Twentieth-Century Sea Surface Temperature Trends M.A. Cane, et al., Science 275, pp (1997) Jason P. Criscio GEOS Apr 2006

Transcription

1 Twentieth-Century Sea Surface Temperature Trends M.A. Cane, et al., Science 275, pp (1997) Jason P. Criscio GEOS Apr 2006

2 Questions 1. What is the proposed mechanism by which a uniform radiative heating from above affects the mean state and variability of the tropical Pacific coupled ocean-atmosphere system? How is the mechanism similar to and different from the ENSO physics we've discussed over the semester? 2. What are the various observational and model results that support the operation of the ocean dynamical thermostat over the past century? What are their various strengths and weaknesses? 3. What do you expect to see from this mechanism as atmospheric CO2 concentrations continue to climb?

3 Current Events Fact Earth surface temperature has risen 0.3ºC - 0.6ºC over the past 100 years Problem Who s on first? Man or Nature Fact Greenhouse gases increase Earth surface temperature Problem Don t fully understand feedbacks or the magnitudes of such feedbacks

4 Current Work Climate models show too great of a temperature increase than what has been observed, ~2x more (e.g., Houghton 1995) Discrepancy can be mitigated via the inclusion sulfates which have a net cooling effect... Sulfate effect still poorly understood so the jury is still out

5 Hypothesis An increased E-W sea-surface temperature gradient across the Equatorial Pacific Ocean is not seen in current model studies E-W SST gradient pattern predicted as a response to increased atmospheric heating (e.g., Neelin and Dijkstra 1995) Therefore, the coupled ocean-atmosphere system can delay global warming and possibly regulate it

6 Bjerknes Mechanism Jul (from Bjerknes 1969) Steeper pressure slope increased EQ easterlies Increased easterlies increased upwelling Increased upwelling stronger temperature gradient Stronger T gradient stronger Walker Circulation Stronger Walker Circulation stronger T gradient Increased heating from above increased evaporation Increased upwelling in E Pac cooler surface waters Net Effect: Average temperature less than expected

7 Bjerknes Mechanism Fig. 1. Annual mean SST anomaly (in degrees Celsius) generated by the Lamont intermediate coupled oceanatmosphere model (12) when forced by an imposed uniform heating. [Adapted from (7)] Cane et al Used simplified ocean-atmosphere model: Lamont Model Imposed 1ºC temperature increase on SST everywhere Achieved expected result: cooler Eastern EQ Pacific, strong E-W temperature gradient Net +0.5ºC increase in the overall mean

8 Problems Fig. 1. Annual mean SST anomaly (in degrees Celsius) generated by the Lamont intermediate coupled oceanatmosphere model (12) when forced by an imposed uniform heating. [Adapted from (7)] Cane et al Lamont model uses specified fixed thermocline temperature Theory relies on upwelling for balance EQ thermocline water come from high latitudes Warmer water less cooling effect on thermocline Different balance achieved

9 So... New Model Ocean GCM simulation using realistic SST dependent heat flux Q = +10 W m-2 Climatological wind stress Warming seen across entire Pacific, though gradient preserved Fig. 2. Results from an ocean GCM simulation (15) with fixed winds and a realistic surface heat flux formulation to which a uniform heating of 10 W m 2 has been added. Changes in SST gradient along the equator from the initial state is shown for years 2, 5, 10, 20, 40, and 70. Most of the adjustment takes place in the first 2 years, and the steady state is closely approached by year 40.

10 Dynamical Ocean Thermostat Seager and Murtugudde, 1997: Ocean Dynamics, Thermocline Adjustment, and Regulation of Tropical SST. J. Climate, 10, Suppose a uniform heat flux is added above the ocean In W Pacific, there is essentially no ocean heat transport so SSTs warm in response to added heat flux In E/C Pacific, heat transport exists due to upwelling Excess heat lost poleward, to the atmosphere, or to downwelling Net effect is an increased E-W temperature gradient

11 Dynamical Ocean Thermostat Seager and Murtugudde, 1997: Ocean Dynamics, Thermocline Adjustment, and Regulation of Tropical SST. J. Climate, 10, Zebiak-Cane model responds with increased convection in W Pacific (enhanced Walker Circulation) Increased easterlies increased upwelling shallow thermocline in E Pacific SST anomaly is negative in E Pacific, positive everywhere else Net change due to ocean dynamics is less than a pure thermodynamic response

12 New Model Problems Ocean GCM doesn t allow downwelled water to affect thermocline temperature However, the water is too shallow not to be affected by mixing Amplitude of adjustment determined by SST at source Complex coupled GCMs with 2x CO2 show E Pacific warmer than W Pacific

13 How To Resolve Hypothesis Strength of coupling between ocean and atmosphere is the key to discerning climate change Strong: Bjerknes feedback (e.g., Lamont, Z-K models) Weak: Thermodynamics dominate (e.g, greenhouse model) Current models lacking in coupling because interannual and ENSO signals do not match observations Problem may be in low resolutions

14 Reconstructed SSTs Fig. 3. A Use observational SST data to see the change of the E-W temperature gradient over the last 100 years Data utilized is optimal smoother (OS) solution to minimizing the cost function e T Wee + m T Wmm Data originates from GOSTA

15 Reconstructed SSTs Warming seen everywhere, except... N Pacific S of Greenland E EQ Pacific Matches Lamont Model Fig. 3. (A) The trend in monthly mean SST anomalies for 1900 to 1991 in tre end was calculated. This was done by removing the ENSO mode, defined

16 Reconstructed SSTs Increased El Niño activity after 1975 may be the result of global warming Increased El Niño may be random occurrence When leading EOF in 2-7 year band removed (i.e., El Niño), E EQ Pacific cooled even more Fig. 3. (A) The trend in monthly mean SST anomalies for 1900 to 1991 in tre end was calculated. This was done by removing the ENSO mode, defined

17 Reconstructed SSTs Analyze OS, GOSTA, and COADS data, checking for change in E-W sea-surface temperature gradient OS dataset is derived from GOSTA GOSTA dataset includes COADS data when data was unavailable GOSTA corrects for systematic bucket error whereas COADS does not

18 E-W Gradient Analysis OS: ± 0.14ºC / 100yr GOSTA: ± 0.15ºC / 100yr COADS: ± 0.14ºC / 100yr Therefore coupling is strong Refutes greenhouse models due to E EQ Pacific cooling ig. 4.

19 Further Study Unknown how atmospheric GCMs will respond to SST E-W gradient trend Expected Dynamics of coupled system + thermodynamics lead to enhanced negative feedback Patterns have cold event characteristics, leading to a reduction in in global temperatures Coupled GCM with 2x CO2 with E EQ Pacific temperatures held fixed led to less temperature change

20 Conclusions Data and theory corroborate hypothesis that the SST warming trend is anthropogenic Strong coupling of ocean-atmosphere system creates a delay in the global warming signal Coupling also regulates global warming Current models do not show observed E-W temperature gradient Current models are over-predicting temperature change due to global warming

21 Questions 1. What is the proposed mechanism by which a uniform radiative heating from above affects the mean state and variability of the tropical Pacific coupled ocean-atmosphere system? How is the mechanism similar to and different from the ENSO physics we've discussed over the semester? 2. What are the various observational and model results that support the operation of the ocean dynamical thermostat over the past century? What are their various strengths and weaknesses? 3. What do you expect to see from this mechanism as atmospheric CO2 concentrations continue to climb?