Hermann Flohn

African environmental and climatic changes and the general atmospheric circulation in late pleistocene and holocene

The paper describes the environmental and climatic changes which took place in Africa from the late Pleistocene through the Holocene and the general atmospheric circulation patterns which likely correspond to them. Three major periods are considered: (1) a period of aridity and dune building c. 20,000-12,000 B.P. in which the Sahara advanced considerably southward; (2) a moist, lacustrine period c. 10,000-8,000 B.P.; and (3) a second moist, lacustrine period toward c. 6,500-4,500 B.P. in which the entire Sahara desert contracted considerably. The prevailing atmospheric circulation patterns are theorized on the basis of corresponding changes of surface boundary conditions-primarily changing thermal character—and known dynamic behavior of the atmosphere.

Behaviour of the centres of action above the Atlantic since 1881. Part I: Characteristics of seasonal and interannual variability

Based on generated time series of the central pressure, latitudinal and longitudinal position of the Iceland Low, Azores High, Inter Tropical Convergence Zone and South Atlantic Subtropical High, the behaviour of these atmospheric centres of action since 1881 is examined with regard to the intra- and interannual variations, persistence of anomalies, linear trends and interactions. The critical analysis of these time series as well as those of the pressure gradients between the neighbouring centres suggests that the existing non-periodic fluctuations of the surface atmospheric circulation on the time-scale of one or more decades suppress the long-term changes. These fluctuations are similar to the ‘internal’ variations of the climate system, recognized in the climate simulations of coupled ocean–atmosphere models.

Background of a geophysical model of the initiation of the next glaciation

Abstract Evidence of (at least) five rapid hemispheric coolings of about 5°C during the last 105 yr has been found, each event spread over not more than about a century, as examples of a global-scale climatic intransitivity. Only some of them lead to a complete glaciation at the northern continents, others ended after a few centuries by a sudden warming (“abortive glaciation”). Starting from a modified version of Wilsons hypothesis of Antarctic ice surges, an air-sea interaction model with realistic geophysical parameters is outlined to interpret the sudden initiation of the North American ice sheet. Special attention is given to the Atlantic section, where the climatic anomalies during the last glaciation appear to have been significantly larger than in other sections.

Recent fluctuations of tropospheric temperature and water vapour content in the tropics

SummaryDuring the last decades the average temperature of the tropical troposphere (200/850 hPa layer) has steadily increased, between 1965 and 1984 by about 0.8°C in the whole equatorial belt. Data series from a section of individual stations verify this trend as seasonally constant, but decreasing from the equator towards both hemispheres. Further evidence is presented by selected mountain stations and glacier retreat in all equatorial mountains.Above the equatorial Pacific, the same stations indicate an increase of moisture content in the middle troposphere (500/700 hPa layer) expressed in precipitable water as well as in relative humidity. This coincides with increasing sea surface temperature in the area around Indonesia and northern Australia. Above Africa the trend is (if real) quite patchy. Due to the short residence time of water vapour in the atmosphere the horizontal (zonal ) distances between its sources and sinks remain near 2000 km, which may explain, in addition to instrumental differences, large regional deviations.

BEHAVIOUR OF THE CENTRES OF ACTION ABOVE THE ATLANTIC SINCE 1881. PART II: ASSOCIATIONS WITH REGIONAL CLIMATE ANOMALIES

This paper constitutes the second part of the analysis of the behaviour of centres of action above the Atlantic since 1881. The present study examines the applicability of the generated time series of the central pressure and geographical location of the Iceland Low (IL), Azores High (AH), Inter Tropical Convergence Zone (ITCZ) and South Atlantic Subtropical High (SSH) to the description of the surface circulation and its influence on the climate anomalies above the Atlantic and adjacent land areas in the period 1950—1989. Using the normalized anomalies of the pressure gradient between the cores of the AH and IL (AH—IL gradient) as a NAG (North Atlantic Oscillation) index, it can be shown that t his index is a practicable tool for the detection of characteristic spatial anomaly patterns in different climate variables above the North Atlantic and surrounding land areas. The ‘strong’ NAG (the AH-IL gradient above ‘normal’) in winter leads to a warming above northern and central Europe as well as to the development of the dipole-like anomaly patterns in the wind speed, sea-surface temperature, surface and tropospheric temperature and other variables above the western and eastern North Atlantic. The composite analysis for the ‘weak’ NAG reveals inverse patterns. The application of the characteristic parameters of the ITCZ and of the two subtropical highs as predictors of the rainfall variability in northeast Brazil and the Sahel for the period 1950-1983 shows that for the most part, these variations can be explained by the anomalies of the latitude position and intensity of the ITCZ.

On time scales and causes of abrupt paleoclimatic events

Abstract During the last 7 × 10 5 years the occurrence of abrupt climatic variations, of an intensity probably reaching 5°C/50 yr and with a duration of the order of several centuries can be demonstrated; their frequency is of the order 10 −4 (sometimes even 10 −3 ) per year. Most impressive examples are sudden coolings in earlier interglacials; in some periods the variability of past climates was obviously much greater than now. Due to the effective spatial coherence of the atmospheric and oceanic circulation their extension, not necessarily of similar intensity, is probably hemispheric or even global. They are modified by feedback mechanisms within the geophysical climatic system; orbital changes play a selective role leading either to suppression or to growth. Any physical interpretation of such abrupt paleoclimatic events remain as yet speculative. One of the most attractive models is the occurrence of clusters of major volcanic eruptions which is more frequent than expected in random series. This is similar to the clustering of severe earthquakes in recent years; both events are probably interrelated responses to the (apparently discontinuous) motions of tectonic plates.

Recent changes of the tropical water and energy budget and of midlatitude circulations

Rising atmospheric H2O content and temperature above the tropical Pacific (Hense et al. 1988) stimulated research on tropical ocean-atmosphere fluxes in the belt 10° S-14° N, based on COADS data for 1949–1979. Increasing sea-surface temperature was accompanied by regionally varying increases in the air-sea temperature and humidity gradients. The apparent rise in wind speed appeared to be only partly biased. Using several assumptions of the wind speed trend, increasing evaporation was found nearly everywhere. The best estimates vary regionally between 7% and 15%, with highest values above the warmest oceans between longitude 66° E and the date line. In the Atlantic, freshening surface waters (Levitus 1989) also suggest an increase of precipitation. Conversion of zonally averaged results into global estimates led to a rise of the energy input into the atmosphere, with a most plausible value of 8–10 W/m2. Since large-scale sea-surface warming appears to be induced by the greenhouse effect of CO2 combined with other trace gases, a powerful feedback mechanism — including H2O phase changes — should be responsible for the intensification of the hydrological cycle. This energy input of tropical origin seems to be larger — by a factor near 4 — than the “dry” greenhouse effect. Such a well-founded conjecture of increasing internal/potential energy in the tropics suggests a similar rise of kinetic energy within the extratropical atmospheric circulation. This can be checked on the basis of daily operational hemispheric analyses of the German Weather Service, here using the period October 1961–March 1988. During the cold season they show, at the surface, a deepening of the Icelandic and Aleutian Lows by 6 and 10 hPa, respectively, and at the 50 kPa level an amplification of the baroclinic westerlies by 20–40%. Upper wind observation series have been used to check this strengthening of the westerlies and an expansion of the Aleutian Low. During the warm season, weaker changes in opposite directions are observed. While the observed facts are incompatible with many of the recent climate models, a few models (Wilson and Mitchell 1987, Hansen et al. 1988) using an advanced parameterization of tropical convection support the evolution of a powerful tropical heat source centred within mid-tropospheric layers.

Contributions to the knowledge of the fluctuations of the Dead Sea level

SummaryAfter an extended critical evaluation of a wealth of historical sources, the history of the Dead Sea level fluctuations is reviewed with accuracy since 1800 (Klein, 1961). Earlier data since 1100 AD based on landmarks and tree-ring data (Klein, 1982, 1985) are also summarized. Since 1930 interannual fluctuations are compared with area-averaged rainfall data from the western and eastern part of the catchment, as well as with rainfall records from Jerusalem and Amman. Correlation analysis lead to two linear regression equations for the period 1930/1931–1962/1963 (undisturbed) and 1963/1964–1983/1984 after diversion of water from Lake Tiberias and Jordan River. From these equations, the average annual effect of this diversion results to about −24 cm; in very moist years this effect is insignificant.ZusammenfassungAuf der Grundlage einer kritischen Auswertung einer Fülle von Quellen werden zunächst die Spiegelschwankungen des Toten Meeres seit 1800 genau behandelt (Klein, 1961). Eine zusammenfassende Darstellung der Schwankungen ab 1100 A. D. wird aus historischen Quellen über die Landmarken des Seeufers und Baumringdaten (Klein, 1982, 1985) abgeleitet. Die interannulären Schwankungen des Seespiegels seit 1930 werden verglichen mit Gebietsmitteln des Niederschlages vom westlichen wie vom östlichen Teil des Einzugsgebietes, ebenso mit den Niederschlagsreihen von Jerusalem und Amman. Eine Korrelationsanalyse führt zu zwei linearen Regressionsgleichungen für die ungestörte Periode 1930/1931 bis 1962/1963, sowie für die Periode 1963/1964 bis 1983/1984, nach der Ableitung von Wasser zu Bewässerungszwecken aus dem See Genezareth und dem Jordan-Fluß. Eine Abschätzung des mittleren Effektes aus diesen Gleichungen ergibt etwa −24 cm pro Jahr; in sehr feuchten Jahren ist der Effekt unbedeutend.

Allgemeine Atmosphärische Zirkulation und Paläoklimatologie

ZusammenfassungNach einleitender Erörterung der z. Z. in voller Entwicklung begriffenen dreidimensionalen Auffassung der allgemeinen Zirkulation der Atmosphäre werden deren Grundgedanken in Beziehung zu paläoklimatischen Problemen, insbesonderen deren der pleistozänen Eiszeit gesetzt. Folgende Ergebnisse erscheinen wesentlich:1.Die nordischen Inlandeisvergletscherungen des Pleistozäns stimmen mit den aktuellen quasistationären Höhentrögen bei meridionaler Zirkulation überein.2.Gleichzeitige Vergletscherung in den Tropen und in außertropischen Breiten sowie Einengung des subtropischen Trockengürtels in den Pluvialzeiten entspricht ebenfalls den heutigen Witterungsanomalien des meridionalen Zirkulationstyps.3.Die eiszeitliche Zirkulation der Atmosphäre ist somit als gesteigerte Meridionalzirkulation — bei gleichzeitiger Abschwächung der Westdrift — zu deuten.4.Aus der Schneegrenzdepression im Äquatorialgebiet ergibt sich eine Temperatursenkung der Tropen um 4°; für die ganze Erde wird eine solche von rund 5° als wahrscheinlich angesehen.5.In Erweiterung der bisherigen Relieftheorie wird das Auftreten orographisch bedingter meridionaler Zirkulationsformen und eisbürtiger (kryogener) Klimate auf die relief starken Perioden im Anschluß an die Hauptorogenesen beschränkt.6.Parallelen zwischen den pleistozänen und historischen Zirkulationsanomalien sowie den mit der Sonnenfleckentätigkeit verbundenen Änderungen führt auf die Diskussion einer gemeinsamen solaren Ursache.7.Die ursächliche Verknüpfung der Vorzeitklimate erfordert eine Kombination von Schwankungen der Sonnenstrahlung (wahrscheinlich nur im Ultraviolett) und terrestrischen Faktoren der Reliefbildung (reliefbürtige Meridionalzirkulation).

Climate and energy: A scenario to a 21st century problem

The energy contribution of anthropogenic climatic fluctuations has been estimated to a gain of 15–20 TW, in comparison with a gain or deficit of 100–300 TW from natural processes responsible for the observed climatic fluctuations of the last 200 years. A dominant role of an increase of CO2 by a factor 2–5 in the next century, accompanied by side effects acting in the same direction, seems to be most likely. Under the assumption of constant natural factors anthropogenic warming and its effects on the Arctic sea-ice may successively lead to climatic states as in 1931–60, in the early Middle Age (900–1200) and in the climatic optimum period ca. 5000 BP. Finally it may result in a complete destruction of the Arctic sea-ice with a drastic shift of all climatic belts towards north, extending even to the interior Tropics.