- Introduction
- Marine climate
- Sea surface temperature
- Sea surface salinity
- Vertical structure
- Ocean currents
- Regional processes
- Tides
- Surface waves
- Conversion factors
- On-line references
- Authors
- Copyright
Vertical profiles of water properties
The average vertical profiles of temperature, salinity, and nutrients (nitrate+nitrite) were computed from series of monthly surface-to-bottom measurements made at 22 45 N 158 W (see below).
Plate 5. Position of the Ocean Station Aloha (22 45 N, 158 W), visited monthly by a research ship in support of global change research programs (circle), and of the NOAA weather buoy 50001 (23 25 N 162 20 W) (asterisk).
Near the surface, the water column is mixed by the wind and has uniform properties; the depth of this turbulent layer varies from nearly 120 m (400 ft) in winter to less than 30 m (100 ft) in summer. Below the mixed layer there is a sharp decrease in temperature (called a thermocline), from 25 C (77 F) at the surface to 5 C (41 F) at 700 m (2300 ft) depth, then a gradual decrease to 1.5 C (36 F) at the bottom.
Plate 6. Average vertical distribution of temperature, salinity, and nutrients (nitrate+nitrite) at Ocean Station Aloha. Period: 1988 to 1995. Source: World Ocean Circulation Experiment, Hawaii Ocean Time Series Project, University of Hawaii. Units: degrees Celsius, part-per-thousand of salt, and micromole/kg of nutrients).
The salinity distribution reflects the sinking of water from the north: higher salinity water of 35.2 ppt at 150 m (500 ft) depth, traceable to the high surface salinity water north of Hawai'i; low salinity water of 34.1 ppt at 500 m (1670 ft) depth, traceable to low surface salinity water further to the north west. Below this depth, salinity increases gradually to 34.7 ppt for abyssal waters.
Section of annual average salinity versus depth along 156 W. From NOAA World Ocean Atlas, 1895-1993. Units in parts per thousand.
The concentration of nutrients (nitrate NO3 and nitrite NO2) is small at the surface (less than 1 umole/kg), but increases steadily to the bottom, reaching 40 umole/kg or more; similar vertical distributions are found for phosphate and silicate. The distribution of nutrients illustrates how small vertical motions generally are in the ocean. The upper 100 m (330 ft) or so is illuminated by sunlight (the euphotic layer), but lack of nutrients limits growth of phytoplankton (microscopic plants), much like lack of fertilizers would limit growth of vegetation over land. Deeper in the water, nutrients are abundant, but there is no light to support photosynthesis.
Where and when upward vertical motions exist, nutrients are brought into the euphotic layer, resulting in increased biological productivity. Upward nutrient transport occurs when strong winds increase the turbulence in the mixed layer (for example, in winter, or during a storm or hurricane), or when surface currents diverge, causing upwelling of deeper water.
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