Connecting wind-driven upwelling and offshore stratification to nearshore internal bores and oxygen variability

1* 2 3 1 Ryan K. Walter , C. Brock Woodson , Paul R. Leary , and Stephen G. Monismith

*[email protected] 1Environmental Fluid Mechanics Laboratory, Stanford University, Stanford, CA, USA 2COBIA Lab, College of Engineering, University of Georgia, Athens, GA, USA 3Hopkins Marine Station of Stanford University, Pacific Grove, CA, USA

Energetic nonlinear internal waves are a widespread feature of the coastal ocean and often play a key role in the cross-shelf exchange of nutrients, sediments, and contaminants; larval transport; hypoxia risk; and turbulent mixing. Yet, many questions still remain with respect to their evolution, fate, and impact on the shallow, nearshore environment, a region that can be thought of as the “swash zone” for the larger scale internal wave field and where waves appear more bore-like than wave-like. In southern Monterey Bay, CA transient stratification and mixing events associated with nearshore internal bores dramatically alter local dynamics and mixing in the stratified interior [Walter et al., 2012], and these bores also have been attributed to low oxygen intrusions [Booth et al., 2012]. The objective of this study is to understand how wind-driven upwelling and offshore stratification influence nearshore internal bores and oxygen variability.

1 Study Site and Experimental Setup 3 Nearshore Bore Strength Index • Potential energy (PE) density anomaly – energy to mix completely the water column (measure of stratification): g HH1 Towe φ=−−( ρρmm )zdz , where ρ = ρdz Monterey Bay, CA HH∫∫00 r • Captures transient, non-canonical structure and various “background states” unlike conventional IW methods Windowed-Standard Deviation (1 day w/ 50% overlap)

Hopkins Marine Station (HMS)

ADV/CT Nearshore (NS) Mooring (15 m) Offshore (OS) Mooring (85 m) T • 3 weeks of very high resolution T, CTD, and ADCP • 2.5 months of T and CTD • T at 0.5 m vertical resolution, 0.5 s sampling • T at 10 m vertical resolution, 30 s sampling • CTD at 2 m vertical resolution, 6 s sampling • CTD at top (3 min) and bottom (30 s) • ADCP w/ 0.5 m bins and 1 s (mode 12) sampling (not discussed here) T CTD • 2.5 months of bottom T and dissolved oxygen (DO) CTD Near-bed temperature “bore index” is highly DO ADCP correlated to PE anomaly index (R2 = 0.98); bottom • Both at 1 meter above the bed (mab) Bore strength index characterizes the magnitude of the nearshore bores • 10 min averages/sampling period temperature “bore index” is available over the 2.5 month record

Offshore to Nearshore Connection 2 General Observations and Trends 4 Windowed-Quantities ~2.5 month study period (1 day w/ 50% overlap)

~3 week study period

Increased (decreased) offshore internal wave activity Deeper (shallower) offshore thermocline leads to leads to increased (decreased) nearshore internal ~ hypoxic level increased (decreased) offshore internal wave activity bore activity 5 Nearshore Oxygen Variability and Low Oxygen Events

Nearshore oxygen variability is linked to bore variability Low-frequency upwelling winds modify the Windowed-Quantities (1 day w/ 50% overlap) offshore stratification and thermocline depth: • Strong upwelling, thermocline shoals Lowest mean oxygen concentrations (left) observed • Weak upwelling, thermocline deepens during periods of decreased bore activity (extended • Normal mode analysis reveals decreased (increased) offshore upwelling, “nearshore pooling” periods) – duration of low internal waves/tides during strong (weak) upwelling ~ hypoxic level oxygen events (i.e., below hypoxic levels) is longer • Winds and thermocline are significantly coherent at upwelling time scales (7-10 d cycles, not shown)

Transient stratification and mixing events in the 3 Minimum oxygen concentrations (right) highlight that strong ~ hypoxic level bores produce transient pulses of low oxygen – onset time nearshore (internal bores) of low oxygen events (i.e., below hypoxic levels) is faster • Bore strength and structure in the nearshore depends on the offshore •Normal mode analysis yields offshore IW phase speed (c): thermocline/stratification (and hence wind-driven upwelling) dN22ψ • Strong upwelling results in cold water accumulation (“nearshore +=ψ 22 0, g ∂ρ pooling”) dz c Nz2 ()= − Acknowledgements: This work was funded by the National Science Foundation through grants OCE-1235552 and DEB-1212124, and by the Singapore Stanford • Oxygen variability at low (upwelling cycles) and high (internal bores) ρo ∂z ψψ(0)0,= (H)0 = Program. R. Walter was supported by the Stanford Graduate Fellowship. We acknowledge helpful discussions with Jeff Koseff, Derek Fong, Oliver Fringer, and Olivia frequencies Cheriton. Offshore data used in this study was provided to R. Walter by the United States Geological Survey’s Pacific Coastal and Marine Science Center. Bathymetry •Proxy for offshore IW activity since IW amplitude ∝ phase speed data used in this study were acquired, processed, archived, and distributed by the Seafloor Mapping Lab of California State University Monterey Bay.