INTERANNUAL VARIABILITY OF REGIONAL CLIMATE AND ITS CHANGE DUE TO THEGREENHOUSE-EFFECT

Interannual variability of regional climate was investigated on a seas onal basis. Observations and two global climate model (GCM) simulation s were intercompared to identify model biases and climate change signa ls due to the enhanced greenhouse effect. Observed record length varie s from 40 to 100 years, while the model output comes from two 100-year equilibrium climate simulations corresponding to atmospheric greenhou se gas concentrations at observed 1990 and projected 2050 levels. The GCM includes an atmosphere based on the NCAR CCM1 with the addition of the radiative effects of CH4, N2O and CFCs, a bulk layer land surface and a mixed-layer ocean with thermodynamic sea-ice and fixed meridion al oceanic heat transport. Because comparisons of interannual variabil ity are sensitive to the time period chosen, a climate ensemble techni que has been developed. This technique provides comparisons between va riance ratios of two time series for all possible contiguous sub-perio ds of a fixed length. The time autocorrelation is thus preserved withi n each sub-period. The optimal sub-period length was found to be 30 ye ars, based on which robust statistics of the ensemble were obtained to identify substantial differences in interannual variability that are both physically important and statistically significant. Several aspec ts of observed interannual variability were reproduced by the GCM. The se include: global surface air temperature; Arctic sea-ice extent; and regional variability of surface air temperature, sea level pressure a nd 500 mb height over about one quarter of the observed data domains. Substantial biases, however, exist over broad regions, where strong se asonality and systematic links between variables were identified. For instance, during summer substantially greater model variability was fo und for both surface air temperature and sea-level pressure over land areas between 20-50 degrees N, while this tendency was confined to 20- 30 degrees N in other seasons. When greenhouse gas concentrations incr ease, atmospheric moisture variability is substantially larger over ar eas that experience the greatest surface warming. This corresponds to an intensified hydrologic cycle and, hence, regional increases in prec ipitation variability. Surface air temperature variability increases w here hydrologic processes vary greatly or where mean soil moisture is much reduced. In contrast, temperature variability decreases substanti ally where sea-ice melts completely. These results indicate that regio nal changes in interannual variability due to the enhanced greenhouse effect are associated with mechanisms that depend on the variable and season.