Suspended particulate layers and internal waves over the southern Monterey Bay continental shelf: An important control on shelf mud belts?

Abstract

[1] Physical and optical measurements taken over the mud belt on the southern continental shelf of Monterey Bay, California documented the frequent occurrence of suspended particulate matter features, the majority of which were detached from the seafloor, centered 9-33 m above the bed. In fall 2011, an automated profiling mooring and fixed instrumentation, including a thermistor chain and upward-looking acoustic Doppler current profiler, were deployed at 70 m depth for 5 weeks, and from 12 to 16 October a long-range autonomous underwater vehicle performed across-shelf transects. Individual SPM events were uncorrelated with local bed shear stress caused by surface waves and bottom currents. Nearly half of all observed SPM layers occurred during 1 week of the study, 9-16 October 2011, and were advected past the fixed profiling mooring by the onshore phase of semidiurnal internal tide bottom currents. At the start of the 9-16 October period, we observed intense near-bed vertical velocities capable of lifting particulates into the middle of the water column. This ''updraft'' event appears to have been associated with nonlinear adjustment of high-amplitude internal tides over the mid and outer shelf. These findings suggest that nonlinear internal tidal motions can erode material over the outer shelf and that, once suspended, this SPM can then be transported shoreward to the middle and shallow sections of the mud belt. This represents a fundamental broadening of our understanding of how shelf mud belts may be built up and sustained.

Transport by breaking internal gravity waves on slopes

“…Both the maximum onshore and offshore transport, as well as the effective cross-shore dispersion, vary with the depth, with larger values occurring near the pycnocline and along the bottom where velocities induced by wave breaking are strongest. The layer of particles transported offshore by the intrusion of mixed fluid after breaking resembled an INL, as observed in previous studies of wave breaking on slopes in the field (McPhee-Shaw et al 2004;McPhee-Shaw 2006;Cheriton et al 2014), in the laboratory (Ivey & Nokes 1989;Helfrich 1992;McPhee-Shaw & Kunze 2002;Nakayama & Imberger 2010) and in numerical models (Bourgault et al 2005(Bourgault et al , 2014Nakayama & Imberger 2010). In order to visualize the INL, an intermediatedensity region was defined within 0.7 ρ of ρ 0 .…”

Vertical convergence of resuspended sediment and subducted phytoplankton to a persistent detached layer over the southern shelf of Monterey Bay, California

“…Over the southern shelf, significant sediment resuspension occurs during surface wave events lasting 2–3 days and during the active passage of nonlinear internal solitary waves over the shelf and slope [ Carter et al ., ; Cheriton et al ., ]. Bottom shear stresses from tidal currents alone rarely exceed resuspension thresholds but are important in establishing a “background” shear level over which surface and internal wave events build upon [ Cheriton et al ., ].…”

“…Cheriton et al . [] proposed that while surface waves and tidal currents play important roles in mobilizing sediment into the benthic boundary layer (BBL), updraft events may be responsible for lifting sediment out of the BBL and into the interior water column.…”

“…In the first year of the present 2‐year field study, Cheriton et al . [] documented the frequent presence of detached SPM layers over the southern Monterey Bay shelf. These layers were on average centered 16 m above the bottom (mab) and were 9 m thick in the vertical.…”

Topographically generated internal waves and boundary layer instabilities

“…6 More broadly, a wide variety of field studies have pointed to a link between increased near-bed sediment concentrations and shoaling internal waves on a slope. [18][19][20][21][22] Quaresma et al 8 have offered clear evidence of nonlinear internal waves influencing sediment resuspension over the northern shelves of Portugal. Bottom shear stress calculations suggested that the strongest nonlinear internal waves were capable of suspending sediment.…”