scholarly journals Revisiting the Relationship among Metrics of Tropical Expansion

2018 ◽  
Vol 31 (18) ◽  
pp. 7565-7581 ◽  
Author(s):  
D. W. Waugh ◽  
K. M. Grise ◽  
W. J. M. Seviour ◽  
S. M. Davis ◽  
N. Davis ◽  
...  
Keyword(s):  
Climate Models ◽  
Longwave Radiation ◽  
Systematic Investigation ◽  
Hadley Cell ◽  
Subtropical High ◽  
Coupled Climate Models ◽  
Dry Zone ◽  
The Tropics ◽  
The Relationship

There is mounting evidence that the width of the tropics has increased over the last few decades, but there are large differences in reported expansion rates. This is, likely, in part due to the wide variety of metrics that have been used to define the tropical width. Here we perform a systematic investigation into the relationship among nine metrics of the zonal-mean tropical width using preindustrial control and abrupt quadrupling of CO2 simulations from a suite of coupled climate models. It is shown that the latitudes of the edge of the Hadley cell, the midlatitude eddy-driven jet, the edge of the subtropical dry zones, and the Southern Hemisphere subtropical high covary interannually and exhibit similar long-term responses to a quadrupling of CO2. However, metrics based on the outgoing longwave radiation, the position of the subtropical jet, the break in the tropopause, and the Northern Hemisphere subtropical high have very weak covariations with the above metrics and/or respond differently to increases in CO2 and thus are not good indicators of the expansion of the Hadley cell or subtropical dry zone. The differing variability and responses to increases in CO2 among metrics highlights that care is needed when choosing metrics for studies of the width of the tropics and that it is important to make sure the metric used is appropriate for the specific phenomena and impacts being examined.

scholarly journals An Evaluation of Northern Australian Wet Season Rainfall Bursts in CMIP5 Models

2018 ◽  
Vol 31 (19) ◽  
pp. 7789-7802 ◽  
Author(s):  
Sugata Narsey ◽  
Michael J. Reeder ◽  
Christian Jakob ◽  
Duncan Ackerley
Keyword(s):  
Outgoing Longwave Radiation ◽  
Climate Models ◽  
Longwave Radiation ◽  
Cmip5 Models ◽  
Madden Julian Oscillation ◽  
Wet Season ◽  
The North ◽  
Coupled Climate Models ◽  
The Tropics ◽  
The Relationship

The simulation of northern Australian wet season rainfall bursts by coupled climate models is evaluated. Individual models produce vastly different amounts of precipitation over the north of Australia during the wet season, and this is found to be related to the number of bursts they produce. The seasonal cycle of bursts is found to be poor in most of the models evaluated. It is known that northern Australian wet season bursts are often associated with midlatitude Rossby wave packets and their surface signature as they are refracted toward the tropics. The relationship between midlatitude waves and the initiation of wet season bursts is simulated well by the models evaluated. Another well-documented influence on the initiation of northern Australian wet season bursts is the Madden–Julian oscillation (MJO). No model adequately simulated the tropical outgoing longwave radiation temporal–spatial patterns seen in the reanalysis-derived OLR. This result suggests that the connection between the MJO and the initiation of northern Australian wet season bursts in models is poor.

scholarly journals Global climatology and trends in convective environments from ERA5 and rawinsonde data

2021 ◽  
Vol 4 (1) ◽  
Author(s):  
Mateusz Taszarek ◽  
John T. Allen ◽  
Mattia Marchio ◽  
Harold E. Brooks
Keyword(s):  
Large Scale ◽  
Climate Models ◽  
Global Climate ◽  
Convective Available Potential Energy ◽  
Global Climate Models ◽  
Convective Precipitation ◽  
The Past ◽  
The Tropics ◽  
Rawinsonde Data

AbstractGlobally, thunderstorms are responsible for a significant fraction of rainfall, and in the mid-latitudes often produce extreme weather, including large hail, tornadoes and damaging winds. Despite this importance, how the global frequency of thunderstorms and their accompanying hazards has changed over the past 4 decades remains unclear. Large-scale diagnostics applied to global climate models have suggested that the frequency of thunderstorms and their intensity is likely to increase in the future. Here, we show that according to ERA5 convective available potential energy (CAPE) and convective precipitation (CP) have decreased over the tropics and subtropics with simultaneous increases in 0–6 km wind shear (BS06). Conversely, rawinsonde observations paint a different picture across the mid-latitudes with increasing CAPE and significant decreases to BS06. Differing trends and disagreement between ERA5 and rawinsondes observed over some regions suggest that results should be interpreted with caution, especially for CAPE and CP across tropics where uncertainty is the highest and reliable long-term rawinsonde observations are missing.

scholarly journals A Multidiagnostic Intercomparison of Tropical-Width Time Series Using Reanalyses and Satellite Observations

2012 ◽  
Vol 25 (4) ◽  
pp. 1061-1078 ◽  
Author(s):  
Sean M. Davis ◽  
Karen H. Rosenlof
Keyword(s):  
Longwave Radiation ◽  
Hadley Cell ◽  
Radiosonde Data ◽  
Full Time ◽  
Robust Detection ◽  
Time Period ◽  
Wide Range ◽  
The Mean ◽  
Yield Trends ◽  
The Tropics

Abstract Poleward migration of the latitudinal edge of the tropics of 0.25°–3.0° decade−1 has been reported in several recent studies based on satellite and radiosonde data and reanalysis output covering the past ~30 yr. The goal of this paper is to identify the extent to which this large range of trends can be explained by the use of different data sources, time periods, and edge definitions, as well as how the widening varies as a function of hemisphere and season. Toward this end, a suite of tropical edge latitude diagnostics based on tropopause height, winds, precipitation–evaporation, and outgoing longwave radiation (OLR) are analyzed using several reanalyses and satellite datasets. These diagnostics include both previously used definitions and new definitions designed for more robust detection. The wide range of widening trends is shown to be primarily due to the use of different datasets and edge definitions and only secondarily due to varying start–end dates. This study also shows that the large trends (>~1° decade−1) previously reported in tropopause and OLR diagnostics are due to the use of subjective definitions based on absolute thresholds. Statistically significant Hadley cell expansion based on the mean meridional streamfunction of 1.0°–1.5° decade−1 is found in three of four reanalyses that cover the full time period (1979–2009), whereas other diagnostics yield trends of −0.5°–0.8° decade−1 that are mostly insignificant. There are indications of hemispheric and seasonal differences in the trends, but the differences are not statistically significant.

scholarly journals Reassessing Sea Ice Drift and Its Relationship to Long‐Term Arctic Sea Ice Loss in Coupled Climate Models

2018 ◽  
Vol 123 (6) ◽  
pp. 4338-4359 ◽  
Author(s):  
Neil F. Tandon ◽  
Paul J. Kushner ◽  
David Docquier ◽  
Justin J. Wettstein ◽  
Camille Li
Keyword(s):  
Sea Ice ◽  
Climate Models ◽  
Arctic Sea Ice ◽  
Coupled Climate Models ◽  
Ice Drift ◽  
Arctic Sea ◽  
Sea Ice Drift ◽  
Arctic Sea Ice Loss

scholarly journals Mechanisms of Future Predicted Changes in the Zonal Mean Mid-Latitude Circulation

2019 ◽  
Vol 5 (4) ◽  
pp. 345-357 ◽  
Author(s):  
Tiffany A. Shaw
Keyword(s):  
Heat Release ◽  
Latent Heat ◽  
Climate Models ◽  
Static Stability ◽  
Hadley Cell ◽  
Specific Humidity ◽  
Latent Heat Release ◽  
Starting Point ◽  
Intensity Changes ◽  
The Tropics

AbstractState-of-the-art climate models predict the zonal mean mid-latitude circulation will undergo a poleward shift and seasonally and hemispherically dependent intensity changes in the future. Here I review the mechanisms put forward to explain the zonal mean mid-latitude circulation response to increased carbon dioxide (CO2) concentration. The mechanisms are grouped according to their thermodynamic starting point, which are thought to arise from processes independent of the zonal mean mid-latitude circulation response. There are 24 mechanisms and 8 thermodynamic starting points: (i) increased latent heat release aloft in the tropics, (ii) increased dry static stability and tropopause height outside the tropics, (iii) radiative cooling of the stratosphere, (iv) Hadley cell expansion, (v) increased specific humidity following the Clausius-Clapeyron relation, (vi) cloud radiative effect changes, (vii) turbulent surface heat flux changes, and (viii) decreased surface meridional temperature gradient. I argue progress can be made by testing the thermodynamic starting points. I review recent tests of the increased latent heat release aloft in the tropics starting point, i.e., prescribing diabatic perturbations, quantifying the transient response to an abrupt CO2 increase and imposing latitudinally dependent CO2 concentration. Finally, I provide a future outlook for improving our understanding of predicted changes in the zonal mean mid-latitude circulation.

scholarly journals Development of a climate record of tropospheric and stratospheric ozone from satellite remote sensing: evidence of an early recovery of global stratospheric ozone

2012 ◽  
Vol 12 (1) ◽  
pp. 3169-3211
Author(s):  
J. R. Ziemke ◽  
S. Chandra
Keyword(s):  
Climate Models ◽  
Stratospheric Ozone ◽  
Steady Increase ◽  
Ozone Monitoring Instrument ◽  
Early Recovery ◽  
Quasi Biennial Oscillation ◽  
Time Record ◽  
The Tropics ◽  
Biennial Oscillation

Abstract. Ozone data beginning October 2004 from the Aura Ozone Monitoring Instrument (OMI) and Aura Microwave Limb Sounder (MLS) are used to evaluate the accuracy of the Cloud Slicing technique in effort to develop long data records of tropospheric and stratospheric ozone and for studying their long-term changes. Using this technique, we have produced a 32-yr (1979–2010) long record of tropospheric and stratospheric ozone from the combined Total Ozone Mapping Spectrometer (TOMS) and OMI. The analyses of these time series suggest that the quasi-biennial oscillation (QBO) is the dominant source of inter-annual variability of stratospheric ozone and is clearest in the Southern Hemisphere during the Aura time record with related inter-annual changes of 30–40 Dobson Units. Tropospheric ozone also indicates a QBO signal in the tropics with peak-to-peak changes varying from 2 to 7 DU. The stratospheric ozone record indicates a steady increase since the mid-1990's with current ozone levels comparable to the mid-1980's. This is earlier than predicted by many of the current climate models which suggest recovery to the mid-1980's levels by year 2020 or later.

Variability of Monthly Diurnal Cycle Composites of TOA Radiative Fluxes in the Tropics

2013 ◽  
Vol 71 (2) ◽  
pp. 754-766 ◽  
Author(s):  
Patrick C. Taylor
Keyword(s):  
Diurnal Cycle ◽  
Climate Models ◽  
Longwave Radiation ◽  
Atmospheric Variability ◽  
Radiative Fluxes ◽  
System Variability ◽  
Synchronous Orbit ◽  
Cloud Evolution ◽  
The Tropics ◽  
The Impact

Abstract Earth system variability is generated by a number of different sources and time scales. Understanding sources of atmospheric variability is critical to reducing the uncertainty in climate models and to understanding the impacts of sampling on observational datasets. The diurnal cycle is a fundamental variability evident in many geophysical variables—including top-of-the-atmosphere (TOA) radiative fluxes. This study considers aspects of the TOA flux diurnal cycle not previously analyzed: namely, deseasonalized variations in the monthly diurnal cycle composites, termed monthly diurnal cycle variability. Significant variability in the monthly diurnal cycle composites is found in both outgoing longwave radiation (OLR) and reflected shortwave (RSW). OLR and RSW monthly diurnal cycle variability exhibits a regional structure that follows traditional, climatological diurnal cycle categorization by prevailing cloud and surface types. The results attribute monthly TOA flux diurnal cycle variability to variations in the diurnal cloud evolution, which is sensitive to monthly atmospheric dynamic- and thermodynamic-state anomalies. The results also suggest that monthly diurnal cycle variability can amplify or buffer monthly TOA flux anomalies, depending on the region. Considering the impact of monthly diurnal cycle variability on monthly TOA flux anomalies, the results suggest that monthly TOA flux diurnal cycle variability must be considered when constructing a TOA flux dataset from sun-synchronous orbit. The magnitude of monthly diurnal composite variability in OLR and RSW is regionally dependent—1–7 W m−2 and 10%–80% relative to interannual TOA flux variability. The largest (4–7 W m−2; 40%–80%) and smallest (1–3 W m−2; 10%–30%) TOA flux uncertainties occur in convective and nonconvective regions, respectively, over both land and ocean.

scholarly journals Fast and Slow Components of the Extratropical Atmospheric Circulation Response to CO2 Forcing

2018 ◽  
Vol 31 (3) ◽  
pp. 1091-1105 ◽  
Author(s):  
Paulo Ceppi ◽  
Giuseppe Zappa ◽  
Theodore G. Shepherd ◽  
Jonathan M. Gregory
Keyword(s):  
Atmospheric Circulation ◽  
Radiative Forcing ◽  
Climate Models ◽  
Fast Response ◽  
Long Term Evolution ◽  
Global Climate Models ◽  
Hadley Cell ◽  
Term Evolution ◽  
Co2 Forcing

Abstract Poleward shifts of the extratropical atmospheric circulation are a common response to CO2 forcing in global climate models (GCMs), but little is known about the time dependence of this response. Here it is shown that in coupled climate models, the long-term evolution of sea surface temperatures (SSTs) induces two distinct time scales of circulation response to steplike CO2 forcing. In most GCMs from phase 5 of the Coupled Model Intercomparison Project as well as in the multimodel mean, all of the poleward shift of the midlatitude jets and Hadley cell edge occurs in a fast response within 5–10 years of the forcing, during which less than half of the expected equilibrium warming is realized. Compared with this fast response, the slow response over subsequent decades to centuries features stronger polar amplification (especially in the Antarctic), enhanced warming in the Southern Ocean, an El Niño–like pattern of tropical Pacific warming, and weaker land–sea contrast. Atmosphere-only GCM experiments demonstrate that the SST evolution drives the difference between the fast and slow circulation responses, although the direct radiative effect of CO2 also contributes to the fast response. It is further shown that the fast and slow responses determine the long-term evolution of the circulation response to warming in the representative concentration pathway 4.5 (RCP4.5) scenario. The results imply that shifts in midlatitude circulation generally scale with the radiative forcing, rather than with global-mean temperature change. A corollary is that time slices taken from a transient simulation at a given level of warming will considerably overestimate the extratropical circulation response in a stabilized climate.

scholarly journals Eocene greenhouse climate revealed by coupled clumped isotope-Mg/Ca thermometry

2018 ◽  
Vol 115 (6) ◽  
pp. 1174-1179 ◽  
Author(s):  
David Evans ◽  
Navjit Sagoo ◽  
Willem Renema ◽  
Laura J. Cotton ◽  
Wolfgang Müller ◽  
...  
Keyword(s):  
Climate Models ◽  
Planktonic Foraminifera ◽  
Deep Ocean ◽  
Seawater Chemistry ◽  
Greenhouse Climate ◽  
Polar Amplification ◽  
Clumped Isotope ◽  
The Tropics ◽  
The Relationship ◽  
Globally Distributed

Past greenhouse periods with elevated atmospheric CO2 were characterized by globally warmer sea-surface temperatures (SST). However, the extent to which the high latitudes warmed to a greater degree than the tropics (polar amplification) remains poorly constrained, in particular because there are only a few temperature reconstructions from the tropics. Consequently, the relationship between increased CO2, the degree of tropical warming, and the resulting latitudinal SST gradient is not well known. Here, we present coupled clumped isotope (Δ47)-Mg/Ca measurements of foraminifera from a set of globally distributed sites in the tropics and midlatitudes. Δ47 is insensitive to seawater chemistry and therefore provides a robust constraint on tropical SST. Crucially, coupling these data with Mg/Ca measurements allows the precise reconstruction of Mg/Casw throughout the Eocene, enabling the reinterpretation of all planktonic foraminifera Mg/Ca data. The combined dataset constrains the range in Eocene tropical SST to 30–36 °C (from sites in all basins). We compare these accurate tropical SST to deep-ocean temperatures, serving as a minimum constraint on high-latitude SST. This results in a robust conservative reconstruction of the early Eocene latitudinal gradient, which was reduced by at least 32 ± 10% compared with present day, demonstrating greater polar amplification than captured by most climate models.

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