Tides and shallow water sea research at NIOZ Royal Netherlands Institute for Sea Research but first: crossing the Agulhas current NIOZ is part of the Netherlands Organisation for Scientific Research (NWO) Herman Ridderinkhof
Crossing the Agulhas current Sea surface temperature
Chlorophyll Crossing the Agulhas current
Temperature from CTD behind the clipper Crossing the Agulhas current 18 april 19 april till 09.00
Crossing the Agulhas current
Tides Herman Ridderinkhof NIOZ and Utrecht University
What are the causes of tides and what are their characteristics?
How do tides propagate in oceans (e.g. Atlantic), shelf seas (e.g. North Sea), shallow tidal basins (e.g. Wadden Sea)?
General pattern of the vertical tide at one station Daily inequality delay: moon phases - tidal phenomena Neap tide Spring tide Two highs and lows per day
Examples: “ Lunar” frequencies - lunar day - 2 x lunar day (dominant) - overtones - eccentricity lunar orbit - tilt lunar orbit - etc... “ Solar” frequencies - similar to moon freq. And all kinds of crossterms A tidal signal can be decomposed in a number of frequencies related to astronomical frequencies
Tides are caused by the gravitational force of the moon and sun and the motion of the earth-moon-sun system
The gravitational force causes a deformation of the ocean surface (equilibrium tide)
Frequencies of planetary motions result in different ‘astronomical frequencies’
Tidal frequencies: lunar day
A lunar day is the time that elapses between when the moon is directly overhead and the next time the moon is directly overhead.
During one complete rotation of Earth (the 24-hour solar day) the moon moves eastward 12.2 degrees, and Earth must rotate an additional 50 minutes to place the moon in the exact same position overhead.
Thus, a lunar day is 24 hours 50 minutes long (and a solar day 24 hours).
Tidal frequencies: spring-neap variability
There are lots of other “astronomical frequencies” : example: relative tilts of plane of the Moon’s orbit and equator plane. These variations are a.o. recognizable in geology.
Tide Patterns The worldwide distribution of the three tidal patterns.
Tide driven by astronomical forces is present only in the Southern Ocean.
Amplitude of tide due to astronomical force, the equilibrium tide, is 0.27 m due to the moon and 0.12 m due to the sun
Tides at any location outside the Southern Ocean are caused by propagation of the tidal wave from the Southern Ocean
How do tides propagate in oceans and shelf seas?
William Thomson (Lord Kelvin) Sea level displacement for a Kelvin wave
Kelvin wave in a square tank: The Coriolis force lets the wave run along the wall on the right side (northern hemisphere)
Amplitudes and phase lines from a world ocean tidal model
The Kelvin waves in the Atlantic Ocean start in the south For the tidal wave it takes about 2-2.5 days to move from the Southern Ocean to the North Sea (spring tide is 2-2.5 after full moon)
Kelvin waves in the North Sea split up in three separate Kelvin waves The “Knots” are called Amphidromic points. Here is no vertical tide
Tides in Confined Basins: no Kelvin wave but possibility of amplification due to resonance Resonance occurs if reflected wave from landward end amplifies the incoming wave (length basin = .5 * tidal wave length)
The largest tidal range in the world occurs in Nova Scotia's Bay of Fundy. Even though the maximum spring tidal range at the mouth of the bay is only 2 meters, amplification of tidal energy causes a maximum tidal range at the northern end of Minas Basin of 17 meters.
Bay of Fundy, High Tide Bay of Fundy, Low Tide
Kelvin wave along Dutch coast
Near-resonance in Wadden Sea combined with strong bottom friction Tides in Wadden Sea have become stronger due to closure with Afsluitdijk!
Tide is excited by astronomical forces. ‘Ideal tide’ is present only in the Southern Ocean (no continents).
Frequencies of the tide are related to astronomical frequencies
Tides propagate through the world oceans as rotating (Kelvin) waves
Amplitude of the tide at a certain location strongly depends on the resonance characteristics of a sea basin
Tides: important issues
Some NIOZ Wadden Sea studies based on our speciality: observations at sea
Ferry based observations and studies
Ecological studies based on (long term) field observations
water Slib Netto fluxen water: naar Noordzee slib: naar Waddenzee (veel groter dan eerdere schattingen, nu als T0 meting ivm Maasvlakte-II aanleg)
Bottom depth of the Marsdiep inlet, and the changes over time showing migrating sandwaves
A Cross section of the bottom profile along section A Migrating sandwaves into the WaddenSea with a height of 2-3 m and a length of 100 m (dark is orinigal profile)
B Cross section of the bottom profile along section B Migrating sandwaves into the WaddenSea with a height of 7-8 m and a length of 300 m (dark is orinigal profile)
Temperature, Salinity since 1861 (van Aken) Now from NIOZ jetty
Into the field
Predicted intake rate of the knot (time-series since 1991)
Concurrent changes Wadden Sea communities Phosphate concentrations early spring Nutrient enrichment & reduction Phytoplankton NIOZ Macrozoobenthos NIOZ/RWS Estuarine birds SOVON Philippart et al. (under review) Ecosystems Observation : Correlation : Possible Cause : 1970 1977 1984 1991 1998 2005 -2 -1 0 1 2 3 Standardised units (-) Estuarine birds (PC2; following winter) Macrozoobenthos (PC1; following winter) Phytoplankton (PC2; summer)
Philippart & Peperzak (2006) Waddenbulletin < 1000 cells/ml > 1000 cells/ml Observation : Correlation : Possible Cause : Variations in seasonality Phaeocystis blooms Nutrients and/or climate change? Phaeocystis globosa December November October September August July June May April March February January 1975 1980 1985 1990 1995 2000 2005