ALBERT

Description: The thermal and denudational history of Ireland is evaluated using an extensive new apatite fission-track (AFT) dataset derived from surface samples. Modelled thermal histories are used to construct maps of denudation for a number of time slices from Triassic time to 10 Ma using a time-dependent palaeogeotherm. The maps illustrate the spatial variability of denudation and subsidence within each time slice. The patterns of denudation are complex, showing considerable variability at the length scale of 101-102 km, with especially high denudation rates found over known igneous centres such as the Mournes of County Down. Based on the onshore AFT data alone, there is no definitive signature of an Irish Sea Dome extending significantly across Ireland in Early Tertiary time. The cumulative amount of denudation during Tertiary time varies depending on the AFT annealing model used, but is generally in the region between 1 and 2 km and without clear spatial trends. High amounts of denudation have been mapped over the Tertiary intrusions in County Down, reflecting their unroofing since emplacement in Paleocene time. The cumulative denudation from Triassic time to 10 Ma shows relatively low amounts of denudation (〈2 km) in the Irish Midlands and the extreme NE of the island, consistent with the observation that Mesozoic-Tertiary sediments and igneous products are preserved in the Ulster Basin. The western flank of Ireland and the region between Dublin and County Down show high cumulative amounts of denudation (〈4 km), the latter being consistent with the high amounts of denudation interpreted for the Irish Sea region. This denudation pattern explains in part the outcrop of Precambrian and Lower Palaeozoic rocks in these areas. The spatial integration of the denudation over the entire landmass gives the average denudation rate and the sediment discharge from Ireland as a function of time. Average denudation rates are moderately high in Triassic time, falling to low values in Cretaceous time, and increasing substantially in Tertiary time. However, the total volumetric discharge of sediment in Tertiary time is an order of magnitude smaller than the preserved solid volume of Tertiary sediment in the basins offshore western Ireland.

Description: In situ measurements of water vapor isotopic composition from Mauna Loa, Hawaii, are merged with soundings from Hilo to show an inverse relationship between the estimated inversion strength (EIS) and isotopically derived measures of lower-tropospheric mixing. Remote sensing estimates of cloud fraction, cloud liquid water path, and cloud top pressure were all found to be higher (lower) under low (high) EIS. Inverse modeling of the isotopic data corresponding to terciles of EIS conditions provide quantitative constraints on the last-saturation temperatures and mixing fractions that govern the humidity above the trade inversion. The mixing fraction of water vapor transported from the boundary layer to Mauna Loa decreases with respect to EIS at a rate of about 3% K−1, corresponding to a mixing ratio decrease of 0.6 g kg−1 K−1. A last-saturation temperature of 240 K can match all observations. This approach can be applied in other settings and may be used to test models of low-cloud climate feedbacks. ©2017. American Geophysical Union. All Rights Reserved.

Description: The humidity of the free troposphere can be modeled, to first order, in terms of cold-point dehydration, followed by moistening via mixing with boundary layer air. The relative balance between these processes is of prime interest for understanding interannual variability of humidity and for understanding the water vapor feedback. Measurements of water vapor isotopic composition can provide quantitative constraints on these processes. The authors developed a stochastic model that parameterizes water vapor isotopic composition in terms of these processes and fit the model parameters to data from the Chajnantor Plateau, Chile (23°S). For August–November 2012, the average mixing ratio was 1680 ppmv, with mean water vapor δD of −234‰ and mean deuterium excess of 21‰. The data were best fit by an asymmetric last-saturation distribution with mean last-saturation mixing ratio rs of 391 (+45, −75) ppmv, a median rs of 368 (+45, −75) ppmv, and a mean mixing fraction between the freeze-dried air and moist boundary layer air of . Measurements from August to November 2014 had an average mixing ratio of 2210 ppmv, an average δD of −220‰, and an average deuterium excess of 14‰. The last-saturation distribution for this period was less skewed than for 2012, with an average rs of 520 (+42, −75) ppmv and a median rs of 507 (+25, −75) ppmv. The mean mixing fraction for 2014 was . The results show that the moistening in 2014, relative to 2012, requires increases in both the last-saturation mixing ratio and the postcondensation moistening and illustrate the utility of isotopic measurements for constraining the processes governing subtropical humidity.

Description: Quantitative, observational constraints on lower-tropospheric mixing are crucial for improved models of low-cloud feedbacks, and analysis of water vapor isotopic composition provides an independent means for generating such constraints. In situ measurements of water vapor isotopic composition from the Chajnantor Plateau, in northern Chile, are merged with sounding data from Antofagasta and satellite measurements of cloud fraction (CF) from the SE Pacific to show an inverse relationship between the estimated inversion strength (EIS) and water vapor export from the marine boundary layer into the free troposphere. When merged with results from the subtropical northern Pacific, the relationship between EIS and water vapor transport is found to be exponential across EIS values ranging from 0 to 15.6 K. The data from Chile are stratified into terciles of EIS with average EIS values of 9, 12.6 and 15.6 K. We show positive relationships between EIS, cloud fraction, and the mixing diagnostic δDv−δDr and negative relationships between EIS and the observed mixing ratios and water vapor δD values at Chajnantor, all of which are consistent with an inverse relationship between inversion strength and water vapor export from the marine boundary layer. Inverse modeling of the isotopic data using a simple process model shows that the average mixing ratios at Chajnantor derived from the marine boundary layer are estimated to be 2.1, 1.15, and 0.84 g/kg, respectively, for the lowest, middle, and highest terciles of EIS. These results can be used to constrain convective parameterizations and models of low-cloud feedbacks. ©2018. American Geophysical Union. All Rights Reserved.

Description: The thermodynamic factors related to tropical cyclone genesis are examined in several simulations of the middle part of the Holocene epoch when the precession of Earth’s orbit altered the seasonal distribution of solar radiation and in one transient simulation of the millennium preceding the industrial era. The thermodynamic properties most crucial for genesis display a broad stability across both periods, although both orbital variations during the mid-Holocene (MH) 6000 years ago (6ka) and volcanic eruptions in the transient simulation have detectable effects. It is shown that the distribution of top-of-the-atmosphere radiation 6ka altered the Northern Hemisphere seasonal cycle of the potential intensity of tropical cyclones in addition to slightly increasing the difference between middle tropospheric and boundary layer entropy, a parameter that has been related to the incubation period required for genesis. The Southern Hemisphere, which receives more solar radiation during its storm season today than it did 6ka, displays slightly more favorable thermodynamic properties during the MH than in the preindustrial era control. Surface temperatures over the ocean in both hemispheres respond to radiation anomalies more slowly than those in upper levels, altering the thermal stability. Volcanism produces a sharp but transient temperature response in the last-millennium simulation that strongly reduces potential intensity during the seasons immediately following a major eruption. Here, too, the differential vertical temperature response is key: temperatures in the lower and middle troposphere cool, while those near the tropopause rise. Aside from these deviations, there is no substantial variation in thermodynamic properties over the 1000-yr simulation.

Description: Water vapor tracers of last saturation were used in an atmospheric tracer transport model to evaluate ENSO variability in the generation of the dry air that defines the subtropical middle troposphere over the North Pacific. Fifteen Northern Hemisphere winters, including El Niño, La Niña, and ENSO-neutral seasons, were evaluated using both Northern Hemisphere and global last saturation water vapor tracer configurations. During El Niño northern winter, the free troposphere over the subtropical North Pacific is both drier and warmer than during La Niña. The probability distributions of the last saturation position for the dry air in the middle troposphere were evaluated over the subtropical North Pacific and were found to be further poleward, at a higher altitude, and more westerly in their components during the warm phase compared to the cold phase. During warm phase (cold phase) northern winter, 57% (49%) of the air at 20°N and 633 hPa over the North Pacific was last saturated poleward of 20°N and above 500 hPa. Coherency was demonstrated between tropical sea surface temperatures, extratropical atmospheric saturation, and subtropical aridity of the middle troposphere. The stronger westerly component of last saturation during the warm phase ties ENSO-variable subtropical aridity to midlatitude westerlies when there is enhanced baroclinicity and an equatorward migration of the Pacific storm track. Humidity reconstructions from the water vapor tracers capture observed ENSO humidity variability and demonstrate that it can be explained in terms of changes in the location of last saturation, and not by changes in the temperature field. This study shows how teleconnections between the tropical ocean and the extratropical upper troposphere can impact the humidity of the middle troposphere of the subtropical dry regions.

Description: The zonal mean relative humidity response to a doubling of CO2 in a climate model is examined using a global climate model and an offline tracer transport model. Offline tracer transport model simulations are driven by the output from two configurations of the climate model, one with 1979 concentrations of atmospheric greenhouse gases and one with doubled CO2. A set of last saturation tracers is applied within the tracer transport model to diagnose the dynamics responsible for features in the water vapor field. Two different methods are used to differentiate the effects of circulation and transport shifts from spatially inhomogeneous temperature changes. The first of these uses the tracer transport model and is achieved by decoupling the input temperature and circulation fields; the second uses the reconstruction of humidity from the last saturation tracers and is achieved by decoupling the tracer concentrations from their saturation specific humidities. The responses of the tropical and subtropical relative humidities are found to be largely dependent on circulation and transport changes, particularly a poleward expansion of the Hadley cell, a deepening of the height of convective detrainment, a poleward shift of the extratropical jets, and an increase in the height of the tropopause. The last saturation tracers are used to illustrate the influence of changes in transport pathways within the GCM on the zonal mean relative humidity, particularly in the tropical upper troposphere and subtropical dry zones. Relative humidity changes near the extratropical tropopause and in the lower troposphere are largely dependent on changes in the distribution and gradients of temperature. Increases in relative humidity near the extratropical tropopause in both hemispheres are coincident with increases in the occurrence of local saturation and high cloud cover.

Description: Large-scale environmental factors that favor tropical cyclogenesis are calculated and examined in simulations of the Last Glacial Maximum (LGM) from the Paleoclimate Modelling Intercomparison Project Phase 2 (PMIP2). Despite universally colder conditions at the LGM, values of tropical cyclone potential intensity, which both serves as an upper bound on thermodynamically achievable intensity and indicates regions supportive of the deep convection required, are broadly similar in magnitude to those in preindustrial era control simulation. Some regions, including large areas of the central and western North Pacific, feature higher potential intensities at the LGM than they do in the control runs, while other regions including much of the Atlantic and Indian Oceans are lower. Changes in potential intensity are strongly correlated with the degree of surface cooling during the LGM. Additionally, two thermodynamic parameters—one that measures midtropospheric entropy deficits relevant for tropical cyclogenesis and another related to the time required for genesis—are broadly more favorable in the LGM simulation than in the preindustrial era control. A genesis potential index yields higher values for the LGM in much of the western Pacific, a feature common to nearly all of the individual models examined.

Description: Austral summer transport of water vapor to the southern South American Altiplano is investigated using in situ measurements of water vapor isotopic composition collected from 1 November 2012 to 10 February 2013 on the Chajnantor Plateau in the Chilean Andes. Onset of the wet season in December was associated with an increase in mixing ratios from an average of 1500 ppmv during the winter dry season to 5400 ppmv in early December. Water vapor isotopes δD and δ18O increased from dry season averages of −235‰ and −31‰, respectively, to wet season averages of −142‰ and −17‰, reaching as high as −70‰ and −17‰, respectively. The highest water vapor δ values were close to those measured in coastal settings, suggesting little condensation during transport to the site. About 5% of the wet season data have δ values that are lower than expected for Rayleigh distillation and are associated with high relative humidity (〉75%), easterly winds, and periods of low outgoing longwave radiation over the Altiplano, consistent with moistening by deep convection. The remainder of the data have δ values that are greater than expected for Rayleigh distillation, up to 250‰ above the Rayleigh curve. These data are consistent with mixing between very dry air and moist air from the boundary layer. The data show intraseasonal variability coherently linked to the position of the Bolivian high, with moist air associated with a southward displacement in the Bolivian high. The humidity over the southern Altiplano during the wet season reflects a balance among advective drying, advective moistening with little condensation, and convective moistening.