The interaction of the Madden-Julian Oscillation with the Maritime Continent in a GCM

Previous studies using the Hadley Centre coupled model (HadCM3) have shown that the islands of the Maritime Continent act as an unrealistic block to the eastward propagation of the Madden-Julian Oscillation (MJO). This blocking effect is investigated using a simplified, aqua-planet version of this GCM, with various idealized configurations of the Maritime Continent islands placed on the equator, and an MJO-like convective signal forced by a propagating sea-surface temperature anomaly dipole. Results suggest that it is the orography of the islands, rather than the presence of the islands themselves, which results in the blocking of the MJO. Although the peak elevation of the orography in the GCM is very much lower than in reality, it appears to act as effective block to the eastward propagation of the low-level Kelvin wave signal which accompanies the MJO. In particular, the representation of Sumatra in the GCM, as a north-south oriented ridge straddling the equator, seems to be particularly effective at blocking the Kelvin wave signal, which in a full GCM would result in the weakening or complete extinction of the MJO signal to the east of the Maritime Continent.

The sensitivity of the tropical circulation and Maritime Continent precipitation to climate model resolution

The dependence of the annual mean tropical precipitation on horizontal resolution is investigated in the atmospheric version of the Hadley Centre General Environment Model (HadGEM1). Reducing the grid spacing from about 350 km to 110 km improves the precipitation distribution in most of the tropics. In particular, characteristic dry biases over South and Southeast Asia including the Maritime Continent as well as wet biases over the western tropical oceans are reduced. The annual-mean precipitation bias is reduced by about one third over the Maritime Continent and the neighbouring ocean basins associated with it via the Walker circulation. Sensitivity experiments show that much of the improvement with resolution in the Maritime Continent region is due to the specification of better resolved surface boundary conditions (land fraction, soil and vegetation parameters) at the higher resolution. It is shown that in particular the formulation of the coastal tiling scheme may cause resolution sensitivity of the mean simulated climate. The improvement in the tropical mean precipitation in this region is not primarily associated with the better representation of orography at the higher resolution, nor with changes in the eddy transport of moisture. Sizeable sensitivity to changes in the surface fields may be one of the reasons for the large variation of the mean tropical precipitation distribution seen across climate models.

Propagation of the Madden-Julian Oscillation through the Maritime Continent and scale interaction with the diurnal cycle of precipitation

The convectively active part of the Madden-Julian Oscillation (MJO) propagates eastward through the warm pool, from the Indian Ocean through the Maritime Continent (the Indonesian archipelago) to the western Pacific. The Maritime Continent's complex topography means the exact nature of the MJO propagation through this region is unclear. Model simulations of the MJO are often poor over the region, leading to local errors in latent heat release and global errors in medium-range weather prediction and climate simulation. Using 14 northern winters of TRMM satellite data it is shown that, where the mean diurnal cycle of precipitation is strong, 80% of the MJO precipitation signal in the Maritime Continent is accounted for by changes in the amplitude of the diurnal cycle. Additionally, the relationship between outgoing long-wave radiation (OLR) and precipitation is weakened here, such that OLR is no longer a reliable proxy for precipitation. The canonical view of the MJO as the smooth eastward propagation of a large-scale precipitation envelope also breaks down over the islands of the Maritime Continent. Instead, a vanguard of precipitation (anomalies of 2.5 mm day^-1 over 10^6 km^2) jumps ahead of the main body by approximately 6 days or 2000 km. Hence, there can be enhanced precipitation over Sumatra, Borneo or New Guinea when the large-scale MJO envelope over the surrounding ocean is one of suppressed precipitation. This behaviour can be accommodated into existing MJO theories. Frictional and topographic moisture convergence and relatively clear skies ahead of the main convective envelope combine with the low thermal inertia of the islands, to allow a rapid response in the diurnal cycle which rectifies onto the lower-frequency MJO. Hence, accurate representations of the diurnal cycle and its scale interaction appear to be necessary for models to simulate the MJO successfully.

The Madden-Julian Oscillation and the diurnal cycle over the Maritime Continent: scale interactions and modelling

The Maritime Continent archipelago, situated on the equator at 95-165E, has the strongest land-based precipitation on Earth. The latent heat release associated with the rainfall affects the atmospheric circulation throughout the tropics and into the extra-tropics. The greatest source of variability in precipitation is the diurnal cycle. The archipelago is within the convective region of the Madden-Julian Oscillation (MJO), which provides the greatest variability on intra-seasonal time scales: large-scale (∼10^7 km^2) active and suppressed convective envelopes propagate slowly (∼5 m s^-1) eastwards between the Indian and Pacific Oceans. High-resolution satellite data show that a strong diurnal cycle is triggered to the east of the advancing MJO envelope, leading the active MJO by one-eighth of an MJO cycle (∼6 days). Where the diurnal cycle is strong its modulation accounts for 81% of the variability in MJO precipitation. Over land this determines the structure of the diagnosed MJO. This is consistent with the equatorial wave dynamics in existing theories of MJO propagation. The MJO also affects the speed of gravity waves propagating offshore from the Maritime Continent islands. This is largely consistent with changes in static stability during the MJO cycle. The MJO and its interaction with the diurnal cycle are investigated in HiGEM, a high-resolution coupled model. Unlike many models, HiGEM represents the MJO well with eastward-propagating variability on intra-seasonal time scales at the correct zonal wavenumber, although the inter-tropical convergence zone's precipitation peaks strongly at the wrong time, interrupting the MJO's spatial structure. However, the modelled diurnal cycle is too weak and its phase is too early over land. The modulation of the diurnal amplitude by the MJO is also too weak and accounts for only 51% of the variability in MJO precipitation. Implications for forecasting and possible causes of the model errors are discussed, and further modelling studies are proposed.

Scale interactions between the MJO and the western maritime continent

State-of-the-art regional climate model simulations that are able to resolve key mesoscale circulations are used, for the first time, to understand the interaction between the large-scale convective environment of the MJO and processes governing the strong diurnal cycle over the islands of the Maritime Continent (MC). Convection is sustained in the late afternoon just inland of the coasts due to sea breeze convergence. Previous work has shown that the variability in MC rainfall associated with the MJO is manifested in changes to this diurnal cycle; land-based rainfall peaks before the active convective envelope of the MJO reaches the MC, whereas oceanic rainfall rates peak whilst the active envelope resides over the region. The model simulations show that the main controls on oceanic MC rainfall in the early active MJO phases are the large-scale environment and atmospheric stability, followed by high oceanic latent heat flux forced by high near-surface winds in the later active MJO phases. Over land, rainfall peaks before the main convective envelope arrives (in agreement with observations), even though the large-scale convective environment is only moderately favourable for convection. The causes of this early rainfall peak are convective triggers from land-sea breeze circulations that are strong due to high surface insolation and surface heating. During the peak MJO phases cloud cover increases and surface insolation decreases, which weakens the strength of the mesoscale circulations and reduces land-based rainfall, even though the large-scale environment remains favourable for convection at this time. Hence, scale interactions are an essential part of the MJO transition across the MC.

Reflections: 50 Years of Medieval Archaeology, 1957-2007

This volume celebrates the 50th anniversary of the Society for Medieval Archaeology (established in 1957), presenting reflections on the history, development and future prospects of the discipline. The papers are drawn from a series of conferences and workshops that took place in 2007-2008, in addition to a number of contributions that were commissioned especially for the volume. They range from personal commentaries on the history of the Society and the growth of the subject, to historiographical, regional and thematic overviews of major trends in the evolution and current practice of medieval archaeology in Britain. Critical overviews are presented of the archaeology of medieval landscapes, buildings and material culture; new developments in the scientific study of medieval health, diet and materials; and innovations in social approaches to medieval archaeology. A series of papers on southern Europe provide a comparative perspective, featuring overviews on medieval archaeology in Italy, Spain and southeastern Europe.

Results from the Covered Catchment Experiment at Gårdsjön, Sweden, after ten years of clean precipitation treatment

The Covered Catchment Experiment at Gordsjon is a large scale forest ecosystem manipulation, where acid precipitation was intercepted by a 7000 m(2) plastic roof and replaced by 'clean precipitation' sprinkled below the roof for ten years between 1991 and 2001. The treatment resulted in a strong positive response of runoff quality. The runoff sulphate, inorganic aluminium and base cations decreased, while there was a strong increase in runoff ANC and a moderate increase in pH. The runoff continued to improve over the whole duration of the experiment. The achieved quality was, however, after ten years still considerably worse than estimated pre-industrial runoff at the site. Stable isotopes of sulphur were analysed to study the soil sulphur cycling. At the initial years of the experiment, the desorption of SO4 from the mineral soil appeared to control the runoff SO4 concentration. However, as the experiment proceeded, there was growing evidence that net mineralisation of soil organic sulphur in the humus layer was an additional source of SO4 in runoff. This might provide a challenge to current acidification models. The experiment convincingly demonstrated on a catchment scale, that reduction in acid deposition causes an immediate improvement of surface water quality even at heavily acidified sites. The improvement of the runoff appeared to be largely a result of cation exchange processes in the soil due to decreasing concentrations of the soil solution, while any potential change in soil base saturation seemed to be less important for the runoff chemistry over the short time period of one decade. These findings should be considered when interpreting and extrapolating regional trends in surface water chemistry to the terrestrial parts of ecosystems.

Stable isotope evidence for 1500 years of human diet at the city of York, UK

We present here the results of a large-scale diachronic palaeodietary (carbon and nitrogen isotopic measurements of bone collagen) study of humans and animals from a single site, the city of York (U.K.) dating from the Roman period to the early 19th century The human sample comprises 313 burials from the cemeteries of Trentholme Drive and Blossom Street (Roman), Belle Vue House (Anglo-Saxon), Fishergate (High and Later Medieval), and All Saints, Pavement (Later and Post-Medieval). In addition, 145 samples of mammal, fish and bird bone from the sites of Tanner Row and Fishergate were analyzed. The isotope data suggest dietary variation between all archaeological periods, although the most significant change was the introduction of significant quantities of marine foods in the Medieval periods. These are first evident in the diet of a small group of individuals from the High Medieval cemetery at Fishergate, although they were consumed almost universally in the following periods. The human isotope values are also remarkable due to unusually elevated delta N-15 ratios that are not sufficiently explained by the comparably small enrichment in C-13 that accompanies them. We discuss the possible reasons behind this and the archaeological significance of the data set.

Metrics for solar wind prediction models: Comparison of empirical, hybrid, and physics-based schemes with 8 years of L1 observations

Space weather effects on technological systems originate with energy carried from the Sun to the terrestrial environment by the solar wind. In this study, we present results of modeling of solar corona-heliosphere processes to predict solar wind conditions at the L1 Lagrangian point upstream of Earth. In particular we calculate performance metrics for (1) empirical, (2) hybrid empirical/physics-based, and (3) full physics-based coupled corona-heliosphere models over an 8-year period (1995–2002). L1 measurements of the radial solar wind speed are the primary basis for validation of the coronal and heliosphere models studied, though other solar wind parameters are also considered. The models are from the Center for Integrated Space-Weather Modeling (CISM) which has developed a coupled model of the whole Sun-to-Earth system, from the solar photosphere to the terrestrial thermosphere. Simple point-by-point analysis techniques, such as mean-square-error and correlation coefficients, indicate that the empirical coronal-heliosphere model currently gives the best forecast of solar wind speed at 1 AU. A more detailed analysis shows that errors in the physics-based models are predominately the result of small timing offsets to solar wind structures and that the large-scale features of the solar wind are actually well modeled. We suggest that additional “tuning” of the coupling between the coronal and heliosphere models could lead to a significant improvement of their accuracy. Furthermore, we note that the physics-based models accurately capture dynamic effects at solar wind stream interaction regions, such as magnetic field compression, flow deflection, and density buildup, which the empirical scheme cannot.

Survival and vigour of ultra-dry seeds after ten years of hermetic storage

Seeds of carrot, groundnut, lettuce, oilseed rape and onion were stored hermetically in laminated aluminium foil packets in four environments (dry or ultra-dry moisture contents combined factorially with temperatures of 20 degrees C or -20 degrees C), replicated at several sites. After ten years' hermetic storage, seed moisture content, equilibrium relative humidity, viability (assessed by ability to germinate normally in standard germination tests) and vigour were determined. After a decade, the change in seed moisture content of samples stored at -20 degrees C was small or nil. Except for groundnut and lettuce (where loss in viability was about 8 and 3%, respectively), no loss in viability was detected after 10 years' hermetic storage at -20 degrees C. In all cases, there was no difference in seed survival between moisture contents at this temperature (P > 0.25). Comparison of seed vigour (root length and rate of germination) also confirmed that drying to moisture contents in equilibrium with 10-12% r.h. had no detrimental effect to longevity when stored at -20 degrees C: the only significant (P < 0.05) differences detected were slightly greater root lengths for ultra-dry storage of four of the six seed lots. Seed moisture content had increased after a decade at 20 degrees C (generally to the level in equilibrium with ambient relative humidity). Hence, sub-zero temperature storage helped maintain the long-term integrity of the laminated aluminium foil packets, as well as that of the seeds within.