In 1922, a historic moment in cartography and oceanography occurred. The USS Stewart used a Navy Sonic Depth Finder to draw a continuous map across the bed of the Atlantic. This involved little guesswork because the idea of sonar is straightforward with pulses being sent from the vessel, which bounce off the ocean floor, then return to the vessel.[33] The deep ocean floor is thought to be fairly flat with occasional deeps, abyssal plains, trenches, seamounts, basins, plateaus, canyons, and some guyots. Various shelves along the margins of the continents constitute about 11% of the bottom topography with few deep channels cut across the continental rise.
During the LGM the Laurentide Ice Sheet covered most of northern North America while Beringia connected Siberia to Alaska. In 1973, late American geoscientist Paul S. Martin proposed a "blitzkrieg" colonization of the Americas by which Clovis hunters migrated into North America around 13,000 years ago in a single wave through an ice-free corridor in the ice sheet and "spread southward explosively, briefly attaining a density sufficiently large to overkill much of their prey."[76] Others later proposed a "three-wave" migration over the Bering Land Bridge.[77] These hypotheses remained the long-held view regarding the settlement of the Americas, a view challenged by more recent archaeological discoveries: the oldest archaeological sites in the Americas have been found in South America; sites in north-east Siberia report virtually no human presence there during the LGM; and most Clovis artefacts have been found in eastern North America along the Atlantic coast.[78] Furthermore, colonisation models based on mtDNA, yDNA, and atDNA data respectively support neither the "blitzkrieg" nor the "three-wave" hypotheses but they also deliver mutually ambiguous results. Contradictory data from archaeology and genetics will most likely deliver future hypotheses that will, eventually, confirm each other.[79] A proposed route across the Pacific to South America could explain early South American finds and another hypothesis proposes a northern path, through the Canadian Arctic and down the North American Atlantic coast.[80]Early settlements across the Atlantic have been suggested by alternative theories, ranging from purely hypothetical to mostly disputed, including the Solutrean hypothesis and some of the Pre-Columbian trans-oceanic contact theories.
Bottom Topography Of Indian Ocean Pdf Free
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Seasonal variability of the ocean bottom pressure (OBP) in the world oceans is investigated using 15 years of GRACE observations and a Pressure Coordinate Ocean Model (PCOM). In boreal winter, negative OBP anomalies appear in the northern North Pacific, subtropical South Pacific and north of 40 S in the Indian Ocean, while OBP anomaly in the Southern Ocean is positive. The summer pattern is opposite to that in winter. The centers of positive (negative) OBP signals have a good coherence with the mass convergence/divergence due to Ekman transport, indicating the importance of wind forcing. The PCOM model reproduces the observed OBP quite well. Sensitivity experiments indicate that wind forcing dominates the regional OBP seasonal variations, while the contributions due to heat flux and freshwater flux are unimportant. Experiments with daily sea level pressure (SLP) forcing suggest that at high frequencies the non-static effect of SLP is not negligible.
Before the launching of GRACE, studies of OBP were mainly based on theoretical diagnosis and numerical models due to the lack of observations. Gill and Niller (1973) pioneered the study of OBP; they derived the equation of OBP and diagnosed the seasonal distribution of OBP in the northern Pacific and northern Atlantic. They pointed out that the OBP represents the barotropic response to changes in the wind stress, which dominates the variations of sea level at high latitudes. Wunsch and Stammer (1997) reviewed the oceanic response to the atmospheric pressure loads. Besides the static response (inverted barometer effect), non-static response occurs at certain periods in some regions (Ponte et al. 1991; Ponte 1992, 1993; Gaspar and Ponte 1997), indicating that the variations of sea level pressure (SLP) may alter the OBP. Ponte (1999) firstly simulated the seasonal variability of OBP using a volume conserved model. The model results indicated that seasonal amplitude of large-scale OBP ranges from less than 1 cm to several centimeters in shallow water and continental shelf. Huang et al. (2001) developed the first model in pressure coordinates, the Pressure Coordinates Ocean Model (PCOM), which exactly conserves the total mass. They found the evolution of free surface elevation and bottom pressure in the model is different from that produced by models based on the Boussinesq approximations (Huang and Jin 2002). Due to limited observations, these diagnostic and model results have not yet been verified.
As shown in Fig. 1, OBP anomalies are closely linked to the Ekman transport divergence/convergence. Figure 3 shows the observed OBP and diagnosed OBP based on Eq. (1) in the North Pacific (Gill and Niiler 1973). Overall, the pattern and amplitude of the signals obtained from these two approaches are similar, especially for the large-scale feature in the open northern North Pacific during boreal winter (summer) (Fig. 3). However, the results from the diagnostic equation are different from observations in the regions with complicated bottom topography. When there exists closed contour of \(\mathcalH\), such as in the southern Indian Ocean and Pacific, the OBP cannot be obtained by integration along \(\mathcalH\) from eastern boundary. Under this circumstance, Eq. (1) will be invalid. At lower latitudes and longer periods, baroclinic processes also make considerable contributions to the OBP variations (Piecuch 2013,2015; Piecuch and Ponte 2014; Piecuch et al. 2015). In addition, the OBP adjusts around the world oceans. Therefore, to fully understand OBP variations, even on regional scale, a global model is needed.
Based on linear water-wave theory, this study investigated the scattering of oblique incident water waves by two unequal surface-piercing thin vertical rigid plates with stepped bottom topography. By using the matched eigenfunction expansion method and a least square approach, the analytical solutions are sought for the established boundary value problem. The effects of the incidence angle, location of step, depth ratio of deep to shallow waters, and column width between two plates, on the reflection coefficients, the horizontal wave forces acting on the two plates, and the mean surface elevation between the two plates, are numerically examined under a variety of wave conditions. The results show that the existence of the stepped bottom between two plates considerably impacts the hydrodynamic performances of the present system. It is found that the effect of stepped bottom on the reflection coefficient of the present two-plate structure is evident only with waves of the low dimensionless frequency. Moreover, the influence of the step location on the hydrodynamic performance of the present two-plate structure is slight if the step is placed in between the two plates.
Black isobars are hundred meters contour intervals and gray isobars are twenty meters contour interval. The inset plot shows the bathymetry bottom topography of the Pemba channel along an east-west transect indicated by a red dotted line.
Information of surface current variations in the Pemba Channel and many other coastal areas in the Western Indian Ocean is limited because of lack of consistent and time-series of oceanographic data. The bottom topography of the Pemba Channel partly hinders installation of equipment needed to collect continuous and high-frequency data. However, since the late 1980s the Pemba Channel has been traversed by drifters from the Global Drifter Program, which offers long-term in-situ observations to study surface currents [22]. Despite the available drifter observations, the Pemba Channel is the least explored coastal waters of Tanzania [17]. This study used drifter observations to understand how surface current speeds vary in time and space in the Pemba Channel. We have also deduced that the EACC does not always flow northwards, but changes its direction based on the monsoon seasons. Therefore, this study provides very important surface current information in the Pemba Channel. And although the drifters have proved to be robust tools for oceanographic data collection, even in areas without oceanographic observations [23], more drifter observations in the shallower parts of the Pemba Channel are required to expand this analysis. 2ff7e9595c
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