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Ocean-Atmosphere Interaction: A Tropical Thermostat for Global Warming? Wallace and Clement Papers Casey Saup.

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Presentation on theme: "Ocean-Atmosphere Interaction: A Tropical Thermostat for Global Warming? Wallace and Clement Papers Casey Saup."— Presentation transcript:

1 Ocean-Atmosphere Interaction: A Tropical Thermostat for Global Warming? Wallace and Clement Papers Casey Saup

2  Wallace Paper  Background  Purpose/goals, Methods, Figures, Conclusions  Clement Paper  Background  Purpose/goals, Methods, Figures, Conclusions  Summary  Future Directions of the Research (IMO) Discussion Plan

3  Water vapor in the atmosphere has a destabilizing effect on climate  As SSTs increase, more water evaporates, creating a positive feedback cycle (traps longwave radiation).  Ramanathan and Collins postulated that cirrus clouds (associated with deep convection and increase locally over warm pool with increasing SST) “braked” this feedback and limited the warm pool (305K)  Claimed this would explain negative skewness Wallace Paper Background

4  Purpose: To suggest that Ramanathan and Collins’ proposed thermostat mechanism may not be required to explain SST distribution.  Large-scale dynamical processes should maintain uniform tropical tropospheric temperatures to within ~2K  Without horizontal temperature contrasts in the atmosphere, a negatively skewed SST frequency distribution will develop due to equilibration btw the atmosphere and SSTs that vary by location.  Cirrus clouds will not necessarily prevent SST from rising above 305K even though they reduce radiation in regions of deep convection. Wallace Paper Purpose/Goals

5 Wallace Paper Figures/Results  April climatological mean geopotential height field on the 200hPa surface—this is situated in the upper troposphere at the level of strongest horizontal gradients.  The range between the highest and lowest values in the tropics is not more than two contour intervals (80 m).  Maps for individual years/months at levels ranging from the surface to the tropopause (boundary btw the troposphere and the stratosphere) are similar in this respect.

6 Wallace Paper Figures/Results  Vertical scale of tropical circulation systems is comparable to the depth of the troposphere, the vertically averaged temperature of the tropical troposphere must be uniform to within 2 K

7 Clement Paper Background Why did no one consider ocean dynamics?  Water vapor in the atmosphere has a destabilizing effect on climate  As SSTs increase, more water evaporates, creating a positive feedback cycle (traps longwave radiation).  Ramanathan and Collins postulated that cirrus clouds (associated with deep convection and increase locally over warm pool with increasing SST) “braked” this feedback and limited the warm pool (305 K)  Claimed this would explain negative skeweness  Wallace pointed out that tropical temperatures would need to be fairly uniform and that efficient ocean-atmosphere heat exchange in areas of deep convection would lead to the negative skewness in the absence of cirrus cloud cover.

8  Many other papers were published suggesting other mechanisms than that of Ramanathan and Collins, the best of which is from Pierrehumbert  Says that clouds have no net effect on the top of atmosphere radiation  Deep convective clouds have no net effect in stabilizing the tropical climate.  Relies on “radiator fins”—in areas of deep convection energy is exported to drier, nonconvecting regions where it is effectively radiated to space.  All of these differing views have something in common: they don’t take “interactive dynamical transports of heat in the ocean” into account. Clement Paper Background (contd)

9  Goal: “To illustrate a possible role for ocean dynamics in regulating sea surface temperatures by including ONLY highly idealized atmospheric thermodynamics.” Clement Paper Purpose/Goals

10 Clement Paper Methods  Zebiak-Cane Model  Coupled ocean-atmosphere model, solves for perturbations about the climatological state  Consists of an atmosphere governed by two shallow-water equations on an equatorial beta plane and a linear reduced gravity ocean model  Model domain extends from 29°N to 29°S, 124°E to 80°W  The Temperature anomaly in the ocean model mixed layer was determined using the equation below.  This experiment was intended to stimulate how the coupled tropical system would respond to a simple forcing.  In the absence of ocean dynamics, the model would generate an SST anomaly T=T*

11 Clement Paper Figures  Figure 1: Surface temperature anomaly in April—four model months after the start of the runs  The temperature change in the eastern equatorial (180° to the eastern boundary and btw 5°S and 5°N) region is less than that of the surrounding region.  SST must change so that the surface heat flux anomaly balances imposed forcing.  In the eastern region here, the imposed forcing can be partially balanced by anomalous horizontal and vertical advection, and the SST will change less.  Positive *T  E-W temperature gradient increased, which strengthens the equatorial easterlies, which will increase upwelling and cause the thermocline to shoal in the east. Both of these processes will further cool the SSTs in the eastern portion of the basin. This will lead to a coupled interaction that establishes a new climatology.

12 Clement Paper Figures/Results  Resulting annual mean SSTs for warming and cooling  Surprisingly, coupled interaction causes temperature anomaly in the NINO3 region (5S-%N, W) to be the opposite sign to that of the forcing.

13 Clement Paper Figures/Results  Terms in Equation 1 averaged over the area of the entire basin.  The forcing αT* is almost equally balanced by the change in heat flux (αT, dashed) and the vertical advection of temperature (change in vertical flux, dotted)

14 Clement Paper Figures/Results  Basin and annual mean temperature anomaly and NINO3 temperature anomaly relative to the standard run as a function of T*.  Meridional advection spreads the upwelled water off of the equator leading to a basin average temperature change that is less than expected.

15 Clement Paper Figures/Results  Seasonal cycle of surface temperature anomaly for the NINO3 region  SST response is smaller in the spring than it is in the fall  Since the sign of the response in this region is opposite to that of the forcing, the seasonal cycle for warming is enhanced and weakened for cooling.

16 Clement Paper Figures/Results  Representative segments of time series of the NINO3 index taken from a 1000-yr run for T*=+2 and T*=-2  Variability is dramatic across T*  ENSO variability is almost completely wiped out in the warming scenario, but the cooling events become more regular

17  Wallace Summary:  Tropical troposphere temperatures are fairly uniform due to large-scale dynamical processes (within about 2 K)  Efficient heat exchange between the ocean and the atmosphere in areas of deep convection will lead to the negative skewness that Ramanathan and Collins attributed to cirrus cloud cover (so…no cirrus cloud cover is necessary for the negative skewness) Summary

18  Clement Summary:  Everyone else considered the ocean to be dynamically inactive in their models which is super wrong.  Using the Zebiak-Cane model, they discovered that ocean dynamics alter/affect the mean tropical sea surface temperature, climatology, amplitude of seasonal cycle, interannual variability.  This indicates that you can’t ignore ocean dynamics when dealing with tropical climate stability, even though it’s hard  Summary

19  IMO:  I think it may be beneficial to look into paleoclimatological records to see if that could shed any light on this area of research  I think models that incorporate finer details of wind, ocean, and atmospheric dynamics and the interactions between the three should be developed. Future Directions of the Research

20 References  Wallace, John M. "Effect of Deep Convection on the Regulation of Tropical Sea Surface Temperature." Nature (1992): Print.  Clement, Amy C., Richard Seager, Mark A. Cane, and Stephen E. Zebiak. "An Ocean Dynamical Thermostat." Journal of Climate: Print.


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