What is water temperature?
Water temperature is a measure of the heat (or kinetic energy) in the water. It is expressed as degrees Celsius (°C) or degrees Fahrenheit (°F). As oceans are warmed by the sun, seawater temperature varies with latitude and depth. Generally, seawater temperature is warmer near the equator than the poles, and warmer near the surface than down deep. However, movement by currents helps to regulate these temperature differences.
Gulf of Maine water temperature
The profiles below show how temperature can vary in the gulf over seasons. The warmest temperatures are at the surface during the summer and autumn. In the Gulf of Maine, summer means longer days with abundant solar energy to heat the surface. Summer winds are typically calm so very little mixing occurs to drive surface water deeper. In autumn, however, the days shorten and winds pick up, allowing the surface and deeper layers to mix. In spring, waters are cool and well mixed: this is the result of shorter days and strong, storm-driven winds
Note the relatively warm water deep in the gulf during spring. To discover its origin, compare Jordan Basin's location with the map of currents.)
NOTE: Data obtained during three cruises (1999-2000) and do not represent seasonal averages
Every hour, GoMOOS buoys measure water temperature (°C and °F) at several depths. These measurements are complemented by data from NOAA's satellite-based AVHRR (Advanced Very High Resolution Radiometer) which measures Gulf of Maine sea surface temperature several times per day.
The image at right shows an AVHRR sea surface temperature taken during winter. Blue shows cooler waters; warmer waters are indicated by orange and red. AVHRR cannot "see" through clouds, thus data images taken on cloudy days will have black areas. Sea surface temperature images are available by selecting AVHRR and entering the location and dates of interest at the Oceanography Lab at the University of Maine website.
Temperature plays a significant role in the movement of water. Why? Seawater density depends on temperature, salinity, and pressure. As water cools, its density increases. Thus cooler water will sink below warmer water (assuming salinities are equal). Ocean heating is primarily from the sun; thus in the absence of mixing, ocean temperature will decrease with depth. The thermocline is a region where temperature increases sharply with depth.
Water temperature is an important factor determining habitat ranges of many organisms. For example, some corals cannot survive in cool waters and are only found in the tropics and subtropics. Other organisms, like whales, migrate with changing water temperatures.
Beyond the Gulf: What are typical ocean temperatures?
Seawater surface temperatures can vary a lot: from less than 0°C (<32°F) to over 30°C (86°F). Can liquid seawater be below freezing? Yes, because dissoved salts lower the freezing point of water.
Deep ocean waters are fairly constant in temperature, ranging from about 0°C to 4°C (32°F to 39°F). However, deep-sea hydrothermal vents are a notable exception: water from these can be well over 100°C (212°F). Can deep ocean waters boil? No, because high pressure increases the boiling point of water.
Away from hydrothermal vents and well below the sun-warmed surface, deep ocean waters are quite frigid. These cold, dense waters tend to remain at depth unless driven upward by thermohaline circulation or wind-induced upwelling.
Why does sea surface temperature vary so very much?
The sun's energy penetrates the top of the ocean, heating these waters. In the tropics, the sun's rays hit the Earth directly and sea surface temperature (SST) is high. Away from the tropics, the sun's rays hit Earth's surface at an angle and spread over a relatively large area; this, coupled with seasonal variation in daylength at higher latitudes, is why SST generally decreases with latitude. Why don't SST patterns correlate exactly with latitude? The movement of surface water by ocean currents disrupts the expected latitudinal pattern. Fortunately, the global network of surface ocean currents helps to moderate Earth's climate.
- deCharon, A., and M. Sieracki. (1999). Ship Mates: Explore the Gulf of Maine as Oceanographers Do. URL: http://www.bigelow.org/shipmates.
- Lalli, C. M. and Parsons, T. R. (1993). Biological Oceanography - An Introduction. Butterworth-Heinemann.
- University of Maine. (2002). Satellite oceanography data laboratory. URL: http://wavy.umeoce.maine.edu/
