Patrick Lonergan

Increased polar stratospheric ozone losses and delayed eventual recovery owing to increasing greenhouse-gas concentrations

The chemical reactions responsible for stratospheric ozone depletion are extremely sensitive to temperature. Greenhouse gases warm the Earths surface but cool the stratosphere radiatively and therefore affect ozone depletion. Here we investigate the interplay between projected future emissions of greenhouse gases and levels of ozone-depleting halogen species using a global climate model that incorporates simplified ozone-depletion chemistry. Temperature and wind changes induced by the increasing greenhouse-gas concentrations alter planetary-wave propagation in our model, reducing the frequency of sudden stratospheric warmings in the Northern Hemisphere. This results in a more stable Arctic polar vortex, with significantly colder temperatures in the lower stratosphere and concomitantly increased ozone depletion. Increased concentrations of greenhouse gases might therefore be at least partly responsible for the very large Arctic ozone losses observed in recent winters. Arctic losses reach a maximum in the decade 2010 to 2019 in our model, roughly a decade after the maximum in stratospheric chlorine abundance. The mean losses are about the same as those over the Antarctic during the early 1990s, with geographically localized losses of up to two-thirds of the Arctic ozone column in the worst years. The severity and the duration of the Antarctic ozone hole are also predicted to increase because of greenhouse-gas-induced stratospheric cooling over the coming decades.

Climate Change and the Middle Atmosphere. Part III: The Doubled CO2 Climate Revisited

Abstract The response of the troposphere–stratosphere system to doubled atmospheric CO2 is investigated in a series of experiments in which sea surface temperatures are allowed to adjust to radiation imbalances. The Goddard Institute for Space Studies (GISS) Global Climate Middle Atmosphere Model (GCMAM) warms by 5.1°C at the surface while the stratosphere cools by up to 10°C. When ozone is allowed to respond photochemically, the stratospheric cooling is reduced by 20%, with little effect in the troposphere. Planetary wave energy increases in the stratosphere, producing dynamical warming at high latitudes, in agreement with previous GCMAM doubled CO2 simulations; the effect is due to increased tropospheric generation and altered refraction, both strongly influenced by the magnitude of warming in the model’s tropical upper troposphere. This warming also results in stronger zonal winds in the lower stratosphere, which appears to reduce stratospheric planetary wave 2 energy and stratospheric warming events. ...

The Relative Importance of Solar and Anthropogenic Forcing of Climate Change between the Maunder Minimum and the Present

Abstract The climate during the Maunder Minimum is compared with current conditions in GCM simulations that include a full stratosphere and parameterized ozone response to solar spectral irradiance variability and trace gas changes. The Goddard Institute for Space Studies (GISS) Global Climate/Middle Atmosphere Model (GCMAM) coupled to a q-flux/mixed-layer model is used for the simulations, which begin in 1500 and extend to the present. Experiments were made to investigate the effect of total versus spectrally varying solar irradiance changes; spectrally varying solar irradiance changes on the stratospheric ozone/climate response with both preindustrial and present trace gases; and the impact on climate and stratospheric ozone of the preindustrial trace gases and aerosols by themselves. The results showed that 1) the Maunder Minimum cooling relative to today was primarily associated with reduced anthropogenic radiative forcing, although the solar reduction added 40% to the overall cooling. There is no obv...

Effects of solar cycle variability on the lower stratosphere and the troposphere

The effects of solar irradiance variability on the lower stratosphere and the troposphere are investigated using observed and general circulation model (GCM)-generated 30 and 100 mbar geopotential heights. The GCM includes changes in UV input (+ or −5% at wavelengths below 0.3 micron and no ozone photochemistry and transport) to roughly approximate the combined effects of UV and ozone changes associated with the solar variability. The annual and seasonal averages of the height differences between solar maximum and solar minimum conditions are evaluated. In the subtropics, observations indicate statistically highly significant increased geopotential heights during solar maximum, compared to solar minimum, in composite annual and seasonal averages. The model simulates this feature reasonably well, although the magnitude and statistical significance of the differences are often weaker than in observations, especially in summer. Both the observations and the model results show a strong dipole pattern of height differences when the data are partitioned according to the phase of the quasi-biennial oscillation (QBO), with the pattern reversing itself with the change in the phase of the QBO. The connection between solar variability and lower atmospheric changes are interpreted as follows: The solar changes directly affect the stratosphere by changing the vertical gradients of temperature and zonal wind. This leads to changes in propagation conditions for planetary waves resulting in changes of E-P flux divergence and then by the downward control principle, affecting the circulation in the lower stratosphere and the troposphere.

Climate Change and the Middle Atmosphere. Part IV: Ozone Response to Doubled CO2

Abstract Parameterized stratospheric ozone photochemistry has been included in the Goddard Institute for Space Studies (GISS) GCM to investigate the coupling between chemistry and climate change for the doubled CO2 climate. The chemical ozone response is of opposite sign to temperature changes, so that radiative cooling in the upper stratosphere results in increased ozone, while warming reduces ozone in the lower stratosphere. The increased overhead column reduces the amount of UV reaching the lower stratosphere, resulting in further ozone decreases there. Changes of up to 15% are seen, including both photochemistry and transport. Good agreement is found between the authors’ results and those in other models for tropical latitudes where the stratospheric temperature responses are similar. However, in the extratropics, there are large differences between present results and those of the other models due to differences in tropospheric warming and tropospheric forcing of the stratospheric residual circulatio...

Dynamic Downscaling of Seasonal Climate Predictions over Brazil

Abstract Climate projections for March–April–May (MAM) 1985 and 1997 made with the NASA Goddard Institute for Space Studies (GISS) GCM over South America on a 4° latitude by 5° longitude grid are “downscaled” to 0.5° grid spacing. This is accomplished by interpolating the GCM simulation product in time and space to create lateral boundary conditions (LBCs) for synchronous nested simulations by the regional climate model (RCM) of the GISS/Columbia University Center for Climate Systems Research. Both the GCM and the RCM simulations use sea surface temperature (SST) predictions based on persisted February SST anomalies. Each downscaled prediction is evaluated from an ensemble of five simulations and each is compared to a control ensemble of five RCM simulations driven by synchronous NCEP reanalysis data. An additional five-run control ensemble for MAM 1997 tests the impact of “perfect” SST predictions on the RCM forecast. Results are compared to observational evidence that includes NCEP reanalysis data, Clim...

Climatic effect of water vapor release in the upper troposphere

Water vapor is released into the Goddard Institute for Space Studies (GISS) global climate middle atmosphere model at the locations and cruise altitude of subsonic aircraft. A range of water vapor values is used to simulate not only current and 2015 projected emissions but also to provide larger signal-noise ratios. The results show that aircraft water vapor emissions do not significantly affect the models climate, either at the surface or in situ. With emissions some 15 times higher than the 2015 projection, a small impact is observed, amounting to a few tenths degrees celsius globally and locally, while with emissions 300 times the 2015 values, a global warming of 1°C results. However, with releases this large, only about 5% actually stays in the atmosphere. The larger emissions increase the specific humidity most in the tropical lower troposphere, partly as a result of increased evaporation due to the global warming; at flight altitudes, relative humidity and cloud cover increase at latitudes of emission, and temperature decreases. Surface warming is relatively independent of latitude, and only a slight longitudinal aircraft footprint is found in the warming for the most extreme experiment. Comparison to increased CO 2 experiments of similar magnitude warming shows that the upper tropospheric response is greater in the water vapor release experiments, but the high-latitude surface temperature response is larger with increased CO 2 due to more effective cryospheric feedbacks.

Modeled impact of cirrus cloud increases along aircraft flight paths

The potential climate impact of contrails and alterations in the lifetime of background cirrus due to subsonic aircraft water and aerosol emissions has been investigated in a set of experiments using the GISS GCM connected to a q-flux ocean. Cirrus clouds at a height of 12–15km, with an optical thickness of 0.33, were input to the model “x” percentage of clear-sky occasions along subsonic aircraft flight paths. The percentage x is varied from 0.05 to 6%. Two types of experiments were performed: one with the percentage of cirrus cloud increase independent of flight density along the flight paths, the other with the percentage related to the density of fuel expenditure. The overall climate impact was similar with the two approaches, due to the feedbacks of the climate system. Fifty years were run for each of the eight experiments, with the following conclusions based on the stable results from years 31–50. The equilibrium global mean response shows that altering high-level clouds by 1% changes the global mean temperature by 0.43°C. The global temperature response is highly linear (linear correlation coefficient of 0.996) for high cloud cover changes between 0.1 and 5%. The warming is amplified in the Northern Hemisphere, more so with greater cloud cover change. The temperature effect maximizes around 10km, more so as the overall warming increases (warming greater than 4°C occurs there with a 4.8% increase in upper level clouds). The surface temperature response is dominated by the feedbacks and shows little geographic relationship to the high cloud input outside of the hemispheric difference. Considering whether these effects would be observable, changing upper level cloud cover by as little as 0.4% produces warming greater than 2 standard deviations in the microwave sounding unit (MSU) channels 4, 2, and 2r (although the effect would be most noticeable in the upper troposphere channel 3 were standard deviations available). Given estimates of current aircraft impacts, this would require some increase relative to present-day effects, but a signal should be clear given the projections for 2050 aircraft. In comparison to increased CO2 experiments, in these runs the Northern Hemisphere clearly warms more relative to the Southern Hemisphere, and warming due to cloud height changes exceeds that due to the water vapor feedback. Despite the simplified nature of these experiments, the results emphasize the sensitivity of the modeled climate to high-level cloud cover changes and thus the potential ability of aircraft to influence climate by altering clouds in the upper troposphere.

A GCM investigation of global warming impacts relevant to tropical cyclone genesis

Two approaches that consider how greenhouse warming might impact the frequency of tropical cyclone (TC) genesis are explored. Results are based on GCM experiments with the q-flux version global climate model of the NASA/Goddard Institute for Space Studies (GISS); one set representing contemporary atmospheric concentrations of CO2, contrasting with the second set representing the global climate in double CO2 equilibrium. July–September means of climate parameters relevant to TC genesis are computed from the simulations and combined to formulate a seasonal genesis parameter (SGP), as suggested in an empirical study by Gray (in Shaw, D.B. (ed.), Meteorology Over the Tropical Oceans, 1979, pp. 155–218). The spatial distribution of the July–September SGP based on the control simulations is compared with the observed distribution and results using other models. The corresponding spatial distribution of the July–September SGP derived from the double CO2 simulations, when compared with the control results, projects a 50% increase in the genesis frequency of TC over the western North Atlantic/Gulf of Mexico basin, but 100–200% increases over the North Pacific Ocean. The increases, most of which are attributable to enhanced ocean temperatures, may be exaggerated, suggesting that the original SGP formulation requires tuning or other revisions. For example, it is noted that SGP computed from the NCEP 1982–1994 re-analysis climatology do not accurately reflect the known spatial distributions of TC genesis frequency. The second approach detects easterly waves over the eastern North Atlantic Ocean by spectral analysis of vorticity and wind component time trends, comparing wave activity in the control and double CO2 simulations. Results indicate a southward shift in future trajectories of easterly waves over West Africa and significant increases in their average amplitude as they cross the African coast and begin to traverse the Eastern Atlantic along 14°N. Copyright

Climate change and the middle atmosphere: 5. Paleostratosphere in cold and warm climates

The GISS Global Climate Middle Atmosphere Model is used to investigate how the stratosphere would have changed during two paleotime periods: the cold Last Glacial Maximum (∼21,000 years ago) and the warm Paleocene (58 million years ago). Uncertainties in sea surface temperatures and mountain wave drag over the ice sheets are investigated in sensitivity experiments. In many respects the climate and dynamical forcing of the stratosphere was opposite in these time periods, with reduced CO2, increased topography, and increased latitudinal temperature gradients during the ice age, and increased CO2, reduced topography and latitudinal temperature gradients during the Paleocene, representative of much of the Tertiary. The results show that the stratospheric response was often of an opposite nature as well, with the ice ages featuring a warmer stratosphere, increased residual circulation in the lower stratosphere (and decreased above), and weakened polar vortices, while the Paleocene simulation had a colder stratosphere, decreased residual circulation in the lower stratosphere (and increased above), with strengthened polar vortices. Analysis shows that the stratospheric response is very individualistic to the particular climate regime, and the opposite effects are not necessarily produced by inversely related mechanisms. Of particular importance in both climates is the reduced latitudinal gradient at high latitudes, which weakens high-latitude zonal winds and limits wave energy vertical propagation. Increased planetary wave forcing in the lower stratosphere accelerates the circulation during the ice ages. A strong increase in zonal winds during the Paleocene is the result of both decreased planetary wave forcing, associated with the reduced topography, and decreased mountain wave drag. The sensitivity experiments show that if tropical sea surface temperatures were warmer, the stratospheric residual circulation was enhanced, while stratospheric warmings are sensitive to the precise sea surface temperature specifications and mountain wave drag.