What can sea level tell us?

Oceanographers read a lot into sea level variability. After all, this is a phenomenon affected by celestial mechanics and climate change, as well as by more mundane factors like the weather. Our historical sea level data, and our educational resources, will help people unpick sea level variability and understand the different processes that affect it.

The Thames barrier

The Thames Barrier protecting London from sea level variability. Courtesy of Prof. P.L. Woodworth

What is the sea level signal made up of? We can divide the different factors into

  • those that change the volume of the water
  • those that move water from one place to another

Thermal Expansion

If you reheat a cup of tea in the microwave, the tea expands. This principle holds for water in the oceans, too. As it warms up, sea level rises. We will show how our historic sea level data can help users decide for themselves about the global warming debate.

Movement

Water movements in the ocean are caused by various processes:

  • Tides

These are the effects of gravitational forces from many astronomical bodies, which pull water around the oceans. We all know that tides are related to the lunar cycle, but our resources will show that there’s more to it than that.

  • Weather

If you push down on one end of a long balloon, the other end pushes up. In the same way, the sea surface is pushed down where atmospheric pressure is high, and pushes up where it’s lower. This is called the “inverse barometer effect”.

As well as its direct effect on the sea surface, air pressure also drives winds, and winds push water around. A sustained wind, blowing across a sea, piles water up at one side.

Our materials will show how, in 1953, weather conditions combined with tides to produce a historic flood, and what that has to do with our sea level data.

  • Ocean currents

Just as atmospheric pressure gradients causes air movements (winds), pressure gradients inside the ocean cause water movements (currents). This also works backwards: ocean currents cause ocean pressure gradients. Pressure gradients in the ocean show up as sea level changes. We will show how oceanographers now use satellites to measure sea level, and therefore ocean currents.

  • Land ice melt

If you add more water to the ocean, sea level goes up, a lot like running extra water into the bath. We will look at why ice melt running off land affects sea level more than icebergs melting into the ocean.

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