Ocean Salinity
Ocean salinity is the concentration of dissolved salts in seawater, primarily sodium chloride (NaCl), along with chloride, sulfate, magnesium, calcium, and potassium ions. Average ocean salinity is approximately 35 parts per thousand (ppt) or 35 g of salt per kilogram of seawater, though it varies regionally due to evaporation, precipitation, river input, sea ice formation, and melting. Salinity directly affects seawater density and is a key driver of thermohaline circulation, marine organism physiology, and the freezing point of seawater.
Key Formula
Salinity S (ppt) = (mass of dissolved salts / mass of seawater) × 1000
LaTeX: S = \frac{m_{salt}}{m_{seawater}} \times 1000 \text{ ppt}
| Symbol | Meaning | Unit |
|---|---|---|
| S | Salinity | ppt (parts per thousand) or PSU |
| m_salt | Mass of dissolved salts | g |
| m_seawater | Total mass of seawater sample | g |
Worked Example
Problem
A 500 g seawater sample from the Red Sea is evaporated and leaves 19.5 g of dissolved salts. Calculate the salinity in ppt. How does this compare to the average ocean salinity of 35 ppt?
Solution
Step 1: Apply the salinity formula: S = (m_salt / m_seawater) × 1000 S = (19.5 g / 500 g) × 1000 S = 0.039 × 1000 S = 39 ppt Step 2: Compare to average: Difference = 39 − 35 = 4 ppt higher than average Percentage above average = (4 / 35) × 100 = 11.4% above average
Answer
Salinity = 39 ppt, which is 4 ppt (11.4%) higher than average ocean salinity, consistent with the Red Sea's high evaporation rate and restricted water exchange.
Salinity Values in Selected Ocean Regions
| Region | Salinity (ppt) | Main Cause of Variation | Effect on Density |
|---|---|---|---|
| Average Ocean | 35 | Baseline | Reference density ~1025 kg/m³ |
| Red Sea | 40–43 | High evaporation, restricted inflow | Higher density |
| Baltic Sea | 7–8 | High river freshwater input | Much lower density |
| Dead Sea | 280–300 | Extreme evaporation, inland basin | Very high density |
| Arctic Ocean surface | 28–30 | Sea ice melt, river input | Lower density |
| Equatorial Pacific | 34–35 | High rainfall dilutes surface | Near average |
Interactive Tools
NOAA World Ocean Atlas
Global climatological data including ocean salinity distributions at all depths
Open ToolWolfram Alpha Seawater Properties
Calculate seawater density and properties from salinity and temperature values
Open ToolKhan Academy: Ocean Salinity
Lesson on salinity sources, variation, and biological significance in the ocean
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Thermohaline Circulation
Thermohaline circulation is a global system of ocean currents driven by differences in water density, which is controlled by temperature (thermo) and salinity (haline). Cold, salty water is denser and sinks in the North Atlantic and around Antarctica, driving a slow, deep circulation that connects all ocean basins in what is often called the "global ocean conveyor belt." This circulation system plays a critical role in regulating Earth's climate by transporting heat from the tropics to higher latitudes and cycling nutrients through the ocean depths.
Thermocline
The thermocline is a distinct layer in the ocean — typically found between 200 and 1000 meters depth — where water temperature decreases rapidly with increasing depth, separating the warm, well-mixed surface layer from the cold, deep ocean. The main thermocline is a permanent feature of the tropical and mid-latitude oceans, with temperature dropping from about 20°C at the surface to 5°C at 1000 m depth, while seasonal thermoclines can form and dissipate in response to summer heating. The thermocline acts as a physical barrier that limits the exchange of nutrients, gases, and heat between the surface and deep ocean.
Marine Ecosystem
A marine ecosystem is a community of living organisms — including phytoplankton, zooplankton, fish, marine mammals, and benthic organisms — interacting with each other and their physical environment (water, light, temperature, salinity, nutrients) in the ocean. Marine ecosystems range from shallow coastal zones and estuaries to the open pelagic ocean, deep-sea hydrothermal vents, and polar seas, with each zone characterized by distinct biological communities adapted to its conditions. These ecosystems provide critical services including oxygen production (phytoplankton generate ~50% of Earth's oxygen), carbon sequestration, food security for billions of people, and climate regulation.
From Latin "salinus" (of salt), derived from "sal" (salt). The systematic measurement of ocean salinity was pioneered during the HMS Challenger expedition (1872–1876), which established that ocean salinity was remarkably uniform at about 35 ppt worldwide.