In Costa Rica Ocean Watch took on two new instruments: a second thermosalinograph for underway measurements of surface water temperature and salinity, and a profile instrument called a CTD for conductivity-temperature-depth. The CTD has a high-precision pH sensor. The CTD is lowered by a rope so a profile of the upper ocean structure can be obtained. The profile is necessarily shallow, but still reveals interesting near-surface structure including solar heating layers, mixed layers, and a strong thermocline. We will use the profiles to demonstrate many important concepts in physical oceanography.
|Figure 1. Captain Mark Schrader and scientist Michael Reynolds prepare the Sea-Bird Electronics, Inc. SeaCat for a cast. The SeaCat is lowered to a depth of 40 meters while it measures water conductivity, temperature, pH, and depth, each four times a second. The electrical conductivity and temperature are used to compute the salt concentration, salinity. Image credit: David Thoreson.|
Layers in the ocean
The waters of the oceans are layered and the structure of the layering tells oceanographers a great deal about the ocean currents, the origins of the water and its fate. To discuss ocean layers one needs to know about density. Density of a substance is the weight of a set volume of that substance. For example, a cup of mercury weighs much more than a cup of water because the density of mercury is about 14 times the density of water. Oil floats on water because oil density is slightly less than water.
In the ocean, the water density is determined, primarily, by two factors: the temperature and the salinity. As the temperature is increased the water becomes lighter. Salinity is the amount of salt in a volume of water. Oceanographers measure salinity in parts per thousand and use the symbol “°/°°“. The average salinity in the ocean is about 34°/°°, meaning 3.4% by volume. The saltier water, e.g. higher salinity water, is heavier. That is, it has a greater density. Density is a measure of weight for a specific volume. For comparison, the salinity of water in the Persian Gulf can reach more than 40°/°°, while the salinity in a river-fed bay might be a brackish 25°/°°.
|Figure 2. A graph of temperature, salinity, and pH versus depth for Ocean Watch cast number 11. In this example, typical near-surface layers can be identified. Starting at the surface, a shallow layer of warm water has developed from solar heating. Below that, a well mixed layer, to about 17 meters, was formed by wave and wind turbulence at the surface. Below the mixed layer, the temperature decreases rapidly in the thermocline, from 17 meters to the bottom of the cast. The salinity here changes little throughout the top 35 meters. However, in the solar layer and the thermocline salinity is highly variable which is a sign of turbulence and internal waves. The pH is almost constant at 8.1 which is typical for oceanic surface water in this region.|
Physical Oceanography pays particular attention to density and to the layers that form in the sea. The interface between layers is important because they form barriers to exchange between layers. There are thousands of examples and we certainly will not be covering them in detail here. I want to discuss instead the structure of the water we are currently transiting, where coastal upwelling breaks down the layers and brings nutrients to the surface thereby fueling the high productivity that supports a fishing industry.
The ocean surface is a region where layers are complex and variable. This is where the ocean and atmosphere exchange heat, moisture (rain and evaporation), and gases (approximately one-third of all CO2 emitted into the atmosphere by humans is absorbed into the ocean through its surface). Figure 2 shows that the top of the ocean in our current location off the Mexican coast is a warm layer over a deeper layer of cold water. Our cast does not go deep enough to register deeper layers, but the very strong thermocline is apparent.
Life in the ocean depends on the plants. In a pyramidal fashion, called the food chain, the plants (algae) come first and are followed by plant eaters such as shrimp, krill, shellfish, and tiny fishes. Then, eating these small creatures are larger fish, squid, and the other carnivores. Finally come the large fish, the tuna, swordfish, dolphin, and porpoises. The food chain depends on the plants, and the plants grow where there is sunlight and nutrients. Productivity is a measure of the rate of creation of carbon in the food chain. Satellites can measure ocean color and determine the amount of chlorophyll, and from that estimate the productivity in different regions. Murkiness in the water is related to the concentration of algae.
Plants grow in the ocean layers where there is ample sunlight, that is, near the surface. The euphotic zone is the top layer of water where there is enough sunlight for photosynthesis. The rest of the food chain follows and life in the upper ocean is good until the nutrients run out. Nutrients for ocean plants are not much different than nutrients needed in your garden. Plants need nitrogen, phosphorous, iron, and other ingredients. If any one of these runs out, productivity comes to a halt. As the food chain prospers there is a slow sinking of nutrients to the bottom. Fish and plants die and animals excrete waste, creating a steady snow of particulates, falling downward, and importantly, through the thermocline barrier. Indeed, the falling debris is called “marine snow” and it is reported by deep divers in all the world’s oceans.
|Figure 3. Wikipedia describes marine snow as follows: “In the deep ocean, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. Its origin lies in activities within the productive photic zone. Consequently, the prevalence of marine snow changes with seasonal fluctuations in photosynthetic activity and ocean currents.” Image credit: Planktos Science|
When the snow falls through the density barrier, in this case the thermocline, it is removed from the surface water and cannot provide nutrients for more algae production. The thermocline is an interface that bars mixing of water. When the nutrients run out the whole food chain is diminished, the water clears and turns blue to the observer, and the euphotic zone becomes much deeper. Productivity will increase only when more nutrients are injected into the euphotic zone, and a major way this happens is by upwelling.
We have described upwelling in previous reports. From the point of view of oceanography, upwelling is caused when the mixed layer is pushed offshore by the winds (along with gyroscopic forces called Coriolis forces) and deeper water moves shoreward lifting the thermocline up to the surface where it can be broken by wind and wave turbulence. With the thermocline broken deeper water can mix to the surface.
Measuring the ocean from Ocean Watch
This spring, Ocean Watch was sailing out of an El Niño region and up the west coast of the U.S., one of the world’s productive upwelling regions. For an ersatz oceanographer like me, it is just too frustrating to be sailing over such fascinating water without trying to look under the surface. Therefore, while I was back in Seattle in April I contacted my friends at Sea-Bird Electronics, Inc. and without hesitation they agreed to loan us two instruments. Sea-Bird is one of the most respected names in oceanographic instrumentation and we are deeply thankful for their support.
First they provided us with an instrument called the thermosalinograph. A thermosalinograph is a device that sucks seawater through a fitting below the water line in the boat’s hull, through a special sensor, and back out to sea. It continuously measures the temperature and salinity of the water as the ship travels. Other sensors, such as oxygen, pH, and opacity can be included. Our SeaKeeper thermosalinograph, described in many of our reports, has operated since we left Seattle. After such a long time we had some concerns about calibration drift, i.e. the accuracy of our measurements. The Sea-Bird thermosalinograph was connected into the same pumping system as the SeaKeeper sensors and it provides a check on any error from the SeaKeeper system.
And I am happy to say that the two key measurements, salinity and temperature, agree nearly perfectly on the two instruments. We are pleased and reassured by this result.
CTD profiles from Ocean Watch
Ocean sub-surface density profiles are measured by an instruments called a CTD. The name stands for conductivity-temperature-depth, the primary sensors. The CTD is the heart of any oceanographic research cruise. Other optional sensors for pH, oxygen, and other chemicals are often included with the package The Ocean Watch probe has a pH sensor (see Figure 2) so it is possible see differences to pH through the surface layers. Figure 1 shows the SeaCat being deployed and Figure 2 shows the profile of salinity, temperature, and pH for a typical cast. A discussion of the CTD program on Ocean Watch is a topic for a future report.
Michael Reynolds, Ph.D., RMR Company