In the benthic studies, emphasis is on those organisms living within the sediments or directly on the sea floor. Populations of actively swimming organisms are known to be associated with water directly over the bottom, but very little is known about them. Highly specialized techniques are required to study them intensively, although a program utilizing baited traps is planned to give some insight into the occurrence and relative importance of these organisms.

This section addresses the ambient pre-mining condition of the ocean environment in the DOMES area. The data base for the DOMES baseline is drawn from two sources: the published and unpublished results of earlier investigations within and/or associated with the DOMES area, and a group of specific studies supported by the DOMES project.

Field data for the DOMES-supported studies (Appendix 1) have been collected in an integrated manner. On each cruise leg, scientists representing several disciplines worked cooperatively on a specific task. This report points out interrelationships among studies and disciplines where applicable. However, in order to present the material in an orderly manner, data for each segment of the water column--upper, middle, and lower--are therefore classified under major oceanographic disciplines.

2.2 Upper Water Column

2.2.1 Physical Oceanography

Climatology. The DOMES area climatology may be generalized as typical of the trade winds zone. The Intertropical Convergence Zone is along the southern limit of the area and the North Pacific High is to the north. The resultant Northeast Tradewinds are the most important climatologic feature of the area. These are very steady and affect the whole of the DOMES are through most of the year. The extreme southern part of the area may have some impingement by the Southeast Trades during northern winter as the thermal equator shifts to the north. Highest rainfall is associated with the thermal equator at the south, and rainfall decreases northward across the area. The principal short-term disturbances are those associated with the passage of tropical storms (figure 5) which occur principally through the northern summer (table 1).

The climatologic description of the surface currents is analogous to the trade winds and consists of three zonal currents: the westward-flowing North Equatorial Current, the westward-flowing South Equatorial Current, and between them, the eastward-flowing Equatorial Countercurrent (figure 6, 7). The currents are relatively shallow; Tsuchiya (1968) indicated that the 500 db (500 m) level was deep enough for estimating the geostrophic flow in the upper layers of the intertropical region. Because there is a marked seasonal and spatial variation in the winds and the wind stress, (e.g., the northsouth migration of the Intertropical Convergence Zone) the magnitude of the currents is not constant. The Countercurrent is stronger in the northern summer than in the winter; and in fact there is doubt about the existence of a continuous countercurrent during some of the winter months.

Figure 5.

180°

170°

160°

150°

140°

130°

120°

Eastern Pacific hurricanes, 1968-1975. The A and B designations in each month serve only to avoid overcrowding the figure.

110°

Table 1. Occurrence of Eastern Pacific tropical storms and hurricanes by months and years (from Monthly Weather Review, 104, No. 4, April 1976)

The equatorial oceans are characterized by a layer of warm, well-mixed water separated from the colder intermediate and deep water by a narrow zone of remarkably high stability. The thickness of the well-mixed surface layer varies from 10 m to 150 m. The intensity of the density gradient below the surface mixed layer is not the same throughout the tropical regions. The strongest density gradients are usually found in the vicinity of the north edge of the countercurrent.

The circulation of the tropical ocean has been inferred from reports of ship's drift and from the distributions of conservative properties like salinity on the constant density surface or from surface height distributions computed from the vertical density field. Direct observations of currents are rare; in fact, according to the National Oceanographic Data Center, no direct current measurements of duration greater than 4 days were made prior to the DOMES investigations in an 8 x 106 km2 region encompassing the three DOMES sites.

Currents. In view of the lack of specific information on measured currents in the DOMES area, the project has initiated a program of field investigations to establish both magnitude and variability of currents of specific interest in terms of the DOMES problem. This effort is continuing, with measurements being made at this time. Initial results are available from site C and a brief summary of these follows.

Current and temperature time series measurements within the upper 300 m were made during the period August 23 to October 28, 1975 (Halpern, 1976a, 1976b). A single-point, compound, taut-line surface mooring was deployed at site C. Current and temperature measurements were made at five depths: 20 m, 50 m, 100 m, 200 m, and 300 m.