Geology topics

M.A. McManus

Publications and source records attributed to M.A. McManus.

4 recordsLinked to original sources

USGS2009

Northern Monterey Bay upwelling shadow front: Observations of a coastally and surface-trapped buoyant plume

During the upwelling season in central California, northwesterly winds along the coast produce a strong upwelling jet that originates at Point A??o Nuevo and flows southward across the mouth of Monterey Bay. A convergent front with a mean temperature change of 3.77 ?? 0.29??C develops between the warm interior waters and the cold offshore upwelling jet. To examine the forcing mechanisms driving the location and movement of the upwelling shadow front and its effects on biological communities in northern Monterey Bay, oceanographic conditions were monitored using cross-shelf mooring arrays, drifters, and hydrographic surveys along a 20 km stretch of coast extending northwestward from Santa Cruz, California, during the upwelling season of 2007 (May-September). The alongshore location of the upwelling shadow front at the northern edge of the bay was driven by: regional wind forcing, through an alongshore pressure gradient; buoyancy forces due to the temperature change across the front; and local wind forcing (the diurnal sea breeze). The upwelling shadow front behaved as a surface-trapped buoyant current, which is superimposed on a poleward barotropic current, moving up and down the coast up to several kilometers each day. We surmise that the front is advected poleward by a preexisting northward barotropic current of 0.10 m s -1 that arises due to an alongshore pressure gradient caused by focused upwelling at Point A??o Nuevo. The frontal circulation (onshore surface currents) breaks the typical two-dimensional wind-driven, cross-shelf circulation (offshore surface currents) and introduces another way for water, and the material it contains (e.g., pollutants, larvae), to go across the shelf toward shore.Copyright 2009 by the American Geophysical Union.

Journal of Geophysical Research C: Oceans↗

USGS2006

Cross-shore velocity shear, eddies and heterogeneity in water column properties over fringing coral reefs: West Maui, Hawaii

A multi-day hydrographic survey cruise was conducted to acquire spatially extensive, but temporally limited, high-resolution, three-dimensional measurements of currents, temperature, salinity and turbidity off West Maui in the summer of 2003 to better understand coastal dynamics along a complex island shoreline with coral reefs. These data complement long-term, high-resolution tide, wave, current, temperature, salinity and turbidity measurements made at a number of fixed locations in the study area starting in 2001. Analyses of these hydrographic data, in conjunction with numerous field observations, evoke the following conceptual model of water and turbidity flux along West Maui. Wave- and wind-driven flows appear to be the primary control on flow over shallower portions of the reefs while tidal and subtidal currents dominate flow over the outer portions of the reefs and insular shelf. When the direction of these flows counter one another, which is quite common, they cause a zone of cross-shore horizontal shear and often form a front, with turbid, lower-salinity water inshore of the front and clear, higher-salinity water offshore of the front. It is not clear whether these zones of high shear and fronts are the cause or the result of the location of the fore reef, but they appear to be correlated alongshore over relatively large horizontal distances (orders of kilometers). When two flows converge or when a single flow is bathymetrically steered, eddies can be generated that, in the absence of large ocean surface waves, tend to accumulate material. Areas of higher turbidity and lower salinity tend to correlate with regions of poor coral health or the absence of well-developed reefs, suggesting that the oceanographic processes that concentrate and/or transport nutrients, contaminants, low-salinity water or suspended sediment might strongly influence coral reef ecosystem health and sustainability.

Continental Shelf Research↗

USGS2005

Local wind forcing of the Monterey Bay area inner shelf

Wind forcing and the seasonal cycles of temperature and currents were investigated on the inner shelf of the Monterey Bay area of the California coast for 460 days, from June 2001 to September 2002. Temperature measurements spanned an approximate 100 km stretch of coastline from a bluff just north of Monterey Bay south to Point Sur. Inner shelf currents were measured at two sites near the bay's northern shore. Seasonal temperature variations were consistent with previous observations from the central California shelf. During the spring, summer and fall, a seasonal mean alongshore current was observed flowing northwestward in the northern bay, in direct opposition to a southeastward wind stress. A barotropic alongshore pressure gradient, potentially driving the northwestward flow, was needed to balance the alongshore momentum equation. With the exception of the winter season, vertical profiles of mean cross-shore currents were consistent with two-dimensional upwelling and existing observations from upwelling regions with poleward subsurface flow. At periods of 15-60 days, temperature fluctuations were coherent both throughout the domain and with the regional wind field. Remote wind forcing was minimal. During the spring upwelling season, alongshore currents and temperatures in the northern bay were most coherent with winds measured at a nearby land meteorological station. This wind site showed relatively low correlations to offshore buoy wind stations, indicating localized wind effects are important to the circulation along this stretch of Monterey Bay's inner shelf. ?? 2004 Elsevier Ltd. All rights reserved.

Continental Shelf Research↗

USGS2003

Long-term, high-frequency current and temperature measurements along central California: Insights into upwelling/relaxation and internal waves on the inner shelf

Thermistor chains and acoustic Doppler current profilers were deployed at the northern and southern ends of Monterey Bay to examine the thermal and hydrodynamic structure of the inner (h ??? 20 m) shelf of central California. These instruments sampled temperature and current velocity at 2-min intervals over a 13-month period from June 2000 to July 2001. Time series of these data, in conjunction with SST imagery and CODAR sea surface current maps, helped to establish the basic hydrography for Monterey Bay. Analysis of time series data revealed that depth integrated flow at both sites was shore parallel (northwest-southeast) with net flows out of the Bay (northwest). The current and temperature records were dominated by semi-diurnal and diurnal tidal signals that lagged the surface tides by 3 h on average. Over the course of an internal tidal cycle these flows were asymmetric, with the flow during the flooding internal tide to the southeast typically lasting only one-third as long as the flow to the northwest during the ebbing internal tide. The transitions from ebb to flood were rapid and bore-like in nature; they were also marked by rapid increases in temperature and high shear. During the spring and summer, when thermal stratification was high, we observed almost 2000 high-frequency (Tp ??? 4-20 min) internal waves in packets of 8-10 following the heads of these bore-like features. Previous studies along the West Coast of the US have concluded that warm water bores and high-frequency internal waves may play a significant role in the onshore transport of larvae.

Continental Shelf Research↗