oceanlecture12deepwater2025

HadISST Data

• Summary of Sea Surface Temperature (SST) changes since the mid-20th century, indicating various temperature fluctuations.

Deep Ocean Circulation

• The role of salinity, heat, and carbon in ocean stratification.

• Depth vs Salinity:

  • Graph shows depth (m) versus salinity (psu) and indicates changes in the stratification of ocean layers.

• Key Zones:

  • Mixed Layer (~0-100 m) with constant density.

  • Pycnocline (~100-1000 m) shows sharp increase in density with cooling.

  • Deep Ocean (beyond 1000 m) with gradually increasing density due to cooling and salinity.

Surface Currents and Their Impact

• Graph illustrates locations of different ocean currents:

  • Cold-water currents (e.g., Peru Current, Kuroshio Current).

  • Warm-water currents (e.g., Gulf Stream, North Atlantic Current).

• Significant Currents:

  • Atlantic Meridional Overturning Circulation (AMOC), including NADW, AABW, and AAIW.

Density Stratification

• Oceans are stratified by density:

  • Low-density water floats on high-density water.

  • Wind influence creates mixed layers at the surface, while deeper water is stratified.

Mechanisms Affecting Stratification

• Clicker Question: Mechanisms that could theoretically cause strat stratification to break down:

  • A. Surface Cooling

  • B. Bottom Warming

  • C. Surface Evaporation

  • D. A and B, but not C

  • E. A, B, and C.

Salinity and Density of Seawater

• Average seawater salinity is approximately 35 g/kg, or 3.5%.

• Salinity increases the density of seawater, contributing to circulation:

  • Increases from ~1.0 g/cm3 to ~1.02-1.03 g/cm3 with increasing salinity.

• Temperature Influence:

  • Density increases with decreasing temperature, especially below 4°C.

Sea Ice Formation

• Sea ice is primarily pure water ice with some salt pockets.

• Brine Rejection:

  • Salt is expelled from ice during formation, increasing the surrounding water's density.

• Types of Sea Ice:

  • Pancake Ice

Sea Surface Temperature and Salinity Maps

• Sea surface temperature (SST) varies across different latitudes:

  • Maximum temperature recorded at the equator is 29.9°C.

  • Minimum values approaching -1.93°C in polar areas.

• Sea surface salinity (SSS) shows variation, with max salinity noted at 40.02 in certain regions.

Water Masses in the Deep Ocean

• Three primary deep ocean water masses with characteristics:

  • North Atlantic Deep Water (NADW): ~3°C, 34.9‰ (salty)

  • Antarctic Bottom Water (AABW): ~0°C, 34.6‰ (cold)

  • Antarctic Intermediate Water (AAIW): ~3°C, 34.2‰ (fresher)

Thermohaline Circulation

• Influenced by wind effects primarily in the upper ocean.

• Downwelling occurs when surface water becomes denser than underlying water.

• The creation of deep upwelling occurs through localized energy sources (e.g., tides, winds over uneven topography).

Ocean Heat Transport

• NADW formation:

  • Warm, salty Gulf Stream flows north, cools and sinks to form NADW.

• Important flux measurements for ocean heat transport: ~15 Sv, ~1.2 PW of heat.

Future Considerations

• Potential for the future collapse of thermohaline circulation.

  • Influences include freshening from melting glaciers and global warming impacts.

Water Mass DIstribution

• T-S diagrams reveal the relationship between temperature and salinity for understanding subsurface water sources.

• Density implications:

  • Determination of water masses based on characteristic lines of equal density called isopycnals.