Characteristics of Sea Level Records

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Characteristics of Sea Level Records

• Philip L. Woodworth
• Permanent Service for Mean Sea Level
• With thanks to Simon Williams

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Definitions

• The expression sea level means different
  things to different people.
• Sea Level for us means
• The height of the sea measured relative to a mark
  on the nearby land called the Tide Gauge
  Benchmark

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Sea levels measured by tide gauges at the Newlyn
Lighthouse tidal observatory are measured
relative to the TGBM in the floor of the building
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Sea Level Measurements

• Sea level is measured by a tide gauge
• Values of sea level (or sea surface height) are
  either spot-measurements at regular time
  intervals, or averages (called integrations) over
  the time intervals.
• For most tidal, storm surge or Mean Sea Level
  work, time intervals of 5, 6, or 15 minutes are
  adequate.
• For tsunami work, time intervals of a minute or
  less are usually needed.

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A Sea Level Record will show

• The ocean tide
• On timescales of minutes to hours, there could be
  signals due to seiches or tsunamis
• On timescales of hours to days, there could be
  signals due to storm surges
• On longer timescales, there could be seasonal and
  longer changes and even, at the longest
  timescales, signals due to climate change

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Tides

• Periodic movements which are directly related in
  amplitude and phase to some periodic geophysical
  force
• The dominant geophysical forcing function is the
  variation of the gravitational field on the
  surface of the earth, caused by regular movements
  of the moon-earth and earth-sun systems.

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Tidal signals in March 1981 at five sites which
have very different tidal regimes Karumba
diurnal Musayid mixed Kilindini
semidiurnal Bermuda semidiurnal Courtown
shallow sea distortion Lunar characteristics
responsible for these tidal patterns
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Frequency distribution of hourly tidal levels
Newlyn, semidiurnal
Karumba, diurnal
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The Tide
To learn about tides read chapters 2-5 of David
Pughs book
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Main Points about the Tide to make here

• Tidal characteristics at any location can be
  determined well from a year of good tide gauge
  data
• The knowledge obtained can be used to produce
  Tables of Predicted Tide Levels (Tide Tables)
  for local use
• Tidal and Non-tidal components of the sea level
  record can be separated easily using Harmonic
  Tidal Analysis of a tide gauge data set.

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Non-Tidal Changes

• Storm surges caused by changes in air pressure
  and winds
• Seiches due to resonant behavior of harbours and
  bays (caused by many things, often wind)
• Seasonal, interannual changes due to changes in
  water density
• Tsunami caused by earthquakes
• And many more non-tidal sources.

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Mean Sea Level

• Daily MSL is calculated by applying a suitable
  filter (see IOC Manual III for details) to the
  measured tide gauge data (i.e. not the tidal or
  non-tidal parts separately, but the measured
  values)
• Monthly MSL is then defined as the arithmetic
  average of the daily MSL values in that month
• Annual MSL is defined as the average of the daily
  MSL values in a year

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Typical analysis of monthly mean sea levels
Periodic tidal terms (nodal, annual and
semi-annual)
Long term mean and trend
residuals
Meteorological effects
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Permanent Service for Mean Sea Level

• Time series of MSL are collected by the PSMSL
• Long records show signals due to seasonal
  processes, El Nino, and sea and land level
  changes
• One goal in ODINAFRICA is to eventually have long
  records of MSL from more African stations

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Sea Level Changes in Last 100 Years

• Past 100 years
• Most PSMSL records show evidence for rising sea
  levels during the past century
• IPCC Third Assessment Report concluded that
  there has been a global rise of approximately
  10-20 cm during the past 100 years

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• Types of Sea Level Records
• Real-time. Data are often transmitted to a
  warning centre which can look out for flooding
  due to storm surges or tsunamis. Delay (latency)
  has to be as short as possible so there is no
  time for detailed inspection of the record
• Delayed Mode. Delay is not a major issue. Data
  are archived by a centre for subsequent detailed
  analysis and removal of errors. Such
  highest-quality data are needed for scientific
  research, for production of a range of products
  (e.g. tide tables), for computation of MSL etc.
• There are almost 2 different communities of
  people involved Operational agencies,
  Scientists/Engineers

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• Why do we care about tidal analysis if we just
  want to provide real-time data and monthly MSL
  (neither of which require tidal analysis in
  theory)?
• Two main reasons
• The separation of the sea level record into tidal
  and non-tidal components is needed if one needs
  to produce tide tables or tidal predictions
• The non-tidal signals (seiches, tsunamis) become
  clearly identified
• Tidal analysis also enables errors in the sea
  level time series to be identified ?
  highest-quality data for subsequent analysis

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How does a tidal analysis work?

• The tide is parameterised in terms of harmonics
  with periods specified by the orbits of the Moon
  and Sun but with unknown amplitudes and phase
  i.e.

Tide
The unknown parameters are Z0 and the (Hj, gj).
The fitting is adjusted so that the sum of the
squares of the difference between the observed
and computed tidal levels is minimized. The
residuals to the fit are considered to be the
non-tidal terms
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An example maybe not the best but one I had
available from Port Stanley Nov-Dec 2004
Measured
Tide
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Non-tidal
In the non-tidal record 1. Variations of a few
days, amplitude of a few cm (this is southern
summer so no big storm surges) 2. A lot of
high-frequency noise of a few cm due to harbour
seiches 3. On 27 December arrival of the Sumatra
tsunami (15 cm or so) ? None of this is evident
from looking at the total measured record.
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• The non-tidal component is often called the
  Residual (or sometimes the tidal residual).
• It will be immediately clear (especially with
  some experience) by looking at the residuals if
  there is
• A spike or jump in the data due to instrumental
  faults
• A timing error due to a section of data being
  given the wrong time
• And many other errors ? see the IOC Manuals
• These errors can then be fixed in the data set.
  The final data set is called the Quality
  Controlled Delayed-Mode data set.

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Such tidal analysis can be undertaken using one
of the software packages available for the GLOSS
and ODINAFRICA programmes, see http//www.pol.ac.
uk/psmsl/training In this course we will be
using the TIDE TASKS FOR WINDOWS package.