On the history of meridional overturning circulation schematic diagrams

Abstract

Recent global warming caused by humans and the prediction of a reduced Atlantic Ocean meridional overturning circulation in the future has increased interest in the role of the overturning circulation in climate change. A schematic diagram of the overturning circulation called the "Great Ocean Conveyor Belt," published by Wallace Broecker in 1987, has become a popular image that emphasizes the inter-connected ocean circulation and the northward flux of heat in the Atlantic. This would appear to be a good time to review the development of the conveyor belt concept and summarize the history of overturning circulation schematics.In the nineteenth century it was thought that symmetric overturning circulation cells were located on either side of the equator in the Atlantic. As new hydrographic measurements were obtained in the late nineteenth century and early twentieth century, circulation schematics in the early twentieth century began to show the inter-hemispheric overturning circulation in the Atlantic. In the second half of the twentieth century schematics showed the global ocean overturning circulation including connections between the Atlantic and the Pacific and Indian Oceans. Some recent schematics of the overturning circulation show its complexities, but as more information is included these schematics have also become complex and not as easy to understand as the simple Lenz 1845 4The earliest recorded deep temperature measurements in the subtropical Atlantic were made in 1750 by Captain Henry Ellis of the British slave ship Earl of Halifax. They revealed that warm subtropical water was confined to a relatively thin surface layer overlying much colder water (Ellis, 1851; Wüst, 1968; Deacon, 1971; Warren, 1981).Count Rumford (1798) who was born American as Benjamin Thompson and who served with the British, was knighted, eventually became Count of the Holy Roman Empire and ran the government of Bavaria, first described a circulation system to explain these temperature observations. He suggested that cold water from north and south polar regions spreads on the bottom of the sea toward the equator, and this produces currents at the surface in the opposite directions. A further development of this idea was made by Emil von Lenz (1845), a German-Russian physicist, who noted that the thermocline was shallower near the equator than in the subtropics and proposed an upwelling of deep water into the surface layer near the equator to compensate for poleward flow of warm near surface water. He derived a conceptual model of the meridional overturning circulation in the Atlantic, which consisted of two circulation cells located symmetrically on either side of the equator. A schematic summarizing this circulation (Fig. 2 The overturning circulation is a complicated system of currents driven by winds, by buoyancy forcing through evaporation, precipitation, heating, and cooling, and by mixing due to winds and tides. Knowledge of it developed slowly as subsurface temperature and salinity began to be accurately measu...

Long‐term variations of circulation in the tropical Atlantic sector and Sahel rainfall

“…As shown in the earlier study (Hastenrath, 1990a), the tropical North Atlantic became significantly cooler (Figure 4 Lele and Lamb (2010), with dots for individual years and solid line for 11-year running mean; (b) Sahel rain from Nicholson (1985), with dots for individual years and solid line for 11-year running mean; (c) SST from NOAA, in July-August-September, 11-year running means, with solid line for TN, dashed line for TS and dotted line for dT (= TN − TS). Shading highlights the DRY regimes 1910-1919and 1979-1988, and the WET regimes 1950-1959and 1999-2008 Hemisphere slowed down ( Figure 5(c)), all consistent with a southward displaced Intertropical Convergence Zone and Sahel drought. In the domain of the Sahel rainfall index during DRY as compared to WET pressure was significantly higher, and wind less from the south, consistent with the patterns over the Atlantic.…”

“…From the rainfall series plotted in Figure 2 four decades with extreme regimes were identified, namely 1910-1919(DRY), 1950-1959(WET), 1979-1988(DRY) and 1999-2008. Differences in the fields between contrasting decades were mapped and significance calculated by t-test, as presented in Figures 4-6. …”

The Ocean Circulation in Thermohaline Coordinates

“…Despite this, descriptive oceanographers have identified specific pathways of net oceanic motion from observed tracers fields (Wü st 1935;Deacon 1937;Gordon 1986). These pathways have been schematized in a variety of ways, with the ''global conveyor'' of Broecker (1991) being the most universally recognized (see Richardson 2008, for a review of such schematics).…”

Crucial times for Spanish physical oceanography

“…However, it was the German school, with Alfred Merz, George Wüst and Albert Defant, that led the physically oriented deep-ocean expeditions. This school proposed the early patterns of subsurface and deep circulation in the Atlantic Ocean, which were not much differ-ent from present conceptions (Richardson 2008), and obtained the first ocean-wide set of physical measurements with the Atlantic expedition of the RV Meteor (1925-1927 The development of physical oceanography in Spain followed quite a different path. The late 19th century was a period of cultural enlightenment in Spain.…”