Intercomparison of the seasonal cycle of tropical surface stress in 17 AMIP atmospheric general circulation models

 The mean state of the tropical atmosphere is important as the nature of the coupling between the ocean and the atmosphere depends nonlinearly on the basic state of the coupled system. The simulation of the annual cycle of the tropical surface wind stress by 17 atmospheric general circulation models (AGCMs) is examined and intercompared. The models considered were part of the Atmospheric Model Intercomparison Project (AMIP) and were integrated with observed sea surface temperature (SST) for the decade 1979–1988. Several measures have been devised to intercompare the performance of the 17 models on global tropical as well as regional scales. Within the limits of observational uncertainties, the models under examination simulate realistic tropical area-averaged zonal and meridional annual mean stresses. This is a noteworthy improvement over older generation low resolution models which were noted for their simulation of surface stresses considerably weaker than the observations. The models also simulate realistic magnitudes of the spatial distribution of the annual mean surface stress field and are seen to reproduce realistically its observed spatial pattern. Similar features are observed in the simulations of the annual variance field. The models perform well over almost all the tropical regions apart from a few. Of these, the simulations over Somali are interesting. Over this region, the models are seen to underestimate the annual mean zonal and meridional stresses. There is also wide variance between the different models in simulating these quantities. Large model-to-model variations were also seen in the simulations of the annual mean meridional stress field over equatorial Indian Ocean, south central Pacific, north east Pacific and equatorial eastern Pacific oceans. It is shown that the systematic errors in simulating the surface winds are related to the systematic errors in simulating the Inter-Tropical Convergence Zone (ITCZ) in its location and intensity. Weaker than observed annual mean southwesterlies simulated by most models over Somali is due to weaker than observed southwesterlies during the Northern Hemisphere summer. This is related to the weaker than observed land precipitation simulated by most models during the Northern Hemisphere summer. The diversity in simulation of the surface wind over Somali and equatorial Indian ocean is related to the diversity of AGCMs in simulating the precipitation zones in these regions. Received: 2 August 1996 / Accepted: 7 February 1997     

Pressure and temperature conditions for crystallization of metamorphic allanite and monazite in metapelites: a case study from the Miyar Valley (high Himalayan Crystalline of Zanskar,NW India)

The textural and chemical evolution of allanite and monazite along a well‐constrained prograde metamorphic suite in the High Himalayan Crystalline of Zanskar was investigated to determine the P–T conditions for the crystallization of these two REE accessory phases. The results of this study reveals that: (i) allanite is the stable REE accessory phase in the biotite and garnet zone and (ii) allanite disappears at the staurolite‐in isograd, simultaneously with the occurrence of the first metamorphic monazite. Both monazite and allanite occur as inclusions in staurolite, indicating that the breakdown of allanite and the formation of monazite proceeded during staurolite crystallization. Staurolite growth modelling indicates that staurolite crystallized between 580 and 610 °C, thus setting the lower temperature limit for the monazite‐forming reaction at ~600 °C. Preservation of allanite and monazite inclusions in garnet (core and rim) constrains the garnet molar composition when the first monazite was overgrown and subsequently encompassed by the garnet crystallization front. Garnet growth modelling and the intersection of isopleths reveal that the monazite closest to the garnet core was overgrown by the garnet advancing crystallization front at 590 °C, which establishes an upper temperature limit for monazite crystallization. Significantly, the substitution of allanite by monazite occurs in close spatial proximity, i.e. at similar P–T conditions, in all rock types investigated, from Al‐rich metapelites to more psammitic metasedimentary rocks. This indicates that major silicate phases, such as staurolite and garnet, do not play a significant role in the monazite‐forming reaction. Our data show that the occurrence of the first metamorphic monazite in these rocks was mainly determined by the P–T conditions, not by bulk chemical composition. In Barrovian terranes, dating prograde monazite in metapelites thus means constraining the time when these rocks reached the 600 °C isotherm.     

Effect of land–ocean contrast on the structure of anomaly tropical circulation

While the important role of land–ocean contrast (LOC) in the mean atmospheric circulation is well-known, an intriguing question remains as to whether LOC can also significantly influence the anomaly circulation. This question is particularly important in the tropics, where a large part of the variabilities is known to be due to convective internal dynamics, which in turn can be significantly affected by LOC. In the present work, we investigate this question using a model of convectively driven anomaly circulation in the tropics. Emphasizing the lower tropospheric flow, we adopt a model describing the horizontal dynamics of the first baroclinic mode on an equatorial β-plane, in the presence of moist feedbacks. We introduce LOC in both latitudinal and longitudinal directions. Land surface dryness is taken into account in the moisture budget through the control of evaporative flux. A constant non-latent heating term is used over land in order to represent radiative and sensible heating effects. First, a control run is performed, numerically, without any LOC using random initial perturbations. The gravest mode that emerges from the control run, which is a wave-2 feature with a period around 20 days, is then used as the initial field for the model runs with LOC. The results show that LOC can significantly influence even the tropical variabilities. A latitudinal LOC, with a land mass in the northern hemisphere (north of 10°N), tends to shift the region of maximum precipitation slightly north of the equator with accompanying steeper gradients near the land–ocean boundary. The implications of this result for our understanding of Asian summer monsoon conditions are discussed. When LOC is only in the longitudinal direction, the dominant wave structure that emerges from the model run has wavenumber one and a period of about 35 days, very similar to the observed 30–60 day oscillation. In our final experiment, which is nearer to the realistic land–ocean configuration in the tropics, it is found that both latitudinal and longitudinal effects of LOC are important aspects of the tropical anomaly circulation. It is suggested that some of the patterns in the precipitation distribution, observed in the tropics and simulated using general circulation models are results of convectively induced internal dynamics, modulated by LOC.     

Effects of atmospheric dynamics and ocean resolution on bi-stability of the thermohaline circulation examined using the Grid ENabled Integrated Earth system modelling (GENIE) framework

We have used the Grid ENabled Integrated Earth system modelling (GENIE) framework to undertake a systematic search for bi-stability of the ocean thermohaline circulation (THC) for different surface grids and resolutions of 3-D ocean (GOLDSTEIN) under a 3-D dynamical atmosphere model (IGCM). A total of 407,000 years were simulated over a three month period using Grid computing. We find bi-stability of the THC despite significant, quasi-periodic variability in its strength driven by variability in the dynamical atmosphere. The position and width of the hysteresis loop depends on the choice of surface grid (longitude-latitude or equal area), but is less sensitive to changes in ocean resolution. For the same ocean resolution, the region of bi-stability is broader with the IGCM than with a simple energy-moisture balance atmosphere model (EMBM). Feedbacks involving both ocean and atmospheric dynamics are found to promote THC bi-stability. THC switch-off leads to increased import of freshwater at the southern boundary of the Atlantic associated with meridional overturning circulation. This is counteracted by decreased freshwater import associated with gyre and diffusive transports. However, these are localised such that the density gradient between North and South is reduced tending to maintain the THC off state. THC switch-off can also generate net atmospheric freshwater input to the Atlantic that tends to maintain the off state. The ocean feedbacks are present in all resolutions, across most of the bi-stable region, whereas the atmosphere feedback is strongest in the longitude–latitude grid and around the transition where the THC off state is disappearing. Here the net oceanic freshwater import due to the overturning mode weakens, promoting THC switch-on, but the atmosphere counteracts this by increasing net freshwater input. This increases the extent of THC bi-stability in this version of the model. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Prediction and monitoring of monsoon intraseasonal oscillations over Indian monsoon region in an ensemble prediction system using CFSv2

An ensemble prediction system (EPS) is devised for the extended range prediction (ERP) of monsoon intraseasonal oscillations (MISO) of Indian summer monsoon (ISM) using National Centers for Environmental Prediction Climate Forecast System model version 2 at T126 horizontal resolution. The EPS is formulated by generating 11 member ensembles through the perturbation of atmospheric initial conditions. The hindcast experiments were conducted at every 5-day interval for 45 days lead time starting from 16th May to 28th September during 2001–2012. The general simulation of ISM characteristics and the ERP skill of the proposed EPS at pentad mean scale are evaluated in the present study. Though the EPS underestimates both the mean and variability of ISM rainfall, it simulates the northward propagation of MISO reasonably well. It is found that the signal-to-noise ratio of the forecasted rainfall becomes unity by about 18 days. The potential predictability error of the forecasted rainfall saturates by about 25 days. Though useful deterministic forecasts could be generated up to 2nd pentad lead, significant correlations are found even up to 4th pentad lead. The skill in predicting large-scale MISO, which is assessed by comparing the predicted and observed MISO indices, is found to be ~17 days. It is noted that the prediction skill of actual rainfall is closely related to the prediction of large-scale MISO amplitude as well as the initial conditions related to the different phases of MISO. An analysis of categorical prediction skills reveals that break is more skillfully predicted, followed by active and then normal. The categorical probability skill scores suggest that useful probabilistic forecasts could be generated even up to 4th pentad lead.     

Floral biodiversity and geology of the Talcher Basin,Orissa, India during the Permian–Triassic interval

The review paper provides an updated account of the previous and recently published records concerning the palaeobiology and the geology of the Talcher Basin of Orissa State, India. We conclude that fossil floral species in this basin originated in the earliest Permian Talchir Formation and evolved and diversified through the Karharbari Fm., Barakar Fm., Barren Measures Fm. and the uppermost Kamthi Fm. (Late Permian–Triassic). The megaflora and the palynology of the different formations of the basin are also discussed briefly. The geological setting of the basin along with the status of different formations (especially the Kamthi Formation) has been redefined. The post‐Barakar Fm. rocks, earlier retained in the Raniganj/Kamthi, Panchet and Mahadeva formations in this basin, have been critically assessed and redefined as the Lower and Upper Kamthi formations of Late Permian and Triassic ages, respectively. Accordingly, the geological map of the basin has been modified. Permian deposits (particularly the Barakar and the lower Kamthi formations) not only have the best preserved flora but also possess the highest diversity, whereas the upper Kamthi Triassic sediments have a meagre number of taxa. The plant diversity of the basin has been discussed in detail to interpret the development of the flora, evolutionary trends and palaeoenvironments of the basin. The patchy Gangamopteris vegetation of the Talchir glacial phase has ultimately evolved and diversified through time (Karharbari Fm. to Lower Kamthi Fm.) and gave rise to the thick dense swampy forests consisting of large Glossopteris trees and other shade‐loving under‐storied pteridophytes. Several groups of plants including spores and pollen have disappeared in a ladder pattern during the Permian–Triassic interval (Lower Kamthi–Upper Kamthi Fm.) and, similarly, in steps, many new fore‐runners appeared in the Upper Kamthi Formation. Records of marine acritarchs and ichnofossils in this basin at various Permian–Triassic levels demonstrate that there were marine influences. These features suggest a paralic (coastal marine to deltaic) mode of origin of the coal beds and associated sediments in the basin. The present study also advocates the continued survival of plants, rather than a mass extinction near the vicinity of the Permian–Triassic (P–T) boundary in this basin. Copyright © 2012 John Wiley & Sons, Ltd.