On Importance of Acoustic Backscatter Corrections for Texture-based Seafloor Characterization

Seafloor segmentation and characterization based on local textural properties of acoustic backscatter has been a subject of research since 1980s due to the highly textured appearance of sonar images. The approach consists of subdivision of sonar image in a set of patches of certain size and calculation of a vector of features reflecting the patch texture. Advance of multibeam echosounders (MBES) allowed application of texture-based techniques to real geographical space, and predicted boundaries between acoustic facies became experimentally verifiable. However, acoustic return from uncalibrated MBES produces artifacts in backscatter mosaics, which in turn affects accuracy of delineation. Development of Geocoder allowed creation of more visually consistent images, and reduced the number of factors influencing mosaic creation. It is intuitively clear that more accurate backscatter mosaics lead to more reliable classification results. However, this statement has never been thoroughly verified. It has not been investigated which corrections are important for texture-based characterization and which are not essential. In this paper the authors are investigating the Stanton Banks common dataset. Raw data files from the dataset have been processed by the Geocoder at different levels of corrections. Each processing resulted in a backscatter mosaic demonstrating artifacts of different levels of severity. Mosaics then underwent textural analysis and unsupervised classification using Matlab package SonarClass. Results of seafloor characterization corresponding to varying levels of corrections were finally compared to the one generated by the best possible mosaic (the one embodying all the available corrections), providing an indicator of classification accuracy and giving guidance about which mosaic corrections are crucial for acoustic classification and which could be safely ignored

Atmospheric carbon dioxide, ice sheet and topographic constraints on palaeo moisture availability in Asia

Today, the hydrological regime in East and South Asia is dominated by the monsoons, whilst central Asia is characterized as arid. Studies that have examined the onset of aridity and the intensification of the monsoons in Asia have generated significant debate, especially in respect to the timing of monsoon onset and how this relates to the potential causal mechanisms. The uplift of the Tibetan Plateau, the retreat of the Paratethys Sea, and the global cooling after the Eocene/Oligocene transition are all considered major drivers of Asian aridity and monsoonal intensification. However, little is known about each of these factor's contribution to the development of modern monsoon behaviour. Here, for the first time, we perform sensitivity simulations of a fully coupled ocean–atmosphere climate model (HadCM3) to investigate the effect of the Greenland and Antarctic ice-sheets formation, atmospheric carbon dioxide (CO2) variability, and Tibetan Plateau uplift on East Central Asian aridity and monsoon driven precipitation. We focus on three individual regions, the South Asian Monsoon, the East Asian Monsoon and the Arid East Central Asia and we present the annual precipitation cycle and the moisture availability over each region. Our results show that of the parameters investigated the primary control on Asian hydroclimate is the topography of the Tibetan Plateau. Furthermore, our results highlight that the significance of each forcing depends on the component of the hydrological region and factors studied, a factor that proxy interpretation need to take into consideration

The role of Central Asian uplift in East Asian Monsoon circulation and its palaeoclimate implication

It has been clearly established that the climate of Asia is significantly affected by high-elevation orogens such as the Tibetan Plateau, Mongolian Plateau and Tian-Shan. The East Asian Monsoon (EAM), one of the most prominent features of Asian climate, has been well studied in a modern context and its dynamics are generally well understood. However, specific features of the EAM are less studied and understood in a palaeoclimate context, largely because of associated uncertainties in palaeotopography for the Cenozoic era. Here, we investigate changes in the individual stages of the EAM in response to increasing topography over Central Asia. We perform a series of sensitivity experiments with different palaeogeographic elevations using a coupled ocean-atmosphere General Circulation Model (HadCM3), to investigate seasonal variability of the EAM, and investigate the emergent critical threshold in elevation where the patterns of atmospheric circulation and climate over Asia attains the characteristics observed in the modern climate system. Our results indicate that above an elevation threshold of 3000 m, EAM circulation follows the modern pattern, but below that threshold, EAM circulation and precipitation follow a distinctly different pattern, where the westerly jet does not propagate into the higher latitudes and monsoonal precipitation is limited to June and July. This shift in circulation pattern has important implications for the successful interpretation of proxy-based palaeoclimate and environmental reconstructions. In addition, our results emphasize the importance of the latitudinal position of high-elevation on the EAM circulation, by showing that low-elevation can produce modern-like EAM conditions, if located at different latitudes than modern