Oceanography: Seawater Properties - Sound, Light, and Density
Sound in the Ocean
Motivation: Whale Communication:
Humpback whales migrate vast distances (thousands of kilometers) from plankton-rich polar feeding grounds to warmer waters for calving and birth.
The ocean is mostly dark (light penetrates only s to meters), making visual communication impossible over long distances.
Whales are social animals and communicate using sound to find mates and maintain social structures.
They utilize special channels, known as SOFAR (Sound Fixing and Ranging) channels, to communicate over long ranges.
Speed of Sound in Water:
The speed at which sound waves travel through water is fundamentally influenced by its physical properties. Generally, sound travels faster with increasing temperature and increasing pressure (which directly corresponds to depth).
To understand why, we can look at the physics behind the speed of sound () in a fluid. The formula is: c = \sqrt{\frac{\text{bulk modulus (K)}}{\text{density (\rho)}}}
Bulk Modulus (): This measures a fluid's resistance to compression, essentially how 'stiff' it is. A higher bulk modulus means the material is harder to compress and will generally transmit sound faster.
Density (): This is the mass per unit volume (). A denser medium might seem like it would transmit sound faster, but it also increases the inertia of the molecules, which can slow down the transmission.
Detailed Effect of Temperature: As water temperature increases:
Density Decreases: Water molecules gain more kinetic energy, move further apart, and the water expands. This means its density ($\rho$) decreases. In the formula, a smaller denominator generally leads to a higher sound speed.
Bulk Modulus Increases: The increased kinetic energy of the molecules makes them more 'springy' and resistant to compression. So, the bulk modulus () increases. In the formula, a larger numerator generally leads to a higher sound speed.
Combined Result: Both the decrease in density and the increase in bulk modulus work together to increase the speed of sound as temperature rises, making this a very strong factor.
Detailed Effect of Pressure (Depth): As pressure increases (due to greater depth):
Density Increases: The immense pressure in deeper water compresses the water molecules, packing them more closely together. This leads to an increase in density ($\rho$). If this were the only factor, it would tend to decrease sound speed.
Bulk Modulus Increases: The water becomes significantly more resistant to further compression; it gets much 'stiffer'. This means the bulk modulus () increases substantially. This factor tends to increase sound speed.
Combined Result: While both density and bulk modulus increase, the bulk modulus increases proportionally more than the density. This means the stiffening effect dominates the increased mass, leading to a net increase in the speed of sound with increasing pressure/depth. Therefore, deep water often has faster sound speeds despite being very cold, because of the overwhelming effect of pressure.
SOFAR Channel Formation (Sound Refraction):
Refraction Principle: Sound waves bend towards regions where their speed is slower. (Analogy: a marching band slowing down when moving from pavement to sand, causing the lines to bend).
Vertical Speed Profile: A typical ocean profile shows:
Fastest sound speed near the surface (due to high temperature).
Fastest sound speed in deep water (due to high pressure).
Slowest sound speed at intermediate depths (around meters) - this is the core of the SOFAR channel.
Sound Trapping: In the SOFAR channel:
Sound rays propagating downwards from the slow-speed layer encounter faster water below, causing them to refract (bend) upwards back into the slow-speed layer.
Sound rays propagating upwards from the slow-speed layer encounter faster water above (warmer surface), causing them to refract (bend) downwards back into the slow-speed layer.
This trapping mechanism allows sound to travel immense distances with minimal loss of energy, making it ideal for long-range communication.
Human Impact on Ocean Sound and Marine Mammals:
Sources of Anthropogenic Sound: Oil exploration (seismic airguns), military activities (sonar), shipping traffic.
Intensity: Human-generated sound in certain locations can be times louder than jet engines, far exceeding the natural sounds of whales.
Consequences for Whales: Mass stranding events of whales and porpoises provide clear evidence of physical harm.
Disorientation and Confusion: Impairs navigation and communication.
Physical Damage: Loud sounds can cause inner ear damage and hemorrhaging.
Legal & Ethical Conflict: Clashes between government's right to conduct naval exercises (national security) and environmental groups advocating for animal rights.
US Supreme Court Ruling (2008): Favored the government's right over animal rights, stating that national security concerns trump animal welfare in this context.
Shadow Zones: Regions where sound rays are deflected away, creating areas where submarines can hide from sonar detection.
Global Warming Detection via Sound Speed: Because sound travels faster in warmer waters, the slight increase in ocean temperature due to global warming can be detected by measuring the increased speed of sound propagation over long distances (e.g., across the Pacific from Hawaii).
Light in the Ocean
Light Penetration: Light only penetrates the uppermost layers of the ocean (typically s to meters Maximum light penetration depth depends on water clarity and angle of sunlight).
Color Absorption: Water preferentially absorbs certain wavelengths (colors) of light.
Red Light: Absorbed most rapidly, disappearing quickly with depth.
Green Light: Also absorbed relatively quickly.
Blue Light: Penetrates deepest, which is why the ocean appears blue.
Observation: Natural underwater environments at even moderate depths appear predominantly blue-green. Vibrant colors (like in discovery channel documentaries) are often only visible due to artificial light sources (camera flashes).
Electromagnetic (EM) Spectrum:
Light as Waves: EM radiation consists of waves with varying amplitudes (wavelengths).
Spectrum Range: Includes radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays, and gamma rays.
Visible Spectrum: Human eyes have evolved to be sensitive only to the visible portion of the EM spectrum.
Wavelength and Energy: Shorter wavelengths correspond to higher energy (e.g., UV, X-rays, gamma rays). Longer wavelengths correspond to lower energy (e.g., radio, microwave, IR).
Temperature Dependence of Emission: Hotter objects emit shorter wavelength radiation (e.g., sun emits visible light, hot stove coil emits visible and IR). Cooler objects primarily emit longer wavelength radiation (e.g., human body emits infrared).
Infrared Camera Examples: Used to visualize temperature differences (e.g., body heat, energy leaks from buildings).
The Special Properties of Water
Water Molecule Composition: One oxygen atom and two hydrogen atoms ().
Electron Sharing and Polarity:
Noble Gas Configuration: Atoms (like oxygen and hydrogen) bond to achieve a stable electron configuration similar to noble gases (e.g., neon for oxygen, helium for hydrogen).
Covalent Bonds: Oxygen and hydrogen atoms share electrons to form water molecules.
Electronegativity: Oxygen is larger and more electronegative than hydrogen, meaning it attracts shared electrons more strongly.
Polarity: This unequal sharing creates a slight negative charge on the oxygen atom and slight positive charges on the hydrogen atoms, making water a polar molecule.
Hydrogen Bonds:
Intermolecular Attraction: The slight positive charge of a hydrogen atom in one water molecule is electrostatically attracted to the slight negative charge of an oxygen atom in an adjacent water molecule. This attraction is called a hydrogen bond.
Significance: Hydrogen bonds are not true chemical bonds (they don't involve electron sharing within a molecule), but they are crucial for many of water's unique properties and are essential for life.
Consequences of Polarity and Hydrogen Bonds:
Three States of Water: Gas (water vapor), liquid (water), and solid (ice).
Solid (Ice): Molecules are locked in a rigid structure, unable to slide past each other.
Liquid (Water): Molecules can easily glide past each other, but hydrogen bonds still exert influence.
Gas (Water Vapor): Molecules are widely dispersed, flying about with high energy.
High Heat Capacity: Water has a high capacity to absorb and store heat energy without a large change in temperature.
Reason: A significant amount of energy is required to break the hydrogen bonds between water molecules before their kinetic energy (and thus temperature) can increase.
Latent Heat: Large amounts of energy are absorbed or released during phase changes without a change in temperature:
Latent Heat of Fusion: Energy needed to melt ice into liquid water at .
Latent Heat of Vaporization: Energy needed to convert liquid water to water vapor at .
Environmental Impact: The high heat capacity of the oceans (which cover of Earth) helps stabilize global climate by moderating temperature fluctuations.
Universal Solvent: Water is an excellent solvent, capable of dissolving many substances, especially ionic compounds (like salt).
Mechanism: The polar nature of water allows its slightly positive hydrogen ends to surround negative ions (anions) and its slightly negative oxygen end to surround positive ions (cations), effectively dissolving them through electrostatic attraction.
Surface Tension: Hydrogen bonds create cohesive forces that lead to high surface tension.
Density of Water
Definition: Mass per unit volume.
Freshwater Density vs. Temperature:
Unlike most substances, freshwater reaches its maximum density at approximately .
Above , water becomes less dense as temperature increases.
Below , water also becomes less dense as temperature decreases (which is why ice floats).
Lake Mixing (Dimictic Behavior):
Winter: Lakes are cold, often below . Edges begin to warm in spring.
Spring Turnover: As surface water warms to , it becomes denser and sinks, leading to vertical mixing until the entire lake reaches . Then, further warming causes surface water to become less dense, creating summer stratification.
Summer Stratification: A warm, less dense cap of water sits atop cooler, denser water.
Fall Turnover: As surface water cools from summer, it again reaches , becomes denser, and sinks, leading to another period of vertical mixing until the entire lake is uniformly mixed (often below in winter).
This seasonal mixing (fall and spring turnover) is characteristic of dimictic lakes (like many in Minnesota).
Seawater Density vs. Temperature and Salinity:
Salinity Effect: As salinity increases, the temperature of maximum density shifts to below .
Monotonic Relationship: For typical ocean salinity (around parts per thousand, or grams of salt per liter), seawater density monotonically increases as temperature decreases (i.e., denser the colder it gets, without a anomaly).
Freezing Point: Seawater freezes at approximately (not ) due to dissolved salts.
Ocean Temperature Profiles:
Early observations (e.g., Ellis, th century sailors) showed that warm water is confined to the very top layer, with cold water generally below.
Modern View: The vast majority of the ocean (below a few hundred meters) is filled with frigid water, often around . Surface waters can be in tropical regions but rapidly decrease to near-freezing at high latitudes.
Vertical Stratification: Due to density differences, warmer, less dense water stays at the surface, while colder, denser water sinks.
Convection: This principle explains why deep ocean waters are cold; they originate from dense, cold waters that sink at the poles (observed by Rumford).
Ocean Salinity Profiles:
Salinity also varies across the ocean and with depth (e.g., Atlantic is saltier than other oceans).
Combined Influence of Temperature & Salinity on Density:
Ocean density is a function of both temperature and salinity.
General Rule: Colder water and saltier water are both denser.
Ocean Structure: Density defines the three-dimensional structure of the ocean. Just as a forest has layers (underground, smaller trees, canopy), the ocean has distinct layers based on density.
Density Profile: In a stable water column, density always increases with depth (heavier water is at the bottom).
Vertical Mixing and Ocean Circulation:
Surface Warming (Summer): Sunlight warms the surface, making it less dense and promoting stratification (stability).
Surface Cooling (Winter): Cold, strong winds cool the surface water. This cooling makes the surface water denser.
Convective Adjustment / Vertical Mixing: If the surface water becomes denser than the water below, it will sink, inducing vertical mixing (deep convection).
Great Ocean Conveyor Belt: These density-driven processes, particularly in polar regions, are fundamental drivers of global ocean circulation. For example, cold, dense waters formed at the poles sink and spread throughout the deep ocean. Strong winds also contribute to mechanical mixing. When mixing occurs, the water column becomes homogenized (a single temperature/salinity vertically).