4. Ocean Temperature

1. A new feedback on climate change from the hydrological cycle Paul Williams1, Eric Guilyardi1,2, Rowan Sutton1, Jonathan Gregory1,3, Gurvan Madec4 (1) Walker Institute, Department of Meteorology, University of Reading, UK; (2) IPSL / Laboratoire des Sciences du Climat et de l'Environnement, France; (3) Hadley Centre, Met Office, Exeter, UK; (4) IPSL / Laboratoire d'Océanographie Dynamique et de Climatologie, Université Paris VI, France 2. Proposed ,,,,,Feedback We recently proposed a specific mechanism by which the feedback described in Section 1 may operate [3]. Precipitation and evaporation maintain salinity and temperature gradients within the ocean.Mixing of waters with different temperatures causes a transfer of heat, as shown schematically below. Global warming-induced changes in precipitation and evaporation will lead to changes in the salinity and temperature gradients. This could modify the heat transfer and potentially alter the temperature structure of the ocean. 3. Experimental ,,,,,Design We wish to investigate the sign, magnitude and linearity of the feedback proposed in Section 2. Using a state-of-the-art climate model, we run two sensitivity experiments in which the net fresh water flux from the atmosphere (consisting of evaporation, precipitation and run-off) is multiplied by 2 (EPR2) and 0 (EPR0) before being passed to the ocean. The two experiments are compared to a control run (CTRL), and all three are initiated from the present-day ocean structure of [4] with constant greenhouse gas forcing. We use the SINTEX ocean-atmosphere model [5,6], which consists of ORCA2 (2o x 0.5-2o with 31 levels) coupled to ECHAM4 (T106). 1. Introduction Climate models predict that global warming will result in an intensified hydrological cycle [1]. Observations suggest that this process has already begun: globally-integrated rainfall has steadily increased in recent decades [2]. But changes in evaporation and precipitation may themselves affect ocean temperature, giving a possible feedback on global warming, as indicated by the dashed arrow: 4. Ocean Temperature The equilibrated ocean surface temperature anomalies in EPR0 and EPR2 are shown below (oC). The responses at low latitudes, where the vertical component of the heat transfer discussed in Section 2 is downward [7], are consistent with our proposed feedback mechanism. Decreased gradients in EPR0 give a decreased downward heat flux and a mean surface warming of 0.6oC, whereas increased gradients in EPR2 give an increased downward heat flux and a mean surface cooling of 0.8oC. 6. Conclusions At low latitudes, our proposed feedback has negative sign under an intensified hydrological cycle. It is linear for multiplying factors in the range studied. For a multiplying factor of 1.1, which is of the order projected to exist at the time of CO2 doubling [1], we estimate by interpolation a negative ocean surface temperature anomaly of magnitude around 0.1oC. We conclude that an intensification of the hydrological cycle is likely to contribute a weak negative feedback to low-latitude climate change. 5. Linearity of the ,,,,,Feedback The feedback’s characteristics are summarized in the figure below, broken down by geographical region. Because of atmospheric feedbacks, the freshwater amplification factor in EPR2 is 1.8 rather than 2. At low latitudes (<40°N/S) the feedback is almost linear for amplification factors in the range studied, but at higher latitudes (>50°N/S) it is highly nonlinear. suppressed hydrological cycle References [1] IPCC (2001). Climate Change 2001: The Scientific Basis. Cambridge University Press. [2] Dai, A., Fung, I. Y. & Del Genio, A. D. (1997). Surface observed global land precipitation variations during 1900-88, J. Clim., 10, 2943-2962. [3] Williams, P. D., Guilyardi, E., Sutton, R. T., Gregory, J. M. & Madec, G. (2006). On the climate response of the low-latitude Pacific ocean to changes in the global freshwater cycle, Clim. Dyn., 27(6), 593-611. [4] Levitus, S. (1982). Climatological atlas of the world ocean. NOAA Professional Paper 13, pp. 173. [5] Gualdi, S., Navarra, A., Guilyardi, E. & Delecluse, P. (2003). Assessment of the tropical Indo-Pacific climate in the SINTEX CGCM, Annals of Geophysics, 46(1), 1-26. [6] Guilyardi, E., Delecluse, P., Gualdi, S. & Navarra, A. (2003). Mechanisms for ENSO phase change in a coupled GCM, J. Clim., 16(8), 1141-1158. [7] Osborne, T. J. (1998). The vertical component of epineutral diffusion and the dianeutral component of horizontal diffusion, J. Phys. Oceanogr., 28, 485-494. amplified hydrological cycle CTRL EPR0 EPR2