Sea level is the average sea level when averaged over the entire globe. An average needs to be made since sea level can locally vary due to local wind, tides, global wind patterns, gravity, water density, and pressure. Sea level is that location where the ocean meets the land. Average sea level is given a value of 0. Any positive value above 0 indicates the distance above average sea level. This writing looks at how sea level changes due to the factors mentioned. Local wind: Wind direction can influence sea level. Wind blowing from the ocean toward land will help pile up water on the beach. This will increase sea level. Tides: Often tides have the biggest influence on sea level on a day to day basis. The tides are caused by forces from the moon and the sun while the earth spins of its axis. Although the sun and moon are at a distance, they are close enough to produce a significant gravitational force. The oceans facing the moon and sun and the oceans opposite to the moon and sun will have high tide. Due to the sun and moon being at different locations, the tides are a complex mix of the influence of both the position of the sun and moon relative to a location on earth’s surface. Global wind pattern: Just like with local winds, global scale winds will either cause water to pile toward the shore or move away from the shore. Global wind patterns are influenced by the positions of global high and low pressure systems. The winds from these pressure systems influence ocean currents and the transport of water. Gravity: Typically gravity is given as a constant on earth’s surface as 9.81 m/s^2. This value can change slightly though depending on factors such as the density of rock below a location, the fact that the earth is not a perfect sphere, and types of rock/substances below a location. Water Density: Water changes density with temperature. As water warms above 4 C, it will decrease in density. Thus warming water expands and takes up a larger volume. Tropical waters have a greater volume than cold waters of the same mass. Pressure: The barometric pressure has an influence on sea level. Lower pressures cause sea level to rise and higher pressures cause sea level to lower. The influence is about 1 centimeter for each millibar change in air pressure. |