Del Mar Mooring

Investigator: U. Send

Introduction

Continuous, long-term monitoring of physical-biogeochemical variables at the Del Mar site is providing a multidisciplinary dataset which enables detection of both short-term events, and their relation to seasonal and interannual variability.

This dataset is enhancing our understanding of complex coastal processes (e.g. Ekman transport, tides, alongshore currents, coastal trapped waves and internal waves), and interactions with basin-scale climatic variability such as the El Niño Southern Oscillation and Pacific Decadal Oscillation. Moreover, the data contribute to current ecological research in better understanding issues such as hypoxia, massive fish kills and bird population decreases.

The Del Mar mooring was originally developed in 2006 in collaboration with the Scripps Hydraulics Laboratory as part of the Southern California Coastal Ocean Observing System (SCCOOS). Instrumentation includes a suite of meteorological sensors, and sensors for water temperature, salinity, oxygen concentration, fluorescence, nutrients and currents. In addition to providing a valuable real-time coastal timeseries for these parameters, the platform has also served as a testbed for developing technologies both in-house at SIO and with partner organizations such as the Environmental Sample Processor (MBARI) and Imaging Flow Cytobot (WHOI), and since 2011 has provided hands-on training for a university class on observational techniques.

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Research Highlights

Amplification of hypoxic and acidic events by La Niña conditions on the continental shelf off California

Abstract: Low-oxygen and low-pH events are an increasing concern and threat in the Eastern Pacific coastal waters, and can be lethal for benthic and demersal organisms on the continental shelf. The normal seasonal cycle includes uplifting of isopycnals during upwelling in spring, which brings low-oxygen and low-pH water onto the shelf. Five years of continuous observations of subsurface dissolved oxygen using the Del Mar mooring, reveal large additional oxygen deficiencies relative to the seasonal cycle during the 2010-2011 La Niña event. While some changes in oxygen related to the isopycnal depression/uplifting during El Niño/La Niña are not unexpected, the observed oxygen changes are 2-3 times larger than what can be explained by cross-shore exchanges. In late summer 2010, oxygen levels at mid-depth of the water column reached values of 2.5 ml/L, which is much lower than normal oxygen levels at this time of the seasons, 4-5 ml/L. The extra uplifting of isopycnals related to the La Niña event can explain oxygen reductions only to roughly 3.5 ml/L. We find that the additional oxygen decrease beyond that is strongly correlated with decreased subsurface primary production and strengthened poleward flows by the California Undercurrent. The combined actions of these three processes created a La Niña-caused oxygen decrease as large and as long as the normal seasonal minimum during upwelling period in spring, but later in the year. With a different timing of a La Niña, the seasonal oxygen minimum and the La Niña anomaly could overlap to potentially create hypoxic events of previously not observed magnitudes.

Relaxation from upwelling: The effect on dissolved oxygen on the continental shelf

Abstract: Continental shelves in upwelling regimes are subject to sequences of upwelling and relaxation events, each on timescales of order 1 week typically. These episodes have pronounced impacts on the temperature and density structure on the shelves and also on the along‐shore and cross‐shore flow regimes. It had previously been demonstrated that relaxation phases advect warm water along shore (poleward) from regions of less intense upwelling, thus adding to the heat balance in upwelling locations and providing a rectification of oscillating heat fluxes. In the current paper it is shown that relaxations also modify the dissolved oxygen (DO) budget of the lower layers. On a narrow shelf, this provides enhanced DO values due to near‐surface exposure, while on a wide shelf decreased DO concentrations are created due to oxygen consumption on the inner shelf. The resulting variations along the coast can lead to along‐shore advection of high or low DO during a relaxation event. Observations are presented from moorings off San Diego and Del Mar, which show large departures from density‐correlated DO values during relaxations and which support the proposed mechanisms.

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Publications

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Acknowledgements

Initial funding for the Del Mar Mooring came from the Southern California Coastal Ocean Observing System (SCCOOS).