Measuring Sea Level Change NSF Grant DRL-1316782 Measuring Sea Level Change by: Ping Wang Denise Davis

Or should we say what is mean sea level? What is sea level? This schematic shows that there are different measures for sea level. Generally when we talk about sea level we are referring to mean 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. The next several slides are showing the different ways that sea level varies on different time scales.

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. This are daily variations of sea level.

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 This is showing seasonal variations of sea level. The bottom graph (second fly-in) is the seasonal variations measured at a gauge at St. Petersburg, FL. It shows that the water levels in the summer are higher than those in the winter months. The points in between the error bars are the average of the water levels for that particular month.

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 This is showing sea level variations in terms of years. El Nino is related to water temperature anomalies. In areas where the water temperature is higher the water expands more resulting in higher water level in those locations.

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. This is showing irregular sea level variation. The first diagram (on the left) shows that the pressure differential and wind pile up of the water creating the storm surge. The second diagram (on the right) shows the surge coming onto land and its relation to the mean sea level. It also shows that the tide can also increase the surge with a high tide. The link at the bottom of the page will take you to a nice animation simulating storm surge. Sometimes you have to click the link twice for it to go there, if not the web address is below the link. 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. This is showing another type of irregular sea level variation. Tsunami is a very large wave that can cause the sea level to fall and rise a very large amount, very rapidly (minutes). Tsunamis are generally a result of tectonic activity or very large landslide in the ocean causing a large displacement of water. They travel extremely fast.

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. The next several slides are related to how we measure sea level.

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) This is how we measured sea level before we had acoustic technology (or better technology).

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. This is how we measure sea level at the coast today with acoustic sensors and computers for the data collection. Now data can be sent out through the antenna rather than someone going to the tide gauge house to retrieve the data.

Let’s take a closer look at a modern water level measurement station Acoustic sensor to measure water level Housing to keep the computer and equipment out of the weather Sensor to measure wind speed and direction Solar panel to supply power These are images of a tide house and most of the components associated with collecting water level data. 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. The tide houses only collect data at the coast. To collect water level data in the deep waters of the ocean we use satellites. Satellites are much more costly that the tide houses.

Measuring Sea Level: Using satellite This is called satellite altimetry. It is quite complicated and we will not going into details here. This is just another way to measure sea level. This is quite expensive to use.

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 These are just some of the examples of causes for sea level change and what actually cause the sea level change.

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 These are the main causes for sea level change related to climate change.

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. On the two graphs, the blue lines are the predicted water levels (tides) and the green lines are the measured water levels. You can see the surge based on the water levels that are much higher than the predicted water levels. Notice on the second graph (bottom) that the measured water levels stop on Oct. 29. The water level measurements stop because the gauge broke due to the water levels surpassing the maximum measurement capability of that particular gauge. The map in the lower right hand corner point out the locations of the two water gauges. The Battery, NY 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. 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. This is showing an example of the impact of storm surge from Superstorm Sandy.

Examples of what your tide gauge can and should do: Example 2: Long term sea level trend This is a graph of water level measurements at a tide gauge in Key West, FL. This gauge has been measuring water level for about 100 years. Based on the trend of the measurements sea level rise in this location is about 2.24 mm per year. In this location there is not significant tectonic or geologic activities occurring.

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. This is a graph of water level measurements at a tide gauge in Grand Isle, LA. This gauge has been measuring water level for about 60+ years. Based on the trend of the measurements sea level rise in this location is about 9.24 mm per year. Notice that the sea level in this location is rising 7 mm per year more than the Key West gauge measured. This is due to the subsidence of the land. This topic should be discussed more 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. This is a graph of water level measurements at a tide gauge in Nikiski, AK. This gauge has been measuring water level for about 40 years, but not with a continuous record. Based on the trend of the measurements the sea level is falling in this location at a rate of about 9.80 mm per year. This is due to the isostatic rebound of the land. Isostatic rebound is the land raising back up after weight (glaciers) that was once pushing down on it is gone, i.e., the glaciers melting. This topic should be discussed more in the geology unit. The last two slides are showing that measuring actual sea level rise is a very difficult topic. Many times when looking at sea level trends you need to take into account the area you are looking at and consider the geologic activities occurring in that particular location.

The questions in the previous slides are complicated 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/