Continental Stations Research Articles

Comparing Long-Term Atmospheric 14C and 3H Records Near Groningen, the Netherlands with Fruholmen, Norway and Izaña, Canary Islands 14C Stations

We present the results of the 14CO2 atmospheric monitoring station at the Smilde observation station, near Groningen, the Netherlands, a typical continental station. We compared these data, for absolute values and annual variation, with data from Fruholmen, Nordkapp, Norway and Izaña, Tenerife, Canary Islands, which are situated in areas less influenced by fossil-fuel CO2. The 20-yr Smilde record shows much seasonal variation (peak-to-trough variation is ∼30‰ in contrast to ∼12‰ for Fruholmen, and ∼5‰ for Izaña) and a lower overall value due to fossil-fuel consumption, in accordance with findings from similar stations in continental Western Europe. The Fruholmen and Izaña data show fairly equal mean Δ14C levels, but differ in seasonal amplitude. This difference could be due partly to the elevation difference between the stations. The Izaña station also has a slow exchange with the ground-level air because of an inversion layer. It is speculative whether annual injection of 14C from the stratosphere also plays a role. We give the Groningen long record of tritium in precipitation, showing profound seasonality.

The French Pilot Experiment OFM-SISMOBS: first scientific results on noise level and event detection

Abstract The seismic data recorded during the French Pilot Experiment OFM-SISMOBS (Observatoire Fond de Mer) were carefully analyzed. This experiment was successfully conducted between 28 April and 11 May 1992. Notwithstanding the technical goal of the experiment, which was to show the feasibility of installing and recovering two sets of three-component broadband seismometers (one inside an Ocean Drilling Program (ODP) borehole and another inside an ocean-bottom seismograph (OBS) sphere in the vicinity of the hole), the second goal of the experiment was to obtain for the first time the seismic noise level in the broadband range 0.5–3600 s, to conduct a comparative study of broadband noise on the sea-floor, downhole and on a continent, and to determine the detection threshold of seismic events. After the installation of both sets of seismometers, seismic signals were recorded continuously for 10 days. The analysis of these signals shows that the seismic noise is smaller in the period range 4–30 s for both sea-floor seismometers (OFM; Observatoire Fond de Mer) and downhole seismometers (OFP; Observatoire Fond de Puits) than at a typical broadband continental station such as GEOSCOPE Station SSB. The noise is smaller than that at SSB up to 600 s for OFM. The noise on vertical components is much smaller than on the horizontal ones. This difference might be explained by instrument settling. It was also observed that the noise level tends to decrease with time for both OFM and OFP, which means that the equilibrium stage was not attained by the end of the experiment. The patterns of microseismic noise in oceanic and continental areas are completely different. The background microseismic noise is shifted towards shorter periods for OFM and OFP compared with a continental station. This might be related to the difference in the crustal structure between oceans and continents. The low level of seismic noise implies that the detection threshold of earthquakes is very low, and it has been possible to record correctly teleseismic earthquakes of magnitude as small as 5.3. It was also possible to extract the Earth tide oceanic signal. Therefore, the experiment was a complete technical and scientific success, and demonstrated that the installation of a permanent broadband seismic and geophysical observatory on the sea-floor is now possible and can provide the scientific community with high-quality seismic data.

Unprecedented 1993 ozone decrease over the United States from Dobson spectrophotometer observations

Dobson spectrophotometer observations conducted since the early‐to‐mid 1960s at Bismarck, North Dakota; Caribou, Maine; Boulder, Colorado; Wallops Island, Virginia; Nashville, Tennessee; and at Fresno, California, since 1983, have revealed record low total ozone values during 1993. The tendency toward the low ozone values began in May 1992, but accelerated in early 1993. During January–August 1993, ozone monthly means at the stations were more than 2 standard deviations below long‐term normal monthly means 72% of the time and more than 3 standard deviations below normals 42% of the time. On average, the January–April 1993 ozone values were 12.6% below normal, with ozone deficits as large as 18% observed at Caribou and Wallops Island in January. Of particular concern are unusually low ozone values that occur in summertime when solar ultraviolet insolation is high. Such record lows occurred at four of the six stations (Caribou, Wallops Island, Fresno, and Nashville). During May–August 1993, ozone was on average 8.5% below normal at these sites. Monthly means at these stations were, furthermore, lower on average by 3.7% than corresponding lowest values observed there in the past. The ozone decrease of 12.6% below normal at the six continental Dobson instrument stations during the winter and spring months of 1993 implies a possible average increase in UV erythemal radiation at that time of 16–25% above normal. The 8.5% decrease in ozone at Caribou, Wallops Island, Fresno, and Nashville, implies that on average, UV erythemal radiation may have been higher than normal at these stations during the summer of 1993 by 11–17%.

Comparison of coastal wind and pressure trends over the tropical atlantic: 1946–1987

AbstractStrengthening of the Atlantic Trade Winds since 1946 has been documented by several workers. This is broadly consistent with changes in the atmospheric pressure patterns over the tropical Atlantic. Here ship‐recorded pressures are compared with sea‐level pressures recorded from 49 coastal sites in order to validate the trends in the pressure patterns. Good correlation is found between the two independent sets of pressure measurements; a comparison of trends reveals statistically significant agreement along the African coast but disagreement in the western Atlantic and Caribbean. Island stations generally show poorer correlation with adjacent marine pressures than do continental stations. A degree of support for the observed marine trends is therefore provided by this analysis but it appears that, particularly for islands, marine and adjacent land areas are not always strongly coupled meteorologically.

Precision and Radiosonde Validation of Satellite Gridpoint Temperature Anomalies. Part II: A Tropospheric Retrieval and Trends during 1979–90

TIROS-N satellite Microwave Sounding Unit (MSU) channel 2 data from different view angles across the MSU man swath are combined to remove the influence of the lower stratosphere and much of the upper troposphere on the measured brightness temperatures. The retrieval provides a sharper averaging kernel than the raw channel 2 weighting function, with a peak lowered from 50 kPa to 70 kPa and with only slightly more surface influence than raw channel 2. Monthly 2.5° gridpoint anomalies of this tropospheric retrieval compared between simultaneously operating satellites indicate close agreement, 0.15°C in the tropics to around 0.30°C over much of the higher latitudes. The agreement is not as close as with raw channel 2 anomalies because synoptic-scale temperature gradient information across the 2000-km swath of the MSU is lost in the retrieval procedure and because the retrieval involves the magnification of a small difference between two large numbers. Single gridpoint monthly anomaly correlations between the satellite measurements and the radiosonde calculations range from around 0.95 at high latitudes to below 0.8 in the tropical west Pacific, with standard errors of estimate of 0.16°C at Guam to around 0.50°C at high-latitude continental stations. Calculation of radiosonde temperature with a static weighting function instead of the radiative transfer equation degrades the standard errors by an average of less than 0.04°C. Of various standard tropospheric layers, the channel 2 retrieval anomalies correlate best with radiosonde 100–50- or 100–40-kPa-thickness anomalies. A comparison between global and hemispheric anomalies computed for raw channel 2 data versus the tropospheric retrieval show a correction in the 1979–90 time series for the volcano-induced stratospheric warming of 1982–83, which was independently observed by MSU channel 4. This correction leads to a slightly greater tropospheric warming trend in the 12-year time series (1979–90) for the tropospheric retrieval [0.039°C (±0.03°C) per decade] than for channel 2 alone [0.022°C (±0.02°C) per decade].

Rayleigh waves normally incident at a continental boundary

SUMMARY Some results of a study of the propagation of monochromatic Rayleigh waves normally incident on the western ocean-continent boundary of the Iberian Peninsula are described. This seismological problem is modelled in two dimensions using the finite element method for periods in the range of 1.5 to 60.0s. The following results for ocean-continent direction of propagation are presented: (i) the estimate of the reflected and transmitted wavefields; (ii) the phase velocity curves for the fundamental mode in the transition zone; (iii) the transmission and reflection coefficients; and (iv) the seismic energy distribution for transmitted and reflected modes. Experimental data from PORTO (PTO), a continental station, and PONTA DELGADA (PDA), an oceanic station, were compared to ascertain the effect on Rayleigh wave propagation across the continental margin. Comparison of the experimental amplitude ratios and transmission factor suggests a thickness of about 4 km for the sediment layer in the western oceanic zone of the Iberian Peninsula.

Locating oceanic earthquakes-the influence of regional models and location criteria

Earthquakes in oceanic areas are normally located using traveltime tables which are representative of continental paths, since most seismic stations lie on continents. It should therefore be possible to improve such locations by employing a set of traveltimes more appropriate to paths from oceanic events to continental stations. A comparison has therefore been made between locations for a number of oceanic events using the recent iasp91 global traveltimes and the times for the pac91 model derived from observations of events in the Pacific. Although there were often significant differences in the location estimates for the two models, these were often no larger than the shifts induced by changing the misfit criterion used for determining the location. For events in purely oceanic regions such as Tonga and the Marianas with little nearby continent, the results from the pac91 model either provided a significantly better fit to the data or produced depth estimates in close accord with independent constraints (e.g. centroid moment tensor locations). In these cases the use of a specific set of ‘oceanic’ traveltimes can be recommended. However for marginal zones and island arcs, the situation is less clear and it is probably best to employ the global traveltime set with the use of additional phases to improve depth estimates.

Temporal and spatial characteristics of annual precipitation over the balkans in the twentieth century

AbstractA study of the annual totals of precipitation is presented for 12 stations in the Balkan area for a period of 92 years (1894–1985) The homogeneity of the precipitation series is examined, and their statistical characteristics are analysed. An abrupt climatic change is found at the station of Belgrad, and a statistically significant trend of annual precipitation is found for various periods for the stations of Kerkyra, Patras, Hvar, and Sarajevo. Persistence is only ascertained for the Belgrad station. The application of principal component analysis has yielded three groups of stations. Group A includes northern and continental stations, group B includes stations of the Ionian and Adriatic Seas, and group C includes the stations of Thessaloniki and Athens. The most important period of fluctuations covers the years 1933–1953, when a humid period is followed by a dry period. The most recent 5‐year period can be described as a very dry one.Finally, comparison between rainfall fluctuations and types of circulation shows that the predominance of meridional circulation over the Balkans results in an increase of precipitation, whilst a zonal circulation results in a decrease of precipitation.

Decadal‐scale changes of the circulation in the tropical atlantic sector associated with Sahel drought

AbstractTrends in the general circulation setting during 1948–1983 accompanying the progressive aggravation of drought in sub‐Saharan Africa are investigated from ship observations over the tropical Atlantic and surface station records in western Africa.Decadal‐scale circulation changes in the Atlantic sector include a pressure rise over the tropical North Atlantic; southward displacement of the near‐equatorial wind confluence and associated maximum cloud belt; acceleration of the north‐east trades and possibly a strengthening of the South Atlantic trades, along with increasing cloudiness in the equatorial zone; increasing cloudiness over the tropical North Atlantic and Central American seas; and cooling of surface waters in a band across the tropical North Atlantic contrasting with warming in the South Atlantic. These changes occurred in all seasons, but were most pronounced at the height of boreal summer.Records at land stations in western Africa show that near the coasts the temperature and pressure trends are consistent with the adjacent ocean areas. In the interior of sub‐Saharan Africa, warming trends prevail in boreal summer, presumably as a direct consequence of reduced inflow of cool maritime monsoon air; the concomitant downward pressure trends at continental stations in sub‐Saharan Africa may reflect the hydrostatic effect of increasing temperature. In contrast to boreal summer, slight cooling trends and pressure rises prevail in the interior during winter.In the course of the past four decades, the near‐equatorial wind confluence over the eastern Atlantic migrated southward by 200 km in July–August and 150 km for the rainy season as a whole. Given the steep meridional precipitation gradients across the Sahel, these circulation shifts are large enough to account for the observed downward trend in Sahel rainfall.

Comment on “Record aurora over North America,” and “Effects of the March 1989 solar activity” by Joe Allen, et al.] southern aurora

The spectacular equatorward extent of auroral displays over North America, along with disruptions of radio communications and a power blackout, on the occasion of the March 1989 solar and geomagnetic disturbances, as reported in Eos (May 16, p. 595; November 14, p. 1479), calls for supplementary information from the southern hemisphere. Immediately after receiving some spontaneous descriptions by private observers in central Patagonia, we started a nationwide enquiry in search of similar data from continental and naval weather stations, as well as from radio and astronomy amateurs.

Anisotropy beneath 9 stations of the GEOSCOPE Broadband Network as deduced from shear‐wave splitting

Polarization of SKS, ScS and S waves has been analyzed on digital 3‐components seismograms recorded at 9 stations of the recently installed GEOSCOPE broadband network. Splitting was observed for about 15 records, and is interpreted as being due to seismic anisotropy beneath the stations. From every record, it is possible to retrieve the fast direction at the station, and the time difference between the fast and slow polarization directions. Some 30 observations of quasi‐linear S‐pulses also help constrain the direction of fast velocity. Synthetic seismograms were built to test the validity of the method used to retrieve the parameters of anisotropy. Typical time‐differences are about 1 sec, corresponding, for example, to a 80 km‐thick zone of perfectly oriented pyrolite. The fast directions obtained at 3 continental stations in America (Westford, Santa Cruz, Heredia), could be related to plate tectonic processes. On the other hand, the fast directions found in 3 oceanic islands (Kipapa, Papete, La Réunion) differ strongly from the oceanic plate velocity direction there.

A comparison of incoming solar radiation at marine and continental stations

In modelling solar radiation from surface weather observations, different approaches have been successful with marine and with land-based data. We apply a simple model to a marine and a mid-latitude continental station to illustrate systematic differences in the dependence of the insolation on solar elevation, cloud, and season. Relative to the continental station, the marine station has less insolation at low solar elevations, particularly in winter, but more insolation at high solar elevations. Two mechanisms are discussed to explain these differences.

A comparison of incoming solar radiation at marine and continental stations

AbstractIn modelling solar radiation from surface weather observations, different approaches have been successful with marine and with land‐based data. We apply a simple model to a marine and a mid‐latitude continental station to illustrate systematic differences in the dependence of the insolation on solar elevation, cloud, and season. Relative to the continental station, the marine station has less insolation at low solar elevations, particularly in winter, but more insolation at high solar elevations. Two mechanisms are discussed to explain these differences.

Oxygen-18 in Estonian Natural Waters

The data of 1982–1985 from the Turikoja meteostation (East Estonia) on the isotopic composition of precipitation are generalized and the results compared to those of some European IAEA/WMO network station. The monthly weighted mean value of δ18 O is equal to −10.4‰ and the seasonal isotope gradient ▵δ18 O/▵t is 0.32. Thus, in spite of the coastal position of Estonia, the temperature-dependence parameters of precipitation are typical of continental stations. This refers to the insignificant role of the Baltic in the formation of precipitation. The isotope records in Estonian lakes enabled to compile a model based on isotopic balance for a steady-state well-mixed waterbody. The δ18 O variations in ground-waters mostly fluctuate between −10.8‰ and −12.8‰, but there are aquifers with an isotopic composition considerably different from that of current precipitation, reaching up to −21‰.

Specification of monthly mean surface temperature anomalies in Europe and Asia from concurrent 700 mb monthly mean height anomalies over the Northern Hemisphere

AbstractSpecification equations for the monthly mean air temperature anomalies at 115 surface stations in Eurasia are derived by applying a stepwise forward screening procedure to the simultaneous monthly mean 700 mb height anomalies at 134 grid points in the northern hemisphere and the previous month's local temperature. These equations explain about 65 per cent of the temperature variance by an average of only 4.7 variables, but with marked regional, seasonal and month‐to‐month variations. The main properties of the equations are highlighted by means of several maps and graphs that delineate important spatial and temporal variations.The average properties of the specification equations in the European sector closely resemble those derived previously for North America. However, the results are consistently poorer for stations in Asia, not only for the annual mean, but also during each of the four seasons. Furthermore, the phases of seasonal variation are opposite in Europe and Asia. In both sectors, however, the equations are better in winter than in summer at coastal stations, but the reverse is true at continental stations. The average error in specifying monthly average surface temperature from perfect 700 mb height forecasts should be less than 0.9°C in Europe and 1.2°C in Asia about two‐thirds of the time.

Rank correlation for screening precipitation chemistry data

Sources of errors in precipitation chemistry data can be detected by nonparametric rank correlation of a measure of analysis error such as the algebraic sum of cations minus anions with the concentrations of each ion. Concentrations of ions which are imprecisely measured or are associated with unmeasured ions will be significantly correlated with the measure of analysis error. This type of analysis is illustrated for one year of NADP precipitation chemistry data from an oceanic, arid continental, and humid continental station.

Surface ozone measurements in the Venezuelan tropical savannah

A one-year set of surface ozone measurements in a four-station network located in the Venezuelan savannah is reported. The diurnal ozone variation is typical of continental stations with a maximum in the afternoon, when vertical turbulent mixing is strongest. The annual O3 average concentration, based on the monthly averages of daily maxima, was 17±2 ppb, which is in good agreement with values reported for similar latitudes. The boundary-layer ozone levels did not fall below 8 ppb, in contrast with previous sporadic measurements made in tropical latitudes. No evidence was found that mesoscale O3 downdrafts in the ITCZ in the South American continent are an important source of surface ozone. Finally, it is suggested that the relatively high ozone levels observed at the end of the dry season are probably of photochemical origin.

The trends of global tritium precipitations.

The trends of global tritium precipitation from 1953 to 1979 were estimated based on the tritium data published in seven volumes of Environmental Isotope Data by the International Atomic Energy Agency (IAEA). Tritium precipitation samples were collected from 342 stations in the world and tritium concentrations were measured by IAEA and 27 laboratories. Due to repeated atomospheric nuclear explosions, tritium precipitations showed maximum peak in 1963. After the agreement of the Partial Test Ban Treaty in 1964, they have gradually decreased until now showing seasonal variations. To obtain clear trends of tritium precipitations, seasonal and irregular factors were eliminated from the original time-series data using a code developed by the Japanese Economic Planning Agency. Results of analyses were as follows; a) Peak concentrations and precipitations of tritium were observed every year around the period of late spring to summer. b) The maximum annual tritium concentration and precipitation were observed in 1963 for northern hemisphere stations. c) A latitude effect was observed in the northern hemisphere. The maximum concentrations and precipitations were seen at the latitude of approximately 50°N. d) Continental stations always showed higher tritium concentrations and precipitations than comparable maritime stations.

Wake Island hydrophone seismic array

The hydrophone array installed near Wake Island (WHA) was reactivated as a seismic station in June 1979 by the Hawaii Institute of Geophysics. WHA consists of six bottom phones, at 5.5-km depth, in an array 40 km across and an additional four pairs of phones, at large spacing, suspended at SOFAR depth. The bottom array lies in a flat area on high Q lithosphere > 108 years old. All of the deep and five of the SOFAR phones are operational. Until September 1982, generally data were recorded from only three phones, on a slow speed analog cassette recorder. The station is now being upgraded for digital recording of all 11 phones. Many circumpacific earthquakes and nuclear tests have been well recorded by WHA, its location being ideally suited for monitoring such events. Nearly pure oceanic paths between WHA and circumpacific sources facilitate studies of the ocean-lithosphere waveguide such as propagation and attenuation of high-frequency, teleseismic Pn and Sn. For frequencies above 3 Hz, WHA is quieter than most quiet continental stations. [Work supported by AFOSR and ONR.]

Aerosol deposition velocities on the Pacific and Atlantic oceans calculated from 7Be measurements

The concentrations of 7Be have been measured in Pacific and Atlantic ocean water for the past several years to determine the deposition velocity of aerosol particles on the ocean surface. 7Be is produced at a relatively constant rate in the atmosphere by spallation reactions of cosmic rays with atmospheric nitrogen and oxygen. Immediately after its formation 7Be becomes attached to aerosol particles, and therefore can serve as tracers of the subsequent behavior of these particles. Isopleths of 7Be surface water concentrations, 7Be inventory in the ocean, and deposition velocity have been prepared for the Pacific Ocean from 30°S to 60°N and for the Atlantic Ocean from 10°N to 55°N. The concentrations, inventories and deposition velocities tended to be higher in regions where precipitation was high, and generally increased with latitude. The average flux of 7Be across the ocean surface was calculated to be 0.027 atoms cm −2 s −1 which is probably not significantly greater than the worldwide average 7Be flux across land and ocean surfaces of 0.022 atoms cm −2 s −1 calculated by Lal and Peters. The average deposition velocity was calculated to be 0.80 cm s −1. This value may be 10–50% too low, since it was calculated using atmospheric 7Be concentrations which were measured at continental stations. Measurements of atmospheric 7Be concentrations at ocean stations suggest that the concentrations at the continental stations averaged 10–50% higher than the concentrations over the ocean.