Interhemispheric climate connections: What can the atmosphere do?

Transcription

1 Interhemispheric climate connections: What can the atmosphere do? Raymond T. Pierrehumbert The University of Chicago 1

2 Uncertain feedbacks plague estimates of climate sensitivity 2

3 Water Vapor

4 Models agree with one another on water vapor feedback, but this doesn t guarantee that they are all correct. 3

5 Clouds * Main source of spread in IPCC forecasts * Major disagreements between models * Little sign of convergence

6 Sea Ice

7 ...And there are also substantial uncertainties about how the ocean circulation responds to climate forcing. 4

8 LGM Cooling as a Test of Climate Sensitivity The LGM is our best argument that climate models are not excessively sensitive to CO 2 changes, and may instead be underestimating sensitivity. We know the magnitude of CO 2 changes at the LGM We know a lot about other climate forcings We know a lot about the state of the climate How can we make the best use of this test? 5

9 How much cooling should the LGM CO 2 drop cause? Baseline calculation: Decrease CO 2 from 280ppm to 180ppm Local radiative-convective model assuming no change in meridional heat transport Conventional water vapor feedback (fixed relative humidity) incorporated No cloud feedback, ice-albedo feedback or ocean dynamic feedback 6

10 Baseline Results In the tropics (300K unperturbed warm pool temperature): 1.45K surface cooling if radiation balance dominated by moist regions 1K surface cooling if radiation dominated by dry regions Compatible with 1.6K-2.5K tropical warming from doubling CO 2 In the midlatitudes (280K, 50% relative humidity),cooling is 1K 7

11 Greater cooling at LGM indicates enhanced climate sensitivity due to additional feedbacks in climate system, but... we need to somehow subtract off the effect of the growth of the Northern Hemisphere continental ice sheets. 8

12 To minimize effect of NH ice sheets, we can focus on: Pacific Tropical Warm Pool Temperature Southern Hemisphere as a test of climate sensitivity.

13 A message from the Tropical Snowline Altitude (meters) Freeze Line Temperature 9

14 Mechanisms of interhemispheric transport (Summer) (Winter) SP Equator NP 10

15 The glacial world according to Manabe and Broccoli In an atmospheric GCM coupled to a mixed-layer ocean: NH ice sheets alone produce little cooling in Northern subtropics, almost none, south of Equator Reduction in meridional latent heat flux compensates increase of meridional sensible heat flux The LGM orbital parameters have little direct effect 11

16 An idealized AGCM simulation Mixed layer ocean, sea ice suppressed Interglacial Case Glacial Case 12

17 90 Surface Temperature, January 90 Surface Temperature, July Latitude 0 Jan, Cold case Jan, Warm case Latitude 0 July, Warm case July, Cold case Temperature (K) Temperature (K) 13

18 90 Surface Temperature Differences 60 Latitude Warm-Cold, July Warm-Cold,Jan Temperature (K) 14

19 Transient Eddy Flux Diagnostics In these simulations, NH glaciation is communicated to the rest of the world solely through changes in transient eddy heat (and momentum) fluxes Is there compensation between sensible and latent heat flux? How much does sensible heat flux change, and in which season? At what latitudes? 15

20 Sensible Heat Flux Latent Heat Flux Net Heat Flux Latitude 0 Latitude 0 Latitude 0-30 Jan SH W Jan SH C -30 Jan LH W Jan LH C -30 Jan net W Jan net C Flux (Petawatts) Flux (Petawatts) Flux (Petawatts) 16

21 Sensible Heat Flux Latent Heat Flux Net Heat Flux Latitude 0 Latitude 0 Latitude Jul SH W Jul SH C Jul LH W Jul LH C Jul net W Jul net C Flux (Petawatts) Flux (Petawatts) Flux (Petawatts) 17

22 What is the Hadley Cell doing? Interhemispheric communication of the climate signal must pass through the Hadley cell. Therefore it is important to understand how the Hadley cell responds to NH glaciation Dynamics tells us that large tropical temperature gradients should be confined to the boundary layer (Nature abhors interhemispheric asymmetry in the Tropics) 18

23 4 January T, Warm -Cold Latitude 19

24 4 July T, Warm -Cold Latitude 20

25 4 July T, Warm -Cold Latitude 21

26 Warm January July Warm Cold Cold 22

27 90 January Precipitation 90 July Precipitation Latitude 0 Latitude Jan, cold case Jan, Warm Case Precipitation, cm/month -60 July, Cold Case July, Warm Case Precipitation, cm/month 23

28 Strengthened Hadley cell dries the Northern subtropics Warm Case July Relative Humidity Cold Case Latitude Latitude 0% 100% 24

29 Enhanced water vapor feedback exerts a cooling effect on the Tropics, which results from tropical-extratropical coupling, but does not show up in the meridional flux diagnostics

30 Cloud Feedback Much rearrangement of patterns, but little net tropical feedback. Net Radiative Forcing (W/m 2 ) Net Tropical Cloud Forcing (30S to 30N) Cold Case Warm Case month Like water vapor, cloud feedbacks are an indirect coupling mechanism that doesn t show up in meridional heat flux diagnostics. 25

31 Conclusions Surface Temperature: Glaciating a NH continent induces only moderate tropical surface cooling, despite a large increase in the NH extratropical temperature gradient (esp. in July). There is almost no surface temperature reduction south of the ITCZ. This is not primarily due to latent/sensible heat flux compensation. The main reason is that transient eddy fluxes are weak at 30N. Though much weaker than NH continental cooling, significant NH tropical cooling penetrates to the Equator, so that the Pacific Warm pool temperature changes shouldn t be thought of as being completely decoupled from the NH glaciation effects, even considering just the atmosphere alone. Suggest a transfer coefficient approach to subtracting off glaciation effect 26

32 The SH midlatitudes are well-insluated from NH glacial cooling, and so provid a good test of CO 2 sensitivity, incorporating amplification by water vapor, clouds, sea-ice, and atmospheric dynamical feedbacks. The dynamic ocean response, amplified by sea ice, complicates the preceding picture. By how much?

33 The vertical structure of the cooling: NH continental cooling in response to glaciation is bottom-heavy, Tropical cooling is top heavy and follows expected pattern of the moist adiabat While low level cooling is confined to N of the ITCZ, the upper level cooling is nearly symmetric across the equator, and penetrates well into the Southern subtropics. However, this has too little low-level expression to couple into the SH storm tracks.

34 Other changes induced by NH glaciation: NH glaciation greatly strengthens the Hadley cell This is due to increased July sensible heat export, carried over into the rest of the year by the thermal inertia of the subtropical ocean The ITCZ shifts somewhat further into the Southern hemisphere, but precipitation does not increase, because reduced bdd. layer humidity compensates increased flow The strengthened Hadley circulation greatly reduces the Northern subtropical relative humidity aloft, given additional cooling due to an anomalous (i.e. stronger than fixed-rh) water vapor feedback. The tropical cloud pattern is considerably redistributed by glaciation, but the net cloud feedback on the tropics is weak in this in this calculation. (Your mileage may vary)

35 Possible ocean effects: The oceanic subtropical gyre could play a significant role in communicating the NH signal into the Northern subtropics. With a dynamic ocean, changes in cold upwelling arising from wind changes could also affect tropical SST, though in these simulations there is little increase of the surface easterlies, despite a dramatic increase in the strength of the Hadley circulation.