Chapter 6 - Temperature, Salinity, and Density

Chapter 6 Contents

6.3 Geographical Distribution of Surface Temperature and Salinity

The distribution of temperature at the sea surface tends to be zonal, that is it tends to be independent of longitude (Figure 6.2). Warmest water is near the equator, coldest water is near the poles. The deviations from zonal are small. Equatorward of 40°, cooler waters tend to be on the eastern side of the basin. North of this latitude, cooler waters tend to be on the western side.

Sea Surface Temperature for July

Sea Surface Temperature for January

Figure 6.2 Mean sea-surface temperature calculated from the optimal interpolation technique (Reynolds and Smith, 1995) using ship reports and AVHRR measurements of temperature.

The anomalies of sea-surface temperature, the deviation from a long term average, are small, less than 1.5°C except in the equatorial Pacific where the deviations can be 3°C (Figure 6.3: top).

The annual range of surface temperature is highest at mid-latitudes, especially on the western side of the ocean (Figure 6.3: bottom). In the west, cold air blows off the continents in winter and cools the ocean. The cooling dominates the heat budget. In the tropics the temperature range is mostly less than 2°C.

Figure 6.3
Top:
Sea-surface temperature anomaly for January 2005 relative to mean temperature shown in Figure 6.2 using data published by Reynolds and Smith (1995) in the Climate Diagnostics Bulletin for February 1995.

Bottom:
Annual range of sea-surface temperature in °C calculated from the Reynolds and Smith (1995) mean sea-surface temperature data set. Contour interval is 1°C with heavy contours at 4°C and 8°C. Shaded areas exceed 8°C.

The distribution of sea-surface salinity also tends to be zonal. The saltiest waters are at mid-latitudes where evaporation is high. Less salty waters are near the equator where rain freshens the surface water, and at high latitudes where melted sea ice freshens the surface waters (Figure 6.4). The zonal (east-west) average of salinity shows a close correlation between salinity and evaporation minus precipitation plus river input (Figure 6.5).

 Figure 6.4 Mean sea-surface salinity. Contour interval is 0.25. Shaded areas exceed 36. From Levitus (1982).

Figure 6.5 Zonal average of sea-surface salinity calculated for all oceans from Levitus (1982) and the difference between evaporation and precipitation (E - P) calculated using global rainfall data from the Global Precipitation Climatology Projects and latent heat flux data from the Data Assimilation Office, both at NASA's Goddard Space Flight Center.

Because many large rivers drain into the Atlantic and the Arctic Sea, why is the Atlantic saltier than the Pacific? Broecker (1997) showed that 0.32 Sv of the water evaporated from the Atlantic does not fall as rain on land. Instead, it is carried by winds into the Pacific (Figure 6.6). Broecker points out that the quantity is small, equivalent to a little more than the flow in the Amazon River, but "were this flux not compensated by an exchange of more salty Atlantic waters for less salty Pacific waters, the salinity of the entire Atlantic would rise about 1 gram per liter per millennium."

Figure 6.6 Water transported by the atmosphere into and out of the Atlantic. Basins draining into the Atlantic are black, deserts are white, and other drainage basins are shaded. Arrows give direction of water transport by the atmosphere, and values are in Sverdrups. Bold numbers give the net transport for the Atlantic. Overall, the Atlantic loses 0.32Sv, an amount approximately equal to the flow in the Amazon River. From Broecker (1997).

Mean Temperature and Salinity of the Ocean The mean temperature of the ocean's waters is: t = 3.5°C; and the mean salinity is S = 34.7. The distribution about the mean is small: 50% of the water is in the range:

1.3°C <t< 3.8°C
34.6 < S < 34.8

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