Intensive Operations Period Planning

Drifter module

A draft of the drifter plan can be found here (drifter_release_notes)

Sampling modules

1. • Non-linear internal waves

     NLIW_SurveyPlan_01 — Straw plan for NLIW surveys with four boats
     a)  Oceano NLIW — 2-3 km spacing between cross-shore lines. Detailed evolution along and across shore evolution of the internal tide and NLIWs
     b) NLIW refraction survey — In the vicinity of Mussel Rock outcrop. Influence of bathymetric variability on internal tide and NLIW evolution
     c) Broad scale NLIW survey — Variability in internal and NLIW wave field over the extent of the inner shelf experiment.
     –  Transect lines assume minimum depth for surveys is 20 m.  The Kalipi could survey into shallower water (e.g., to 10 m)
     –  Transects repeated rapidly in order to resolve internal tide variability. In figures the lines are 3.7 km long allowing completion of transect in 1 hr (out and back, travel speed 4 kts).

   • Wind-driven mean flow and mesoscale to submesoscale variability

Guiding questions:

1) Can we reliably observe the following processes?
a) Mixed layer (baroclinic) instability eddies and other mixed layer processes;
b) Shoaling internal tides, NLIWs and their effect on the mesoscale/submesoscale flow;
c) Wakes of headland eddies and (if any) their instabilities (baroclinic, barotropic, centrifugal, mixed);
d) Mesoscale eddies arising from baroclinic instability of the wind-driven mean flow.
e) Tidal and subtidal along-shelf mean flow (good descriptors of the large-scale setting);

2) Length scales/anisotropy: What are the along-/cross-shelf decorrelation lengths of T, S, u, v, Fluorescence in the ~inner- and ~mid-shelf (e.g., 20 m and 40 m isobaths)? How do these length scales change in the surface mixed layer, bottom mixed layer and geostrophic interior (if present)?

 
3) Other flow statistics and relation to dynamical budgets: What is the vertical structure of vorticity, strain, divergence and their Probability Density Functions (PDFs) and Joint PDFs, and are they different in ~inner-shelf and ~mid-shelf regimes? Does this answer tell us anything about the transition between physical regimes from deeper to shallower water? Can we estimate crude energy and momentum budgets?

4) Physical-biological-chemical processes: How does the chlorophyll fluorescence and the phytoplankton community structure change in response to physical processes, especially NLIWs and tidal/subtidal heaving of the isopycnals?

Our plan is to approach these questions with repeated occupations of mostly fixed, approximately 1 km-5 km long lines in the along-shelf and cross-shelf directions bounded by the ~10 m and ~40 m isobaths, so that the resulting dataset allows us to make statistically meaningful comparisons with numerical model outputs and simple theoretical ideas. This will be sought mostly in Modules A and C (track maps below), and will greatly benefit from utilizing multiple ships simultaneously. In Module B, we will also join the other vessels for part of the headland eddy sampling (track maps below).

For more details, see the R/V Sproul’s cruise plan.

• Headland eddies
  

  Questions to answer —
  a) what is the vorticity generated
  b) what role does the tide, or specifically the periodic superposition of the tide and low-frequency flow play in the strength, timing, or sign of that vorticity,
  c) how quickly does that vorticity decay (from bottom drag, ’tilted vortex’ instability, centrifugal instability, other…) , and
  d) what is the net contribution of the form drag associated with the eddy pressure signal on momentum removal from the low-frequency along-shelf flows. Previous experiments have suggested the latter may be quite significant.

Headland Eddy Approach
A) drifter release at or just upstream of the headland at the time when the along-coast barotropic tidal flow is maximally in the direction of the low-frequency currents. These should be the strongest eddies produced. Of course the ‘direction of low-frequency currents’ is something we won’t know till closer to the time, from a combination of the few-day-forecast model and direct observations.

B) drifter release 6 hours later, on the opposite phase of the tide/mean flow alignment(which, if either, of these is feasible depends how the tide+mean flow alignment projects onto the times of day in which the small boat can work, TBD).

C) repeated box surveys on both sides of the headland, from multiple ships.  If two ships are available, these could be done simultaneously on both sides of the headland, for time enough to see a lee eddy evolve, separate, start to decay, and possibly advect across the headland the other way when the tide changes.  If a small boat plus two bigger boats are available, the small boat could do a box that’s within and in shallower water than the big boats, which are limited to the 20+ isobaths.  Simple velocity from ship-board / pole-mounted ADCPs would be awesome enough here to get a great story, but if we could also tow thermistors, or do a few microstructure profiles, that would be even better.

General Timeline

Sept 8-12 – Moorings 0600-2000, Survey 2000-0600

Sept 12-21 – Survey

Ship sampling straw plan timeline

APL-UW Aircraft sampling plan

Timeline:

IOP1: Sept 8-19

IOP2: Oct 5-17

Typical flight day will be 4 continuous hours (max before refueling), with occasional 8 hour days with refuel. Flights will be coordinated with satellite and vessel sampling.