Thermocline & Pycnocline Gradients of Temperature & Density: the Stratification (Layering) of the Ocean Also, the SOFAR Channel, Marine Mammals & Submarines p. 122-123

Thermocline & Pycnocline Based on what we know about the uneven heating of the Earth, predict the temperature profile of the water column for the low latitudes, mid-latitudes, and high latitudes. very cold to cold very cold to cold cool to warm warm to very warm cool to warm very cold very cold very cold p. 161

Mean Annual Temperature of the Surface Ocean Observations? warmer in the tropics, colder at the poles temperature patterns are +/- zonal eastern sides of the ocean basins differ from the west sides of the ocean basins

note the different scale Mean Annual Temperature at 4000 m Depth note the different scale Observations? the gray areas are areas shallower than 4000 m (e.g., spreading centers) deep waters are very cold! <3.5°C the deep Atlantic is the warmest, the Southern Ocean is the coldest, and the Indian & Pacific are intermediate

The Thermocline solar energy heats the surface waters in the low to mid-latitudes net solar gain in the tropics & subtropics this creates a warm, less dense surface layer over very cold and dense deep waters the permanent thermocline is the interval through which temperature decreases rapidly with increasing water depth p. 123

The permanent thermocline extends from the base of the surface mixed layer (~50-100 m) to approximately 1000 m water depth. The depth of the mixed layer is a function of mixing (homogenization) of the warmed surface waters by the day-to-day winds and storms, waves and surface currents. p. 122-123

pronounced seasonality Winter storms tend to be bigger than summer storms, therefore the mixed layer tends to be deeper during winter months. Summer heating causes the creation of a seasonal thermocline (a steeper temperature gradient than during the winter), particularly in the mid-latitudes. pronounced seasonality is the hallmark of the mid-latitudes

A permanent thermocline is absent in polar regions because surface waters are very cold and deep waters are very cold. Therefore, there is little temperature contrast/gradient between polar surface and deep waters. p. 122-123

Temperature Profiles Across Latitude

Mean Annual Sea Surface Temperatue http://en.wikipedia.org/wiki/World_Ocean_Atlas

Plot of seawater density versus depth Why are deep waters more dense than surface waters? p. 162

Now to say something about the speed of sound in water.

For the range of temperatures found in the ocean (0-40oC), the colder the water the slower the speed of sound.

For the range of pressures found in the oceans (1-1100 atmospheres), the greater the pressure the faster the speed of sound.

From the surface to the base of the thermocline the effect of the decreasing water temperature is to decrease the speed of sound in water. Below the thermocline the pressure effect dominates and the speed of sound increases to the seafloor. The velocity of sound in water is much faster than in air: ~1484 m/sec (= 3320 mi/hr) compared with ~343 m/sec (= 767 mi/hr)

SOFAR Channel In most deep water, there is a sound channel formed near the bottom of the thermocline where there is a zone of minimum sound velocity. This is known as the SOFAR (Sound Fixing and Ranging) Channel after a wartime system for rescue of aviators downed at sea. Commonly in the tropics it is at a depth of 1 km, but in the polar regions where there is no thermocline, it reaches the surface. http://www.dealbreaker.com/images/entries/submarines.jpg http://www.divediscover.whoi.edu/expedition12/hottopics/images/sound4-en.jpg

Downward refraction caused by the thermocline meets upward refraction caused by high pressure. Sound propagating upward is bent down and sound propagating downward is bent up. Therefore, sound is focused in the sound channel axis. In this channel, little energy is absorbed by the bottom or scattered by waves, so sound tends to propagate for long distances. The sound waves trace a path that oscillates across the SOFAR channel axis. This principle is similar to long distance transmission of light in an optical fiber. Sound signals that originate in the SOFAR Channel tend to stay in the channel rather than escaping. The sound may travel enormous distances in this channel; explosions set off in the channel in Australia have been heard in Bermuda (25,000 km or 15,500 miles away!).

SOFAR SOFAR SOFAR p. 120, 123 Minimum sound velocity (SOFAR Channel) is ~1000 m in the subtropics, but occurs at shallower depths in the mid-latitudes and near the surface in the high latitudes because of changing temperature profile of the water column

The ocean’s acoustical layering can be subdivided: Surface Channel: the low velocity sound passage between the surface and the SLD. SLD-Sonic Layer Depth of maximum sound velocity SOFAR: can be divided into a Secondary Sound Channel and the main Primary Sound Channel. The best depth for a submarine to avoid detection by a hull-mounted sonar is conventionally regarded as the Sonic Layer Depth plus 100 meters.

http://oceanexplorer. noaa http://oceanexplorer.noaa.gov/technology/tools/acoustics/media/haru_mooring.html

Marine mammals are very sensitive to sounds in the sea http://www.noaanews.noaa.gov/stories2007/images/humpbackwhale_hires.jpg

The destroyer Shoup, background, conducting sonar exercises in the San Juan Islands of Washington State with orcas nearby. Photo by Kenneth C. Balcomb III/Center for Whale Research (NY Times June 24, 2008).