The monsoon imprint during the ‘atypical’ MIS 13 as seen through north and equatorial Indian Ocean records

Previous studies have suggested that Marine Isotope Stage (MIS) 13, recognized as atypical in many paleoclimate records, is marked by the development of anomalously strong summer monsoons in the northern tropical areas. To test this hypothesis, we performed a multi-proxy study on three marine records from the tropical Indian Ocean in order to reconstruct and analyse changes in the summer Indian monsoon winds and precipitations during MIS 13. Our data confirm the existence of a low-salinity event during MIS 13 in the equatorial Indian Ocean but we argue that this event should not be considered as “atypical”. Taking only into account a smaller precession does not make it possible to explain such precipitation episode. However, when considering also the larger obliquity in a more complete orbitally driven monsoon “model,” one can successfully explain this event. In addition, our data suggest that intense summer monsoon winds, although not atypical in strength, prevailed during MIS 13 in the western Arabian Sea. These strong monsoon winds, transporting important moisture, together with the effect of insolation and Eurasian ice sheet, are likely one of the factors responsible for the intense monsoon precipitation signal recorded in China loess, as suggested by model simulations.     

Assessment of MTBE biodegradation in contaminated groundwater using C and C analysis: Field and laboratory microcosm studies

Radiolabelled assays and compound-specific stable isotope analysis (CSIA) were used to assess methyl tert-butyl ether (MTBE) biodegradation in an unleaded fuel plume in a UK chalk aquifer, both in the field and in laboratory microcosm experiments. The ¹⁴C-MTBE radiorespirometry studies demonstrated widespread potential for aerobic and anaerobic MTBE biodegradation in the aquifer. However, δ¹³C compositions of MTBE in groundwater samples from the plume showed no significant ¹³C enrichment that would indicate MTBE biodegradation at the field scale. Carbon isotope enrichment during MTBE biodegradation was assessed in the microcosms when dissolved O₂ was not limiting, compared with low in situ concentrations (2 mg/L) in the aquifer, and in the absence of O₂. The microcosm experiments showed ubiquitous potential for aerobic MTBE biodegradation in the aquifer within hundreds of days. Aerobic MTBE biodegradation in the microcosms produced an enrichment of 7‰ in the MTBE δ¹³C composition and an isotope enrichment factor (ε) of −1.53‰ when dissolved O₂ was not limiting. However, for the low dissolved O₂ concentration of up to 2 mg/L that characterizes most of the MTBE plume fringe, aerobic MTBE biodegradation produced an enrichment of 0.5-0.7‰, corresponding to an ε value of −0.22‰ to −0.24‰. No anaerobic MTBE biodegradation occurred under these experimental conditions. These results suggest the existence of a complex MTBE-biodegrading community in the aquifer, which may consist of different aerobic species competing for MTBE and dissolved O₂. Under low O₂ conditions, the lower fractionating species have been shown to govern overall MTBE C-isotope fractionation during biodegradation, confirming the results of previous laboratory experiments mixing pure cultures. This implies that significant aerobic MTBE biodegradation could occur under the low dissolved O₂ concentration that typifies the reactive fringe zone of MTBE plumes, without producing detectable changes in the MTBE δ¹³C composition. This observed insensitivity of C isotope enrichment to MTBE biodegradation could lead to significant underestimation of aerobic MTBE biodegradation at field scale, with an unnecessarily pessimistic performance assessment for natural attenuation. Site-specific C isotope enrichment factors are, therefore, required to reliably quantify MTBE biodegradation, which may limit CSIA as a tool for the in situ assessment of MTBE biodegradation in groundwater using only C isotopes.     

Simulation of the Hurricane Dennis storm surge and considerations for vertical resolution

A high-resolution storm surge model of Apalachee Bay in the northeastern Gulf of Mexico is developed using an unstructured grid finite-volume coastal ocean model (FVCOM). The model is applied to the case of Hurricane Dennis (July 2005). This storm caused underpredicted severe flooding of the Apalachee Bay coastal area and upriver inland communities. Accurate resolution of complicated geometry of the coastal region and waterways in the model reveals processes responsible for the unanticipated high storm tide in the area. Model results are validated with available observations of the storm tide. Model experiments suggest that during Dennis, excessive flooding in the coastal zone and the town of St. Marks, located up the St. Marks River, was caused by additive effects of coincident high tides (~10–15% of the total sea-level rise) and a propagating shelf wave (~30%) that added to the locally wind-generated surge. Wave setup, the biggest uncertainty, is estimated on the basis of empirical and analytical relations. The Dennis case is then used to test the sensitivity of the model solution to vertical discretization. A suite of model experiments is performed with varying numbers of vertical sigma (σ) levels, with different distribution of σ-levels within the water column and a varying bottom drag coefficient. The major finding is that the storm surge solution is more sensitive to resolution within the velocity shear zone at mid-depths compared to resolution of the upper and bottom layer or values of the bottom drag coefficient.     

The impact of natural versus anthropogenic aerosols on atmospheric circulation in the Community Atmosphere Model

The equilibrium response of atmospheric circulation to the direct radiative effects of natural or anthropogenic aerosols is investigated using the Community Atmosphere Model (CAM3) coupled to two different ocean boundary conditions: prescribed climatological sea surface temperatures (SSTs) and a slab ocean model. Anthropogenic and natural aerosols significantly affect the circulation but in nearly opposite ways, because anthropogenic aerosols tend to have a net local warming effect and natural aerosols a net cooling. Aerosol forcings shift the Intertropical Convergence Zone and alter the strength of the Hadley circulation as found in previous studies, but also affect the Hadley cell width. These effects are due to meridional gradients in warming caused by heterogeneous net heating, and are stronger with interactive SST. Aerosols also drive model responses at high latitudes, including polar near-surface warming by anthropogenic aerosols in summer and an Arctic Oscillation (AO)-type responses in winter: anthropogenic aerosols strengthen wintertime zonal wind near 60°N, weaken it near 30°N, warm the troposphere, cool the stratosphere, and reduce Arctic surface pressure, while natural aerosols produce nearly opposite changes. These responses are shown to be due to modulation of stratospheric wave-driving consistent with meridional forcing gradients in midlatitudes. They are more pronounced when SST is fixed, apparently because the contrast in land-ocean heating drives a predominantly wavenumber-2 response in the northern hemisphere which is more efficient in reaching the stratosphere, showing that zonal heating variations also affect this particular response. The results suggest that recent shifts from reflecting to absorbing aerosol types probably contributed to the observed decadal variations in tropical width and AO, although studies with more realistic temporal variations in forcing would be needed to quantify this contribution.     

Modeling the time-dependent response of the Asian summer monsoon to obliquity forcing in a coupled GCM: a PHASEMAP sensitivity experiment

To study the time-dependent response of the Asian summer monsoon to obliquity forcing, we analyze a 284,000-year long transient simulation produced by a fully coupled global climate model (GCM) using a new phase mapping (PHASEMAP) approach. Here we focus on understanding the phase response of monsoonal circulation to insolation forcing at the Earth-orbital obliquity band (41 Kyr). Our results show that the East Asian summer monsoon (EASM) can be divided into two geographic regions: the North East Asian summer monsoon (NEASM) and the South East Asian summer monsoon (SEASM). The Indian summer monsoon (ISM) and the SEASM are in phase at the obliquity band, strengthened with an increase in obliquity from Obliquity minima (Omin) to Obliquity maxima (Omax). The NEASM is out of phase with the ISM and SEASM, weakened with an increase in obliquity from Omin to Omax. We hypothesize that the inverse phase between the NEASM and the ISM at the obliquity band results from an ISM–NEASM teleconnection linked to the formation mechanism of the Bonin High.     

Consistent scale-dependency of future increases in hourly extreme precipitation in two convection-permitting climate models

Climate Dynamics - Convection-permitting models (CPMs) have been proven successful in simulating extreme precipitation statistics. However, when such models are used to study climate change,...     

Modeling Use of Space from Social Media Data Using a Biased Random Walker

Individuals and other entities move through space as a function of local characteristics of place, their internal behavioral models, and the topological structure of the underlying space. When a collection of locations (i.e. geotagged photos or other geotagged social media information) from a large number of individuals is assembled, it becomes possible to understand the interrelationship between the individuals and the space they occupy. This research systematically considers this interrelationship through an examination of the effect of the intersection of behavioral and spatial characteristics on individuals moving on street networks. The research illustrates how social media data, in combination with a biased random walker, can be used to understand and model the interaction of spatial structure and social‐environmental factors on influencing individuals' use of their environment. The biased walker offers a flexible approach to incorporate consideration of both social‐environmental and structural factors into a model and we demonstrate this through a case study wherein we are able to use the random walker to model the characteristics of Flickr users in New York City.     

Spring-summer temperatures since AD 1780 reconstructed from stable oxygen isotope ratios in white spruce tree-rings from the Mackenzie Delta,northwestern Canada

High-latitude δ¹⁸O archives deriving from meteoric water (e.g., tree-rings and ice-cores) can provide valuable information on past temperature variability, but stationarity of temperature signals in these archives depends on the stability of moisture source/trajectory and precipitation seasonality, both of which can be affected by atmospheric circulation changes. A tree-ring δ¹⁸O record (AD 1780–2003) from the Mackenzie Delta is evaluated as a temperature proxy based on linear regression diagnostics. The primary source of moisture for this region is the North Pacific and, thus, North Pacific atmospheric circulation variability could potentially affect the tree-ring δ¹⁸O-temperature signal. Over the instrumental period (AD 1892–2003), tree-ring δ¹⁸O explained 29 % of interannual variability in April–July minimum temperatures, and the explained variability increases substantially at lower-frequencies. A split-period calibration/verification analysis found the δ¹⁸O-temperature relation was time-stable, which supported a temperature reconstruction back to AD 1780. The stability of the δ¹⁸O-temperature signal indirectly implies the study region is insensitive to North Pacific circulation effects, since North Pacific circulation was not constant over the calibration period. Simulations from the NASA-GISS ModelE isotope-enabled general circulation model confirm that meteoric δ¹⁸O and precipitation seasonality in the study region are likely insensitive to North Pacific circulation effects, highlighting the paleoclimatic value of tree-ring and possibly other δ¹⁸O records from this region. Our δ¹⁸O-based temperature reconstruction is the first of its kind in northwestern North America, and one of few worldwide, and provides a long-term context for evaluating recent climate warming in the Mackenzie Delta region.     

Analysis of the present and future winter Pacific-North American teleconnection in the ECHAM5 global and RegCM3 regional climate models

We use the NCEP/NCAR Reanalysis (NCEP) and the MPI/ECHAM5 general circulation model to drive the RegCM3 regional climate model to assess the ability of the models to reproduce the spatiotemporal aspects of the Pacific-North American teleconnection (PNA) pattern. Composite anomalies of the NCEP-driven RegCM3 simulations for 1982–2000 indicate that the regional model is capable of accurately simulating the key features (500-hPa heights, surface temperature, and precipitation) of the positive and negative phases of the PNA with little loss of information in the downscaling process. The basic structure of the PNA is captured in both the ECHAM5 global and ECHAM5-driven RegCM3 simulations. The 1950–2000 ECHAM5 simulation displays similar temporal and spatial variability in the PNA index as that of NCEP; however, the magnitudes of the positive and negative phases are weaker than those of NCEP. The RegCM3 simulations clearly differentiate the climatology and associated anomalies of snow water equivalent and soil moisture of the positive and negative PNA phases. In the RegCM3 simulations of the future (2050–2100), changes in the location and extent of the Aleutian low and the continental high over North America alter the dominant flow patterns associated with positive and negative PNA modes. The future projections display a shift in the patterns of the relationship between the PNA and surface climate variables, which suggest the potential for changes in the PNA-related surface hydrology of North America.