The observed summertime drying over Northern Central India (NCI) during the latter half of the twentieth century is not reproduced by the Coupled Model Intercomparison Project Phase 5 (CMIP5) model ensemble average. At the same time, the spread between precipitation trends from individual model realizations is large, indicating that internal variability potentially plays an important role in explaining the observed trend. Here we show that the drying is indeed related to the observed 1950–1999 positive trend of the Pacific Decadal Oscillation (PDO) index and that the relationship is even stronger for a simpler index (S1). Adjusting the CMIP5-simulated precipitation trends to account for the difference between the observed and simulated S1 trend increases the original multimodel average NCI drying trend from −0.09 ± 0.31 mm d−1 (50 years)−1 to −0.54 ± 0.40 mm d−1 (50 years)−1. Thus, our estimate of the 1950–1999 NCI drying associated with Pacific decadal variability is of similar magnitude as our previous CMIP5-based estimate of the drying due to anthropogenic aerosol. The drying (moistening) associated with increasing (decreasing) S1 can partially be attributed to a southeastward (northwestward) shift of the boundary between ascent and descent affecting NCI. This shift of the ascent region strongly affects NCI but not Southeast Asia and south China. The average spread between individual model realizations is only slightly reduced when adjusting for S1 as smaller-scale variability also plays an important role.

No description available.

Proxy-based quantitative estimates of Eocene climates can be made from marine isotope records for ocean conditions or fossil plants for terrestrial environment. However, our understanding about Eocene terrestrial climates is derived mainly from North America and Europe, and little is known about East Asia. Previous qualitative paleoclimate studies briefly revealed three climatic regimes across China during the Eocene with a planetary wind-dominated subtropical to tropical arid zone in the central part (i.e., the subtropical highs), which was flanked by the subtropical climate zone in the north and tropical climate zone in the south. But such a pattern of paleoclimatic zonation still requires a test from quantitative study. Based on analyses of 66 plant assemblages, carefully selected from 37 fossil sites throughout China, we here report the first large-scale quantitative climatic results and discuss the Eocene climatic patterns in China. Our results demonstrate that the Eocene monsoonal climate must have been more or less developed over China, judging from the presence of apparent seasonality of both temperature and precipitation revealed by our quantitative estimation. This appears not to support the previously claimed Eocene planetary wind-dominated climate system, at least in the region of eastern China. In addition, the research indicates that, with a slight declining trend of MAT during the Eocene, the winter temperature substantially dropped in tropical southern China during the middle to late Eocene interval. This might be related to the development of a weak Eocene Kuroshio Current in the southwestern Pacific, and/or a significantly enhanced paleo-winter monsoon from Siberia.

No description available.

Information from the TRMM-CSH and TRMM-2A12 datasets is used to examine the four-dimensional latent heating (LH) structures over the Asian monsoon region between 1998 and 2006. High sea surface temperatures, ocean-land contrasts and complex terrain produce large precipitation and atmospheric heating rates whose spatial and temporal characteristics are relatively undocumented. Analyses identify interannual and intraseasonal LH variations, with a large fraction of the interannual variability induced by internal intraseasonal variability. Also, the analyses identify a spatial dipole of LH anomalies between the equatorial Indian Ocean and the Bay of Bengal regions occurring during the summer active and suppressed phases of the monsoon intraseasonal oscillation. Comparisons made between the TRMM-CSH and TRMM-2A12 datasets indicate significant differences in the shape of the vertical profile of LH. Comparison of TRMM-LH retrievals with sounding budget observations made during the South China Sea Monsoon experiment shows a high correspondence in the timing of positive LH episodes during rainy periods. Negative values of LH, associated with radiative cooling and with higher troposphere cooling from non-precipitating clouds, are not captured by any of the TRMM datasets. In summary, LH algorithms based on satellite information are capable of representing the spatial and temporal characteristics of the vertically integrated heating in the Asian monsoon region. The TRMM-CSH presents better performance than TRMM-2A12. However, the vertical distribution of atmospheric heating is not captured accurately throughout all different convective phases. It is suggested that satellite derived radiative heating/cooling products are needed to supplement the LH products in order to give an overall better depiction of atmospheric heating.

No description available.

East Asian Paleogene climates have long been regarded as controlled by the planetary wind system, which might result in a climate pattern with three latitudinally distributed zones. Two humid zones located separately in the north and south were lithologically designated by coals and oil shales, while an arid zone in the middle was represented by red beds and evaporites. Because the middle arid zone was located along ~. 30° N paleolatitude, its presence had been further linked with a then subtropical high. However, this long-standing model has recently been challenged by growing evidence from petrology, sedimentology, paleontology, paleobiogeography, paleoclimatology, and climate modeling. Here we review the primary data from these disciplines and reinterpret their climate significances to revisit the East Asian climate pattern during the Paleogene. Petrologically, while the occurrence of coals and/or oil shales is accepted as an indicator for overall humid climates, that of red beds and/or evaporites is highly equivocal to exclusively indicate perennial arid climates unless their origins are carefully investigated. In reality, generic red beds merely represent an oxidizing environment, not essentially associated with a single specific climate type. Meanwhile evaporites, although typically precipitated in arid environments, may be deposited in either perennial dry or seasonal/monsoonal climates. There is no solid evidence so far to convincingly support that the landscape of the so-called middle arid zone was dominated by desert and/or steppe under a then subtropical high during most of the Paleogene. The plant function type study additionally suggests that the "middle arid zone" appears to be lack of xerophytic vegetation, even though some xerophytic or sclerophyllous plant taxa did sporadically occur. Interestingly, paleozoological data show that the Paleogene mammalian faunas were somewhat equably distributed over East Asia, strongly suggesting the evident absence of a critical biogeographical or climatic barrier stretched across the "middle arid zone" as the planetary wind model implied. In contrast to the planetary wind model, monsoonal or monsoon-like Paleogene climates have been broadly reported from the northern, middle, and southern East Asia, as well as adjacent regions of Russia and Kazakhstan. If only the indicators for humid climates are considered, simply due to the uncertainty of those for perennial arid climates, East Asia must have had a relatively dry region in the continental interior during the late Eocene to Oligocene transition, likely caused by the continentality and/or the rain shadow effect along with the global cooling. The monsoonal interpretation is highly in agreement with the evidence from floras, faunas, basin analyses, and modeling experiments, and well explicates the Paleogene climate distribution and seasonal dynamics of East Asia. However, further studies will be largely needed to verify whether, uniformly according to the modern criteria, the Paleogene climates of the East Asia interior can be accurately attributed to the arid category.

The role of feedback between erosional unloading and tectonics controlling the development of the Himalaya is a matter of current debate. The distribution of precipitation is thought to control surface erosion, which in turn results in tectonic exhumation as an isostatic compensation process. Alternatively, subsurface structures can have significant influence in the evolution of this actively growing orogen. <br><br>Along the southern Himalayan front new 40Ar/39Ar white mica and apatite fission track (AFT) thermochronologic data provide the opportunity to determine the history of rock-uplift and exhumation paths along an approximately 120-km-wide NE-SW transect spanning the greater Sutlej region of the northwest Himalaya, India. 40Ar/39Ar data indicate, consistent with earlier studies that first the High Himalayan Crystalline, and subsequently the Lesser Himalayan Crystalline nappes were exhumed rapidly during Miocene time, while the deformation front propagated to the south. In contrast, new AFT data delineate synchronous exhumation of an elliptically shaped, NE-SW-oriented ~80 x 40 km region spanning both crystalline nappes during Pliocene-Quaternary time. The AFT ages correlate with elevation, but show within the resolution of the method no spatial relationship to preexisting major tectonic structures, such as the Main Central Thrust or the Southern Tibetan Fault System. Assuming constant exhumation rates and geothermal gradient, the rocks of two age vs. elevation transects were exhumed at ~1.4 &#177;0.2 and ~1.1 &#177;0.4 mm/a with an average cooling rate of ~50-60 &#176;C/Ma during Pliocene-Quaternary time. The locus of pronounced exhumation defined by the AFT data coincides with a region of enhanced precipitation, high discharge, and sediment flux rates under present conditions. We therefore hypothesize that the distribution of AFT cooling ages might reflect the efficiency of surface processes and fluvial erosion, and thus demonstrate the influence of erosion in localizing rock-uplift and exhumation along southern Himalayan front, rather than encompassing the entire orogen.<br><br>Despite a possible feedback between erosion and exhumation along the southern Himalayan front, we observe tectonically driven, crustal exhumation within the arid region behind the orographic barrier of the High Himalaya, which might be related to and driven by internal plateau forces. Several metamorphic-igneous gneiss dome complexes have been exhumed between the High Himalaya to the south and Indus-Tsangpo suture zone to the north since the onset of Indian-Eurasian collision ~50 Ma ago. Although the overall tectonic setting is characterized by convergence the exhumation of these domes is accommodated by extensional fault systems.<br><br>Along the Indian-Tibetan border the poorly described Leo Pargil metamorphic-igneous gneiss dome (31-34&#176;N/77-78&#176;E) is located within the Tethyan Himalaya. New field mapping, structural, and geochronologic data document that the western flank of the Leo Pargil dome was formed by extension along temporally linked normal fault systems. Motion on a major detachment system, referred to as the Leo Pargil detachment zone (LPDZ) has led to the juxtaposition of low-grade metamorphic, sedimentary rocks in the hanging wall and high-grade metamorphic gneisses in the footwall. However, the distribution of new 40Ar/39Ar white mica data indicate a regional cooling event during middle Miocene time. New apatite fission track (AFT) data demonstrate that subsequently more of the footwall was extruded along the LPDZ in a brittle stage between 10 and 2 Ma with a minimum displacement of ~9 km. Additionally, AFT-data indicate a regional accelerated cooling and exhumation episode starting at ~4 Ma. <br><br>Thus, tectonic processes can affect the entire orogenic system, while potential feedbacks between erosion and tectonics appear to be limited to the windward sides of an orogenic systems. / Welche Rolle Wechselwirkungen zwischen der Verteilung des Niederschlags, Erosion und Tektonik während der Entwicklung des Himalayas über geologische Zeiträume gespielt haben bzw. heute spielen, ist umstritten. Dabei ist von besonderem Interesse, ob Erosion ausschliesslich in Folge tiefkrustaler Hebungsprozesse entsteht und gesteuert wird, oder ob Regionen besonders effektiver Erosion, bedingt durch isostatische Kompensation, die Lokation tektonischer Deformation innerhalb aktiver Orogene beeinflussen können. <br><br>Entlang der südlichen Himalayafront ermöglichen neue thermochronologische 40Ar/39Ar-Hellglimmer- und Apatite-Spaltspur-Alter die Bestimmung der Exhumationspfade entlang eines 120-km-langen NE-SW-gerichteten Profils, dass quer durch die gesamte Sutlej-Region des nordwestlichen, indischen Himalayas verläuft. Dabei deuten die 40Ar/39Ar-Daten in übereinstimmung mit früheren Studien darauf hin, dass zuerst das Kristallin des Hohen Himalayas und anschliessend, südwärts propagierend, das Kristallin des Niederen Himalayas während des Miozäns exhumiert worden ist. Im Gegensatz dazu weisen die neuen Apatit-Spaltspur-Alter auf eine gleichmässige und zeitgleiche Exhumation beider kristallinen Decken entlang des Sutlejflusses. Dieser 80x40 km weite Bereich formt einen elliptischen, nordost-südwest orientierten Sektor erhöhter Exhumationsraten während des Pliozäns und Quartärs. Innerhalb des Fehlerbereichs der Spaltspurmethode zeigen die Alter eine gute Korrelation mit der Höhe, zeigen aber gleichzeitig keine Abhängigkeit zu bedeutenden tektonischen Störungen, wie die "Main Central Thrust" oder dem "Southern Tibetan Fault System". Unter der vereinfachten Annahme konstanter Exhumationsraten deuten zwei verschiedene Höhenprofile auf Exhumationraten in der Grössenordnung von ~1,4 &#177;0,2 und ~1,1 &#177;0,4 mm/a bei einer durchschnittlichen Abkühlrate von ~50-60 &#176;C/m.y. während des Pliozäns bzw. Quartärs hin. Der anhand von Spaltspuraltern bestimmte Sektor verstärkter Exhumation korreliert mit dem Gebiet, das während des Holozäns hohen Niederschlags-, Erosion- bzw. Sedimenttransportraten ausgesetzt ist. Daher vermuten wir, dass die Verteilung von jungen Spaltspuraltern den regionalen Grad der Effiziens von Oberflächenprozessen und fluviatiler Erosion wiederspiegelt. Dies deutet auf einen Zusammenhang zwischen Erosion und der Lokalisierung von Hebung und Exhumation entlang der südlichen Front des Himalayas hin, und zeigt gleichzeitig, dass die Exhumation nicht einfach über die gesamte Front gleichmässig verteilt ist.<br><br>Trotz der Wechselwirkungen zwischen Exhumation und Erosion, die möglicherweise die Entwicklung der südlichen Himalayafront beeinflussen, beobachten wir auch tiefkrustale tektonische Exhumation in ariden Gebieten nördlich des Hohen Himalayas, die vermutlich im Zusammenhang mit plateauinternenen Deformationsprozessen steht. So haben sich zum Beispiel mehrere metaplutonische Gneissdomkomplexe zwischen dem Hohen Himalaya im Süden und der Indus-Tsangpo Suturzone im Norden seit der Indien-Asien Kollision vor ca. 50 Millionen Jahren entwickelt. Obwohl die Dome sich grossräumig in einem kommpressiven Spannungsfeld befinden, werden sie lokal entlang von Extensionsstrukturen exhumiert. Bis heute sind die Ursachen für die Entstehung dieser Prozesse umstritten.<br><br>Entlang der Indisch-Tibetischen Grenze erstreckt sich der fast vollkommen unbeschriebene Leo-Pargil-Gneissdomkomplex (31-34&#176;N/77-78&#176;E) innerhalb des Tethyschen Himalayas. Neue Geländekartierungen, strukturelle und geochronologische Daten der westliche Flanke des Leo Pargil Domes dokumentieren, dass dieser sich entlang zeitlich verbundener Abschiebungssysteme in einem extensionalen Regime entwickelt hat. Im Gelände wird der Dome von einem mächtigen Störungssystem begrenzt, die "Leo Pargil Detachment Zone" (LPDZ). Durch den tektonischen Versatz entlang der LPDZ liegen heute niedriggradig metamorphe Sedimentgesteine im Hangenden neben hochgradigen Gneisen in Liegenden. Unabhängig von der Probenlokation entlang des aufgeschlossenen Störungssystemes ergeben alle neuen 40Ar/39Ar-Hellglimmeralter um die 15 Ma und deuten auf ein regional wichtiges Abkühlungsereignis hin. Im Gegensatz dazu deuten die neuen Apatit-Spaltspuralter (AFT) auf eine kontinuierliche Exhumation der hochmetamorphen Einheiten im Liegenden der LPDZ unter sprödtektonischen Bedingungen zwischen 10 und 2 Ma hin, bei einem minimalen Versatz von ungefähr 9 km. Desweiteren deuten die Apatit-Spaltspur-Daten auf überregionale beschleunigte Abkülhlungs- bzw Exhumationsphase seit 4 Ma.<br><br>Daraus kann gefolgert werden, dass die tektonischen Prozesse die Entwicklung des gesamten Gebirges beflussen können, während potenzielle Wechselwirkungen zwischen Erosion und Tektonik auf die luvwärtigen Gebirgsflanken beschränkt zu bleiben scheinen.

Monsoons and Intertropical Convergence Zones (ITCZ) exhibit variability at various temporal and spatial scales. The temporal scale of variability encompasses scales from the intraseasonal through interannual to interdecadal time scales. Anthropogenic climate change can also have an impact on ITCZ and monsoons. Thus it is necessary to assess the ability of coupled ocean atmospheric models (commonly known as AOGCM) to simulate these aspects of variability of tropical climate. This has been studied with simulations from 20 AOGCMs and their AGCM from IPCCAR4 archive. In addition, we have used our own 100 year simulation with CCSM2 and also simulations with its AGCM viz. CAM2. Our analysis shows that most model have significant bias in tropical rainfall and SST. Most models underestimate SST except over a few regions such as the Eastern boundaries of Atlantic and Pacific Oceans. The AGCMs which are forced with observed SSTs have much higher annual mean rainfall as compared to AOGCMs. There is a strong correlation between error in shortwave reflectance at the top of the atmosphere and error in SST. The ability of coupled ocean-atmosphere models and their atmosphere-alone counterparts to simulate the seasonal cycle of rainfall over major monsoon regions and also over oceanic ITCZ. It is found that over the Indian monsoon region, most AGCMs overestimate the seasonal cycle while AOGCMs have a more realistic seasonal cycle. This inspite of the fact that most AOGCMs underestimate the SST over the Indian region. It is shown that this is related to errors in precipitable water-rainfall relationship in most models i.e. for a given amount of precipitable water, most models overestimate the rainfall. Thus lower SST reduces the precipitable water and hence the amount of rainfall is reduced. Therefore, the mutual cancellation of errors leads to a more realistic seasonal cycle in AOGCMs. The seasonal cycle over Africa was analysed with the help of a diagnostic model. Over Southern Africa, most models show simulate a less stable atmosphere and hence the rainfall is overestimated. A technique based on Continous Wavelet Transform in Space and Time (CWTST) has been modified to seperate northward and southward propagating modes of BSISO over the Indian and West Pacific regions. It was seen that over the Indian region, northward propagating modes were more prominent in comparison to southward modes. It was also found that the predominant spatial scale (of about 30o) did not show much interannual variability but the associated temporal scale showed significant variation. Both AOGCMs and AGCMs simulations were analysed to investigate the impact of coupling on intraseasonal activity. Most AOGCMs were able to simulate the predominant spatial scale but were unable to simulate the associated temporal scale correctly. These problems persisted with AGCMs also. It was also found that for AGCMs, there were some variations between ensemble members of the AGCMs. Comparing BSISO in increased GHG scenarios with present day simulations we found that in general, power in the spectrum increases. This could be related to higher mean precipitation that has been simulated by most AOGCMs when GHG are increased. The interannual variability in the tropics with special reference to Tropical Biennial Oscillation (TBO) and ENSO has been studied. The changes in these modes of variability due to anthropogenic climate change has also been assessed. We found that in most models over the Nino3.4 region, the mode of variation shifts from a near-four period (in pre-industrial simulations) to that of TBO mode in increased GHG (green house gas) scenario. This suggests that with increasing GHGs, ENSO quasi-periodicity might shift to about two years. It is also interesting to note that for observed rainfall, OLR and 850 hPa winds, the TBO mode has higher variance over the Eastern Indian Ocean, indicating that the TBO mode might be related to Indian Ocean Dipole Mode and EQUINOO (Equatorial Indian Ocean Oscillation).

Several previous studies have found that various characteristics of the Indian monsoon rainfall have shown secular changes over the past century. In this study, using a gridded (1degree) daily rainfall dataset, we analyse the spatio-temporal characteristics of the intensity and duration of monsoon (June through September) rainfall for secular changes over the last 50 years. The characteristics of the duration of rain events are described by wet and dry spells. A wet/dry spell is defined as a period of consecutive days with rainfall above/below a particular threshold. We choose to use a threshold that is a function of the local climatological mean, given the spatial heterogeneity of mean monsoon rainfall. The wet and dry spells are then divided into three categories: short [1 to 7 days], moderate [8 to 10 days], long [11 and more days] and analysed for changes over the past 50 years [19512000]. We find that while the number of short duration wet spells show a significant increase over the last 50 years (~15% change), the number of long duration wet spells show a significant decrease (~25%). Furthermore, while the numbers of short duration dry periods have shown a significant increase, the moderate and long duration dry spells do not shown an appreciable change. This increase and decrease in the short and long duration wet spells offset each other and consequently the total number of rainy days during the season has not shown any significant change over the past 50 years. In addition to the duration of wet and dry spells, we also analysed for changes in the accumulated rainfall of the short, medium and long duration wet spells. Our analysis suggests that while the depth of accumulated rainfall in short duration wet spells has shown a significant increase (~20%), the depth of rain in the long duration spells has shown a significant decrease (~30%) in the past fifty years.