Page 162 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

126 part I The energy–atmosphere System
▲Figure 5.8 The Gulf Stream. Satellite instruments sensitive to thermal infrared wavelengths imaged the gulf Stream. Temperature differences are noted by computer-enhanced false colours: reds and oranges = 25° to 29°C, yellows and greens = 17° to 24°C; blues = 10° to 16°C; purples = 2° to 9°C. [imagery by rSMaS, University of Miami.]
Ocean Currents and Sea-Surface Temperatures Al- though our full discussion of ocean circulation is in Chapter 6, we include a brief discussion of currents here because they influence temperature in coastal loca- tions. Ocean currents affect land temperatures in differ- ent ways, depending on whether the currents are warm or cold. Along midlatitude and subtropical west coasts of continents, cool ocean currents flowing toward the equator moderate air temperatures on land. An example is the effect of the cold Humboldt Current flowing off- shore from Lima, Peru, which has a cooler climate than might be expected at that latitude. When conditions in these regions are warm and moist, fog frequently forms in the chilled air over the cooler currents.
The warm current known as the Gulf Stream moves northward off the east coast of North America, carrying warm water far into the North Atlantic (Figure 5.8). As a result, the southern third of Iceland experiences much milder temperatures than would be expected for a lati- tude of 65° N, just south of the Arctic Circle (66.5° N). In Reykjavík, on the southwestern coast of Iceland, monthly temperatures average above freezing during all months of the year. The Gulf Stream also moderates temperatures in coastal Scandinavia and northwestern Europe. In the western Pacific Ocean, the warm Kuroshio, or Japan Cur- rent, functions much the same as the Gulf Stream, having a warming effect on temperatures in Japan, the Aleutians, and along the northwestern margin of North America.
Across the globe, ocean water is rarely found warmer than 31°C, although in 2005, Hurricanes Katrina, Rita, and Wilma intensified as they moved over 33°C sea-surface
temperatures in the Gulf of Mexico. Higher ocean temper- atures produce higher evaporation rates, and more energy is dissipated from the ocean as latent heat. As the water vapour content of the overlying air increases, the ability of that air to absorb longwave radiation also increases, leading to warming. The warmer the air and the ocean become, the more evaporation that occurs, increasing the amount of water vapour entering the air. More water va- pour leads to cloud formation, which reflects insolation and produces lower temperatures. Lower temperatures of air and ocean reduce evaporation rates and the ability of the air mass to absorb water vapour—an interesting nega- tive feedback mechanism.
Ocean temperatures are typically measured at the surface and recorded as the sea-surface temperature, or SST. Maps of global average SSTs measured from satel- lites, such as those in Figure 5.9, reveal that the region with the highest average ocean temperatures in the world is the Western Pacific Warm Pool in the southwestern Pacific Ocean, where temperatures are often above 30°C. Although the difference in SSTs between the equator and the poles is apparent on both maps, note the seasonal changes in ocean temperatures, such as the northward shifting of the Western Pacific Warm Pool in July. The warm Gulf Stream is apparent off the coast of Florida in both images; cooler currents occur off the west coasts of North and South America, Europe, and Africa.
Following the same recent trends as global air tem- peratures, average annual SSTs increased steadily from 1982 through 2010 to record-high levels—2010 breaking records for both ocean and land temperatures. Increasing warmth is measured at depths to 1000 m, and in 2004, scientists reported slight increases even in the tempera- ture of deep bottom water. These data suggest that the ocean’s ability to absorb excess heat energy from the at- mosphere may be nearing its capacity.
Examples of Marine Effects and Continental Effects
The land–water heating differences that affect temper- ature regimes worldwide can be summarized in terms of continental and marine effects. The marine effect, or maritime effect, refers to the moderating influences of the ocean and usually occurs in locations along coastlines or on islands. The continental effect, or con- dition of continentality, refers to the greater range be- tween maximum and minimum temperatures on both a daily and a yearly basis that occurs in areas that are inland from the ocean or distant from other large water bodies.
Vancouver, British Columbia, and Winnipeg, Manitoba, exemplify marine and continental conditions (Figure 5.10). Both cities have similar latitudes, about 49° N and 50° N, respectively. However, Vancouver has a more moderate pattern of average maximum and minimum temperatures. Vancouver’s annual range of 16.0 C° is far less than Win- nipeg’s 38.0 C° range. In fact, Winnipeg’s continental tem- perature pattern is more extreme in every aspect than that of maritime Vancouver.