The great “oceanic heat pump”- Will the Gulf Stream disappear with global warming…?

Motivation Atmosphere produces its circulation the hard way, through heating/cooling (heat engine): no external force pushes air parcels from West to East to create the Jet Stream… What about the Ocean? Tropopause Stefan-Boltzman radiation (cooling) Enthalpy flux (heating) Sea surface

(i) Surface winds can accelerate water parcels Annual mean 10m wind from ERA40 Atlas (2006)

(ii) Heating and cooling only occur at the surface Enthalpy flux+surface radiation exceed solar+greenhouse radiation Heating: Solar+greenhouse radiation exceed Enthalpy flux+surface radiation Surface heat flux (Wm-2, color) and temperature (contours, °C) -NOC climatology

Outline The thermohaline circulation (THC) The “Great oceanic heat pump” Gulf Stream and Global Warming

Captain Ellis and the Earl of Halifax “measurements” (1751) “Upon the passage, I made several trials with the bucket sea-gage. I charged it and let it down to different depths, from 360 feet to 5346 feet; when I discovered, by a small thermometer of Fahrenheit’s, made by Mr. Bird, which went down to it, that the cold increased regularly, in proportion to the depths, till it descended to 3900 feet: from whence the mercury in the thermometer came up to 53 degrees; and tho’ I afterwards sunk it to the depth of 5346 feet, it came up no lower. The warmth of the water upon the surface, and that of the air, was at the time by the thermometer 84 degrees.”

Captain Ellis and the Earl of Halifax “measurements” (1751) “The experiment, which seem’d at first but mere food for curiosity, became in the interim very useful to us. By its means we supplied our cold bath, and cooled our wines or water at pleasure; which is vastly agreeable to us in this burning climate” NB: Captain Ellis was at 25N, 25W

Count Rumford explains the measurements… But a still more striking, and I might, I believe, say, an incontrovertible proof of the existence of currents of cold water at the bottom of the Sea, setting from the poles towards the equator, is the very remarkable difference that has been found to subsist between the temperature of the sea at the surface and at great depth, at the tropic –though the temperature of the atmosphere there is so constant that the greatest changes produced in it by the seasons seldom amounts to more than five or six degrees; yet the difference between the heat of water at the surface of the Sea, and that at the depth of 3600 feet, has been found to amount to no less than 31 degrees; the temperature above or at the surface being 84 degrees, and at the given depth below no more than 53 degrees. It appears to me to be extremely difficult, if not quite impossible, to account for this degree of cold at the bottom of the sea in the torrid zone, on any other supposition than that of cold currents from the poles. born Benjamin Thompson (1753-1814)

The Stommel-Aarons-Faller experiments (1958) Ω Surface cooling WARM So So COLD Water Volume of cold layer increases at the expense of the volume of warm water

Results So A “western boundary current” is produced, “North” So A “western boundary current” is produced, flowing towards the “South” (equator)! “East” “West” “South”

Salinity at 50m depth (in g/kg of seawater) 40 Higher salinity Through excess evaporation Lower salinity through excess precipitation 30 World Ocean Atlas 2001

The “great oceanic conveyor belt”

The “great oceanic conveyor belt”

Summary (THC part) Temperature at depth is much colder than at the surface as a result of deep currents from the poles. Salty water flows southward, deep in the Atlantic Ocean. It upwells over the surface Southern Ocean where it mixes with fresher surrounding waters, before subsequently flowing northward at subsurface as a fresh current. The Gulf Stream closes this “thermohaline cell” by replenishing the volume of warm water in the North Atlantic.

What drives this thermohaline cell?

Thermodynamic cycle of the THC Z Warm and salty B C D A Temperature C A Cold and fresh B SH NH Equator Entropy Separation between the surface (diabatic) layer and the deep (adiabatic) layer

The “Great Oceanic heat pump” Cold Southern Ocean D Warm and salty Temperature C A Cold and fresh Atlantic Ocean B Mechanical Energy input Entropy Warm North Atlantic

Work done by the surface wind on the ocean (≈2TW total) In units of Wunsch (1998)

The “great oceanic heat pump” Southern Ocean Jet stream D Z Warm and salty B C D A Temperature C A Cold and fresh B SH NH Equator Entropy Atlantic heat transport Wind forcing over S. Ocean

The Thermohaline circulation and Global Warming Movie Scenario: -CO2 increases -Hydrological cycle strengthens -Atlantic Ocean becomes much less salty -The heat pump structure breaks down: the mechanical energy input by the wind can not be converted to a heat transport -Sea-ice margin moves southward -Sea-ice albedo kicks in… Global Freeze! … irrealistic

The Gulf Stream and Global warming The Gulf Stream would exist even if there was no such thing as the thermohaline circulation Jet Stream Surface winds Water (spherical) Earth Rotating Gulf Stream Trade winds

Conclusions The thermohaline circulation works like a heat pump The Gulf Stream will not disappear with Global Warming

…I leave it to Jenny… Tuesdays, 2-3pm in Huxley, Room 726 But I maintain office hours until the end of term: Tuesdays, 2-3pm in Huxley, Room 726 NB: Except on week 10