwen Kong ◽  
...  
Keyword(s):  
Coupled Model ◽  
Internal Variability ◽  
Forced Response ◽  
Meridional Overturning Circulation ◽  
Large Magnitude ◽  
Sst Cooling ◽  
Sst Variability ◽  
External Forcings ◽  
Sst Warming ◽  
Instrumental Period

AbstractThe sea surface temperature (SST) contrast between the Northern Hemisphere (NH) and Southern Hemisphere (SH) influences the location of the intertropical convergence zone (ITCZ) and the intensity of the monsoon systems. This study examines the contributions of external forcing and unforced internal variability to the interhemispheric SST contrast in HadSST3 and ERSSTv5 observations, and 10 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) from 1881 to 2012. Using multimodel mean fingerprints, a significant influence of anthropogenic, but not natural, forcing is detected in the interhemispheric SST contrast, with the observed response larger than that of the model mean in ERSSTv5. The forced response consists of asymmetric NH–SH SST cooling from the mid-twentieth century to around 1980, followed by opposite NH–SH SST warming. The remaining best-estimate residual or unforced component is marked by NH–SH SST maxima in the 1930s and mid-1960s, and a rapid NH–SH SST decrease around 1970. Examination of decadal shifts in the observed interhemispheric SST contrast highlights the shift around 1970 as the most prominent from 1881 to 2012. Both NH and SH SST variability contributed to the shift, which appears not to be attributable to external forcings. Most models examined fail to capture such large-magnitude shifts in their control simulations, although some models with high interhemispheric SST variability are able to produce them. Large-magnitude shifts produced by the control simulations feature disparate spatial SST patterns, some of which are consistent with changes typically associated with the Atlantic meridional overturning circulation (AMOC).

Internal variability of surface solar radiation and associated PV production

2020 ◽  
Author(s):  
Doris Folini
Keyword(s):  
Solar Radiation ◽  
Coupled Model ◽  
Internal Variability ◽  
Industrial Control ◽  
Surface Solar Radiation ◽  
Decadal Scale ◽  
Intercomparison Project ◽  
Analytical Relations ◽  
Average Uncertainty

<p>Results on the statistical properties of internal variability of annual mean surface solar radiation (SSR) and associated decadal scale trends are presented, following in part Folini et al. 2017 (doi:10.1002/2016JD025869). Estimates are based on 43 pre-industrial control (piControl) experiments of the Coupled Model Intercomparison Project Phase 5 (CMIP5). Trends are shown to depend strongly on geographical region and on whether they are quantified in absolute units or relative to the long term mean SSR. Providing one map for absolute and one map for relative trends is sufficient, as approximate analytical relations are shown to hold between trends of different length and likelihood and the standard deviation of the underlying SSR time series. Comparison with present-day observations and inter-model spread suggest an average uncertainty of these estimates of about 30%.  Intermodel spread suggests that regional uncertainties can be up to about three times larger or smaller. Using the model by Crook et al. 2011 (doi:10.1039/C1EE01495A) to translate SSR into PV production, associated internal variability of photo voltaic (PV) energy production is inferred. Results suggest that it is plausible for PV production to change by several per cent over a decade just because of internal variability.</p>

scholarly journals An Assessment of Future Southern Hemisphere Blocking Using CMIP5 Projections from Four GCMs

2016 ◽  
Vol 29 (21) ◽  
pp. 7599-7611 ◽  
Author(s):  
Simon Parsons ◽  
James A. Renwick ◽  
Adrian J. McDonald
Keyword(s):  
Southern Hemisphere ◽  
General Circulation ◽  
Coupled Model ◽  
General Circulation Models ◽  
Southern Annular Mode ◽  
Model Intercomparison ◽  
Future Projections ◽  
South Pacific Ocean ◽  
Blocking Activity ◽  
Intercomparison Project

AbstractThis study is concerned with blocking events (BEs) in the Southern Hemisphere (SH), their past variability, and future projections. ERA-Interim (ERA-I) is used to compare the historical output from four general circulation models (GCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5); the output of the representative concentration pathway 4.5 and 8.5 (RCP4.5 and RCP8.5) projections are also examined. ERA-I shows that the higher latitudes of the South Pacific Ocean (SPO) are the main blocking region, with blocking occurring predominantly in winter. The CMIP5 historical simulations also agree well with ERA-I for annual and seasonal BE locations and frequencies. A reduction in BEs is observed in the SPO in the 2071–2100 period in the RCP4.5 projections, and this is more pronounced for the RCP8.5 projections and occurs predominantly during the spring and summer seasons. Preliminary investigations imply that the southern annular mode (SAM) is negatively correlated with blocking activity in the SPO in all seasons in the reanalysis. This negative correlation is also observed in the GCM historical output. However, in the RCP projections this correlation is reduced in three of the four models during summer, suggesting that SAM may be less influential in summertime blocking in the future.

Sixty Years of Widespread Warming in the Southern Middle and High Latitudes (1957–2016)

2019 ◽  
Vol 32 (20) ◽  
pp. 6875-6898 ◽  
Author(s):  
Megan E. Jones ◽  
David H. Bromwich ◽  
Julien P. Nicolas ◽  
Jorge Carrasco ◽  
Eva Plavcová ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Southern Hemisphere ◽  
Climate Models ◽  
Coupled Model ◽  
Southern Annular Mode ◽  
Cmip5 Models ◽  
High Latitudes ◽  
West Antarctica ◽  
Temporal Perspective ◽  
Temperature Trends

Abstract Temperature trends across Antarctica over the last few decades reveal strong and statistically significant warming in West Antarctica and the Antarctic Peninsula (AP) contrasting with no significant change overall in East Antarctica. However, recent studies have documented cooling in the AP since the late 1990s. This study aims to place temperature changes in the AP and West Antarctica into a larger spatial and temporal perspective by analyzing monthly station-based surface temperature observations since 1957 across the extratropical Southern Hemisphere, along with sea surface temperature (SST) data and mean sea level pressure reanalysis data. The results confirm statistically significant cooling in station observations and SST trends throughout the AP region since 1999. However, the full 60-yr period shows statistically significant, widespread warming across most of the Southern Hemisphere middle and high latitudes. Positive SST trends broadly reflect these warming trends, especially in the midlatitudes. After confirming the importance of the southern annular mode (SAM) on southern high-latitude climate variability, the influence is removed from the station temperature records, revealing statistically significant background warming across all of the extratropical Southern Hemisphere. Antarctic temperature trends in a suite of climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are then investigated. Consistent with previous work the CMIP5 models warm Antarctica at the background temperature rate that is 2 times faster than that observed. However, removing the SAM influence from both CMIP5 and observed temperatures results in Antarctic trends that differ only modestly, perhaps due to natural multidecadal variability remaining in the observations.

scholarly journals Physical processes of cooling and mega-drought during the 4.2 ka BP event: results from TraCE-21ka simulations

2019 ◽  
Vol 15 (1) ◽  
pp. 265-277 ◽  
Author(s):  
Mi Yan ◽  
Jian Liu
Keyword(s):  
North Atlantic ◽  
Wave Train ◽  
Internal Variability ◽  
Meridional Overturning Circulation ◽  
Physical Processes ◽  
Dynamic Background ◽  
External Forcings ◽  
Climate Simulations ◽  
Meridional Overturning

Abstract. It is widely believed that multi-decadal to centennial cooling and drought occurred from 4500 to 3900 BP, known as the 4.2 ka BP event that triggered the collapse of several cultures. However, whether this event was a global event or a regional event and what caused this event remains unclear. In this study, we investigated the spatiotemporal characteristics, the possible causes and the related physical processes of the event using a set of long-term climate simulations, including one all-forcing experiment and four single-forcing experiments. The results derived from the all-forcing experiment show that this event occurs over most parts of the Northern Hemisphere (NH), indicating that this event could have been a hemispheric event. The cooler NH and warmer Southern Hemisphere (SH) illustrate that this event could be related to the slowdown of the Atlantic Meridional Overturning Circulation (AMOC). The comparison between the all-forcing experiment and the single-forcing experiments indicates that this event might have been caused by internal variability, while external forcings such as orbital and greenhouse gases might have modulation effects. A positive North Atlantic Oscillation (NAO)-like pattern in the atmosphere (low troposphere) triggered a negative Atlantic Multi-decadal Oscillation (AMO)-like pattern in the ocean, which then triggered a circum-global teleconnection (CGT)-like wave train pattern in the atmosphere (high troposphere). The positive NAO-like pattern and the CGT-like pattern are the direct physical processes that led to the NH cooling and mega-drought. The AMO-like pattern played a “bridge” role in maintaining this barotropic structure in the atmosphere at a multi-decadal–centennial timescale. Our work provides a global image and dynamic background to help better understand the 4.2 ka BP event.

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