Analysis of Historical and Projected Future Climate of Mali, West African Sahel

The West African country of Mali experienced devastating drought from the late 1960s to the mid 1980s, followed by partial rainfall recovery that remains below pre-1960s climatology. Here, we examine the historical and projected temperature and rainfall variability across the Malian Sahel in an effort to assist future planning for food security. Particular emphasis is placed on clarifying the teleconnection between sea-surface temperatures (SST) expressed by the Atlantic Multidecadal Oscillation (AMO) and the West African monsoon (WAM). Using gridded observations and reanalysis, we show that cool/wet (1950-1967) and warm/dry (1980-1997) end-member climates over Mali correspond to warm and cool phases of the AMO, respectively, with associated atmospheric patterns consistent with negative (slow winds, shallow poleward pressure gradient) and positive (fast winds, steep poleward gradient) modes of the North Atlantic Oscillation (NAO). These opposing climate regimes (AMO-warm/NAO-negative and AMO-cool/NAO-positive) are coupled to strength of the Sahara Low and latitudinal position of the Intertropical Convergence Zone (ITCZ), thereby accounting for changes in strength of the WAM. A case is made that multi-decade North Atlantic SST variability arises naturally from volcanic forcing and resultant changes in strength and position of the westerly winds. However, greenhouse-gas warming, stratospheric ozone depletion, and Arctic sea-ice loss is likely modifying the system behavior. In an examination of CMIP5 general circulation model (GCM) output for Mali, we find that temperature and precipitation in historical simulations fail to validate against observations, and notably do not show multi-decade variability. Atmospheric circulation across the North Atlantic is furthermore anomalously strong in CMIP5 due to an unrealistically steep pressure gradient between Azores and Iceland. These shortcomings undermine the fidelity and meaning of CMIP5 future rainfall projections across the western Sahel. We conclude here that downscale studies should target reanalysis, and investigate extreme years (e.g., cool/wet or hot/dry) as future climate analogs rather than depend on CMIP5. In all from this work, we suggest five plausible future climate scenarios for 2030-2050: 1) standard CMIP5 projection of 2 °C warming with slight rainfall decline; 2) Warming > 1 °C with rainfall remaining at present norm, or increasingly slightly due to poleward displacement of the ITCZ; 3) Warming > 1 °C with diminished rainfall or drought from southward displacement of the ITCZ; 4) Warming > 1 °C with onset of severe drought arising from renewal of high volcanic activity, and subsequent development of strong NAO-positive circulation and cool-AMO sea-surface temperature distribution; 5) Abrupt climate shift in response to collapse of summer Arctic sea ice, wherein any of the scenarios above could develop within a decade

A trip down the Alton Bay flow line

Geology of the coastal lowlands, Boston to Kennebunk, Maine: The 76th annual meeting New England Intercollegiate Geological Conference, Danvers, Massachusetts, October 12-14, 1984: Trip C-

A New Technique for Firn Grain-Size Measurement Using Sem Image Analysis

Firn microstructure is accurately characterized using images obtained from scanning electron microscopy (SEM). Visibly etched grain boundaries within images are used to create a skeleton outline of the microstructure. A pixel-counting utility is applied to the outline to determine grain area. Firn grain sizes calculated using the technique described here are compared to those calculated using the techniques of Gow (1969) and Gay and Weiss (1999) on samples of the same material, and are found to be substantially smaller. The differences in grain size between the techniques are attributed to sampling deficiencies (e.g. the inclusion of pore filler in the grain area) in earlier methods. The new technique offers the advantages of greater accuracy and the ability to determine individual components of the microstructure (grain and pore), which have important applications in ice-core analyses. The new method is validated by calculating activation energies of grain boundary diffusion using predicted values based on the ratio of grain-size measurements between the new and existing techniques. The resulting activation energy falls within the range of values previously reported for firn/ice

Antarctic Climate Change and the Environment

The Antarctic climate system varies on timescales from orbital, through millennial to sub-annual, and is closely coupled to other parts of the global climate system. We review these variations from the perspective of the geological and glaciological records and the recent historical period from which we have instrumental data (the last 50 years). We consider their consequences for the biosphere, and show how the latest numerical models project changes into the future, taking into account human actions in the form of the release of greenhouse gases and chlorofluorocarbons into the atmosphere. In doing so, we provide an essential Southern Hemisphere companion to the Arctic Climate Impact Assessment

A multi-disciplinary perspective on climate model evaluation for Antarctica

A workshop was organized by Antarctic Climate 21 (AntClim21), with the topic 'evaluation of climate models' representation of Antarctic climate from the perspective of long-term twenty-first-century climate change.' The suggested approach for evaluating whether climate models over- or underestimate the effects of ozone depletion is to diagnose simulated historical trends in lower-stratospheric temperature and compare these to observational estimates. With regard to more regional changes over Antarctica, such as West Antarctic warming, the simulation of teleconnection patterns to the tropical Pacific was highlighted. To improve the evaluation of low-frequency variability and trends in climate models, the use and development of approaches to emulate ice-core proxies in models was recommended. It is recommended that effort be put into improving datasets of ice thickness, motion, and composition to allow for a more complete evaluation of sea ice in climate models. One process that was highlighted in particular is the representation of Antarctic clouds and resulting precipitation. It is recommended that increased effort be put into observations of clouds over Antarctica, such as the use of instruments that can detect cloud-base height or the use of remote sensing resources

On the timing and mechanism of millennial-scale climate variability during the last glacial cycle

The demonstration that natural climate variability during the last glacial cycle shifted rapidly between remarkable extremes has dramatically revised the understanding of climate change. To further advance our understanding, research continues into the timings, geographic distribution, and nature of the millennial-scale climate extremes, and into the mechanisms for intra- and inter-hemispheric transmission of variability through the climate/ocean system. Complementing the traditional definition of the timings of millennial-scale climate variability from ice-core d18O records, we here further narrow down the temporal constraints by determining statistically significant anomalies in the major ion series of the GISP2 ice core. This exercise offers an objective definition of the timing of climatic anomalies in Northern Hemisphere palaeoclimate proxy records of the last 110,000 years that significantly improves the potential for inter-calibration of ‘ice-core tuned’ chronostratigraphies. We then present a process-oriented synthesis of proxy records from the Northern Hemisphere. This leads to a conclusion that the Dansgaard-Oeschger (D-O) style fluctuations in these records are (virtually) in phase, since all fall within a clear (atmospheric) pattern of concerted relative dominance shifts between polar/westerly dominated winter-type conditions and tropical/monsoon dominated summer-type conditions. Finally, we speculate on a monsoon-related mechanism that could help explain the anomalously long duration of D-O interstadials 12, 8, and 1, which coincided with cooling trends in Antarctic records