Measuring Sea Level Change

by: Ping Wang

Denise Davis

NSF Grant DRL-1316782

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 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: Seasonal changes

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

Housing to keep the computer and equipment out of the weather

Sensor to measure wind speed and directionSolar panel

to supply power

Satellite antenna to send the data to the internet

Computer to process and store data

Acoustic sensor to measure water level

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.

This is called satellite altimetry. It is quite complicated and we will not going into details here.

Measuring Sea Level: Using satellite

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

Sandy Hook, NJ

The Battery, NY

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 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 1: Superstorm Sandy

Examples of what your tide gauge can and should do:Example 2: 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 3: 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.

Examples of what your tide gauge can and should do:Example 4: Long term sea level trend

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/