Paleolimnological records of recent glacier recession in the Rwenzori Mountains,Uganda-D. R. Congo

The status of tropical glaciers is enormously important to our understanding of past, present, and future climate change, yet lack of continuous quantitative records of alpine glacier extent on the highest mountains of tropical East Africa prior to the 20th century has left the timing and drivers of recent glacier recession in the region equivocal. Here we investigate recent changes (the last 150–700 years) in lacustrine sedimentation, glacier extent, and biogeochemical processes in the Rwenzori Mountains (Uganda- Democratic Republic of Congo) by comparing sedimentological (organic and siliciclastic component determined by loss-on-ignition; LOI) and organic geochemical profiles (carbon and nitrogen abundance, ratio, and isotopic composition of sedimentary organic matter) from lakes occupying presently glaciated catchments against similar profiles from lakes located in catchments lacking glaciers. The siliciclastic content of sediments in the ‘glacial lakes’ significantly decreases towards the present, whereas ‘non-glacial lakes’ generally show weak trends in their siliciclastic content over time, demonstrating that changes in the siliciclastic content of glacial lake sediments primarily record fluctuations in glacier extent. Radiometric dating of our sediment cores indicates that prior to their late 19th-century recession Rwenzori glaciers stood at expanded ‘Little Ice Age’ positions for several centuries under a regionally dry climate regime, and that recession was underway by 1870 AD, during a regionally wet episode. These findings suggest that the influence of late 19th century reductions in precipitation in triggering Rwenzori glacier recession is weaker than previously thought. Our organic geochemical data indicate that glacier retreat has significantly affected carbon cycling in Afroalpine lakes, but trends in aquatic ecosystem functioning are variable among lakes and require more detailed analysis.     

Comparison of polar mesospheric cloud measurements from the Cloud Imaging and Particle Size experiment and the solar backscatter ultraviolet instrument in 2007

We compare measurements from the Aeronomy of Ice in the Mesosphere (AIM) Cloud Imaging and Particle Size (CIPS) experiment to the NOAA-17 solar backscatter ultraviolet (SBUV/2) instrument during the 2007 Northern Hemisphere polar mesospheric cloud (PMC) season. Daily average Rayleigh scattering albedos determined from identical footprints from the CIPS nadir camera and SBUV/2 agree to better than ～5% throughout the season. Average PMC brightness values derived from the two instruments agree to within ±10%. PMC occurrence frequencies are on average ～5% to nearly a factor of two higher in CIPS, depending on latitude. Agreement is best at high latitudes where clouds are brighter and more frequent. The comparisons indicate that AIM CIPS data are valid for scientific analyses. They also show that CIPS measurements can be linked to the long time series of SBUV/2 data to investigate long-term variability in PMCs.     

The seasonal cycle in coupled ocean-atmosphere general circulation models

We examine the seasonal cycle of near-surface air temperature simulated by 17 coupled ocean-atmosphere general circulation models participating in the Coupled Model Intercomparison Project (CMIP). Nine of the models use ad hoc “flux adjustment” at the ocean surface to bring model simulations close to observations of the present-day climate. We group flux-adjusted and non-flux-adjusted models separately and examine the behavior of each class. When averaged over all of the flux-adjusted model simulations, near-surface air temperature falls within 2?K of observed values over the oceans. The corresponding average over non-flux-adjusted models shows errors up to ～6?K in extensive ocean areas. Flux adjustments are not directly applied over land, and near-surface land temperature errors are substantial in the average over flux-adjusted models, which systematically underestimates (by ～5?K) temperature in areas of elevated terrain. The corresponding average over non-flux-adjusted models forms a similar error pattern (with somewhat increased amplitude) over land. We use the temperature difference between July and January to measure seasonal cycle amplitude. Zonal means of this quantity from the individual flux-adjusted models form a fairly tight cluster (all within ～30% of the mean) centered on the observed values. The non-flux-adjusted models perform nearly as well at most latitudes. In Southern Ocean mid-latitudes, however, the non-flux-adjusted models overestimate the magnitude of January-minus-July temperature differences by ～5?K due to an overestimate of summer (January) near-surface temperature. This error is common to five of the eight non-flux-adjusted models. Also, over Northern Hemisphere mid-latitude land areas, zonal mean differences between July and January temperatures simulated by the non-flux-adjusted models show a greater spread (positive and negative) about observed values than results from the flux-adjusted models. Elsewhere, differences between the two classes of models are less obvious. At no latitude is the zonal mean difference between averages over the two classes of models greater than the standard deviation over models. The ability of coupled GCMs to simulate a reasonable seasonal cycle is a necessary condition for confidence in their prediction of long-term climatic changes (such as global warming), but it is not a sufficient condition unless the seasonal cycle and long-term changes involve similar climatic processes. To test this possible connection, we compare seasonal cycle amplitude with equilibrium warming under doubled atmospheric carbon dioxide for the models in our data base. A small but positive correlation exists between these two quantities. This result is predicted by a simple conceptual model of the climate system, and it is consistent with other modeling experience, which indicates that the seasonal cycle depends only weakly on climate sensitivity.     

A test of the precursory accelerating moment release model on some recent New Zealand earthquakes

The proposal that the moment release rate increases in a systematic way in a large region around a forthcoming large earthquake is tested using three recent, large New Zealand events. The three events, 1993–1995, magnitudes 6.7–7.0, occurred in varied tectonic settings. For all three events, a circular precursory region can be found such that the moment release rate of the included seismicity is modelled significantly better by the proposed accelerating model than by a linear moment release model, although in one case the result is dubious. The 'best' such regions have radii from 122 to 167 km, roughly in accord with previous observations world-wide, but are offset by 50–60 km from the associated main shock epicentre. A grid-search procedure is used to test whether these three earthquakes could have been forecast using the accelerating moment release model. For two of the earthquakes the result is positive in terms of location, but the main shock times are only loosely constrained.     

Mid-holocene climate change in Lake Bosumtwi, Ghana

Lake Bosumtwi is one of the most widely studied palaeoclimate archives in West Africa. Results from numerous AMS ¹⁴C dates of samples from four piston cores from Lake Bosumtwi show that an abrupt sedimentary transition from a mid-Holocene sapropel to calcareous laminated muds occurred at about 3200 cal yr B.P. High-resolution analyses of the nitrogen isotopic composition of organic matter across this transition confirm its abrupt nature, and suggest that the change may signal a step toward increased aridity and intensified surface winds that affected western equatorial Africa from Ghana to the Congo basin. Northern and Eastern Africa experienced a similar abrupt shift toward aridity during the late Holocene, but at about 5000 cal yr B.P., a difference in timing that illustrates the regional nature of climate changes during the Holocene and the importance of feedback mechanisms in regulating Holocene climate variability. Furthermore, an abrupt change at about 3000 cal yr B.P. occurs at several sites adjacent to the tropical and subtropical Atlantic, which may hint at major changes in the surface temperatures of the tropical Atlantic and/or Pacific at this time.     

Terrain-influenced local wind forecasting for the Sasebo typhoon haven: Improved empirical techniques

Although the Sasebo, Japan harbor is usually a “typhoon haven” from tropical cyclone (TC) winds due to terrain-blocking effects, in rare cases damaging winds occur that may be attributed to terrain channeling. An empirical parametric model technique is developed and tested that includes consideration of the TC wind structure, land frictional effects, and terrain influences affecting the maximum wind speeds in the harbor when TCs pass within 200 nautical miles of Sasebo. The terrain influence is represented by two sets of wind direction-dependent acceleration factors. The first set, which is directly from the ratio of the local wind to the adjusted parametric wind for TCs passages during 2003–2010, provides mean values that represent the terrain blocking and channeling effects, but the variability with wind direction may be suspect. The second set derived from a large sample of reanalysis winds not limited to TCs has better variability properties, but is not easily related to just the TC passages. A new nomogram modified to include TC wind structure has higher estimates of Sasebo sustained winds for some TC tracks that may be related to terrain influences, but is limited due to the number of TC structure estimates in the developmental sample. These empirical models have the advantage of ease and low cost for future use in also estimating the combined uncertainty in the local winds in Sasebo harbor due to TCs.     

Advances in understanding the “Perfect Monsoon-influenced Typhoon”: Summary from International Conference on Typhoon Morakot (2009)

Typhoon Morakot (2009) produced 2855 mm of rain and was the deadliest typhoon to impact Taiwan with 619 deaths and 76 missing persons, including a landslide that wiped out an entire village. While Morakot did not exceed the heaviest 24-h rain record, the combination of heavy rain and long duration that led to the record accumulation is attributed to the southwest summer monsoon influence on the typhoon. Thus, a special combination of factors was involved in the Morakot disaster: (i) Strong southwesterly monsoon winds; (ii) Convergence between the typhoon circulation and monsoon flow to form an east-west oriented convective band over the Taiwan Strait that was quasi-stationary and long-lasting; (iii) A typhoon in a specific location relative to the Central Mountain Range and moving slowly; and (iv) Steep topography that provided rapid lifting of the moist air stream. The contributions of each of these four factors in leading to the Morakot disaster are reviewed primarily based on new research presented at the International Conference on Typhoon Morakot (2009). Historical data sets, new Doppler radar observations, and numerical modeling have advanced the understanding of the special conditions of monsoon-influenced typhoons such as Morakot. This research is also leading to modifications of existing and development of new forecasting tools. Gaps in scientific understanding, limits to the predictability, and requirements for advanced forecast guidance tools are described that are challenges to improved warnings of these extreme precipitation and flooding events in monsoon-influenced typhoons.     

An observational program for a unified optical-radio reference frame

An observational pilot program is described which provides a first step towards the construction of an inertial extragalactic reference frame from optical and radio measurements. Candidate objects will be selected from extragalactic compact radio sources which display optical counterparts. Galactic radio stars will be secondary reference objects. The reference frame will be defined by the radio positions of the extragalactic objects. Based on a coordinated effort in both hemispheres, a global homogeneous net of about 400 quasars and 100 radio stars is expected to result from a five year campaign.