Oceanography Introduction & Seawater Properties
Radiation and energy transfer to the ocean
Sunlight (radiant energy) strikes the ocean surface; some energy is reflected back to the atmosphere and does not enter the ocean. The portion that enters can be absorbed by water or by substances in the water.
Absorbed solar energy heats the water; energy transfer occurs both by molecular interactions (conduction) and by the movement of the fluid as a whole (convection, i.e., the water column behaving as a fluid).
At the surface, solar heating creates energy gradients that drive surface currents (e.g., Gulf Stream) and turbulent mixing when wind blows across the water surface.
Turbulent mixing is enhanced by wind, waves, and interactions with the surface; this mixing transfers heat from the surface to adjacent layers and helps explain why the upper ocean becomes stratified with warmer water on top of colder water.
Over larger scales, the solar energy input and subsequent redistribution of heat contribute to the global heat budget; chapter six will address the accounting of this budget more formally.
In essence: radiation enters the ocean, some energy is reflected, some is absorbed and redistributed by molecular (conduction, diffusion) and fluid (advection, mixing) processes, producing surface warming and currents.
Light in seawater: wavelength, energy, absorption, scattering, and attenuation
Visible light spans a spectrum from violet to red; shorter wavelengths have more energy than longer wavelengths: violet (shortest wavelength) has the most energy, red (longest visible wavelength) the least.
In seawater, blue light penetrates deepest because it is higher energy than red light and experiences less rapid absorption at depth in clear water.
Attenuation describes the exponential decrease of light intensity with depth due to absorption and scattering: the transmitted light I(z) declines roughly as
I(z) = I_0 \, e^{-\alpha z}
where (\alpha) is the attenuation coefficient, a function of absorption and scattering properties.
Absorption: conversion of electromagnetic energy into other forms (e.g., heat) by molecules in the water. Photosynthetic organisms (plants and microalgae with chloroplasts) absorb light to drive chemical energy production (sugar) via photosynthesis. Inorganic and organic molecules can also absorb light; sometimes absorbed energy is re-emitted or transferred to other particles.
Scattering: light changes direction when it hits particles or surfaces. This can occur at the water surface (reflection) or within the water by sediment, plankton, and organic matter. Scattering reduces the amount of light that penetrates straight downward and contributes to the lateral redistribution of light energy.
The combination of absorption and scattering defines attenuation; higher particle content and different particle types (sediment, phytoplankton, zooplankton) increase attenuation and shorten the penetration depth of light.
Open ocean vs coastal (neritic) water and the role of particles
Clear open-ocean water (few particulates) allows blue light to penetrate deepest, with blue wavelengths traveling farther down the water column relative to red.
Coastal or neritic waters contain more particles (sediment, phytoplankton, organic matter), increasing scattering and absorption, thus reducing light penetration depth compared with open-ocean water.
Sediment-rich estuaries and coastal shelves show dramatic attenuation due to high scattering and absorption by suspended solids and phytoplankton, often yielding very shallow penetration depths (e.g., coastal waters where light may only reach a few meters or tens of meters).
The attenuation and light penetration depth depend on particle concentration, particle type, phytoplankton abundance, and river input (which brings sediments and nutrients).
The general consequences: light penetration depth is shallower in productive, particle-rich coastal zones and deeper in relatively clear open-ocean waters.
Axial modulus, density, and speed of sound in seawater
Axial modulus (often discussed as a bulk or compressional modulus) describes how compressible a fluid is; higher axial modulus means lower compressibility (harder to compress).
Density ((\rho)) of water increases with salinity and decreases with temperature. Freshwater is less dense than saltwater.
Relationship between density, compressibility, and sound speed: in general, the speed of sound in a fluid is given by
c = \sqrt{\dfrac{K}{\rho}}
where (K) is the bulk (acoustic) modulus and (\rho) is density. This reflects that higher stiffness (larger (K)) and lower density yield higher sound speed.
In the spoken content, two related ideas were discussed (and a caveat noted):
A higher axial modulus and higher density would yield a certain effect on speed of sound per the equation above; the lecturer suggested that high density with high modulus could yield a slower speed of sound in that context, though the standard relation is c = sqrt(K/ρ).
More general trends presented: the speed of sound increases with increasing temperature, salinity, and depth, and decreases with decreasing temperature, salinity, and depth. In other words,
\frac{\partial c}{\partial T} > 0, \quad \frac{\partial c}{\partial S} > 0, \quad \frac{\partial c}{\partial z} > 0,
and conversely the speed decreases as temperature, salinity, or depth decrease.
Practically, freshwater at lower temperatures has a slower speed of sound than warmer freshwater; seawater (higher salinity) generally supports a higher speed of sound than freshwater at the same temperature. The coldest freshwater (near 0°C) tends to have the lowest speed among freshwater examples, while seawater at 0°C and 35 ppt salinity has a relatively low speed compared with warmer seawater of the same salinity.
Sound in the sea: propagation, sonar, and biological/ecological implications
Sound travels about five times faster in seawater than in air, making the ocean an excellent medium for long-range sound transmission.
Key factors affecting sound speed in seawater: temperature, salinity, and depth (pressure) tend to increase speed; lower temperature, lower salinity, and shallower depths tend to decrease it.
Uses of sound in ocean science and industry:
Sonar (Sound Navigation and Ranging): determine distance to objects by sending sound and measuring travel time, using the relation
d = \dfrac{c t}{2}
where (d) is one-way distance, (c) is the speed of sound in water, and (t) is the two-way travel time.
Bathymetry: using sonar to map bottom contours, features, and density contrasts to infer bottom type and depth.
Side-scan sonar: emits sound to profile the seafloor laterally and generate 2D or 3D images of bottom features; useful for locating shipwrecks, coastal features, and seafloor habitat type.
Echolocation: dolphins and some whales use focused sound pulses to locate prey; the returning echoes are interpreted by the animal’s nervous system to navigate and hunt.
Biological soundscapes: many marine organisms produce sounds (e.g., reef fishes, crustaceans, marine mammals) that convey habitat information, mediate communications, and influence navigation and behavior. The underwater soundscape is complex and biologically meaningful.
Tuna and seamounts example: side-scan/sonar data reveal layers of organisms and prey (e.g., micronecton), and layers of tuna aggregations around seamounts; predators follow prey layers, which vary with time of day due to illumination, prey distribution, and water column structure.
Practical illustrations of acoustic use and interpretation
Side-scan sonar images can reveal 3D features of the seafloor by interpreting variations in reflected sound density (bottom type, density contrasts, and layer thickness).
Cross-seamount studies show how density contrasts along the bottom create discernible acoustic reflections that map bathymetric features and habitat zoning, aiding fisheries science and habitat assessment.
Acoustic methods help identify layers of organisms (e.g., micronecton) at various depths and times of day, informing our understanding of predator-prey dynamics and feeding zones.
Salinity distribution and the latitudinal pattern
Seawater salinity is not uniform at the surface; there are distinct latitudinal patterns tied to the hydrologic cycle (evaporation, precipitation, river input, and sea-ice processes).
An isohaline is a line of constant salinity (e.g., the 34 and 35 practical salinity units, psu, or ppt). An isovalue line marks a constant salinity, and the 34 isohaline is commonly highlighted; the concept helps map surface salinity variations.
Latitudinal controls on surface salinity include:
Evaporation zones (generally around mid-latitudes) increase salinity by removing freshwater.
Precipitation zones (near the equator) decrease salinity by adding freshwater.
River discharge near coastlines lowers salinity in coastal waters.
Polar regions experience ice formation, which excludes salt from the ice; as ice forms, remaining seawater becomes saltier (increased surface salinity) while ice forms and sea ice formation effectively concentrates salinity in the surrounding water.
A schematic interpretation: at the equator and nearby, high precipitation lowers surface salinity; at mid-latitudes, high evaporation raises surface salinity; at high latitudes near the poles, sea-ice formation and other processes modify salinity, often increasing it locally as ice forms and rejects salt; in coastal zones, runoff and river input generally lower surface salinity.
A graphical summary ties salinity to the difference between evaporation minus precipitation: higher evaporation relative to precipitation tends to raise surface salinity; higher precipitation relative to evaporation lowers surface salinity. The red line (salinity) often tracks the blue line (evaporation minus precipitation) with a lag or regional offset depending on ocean-atmosphere interactions.
The key takeaway: surface salinity is controlled by the balance of evaporation, precipitation, river inputs, and sea-ice processes, producing a latitudinal pattern described by the hydrologic cycle.
Isohalines, salinity variation, and their significance for marine processes
Isohalines are lines of constant salinity, used to visualize salinity structure in the ocean’s surface layer and throughout the water column.
Salinity distributions influence water density, which in turn affects vertical stratification, circulation, and nutrient mixing.
The latitudinal salinity pattern, along with temperature and depth, influences the attenuation and penetration of light (through water clarity and particle content) and the density-driven layering relevant to ocean dynamics.
Connections to foundational principles and real-world relevance
Energy transfer: solar radiation drives the heat content of the ocean, influencing climate and weather patterns; understanding the pathways of energy absorption and mixing helps explain surface warming and vertical heat transport.
Light propagation and optics in water: absorption, scattering, and attenuation govern how far light penetrates, which affects photosynthesis, primary production, and the distribution of phytoplankton, with implications for global carbon cycling and marine food webs.
Acoustic physics and navigation: the speed of sound in seawater and its dependence on temperature, salinity, and depth underpin sonar, navigation, and the detection of seafloor features; animal echolocation demonstrates the biological significance of underwater sound.
Hydrologic cycle and salinity: the distribution of surface salinity is a useful proxy for understanding evaporation, precipitation, and freshwater inputs; salinity patterns feed back into ocean circulation, density stratification, and climate interactions.
Key numerical references and relationships mentioned in the lecture
Light energy and wavelength: shorter wavelengths have higher energy; violet highest energy, red lowest among visible light.
Light penetration depth: blue light penetrates deeper than red in clear ocean water; attenuation depends on wavelength and particle content.
Attenuation of light with depth: described by an exponential model
I(z) = I_0 \, e^{-\alpha z}
where (\alpha) depends on absorption by dissolved/submerged substances and scattering by particulates.
Speed of sound in seawater: general dependencies
Key qualitative rules for sound speed (as stated in the lecture):
Increases with temperature: (\partial c/\partial T > 0)
Increases with salinity: (\partial c/\partial S > 0)
Increases with depth/pressure: (\partial c/\partial z > 0)
Decreases with decreasing temperature, salinity, and depth (opposite trends).
Relation between density and speed of sound (conceptual):
c = \sqrt{\dfrac{K}{\rho}}
where (K) is the bulk (acoustic) modulus and (\rho) is density; higher density and higher modulus have competing effects on (c).
Practical sonar calculation example from the lecture: distance from speed of sound and travel time
d = \dfrac{c t}{2}
Terminology highlights from the lecture
Surface currents: large-scale flows at the ocean surface driven by wind, rotation (Coriolis), and density gradients; e.g., Gulf Stream as a familiar example.
Water masses and lenses: blocks or blobs of water at the surface with similar temperature and salinity properties that can be advected together; these relate to surface currents and mixing.
Phytoplankton, zooplankton, and micronecton: constituents of the upper ocean that influence light attenuation and food webs; zooplankton shells contribute to scattering.
Acoustic sensing and bathymetry: sonar-based tools can reveal bottom features, layers, and habitat types, enabling three-dimensional reconstructions of submarine terrain.
Hydrologic cycle context: evaporation concentrates salts, precipitation dilutes, rivers add freshwater, ice formation concentrates salt in seawater; these processes shape the latitudinal salinity distribution.
Practical implications and takeaways for exam-ready understanding
Radiative energy from the sun drives the thermal structure of the ocean; energy distribution and mixing govern sea surface temperatures and currents, which in turn influence climate patterns and marine ecosystems.
Light penetration in seawater is strongly modulated by wavelength and by the abundance and type of particulates; this affects primary production, nutrient dynamics, and the depth distribution of phytoplankton.
Sound is a powerful diagnostic tool in oceanography and marine biology; its speed and propagation depend on temperature, salinity, depth, and pressure, and it underpins sonar-based mapping and biological communication/navigation.
Salinity is not uniform and is governed by the hydrologic cycle; understanding isohalines helps predict water mass properties, density stratification, and regional ocean circulation patterns.
Quick reference formulas and constants (LaTeX-ready)
Attenuation of light with depth (conceptual): I(z) = I_0 e^{-\alpha z}
Light energy and wavelength (conceptual): shorter wavelength implies higher energy, since E \propto \dfrac{1}{\lambda} or, more fundamentally, E = h\nu = \dfrac{hc}{\lambda}
Speed of sound in a fluid (bulk modulus form): c = \sqrt{\dfrac{K}{\rho}}
Typical sonar distance relation: d = \dfrac{c t}{2}
Salinity units mentioned: approximately 34–35 psu (ppt) for typical seawater surface salinity; isohalines denote lines of constant salinity (e.g., 34, 35 psu).
Note on scope and order
The lecture moves from radiation and heat transfer to optical properties of seawater, then to acoustic properties and their uses, followed by salinity patterns and their hydrologic explanations. The overall arc ties solar energy input to physical (temperature, density, sound speed) and chemical (salinity) structure of the ocean, and then to observational tools (sonar, side-scan, echolocation) used to study these processes.
Connections to later topics
Chapter six will formalize the global heat budget and the ocean’s role in climate systems.
Deeper exploration of water mass formation, stratification, and their impact on nutrient transport and marine ecosystems.
Further study of bioacoustics and the underwater soundscape, including its use in assessing habitat distinction and species distributions.
Summary takeaways
Radiation is partly reflected and partly absorbed; the absorbed portion heats the water and drives mixing and surface currents.
Light in the ocean attenuates exponentially with depth; blue light penetrates deepest, while red is absorbed near the surface, with attenuation amplified by particulates and phytoplankton in coastal waters.
The speed of sound in seawater increases with temperature, salinity, and depth; the seawater medium supports long-range acoustic propagation, enabling sonar, echolocation, and hydroacoustic studies.
Salinity at the sea surface is governed by evaporation, precipitation, river input, and sea-ice processes; isohalines map the spatial variation of salinity and help predict water mass properties and circulation.