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P. Flament, J. Firing, M. Sawyer, and C. Trefois

Abstract

Intense diurnal warming of the ocean surface was observed in April 1982 off California, using a combination of mooring, hydrographic, and satellite infrared and satellite pigment measurements. The event corresponded to a spatial and temooral minimum of the wind stress. The diurnal surface temperature amplitude exceeded 6.6°C locally despite a 490-nm optical depth of 20 m, suggesting that phytoplankton was not responsible for the shallow heat trapping. Coherent horizontal temperature streaks at least 50 km long and 4-8 km wide formedduring the subsequent erosion of the shallow warm layers. It is hypothesized that thcfr scale was set by planetary boundary-layer circulations.

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T. K. Chereskin, E. Firing, and J. A. Gast

Abstract

Acoustic Doppler current profiler (ADCP) velocity measurements an subject to bias due to the effect of the signal processing filters on the spectrum of the Doppler-shifted signal and on the noise. Bias will occur when the filter is not centered on the signal. Numerical models of the received signal and the processing filter used in RD Instruments profilers show that biases on the order of 10 cm s−1 can occur in the lower half of the current profile in regions of high current shear. Errors tend to increase with the width of the acoustic beam and with the speed of the ship, and decrease with the pulse length. These biases are identified in ADCP velocity measurements made in the high shear of the equatorial undercurrent. We suggest criteria for editing existing ADCP data to remove excessive bias, and we recommend changes in profiler parameters which should greatly reduce the bias in future datasets.

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Gregory C. Johnson, Michael J. McPhaden, and Eric Firing

Abstract

Upper-ocean horizontal velocity and divergence were estimated from shipboard observations taken from 1991 to 1999 in the equatorial Pacific between 170°W and 95°W. Mean transports were estimated for the zonal currents at the mean longitude of the sections, 136°W. Mean meridional currents for the entire longitude range included poleward surface flows reaching −0.09 m s−1 in the south and 0.13 m s−1 in the north as well as equatorward flow within the thermocline reaching 0.05 m s−1 in the south and −0.04 m s−1 in the north near 23°C (85 m). Vertical velocity was diagnosed by integrating horizontal divergence estimated for the entire region down from the surface. Equatorial upwelling velocities peaked at 1.9 (±0.9) × 10−5 m s−1 at 50 m. The upwelling transport in the area bounded by 3.6°S–5.2°N, 170°W–95°W was 62 (±18) × 106 m3 s−1 at 50 m. Strong downwelling was apparent within the North Equatorial Countercurrent. An asymmetry in the meridional flows suggested that on the order of 10 × 106 m3 s−1 of thermocline water from the Southern Hemisphere was upwelled at the equator and moved into the Northern Hemisphere as surface water. This interhemispheric exchange path could be part of the route for water from the Southern Hemisphere to supply the Indonesian Throughflow.

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François Ascani, Eric Firing, Julian P. McCreary, Peter Brandt, and Richard J. Greatbatch

Abstract

We perform eddy-resolving and high vertical resolution numerical simulations of the circulation in an idealized equatorial Atlantic Ocean in order to explore the formation of the deep equatorial circulation (DEC) in this basin. Unlike in previous studies, the deep equatorial intraseasonal variability (DEIV) that is believed to be the source of the DEC is generated internally by instabilities of the upper-ocean currents. Two main simulations are discussed: solution 1, configured with a rectangular basin and with wind forcing that is zonally and temporally uniform, and solution 2, with realistic coastlines and an annual cycle of wind forcing varying zonally. Somewhat surprisingly, solution 1 produces the more realistic DEC; the large, vertical-scale currents [equatorial intermediate currents (EICs)] are found over a large zonal portion of the basin, and the small, vertical-scale equatorial currents [equatorial deep jets (EDJs)] form low-frequency, quasi-resonant, baroclinic equatorial basin modes with phase propagating mostly downward, consistent with observations. This study demonstrates that both types of currents arise from the rectification of DEIV, consistent with previous theories. The authors also find that the EDJs contribute to maintaining the EICs, suggesting that the nonlinear energy transfer is more complex than previously thought. In solution 2, the DEC is unrealistically weak and less spatially coherent than in the first simulation probably because of its weaker DEIV. Using intermediate solutions, this study finds that the main reason for this weaker DEIV is the use of realistic coastlines in solution 2. It remains to be determined what needs to be modified or included to obtain a realistic DEC in the more realistic configuration.

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G. S. Carter, M. A. Merrifield, J. M. Becker, K. Katsumata, M. C. Gregg, D. S. Luther, M. D. Levine, T. J. Boyd, and Y. L. Firing

Abstract

A high-resolution primitive equation model simulation is used to form an energy budget for the principal semidiurnal tide (M 2) over a region of the Hawaiian Ridge from Niihau to Maui. This region includes the Kaena Ridge, one of the three main internal tide generation sites along the Hawaiian Ridge and the main study site of the Hawaii Ocean Mixing Experiment. The 0.01°–horizontal resolution simulation has a high level of skill when compared to satellite and in situ sea level observations, moored ADCP currents, and notably reasonable agreement with microstructure data. Barotropic and baroclinic energy equations are derived from the model’s sigma coordinate governing equations and are evaluated from the model simulation to form an energy budget. The M 2 barotropic tide loses 2.7 GW of energy over the study region. Of this, 163 MW (6%) is dissipated by bottom friction and 2.3 GW (85%) is converted into internal tides. Internal tide generation primarily occurs along the flanks of the Kaena Ridge and south of Niihau and Kauai. The majority of the baroclinic energy (1.7 GW) is radiated out of the model domain, while 0.45 GW is dissipated close to the generation regions. The modeled baroclinic dissipation within the 1000-m isobath for the Kaena Ridge agrees to within a factor of 2 with the area-weighted dissipation from 313 microstructure profiles. Topographic resolution is important, with the present 0.01° resolution model resulting in 20% more barotropic-to-baroclinic conversion compared to when the same analysis is performed on a 4-km resolution simulation. A simple extrapolation of these results to the entire Hawaiian Ridge is in qualitative agreement with recent estimates based on satellite altimetry data.

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