Buoys, Tide Gauges, Nansen Bottles and the Like

It is not enough to rely on satellite observations from space. Measurement data are still collected by older, more traditional methods such as buoys, tide gauges, and Nansen ocean-sampling bottles. The measurements from these observation instruments are necessary to verify the observations from satellites.

Nansen bottles, invented by the Norwegian oceanographer Fridtjof Nansen, are cylindrical containers that sample sea temperature and salinity. They were typical of tools used in the mid-twentieth century from which data were obtained to model ocean circulation. They have generally been replaced by modern electronic instruments, but measurements are still made from ships in the same manner. Samples are usually taken at a number of (different) depths at fixed (same sites) locations at sea, called hydrographic stations. The devices are lowered in the water on cables and measure water properties from the surface to the bottom. The ship then moves on to the next hydrographic station. The process is slow and the regions that can be covered are small compared to the size of the ocean. Still, oceanographic charts are based on the analysis of samples such as these--collected from many ships at different times from different regions and different depths within the sea. This data produces very general maps of the average arrangement of water masses and average patterns of circulation. They could not show how masses of warm or cold water vary or how they swell or diminish, nor how their positions change with the seasons. Below is an artists rendering of a Nansen bottle.

Nansen water bottles before (I), during (II), and after (III) reversing. (From Dietrich et al. 1980)
This is a photograph of an S4 current meter used for shipboard observations.

Before we look at current meters, we should make a note of the differences between two types of velocity (which is how fast something, such as water, is moving in a specific direction) --Lagrangian and Eulerian. The best explanation of each type is found in the methods used to measure the two velocities. Lagrangian velocity in theory is done by tracking a parcel of water wherever it may go - Imagine the current as a car which you followed all over town. Eulerian velocity is a measure of ocean currents which flow past a fixed position - Now imagine sitting at the intersection of two streets, counting the number of cars which drive by your intersection. Now that you understand the distinction, we can look at the different types of current meters.

Lagrangian current meters come in all shapes and sizes- there are both surface and subsurface (below the surface) drifters. Lets talk about surface drifters. The velocity of the surface drifters is used to calculate the currents of the water around the drifter- the idea is that the ocean currents will determine the drifters path. To determine the velocity we must know the location of the drifter from day to day and so each drifter carries a radio transmitter (which sends a signal that tells where it is). Weather satellites carrying a receiver (which hears the signal of the transmitter) are used to track the position of the drifter.

Holey Sock Subsurface Drifter (Photo courtesy http://dbcp.amverseas.noaa.gov)
This is a simplified diagram of a subsurface moored current meter.

Eulerian current meters, since they must remain stationary (not moving), are generally moored (anchored) to the sea floor and they do not extend entirely to the surface (typically the top of the current meter is 500 m or about 1500 ft below the surface) and therefore not visible from a boat. To get a vertical profile of the current, several instrument stations are attatched to the mooring line. Just like the Lagrangian current meters, the eulerian current meters have a radio transmitter which sends the data to satellite receivers. Compare the complexity of this archaic (old) current meter the meter just described.