The term sea level is used to describe the average position of the ocean surface. Sea level is measured in two ways: relative and absolute. Relative sea level change is a measure of how the ocean height has changed in comparison to a point on land. Absolute sea level change is a measure independent of any points on land. Changes in sea level can be measured in a number of ways. Sea level changes can be measured from land using fixed-point tidal gauges and at sea using gauges anchored to the sea floor. Sea level changes can also be measured remotely using satellites orbiting the earth.
Because of the effects of wind, currents, and underwater features, the surface of the ocean is not flat—maps show differences in elevation on the surface of the sea (Fig. 3.20). Winds cause water to pile up in some places and form low places in others, which in turn affects water movement. For example, the Atlantic ocean basin has a lower average sea level than the Pacific ocean basin because water flows out of the Atlantic ocean basin faster than it is replenished. Another cause of sea level differences is the effects of gyres, which pile water in the center of the their circulation; thus, the sea level tends to be higher in the center of ocean basins than at the edges of the basins.
Fig. 3.20. Differences in sea level height across the world ocean can be measured using NASA satellites. The red colors are areas of higher sea level, the dark blue areas indicate the lowest sea level height.
Image courtesy of National Aeronautics and Space Administration (NASA)
As water travels in a circular motion in the large currents that comprise gyres, Coriolis effects and Ekman transport direct water to the center of the gyre. When this happens, water piles up above normal sea level. Thus, sea level tends to be higher in the center of ocean basins.
Fig. 3.22. Coriolis forces, the rotational velocity of Earth, and gravity are responsible for generating geostrophic flow.
Image by Bryon Inouye
If the bulge in the middle of gyres was not rotating, the water would radiate back towards low points due to gravity. However, since the gyre is spinning, these gravity-induced currents are also subject to Coriolis effects and Ekman transport. The result is that the gravity-induced currents are turned such that they flow in concentric circles around the center of the gyre. This phenomenon is known as geostrophic flow (Fig. 3.22). Geostrophic flow is the balance between Coriolis effects causing currents to form a bulge of water in the center of ocean gyres and gravity currents also affected by the Coriolis effect. The geostropic flow is shown as concentric rings around the bulge in Fig. 3.22.
In many parts of the world winds blow predominately one way along the coast. This wind pattern results in water currents moving out to sea away from shore via Ekman transport (Fig. 3.23 A). For example, along the coast of California, the winds usually blow towards the south. Ekman transport in the Northern Hemisphere is 90° to the right of the direction of the wind, therefore the surface waters are transported west away from the coastline, allowing deeper water to move up to replace it. Deeper water tends to be higher in oxygen than the warmer surface water because gasses such as oxygen dissolve better in colder water. It is also higher in nutrients because the higher oxygen levels allow bacteria to cycle important nutrients such as nitrate and phosphate from decomposing organic matter. As the surface water moves to the west it is replaced by cold, nutrient-rich waters in a process called upwelling.
Fig. 3.23. The processes of upwelling. The combination of prevailing winds and Ekman transport moves water away from an area. Cool, nutrient-rich bottom water replaces it. (A) coastal upwelling and (B) equatorial upwelling in the Northern Hemisphere.
Image by Byron Inouye
Areas of coastal upwelling tend to be some of the most biologically productive areas in the ocean. Coastal upwelling is largely responsible for the lush kelp forests and productive fisheries found off the California coast (Fig. 3.24). There are areas of upwelling off the eastern and western shores of all continents.
Fig. 3.24. (A) A sea surface temperature map of the ocean showing upwelling off the coast of California. Red indicates areas of warmer surface water. Blue indicates areas of cooler surface water.
Image courtesy of National Oceanic and Atmospheric Administration (NOAA)
Fig. 3.24. (B) Kelp forest, Monterey, California
Image courtesy of Daderot, Wikimedia Commons
A coastal downwelling is the opposite of a coastal upwelling. Warm surface water can be blown towards the shore by Ekman transport. This warm water mass pushes cold water down and away from the shore. Another kind of downwelling occurs when there is a buildup of the water in the center of a hurricane or in other low-pressure centers.
The term Langmuir circulation describes shallow water currents that rotate near the ocean surface and align with the direction of steady wind patterns (Fig. 3.25 A). In Fig. 3.25 A, blue arrows show the direction of wind flow that matches the direction of the current. This acceleration of the current caused by wind over the water surface creates vortices and mixing of water surrounding these areas (blue circles in Fig. 3.25 A). The vortices on either side of the wind flow rotate away from the center of the wind flow and are shown by the short gray arrows. This type of water circulation can be seen as long streaks or stripes in the water, sometimes with floating debris collecting between adjacent Langmuir currents (Fig. 3.25 B). Wind-driven Langmuir circulation plays an important roll in mixing water near the ocean surface.
Fig. 3.25. (A) Water movement in Langmuir circulation
Image courtesy of BannSaenger, Wikimedia Commons
Fig. 3.25. (B) Example of Langmuir currents, Rodeo Lagoon, California
Image courtesy of Mmelugin, Wikimedia Commons
Fig. 3.26. Basin-wide tilt on a cross section of the Pacific ocean basin at the equator. Sea level is higher, and the thermocline deeper, at the western end of the ocean basin. Note that the diagram is exaggerated and not drawn to scale.
Image by Byron Inouye
Trade winds produce wind-generated currents in the tropics that travel from the eastern part of the Pacific and Atlantic ocean basins to the western part of the ocean basins. These currents are called the Northern and Southern equatorial currents. The trade winds and equatorial currents pile up warm surface water in the western regions of basins. For example, the sea surface height at the western end of the Pacific is approximately 40 cm higher than at the eastern end. This is similar to positioning a fan or hairdryer on one end of a tub filled with water. When the fan is turned on, the water level on the opposite side of the tub from the fan is much higher than near the fan. This phenomenon is known as the basin-wide tilt (Fig. 3.26). In addition to an increase in sea surface height, the thermocline (transition between warm surface water and cooler deep water) is also affected. The warm, well-mixed layer of surface water is thicker in the western part than in the eastern part of ocean basins.
We have been discussing the global prevailing wind and ocean current patterns as they would appear in an ideal world (Fig. 3.13 and Fig. 3.14). But in the real world, these patterns are not always consistent. For example, in a typical year strong trade winds push warm surface waters away from the coast of Peru near the equator, resulting in upwelling of cold, nutrient-rich waters. When the trade winds are reduced, the basin-wide tilt in the Pacific ocean basin is reduced, and warmer ocean water from the western Pacific flows back towards the typically colder eastern Pacific. This phenomenon is known as El Niño. In El Niño years, the surface water in the Pacific ocean basin near Central and South America is abnormally warm. El Niño events have been reported since the 1600s and affect global climate patterns (Fig. 3.27 A).
Fig. 3.13. Oceanic high-pressure centers and their simplified wind patterns. Individual surface currents are identified in Table 3.1.
Image by Byron Inouye
Fig. 3.14. Major surface currents of the world ocean. Individual surface currents are identified in Table 3.1.
Image by Byron Inouye
Fig. 3.27. Global maps of the Pacific ocean basin show patterns of sea surface temperature during (A) El Niño and (B) La Niña events. Sea surface temperature is presented as compared to long-term average values in those locations. The red- and blue-colored streaks along the equator illustrate hotter-than-average and colder-than-average sea surface temperatures associated with El Niño and La Niña events, respectively.
Image courtesy of National Oceanic and Atmospheric Administration (NOAA)
El Niño was originally named for a warm current that appeared periodically near Peru in summer, from December to February. El Niño, meaning “the little boy” in Spanish, refers to the birth of Jesus celebrated at Christmas, which in Peru is at the beginning of summer. In recent years, long El Niño events—lasting from nine months to two years—have become more frequent. El Niño events also occur in cycles, reappearing approximately every two to seven years.
During El Niño events, warm water also moves along the western boundaries of North America against the California Current, and South America against the Peru Current. This current movement interferes with coastal upwelling as warm water replaces cold water, replacing normally nutrient rich water with nutrient-poor surface waters near the coast.
La Niña refers a natural phenomenon of colder than usual sea surface temperatures across the Pacific ocean basin. This is largely due to an increase in trade wind strength that pushes warm surface waters further offshore than normal from Central and South America westward (Fig. 3.27 B). These stronger trade winds increase the degree of basin-wide tilt in the Pacific. The name of La Niña (meaning “the little girl”) refers to how this phenomenon is the strong reversal of the effects of an El Niño event.
The term El Niño Southern Oscillation (ENSO) refers to the combined cycle of warmer El Niño periods and colder La Niña periods with normal “neutral” periods in between. More than half of all years observed by climate scientists are typical neutral periods.
El Niño events can have profound impacts. Any decrease in nutrients affects the entire food chain and typically reduces the abundance of commercially important fish. Economic losses can be catastrophic. For example, the El Niño event of 1997 caused over 20 billion U.S. dollars of lost revenue to the Peruvian economy. El Niño events also influence climate. In Peru El Niño creates large masses of warm, humid, low-pressure air due to the presence of warm ocean water off the coast. This warms the land and disturbs air circulation, and can lead to increased chances of torrential rains and flooding. The opposite is true on the eastern edge of the Pacific ocean basin. In areas like Indonesia, El Niño reduces the amount of humid ocean air and causes droughts.
Coral reefs are particularly vulnerable to the warmer waters caused by El Niño events. The 1997–98 El Niño warming raised seawater temperatures to such a level that coral bleaching occurred, resulting in mass mortality. Coral bleaching is caused when the coral animal expels its algal symbionts, plant cells that live within the coral tissue, as a result of stress. The algal symbionts produce up to 90 percent of a coral’s food through photosynthesis. If the water temperature returns to normal levels within a few weeks, the symbionts return and the coral can recover. If the water temperatures remain elevated for more than eight weeks, death of the coral animal can occur. An El Niño coral bleaching event in 2014–15 was responsible for bleaching and coral death across the Pacific ocean, including reefs in Hawaii (Fig. 3.28).
Fig. 3.28. (A) A healthy Porites lobata coral colony at Wai‘ōpa‘e on the Island of Hawai‘i during a period of normal seawater temperature
Image courtesy of Narrissa Spies, Wikimedia Commons
Fig. 3.28. (B) A bleached P. lobata coral colony at Olowalu on the island of Maui during an El Niño bleaching event in 2015.
Image courtesy of Francois Seneca, Wikimedia Commons
Both El Niño and La Niña can influence weather as far away as Australia, the East Indies, Japan, North America, and Africa because the global wind and water current systems are connected. Disruptions in one area affect weather throughout the world. For example, during the 1997 El Niño, there were devastating droughts in Australia and Africa. Significant El Niño events occur approximately every five years, although this pattern can be irregular and ranges from two to seven years.
