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FY 1996

Subtidal bottom pressure observed at Axial Seamount in the northeastern continental margin of the Pacific Ocean

Mofjeld, H.O., F.I. González, and M.C. Eble

J. Geophys. Res., 101(C7), doi: 10.1029/96JC01451, 16,381–16,390 (1996)

Since September 1987, bottom pressure (BP) has been observed at Axial Seamount (45°57.5′N, 130°00.3′W; 1540-m depth), located 450 km off the Oregon Coast on the Juan de Fuca Ridge. Observations from 1987 to 1992 show that subtidal Axial BP is dominated by barotropic 2- to 5-mbar fluctuations with frequencies ≤ 1/9 cycles per day (cpd). They are slightly smaller than subtidal BP observed 720 km to the west during the Ocean Storms Experiment and a factor of 2–4 smaller than the subtidal fluctuations of adjusted sea level (ASL) at the coast. For the period September 1987 to July 1988, Axial BP and Ocean Storms BP are significantly coherent (coh2 ≥ 0.5), except at intraseasonal frequencies (1/100–1/20 cpd) and in a band (1/8–1/6 cpd) of low Axial BP variance. In the coherent frequency bands, the phase relationships between the two BP series suggest independent atmospheric forcing. On the other hand, Axial BP and coastal ASL are not coherent except at the fortnightly tidal frequency. Looking for possible forcing of Axial BP by the curl of the wind stress (curl) over the NE Pacific, data analyses show that Axial BP and curl (derived from European Centre for Medium-Range Weather Forecasts winds) are significantly coherent in only two frequency bands: one band narrowly peaked near 1/9.1 cpd for curl in regions south of Axial Seamount and a 1/5–1/3.5 cpd band for curl in a large region NW of the seamount. In the 1/5–1/3.5 band, Axial BP also has a small-amplitude spectral peak associated with bottom-trapped (baroclinic) currents near Axial Seamount. Remote Axial BP/curl coherence at some frequencies is qualitatively consistent with the local-forcing theory of Lippert and Müller [1995] that attributes the coherence to stable phase relationships and the spatial characteristics of the BP/curl transfer function. However, the Axial BP/curl comparison is inhibited by the lack of accurate estimates of curl within 400 km of the coast. Fully understanding the origins and dynamics of subtidal BP at Axial Seamount will require a knowledge of how the complex topography in the region affects the barotropic and baroclinic responses to atmospheric forcing, as well as the propagation of Rossby and other waves.

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