The Urban Heat Island (UHI), the tendency for urban areas to be hotter than rural regions, represents a significant health concern in summer as urban populations are exposed to elevated temperatures. ...A number of studies suggest that the UHI increases during warmer conditions, however there has been no investigation of this for a large ensemble of cities. Here we compare urban and rural temperatures in 54 US cities for 2000-2015 and show that the intensity of the Urban Heat Island, measured here as the differences in daily-minimum or daily-maximum temperatures between urban and rural stations or ΔT, in fact tends to decrease with increasing temperature in most cities (38/54). This holds when investigating daily variability, heat extremes, and variability across climate zones and is primarily driven by changes in rural areas. We relate this change to large-scale or synoptic weather conditions, and find that the lowest ΔT nights occur during moist weather conditions. We also find that warming cities have not experienced an increasing Urban Heat Island effect.
Observational evidence indicates that the southern edge of the Hadley cell (HC) has shifted southward during austral summer in recent decades. However, there is no consensus on the cause of this ...shift, with several studies reaching opposite conclusions as to the relative role of changes in sea surface temperatures (SSTs) and stratospheric ozone depletion in causing this shift. Here, the authors perform a meta-analysis of the extant literature on this subject and quantitatively compare the results of all published studies that have used single-forcing model integrations to isolate the role of different factors on the HC expansion during austral summer. It is shown that the weight of the evidence clearly points to stratospheric ozone depletion as the dominant driver of the tropical summertime expansion over the period in which an ozone hole was formed (1979 to late 1990s), although SST trends have contributed to trends since then. Studies that have claimed SSTs as the major driver of tropical expansion since 1979 have used prescribed ozone fields that underrepresent the observed Antarctic ozone depletion.
Surface westerly winds in the Southern Hemisphere have intensified over the past few decades, primarily in response to the formation of the Antarctic ozone hole, and there is intense debate on the ...impact of this on the ocean's circulation and uptake and redistribution of atmospheric gases. We used measurements of chlorofluorocarbon-12 (CFC-12) made in the southern oceans in the early 1990s and mid- to late 2000s to examine changes in ocean ventilation. Our analysis of the CFC-12 data reveals a decrease in the age of subtropical subantarctic mode waters and an increase in the age of circumpolar deep waters, suggesting that the formation of the Antarctic ozone hole has caused large-scale coherent changes in the ventilation of the southern oceans.
Abstract
Zonal jets are common in planetary atmospheres. Their character, structure, and seasonal variability depend on the planetary parameters. During solstice on Earth and Mars, there is a strong ...westerly jet in the winter hemisphere and weak, low-level westerlies in the ascending regions of the Hadley cell in the summer hemisphere. This summer jet has been less explored in a broad planetary context, both due to the dominance of the winter jet and since the balances controlling it are more complex, and understanding them requires exploring a broader parameter regime. To better understand the jet characteristics on terrestrial planets and the transition between winter- and summer-dominated jet regimes, we explore the jet’s dependence on rotation rate and obliquity. Across a significant portion of the parameter space, the dominant jet is in the winter hemisphere, and the summer jet is weaker and restricted to the boundary layer. However, we show that for slow rotation rates and high obliquities, the strongest jet is in the summer rather than the winter hemisphere. Analysis of the summer jet’s momentum balance reveals that the balance is not simply cyclostrophic and that both boundary layer drag and vertical advection are essential. At high obliquities and slow rotation rates, the cross-equatorial winter cell is wide and strong. The returning poleward flow in the summer hemisphere is balanced by low-level westerlies through an Ekman balance and momentum is advected upward close to the ascending branch, resulting in a midtroposphere summer jet.
Stratospheric Ozone Depletion Polvani, Lorenzo M.; Waugh, Darryn W.; Correa, Gustavo J. P. ...
Journal of climate,
02/2011, Volume:
24, Issue:
3
Journal Article
Peer reviewed
Open access
The importance of stratospheric ozone depletion on the atmospheric circulation of the troposphere is studied with an atmospheric general circulation model, the Community Atmospheric Model, version 3 ...(CAM3), for the second half of the twentieth century. In particular, the relative importance of ozone depletion is contrasted with that of increased greenhouse gases and accompanying sea surface temperature changes. By specifying ozone and greenhouse gas forcings independently, and performing long, time-slice integrations, it is shown that the impacts of ozone depletion are roughly 2–3 times larger than those associated with increased greenhouse gases, for the Southern Hemisphere tropospheric summer circulation. The formation of the ozone hole is shown to affect not only the polar tropopause and the latitudinal position of the midlatitude jet; it extends to the entire hemisphere, resulting in a broadening of the Hadley cell and a poleward extension of the subtropical dry zones. The CAM3 results are compared to and found to be in excellent agreement with those of the multimodel means of the recent Coupled Model Intercomparison Project (CMIP3) and Chemistry–Climate Model Validation (CCMVal2) simulations. This study, therefore, strongly suggests that most Southern Hemisphere tropospheric circulation changes, in austral summer over the second half of the twentieth century, have been caused by polar stratospheric ozone depletion.
The impacts of stratospheric ozone recovery on Southern Ocean surface and interior temperature, heat content, heat uptake, and heat transport are investigated by contrasting two ensemble ...chemistry-climate model simulations in 2005–2099: one with fixed ozone depleting substances (ODSs) and another with decreasing ODSs. In our simulations ozone recovery significantly affects Southern Ocean temperature, with large latitudinal and vertical variations. Ozone recovery causes a dipole change of the full-depth ocean heat content (OHC) with an increase south of 60°S and a decrease between 45°S and 60°S. Integrated over latitudes south of 40°S, OHC decreases in response to ozone recovery. This ocean heat loss is shown to be driven by weakened poleward ocean heat transport (OHT) across 40°S, which is partly canceled by enhanced heat uptake. The weakening of poleward OHT into the Southern Ocean is caused by the ozone-induced equatorward shift of the meridional overturning circulation.
The body of literature on ambient air pollution suggests that atmospheric stagnation events trigger high levels of air pollution. In this paper we use fifteen years (2000-2014) of summertime in situ ...air quality measurements together with meteorological reanalysis data to examine the temporal correlation of pollutants with the Air Stagnation Index (ASI) on daily timescales. We find that while the direction of the relationship between the ASI and summertime PM2.5 and O3 ranges from near-zero to positive throughout regions comprising the contiguous United States (US), the strength of the relationship is very weak (e.g. in the Northeast the correlation coefficient between the ASI and PM2.5 is 0.09). Moreover, similar to our analysis of the correlation of day-to-day variations of the ASI and pollutants, the percentage of co-occurring extreme pollution and stagnation events is small (e.g. days with a high coverage of stagnation only co-occur with extreme pollution events about one-third of the time in the Northeast). The southern US is an exception to our overall findings as the strength of the relationship between the ASI and pollution is stronger and the percentage of co-occurring events is higher compared with other regions. The results of this study suggest a reevaluation of the ASI as an index to assess meteorological and climatic impacts to air quality.
Several studies have reported that global climate models underestimate the observed trend in tropical expansion, with the implication that such models are missing key processes of the climate system. ...We show here that integrations of a chemistry‐climate model forced with observed sea surface temperatures (SSTs), greenhouse gases, and ozone‐depleting substances can produce 1980 to 2009 expansion trends comparable to those found in most reanalyses data products. Correct representation of the SSTs changes is important for the Northern Hemisphere, while correct representation of stratospheric ozone changes is important for the Southern Hemisphere. The ensemble mean trend (which captures only the forced response) is nearly always much weaker than trends in reanalyses. This suggests that a large fraction of the recently observed changes may, in fact, be a consequence of internal atmospheric variability and not a response of the climate system to anthropogenic forcings.
Key Points
Climate models can capture recent HC expansion if internal variability is considered
SSTs important for Northern Hemisphere, ozone depletion for Southern Hemisphere
Models are not necessarily missing key processes
A dry general circulation model is used to investigate how coupling between the stratospheric polar vortex and the extratropical tropospheric circulation depends on the latitude of the tropospheric ...jet. The tropospheric response to an identical stratospheric vortex configuration is shown to be strongest for a jet centered near 40° and weaker for jets near either 30° or 50° by more than a factor of 3. Stratosphere-focused mechanisms based on stratospheric potential vorticity inversion, eddy phase speed, and planetary wave reflection, as well as arguments based on tropospheric eddy heat flux and zonal length scale, appear to be incapable of explaining the differences in the magnitude of the jet shift. In contrast, arguments based purely on tropospheric variability involving the strength of eddy–zonal mean flow feedbacks and jet persistence, and related changes in the synoptic eddy momentum flux, appear to explain this effect. The dependence of coupling between the stratospheric polar vortex and the troposphere on tropospheric jet latitude found here is consistent with 1) the observed variability in the North Atlantic and the North Pacific and 2) the trend in the Southern Hemisphere as projected by comprehensive models.
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