scholarly journals Recent Tropical Expansion: Natural Variability or Forced Response?

2019 ◽  
Vol 32 (5) ◽  
pp. 1551-1571 ◽  
Author(s):  
Kevin M. Grise ◽  
Sean M. Davis ◽  
Isla R. Simpson ◽  
Darryn W. Waugh ◽  
Qiang Fu ◽  
...  
Keyword(s):  
Climate Models ◽  
Global Climate ◽  
Stratospheric Ozone ◽  
Hadley Circulation ◽  
Natural Variability ◽  
Forced Response ◽  
Global Climate Models ◽  
First Century ◽  
Atmospheric Variability ◽  
Twenty First Century

AbstractPrevious studies have documented a poleward shift in the subsiding branches of Earth’s Hadley circulation since 1979 but have disagreed on the causes of these observed changes and the ability of global climate models to capture them. This synthesis paper reexamines a number of contradictory claims in the past literature and finds that the tropical expansion indicated by modern reanalyses is within the bounds of models’ historical simulations for the period 1979–2005. Earlier conclusions that models were underestimating the observed trends relied on defining the Hadley circulation using the mass streamfunction from older reanalyses. The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but models suggest that the factors driving the expansion differ between the hemispheres. In the SH, increasing greenhouse gases (GHGs) and stratospheric ozone depletion contributed to tropical expansion over the late twentieth century, and if GHGs continue increasing, the SH tropical edge is projected to shift further poleward over the twenty-first century, even as stratospheric ozone concentrations recover. In the NH, the contribution of GHGs to tropical expansion is much smaller and will remain difficult to detect in a background of large natural variability, even by the end of the twenty-first century. To explain similar recent tropical expansion rates in the two hemispheres, natural variability must be taken into account. Recent coupled atmosphere–ocean variability, including the Pacific decadal oscillation, has contributed to tropical expansion. However, in models forced with observed sea surface temperatures, tropical expansion rates still vary widely because of internal atmospheric variability.

Drivers of Twenty-First Century U.S. Winter Precipitation Trends in CMIP6 Models: A Storyline-Based Approach

2021 ◽  
pp. 1-48
Author(s):  
Daniel F. Schmidt ◽  
Kevin M. Grise
Keyword(s):  
Climate Models ◽  
Southern Oscillation ◽  
Global Climate ◽  
The United States ◽  
Winter Precipitation ◽  
Global Climate Models ◽  
First Century ◽  
Precipitation Trend ◽  
Twenty First Century ◽  
Precipitation Trends

AbstractClimate change during the twenty-first century has the potential to substantially alter geographic patterns of precipitation. However, regional precipitation changes can be very difficult to project, and in some regions, global climate models do not even agree on the sign of the precipitation trend. Since some of this uncertainty is due to internal variability rather than model bias, models cannot be used to narrow the possibilities to a single outcome, but they can usefully quantify the range of plausible outcomes and identify the combination of dynamical drivers that would be likely to produce each.This study uses a storylines approach—a type of regression-based analysis—to identify some of the key dynamical drivers that explain the variance in 21st century U.S. winter precipitation trends across CMIP6 models under the SSP3-7.0 emissions scenario. This analysis shows that the spread in precipitation trends is not primarily driven by differences in modeled climate sensitivity. Key drivers include global-mean surface temperature, but also tropical upper-troposphere temperature, the El Niño-Southern Oscillation (ENSO), the Pacific-North America (PNA) pattern, and the East Pacific (EP) dipole (a dipole pattern in geopotential heights over North America’s Pacific coast). Combinations of these drivers can reinforce or cancel to produce various high- or low-impact scenarios for winter precipitation trends in various regions of the United States. For example, the most extreme winter precipitation trends in the southwestern U.S. result from opposite trends in ENSO and EP, whereas the wettest winter precipitation trends in the midwestern U.S. result from a combination of strong global warming and a negative PNA trend.

An updated view of Hadley cell expansion from CMIP6 models

2020 ◽  
Author(s):  
Kevin Grise ◽  
Sean Davis
Keyword(s):  
Greenhouse Gases ◽  
Climate Models ◽  
Global Climate ◽  
Stratospheric Ozone ◽  
Hadley Circulation ◽  
Global Climate Models ◽  
Hadley Cell ◽  
Cmip5 Models ◽  
Anthropogenic Aerosols ◽  
Poleward Shift

<p><strong>            </strong>One of the most robust aspects of the atmospheric circulation response to increasing greenhouse gases is the poleward shift in the subsiding branches of the Hadley circulation, potentially pushing subtropical dry zones poleward toward midlatitudes.  Numerous lines of observational evidence suggest that this tropical expansion may have already begun.  Yet, the degree to which the observed tropical widening is anthropogenically forced has remained a topic of great debate, as previous studies have attributed the recent circulation trends to some combination of increasing greenhouse gases, stratospheric ozone depletion, anthropogenic aerosols, and natural variability.  During the past few years, two international working groups have synthesized recent findings about the magnitude and causes of the observed tropical widening, primarily using output from global climate models that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5).  In this presentation, we update those findings using the recently released CMIP6 global climate models.</p><p>            Over recent decades, the poleward expansion of the Hadley circulation estimated from modern reanalyses is relatively modest (< 0.5 degrees latitude per decade).  The reanalysis trends have similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but both CMIP5 and CMIP6 models suggest that increasing greenhouse gases should drive 2–3 times larger circulation shifts in the SH.  The reanalysis trends fall within the bounds of the models’ simulations of the late 20<sup>th</sup> century and early 21<sup>st</sup> century, although prescribing observed coupled atmosphere-ocean variability allows the models to better capture the observed trends in the NH.  We find two notable differences between CMIP5 and CMIP6 models.  First, both CMIP5 and CMIP6 models contract the NH summertime Hadley circulation equatorward (particularly over the Pacific sector) in response to increasing greenhouse gases, but this contraction is larger in CMIP6 models due to their higher average climate sensitivity.  Second, in recent decades, the poleward shift of the NH annual-mean Hadley cell edge is slightly larger in the historical runs of CMIP6 models.  Increasing greenhouse gases drive similar trends in CMIP5 and CMIP6 models, so CMIP6 models imply a stronger role for other forcings (such as aerosols) in recent circulation trends than CMIP5 models.</p>

scholarly journals Simulations of Arctic Temperature and Pressure by Global Coupled Models

2007 ◽  
Vol 20 (4) ◽  
pp. 609-632 ◽  
Author(s):  
William L. Chapman ◽  
John E. Walsh
Keyword(s):  
Sea Level ◽  
Climate Models ◽  
Barents Sea ◽  
Global Climate ◽  
Global Climate Models ◽  
First Century ◽  
The Arctic ◽  
Sea Level Pressure ◽  
Level Pressure ◽  
Twenty First Century

Abstract Simulations of Arctic surface air temperature and sea level pressure by 14 global climate models used in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change are synthesized in an analysis of biases and trends. Simulated composite GCM surface air temperatures for 1981–2000 are generally 1°–2°C colder than corresponding observations with the exception of a cold bias maximum of 6°–8°C in the Barents Sea. The Barents Sea bias, most prominent in winter and spring, occurs in 12 of the 14 GCMs and corresponds to a region of oversimulated sea ice. All models project a twenty-first-century warming that is largest in the autumn and winter, although the rates of the projected warming vary considerably among the models. The across-model and across-scenario uncertainties in the projected temperatures are comparable through the first half of the twenty-first century, but increases in variability associated with the choice of scenario begin to outpace increases in across-model variability by about the year 2070. By the end of the twenty-first century, the cross-scenario variability is about 50% greater than the across-model variability. The biases of sea level pressure are smaller than in the previous generation of global climate models, although the models still show a positive bias of sea level pressure in the Eurasian sector of the Arctic Ocean, surrounded by an area of negative pressure biases. This bias is consistent with an inability of the North Atlantic storm track to penetrate the Eurasian portion of the Arctic Ocean. The changes of sea level pressure projected for the twenty-first century are negative over essentially the entire Arctic. The most significant decreases of pressure are projected for the Bering Strait region, primarily in autumn and winter.

scholarly journals Tropical Widening: From Global Variations to Regional Impacts

2020 ◽  
Vol 101 (6) ◽  
pp. E897-E904 ◽  
Author(s):  
Paul W. Staten ◽  
Kevin M. Grise ◽  
Sean M. Davis ◽  
Kristopher B. Karnauskas ◽  
Darryn W. Waugh ◽  
...  
Keyword(s):  
Climate Models ◽  
Global Climate ◽  
Stratospheric Ozone ◽  
Hadley Circulation ◽  
Global Climate Models ◽  
Regional Patterns ◽  
Regional Impacts ◽  
Early Twenty ◽  
Working Groups ◽  
Late Twentieth

Abstract Over the past 15 years, numerous studies have suggested that the sinking branches of Earth’s Hadley circulation and the associated subtropical dry zones have shifted poleward over the late twentieth century and early twenty-first century. Early estimates of this tropical widening from satellite observations and reanalyses varied from 0.25° to 3° latitude per decade, while estimates from global climate models show widening at the lower end of the observed range. In 2016, two working groups, the U.S. Climate Variability and Predictability (CLIVAR) working group on the Changing Width of the Tropical Belt and the International Space Science Institute (ISSI) Tropical Width Diagnostics Intercomparison Project, were formed to synthesize current understanding of the magnitude, causes, and impacts of the recent tropical widening evident in observations. These working groups concluded that the large rates of observed tropical widening noted by earlier studies resulted from their use of metrics that poorly capture changes in the Hadley circulation, or from the use of reanalyses that contained spurious trends. Accounting for these issues reduces the range of observed expansion rates to 0.25°–0.5° latitude decade‒1—within the range from model simulations. Models indicate that most of the recent Northern Hemisphere tropical widening is consistent with natural variability, whereas increasing greenhouse gases and decreasing stratospheric ozone likely played an important role in Southern Hemisphere widening. Whatever the cause or rate of expansion, understanding the regional impacts of tropical widening requires additional work, as different forcings can produce different regional patterns of widening.

Future Changes in Northern Hemisphere Snowfall

2013 ◽  
Vol 26 (20) ◽  
pp. 7813-7828 ◽  
Author(s):  
John P. Krasting ◽  
Anthony J. Broccoli ◽  
Keith W. Dixon ◽  
John R. Lanzante
Keyword(s):  
Twentieth Century ◽  
Northern Hemisphere ◽  
Climate Models ◽  
Global Climate ◽  
Noise Analysis ◽  
Coupled Model ◽  
Surface Air Temperature ◽  
Global Climate Models ◽  
First Century ◽  
Twenty First Century

Abstract Using simulations performed with 18 coupled atmosphere–ocean global climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), projections of the Northern Hemisphere snowfall under the representative concentration pathway (RCP4.5) scenario are analyzed for the period 2006–2100. These models perform well in simulating twentieth-century snowfall, although there is a positive bias in many regions. Annual snowfall is projected to decrease across much of the Northern Hemisphere during the twenty-first century, with increases projected at higher latitudes. On a seasonal basis, the transition zone between negative and positive snowfall trends corresponds approximately to the −10°C isotherm of the late twentieth-century mean surface air temperature, such that positive trends prevail in winter over large regions of Eurasia and North America. Redistributions of snowfall throughout the entire snow season are projected to occur—even in locations where there is little change in annual snowfall. Changes in the fraction of precipitation falling as snow contribute to decreases in snowfall across most Northern Hemisphere regions, while changes in total precipitation typically contribute to increases in snowfall. A signal-to-noise analysis reveals that the projected changes in snowfall, based on the RCP4.5 scenario, are likely to become apparent during the twenty-first century for most locations in the Northern Hemisphere. The snowfall signal emerges more slowly than the temperature signal, suggesting that changes in snowfall are not likely to be early indicators of regional climate change.

Regional and global projections of twenty-first century glacier mass changes in response to climate scenarios from global climate models

2013 ◽  
Vol 42 (1-2) ◽  
pp. 37-58 ◽  
Author(s):  
Valentina Radić ◽  
Andrew Bliss ◽  
A. Cody Beedlow ◽  
Regine Hock ◽  
Evan Miles ◽  
...  
Keyword(s):  
Climate Models ◽  
Global Climate ◽  
Global Climate Models ◽  
First Century ◽  
Climate Scenarios ◽  
Twenty First Century

scholarly journals What Caused the Observed Twentieth-Century Weakening of the Walker Circulation?

2011 ◽  
Vol 24 (24) ◽  
pp. 6501-6514 ◽  
Author(s):  
Scott B. Power ◽  
Greg Kociuba
Keyword(s):  
Twentieth Century ◽  
Climate Models ◽  
Southern Oscillation ◽  
Walker Circulation ◽  
Natural Variability ◽  
First Century ◽  
External Forcing ◽  
Wide Range ◽  
Twenty First Century ◽  
The Walker Circulation

Abstract The Walker circulation (WC) is one of the world’s most prominent and important atmospheric systems. The WC weakened during the twentieth century, reaching record low levels in recent decades. This weakening is thought to be partly due to global warming and partly due to internally generated natural variability. There is, however, no consensus in the literature on the relative contribution of external forcing and natural variability to the observed weakening of the WC. This paper examines changes in the strength of the WC using an index called BoxΔP, which is equal to the difference in mean sea level pressure across the equatorial Pacific. Change in both the observations and in World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) climate models are examined. The annual average BoxΔP declines in the observations and in 15 out of 23 models during the twentieth century (results that are significant at or above the 95% level), consistent with earlier work. However, the magnitude of the multimodel ensemble mean (MMEM) 1901–99 trend (−0.10 Pa yr−1) is much smaller than the magnitude of the observed trend (−0.52 Pa yr−1). While a wide range of trends is evident in the models with approximately 90% of the model trends in the range (−0.25 to +0.1 Pa yr−1), even this range is too narrow to encompass the magnitude of the observed trend. Twenty-first-century changes in BoxΔP under the Special Report on Emissions Scenarios (SRES) A1B and A2 are also examined. Negative trends (i.e., weaker WCs) are evident in all seasons. However, the MMEM trends for the A1B and A2 scenarios are smaller in magnitude than the magnitude of the observed trend. Given that external forcing linked to greenhouse gases is much larger in the twenty-first-century scenarios than twentieth-century forcing, this, together with the twentieth-century results mentioned above, would seem to suggest that external forcing has not been the primary driver of the observed weakening of the WC. However, 9 of the 23 models are unable to account for the observed change unless the internally generated component of the trend is very large. But indicators of observed variability linked to El Niño–Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation have modest trends, suggesting that internally variability has been modest. Furthermore, many of the nine “inconsistent” models tend to have poorer simulations of climatic features linked to ENSO. In addition, the externally forced component of the trend tends to be larger in magnitude and more closely matches the observed trend in the models that are better able to reproduce ENSO-related variability. The “best” four models, for example, have a MMEM of −0.2 Pa yr−1 (i.e., approximately 40% of the observed change), suggesting a greater role for external forcing in driving the observed trend. These and other considerations outlined below lead the authors to conclude that (i) both external forcing and internally generated variability contributed to the observed weakening of the WC over the twentieth century and (ii) external forcing accounts for approximately 30%–70% of the observed weakening with internally generated climate variability making up the rest.

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