(Article by Javier Vinos republished from JudithCurry.com)
Here is the text of Chapter 17 of my new book
The ocean is the primary source of poleward heat transport in the tropics, with the tropical Pacific being the dominant player due to its size. It exports heat to the Atlantic and Indian Oceans, which are the only ones to transport heat across the equator. However, inter-basin exchanges are relatively small, indicating that global seawater pathways play a minor role in heat transport. The Atlantic is unique in having an exclusively northward net heat transport due to its meridional overturning circulation, which accounts for about 60% of the heat transported in the North Atlantic. Oceanic heat transport from the North Atlantic to the Nordic Seas and the Arctic increased significantly between 1998 and 2002, during a period of Arctic and global climate shift.
Most of the heat transported by the global ocean is carried by water above 10°C (50 °F), located between 40°N and 40°S at depths of less than 500 m. This transport is primarily due to wind-driven circulation. Even the Atlantic Meridional Overturning Circulation is as sensitive to winds as it is to the formation of high-latitude deep water.
Analysis of the critical tropical upper-layer heat budget has revealed a remarkable 11-year variability associated with the solar cycle that is ten times larger than can be accounted for by changes in solar radiation. In addition, model studies of the Atlantic meridional circulation show that solar forcing is its most important natural determinant. These studies underscore the critical role of the Sun in modulating ocean heat transport by inducing changes in atmospheric circulation.
The ocean plays a critical role in the Earth’s climate system, providing thermal stability and storing a large fraction of the system’s energy. With a total mass 265 times that of the atmosphere and a heat capacity 1000 times greater, the ocean stores 96% of the energy in the climate system and receives 75% of the energy delivered by the Sun to the planet’s surface. This essential feature of the ocean has allowed the existence of complex life. However, because the Earth is currently in an ice age that began 34 million years ago (the Late Cenozoic Ice Age), the ocean has reached a cold state with an average temperature of about 4°C (39 °F), and only the upper mixed layer is substantially warmer due to solar heating and wind-induced turbulence. The sea surface temperature of the open ocean is limited to 30°C (86 °F) because deep convection occurs above 27°C (80 °F), increasing evaporation and forming clouds that effectively cool the surface. Although the upper 2.5 m of the ocean contains as much heat as the entire atmosphere, its main function in climate change is to absorb heat as the planet warms and release it as it cools, providing thermal inertia.
The ocean contributes about 25% of global poleward heat transport (ch. 10). In the tropics, the ocean is the most important heat transporter. Its contribution is even greater in the Northern Hemisphere, where it accounts for about 30% of heat transport. However, the Atlantic Ocean has a unique heat transport pattern. The South Atlantic has a net heat transport towards the equator (fig. 25).
Figure 25. Ocean heat transport. Mean meridional ocean heat transport (in petawatts) for the global ocean (solid black), Atlantic (dashed red), and Indo-Pacific (dotted blue).
Most of the ocean’s heat is transported by water with a temperature above 10°C (50 °F), mainly in the band of the ocean between 40°S and 40°N and above a depth of 500 m. This is the main reason why meridional ocean heat transport is more important at these latitudes, where the Hadley cell is not very effective in transporting heat poleward (ch. 13).
Global ocean heat transport is dominated by heat export from the tropical Pacific, which has the largest tropical surface area and receives the most solar energy. However, it’s striking how much the tropical Pacific dominates heat export to other oceans, exporting four times more heat than is imported into the Atlantic and Arctic oceans. The Atlantic and Indian Oceans transport heat north and south across the equator, respectively, but the Pacific provides this heat through the Drake Passage and the Indonesian Throughflow. While there is some exchange between the basins, it’s relatively small, suggesting that global seawater pathways play a minor role in the Earth’s heat budget.
The Atlantic Ocean has northward heat transport in both hemispheres and across the equator due to the Atlantic Meridional Overturning Circulation. This circulation is part of the thermohaline circulation, which involves the northward flow of warmer, lighter water in the upper layers of the Atlantic and the southward flow of cooler, denser water at depth. Although the two branches are mechanically driven, they are linked by the transformation of warm to cold water masses at high latitudes (ch. 10).
The uniqueness of Atlantic heat transport is highlighted in figure 25 and is related to the asymmetry of the latitudinal temperature gradient between the two hemispheres. Each year, the Southern Hemisphere receives more solar energy than the Northern Hemisphere. This is due to the Earth’s current axial precession, which causes the Southern Hemisphere to be oriented toward the Sun when the Earth is closer to it. Albedo does not correct for this difference due to its interhemispheric symmetry (box 2, ch. 3). Despite receiving a greater annual influx of solar energy, the Southern Hemisphere is about 2°C cooler than the Northern Hemisphere, and the Earth maintains a steeper temperature gradient toward the colder Antarctic than toward the warmer Arctic (ch. 9, fig. 13). Transport theory states that more heat should flow toward the colder pole since temperature differences drive transport. However, the Atlantic transports more heat from the Southern to the Northern Hemisphere, suggesting that energy transport is not solely determined by entropy production. Rather, it is strongly influenced by geographic and climatic factors and thus may be a forcing mechanism for climate change.
The exceptional nature of the Atlantic Ocean’s heat transport has important implications for the climate of the surrounding regions of the North Atlantic, the Arctic, and the global climate. Sea surface temperature in the North Atlantic exhibits a multidecadal oscillation that correlates with global temperature (ch. 19). Analysis of the Atlantic heat flux over time shows a clear relationship between oceanic heat transport and North Atlantic sea surface temperatures (fig. 26). This evidence supports the notion that the oscillation in North Atlantic sea surface temperature is a result of changes in meridional heat transport. Surprisingly, despite this evidence, ocean oscillations are rarely considered in terms of heat transport.+
Figure 26. Atlantic heat transport and North Atlantic sea surface temperature. a) Atlantic integrated meridional heat transport over time in petawatts from reanalysis. b) North Atlantic sea surface temperature record for the same period.
The transport of Atlantic water to the Arctic occurs through the Nordic Seas, and the volume and temperature of the transported water strongly influence the climate of northern Europe and the Arctic. The transformation of warm to cold water masses necessary for the Atlantic Meridional Overturning Circulation occurs in the Nordic Seas and the Arctic Ocean. Although oceanic heat transport is a small part of the Arctic heat budget (ch. 11 & 16), its analysis can be very informative. A recent study of ocean heat transport in the Nordic Seas and the Arctic Ocean found a sudden increase in transport. From the 1993-98 average to the 2002-2016 average, oceanic heat transport in this important climate region, the “bellwether” for climate change, increased by 25 terawatts (9%) between 1998 and 2002 (fig. 27).
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