The Eocene-Oligocene transition in the eastern Gulf Coastal Plain of Alabama and Mississippi occurs within a sequence of marine clastic rocks making up the Jackson Group and the lower Vicksburg Group. The placement and the nature of the Eocene-Oligocene boundary remain controversial after more than 20 yr of detailed study. In Alabama, the Eocene-Oligocene boundary is placed within a condensed section at the contact between the Shubuta Member of the Yazoo Clay and the Red Bluff Clay. In eastern Mississippi, the Eocene-Oligocene boundary is recognized through the use of planktonic foraminifera and calcareous nannofossils as also occurring at the Yazoo–Red Bluff contact, but this surface is considered an unconformity representing a sequence boundary. In the Mossy Grove core of western Mississippi, planktonic foraminifera, in conjunction with radiometrically dated bentonites, place the Eocene-Oligocene boundary within the upper Yazoo Clay.
Abstract. The Aura satellite Tropospheric Emission Spectrometer (TES) instrument is capable of measuring the HDO/H2O ratio in the lower troposphere using thermal infrared radiances between 1200 and 1350 cm−1. However, direct validation of these measurements is challenging due to a lack of in situ measured vertical profiles of the HDO/H2O ratio that are spatially and temporally co-located with the TES observations. From 11 October through 5 November 2008, we undertook a campaign to measure HDO and H2O at the Mauna Loa observatory in Hawaii for comparison with TES observations. The Mauna Loa observatory is situated at 3.1 km above sea level or approximately 680 hPa, which is approximately the altitude where the TES HDO/H2O observations show the most sensitivity. Another advantage of comparing in situ data from this site to estimates derived from thermal IR radiances is that the volcanic rock is heated by sunlight during the day, thus providing significant thermal contrast between the surface and atmosphere; this thermal contrast increases the sensitivity to near surface estimates of tropospheric trace gases. The objective of this inter-comparison is to better characterize a bias in the TES HDO data, which had been previously estimated to be approximately 5 % too high for a column integrated value between 850 hPa and 500 hPa. We estimate that the TES HDO profiles should be corrected downwards by approximately 4.8 % and 6.3 % for Versions 3 and 4 of the data respectively. These corrections must account for the vertical sensitivity of the TES HDO estimates. We estimate that the precision of this bias correction is approximately 1.9 %. The accuracy is driven by the corrections applied to the in situ HDO and H2O measurements using flask data taken during the inter-comparison campaign and is estimated to be less than 1 %. Future comparisons of TES data to accurate vertical profiles of in situ measurements are needed to refine this bias estimate.
A large fraction of the rain received by continental India is produced by cyclonic vortices with outer radii of about 1000 km that are contained within the larger scale South Asian monsoon flow. The more intense occurrences of these vortices are called monsoon depressions; these consist of bottom‐heavy columns of relative vorticity that propagate to the northwest in time‐mean low‐level eastward flow. Previous studies have argued that this apparent upstream propagation is caused by dynamical lifting west of the vortex centre, with the resulting ascent producing vortex stretching that shifts the vortex to the west. Here, analysis of over 100 Indian monsoon depressions is used to show that low‐level vortex stretching has a spatial structure inconsistent with the observed propagation and is balanced by other terms in the low‐level vorticity budget. Instead, monsoon depressions are shown to consist of potential vorticity maxima that have peak amplitude in the middle troposphere and propagate westward by nonlinear, horizontal adiabatic advection (i.e. beta drift). The precipitating ascent in monsoon depressions makes a more minor contribution to the total storm motion and primarily acts to maintain the upright structure of the vortex. These results suggest a new view of Indian monsoon depressions as potential vorticity columns that propagate primarily by adiabatic dynamics.
[1] Water vapor in the subtropical troposphere plays an important role in the radiative balance, the distribution of precipitation, and the chemistry of the Earth's atmosphere. Measurements of the water vapor mixing ratio paired with stable isotope ratios provide unique information on transport processes and moisture sources that is not available with mixing ratio data alone. Measurements of the D/H isotope ratio of water vapor from Mauna Loa Observatory over 4 weeks in October–November 2008 were used to identify components of the regional hydrological cycle. A mixing model exploits the isotope information to identify water fluxes from time series data. Mixing is associated with exchange between marine boundary layer air and tropospheric air on diurnal time scales and between different tropospheric air masses with characteristics that evolve on the synoptic time scale. Diurnal variations are associated with upslope flow and the transition from nighttime air above the marine trade inversion to marine boundary layer air during daytime. During easterly trade wind conditions, growth and decay of the boundary layer are largely conservative in a regional context but contribute ∼12% of the nighttime water vapor at Mauna Loa. Tropospheric moisture is associated with convective outflow and exchange with drier air originating from higher latitude or higher altitude. During the passage of a moist filament, boundary layer exchange is enhanced. Isotopic data reflect the combination of processes that control the water balance, which highlights the utility for baseline measurements of water vapor isotopologues in monitoring the response of the hydrological cycle to climate change.
The first global climatology of monsoon low‐pressure systems is presented here, based on the ERA‐Interim reanalysis. Low‐pressure systems are classified into three intensity categories and particular focus is given to systems in the category corresponding to a traditional definition of monsoon depressions. Vortex tracks are identified using an automated algorithm applied to the distributions of 850 hPa relative vorticity, sea‐level pressure and surface wind speed for 1979–2012. Roughly two to three times as many monsoon low‐pressure systems form in the Northern Hemisphere as in the Southern Hemisphere during local summer. The frequency of genesis typically peaks in local summer, but low‐pressure systems form throughout the year in every monsoon region. Interannual variability is weak, with standard deviations of summer counts typically being below 10% of the long‐term summer mean. Regional composites reveal that monsoon depressions in India, the western Pacific and northern Australia share a common structure, consisting of a warm‐over‐cold core and a top‐heavy column of potential vorticity that extends from the surface to the upper troposphere. A separate class of monsoon low‐pressure systems develops over dry regions of West Africa and western Australia, with a shallow composite structure having a warm core in the lower troposphere and cyclonic potential vorticity confined to a thin near‐surface layer. Low‐pressure systems in nearly all monsoon regions are estimated to account for a large fraction, from about 40% to more than 80%, of summer precipitation on the poleward edge of the climatological mean precipitation maxima.
Abstract 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.
Abstract Quelccaya Ice Cap in the Andes of Peru contains an annually resolved δ 18 O record covering the past 1800 years; yet atmospheric dynamics associated with snow deposition and δ 18 O variability at this site are poorly understood. Here we make use of 10 years of snow pit and short core δ 18 O data and hourly snow‐height measurements obtained by an automated weather station deployed at the ice cap's summit to analyze linkages between snowfall, δ 18 O, and the South American summer monsoon (SASM). Snow accumulation peaks in December and is negative May–September. Snow δ 18 O values decrease gradually through austral summer from about −17 to −24‰. Surface snow δ 18 O is altered after deposition during austral winter from about −24 to −15‰. More than 70% of the total snow accumulation is tied to convection along the leading edge of cold air incursions of midlatitude air advected equatorward from southern South America. Snowfall amplitude at Quelccaya Ice Cap varies systematically with regional precipitation, atmospheric dynamics, midtroposphere humidity, and water vapor δ D. Strongest snowfall gains correspond with positive precipitation anomalies over the western Amazon Basin, increased humidity, and lowered water vapor δ D values, consistent with the “amount effect.” We discuss ventilation of the monsoon, modulated by midlatitude cold air advection, as potentially diagnostic of the relationship between SASM dynamics and Quelccaya snowfall. Results will serve as a basis for development of a comprehensive isotopic forward model to reconstruct past monsoon dynamics using the ice core δ 18 O record.
Abstract Here it is shown that almost all models participating in the Coupled Model Intercomparison Project (CMIP) exhibit a common bias in the thermodynamic structure of boreal summer monsoons. The strongest bias lies over South Asia, where the upper-tropospheric temperature maximum is too weak, is shifted southeast of its observed location, and does not extend as far west over Africa as it does in observations. Simulated Asian maxima of surface air moist static energy are also too weak and are located over coastal oceans rather than in their observed continental position. The spatial structure of this bias suggests that it is caused by an overly smoothed representation of topography west of the Tibetan Plateau, which allows dry air from the deserts of western Asia to penetrate the monsoon thermal maximum, suppressing moist convection and cooling the upper troposphere. In a climate model with a decent representation of the thermodynamic state of the Asian monsoon, the qualitative characteristics of this bias can be recreated by truncating topography just west of the Tibetan Plateau. This relatively minor topographic modification also produces a negative anomaly of Indian precipitation of similar sign and amplitude to the CMIP continental Indian monsoon precipitation bias. Furthermore, in simulations of next-century climate warming, this topographic modification reduces the amplitude of the increase in Indian monsoon precipitation. These results confirm the importance of topography west of the Tibetan Plateau for South Asian climate and illustrate the need for careful assessments of the thermodynamic state of model monsoons.
