Intermediate waters of the North Pacific
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Deep to middle layer
The previous course explained the formation of bottom and deep water. Let's take a look at the vertical distribution of water temperature in the North Pacific Ocean (170ºE) (upper part of the figure below). The purple area (water temperature below 2°C) is the North Pacific deep water. In the North Pacific deep water (2500-4500m), water temperature and density are fairly uniform, with a temperature drop of less than 0.5°C even if the depth increases by 2000m. Above deep water (500-2500m), the water temperature drops by 5-10°C when the depth increases by 2000m.
The water temperature (density) changes rapidly in the layers above the deep water. A layer in which the water temperature (and density) changes suddenly is called a thermocline (or densocline). This cline can be seen semi-permanently (probably as long as the deep circulation continues), so it is also called the perpetual cline.
亜熱帯subtropical 亜寒帯subarctic 海面sea surface 低塩分水の貫入Intrusion of low-salinity water 永久躍層Perpetual Cline 深層水deep water 赤道equator 中層水intermediate water
Next, look at the vertical distribution of salinity at the same location (bottom of the above figure).From the surface of the subarctic zone, you can see how low-salinity water intrudes into the subtropical water depth of 300-1000m. This is called North Pacific Intermediate Water. The origin of the intermediate water is off the east coast of the Japanese archipelago, and the intermediate water spreads over the mid-latitude region of the North Pacific Ocean. This middle-layer water belongs to water with a water temperature of 10°C or less in the subtropical zone (yellowish green in the upper figure (top)) and a salinity of 33.8 or higher in the subarctic zone (light blue in the upper figure (bottom)).intermediate water
Briefly describe the formation of intermediate water. In the subarctic-subtropical transition zone (mixed zone), subarctic waters (low salinity) and subtropical waters (high salinity) compete with each other. In summer, surface water in the subarctic zone also warms up to nearly 20°C and becomes low density (approximately 23σ = 1.023 g cm-3).
In winter, the subarctic zone (north of latitude 40°N) experiences significant sea surface cooling, with surface water temperature below 5°C and high density (approximately 26.6-26.9σ = 1.0266～1.0269 g cm-3). On the other hand, in the subtropical surface, sea surface cooling is not so great even in winter, and the temperature remains above 15°C and the density remains low (approximately 25.5σ = 1.0255 g cm-3). Since the subarctic and subtropical waters are competing with each other in the transition zone, the subarctic water, which has become denser due to a decrease in temperature, sinks under the subtropical surface layer. Every winter, a large amount of subarctic water is pushed under the subtropical surface, forming the "North Pacific Intermediate Water" at a depth of around 300-700m. This middle-layer water is characterized by low salinity because it originated in subarctic water, and the density is 26.6 to 26.9 σ. Intermediate waters are generally found only in the low to middle latitudes of each ocean (figure below).
北太平洋中層水の分布Distribution of middle-level water in the North Pacific親潮Oyashio current 黒潮Kuroshio current 形成域formation area 北太平洋中層水North Pacific Intermediate Water-----------------------------------------------------------------------
Intermediate waters of the North Pacific (summary)
Surface water is cooled and densified in the high latitudes of the North Pacific in winter
Its dense water is less heavy than deep water from the North Atlantic.
It spreads in the middle layers of the North Pacific above deep water.
Distributed throughout the subtropics as "North Pacific Intermediate Water"
permanent densocline
Density increases rapidly at depths below the surface. It is called the (permanent) densocline.
Density does not change much at depth.
In contrast to the permanent densocline, the density distribution in the surface layer changes with the seasons, so it is sometimes called a "seasonal densocline".
*Is deep circulation really permanent? Let's summarize your thoughts on report assignment ①
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