Temperature of Ocean Waters

Temperature of Ocean Waters

Ocean waters, like land, are heated by solar energy, but the heating and cooling process is slower in water compared to land. The temperature distribution of ocean water varies both horizontally (spatial) and vertically (with depth).

Factors Affecting Temperature Distribution

1. Latitude
   • Surface water temperature decreases from the equator towards the poles due to reduced insolation.
2. Unequal Distribution of Land and Water
   • Oceans in the northern hemisphere are relatively warmer than those in the southern hemisphere because of greater land contact, which provides more heat.
3. Prevailing Winds
   • Winds blowing from land to ocean drive warm surface water away, causing upwelling of cold water from below.
   • Conversely, onshore winds pile up warm water, raising coastal temperatures.
4. Ocean Currents
   • Warm currents raise temperatures in cold regions; cold currents lower temperatures in warm regions.
   • Example: Gulf Stream warms the eastern coast of North America and western Europe; Labrador Current cools the northeast coast of North America.
5. Enclosed Seas
   • Low-latitude enclosed seas → higher temperatures than open seas.
   • High-latitude enclosed seas → lower temperatures than open seas.

Horizontal and Vertical Temperature Distribution

• Surface Temperature
  • Maximum at the ocean surface due to direct solar heating.
  • Heat is transmitted to lower layers through convection, causing temperature decrease with depth.
  • Rate of decrease: Rapid up to 200 m, then slows down.
  • Average surface temperature: ~27°C, decreasing poleward:

    Latitude	Average Temp (°C)
    0° (Equator)	27
    20°	22
    40°	14
    Poles	0

• Vertical Temperature Profile
  • Three-layer system (tropical/mid-latitudes):
    1. Surface Layer: Warm, 0–500 m, 20–25°C (present year-round in tropics; only summer in mid-latitudes).
    2. Thermocline Layer: 500–1,000 m, rapid temperature decrease.
    3. Deep Ocean Layer: Very cold, extends to ocean floor, ~0°C.
  • Polar Regions: Only one cold layer exists (surface to deep floor) with slight temperature change.
• Hemisphere Differences
  • Northern hemisphere average annual temperature: ~19°C
  • Southern hemisphere average annual temperature: ~16°C
  • Highest ocean temperatures occur slightly north of the equator, not exactly at the equator.

Key Terms to Remember

• Thermocline: Boundary region (100–400 m below surface) where temperature drops rapidly.
• Upwelling: Rise of cold water to replace displaced warm surface water.
• Convection: Process transmitting heat from surface to lower layers.
• Gulf Stream: Warm current raising temperatures.
• Labrador Current: Cold current lowering temperatures.

Salinity of Ocean Waters

Salinity refers to the total content of dissolved mineral salts in seawater, expressed in parts per thousand (‰ or ppt). It is measured as the grams of salt dissolved in 1 kg of seawater. Salinity is an important property of seawater, influencing its density and temperature. Waters with salinity below 24.7‰ are considered brackish.

Factors Affecting Salinity

1. Evaporation and Precipitation
   • Surface salinity depends mainly on evaporation (increases salinity) and precipitation (decreases salinity).
2. Freshwater Input
   • Coastal regions: Salinity affected by river inflow.
   • Polar regions: Salinity influenced by freezing and thawing of ice.
3. Wind
   • Transfers water from one area to another, affecting salinity locally.
4. Ocean Currents
   • Influence distribution of salinity, since salinity, temperature, and density are interrelated.

Highest salinity in water bodies:

Horizontal Distribution of Salinity

• Normal open ocean: 33–37‰
• Land-locked seas and extremes:
  • Red Sea: 41‰ (high evaporation)
  • Estuaries/Arctic: 0–35‰ (seasonal variation)
  • Hot/dry regions: up to 70‰

Ocean-specific trends:

Vertical Distribution of Salinity

• Surface Layer:
  • Salinity varies with evaporation, ice formation, or freshwater input.
• Deep Ocean:
  • Salinity is stable, as salt cannot be added or removed.
  • Lower salinity water rests above higher salinity, denser water → leads to stratification.
• Halocline:
  • Zone where salinity increases sharply with depth.
• Effect on density:
  • Higher salinity → higher density → sinks below lower salinity water.

Key Terms to Remember

• Salinity (‰ / ppt): Grams of salt per 1 kg seawater.
• Brackish water: Salinity ≤ 24.7‰
• Halocline: Zone with rapid increase in salinity with depth.
• Stratification: Layering of seawater by salinity (denser water below).

Photosynthesis in the Ocean

1. Definition

• Photosynthesis in the ocean is carried out by phytoplankton, microscopic algae, and cyanobacteria.
• It requires sunlight and occurs only in the photic/euphotic zone, the layer of ocean where light intensity is sufficient for net positive photosynthesis.
• Measured in terms of primary production, which is the amount of carbon fixed by photosynthetic organisms.

2. Depth Zones and Photosynthesis

3. Depth Variation of Photosynthesis

• Highest rates occur near the surface due to abundant sunlight.
• Below ~200 m, light decreases rapidly, and net photosynthesis stops at the compensation depth, where photosynthesis = respiration.
• Light attenuation with depth is the main limiting factor.
• Some phytoplankton can photosynthesize at very low light levels, but ≈95% of ocean photosynthesis occurs in the euphotic zone.

4. Key Concepts

• Photic/Euphotic Zone: Ocean layer where light supports net photosynthesis.
• Compensation Depth: Depth where photosynthesis equals respiration; no net growth occurs below this.
• Light Attenuation: Reduction in sunlight intensity with depth, limiting photosynthesis.
• Primary Production: Total carbon fixed via photosynthesis; forms the base of the marine food web.