Under the sea
When we think about the factors that influence climate, we often focus on the atmosphere, including temperature, precipitation, winds, and weather patterns. However, one of the most important drivers of Earth's climate is found beneath the surface of the oceans. Covering approximately 71 percent of the planet, the oceans store enormous amounts of heat and continuously redistribute that energy around the globe.
Ocean currents transport warm water from tropical regions toward higher latitudes and return colder water toward the equator. These movements influence regional temperatures, atmospheric circulation, precipitation patterns, and even the development of major weather systems. Understanding how the oceans circulate is therefore essential to understanding why different regions of the world experience different climates.
The Oceans as a Global Heat Transport System
The tropics receive substantially more solar energy than the polar regions. This unequal heating creates a temperature imbalance that both the atmosphere and oceans work to reduce. Ocean currents play an important role in this redistribution of energy. Warm currents transport heat away from the tropics, while cold currents carry cooler water toward lower latitudes. One of the most familiar examples is the Gulf Stream, which carries warm water northward along the eastern United States before continuing into the North Atlantic. The Gulf Stream and its extension into the North Atlantic transport considerable amounts of heat, influencing sea surface temperatures and the exchange of energy between the ocean and atmosphere.
This heat exchange contributes to the relatively mild maritime climate of western Europe, although atmospheric circulation also plays a major role in determining European temperatures. Cold ocean currents have different effects. Along the western United States, the California Current transports relatively cool water southward, helping maintain cooler coastal temperatures. Similarly, the Peru Current along the western coast of South America contributes to cool sea surface temperatures and stable atmospheric conditions that help support the extremely dry climate of the Atacama Desert. These examples demonstrate how ocean circulation can create substantial climate differences between regions located at similar latitudes.
The oceans have generally warmed at a brisk rate over the last 30 years, with the most concentrated warming occurring in the North Pacific. The cooler trend in the eastern equatorial Pacific is partly due to the increased frequency of La Nina events during the analysis period. Another area of interest is the colder water to the southeast of Greenland to and the south of Iceland. More on that in a little bit!
Figure 1. Sea surface temperature trend
What Causes Ocean Currents?
Ocean circulation is driven by several interacting physical processes. Near the ocean surface, prevailing winds provide much of the energy responsible for moving water horizontally. Earth's rotation, through the Coriolis effect, influences the direction of these currents and contributes to the development of large circulating systems known as ocean gyres. Note that the warm currents in the oceans tend to be on poleward on the return flow from high pressure. For example, the Gulf Stream's north-northeast movement has a relationship to the sub-tropical Bermuda High.
However, not all ocean circulation is driven directly by the wind. Differences in seawater density also play an important role, particularly in the deeper ocean. Seawater density is primarily influenced by two properties: temperature and salinity. Water with higher salt concentrations is denser than less saline water at the same temperature. Cold seawater is generally denser than warm seawater. These density differences help determine whether water remains near the surface or sinks into deeper layers.
Figure 2. Average ocean salinity in g/kg.
Salinity itself is influenced by the balance between evaporation, precipitation, freshwater runoff, and the formation and melting of sea ice. Evaporation removes freshwater while leaving most dissolved salts behind, increasing salinity. Precipitation and river runoff introduce freshwater, reducing salinity. When sea ice forms, much of the salt is excluded from the ice, potentially increasing the salinity of surrounding water. The resulting differences in temperature and salinity create important variations in seawater density across the global oceans.
Thermohaline Circulation: Connecting the World's Oceans
One of the most fascinating aspects of ocean circulation is the movement of water between the surface and the deep ocean. This density-related circulation is commonly called thermohaline circulation, combining the words associated with temperature (thermo) and salinity (haline).
Figure 3. The thermohaline circulation
In certain high-latitude regions, ocean water becomes sufficiently cold and dense to sink. Important regions of deep-water formation include the North Atlantic near Greenland and Labrador, along with areas surrounding Antarctica. Once water sinks, it becomes part of the deep ocean circulation system, spreading into other ocean basins. Deep waters eventually return toward the surface through mixing and upwelling, completing portions of a complex global overturning circulation. This system is sometimes described as the global ocean conveyor belt.
Although the conveyor belt analogy is useful for visualizing the connection between ocean basins, actual circulation is considerably more complicated. Winds, density differences, mixing, and interactions with the ocean floor all contribute to the movement of water. Importantly, this circulation operates over much longer timescales than many atmospheric processes. While weather systems may develop and dissipate within days, some deep ocean waters remain isolated from the atmosphere for centuries or longer. The ocean therefore provides a mechanism for storing and redistributing heat over extended periods.
Atlantic Meridional Overturning Circulation
A particularly important component of global ocean circulation is the Atlantic Meridional Overturning Circulation, commonly abbreviated as AMOC. The AMOC includes the northward transport of relatively warm upper-ocean waters and the southward return of colder, deeper waters. As warm water moves northward through the Atlantic, heat is transferred from the ocean to the atmosphere. This exchange influences regional temperature patterns, atmospheric circulation, and the distribution of heat across the Northern Hemisphere.
The strength of this circulation is influenced by changes in temperature, salinity, winds, and the formation of dense water at high latitudes. An influx of freshwater from increased precipitation, melting glaciers, or the Greenland Ice Sheet can reduce surface salinity in portions of the North Atlantic. Fresher water is less dense, potentially making it more difficult for surface water to sink. This is one mechanism through which changes in the climate system could influence Atlantic overturning circulation. A substantial weakening of the AMOC could alter heat transport across the Atlantic, affecting regional temperature and precipitation patterns. However, the magnitude and timing of future circulation changes remain areas of active scientific research.
Ocean Temperatures and Atmospheric Circulation
The influence of the oceans extends well beyond the movement of water itself. Ocean temperatures affect the atmosphere through exchanges of heat and moisture at the sea surface. Warm ocean surfaces generally favor greater evaporation and can provide additional moisture and energy to the atmosphere. This energy exchange can influence cloud formation, precipitation, and the development of tropical weather systems.
Cooler ocean surfaces often reduce evaporation and can contribute to more stable conditions in the lower atmosphere. Differences in sea surface temperature can also influence large-scale atmospheric circulation, including the position and strength of pressure systems and prevailing wind patterns. These interactions operate in both directions. Atmospheric winds influence ocean currents and sea surface temperatures, while the resulting ocean temperature patterns can feed back onto atmospheric circulation. This continuous exchange of energy and momentum makes the oceans and atmosphere closely connected parts of the climate system.
Ocean Circulation and Long-Term Climate Change
The oceans also play a central role in the response of Earth's climate system to increasing greenhouse gas concentrations. Because water has a high heat capacity, the oceans can absorb and store enormous quantities of thermal energy. Much of the excess heat accumulating in the climate system is stored in the oceans rather than the atmosphere. This ocean heat uptake moderates the rate of atmospheric warming but also produces changes in ocean temperatures, density, and circulation. As seawater warms, it generally expands, contributing to global sea level rise. Changes in ocean circulation can also redistribute heat, producing regional differences in sea surface temperature and sea level.
Figure 4. Factors contributing to sea level rise since 1993
The melting of land-based glaciers and ice sheets adds water to the oceans, further contributing to rising sea levels. Since the late nineteenth century, global mean sea level has risen substantially, with observations showing an acceleration in the rate of increase. Thermal expansion and the loss of land-based ice are the two principal contributors. While sea level rise is one of the most visible consequences of ocean warming, changes in ocean heat storage and circulation have broader implications for the global climate system.
Figure 5. Sea level rise since the last major ice age.
Why the Oceans Matter for Nebraska
Nebraska is located more than a thousand miles from the nearest ocean coastline, yet the oceans still influence the state's climate. The connection occurs primarily through large-scale atmospheric circulation. Ocean temperature patterns can influence the position of the jet stream, the development of persistent high- and low-pressure systems, and the transport of atmospheric moisture. For example, tropical Pacific temperature patterns associated with El Niño and La Niña can affect atmospheric circulation across North America. Similarly, changes in North Atlantic sea surface temperatures and circulation can influence broader atmospheric patterns, although the connections to Nebraska are more complex and less consistent.
These ocean-atmosphere relationships are important for seasonal climate prediction because ocean temperatures generally change more slowly than atmospheric conditions. As a result, ocean temperature anomalies can provide useful information about the likelihood of certain temperature and precipitation patterns months in advance. However, the atmosphere is highly variable, and ocean conditions represent only one component of the climate system. Land surface conditions, soil moisture, snow cover, and internal atmospheric variability also influence Nebraska's weather and climate.
