Collaborative Research: Did the Laurentine Ice Sheet Control Abrupt Climate Change?

This is a collaborative project with the University of Maine and Ohio State University. The Principal Investigators will model the late glacial Laurentide Ice Sheet from near steady-state equilibrium at - 25,000 BP (years before present), through reversible stadial/interstadial transitions associated with Laurentide iceberg outbursts (Heinrich events 2 and 1), and across the threshold of irreversible Laurentide collapse after the last iceberg outburst at - 1 1,000 BP (Heinrich event 0). The goals are to determine if ice-sheet changes could have triggered climate changes by abrupt ice sheet change and to investigate the structure of these changes. The Principal Investigators will isolate mechanisms of abrupt change over hundreds of years in the ice sheet that are large enough to trigger climate changes captured as time snapshots by coupled global and regional atmospheric climate models. Specific modeling tasks are: 1) to provide the climate settings surrounding the Laurentide Ice Sheet at snapshots of time during this late glacial period. This includes the wind field over the ice sheet, proglacial lakes along the border, the fine-resolution mesoscale climate of North America, and global climate; 2) to provide the basal boundary conditions that, together with the internal flow and temperature fields, are used to calculate the basal mass balance. This includes the pattern of basal temperatures, melting and freezing rates, and the associated subglacial hydrology; 3) to model the Laurentide Ice Sheet basal thermal, hydrological, and mechanical conditions within the imposed and basal boundary constraints for the chosen timeframe; and 4) to determine whether modeling will isolate mechanisms of abrupt change that allow rapid advance and retreat of Laurentide ice, with areal, elevation, and volume changes large enough to trigger climate changes that are captured by our snapshots of regional and global climate.This project has significance for educational outreach and the possible behavior of present-day ice sheets. The education outreach program will be interactive with high school students. They will be able to manipulate the major variables so that they can view three-dimensional computer simulations of how the Laurentide Ice Sheet responds to each variable. This program will be disseminated on the world-wide web. If fluctuations in the Laurentide Ice Sheet triggered climate changes, then the possibility exists that present-day ice sheets covering Greenland and Antarctica could trigger similar climate changes, with major social, economic, and political consequences. A way to assess this possibility is to understand the internal instability mechanisms that could have caused abrupt changes in Laurentide ice extent, and to tie them firmly to known late glacial climate changes

Atmospheric Scaling of Cosmogenic Nuclide Production: Climate Effect

Absorption of cosmic rays by atmospheric mass varies temporally due to a redistribution of atmospheric pressure by ice sheets during glaciations, the compression and expansion of the atmosphere due to cooling and warming, and changes in katabatic winds near large ice masses. These atmospheric processes can result in changes in production rates of cosmogenic nuclides which, when integrated over long exposure durations may result in 0% to \u3e5% adjustments in site production rates depending on location. Combining a CCM3 model with imbedded ice sheets for 20 ka, we show that production rates changes (relative to today) are greatest at high elevations (6–7% at 5 km altitude) due to atmospheric compression from decreased temperature. Production rates at specific times for sites near ice sheet margins can be reduced more than 10% due to a combination of katabatic winds draining off the ice sheet margins and atmospheric cooling. Nunatak settings may be significantly affected by the climate effect due to persistent glacial atmospheric conditions. Atmospheric variability may explain some of the disparities among cosmogenic nuclide production rate calibrations

Ground-based detection of sprites and their parent lightning flashes over Africa during the 2006 AMMA campaign

Sprites have been detected in video camera observations from Niger over mesoscale convective systems in Nigeria during the 2006 AMMA (African Monsoon Multidisciplinary Analysis) campaign. The parent lightning flashes have been detected by multiple Extremely Low Frequency (ELF) receiving stations worldwide. The recorded charge moments of the parent lightning flashes are often in excellent agreement between different receiving sites, and are furthermore consistent with conventional dielectric breakdown in the mesosphere as the origin of the sprites. Analysis of the polarization of the horizontal magnetic field at the distant receivers provides evidence that the departure from linear magnetic polarization at ELF is caused primarily by the day–night asymmetry of the Earth–ionosphere cavity. Copyright © 2009 Royal Meteorological SocietyPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69175/1/489_ftp.pd

Reconstruction of sea surface temperature variations in the Arabian Sea over the last 23 kyr using organic proxies (TEX86 and U37K')

Two sediment cores from the western Arabian Sea, NIOP905 and 74KL, were analyzed to determine sea surface temperature (SST) variations over the last 23 kyr. Two organic molecular SST proxies were used, the well-established U37K' based on long-chain unsaturated ketones synthesized by haptophyte algae and the newly proposed TEX86 derived from the membrane lipids of Crenarchaeota. Comparison of NIOP905 and 74KL core top data with present-day SST (0-10 m) values indicates that both proxies yield temperatures similar to local annual mean SSTs. However, TEX86 and U37K' SST down-core records derived from the same cores differ in magnitude and phasing. The alkenone SST record of NIOP905 shows small changes in SST (∼0.5°C) over the last 23 kyr, while that of core 74KL shows a ∼2°C increase from the Last Glacial Maximum (LGM) (23-19 calendar (cal) kyr B.P.) through the Holocene (the last 11.5 cal kyr B.P.) synchronous with changes in the Northern Hemisphere. In contrast, the TEX86 records of both cores show a large increase in SST from 22°-23°C in the LGM to 28°-30°C during Termination I (19-11.5 cal kyr B.P.), decreasing to present-day annual means of ∼26°C. A cold phase between 14.5 and 12 cal kyr B.P. that may correspond to the Antarctic cold reversal is also observed. This implies a Southern Hemisphere control on tropical SST reconstructed by the TEX86, possibly related to SW monsoon. Our results suggest that the application of both TEX86 and U37K' give different but complementary information on SST developments in past marine environments

Convective Squalls over the Eastern Equatorial Atlantic

The Congo Basin and the adjacent equatorial eastern Atlantic are among the most active regions of the world in terms of intense deep moist convection, leading to frequent lightning and severe squalls. Studying the dynamics and climatology of this convection is difficult due to a very sparse operational network of ground-based observations. Here, a detailed analysis of recently available high temporal resolution meteorological observations from three oil platforms off the coast of Angola spanning the three wet seasons from 2006/07 to 2008/09 is presented. The annual cycle of squall days as identified from wind data closely follows that of convective available potential energy (CAPE) and therefore mirrors the cycle of wet and dry seasons. The diurnal cycle of squall occurrence varies from station to station, most likely related to local features such as coastlines and orography, which control the initiation of storms. An attempt to classify squalls based on the time evolution of the station meteorology and satellite imagery suggests that microbursts account for at least one-third of the strong gusts, while mesoscale squall lines appear to be quite rare. On a daily basis the probability of squall occurrence increases with increasing values ofCAPE, downdraft CAPE, and 925-700-hPa wind shear, and decreases for high convective inhibition, all calculated from vertical profiles of temperature and humidity at the nearest grid point in the NCEP-NCAR and ECMWF reanalysis datasets. Both the climatological results and the stability indices can be used for local forecasting to avoid squalls impacting on operations on the offshore platforms

LGM Summer Climate on the Southern Margin of the Laurentide Ice Sheet: Wet Or Dry?

Regional climate simulations are conducted using the Polar fifth-generation Pennsylvania State University (PSU)-NCAR Mesoscale Model (MM5) with a 60-km horizontal resolution domain over North America to explore the summer climate of the Last Glacial Maximum (LGM: 21 000 calendar years ago), when much of the continent was covered by the Laurentide Ice Sheet (LIS). Output from a tailored NCAR Community Climate Model version 3 (CCM3) simulation of the LGM climate is used to provide the initial and lateral boundary conditions for Polar MM5. LGM boundary conditions include continental ice sheets, appropriate orbital forcing, reduced CO2 concentration, paleovegetation, modified sea surface temperatures, and lowered sea level. The simulated LGM summer climate is characterized by a pronounced low-level thermal gradient along the southern margin of the LIS resulting from the juxtaposition of the cold ice sheet and adjacent warm ice-free land surface. This sharp thermal gradient anchors the midtropospheric jet stream and facilitates the development of synoptic cyclones that track over the ice sheet, some of which produce copious liquid precipitation along and south of the LIS terminus. Precipitation on the southern margin is orographically enhanced as moist southerly low-level flow (resembling a contemporary, Great Plains low-level jet configuration) in advance of the cyclone is drawn up the ice sheet slope. Composites of wet and dry periods on the LIS southern margin illustrate two distinctly different atmospheric flow regimes. Given the episodic nature of the summer rain events, it may be possible to reconcile the model depiction of wet conditions on the LIS southern margin during the LGM summer with the widely accepted interpretation of aridity across the Great Plains based on geological proxy evidence

Polar MM5 Simulations of the Winter Climate of the Laurentide Ice Sheet at the LGM

Optimized regional climate simulations are conducted using the Polar MM5, a version of the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5), with a 60-km horizontal resolution domain over North America during the Last Glacial Maximum (LGM, 21 000 calendar years ago), when much of the continent was covered by the Laurentide Ice Sheet (LIS). The objective is to describe the LGM annual cycle at high spatial resolution with an emphasis on the winter atmospheric circulation. Output from a tailored NCAR Community Climate Model version 3 (CCM3) simulation of the LGM climate is used to provide the initial and lateral boundary conditions for Polar MM5. LGM boundary conditions include continental ice sheets, appropriate orbital forcing, reduced CO2 concentration, paleovegetation, modified sea surface temperatures, and lowered sea level. Polar MM5 produces a substantially different atmospheric response to the LGM boundary conditions than CCM3 and other recent GCM simulations. In particular, from November to April the upper-level flow is split around a blocking anticyclone over the LIS, with a northern branch over the Canadian Arctic and a southern branch impacting southern North America. The split flow pattern is most pronounced in January and transitions into a single, consolidated jet stream that migrates northward over the LIS during summer. Sensitivity experiments indicate that the winter split flow in Polar MM5 is primarily due to mechanical forcing by LIS, although model physics and resolution also contribute to the simulated flow configuration. Polar MM5 LGM results are generally consistent with proxy climate estimates in the western United States, Alaska, and the Canadian Arctic and may help resolve some long-standing discrepancies between proxy data and previous simulations of the LGM climate