Predictability of Interannual Variability in the Kuroshio Transport South of Japan Based on Wind Stress Data over the North Pacific

It is expected that a roughly two-year forecast of the Kuroshio transport variation can be made from a past record of wind stress data over the ocean, since it takes nearly ten years for the first-mode baroclinic Rossby wave to traverse the entire basin in the midlatitude North Pacific (∼30°N). We therefore investigated the predictability using an ocean general circulation model driven by the wind stress data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. Referring to a hindcast experiment as the control run, we carried out fifteen forecast experiments, the initial conditions of which are taken from the hindcast experiment at intervals of two years during the period from the end of 1969 to the end of 1997. Each of the forecast experiments is driven only by wind stress in the year preceding each experiment. The forecasted Kuroshio transport anomaly south of Japan agrees better with the hindcasted one during the first two years of the forecast in most cases. In some cases, however, significant disagreements occur, most of which are likely due to larger unpredictable variations caused by wind stress anomalies near Japan. At the end of forecast year 2, the anomaly correlation coefficient is about 0.7, and rms of the forecast error is smaller than rms of the hindcasted anomaly. These results indicate that the prediction of the interannual variability in the Kuroshio transport could be made two years in advance at a statistically significant level. This revised version was published online in July 2006 with corrections to the Cover Date.     

Construction and Visualization of a Three Dimensional Geologic Model Using GRASS GIS

The present work aims at introducing a basic theory, implementing methodology and algorithms for 3‐D modeling, and visualizing a geologic model using the Open Source Free GIS GRASS environment. A 3‐D geologic model is constructed from the boundary surfaces of geologic units and the logical model of geologic structure. The algorithms for construction and visualization of the proposed model are based on the geologic function g . The geologic function g assigns a unique geologic unit to every point in the objective 3‐D space. The boundary surface that divides the objective space into two subspaces is estimated using data from field survey. The logical model showing the hierarchical relationship between these boundary surfaces and geologic units can be automatically generated based on the stratigraphic sequence and knowledge of geologic structures. Based on these algorithms, a 3‐D geologic model can be constructed virtually in the GRASS GIS. Applying this model, various geologic surfaces and section models can be visualized in the GRASS GIS environment. “Nviz” was used for dynamic visualization of geologic cross‐sections and generation of animated image sequences. Further, the described algorithms and methods are applied and an online 3‐D geologic modeling system is developed.     

Characteristics of coastal trapped waves along the southern and eastern coasts of Australia

The spatial structures and propagation characteristics of coastal trapped waves (CTWs) along the southern and eastern coasts of Australia are investigated using observed daily mean sea level data and results from a high-resolution ocean general circulation model (OGCM), and by conducting sensitivity studies with idealized numerical models. The results obtained from the sea level observations show that shortterm variations, with a typical period of 1 to 2 weeks, dominate the sea level variability in the southern half of Australia. The signal propagates anticlockwise around Australia with a propagation speed of 4.5 m/s or faster in the western and southern coasts and 2.1 to 3.6 m/s in the eastern coast. Strong seasonality of the wave activity, with large amplitude during austral winter, is also observed. It turns out that the waves are mainly generated by synoptic weather disturbances in the southwestern and southeastern regions. The numerical experiment with idealized wind forcing and realistic topography confirms that the propagating signals have characteristics of the CTW both in the southern and eastern coasts. Sensitivity experiments demonstrate that the difference in the phase speed between the coasts and reduction of the amplitude of the waves in the eastern coast are attributed to the different shape of the continental shelf in each region. The structures and the propagation characteristics of the CTWs around Australia are well reproduced in OFES (OGCM for the Earth Simulator) with dominant contribution from the first mode, although meso-scale eddies may modify the structure of the CTWs in the eastern coast. It is also found that generation or reinforcement of the waves by the wind forcing in the southern part of the eastern coast is necessary to obtain realistically large amplitude of the CTWs in the eastern coast.     

What controls equatorial Atlantic winds in boreal spring?

The factors controlling equatorial Atlantic winds in boreal spring are examined using both observations and general circulation model (GCM) simulations from the coupled model intercomparison phase 5. The results show that the prevailing surface easterlies flow against the attendant pressure gradient and must therefore be maintained by other terms in the momentum budget. An important contribution comes from meridional advection of zonal momentum but the dominant contribution is the vertical transport of zonal momentum from the free troposphere to the surface. This implies that surface winds are strongly influenced by conditions in the free troposphere, chiefly pressure gradients and, to a lesser extent, meridional advection. Both factors are linked to the patterns of deep convection. Applying these findings to GCM errors indicates, that, consistent with the results of previous studies, the persistent westerly surface wind bias found in most GCMs is due mostly to precipitation errors, in particular excessive precipitation south of the equator over the ocean and deficient precipitation over equatorial South America. Free tropospheric influences also dominate the interannual variability of surface winds in boreal spring. GCM experiments with prescribed climatological sea-surface temperatures (SSTs) indicate that the free tropospheric influences are mostly associated with internal atmospheric variability. Since the surface wind anomalies in boreal spring are crucial to the development of warm SST events (Atlantic Niños), the results imply that interannual variability in the region may rely far less on coupled air–sea feedbacks than is the case in the tropical Pacific.     

Equatorial Atlantic variability and its relation to mean state biases in CMIP5

Coupled general circulation model (GCM) simulations participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are analyzed with respect to their performance in the equatorial Atlantic. In terms of the mean state, 29 out of 33 models examined continue to suffer from serious biases including an annual mean zonal equatorial SST gradient whose sign is opposite to observations. Westerly surface wind biases in boreal spring play an important role in the reversed SST gradient by deepening the thermocline in the eastern equatorial Atlantic and thus reducing upwelling efficiency and SST cooling in the following months. Both magnitude and seasonal evolution of the biases are very similar to what was found previously for CMIP3 models, indicating that improvements have only been modest. The weaker than observed equatorial easterlies are also simulated by atmospheric GCMs forced with observed SST. They are related to both continental convection and the latitudinal position of the intertropical convergence zone (ITCZ). Particularly the latter has a strong influence on equatorial zonal winds in both the seasonal cycle and interannual variability. The dependence of equatorial easterlies on ITCZ latitude shows a marked asymmetry. From the equator to 15°N, the equatorial easterlies intensify approximately linearly with ITCZ latitude. When the ITCZ is south of the equator, on the other hand, the equatorial easterlies are uniformly weak. Despite serious mean state biases, several models are able to capture some aspects of the equatorial mode of interannual SST variability, including amplitude, pattern, phase locking to boreal summer, and duration of events. The latitudinal position of the boreal spring ITCZ, through its influence on equatorial surface winds, appears to play an important role in initiating warm events.     

Anomalous climatic conditions associated with the El Ni?o Modoki during boreal winter of 2009

The winter months from December 2009 to February 2010 witnessed extreme conditions affecting lives of millions of people around the globe. During this winter, the El Ni?o Modoki in the tropical Pacific was a dominant climatic mode. In this study, exclusive impacts of the El Ni?o Modoki are evaluated with an Atmospheric General Circulation Model. Sensitivity experiments are conducted by selectively specifying anomalies of the observed sea surface temperature in the tropical Pacific. Observed data are also used in the diagnostics to trace the source of forced Rossby waves. Both the observational results and the model simulated results show that the heating associated with the El Ni?o Modoki in the central tropical Pacific accounted for most of the anomalous conditions observed over southern parts of North America, Europe and over most countries in the Southern Hemisphere viz. Uruguay. Unlike those, the model-simulated results suggest that the anomalously high precipitation observed over Australia and Florida might be associated with the narrow eastern Pacific heating observed during the season.