Study Of The Benthic Boundary Layer In The North Equatorial Pacific
Authors: Hayes Stanley P. 
Published: October 2008

ABSTRACT
From April to November 1976 moored current meter measurements were made at three depths in the bottom 50 mat two locations, 11043'N, l38°23'W and 8°27'N 150049'W. Hydrographic measurements were obtained near the mooring upon deployment and recovery. The latter data were collected with a high resolution Neil Brown CTD system. These measurements reveal similarities and differences in the benthic boundary layer at the two locations. The mean potential temperature at the bottom was 1.034°C at the eastern site and 0.982°C at the western site in agreement with historical data. Near bottom vertical temperature gradients were approximately adiabatic. Within instrumental uncertainty (.001½) the potential temperature of the bottom 200 m was often isothermal. At the western site some profiles showed hyperadiabatic temperature gradients extending over the bottom 50 m. Bottom currents were low and, on average, westward at both locations. The average speed at the eastern site was more than double the average speed at the western site. Both records showed the importance of low frequency (less than inertial) oscillations which accounted for about 80% of the observed variance.

INTRODUCTION
During the 6 month period from 28 April to 11 November 1976, current and temperature time-series measurements were made within the bottom 50 mat two locations (called EBW-l, EBW-2) located in the Northeastern tropical Pacific Ocean (Figure 1). Based upon historical analysis of water mass characteristics (Wong, 1972) and theoretical models of abyssal circulation (Stommel, 1958) one expects mean currents to the east in the vicinity of the arrays., Direct current measurements near the bottom in the Central North Pacific Ocean (Earle, 1975; Ramp 1976) generall) show weak mean flows with vector averaged velocities of order 1 cm/sec. However, preliminary measurements about 800 nm northeast of EBW-l (Amos, et al 1976) indicated a mean westward velocity of 6 cm/sec 10 m above the bottom. In the present study a preliminary description of the near bottom hydrography and temporal 'variations in the abyssal currents at the two buoy stations is discussed. The EBW-l and 2 moorings were located at 11°42.5'N, 138°23.5'W (depth 4938 m) and 8°27' N, 150°49. l"W (depth 4984 m), respectively. This abyssal circulation study was one component of a multi-institutional interdisciplinary program to investigate environmental baseline conditions in a manganese nodule province.
Vertical profiles of pressure, temperature, and conductivity were taken from the NOAA Ship OCEANO-. GRAPHER on the deployment and recovery cruises. The deployment survey contained about 5 deep casts around each buoy location. These stations, made with a Plessey model 9040 Conductivity/Temperature/Depth (CTD instrument, were designed to give a preliminary measure of the hydrographic variability around each location. Instrument operation procedures and data processing techniques for the Plessey CTD are discussed in previous reports (e.g., Holbrook and Halpern 1974). On the recovery cruise a more detailed survey at each location (Figure 1) was performed using a Neil Brown Instrument System (NBIS) CTD profiler.