Transcript Sea Level - University of South Florida

NSF Grant
DRL-1316782
Measuring Sea Level
Change
by: Ping Wang
Denise Davis
What is sea level?
Or should we say what is mean sea level?
Mean Sea Level
Before we answer the question “What is mean sea
level?”
Let’s take a closer look at how the sea level
changes.
Sea Level Variations: Daily changes or
tides
 Sea level rises and falls every 12 hours at most
places or every 24 hours at some locations
 This is called ocean tides.
Sea Level
Variations:
Seasonal changes
 Sea level also varies
seasonally
 e.g., in Tampa Bay the
water level tends to be
higher in the summer
and lower in the winter
Sea Level Variations: Multi-year changes
 Sea level also varies
in terms of years
 e.g., during El Nino
years, the sea level
along the US Pacific
coast tends to be
higher than during La
Nina years
Sea Level Variations: Irregular changes
Sea level also varies irregularly.
 During a storm the sea level can rise very fast to a level that
is much higher than normal level, this is called storm surge.
Storm surge animation (click here)
http://www.nhc.noaa.gov/surge/animations/hurricane_stormsurge.swf
Sea Level Variations: Irregular changes
Sea level also varies irregularly.
 During a Tsunami the sea level can rise and fall
EXTREMELY fast in a few minutes.
Then when we say global sea-level rise,
what do we really mean?
We mean: the mean sea level, many times the mean sea level
of a large region, or even the entire earth.
Then how do we “get” the mean sea level?
Before we can get the “mean sea level”, how do we get “just”
the sea level?
Like many other earth phenomena, we measure sea level.
It turns out the measuring sea level is not very difficult and we
have been measuring sea level for several hundreds of years.
Let’s take a look at the History and
Technology of measuring sea level
Back in the old days, we measure sea level with a stilling well,
(just like we measured the water table on land, in a well….)
As the water level rises
and falls, the float rises
and falls. The
mechanical device (at the
top) “draws” the sea level
on a roll of paper
The stilling well is
designed to “filter” out the
wave motion (with the
small holes near the
bottom)
Let’s take a look at the History and
Technology of measuring sea level
Today, we measure sea level with much more sophisticated
equipment than the stilling well, but the principal remains
similar.
The water level is
not measured
with a float any
more, instead it is
measured with
sound (called
acoustic sensor),
much like an
echo sounder on
a boat. The drum
recorder is
replaced by a
digital computer.
Let’s take a closer look at a modern water
level measurement station
Acoustic sensor to
Housing to keep
the computer
and equipment
Solar panel
out of the
to supply
weather
power
measure water level
Sensor to measure
wind speed and
direction
Satellite
antenna to
send the data
to the internet
Computer to
process and
store data
Stilling well to
“filter” out
wave motion
The previous slides illustrate how we
measure water level at the coast, then how
do we measure water level in the vast
ocean?
It turns out that we could not measure water level in
the deep ocean until about 20 years ago, when we
began to have satellites that can accurately
measure elevation.
Measuring Sea Level: Using satellite
This is called satellite altimetry. It is quite complicated and we
will not going into details here.
What causes sea level change?
An introduction: we will get into much details of these in the
chemistry, physics, and geology units.
Here are some examples (regardless of time scales):
 Tides
 The attractions from the Sun and the Moon
 Thermal expansion
 Water expands when becomes warmer
 Note that the average ocean depth is 4000 m, a slight expansion may result
in a “significant” rise of the sea. For example, a 0.03% (a tiny bit) expansion
results in a sea level rise of 1.2 m (or 4 ft).
 Continental ice melting
 Will add water to the ocean
 Strong wind associated with storms
 Will “pile” water up along the shoreline, call storm surge
 Plate tectonics
 Will change the size of the ocean basin
What causes sea level change?
Global warming, which scientists have pretty solid theory and
data to support, will result in the following:
 Thermal expansion
 Water expands when becomes warmer
 Note that the average ocean depth is 4000 m, a slight expansion may
result in a “significant” rise of the sea. For example, a 0.03% (a tiny bit)
expansion results in a sea level rise of 1.2 m (or 4 ft).
 Continental ice melting
 Will add water to the ocean
In this History and Technology unit,
you will be challenged (with a
computer game) to build a water
level gauge, just like the ones
NOAA use.
Examples of what your tide gauge can and should do:
Example 1: Superstorm Sandy
The Battery is where
Wall Street and part of
the subway system in
NY is located. The
water level here
recorded to about 14
ft. during Superstorm
Sandy. This water
level is several feet
higher than the
entrances to the
subway resulting in
the flooding of the
subway.
Notice that the Sandy
Hook water level
measurements stop
on Oct. 29th. Once the
water level reached 13
ft. the gauge broke.
The water level
surpassed the
maximum limit of the
gauge measurement
capability.
The Battery, NY
Sandy
Hook, NJ
Examples of what your tide gauge can and should do:
Example 1: Superstorm Sandy
The roller coaster on the beach is “under water” due to the
rising of the sea.
Examples of what your tide gauge can and should do:
Example 2: Long term sea level trend
Examples of what your tide gauge can and should do:
Example 3: Long term sea level trend
Compared to Example 2 (2.24 mm/yr), why is the sea level
rising much faster here (9.24 mm/yr)?
We will answer this question in the geology unit.
Examples of what your tide gauge can and should do:
Example 4: Long term sea level trend
Why is the sea level falling here (-9.80 mm/yr)?
We will answer this question in the geology unit.
The questions in the previous
slides are complicated. We do not
have the knowledge in this unit yet
to answer them. We will answer
them in the geology unit, after we
lean more about sea-level in the
chemistry and physics units.
Your (game) goal for this unit: to
design a gauge that can measure
surge by, e.g., Superstorm Sandy
and long-term trend.
This is not an easy task. Will your
goal be achieved?
Your can get LOTS of information
from:
http://tidesonline.noaa.gov/