Radiative Effects of Stratospheric Seasonal Cycles in the Tropical Upper Troposphere and Lower Stratosphere

Water vapor and ozone are powerful radiative constituents in the tropical lower stratosphere, impacting the local heating budget and nonlocally forcing the troposphere below. Their near-tropopause seasonal cycle structures imply associated "radiative seasonal cycles" in heating rates that could affect the amplitude and phase of the local temperature seasonal cycle. Overlying stratospheric seasonal cycles of water vapor and ozone could also play a role in the lower stratosphere and upper troposphere heat budgets through nonlocal propagation of radiation. Previous studies suggest that the tropical lower stratospheric ozone seasonal cycle radiatively amplifies the local temperature seasonal cycle by up to 35%, while water vapor is thought to have a damping effect an order of magnitude smaller. This study uses Aura Microwave Limb Sounder observations and an offline radiative transfer model to examine ozone, water vapor, and temperature seasonal cycles and their radiative linkages in the lower stratosphere and upper troposphere. Radiative sensitivities to ozone and water vapor vertical structures are explicitly calculated, which has not been previously done in a seasonal cycle context. Results show that the water vapor radiative seasonal cycle in the lower stratosphere is not sensitive to the overlying water vapor structure. In contrast, about one-third of ozone's radiative seasonal cycle amplitude at 85 hPa is associated with longwave emission above 85 hPa. Ozone's radiative effects are not spatially homogenous: for example, the Northern Hemisphere tropics have a seasonal cycle of radiative temperature adjustments with an amplitude 0.8 K larger than the Southern Hemisphere tropics. Keywords: Stratosphere; Tropopause; Ozone; Radiation budgets; Water vapor; Seasonal cycleNational Science Foundation (U.S.) (Grant AGS-1461517)United States. National Aeronautics and Space Administration. Earth and Space Science Fellowship Program (Grant NNX14AK83H

The tropopause region thermal structure and tropical cyclones

Thesis: Ph. D. in Atmospheric Science, Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 187-207).This thesis is an exploration of two seemingly unrelated questions: First, how do water vapor and ozone variations radiatively influence the thermal structure of the tropopause region? Second, what sets the thermodynamic limits of tropical cyclone intensity across the seasonal cycle? The link between these subjects is tropical cyclone outflow, which often reaches into the tropopause region, allowing the thermal structure there to impact tropical cyclone potential intensity. A radiative transfer model is employed to calculate the radiative effects of the 2000 and 2011 tropopause region abrupt drops -- events in which temperatures, water vapor, and ozone plunge suddenly to anomalously low levels. Results show that radiative effects partially offset in the region above the tropopause, but nonlocally combine to cool the layers below the tropopause. Persistently low water vapor concentrations associated with the abrupt drops spread to extratropical latitudes, and produce a total negative radiative forcing that offsets <12% of the carbon dioxide forcing over 1990-2013. Next, the importance of local and nonlocal radiative heating/cooling for tropopause region temperature seasonal cycles is examined. The radiative effects of water vapor seasonality are weak and local to the tropopause, whereas ozone radiatively amplifies temperature seasonality in the tropopause region by 30%, in part because stratospheric ozone seasonality nonlocally affects the tropopause region thermal structure. To determine how the tropopause region thermal structure affects thermodynamic limits on tropical cyclone intensity, this study presents the first comprehensive seasonal cycle climatology of potential intensity. Perennially warm sea surface temperatures in the Western Pacific result in outflow altitudes that are near the tropical tropopause region throughout the seasonal cycle, whereas the seasonalities of other ocean basins are less influenced by the tropopause region. Probing the potential intensity environmental drivers reveals that the seasonality of near-tropopause temperatures in the Western Pacific damps potential intensity seasonal variability by <30%. Incorporating a best track tropical cyclone archive shows that this result is relevant for real-world tropical cyclones: the tropopause region thermal structure permits intense Western Pacific tropical cyclones in every month of the year, which may have critical consequences for coastal societies.by Daniel Michael Gilford.Ph. D. in Atmospheric Scienc

Radiative Impacts of the 2011 Abrupt Drops in Water Vapor and Ozone in the Tropical Tropopause Layer

An abrupt drop in tropical tropopause layer (TTL) water vapor, similar to that observed in 2000, recently occurred in 2011, and was concurrent with reductions in TTL temperature and ozone. Previous studies have indicated that such large water vapor variability can have significant radiative impacts. This study uses Aura Microwave Limb Sounder observations, the Stratospheric Water Vapor and Ozone Satellite Homogenized dataset, and two radiative transfer models to examine the radiative effects of the observed changes in TTL water vapor and ozone on TTL temperatures and global radiative forcing (RF). The analyses herein suggest that quasi-isentropic poleward propagation of TTL water vapor reductions results in a zonal-mean structure with “wings” of extratropical water vapor reductions, which account for about half of the 2011 abrupt drop global radiative impact. RF values associated with the mean water vapor concentrations differences between 2012/13 and 2010/11 are between −0.01 and −0.09 W m⁻², depending upon the altitude above which perturbations are considered. TTL water vapor and ozone variability during this period jointly lead to a transient radiative cooling of ~0.25–0.5 K in layers below the tropopause. The 2011 abrupt drop also prolonged the reduction in stratospheric water vapor that followed the 2000 abrupt drop, providing a longer-term radiative forcing of climate. Water vapor concentrations from 2005 to 2013 are lower than those from 1990 to 1999, resulting in a RF between these periods of about −0.045 W m⁻², approximately 12% as large as, but of opposite sign to, the concurrent estimated CO[subscript 2] forcing.United States. National Aeronautics and Space Administration (NNX14AK83H)National Science Foundation (U.S.) (AGS-1342810)National Science Foundation (U.S.) (AGS-1461517

Centuries of thermal sea-level rise due to anthropogenic emissions of short-lived greenhouse gases

Mitigation of anthropogenic greenhouse gases with short lifetimes (order of a year to decades) can contribute to limiting warming, but less attention has been paid to their impacts on longer-term sea-level rise. We show that short-lived greenhouse gases contribute to sea-level rise through thermal expansion (TSLR) over much longer time scales than their atmospheric lifetimes. For example, at least half of the TSLR due to increases in methane is expected to remain present for more than 200 y, even if anthropogenic emissions cease altogether, despite the 10-y atmospheric lifetime of this gas. Chlorofluorocarbons and hydrochlorofluorocarbons have already been phased out under the Montreal Protocol due to concerns about ozone depletion and provide an illustration of how emission reductions avoid multiple centuries of future TSLR. We examine the “world avoided” by the Montreal Protocol by showing that if these gases had instead been eliminated in 2050, additional TSLR of up to about 14 cm would be expected in the 21st century, with continuing contributions lasting more than 500 y. Emissions of the hydrofluorocarbon substitutes in the next half-century would also contribute to centuries of future TSLR. Consideration of the time scales of reversibility of TSLR due to short-lived substances provides insights into physical processes: sea-level rise is often assumed to follow air temperature, but this assumption holds only for TSLR when temperatures are increasing. We present a more complete formulation that is accurate even when atmospheric temperatures are stable or decreasing due to reductions in short-lived gases or net radiative forcing

On the Seasonal Cycles of Tropical Cyclone Potential Intensity

Recent studies have investigated trends and interannual variability in the potential intensity (PI) of tropical cyclones (TCs), but relatively few have examined TC PI seasonality or its controlling factors. Potential intensity is a function of environmental conditions that influence thermodynamic atmosphere-ocean disequilibrium and the TC thermodynamic efficiency-primarily sea surface temperatures and the TC outflow temperatures-and therefore varies spatially across ocean basins with different ambient conditions. This study analyzes the seasonal cycles of TC PI in each main development region using reanalysis data from 1980 to 2013. TC outflow in the western North Pacific (WNP) region is found above the tropopause throughout the seasonal cycle. Consequently, WNP TC PI is strongly influenced by the seasonal cycle of lower-stratospheric temperatures, which act to damp its seasonal variability and thereby permit powerful TCs any time during the year. In contrast, the other main development regions (such as the North Atlantic) exhibit outflow levels in the troposphere through much of the year, except during their peak seasons. Mathematical decomposition of the TC PI metric shows that outflow temperatures damp WNP TC PI seasonality through thermodynamic efficiency by a quarter to a third, whereas disequilibrium between SSTs and the troposphere drives 72%-85% of the seasonal amplitude in the other ocean basins. Strong linkages between disequilibrium and TC PI seasonality in these basins result in thermodynamic support for powerful TCs only during their peak seasons. Decomposition also shows that the stratospheric influence on outflow temperatures in the WNP delays the peak month of TC PI by a month. Keywords: Sea surface temperature; Stratosphere-troposphere coupling; Tropical cyclones; Thermodynamics; Reanalysis data; Seasonal cycleUnited States. National Aeronautics and Space Administration (Grant NNX14AK83H)National Science Foundation (U.S.) (Grant AGS-1461517

Understanding of Contemporary Regional Sea-Level Change and the Implications for the Future

Global sea level provides an important indicator of the state of the warming climate, but changes in regional sea level are most relevant for coastal communities around the world. With improvements to the sea-level observing system, the knowledge of regional sea-level change has advanced dramatically in recent years. Satellite measurements coupled with in situ observations have allowed for comprehensive study and improved understanding of the diverse set of drivers that lead to variations in sea level in space and time. Despite the advances, gaps in the understanding of contemporary sea-level change remain and inhibit the ability to predict how the relevant processes may lead to future change. These gaps arise in part due to the complexity of the linkages between the drivers of sea-level change. Here we review the individual processes which lead to sea-level change and then describe how they combine and vary regionally. The intent of the paper is to provide an overview of the current state of understanding of the processes that cause regional sea-level change and to identify and discuss limitations and uncertainty in our understanding of these processes. Areas where the lack of understanding or gaps in knowledge inhibit the ability to provide the needed information for comprehensive planning efforts are of particular focus. Finally, a goal of this paper is to highlight the role of the expanded sea-level observation network—particularly as related to satellite observations—in the improved scientific understanding of the contributors to regional sea-level change