Simulating present climate of the global ocean–ice system using the Meteorological Research Institute Community Ocean Model (MRI.COM): simulation characteristics and variability in the Pacific sector

A long-term spin-up and a subsequent interannual simulation are conducted for the ocean–ice component of the climate model intercomparison project (CMIP)-class earth system model of the Japan Meteorological Agency/Meteorological Research Institute. This experiment has three purposes: first is to assess the ability of our model with the Coordinated Ocean–ice Reference Experiments (COREs) forcing in reproducing the present ocean-climate; second is to understand the ocean-climate variability for the past 60 years; third is to present an example of evaluating an ocean–ice interannual variability simulation. The Pacific Ocean is focused on for the last two purposes. After integrating for about 1500 years with repeated use of a detrended CORE interannual forcing, the model reaches a quasi-steady state where the present climate is reproduced satisfactorily. Then, the interannual variability simulation is conducted with the retrieved forcing trend and the result is analyzed. The simulation is successful at reproducing the long-term variability in the Pacific and surrounding oceans. Brief analyses of the tropical and mid-latitude upper layer, deep circulation, and the Arctic sea ice are presented. A caveat in treating other parts of the globe is due to the recent intense convection in the Southern Ocean caused by a remarkably increasing trend of the Southern Hemisphere westerly. Overall, the current simulation with our CMIP-class ocean–ice model is shown to be useful for studying the present ocean-climate variability, specifically in the Pacific sector. It could also be used as a benchmark control experiment that facilitates further research, model development, and intercomparison.     

Biogeochemical evidence of large diapycnal diffusivity associated with the subtropical mode water of the North Pacific

A profiling float equipped with a fluorimeter, a dissolved oxygen (DO) sensor, and temperature and salinity sensors was deployed in the subtropical mode water (STMW) formation region of the North Pacific. It acquired quasi-Lagrangian, 5-day-interval time-series records from March to July 2006. The time-series distribution of chlorophyll showed a sustained and sizable subsurface maximum at 50–100 m, just above the upper boundary of the STMW, throughout early summer (May–July). The DO concentration in this lower euphotic zone (50–100 m) was almost constant and supersaturated in the same period, becoming more supersaturated with time. On the other hand, the DO concentration at 100–150 m near the upper boundary of the STMW decreased much more slowly compared with the main layer of STMW below 150 m, even though oxygen consumption by organisms was expected to be larger in the former depth range. The small temporal variations of DO in the lower euphotic zone and near the upper boundary of the STMW were reasonably explained by downward oxygen transport because of large diapycnal diffusion near the top of the STMW. Assuming that the oxygen consumption rate at 100–150 m was the same as that in the main layer of STMW and compensated by the downward oxygen flux, the diapycnal diffusivity was estimated to be 1.7 × 10⁻⁴ m² s⁻¹. Nitrate transport into the euphotic zone by the same large diffusion was estimated to be 0.8 mmol N m⁻² day⁻¹. All of the transported nitrate could have been used for photosynthesis by the phytoplankton; net community production was estimated to be 5.3 mmol C m⁻² day⁻¹.     

Causes of large-scale landslides in the Lesser Himalaya of central Nepal

Geologically and tectonically active Himalayan Range is characterized by highly elevated mountains and deep river valleys. Because of steep mountain slopes, and dynamic geological conditions, large-scale landslides are very common in Lesser and Higher Himalayan zones of Nepal Himalaya. Slopes along the major highways of central Nepal namely Prithvi Highway, Narayangadh-Mugling Road and Tribhuvan Highway are considered in this study of large-scale landslides. Geologically, the highways in consideration pass through crushed and jointed Kathmandu Nappe affected by numerous faults and folds. The relict large-scale landslides have been contributing to debris flows and slides along the highways. Most of the slope failures are mainly bechanced in geological formations consisting phyllite, schist and gneiss. Laboratory test on the soil samples collected from the failure zones and field investigation suggested significant hydrothermal alteration in the area. The substantial hydrothermal alteration in the Lesser Himalaya during advancement of the Main Central Thrust (MCT) and thereby clay mineralization in sliding zones of large-scale landslide are the main causes of large-scale landslides in the highways of central Nepal. This research also suggests that large-scale landslides are the major cause of slope failure during monsoon in the Lesser Himalaya of Nepal. Similarly, hydrothermal alteration is also significant in failure zone of the large-scale landslides. For the sustainable road maintenance in Nepal, it is of utmost importance to study the nature of sliding zones of large-scale landslides along the highways and their role to cause debris flows and slides during monsoon period.     

The relationship between the isotopic content of precipitation and the precipitation amount in tropical regions

General circulation models (GCMs) fitted with stable isotope schemes are widely used to interpret the isotope–climate relationship. However, previous studies have found that the spatiotemporal isotope/precipitation correlation simulated by GCMs is stronger and more widespread than the observed value. To understand the reason for this failure, we investigated the factors influencing the empirically well-known isotope/precipitation relationship, or precipitation amount effect, in the tropics using newly obtained daily precipitation isotope monitoring data over Asia. As in previous studies, we found an apparent correlation between the long-term monthly mean isotopic content and the corresponding precipitation amount (local precipitation) observed at sub-tropical island stations. Furthermore, on a monthly timescale, the isotopic variability of precipitation for these stations was more clearly related to the regional precipitation amount than to local precipitation. This correlation of isotopic content with the regional precipitation amount was observed at the equatorial (Maritime Continent) stations. For these stations, isotope/local precipitation relationships only appeared over longer timescales, with different regression line slopes at each station. However, at the coastal stations, there was a strong linear relationship between the monthly mean isotopic content and corresponding regional precipitation, and regression line slopes were spatially uniform. For the two sub-tropical terrestrial (Indochina Peninsula) stations, the isotopic minimum appeared without any relationship to rainfall amount but usually occurred at the leeward station during the rainy season. These results suggest that the isotopic variations of precipitation did not depend on the ’local’ rain-out history but on the rain-out process in the surrounding region. However, local rainfall events were associated not only with large-scale disturbances but also with regional circulation. Thus, the scale difference of controlling factors between local rainfall amount and isotopic value results in the weakening of the rainfall amount effect at the observation site and in the discrepancy between GCM simulations and observations. This finding suggests that regional precipitation–isotope relationships should be compared with GCM results. Additionally, because the isotope signal reflects the rain-out history at a regional scale, evaluation of the isotopic field using isotopic GCMs will be useful not only to reconstruct paleoclimate conditions but also to examine how GCMs can reproduce real atmospheric circulation over the tropics.     

A static predictor of seismic demand on frames based on a post‐elastic deflected shape

Predictors of seismic structural demands (such as inter‐storey drift angles) that are less time‐consuming than nonlinear dynamic analysis have proven useful for structural performance assessment and for design. Luco and Cornell previously proposed a simple predictor that extends the idea of modal superposition (of the first two modes) with the square‐root‐of‐sum‐of‐squares (SRSS) rule by taking a first‐mode inelastic spectral displacement into account. This predictor achieved a significant improvement over simply using the response of an elastic oscillator; however, it cannot capture well large displacements caused by local yielding. A possible improvement of Luco's predictor is discussed in this paper, where it is proposed to consider three enhancements: (i) a post‐elastic first‐mode shape approximated by the deflected shape from a nonlinear static pushover analysis (NSPA) at the step corresponding to the maximum drift of an equivalent inelastic single‐degree‐of‐freedom (SDOF) system, (ii) a trilinear backbone curve for the SDOF system, and (iii) the elastic third‐mode response for long‐period buildings. Numerical examples demonstrate that the proposed predictor is less biased and results in less dispersion than Luco's original predictor. Copyright © 2006 John Wiley & Sons, Ltd.     

The three-dimensional shear wave structure in the mantle by overtone waveform inversion - I. Radial seismogram inversion

Summary. The three-dimensional (3-D) shear wave structure of the mantle, down to the depth of about 900 km, is obtained by inverting waveforms of radial component seismograms. Radial component seismograms contain large amplitude overtone signals which circle the Earth as wave packets and are sometimes called X1, X2, X3, … We use data which contain R1, X1 and X2 and filtered between 2 and 10mHz. It is shown that, unless each seismogram is weighted, all seismograms are not fitted uniformly. Only data from large earthquakes are fitted and the final velocity anomalies are biased by the small number of large earthquake data. Resolution is good at shallow depths, becomes worse in the intermediate depth range between about 400 and 500 km and then becomes better at greater depth ranges (600–900km). Even though we use only spheroidal mode data, velocity anomalies in the shallow structure show excellent correlation with the age of the surface rocks of the Earth. In the deeper regions, between about 600 and 900km, South America shows a fast velocity anomaly which may indicate the slab penetration beyond 700 km there. Another region which shows a fast velocity anomaly is the Mariana trench, but other subduction regions do not show such features.