The Oceans Role in Climate Martin H. Visbeck A Numerical Portrait of the Oceans The oceans of the world cover nearly seventy percent of its surface. The largest is the Pacific, which contains fifty percent of the volume of all the oceans combined, followed by the Atlantic and Southern Oceans. The total mass of the ocean is small compared to that of the solid Earth (10,000 times less) but large when compared to that of the atmosphere (300 times larger). A typical ocean depth is 3,000 meters, which is about 0.05 percent of the Earth s radius, so the ocean is a shallow Martin H. Visbeck is an Associate Professor in the Department of Earth and Environmental Science at Columbia University s Lamont-Doherty Earth Observatory.
layer of fluid covering much of the Earth s crust. The oceans account for ninety-seven percent percent of the total water on Earth. Seawater itself is a mixture of ninety-six percent fresh water and 3.5 percent dissolved salts. Its temperatures range from the freezing point of ocean salt water (-1.8 C) in the polar oceans to a maximum of 34 C in the tropics, with an overall average temperature of 3.8 C. About five percent of the ocean s surface is covered by sea ice. The viscosity of the oceans, or their resistance to flow, is much lower than that of the solid Earth, but significantly higher than that of the atmosphere. This means that ocean currents travel several orders of magnitude faster than the solid Earth, but considerably slower than the atmosphere. Ocean currents travel at speeds of up to three meters per second, while winds move at least ten times faster and the solid Earth about a trillion times slower. Ocean Dynamics Ocean currents can be driven by the wind The wind blowing over the surface of the water drives the ocean s major surface currents. These winds in turn are driven by atmospheric circulation, generated by unequal temperatures in the atmosphere. The main features of this wind-driven surface circulation are large, roughly circular current systems, called gyres. Gyres are found in most major ocean basins (Figure 1). Driven by the prevailing wind systems and deflected by continental boundaries and the Coriolis force resulting from the Earth s rotation, gyres help redistribute heat from the low latitudes to the polar regions. Along the western margins of the ocean basins, warm ocean currents like the Gulf Stream transport heat towards the poles. Along the eastern margins, currents such as the California Current transport cold water to the lower latitudes. In the landlocked higher latitudes of the Northern Hemisphere, the prevailing winds drive smaller gyres that effectively redistribute heat to the polar regions. In the Southern Figure 1: Ocean Surface Currents. Surface currents are driven by the winds, and in turn influence atmospheric circulation. Surface currents form huge gyres black arrows are warm currents and blue arrows are cold currents.
EARTH: INSIDE AND OUT Hemisphere, where no continents block ocean movement, strong westerly winds drive the largest flows of ocean water in the world around Antarctica. These flows move 180 million cubic meters of water per second. (In comparison, the Amazon River moves one million cubic meters per second.) Variations in water density also drive ocean currents The ocean is a layered system. Warm water lies close to its surface with cold water below. Winds, waves, and currents stir the ocean surface to form a mixed-layer a few tens of meters deep. In the low- and mid-latitudes, other layers lie below this warm mixed-layer. These include: intermediate water immediately below the mixed-layer, deep water, which extends from below the intermediate water to near the ocean bottom, and bottom water, which is in contact with the seafloor. Surface seawater becomes denser as it cools or becomes saltier due to evaporation. It becomes lighter as it warms by heating due to the Sun or its salinity decreases due to the addition of fresh water from river outflows and rain. These changes in the density of seawater, even when small, drive the layered circulation of the ocean. There are only a few places on Earth where surface seawater becomes dense enough to sink to great ocean depths. This sinking water draws up surface waters from lower latitudes to replace it, and thereby generates movement on a large scale. In the Northern Hemisphere, the locations where surface water sinks are at the centers of the Greenland and Labrador Seas. In the Southern Hemisphere, surface water sinks to deep ocean depths along the shallow ocean margins around Antarctica, where dense water is produced by an intricate process involving heat loss to the atmosphere and interactions with sea ice. Since the density variations that drive the deep circulation are due to differences in temperature (thermal) and salinity (haline), the density-driven ocean circulation is also referred to as thermohaline circulation. The resulting largescale ocean circulation plays a fundamental role in the Earth s overall climate. Changes in ocean circulation can result in dramatic regional and global climate change. How the Oceans Help Regulate Earth s Climate The oceans modulate the planet s heat distribution The Sun is the ultimate source of the energy that brings about atmospheric and oceanic circulation. Because of astronomical and atmospheric factors, tropical regions receive more of the Sun s energy over the course of a year than do regions at higher latitudes. However, latitude has much less of an effect on outgoing radiation, which remains relatively constant. This creates an imbalance at different latitudes between gains and losses of energy from the Sun. This radiative imbalance is balanced by heat flows from the equator towards the poles with the atmosphere and oceans. This heat transport is accomplished in about equal parts by the circulation of the atmosphere and that of the ocean. However, the transport within the ocean basins is not evenly split. Most heat is transported by the Atlantic Ocean. The oceans modulate Earth s distribution of fresh water The strong heating of the subtropical and tropical oceans causes evaporation at the surface. This moist air moves towards the equator, where it rises to the top of the lower atmosphere (to a height of about ten kilometers) and most of its water rains out. Strong rainfall reduces the salinity of the surface water. The generally poleward ocean
THE OCEANS ROLE IN CLIMATE downwelling upwelling Deep currents carry cold, salty water from the Atlantic to the Pacific. Fresh water is carried eastward by rain clouds. Water takes thousands of years to circulate through the oceans. circulation transports the fresh water back to the subtropical latitudes where it can evaporate again. A similar hydrological cycle exists between the subtropical and middle latitudes. More precipitation occurs over the middle latitudes and fresh water is returned to the subtropics by oceanic flow. How the oceans affect climate variability Changes in the climate have different durations. Strong climate variations occur every season. Subtler but still noticeable changes occur on an interannual scale; some last from a few decades to a century or more. Dramatic changes are linked to ice ages, which occur over time scales greater than 10,000 years. The oceans affect all of these patterns of variability. Figure 2: Water warmed at the equator by the Sun flows into the North Atlantic, where it is cooled and becomes more salty because of evaporation. This cold, salty water sinks to the seafloor and forms a huge undersea river. The deep water flows through the oceans, welling up where the winds push away warm surface water. This transfer of salty water is balanced by fresh water evaporated from the Atlantic and carried to the Pacific by the atmosphere. There, it falls as rain, diluting the upwelling salty water with fresh water. The seasonal cycle The effect of the oceans in moderating seasonal climate is the most familiar to us. The enormous heat capacity of the oceans and of large lakes, relative to that of the land, buffers seasonal changes in solar radiation. During winter, the ocean warms the lower atmosphere above it, which results in milder temperatures along the coasts and downwind of the major
EARTH: INSIDE AND OUT oceans (along the west coasts of the United States and Europe, for example). In the summer, however, strong radiation heats the ocean more slowly than the land surface, so the atmosphere over the ocean remains cool. Climatologists note this difference by referring to maritime and continental climates. These seasonal differences between air temperatures over land and water drive changes in large-scale atmospheric circulation. The most dramatic effects occur between the Indian Ocean and the Asian continent when winds called monsoons reverse seasonally. In the summer, air rises over the warm continent, which causes cooler, moist air masses to flow in from the Indian Ocean. As this air approaches the Himalayan mountains it rises, and it begins to rain over India. A similar phenomenon affects the southern United States when moist summer winds from the Pacific Ocean and Gulf of Mexico bring rain to the central U.S. Variability on a scale of one to ten years Not every winter is like the previous one; some are mild and some are harsh. Scientists refer to this as interannual climate variability. In the last century, climatologists have discovered several interannual climate phenomena, most of which are coupled to changes in the way the oceans store or transport heat. The prime example of this interannual climate variability is called ENSO, which stands for El Niño Southern Oscillation. In the tropical Pacific Ocean, warm temperatures near Indonesia cause the air to rise to great height. This is called atmospheric deep convection. At the same time, ocean currents bring cooler waters toward the equator along the coast of South and North America, thereby lowering the surface temperature. Consequently the atmospheric temperatures drop, and a large atmospheric circulation cell develops. Warm air rises in the western tropical Pacific, near Indonesia, and sinks in the eastern Pacific, off the coast of Central America. Associated surface winds blow water from east to west, generating a large pool of warm water in the western tropical Pacific, between Australia and Japan. Sometimes, for reasons that are not yet entirely clear, the winds relax. If this happens, some of the warm water flows back eastward, carrying heavy rainfall with it. This reduces the temperature difference between the western and the eastern Pacific, so the wind relaxes even more. The warm water then flows all the way to the eastern Pacific, which suppresses the cold ocean currents along the coasts of the Americas. This phenomenon occurs to some extent every year around Christmas time and was named El Niño (after the Christ Child) by fishermen in South America who were not able to catch fish at that time, which normally thrive in the cold currents. However, every three to seven years, the changes in the tropical temperatures are large enough to cause shifts in the atmospheric circulation and rainfall patterns over much of the globe. The term El Niño has come to be reserved for these large-scale events. Once the warm water has cooled, the westward wind starts to develop again, and the climate system returns to its normal state. Sometimes an El Niño event is followed by the movement of colder-thannormal surface waters across the Pacific towards Indonesia. This cold phase is called La Niña. Changes in the tropical sea surface temperatures also change the north-south, or meridional, direction of atmospheric circulation. During El Niño, an area of the tropical Pacific much larger than normal is covered with warm water. This enhances atmospheric winds and the transport of heat towards the poles. The ocean outside of the tropics responds to those
THE OCEANS ROLE IN CLIMATE changes in the wind, and the ocean transport of heat also increases. However, the slow response time of the large oceans (remember, ocean currents travel at least ten times slower than atmosphere currents) can preserve the signal the memory of unusual temperature patterns for several years. This results in longer-term, or decadal, variability in sea surface temperatures in the North Pacific and North Atlantic Oceans. Some scientists think that these changes in sea surface temperatures outside of the tropics also affect the strength and positions of winter storm tracks. Climatologists call these decadal modes of variability the Pacific Decadal Oscillation and the North Atlantic Oscillation. The North Atlantic Oscillation was discovered during the last century. Seamen traveling across the Atlantic Ocean noted that the winters tended to be mild in Greenland when they were harsh in Denmark, and vice versa. Today, we have a great deal more data to investigate such climate phenomena on a global scale. (To learn more about El Niño and predicting such phenomena, read Dr. Cane s essay in this section) Centennial variability Climate also varies over longer periods of time. About 200 400 years ago, glaciers advanced in Europe and northeastern America, a development referred to as the Little Ice Age. The extremely slow ocean currents in the deep ocean are thought to be connected to such centennial climate variability. It takes about 1,000 years for a water molecule to travel from the North Atlantic Ocean to the deep Pacific Ocean. There it might well up into the surface layer, and then return via warm currents through Indonesia, around Africa, and back to the northern North Atlantic. The lack of accurate ocean data for periods preceding the twentieth century makes it very hard to study such phenomena in great detail. Most of what we know comes from several sources: changes in the thickness and chemical composition of layers deposited annually on large ice sheets and recorded in ice core samples, changes in the thickness and density of annual tree rings, and changes in sediment deposits in lakes or in the ocean. (Paul A. Mayewski s essay in this section describes how ice core samples record past climate.) However, these measured parameters reflect responses to changes in sea surface temperature, precipitation or winds, as opposed to the actual changes, so they must be interpreted with great care. Ice Age variability Climate change also occurs on time scales of periods 10,000 years or longer, referred to as glacial periods. Twenty thousand years ago, at the peak of the last big ice age, the oceans were dramatically different. For example, the sea level during this time was 150 300 meters below its current level, which allowed people to walk from Asia to America over the Aleutian Island chain. The Mediterranean Sea was a lake. Atmospheric and oceanic circulations were probably quite different, too. Enormous masses of water were accumulating on the land in the form of large ice shields, which might have increased the frequency and severity of storms. Scientists are just beginning to understand these past events. They discovered that just as the last ice age had terminated, a brief return back to glacial conditions occurred. This rapid change back to cold conditions (called the Younger Dryas; see Paul A. Mayewski s essay in this section) occurred within a few decades. It demonstrates that the climate is dynamic and can respond quickly to slow changes in the intensity of the Sun s radiation or other climatechanging influences.