An evaluation of ENSO and climate change using the GCM/ZC model

Since ENSO has well-established effects on global climate, it is logical to ask how ENSO's intensity and frequency may change due to climate change. To address this question, a model has been developed at the Goddard Institute for Space Studies (GISS) which couples the GISS gcm with the El Nino-generating model developed by Dr. Stephen Zebiak and Dr. Mark Cane (ZC) of the Lamont Doherty Earth Observatory (LDEO).; The first chapter of the thesis details the coupling technique used for the model, while chapter 2 describes the model's ENSO response. In particular, the GCM/ZC model is found to generate a realistic ENSO response, though at a higher frequency than observed. In addition, the ENSO response is found to weaken the Walker Circulation and strengthen the Hadley Circulation when the model is in the El Nino phase. The model's ENSO response is due to the dynamical heating term inputted from the ZC model, with latent heat and shortwave fluxes from the ocean's surface acting to modify the warm anomalies. The global response of the model to ENSO shows the GCM/ZC model produces jet-stream configurations much like the observed ENSOs. Chapter 3 analyzes the possible effect of volcanoes on climate. 10 3-year control runs are compared with 10 3-year runs with a volcanic eruption in month 1 of the runs. The results suggest an amplification of the ENSO cycle due to volcanic aerosol in six of the 10 simulations while four of the runs show a deamplification. The trigger mechanism for the two solutions appears to be the wind stress anomaly field. Chapter 4 tests the effects of increased CO2 on ENSO variability. The 4XCO₂ equilibrium solution shows a 50 percent decrease in magnitude of ENSO variability relative to control, due to increasingly warm upwelled water being incorporated into the mixed-layer of the ZC model. Decadal standard deviation of NINO3 shows increased variability of ENSO from decade 2 to decade 7 of the simulation as the Hadley Circulation strengthens due to decreased longitudinal SST gradient in the Equatorial Pacific. However, as warmer water is upwelled, the variability subsequently decreases after decade 7. The addition of ENSO variability to a 4XCO₂ environment results in further weakening of the longitudinal SST gradient. Accompanying this is a weakening of the wind stress in the Equatorial Pacific resulting in an ENSO-like climatological state. One result of this new climatology is a positive feedback on global warming. In addition, ENSO propagation becomes predominantly eastward. Chapter five focuses on how hurricane frequency changes due to ENSO variability and increased CO2 via Gray's Parameter (GP). Differences in GP between ENSO/Non-ENSO show a rearrangement of GP due to shifting SST and upper-level wind shear patterns (i.e the dynamical term (DP)). Differencing ENSO/Non-ENSO in a 4XCO₂ scenario shows that in a 4XCO₂ equilibrium climate, thermal potential plays a much greater role than dynamic potential in hurricane frequency. (Abstract shortened by UMI.)...