Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 4The Atlantic branch of this global MOC (seeFig. 4.1) consists of two primary overturningcells: (1) an “upper” cell in which warm upperocean waters flow northward in the upper1,000 meters (m) to supply the formation ofNorth Atlantic Deep Water (NADW), whichreturns southward at depths of approximately1,500–4,500 m and (2) a “deep” cell in whichAntarctic Bottom Waters (AABW) f lownorthward below depths of about 4,500 mand gradually rise into the lower part of thesouthward-flowing NADW. Of these two cells,the upper cell is by far the stronger and is themost important to the meridional transportof heat in the Atlantic, owing to the largetemperature difference (~15 °C) between thenorthward-flowing upper ocean waters and thesouthward-flowing NADW.In assessing the “state of the AMOC,” we mustbe clear to define what this means and howit relates to other common terminology. Theterms Atlantic Meridional Overturning Circulation(AMOC) and Thermohaline Circulation(THC) are often used interchangeably but havedistinctly different meanings. The AMOC isdefined as the total (basin-wide) circulation inthe latitude depth plane, as typically quantifiedby a meridional transport streamfunction. Thus,at any given latitude, the maximum value ofthis streamfunction, and the depth at whichthis occurs, specifies the total amount of watermoving meridionally above this depth (andbelow it, in the reverse direction). The AMOC,by itself, does not include any information onwhat drives the circulation.In contrast, the term “THC” implies a specificdriving mechanism related to creationand destruction of buoyancy. Rahmstorf (2002)defines this as “currents driven by fluxes ofheat and freshwater across the sea surface andsubsequent interior mixing of heat and salt.”The total AMOC at any specific location mayinclude contributions from the THC, as wellas contributions from wind-driven overturningcells. It is difficult to cleanly separateoverturning circulations into a “wind-driven”and “buoyancy-driven” contribution. Therefore,nearly all modern investigations of theoverturning circulation have focused on thestrictly quantifiable definition of the AMOC asgiven above. We will follow the same approachin this report, while recognizing that changesin the thermohaline forcing of the AMOC, andparticularly those taking place in the high latitudesof the North Atlantic, are ultimately mostrelevant to the issue of abrupt climate change.There is growing evidence that fluctuationsin Atlantic sea surface temperatures (SSTs),hypothesized to be related to fluctuations inthe AMOC, have played a prominent role insignificant climate fluctuations around theglobe on a variety of time scales. Evidencefrom the instrumental record (based on the last~130 years) shows pronounced, multidecadalswings in SST averaged over the North Atlantic.These multidecadal fluctuations may be at leastpartly a consequence of fluctuations in theAMOC. Recent modeling and observationalanalyses have shown that these multidecadalshifts in Atlantic temperature exert a substantialinfluence on the climate system ranging frommodulating African and Indian monsoonal rainfallto influencing tropical Atlantic atmosphericcirculation conditions relevant to hurricanes.Atlantic SSTs also influence summer climateconditions over North America and WesternEurope.Evidence from paleorecords (discussed morecompletely in subsequent sections) suggests thatthere have been large, decadal-scale changes inthe AMOC, particularly during glacial times.These abrupt change events have had a profoundimpact on climate, both locally in the Atlanticand in remote locations around the globe.Research suggests that these abrupt events wererelated to massive discharges of freshwater intothe North Atlantic from collapsing land-basedice sheets. Temperature changes of more than120