Slope turbulence, internal waves and phytoplankton growth at the Celtic Sea shelf-break

Bowers R . & Bishop D . G. 1966 In Proc. I.E .R .E . Cons. Electronic Engineering in Southam pton 1966 pp. 711-714.

10.1098/rspa.1959.0190

Stride A . H . & Tucker M . J . i 960 Nature Lond. 188 933.

10.1038/159579c0

10.1017/S0025315400012923

10.1017/S0025315400053054

Zeilon N. 1912 K. svenska. Vetenskakad. Handl.

D. H . P eregrine ( Schoolo f M athematics University o f Bristol B S 8 1 U .K.). A d istin c

tiv e featu re o f th e m easurem en ts presented is th at th e th erm o clin e over th e co n tin en ta l sh e If

slope is very m uch thicker th a n it is either over th e oceanic deeps o r th e co n tin en tal shelf. A n

explanation of this feature m ight be found by investigating the p ro p ag atio n o f in tern al waves on

the therm ocline. In te rn a l waves over the co n tin en tal shelf are p ro p ag atin g on tid al currents

w hich have velocities o f the sam e o rd er o f m ag n itu d e as the phase velocities o f the in tern al

waves. W hen these currents are taken in to account I expect th a t th e in tern al w ave field w ould

have m any features in com m on w ith surface waves on currents. T h e m ost relevant p ro p erty for this case is the short wave singularity th a t occurs w hen waves

are being swept dow nstream by a c u rre n t to a region w here th a t cu rre n t drops to n ear near-zero

speed. This is illustrated w ith o u t com m ent in figures 4 an d 11 o f P eregrine (1976) an d figure

of the sam e p ap er is a sketch o f a m ore com plicated b u t m ore general exam ple o f w ave

refraction by a non-uniform c u rre n t w hich leads to a considerable red u ctio n in w avelength.

M ore details o f the case o f a directly adverse c u rre n t are given in P eregrine & T h o m as (1979).

Some aspects o f the general problem are discussed in P eregrine & S m ith (1979). O n the continental slope there is a relatively high g rad ie n t o f tid al cu rren t a n d it is likely

th a t a high pro p o rtio n o f the in tern al waves sw ept in to th a t region off the shelf suffer a sub

stantial reduction o f w avelength. T his leads to th e w aves' kinetic energy being co n cen trated

n earer to the therm ocline an d p ro b ab Iy to a sub stan tial loss o f energy by breaking. B reaking

occurs a t the therm ocline an d m uch o f th e energy is directly available for m IX in g : this broadens

the therm ocline. Even w ithout currents the thicker therm ocline could cause in c id en t in te rn a l waves to break.

T h e waves p ro p ag ate slower on the b ro ad er therm ocline an d becom e sh o rter an d steeper to

m ain tain the sam e w ave action flux. O bservations clearly show th a t some strong in tern al w ave activity is g en erated by th e tid a l

currents. A ny non-uniform ity in th e bed leads to gen eratio n o f statio n ary waves on a uniform

cu rren t o f F roude n u m b er n ear unity. I f the cu rre n t varies in tim e as in this case th e waves so

generated becom e freely p ro p ag atin g waves. Since such waves o rig in ate w ith th e ir gro u p

velocity less th a n the cu rren t velocity they have a very good chance o f being sw ept to th e shelf

edge and hence co n trib u tin g to m ixing in the therm ocline. O bservations a t a single station are unlikely to provide m uch info rm atio n on the in te rn a l w ave

properties even if the c u rre n t is m easured an d the th erm ocline track ed in tim e. M ore extensive

observations are needed to assess the o rien tatio n o f the in te rn a l waves. S atellite an d aerial

photographs are likely to show only the largest waves b u t could be very helpful in in d icatin g the

scatter o f wave directions.

Peregrine D ., 1976, Adv.appl, Mech., 16, 9

10.1098/rsta.1979.0065

10.1098/rsta.1979.0043