Lecture 10: convection and the ocean-atmosphere heat engine

The air temperature on 2 Jan 1993 at the surface of the Earth. The cold (blue) air forms a dome in the Arctic, which is dense(heavy) and tends to slide southwater beneath less dense air. This leads to both the overturning circulation and the westerly winds and jet stream.

• Flow of incoming energy from sun: Fin = 1372 watts/m2

average night and day, tropics, poles:=> Fin /4 mostly as visible light

(wavelengths 400-700 x 10-9m)Flow of outgoing energy upward from Earth:

use black-body formula from physics,Fout = σT4 where

σ= 5.67 x10-8 J/m2 sec oK4

• soFin /4 = σ T4 T = Fin /4σ = 255 K = -18oCsnowball Earth? No, 290 K is about our average surface

temperature. Albedo effect: some of the sunlight reflects from

the whiter regions of Earth (snow, ice particularly), reducing the available sunlight

Greenhouse effect: a blanket of water vapor and CO2 and methane CH4 traps the upward infra-red ‘long-wave’ or ‘heat’ radiation.

Tropics/poles effect: more sunshine in low latitudes, more upward radiation in high latitudes: =>need a flow of heat from Equator toward poles.

Heat moving north, fresh water moving south in ocean….Subpolar Atlantic and exchange with the Arctic: well-

defined passages, ridges, gaps.

AGU, 2003

potential density profiles

30N61N eastern SP gyre57 N Labrador Sea

the red line is lies along the sectionplotted in the next figure

Labrador Greenland Iceland Rockall Ireland

Erika Dan section, 1962L.V. Worthington, R.Wright Location is about 60N latitude, shown on previous figure

Global meridional heat transport divides roughly equally into 3 modes: 1. atmosphere (dry static energy) (Bryden+Imawaki 2002)2. ocean (sensible heat)3. joint atmosphere/ocean mode: water vapor/latent heat transport

The three modes of poleward transport are comparable in amplitude, and distinct in character (sensible heat flux divergence focused in tropics, latent heat flux divergence focus in the subtropics) (based on Keith (Tellus 1995) climatology, and not very differently with Trenberth et al. J.Clim 2003)

Error est.: ± 9% at mid-latitude; Bryden est 2.0 ±0.42 pW at 24N

(residual method, (residual method, TOA radiation TOA radiation 19851985--89 and 89 and ECMWF/NMC ECMWF/NMC atmosatmos obsobs))

90S 90N

the northern subtropics show extremely active upward air/sea moisture flux

water evaporates from the Great Lakes when cold north winds blow over them. It soon condenses back into water, as cloud droplets which then rain or snow out…The lake water has become cloud, and then the cloud piles up as deep snow, downwind of the lakes. This is called ‘lake-effect snow’.

Satellite image of water vapor (not cloud…its gaseous water which is normally invisible). This water vapor carries heat to the Arctic from the tropics. moisture streamers: (1 Sverdrup…106 m3/sec tranport of water carries 2.5 x 1015 watt thermal energy)

polewardpolewardmoisturemoistureflux at 70Nflux at 70N((Dickson et al. 2000Dickson et al. 2000)

more water vapor, beingcarried by storms. The bright colors are towering cumulusclouds near the Equator

The ocean circulation is complex; here the warm (red) currents carry waterand heat northwardwhile deep cold currents(black) return the flow.

Melted ice floats as fresh water (or low salinity water) near the surface (purple) and moves southward. It insulates the ocean from the cold atmosphere above. This buoyant water is the basis for the idea that global warming may slow down the ocean circulation.

Lab model of the overturning circulation of the oceanthe horizontal velocity is much greater than the vertical velocity because of the ratio width:depth of oceans (~ 1000:1 or greater)

www.ocean.washington.edu/research/gfd/gfd.html

RADARSAT& SeaWIFS

‘Climate services: prediction of physical climate is not our only goal.

Spring productivity in Baffin Bay: North Water Polynya

The ocean’s overturning circulation makes possible the great biological food chain of the Arctic.

Here the dovekie (bird) relationship with oceanplankton is suggested..with fish being an intermediary.