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Mediterranean climates are characterized by strong seasonal contrasts between dry summers and wet winters. Changes in winter rainfall are critical for regional socioeconomic development, but are difficult to simulate accurately.sup.1 and reconstruct on Quaternary timescales. This is partly because regional hydroclimate records that cover multiple glacial-interglacial cycles.sup.2,3 with different orbital geometries, global ice volume and atmospheric greenhouse gas concentrations are scarce. Moreover, the underlying mechanisms of change and their persistence remain unexplored. Here we show that, over the past 1.36 million years, wet winters in the northcentral Mediterranean tend to occur with high contrasts in local, seasonal insolation and a vigorous African summer monsoon. Our proxy time series from Lake Ohrid on the Balkan Peninsula, together with a 784,000-year transient climate model hindcast, suggest that increased sea surface temperatures amplify local cyclone development and refuel North Atlantic low-pressure systems that enter the Mediterranean during phases of low continental ice volume and high concentrations of atmospheric greenhouse gases. A comparison with modern reanalysis data shows that current drivers of the amount of rainfall in the Mediterranean share some similarities to those that drive the reconstructed increases in precipitation. Our data cover multiple insolation maxima and are therefore an important benchmark for testing climate model performance. Comparisons between past regional drivers of precipitation extremes found time series data from Lake Ohrid and modern climate models of the Mediterranean may help to reduce simulation uncertainties in predictions of the Mediterranean climate., Author(s): Bernd Wagner [sup.1] , Hendrik Vogel [sup.2] , Alexander Francke [sup.1] [sup.3] , Tobias Friedrich [sup.4] , Timme Donders [sup.5] , Jack H. Lacey [sup.6] , Melanie J. Leng [...]

This study presents a long-term, multi-proxy reconstruction of the Asian southwest monsoon during the Tortonian to Piacenzian, based on a 4.78 Ma record from Coastal Makran, northwestern Gulf of Oman, southeast Iran. The integration of humidity proxies (clay minerals, Th/K, volume magnetic susceptibility, and grain size analysis), marine redox sensitivity (Th/U), total organic matter, carbonate content, [.sup.87]Sr/[.sup.86]Sr ratio, and spectral gamma-ray data conducted here provide valuable information that fill the existing gap in marine palaeoclimate records. The results show that a strong winter monsoon condition associated with relatively low precipitation and subsequently low physical and chemical weathering dominated the region during late Tortonian--late Messinian (7.65-5.83 Ma). However, a few episodes of intense physical and chemical weathering related to high precipitation are observed during this period (6.23-6.01 Ma), which is consistent with increased organic matter input from continental reservoirs to the oceans. In addition, the data indicate that from the latest Messinian (5.82-5.33 Ma) to Zanclean-Piacenzian (5.33-2.87 Ma), a strong summer monsoon accompanied by a relatively wetter condition and higher physical and chemical weathering resulted in a high detrital input into the basin. This higher weathering period is associated with the highest rate of Himalayan uplift, causing enhanced precipitation. Wavelet analysis of spectral gamma-ray data revealed notable periodicities at 750 Ka and 1.7 Ma, with significant periodicities centered around 5.75-6.03 Ma over the latest Messinian--Zanclean. Comparison with palaeoclimate records from other sites indicates a teleconnection with respect to precipitation, weathering, and productivity, especially during the Messinian--Zanclean transition. Resume : Nous presentons une reconstitution de longue duree reposant sur plusieurs variables substitutives de la mousson du Sud-Ouest asiatique, du Tortonien au Piacenzien, etablie a la lumiere d'un registre de 4,78 millions d'annees de la region cotiere de Makran du nord-ouest du golfe d'Oman (sud-est de l'Iran). L'integration de variables substitutives de l'humidite (mineraux argileux, Th/K, susceptibilite magnetique volumique et granulometrie), de la sensibilite a l'oxydoreduction marine (Th/U), des matieres organiques totales, du contenu en carbonate, du [.sup.87]Sr/[.sup.86]Sr et de donnees de spectrometrie gamma fournit de l'information importante qui comble une lacune existante dans les profils paleoclimatiques marins durant la periode etudiee dans cette region. Les resultats montrent que des conditions de forte mousson hivernale associees a des precipitations relativement faibles, suivies d'une faible alteration physique et chimique predominaient dans la region du Tortonien tardif au Messinien tardif (de 7,65 a 5,83 Ma). Quelques episodes d'alteration physique et chimique intense associee a de fortes precipitations sont toutefois observes durant cette periode (de 6,23 a 6,01 Ma), ce qui concorde avec un apport accru aux oceans de matiere organique provenant de reservoirs continentaux. En outre, les donnees indiquent que, du Messinien sommital (de 5,82 a 5,33 Ma) au Zancleen-Piacenzien (de 5,33 a 2,87 Ma), une forte mousson estivale etait accompagnee de conditions relativement humides et d'une plus forte alteration physique et chimique qui se sont traduites par un fort apport detritique dans le bassin. Cette periode d'alteration accrue est associee au taux le plus eleve de soulevement himalayen a l'origine des precipitations accrues. L'analyse par ondelettes de donnees de spectrometrie gamma revele des periodicites marquees a 750 Ka et 1,7 Ma, dont des periodicites significatives centrees autour de 5,75 a 6,03 Ma au Messinien sommital-Zancleen. La comparaison des resultats aux archives paleoclimatiques pour d'autres sites indique une teleconnexion des precipitations, de l'alteration et de la productivite, particulierement durant la transition du Messinien au Zancleen. [Traduit par la Redaction], Introduction The Asian monsoon, an atmospheric circulation pattern, governs the climatic conditions of the entire continent of Asia. It is comprised of the Indian Summer monsoon, East Asian monsoon, and [...]

The monsoon onset is a critical event in the Bangladesh calendar, especially for the domestic agricultural sector. Providing information about the monsoon onset for the past, present, and future has [...]

The Tibetan plateau, like any landmass, emits energy into the atmosphere in the form of dry heat and water vapour, but its mean surface elevation is more than 5 km above sea level. This elevation is widely held to cause the plateau to serve as a heat source that drives the South Asian summer monsoon, potentially coupling uplift of the plateau to climate changes on geologic timescales'-5. Observations of the present climate, however, do not clearly establish the Tibetan plateau as the dominant thermal forcing in the region: peak upper-troposphric temperatures during boreal summer are located over continental India, south of the plateau. Here we show that, although Tibetan plateau heating locally enhances rainfall along its southern edge in an atmospheric model, the large-scale South Asian summer monsoon circulation is otherwise unaffected by removal of the plateau, provided that the narrow orography of the Himalayas and adjacent mountain ranges is preserved. Additional observational and model results suggest that these mountains produce a strong monsoon by insulating warm, moist air over continental India from the cold and dry extratropics. These results call for both a reinterpretation of how South Asian climate may have responded to orographic uplift, and a re-evaluation of how this climate may respond to modified land surface and radiative forcings in coming decades., The Tibetan plateau has long been held to serve as an elevated heat source that drives the thermally direct circulation of the South Asian summer monsoon, with air at a [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.jenvman.2009.04.017 Byline: Mei-Lin Wu (a), You-Shao Wang (a), Cui-Ci Sun (a), Haili Wang (b), Jun-De Dong (a), Shu-Hua Han (c) Abstract: Thirteen water quality parameters from 12 monitoring stations were collected to study the effects caused by climate and anthropogenic activities on water quality in 2003 in Daya Bay, South China Sea. The data matrix has been built according to the results, which were analyzed by fuzzy c-mean cluster (FCM) and cluster analysis (CA). This analysis has identified anthropogenic effects and seasonal characters of water quality. The dry and wet seasonality have been demonstrated with FCM and CA. The precipitation and the Southeast Asian monsoons, northeasterly from October to the next April and southwesterly from May to September have also an important influence on water quality in Daya Bay. In the spatial pattern, two groups have been classified with FCM and CA. Cluster I consisted of the sites S3, S8, S10 and S11 in the west and north coastal parts of Daya Bay. Cluster I is mainly related to anthropogenic activities such as fish-farming. Cluster II consisted of the rest of the stations in the center, east and south parts of Daya Bay. Cluster II is mainly related to seawater input from South China Sea. Thus, fuzzy c-mean cluster and cluster analysis can be an important tool for the successful characterization of regional-scale water quality. Author Affiliation: (a) Key Laboratory of Tropical Marine Environmental Dynamics, South China Sea Institute of Oceanology, Chinese Academy of Sciences, 164 West Xingang Road, Guangzhou 510301, China (b) Scripps Institution of Oceanography, University of California, San Diego, CA 92093-0218, USA (c) College of Chemistry, Shandong University, Jinan 265200, China Article History: Received 16 September 2008; Revised 18 March 2009; Accepted 22 April 2009

A research effort primarily involving pilot balloon observations was carried out during the summer of 1997 to study rainfall variability over Central America. This activity, supported by NOAA's Pan American Climate Studies (PACS) program, grew in scope in response to the strong E1 Nino event of 1997/98 and subsequently evolved into a network ranging from Mexico to Paraguay. The overall goal of the PACS-Sounding Network (PACS-SONET) was to obtain relatively inexpensive wind profiles for describing climate variability over parts of the intertropical Americas that were not well covered by routine radiosonde observations. Major portions of the project supported climate research programs focusing on both the South and North American monsoon systems, while other parts of the network provided multiyear observations across important gaps in the Central American cordillera and also helped to describe cross-equatorial flow variations in the eastern Pacific. Approximately 50,000 observations were made by the PACS-SONET over its 10-yr operation. This paper describes the motivation for and evolution of the network, the logistical complications that were involved in establishing and operating a long-term multinational network, and some of the important results from analysis of the data. We conclude by discussing some of our perspectives on why the network was unable to make a transition from research funding to one supported by meteorological services of the region. (Page 1709)

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2007.09.005 Byline: Thomas R. Anderson (a), Vladimir A. Ryabchenko (b), Michael J.R. Fasham (a), Victor A. Gorchakov (b) Keywords: Denitrification; Biogeochemical cycles; Ecosystem modelling; Detritus; Export flux; Arabian Sea Abstract: A three-dimensional hydrodynamic-ecosystem model was used to examine the factors determining the spatio-temporal distribution of denitrification in the Arabian Sea. The ecosystem model includes carbon and nitrogen as currencies, cycling of organic matter via detritus and dissolved organic matter, and both remineralization and denitrification as sinks for material exported below the euphotic zone. Model results captured the marked seasonality in plankton dynamics of the region, with characteristic blooms of chlorophyll in the coastal upwelling regions and central Arabian Sea during the southwest monsoon, and also in the northern Arabian Sea during the northeast monsoon as the mixed layer shoals. Predicted denitrification was 26.2TgNyr.sup.-1,the greatest seasonal contribution being during the northeast monsoon when primary production is co-located with the zone of anoxia. Detritus was the primary organic substrate consumed in denitrification (97%), with a small (3%) contribution by dissolved organic matter. Denitrification in the oxygen minimum zone was predicted to be fuelled almost entirely by organic matter supplied by particles sinking vertically from the euphotic zone above (0.73mmolNm.sup.-2 d.sup.-1) rather than from lateral transport of organic matter from elsewhere in the Arabian Sea (less than 0.01mmolNm.sup.-2 d.sup.-1). Analysis of the carbon budget in the zone of denitrification (north of 10[degrees]N and east of 55[degrees]E) indicates that the modelled vertical export flux of detritus, which is similar in magnitude to estimates from field data based on the.sup.234Th method, is sufficient to account for measured bacterial production below the euphotic zone in the Arabian Sea. Author Affiliation: (a) National Oceanography Centre, University of Southampton, Waterfront Campus, Southampton SO14 3ZH, UK (b) St. Petersburg Branch, P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, 30, Pervaya Liniya, 199053 St. Petersburg, Russia Article History: Received 6 June 2006; Revised 3 September 2007; Accepted 11 September 2007

A packet of equatorial Rossby (ER) waves that lasted 2.5 months is identified in the lower troposphere of the northwest Pacific. Waves within the packet had a period of 22 days, a wavelength of 3600 km, a westward phase speed of 1.9 m[s.sup.-1], and a near-zero group speed. The wave properties followed the ER wave dispersion relation with an equivalent depth near 25 m. The packet was associated with the development of at least 8 of the 13 tropical cyclones that formed during the period. A composite was constructed around the genesis locations. Tropical cyclones formed east of the center of the composite ER wave low in a region of strong convection and a separate 850-hPa vorticity maximum. The background flow during the life of the packet was characterized by 850-hPa zonal wind convergence and easterly vertical wind shear. Wave amplitude peaked at the west end of the convergent region, suggesting that wave accumulation played a significant role in the growth of the packet. The presence of easterly vertical wind shear provided an environment that trapped energy in the lower troposphere. Each of these processes increases wave amplitude and thus the likelihood of tropical cyclone formation within the waves. The initial low pressure region within the wave packet met Lander's definition of a monsoon gyre. It developed to the west of persistent localized convection that followed the penetration of an upper-tropospheric trough into the subtropics. It is argued that the monsoon gyre represented the initial ER wave low within the packet.

A high-resolution proxy record of precipitation and evaporation variability through the past 1700 yr from [delta][sup.18]O analysis of a varved lake sequence from central Turkey shows rapid shifts between dry periods (A.D. 300-500 and A.D. 1400-1950) and wetter intervals (A.D. 560-750 and A.D. 1000-1350). Changes are consistent with changes in instrumental and proxy records of the Indian monsoon, dry summers in the Eastern Mediterranean being associated with periods of enhanced monsoon rainfall. In addition, major shifts in the record are coherent with changes in North Atlantic winter climate, with cold, wet periods in the Alps occurring at times of dry Turkish climate. Keywords: Turkey, paleoclimate, monsoon, isotopes, lake sediments, North Sea Caspian pattern.

The aim of this article is to consider the main elements of Indian monsoon rainfall - its timing, intensity, amount and distribution. The monsoon rains are examined using a range of different spatial and temporal scales. First, the Indian mean annual and seasonal rainfall distributions are described. These distributions are explained in terms of surface elevation, tropical cyclone tracks, and summer monsoon duration period. A review is then presented of important shorter-term regional variations in the rainfall supply produced by (a) individual tropical weather disturbances including tropical cyclones and thunderstorms and (b) natural break periods in the summer monsoon. A detailed discussion of the rainfall generating powers of the four major types of tropical cyclone, i.e. tropical lows, depressions, storms and hurricanes together with tropical thunderstorms, highlights their varying individual contribution to the monsoon rains. Inter-annual or year to year vacations in the Indian monsoon rainfall between 1871-1990 are considered next. A spatial analysis of inter-annual rainfall distribution reveals major geographical variations between years of deficit, normal and excess rainfall. A temporal analysis of inter-annual rainfall shows marked year to year variability, though longer-term alternating epochs with excess and deficient rain are observed when the Indian Summer Monsoon Rainfall (ISMR) index is used. In a review of external monsoon forcings, ENSO events including both warm phase El Ninos and cold phase La Ninas are shown to be the most significant factors. Nevertheless, because of the great number of factors involved in monsoon development, accurate summer monsoon forecasting remains a difficult task.

The effect of latent heat of condensation on baroclinic instability process is studied using the model developed by Bonatti and Rao. This model includes the effect of latent heat of condensation as parameterized by Mak, a zonal wind profile that varies linearly with pressure, and a constant lapse rate of -6 [degrees] C [km.sup.-1]. The effect of dissipative terms and vertical variation of heating on baroclinic instability with positive and negative zonal wind shears is studied. Dissipative terms and the type and intensity of heating change the selection of the most unstable wave. Viscosity and Newtonian cooling stabilize short waves. The characteristics of the most unstable wave obtained using a double maximum in the vertical distribution of heating and observed vertical profiles of zonal wind and temperature representative of the summer monsoon of India resemble those of observed monsoon depressions. The inclusion of dissipative effects makes the Green mode the most unstable wave.