Large-scale explosive eruptions can disperse tephra over large areas of the Earth's surface by injecting particles to high levels in the atmosphere in convective plumes (Sparks et al., 1997). The extent and direction of dispersal is a function of (1) eruption column height, (2) direction and intensity of the prevailing winds, and (3) size distribution of tephra from the source eruption. In order to evaluate potential source volcanoes for tephra fall deposits in a certain area, it is necessary to know the regional wind patterns at various elevations in the atmosphere. The western Caribbean area is characterized by a surface flow that is dominated by the trade winds that blow from east to west (Fig. 12). However, at higher elevations there are significant changes in both the magnitude and direction of the prevailing winds. For example, at the 250 millibar level (~11 km elevation) the prevailing winds are switched by 180°, blowing from the west to the east (Fig. 13). Furthermore, the wind velocities at this altitude are two to three times as strong as the surface trade winds. At higher elevations (>20 km) the winds shift direction once again to a more east-southeasterly direction.
The vertical variations in wind intensity and direction are also complicated by a seasonal component. This can be seen from detailed records collected at Guatemala City, Guatemala, located at a latitude that is representative of the atmospheric circulation in the western Caribbean area (Fig. 14). Strong westerly winds in the lower stratosphere are most pronounced in winter and spring, but become more subdued in the summer and fall. As these winds decrease there is a strengthening of upper level winds >20 km height (Fig. 14).
Transport of tephra in the western Caribbean is likely to be complicated because of the observed reversal in transport direction with altitude (Fig. 14). However, some generalizations can be made regarding the importance of transport at different levels. In order for a widespread tephra layer to be formed in deep-sea sediments, it is necessary to have a sufficiently large source eruption. The location of Sites 998, 999, and 1000 from the Miocene to present indicate that the closest possible source area (Central America) is at a distance of at least 700 km (Fig. 15). In order to form tephra fall layers of several to tens of centimeters in thickness at such distances would require eruptions of several tens to hundreds of cubic kilometers of magma (e.g., Ninkovich et al., 1978). Eruptions of this type invariably produce convective eruption columns that inject large quantities of tephra well into the stratosphere (Carey and Sigurdsson, 1989). The tops of such columns may reach altitudes of 40 km or more and spread out to form a giant mushroom-shaped plume. Fallout of particles in these plumes occurs primarily from the base of the expanding mushroom-shaped cap, which will be of the order of 20 km height (Carey and Sparks, 1986). Dispersal of fine-grained tephra will therefore take place at levels above the tropopause, and in the western Caribbean it is likely to be controlled by strong westerly winds for at least a significant part of the year. It is possible, however, that easterly transport of some material would also take place at higher and lower altitudes. This bi-directional transport mechanism is well illustrated by the dispersal of tephra from a 84-ka eruption of Atitlan caldera in Guatemala (Drexler et al., 1980). The widespread Los Chocoyos tephra layer from this eruption is found in Pacific, Gulf of Mexico, and Caribbean sediments (Ledbetter, 1985; Drexler et al., 1980). Estimates of the total volume of the eruption are of the order of 200 km3 of magma.