Page 204 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

168 part I The energy–atmosphere System
Animation
Ocean Circulation
Greenland Sea
EUROPE AFRICA
ANTARCTICA
ASIA
AUST.
N.A.
Labrador Sea
N.A. S.A.
Warm shallow current
Cold and salty deep current
Cool water subsides to feed deep current
the Indonesian archipelago receive more than 254 cm under dominant low pres­ sure. This normal alignment of pressure is shown in Figure 6.21a.
El Niño—ENSo’s Warm Phase Occasionally, for un­ explained reasons, pres­ sure patterns and surface ocean temperatures shift from their usual locations
▲Figure 6.20 Deep-ocean thermohaline circulation. This vast conveyor belt of water draws heat energy from warm, shallow currents and transports it to higher latitudes for release in the depths of the ocean basins in cold, deep, salty currents. Four blue areas at high latitudes are where surface water cools, sinks, and feeds the deep circulation.
Natural Oscillations in Global Circulation
in the Pacific. Higher pres­ sure than normal develops over the western Pacific, and lower pressure de­ velops over the eastern Pacific. Trade winds normally moving from east to west weaken and can be reduced or even replaced by an eastward (west­to­east) flow. The shifting of atmospheric pressure and wind patterns
across the Pacific is the Southern Oscillation. Sea­surface temperatures may increase to more than 8 C° above normal in the central and eastern Pacific dur­ ing an ENSO, replacing the normally cold, nutrient­rich water along Peru’s coastline. Such ocean­surface warm­ ing, creating the “warm pool,” may extend to the Inter­ national Date Line. This surface pool of warm water is the El Niño (Figure 6.21b), leading to the designation ENSO—El Niño–Southern Oscillation. During El Niño conditions, the thermocline (the transition layer between surface water and colder deep­water beneath) lowers in depth in the eastern Pacific Ocean, blocking upwelling. The change in wind direction and the warmer surface water slow the normal upwelling currents that control nutrient availability off the South American coast. This loss of nutrients affects the phytoplankton and food chain, depriving fish, marine mammals, and predatory
birds of nourishment.
The expected interval for ENSO recurrence is 3 to
5 years, but the interval may range from 2 to 12 years. The frequency and intensity of ENSO events increased through the 20th century, a topic of extensive scientific research looking for a link to global climate change. Although re­ cent studies suggest that this phenomenon might be more responsive to global change than previously thought, sci­ entists have found no definitive connection.
The two strongest ENSO events in 120 years occurred in 1982–1983 and 1997–1998. The latest El Niño subsided in May 2010 (Figure 6.21d). Although the pattern began to build again in late summer 2012, it resulted in a weak El Niño that ended in early 2013.
La Niña—ENSo’s Cool Phase When surface waters in the central and eastern Pacific cool to below normal by 0.4 C° or more, the condition is dubbed La Niña, Spanish for “the girl.” This condition is weaker and less consis­ tent than El Niño; otherwise, there is no correlation in strength or weakness between the two phases. For instance, following the record 1997–1998 ENSO event,
Several system fluctuations that occur in multiyear or shorter periods are important in the global circulation picture. Multiyear oscillations affect temperatures and air pressure patterns and thus affect global winds and cli­ mates. The most famous of these is the El Niño–Southern Oscillation (ENSO) phenomenon, which affects tempera­ ture and precipitation on a global scale. Here we describe ENSO and briefly introduce three other hemisphere­scale oscillations.
El Niño–Southern oscillation
Climate is the consistent behaviour of weather over time, but normal weather conditions can include extremes that depart from the average in a given region. The El Niño– Southern Oscillation (ENSO) in the Pacific Ocean forces the greatest interannual variability of temperature and precipitation on a global scale. Peruvians coined the name El Niño (“the boy child”) because these episodes seem to occur around the time of the traditional Decem­ ber celebration of Christ’s birth. Actually, El Niños can occur as early as spring and summer and persist through the year.
The cold Peru Current (also known as the Hum­ boldt Current) flows northward off South Amer­ ica’s west coast, joining the westward movement of the South Equatorial Current near the equator (Figure 6.18). The Peru Current is part of the normal counterclockwise circulation of winds and surface ocean currents around the subtropical high­pressure cell dominating the eastern Pacific in the Southern Hemisphere. As a result, a location such as Guayaquil, Ecuador, normally receives 91.4 cm of precipitation each year under dominant high pressure, whereas islands in