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Oceanography – Physical Properties & Structure
Ocean Temperature
- Ocean Mean Temperature (OMT):
- Measured up to the depth of the 26°C isotherm.
- In the south-western Indian Ocean during January-March, the 26°C isotherm is at a depth of 129 meters.
- OMT data from January-March is a useful indicator for forecasting whether the upcoming monsoon rainfall in India will be above or below the long-term average.
Ocean Floor Topography
- The most extensive part of the ocean floor is the Deep Sea Plains (abyssal plains).
- Mid-Oceanic Ridge:
- A massive, continuous underwater mountain range.
- Approximately 64,000 km long and 2000-2400 km wide.
- It passes through the North and South Atlantic Ocean, enters the Indian Ocean, and continues into the South Pacific basin between Australia and Antarctica.
- Indian Ocean Features:
- Ninety East Ridge: A major submarine ridge.
- Telegraph Plateau: A distinct feature.
Global Distribution of Water
Saltwater vs. Freshwater
- Approximately two-thirds (2/3) of the Earth’s surface is covered by water.
- Freshwater available for human use constitutes less than 1% of the total water on Earth.
- The largest single portion of the Earth’s hydrosphere is the Pacific Ocean.
Freshwater Distribution
- The largest reservoir of freshwater on Earth is locked in polar ice caps and glaciers.
- After ice caps and glaciers, the largest proportion of the remaining freshwater exists as groundwater.
- Hierarchy of Freshwater Amounts: Water in polar ice caps and glaciers > Groundwater > Water in rivers and lakes.
Seas and Ocean Depths
- Among the listed seas (Bering Sea, South China Sea, Mediterranean Sea, Japan Sea), the Bering Sea is the deepest.
Earth’s Geometry and Measurement
Earth’s Shape
- Due to the Earth’s oblate spheroid shape (flattened at the poles), the place nearest to the center of the Earth at sea level is the North Pole.
Surveying Terminology
- Datum Line: A horizontal reference line, typically based on mean sea level, from which heights and depths on Earth are measured.
Soil Water (Hydrogeology)
- Gravitational Water: Water held in the ground by a tension of below 1/3 atmosphere. This water can drain freely under the influence of gravity.
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