Study of the air-sea interactions at the mesoscale: the SEMAPHORE experiment

The SEMAPHORE (Structure des Echanges Mer-Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherche Expérimentale) experiment has been conducted from June to November 1993 in the Northeast Atlantic between the Azores and Madeira. It was centered on the study of the mesoscale ocean circulation and air-sea interactions. The experimental investigation was achieved at the mesoscale using moorings, floats, and ship hydrological survey, and at a smaller scale by one dedicated ship, two instrumented aircraft, and surface drifting buoys, for one and a half month in October-November (IOP: intense observing period). Observations from meteorological operational satellites as well as spaceborne microwave sensors were used in complement. The main studies undertaken concern the mesoscale ocean, the upper ocean, the atmospheric boundary layer, and the sea surface, and first results are presented for the various topics. From data analysis and model simulations, the main characteristics of the ocean circulation were deduced, showing the close relationship between the Azores front meander and the occurrence of Mediterranean water lenses (meddies), and the shift between the Azores current frontal signature at the surface and within the thermocline. Using drifting buoys and ship data in the upper ocean, the gap between the scales of the atmospheric forcing and the oceanic variability was made evident. A 2 °C decrease and a 40-m deepening of the mixed layer were measured within the IOP, associated with a heating loss of about 100 W m⁻². This evolution was shown to be strongly connected to the occurrence of storms at the beginning and the end of October. Above the surface, turbulent measurements from ship and aircraft were analyzed across the surface thermal front, showing a 30% difference in heat fluxes between both sides during a 4-day period, and the respective contributions of the wind and the surface temperature were evaluated. The classical momentum flux bulk parameterization was found to fail in low wind and unstable conditions. Finally, the sea surface was investigated using airborne and satellite radars and wave buoys. A wave model, operationally used, was found to get better results compared with radar and wave-buoy measurements, when initialized using an improved wind field, obtained by assimilating satellite and buoy wind data in a meteorological model. A detailed analysis of a 2-day period showed that the swell component, propagating from a far source area, is underestimated in the wave model. A data base has been created, containing all experimental measurements. It will allow us to pursue the interpretation of observations and to test model simulations in the ocean, at the surface and in the atmospheric boundary layer, and to investigate the ocean-atmosphere coupling at the local and mesoscales.     

Finding regions of influence on SO2 and SO= 4 daily concentration measurements at four sites in Spain

Contamination by the pollutants SO₂ and SO⁼₄ was analyzed for the 1989–1992 period at four regional stations in Spain under the auspices of the EMEP-BAPMON program. The evolution of the time series of the daily pollution has also been assessed, and high mean concentrations at La Cartuja and Logroño observed, with values of 3.8 and 4.5 g m⁻³ for SO₂, respectively. Maximum annual concentrations were recorded in 1989, when SO₂ reached values of 6.24, 5.39, 5.71, and 9.30 g m⁻³ for the stations of La Cartuja, San Pablo de los Montes, Roquetas, and Logroño, respectively. This work attempts to establish a relationship between the concentrations of the pollutants - both SO₂ gas and SO⁼₄ aerosol - and the zones of emission or persistence of these long-range transported pollutants. In this way, those regions showing a greater impact on the air quality in each season have been determined. To achieve this, the trajectories of the air masses carrying away the pollution to each of the receiving stations were considered and followed by a sectorial analysis. Nonparametric statistical methods were implemented to contrast the chemical homogeneity among the different sectors. The criterion that several homogeneous sectors form a chemically homogeneous region was used. To improve this sectorial analysis, we have proposed a new technique based on the Potential-Source-Contribution Function (PSCF). Starting out from a set of specified regions, considered to be chemically homogeneous domains, it is possible to determine the likelihood that an air mass with particular characteristics (e.g., that a value of the daily concentration higher than the mean recorded at the station has been obtained) will arrive at a given station after having crossed one of the previously defined regions. Using this technique, it is possible to determine the source regions through which the air masses circulate and bring high pollution concentrations to the studied stations. Thanks to the PSCF, these statistical methods offer, through a sectorial analysis, the possibility to pass from a qualitative to a more quantitative view.     

A comparative allometric study of the morphometry of the gills of an alkalinity adapted cichlid fish,Oreochromis alcalicus grahami,of Lake Magadi,Kenya

A morphometric analysis of the gills ofOreochromis alcalicus grahami has been carried out on specimens from ecologically distinct lagoons and a water-holding tank of Lake Magadi, a highly alkaline salt lake situated in a volcanically active region of the southern part of the Great Rift Valley in Kenya. The data were compared with those fromOreochromis niloticus, a close relative that lives in fresh water and with data from other fresh water and marine fish. Our primary goal was to identify the possible adaptive features which enable the fish to survive in an environment characterized by severely fluctuating levels of oxygen, a condition exacerbated by factors such as high temperature, alkalinity and osmolarity. The specimens ofO. a. grahami from the south-western lagoons of the lake had gills better adapted for gas exchange with a body mass specific diffusing capacity for oxygen which was about 2 times greater than that of the gills of the specimens from the fish spring lagoons and 2.5 times that of those from the water-holding tanks. Some parameters of the gills ofO. a. grahami, e.g. the gill filament length and number of gill filaments are significantly greater than those ofO. niloticus but the number of secondary lamellae, area of secondary lamellae and the diffusing capacity of the gills are similar in the two species. Compared with most other fish, the gills ofO. a. grahami appear to be particularly well adapted for gas exchange especially by having a thin water-blood barrier. Perhaps in no other extant fish have the gills had to be so exquisitely designed to meet environmental extremes and regulate complex and at times conflicting functions such as gas exchange, iono-regulation, acid-base balance and nitrogenous waste excretion as inO. a. grahami     

Monthly summaries of merchant ship surface marine observations and implications for climate variability studies

We compute the interannual fluctuations of the surface heat budget of the North Atlantic using the trimmed monthly summaries of the Comprehensive Ocean-Atmosphere DataSet (COADS) for the period 1950–1979. The presence of long-period trends in the heat budget imply large variations of the northward cross-equatorial heat transport over the years. To assess the reliability of these variations, we compare the COADS climate signal to that derived from the ocean weather stations (OWSs) of the North Atlantic. The sea surface temperature, air temperature and sea level pressure show good correlation between the anomaly time series derived from the merchant ship monthly summaries of COADS, and those derived from OWS monthly summaries, except for northernmost locations during winter. In contrast, the sensible and latent heat parameters, which require simultaneous measurements of various variables, have merchant ships and ocean weather stations anomaly time series that are poorly correlated. Only in heavily travelled latitudes and during winter, when the air-sea heat exchange anomalies are large, are the merchant ship measurements able to reproduce the interannual fluctuations of the heat fluxes. The long-period trends in the surface heat budget of North Atlantic equatorward of 40° N implied by COADS thus appear unrepresentative of true climate trends. The COADS trends result from a gradual increase in the magnitude of the reported winds over the years due probably to variations in the ratio of measured to estimated winds, as well as from long period fluctuations in the near surface vertical temperature and humidity gradients. Offprint requests to: R Michaud     

Compression and coordination changes in pyroxenoids: an EXAFS study of MgGeO3 enstatite and CaGeO3 wollastonite

The evolution of the distortion of MgGeO₃ enstatite and CaGeO₃ wollastonite with increasing pressure, has been investigated using X-ray absorption spectroscopy (XAS) in a diamond anvil cell. At room temperature and low pressure (P<7 GPa), the compressibility of the GeO₄ tetrahedron is higher in MgGeO₃ enstatite (K _[GeO4]∼135 GPa) than in CaGeO₃ wollastonite (K _[GeO4]≥ 280 GPa). The compression mechanisms of the two compounds are different: the whole mineral compressibility of Ge-enstatite appears to be very homogeneous, in contrast to that of Ge-wollastonite which exhibits an inhomogeneous tretrahedral compressibility. This result is consistent with the variation of the Debye-Waller factors of the two compounds with increasing pressure. At higher pressures, the coordination of germanium atoms in the two compounds gradually changes from fourfold to sixfold. For CaGeO₃ the coordination change starts at 7 GPa and is complete a 12 GPa, whereas it starts at about 8.5 GPa for MgGeO₃ and is not complete at 31 GPa. The progressive evolution of the measured Ge-O distances as well as the modification in the X-ray absorption near-edge structure indicate two coexisting different sites rather than a progressive site modification. The transformation is found to be partially reversible in CaGeO₃ wollastonite, whereas it is totally reversible in MgGeO₃ enstatite.     

Assessment of loess as parent material for agroecological potential

Due to its physical and chemical properties, areas with a loess mantle of proper thickness favour arable farming. Loess as a parent material promotes the formation of soils of good structure, nutrient supply and water budget. The ongoing land evaluation project places particular emphasis on the consideration of parent material in its scoring system as soil and climatic parameters are assessed in conjunction with the grain size composition of subsurface material and soil texture.A test area most suitable for the presentation of the method seems to be a sandy alluvial fan with loess mantle and this circumstance allows the contrasting of the agroecological potentials of soils developed on the two parent materials of different nature.The favourable water storage properties of loess make this deposit particularly valuable under the conditions of climatic aridification. The geographical information system applied here is capable of simulating the agroecological potential changes caused by reduced precipitation, increased evaporation and dropping groundwater levels.The research for this paper was carried out as part of the MEDALUS II (Mediterranean Desertification and Land Use) collaborative research project. MEDALUS II was funded by the EC under its Environment Progamme, contract no. EV5V 0128/0166. A grant was also provided by the OTKA (Hungarian National Science Foundation), project no 1278. The support is gratefully acknowledged.     

The role of principles and methods in loess-paleosol investigations

In the process of loessification, the development of loess fabric, the role of zonal, regional and partly of local environmental factors is regarded decisive. In the loess sequences various genetic types of loess pockets and paleosols occur and reflect different climatic and paleogeographical conditions.As a consequence, even within a single profile the colour, grain size and mineral composition, CaCO₃ content, degree of weathering and fabric of loess may vary. Zonal, regional or even local variations in such loess properties may equally derive from syngenetic or postgenetic processes. For similar reasons, spatially and temporally different forms, types and subtypes of paleosol also developed.This way, loess and soil formation resulted in various spatial types in the same glacial, interstadial or interglacial phase in the various geographical zones and regions. Although there exist major loess regions where the interglacial soils (eg brown forest soils) are markedly distinct from interstadial steppe soils or from other types of humic soils, continental loess areas can also be observed where the paleosols developed under interglacial and interstadial conditions (eg steppe or forest steppe soils) cannot be referred into different genetic types. In such cases the changes in paleoenvironments are difficult and uncertain to reconstruct.The interregional correlation of paleosols is occasionally hindered by the various terminologies applied to denote them. In addition, there are uncertainties concerning the dating and identification of paleoenvironments of polygenetic soils and of soil complexes consisting of paleosol units: how many soil formation phases they represent and how long erosional gaps can be reckoned with between these phases.     

Long-term change in precipitation in southern Italy

Summary Annual average precipitation values obtained at 765 Italian raingauge stations south of the Rome parallel and in the Italian Isles, are analyzed. The analysis is extended over 18 years and marked variations in precipitation with the Zürich annual relative sunspot numbers are found. It is found furthermore that the magnitude of change appears to depend also on the average altitude of the 5 categories of stations considered.

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Riassunto In questo articolo viene fatta un'analisi sulle variazioni delle precipitazioni medie annue in base ai dati forniti da 765 stazioni pluviometriche dell'Italia meridionale, Sicilia e Sardegna, negli anni dal 1926 al 1943 incluso. Tali variazioni sembrano essere legate al numero medio annuo di macchie solari e dipendono anche dall'altezza delle stazioni.

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Contribution of the Centro Nucleazione Aerosoli of the National Research Council of Italy; Via Vettore 4 (Monte Sacro)     

On the chemical composition of atmospheric aerosols — Part III: Trends of the total annual precipitation with respect to the chemical content of rainwater over the United States

Summary Following a recent hypothesis on the role of activated sodium chloride particles as potential nuclei of atmospheric precipitation, further analysis is made of average annual data due toC. Junge on the chemical composition of rainwater salts, and correlation of some parameters with the total amount of rainfall is attempted. It is found that the results obtained are in agreement with the proposed role of activated sodium chloride as a nucleus of a substantial part of the atmospheric precipitation.