Structure of Velocity and Temperature in the Northeast Pacific as Measured with Lagrangian Drifters in Fall 1987

Abstract

In October 1987, 49 Lagrangian surface drifters (TRISTAR-II) were released in a 200-km ? 200-km square area southeast of Ocean Station Papa as part of the OCEAN STORMS Experiment. The drifters measured temperature at the drogue level and reported their position through ARGOS approximately 11 times per day. Thirty-one of the drifters retained drogues for longer than three months, and data from those instruments are used to describe the evolving fall 1987 pattern of current and temperature structures at 15 m in the area between 46° and 49°N, 142°W and 132°W. Time variable currents were dominated by mesoscale eddies of anticyclonic rotation with horizontal radii of 53?86 km and rotational speeds of 10?20 cm s?1. These eddies persisted for at least 90 days as evidenced by successive drifter trajectories through the eddies. Currents with periods longer than 1 day had a mean to the east of 4.4 cm s?1 and a mean to the north of 0.7 cm s?1. Background eddy kinetic energy levels were 40 cm s?2. Thus, eddy kinetic energy was four times larger than mean kinetic energy. The eastward single particle diffusivity was 1100 m2 s?1 and northward diffusivity was 1600 m2 s?1. The local change of thermal energy at 15-m depth was ?2.9 W m?3, while on average, flow advected cold water to the east at a rate of 0.8 W m?3. Therefore, large-scale advective processes accounted for 28% of the thermal energy balance at 15 m. This horizontal heat convergence in the open ocean is comparable in magnitude to that produced by powerful equatorial currents in the eastern Pacific cold tongue.