As an alternative to the frequently used mixed boundary conditions in ocean GCM's, we present a dynamic atmospheric model (ECBILT) that is simple and yet describes the relevant dynamic and thermodynamic feedback processes to the ocean. This provides the possibility of studying atmosphere/ocean dynamics on very long time‐scales of the order of a thousand years. The model is two orders of magnitude faster than AGCMs. We have been running ECBILT with prescribed SSTs for a period of 500 years with seasonal cycle included both in the solar forcing and in the climatological SSTs. The mean state and the variability properties of ECBILT are reasonably realistic. The simulation of the surface fluxes is quite realistic and justifies coupling ECBILT to an ocean GCM. We have done two SST anomaly experiments, one with a tropical SST anomaly as observed during January 1983 and one with an SST anomaly in the northern extra‐tropical Atlantic ocean. For the tropical SST anomaly experiment the amount of anomalous precipitation agrees well with what has been found with atmospheric GCM's, but the resulting extra‐tropical teleconnection pattern is very weak. The atmospheric response pattern to extra‐tropical SST anomalies agrees well with similar SST anomaly experiments performed with atmospheric GCM's. We have tested the performance of ECBILT in coupled mode by coupling it to a simple ocean GCM and thermodynamic sea‐ice model and integrating the coupled system for a period of thousand years after a spin up of 6000 years. The simulation of the mean water mass distribution and the mean circulation of the ocean resembles the observed ocean circulation. It has a warm and fresh bias and the circulation and associated transports are too diffuse and too weak. The ocean's variability is realistic, considering the simplicity of the ocean model, although a bit too weak. We have explored the covariability between the atmosphere and ocean over the Northern Atlantic ocean by performing a singular value decomposition of SST anomalies and 800 hPa geopotential height anomalies. The 2nd mode shows a peak in both spectra at a timescale of about 18 years. The time scale of this mode is set by the ocean but the physical mechanisms that are operating are not yet unambiguously identified. The simulation of this coupled extra tropical decadal mode of variability, which also shows up in the observations and in much more complex coupled models provides strong evidence for the potential usefullness of ECBILT when studying atmosphere/ocean interaction and the associated ocean variability on time scales ranging from decades to many thousands of years.

Adopting the viewpoint that atmospheric flow regimes can be associated with steady states, this work investigates the hypothesis that regime transitions in deterministic atmosphere models are related to the existence of heteroclinic connections between these steady states. A low-order barotropic model with topography is studied, in which topographic and barotropic instabilities are the mechanisms dominating the dynamics. By parameter tuning, the Hopf bifurcation corresponding to barotropic instability can be made to coincide with one of the saddle-node bifurcations that are due to the topography in the model. This coincidence is called a fold-Hopf bifurcation. Among the dynamical structures related to such a bifurcation are heteroclinic connections and homoclinic orbits, connected to the equilibria. A heteroclinic cycle back and forth between the equilibria, existing in the truncated normal form of the fold-Hopf bifuraction, will be perturbed in the full model, leaving orbits homoclinic to one of the equilibria. The impact of these mathematical structures explains several characteristics of regime behavior known from previous model studies.

A multi‐millennia simulation performed with a three‐dimensional climate model under constant forcing shows abrupt climate events lasting for several centuries caused by a spontaneous transition to an infrequently visited state of the oceanic thermohaline circulation. This state is characterized by a more southern location of the main area of deep ocean convection in the North Atlantic and implies a large cooling in the mid and high latitudes of the northern hemisphere. This transition of the thermohaline circulation occurs spontaneously less than once in 5000 years in the model, but such transitions can also be triggered by a reduction of the solar irradiance.

1] The vulnerability of society on extreme weather has resulted in extensive research on the statistics of extremes. Although the theoretical framework of extreme value statistics is well developed, meteorological applications are often limited by the relative shortness of the available datasets. In order to overcome this problem, we use archived data from all past seasonal forecast ensemble runs of the European Centre for Medium-Range Weather Forecasts (ECMWF). For regions where the forecasts have very little seasonal skill the archived seasonal forecast ensembles provide independent sets that cumulate to over 1500 years. We illustrate this approach by estimating 10 4 -year sea-surge levels at high-tide along the Dutch coast. No physical mechanisms occur in the ECMWF model that make the distribution of very extreme surges different from what is inferred from a direct analysis of the observations. In comparison with the observational sets, the ECMWF set shows a decrease in the statistical uncertainty of the estimated 10 4 -year return value by a factor four.

Meteorological extremes have large impacts on society. The fact that approximately 40% of the Netherlands is below sea level makes this country especially vulnerable to flooding, both from the sea and from rivers. This has resulted in extensive research on the statistics of extremes. However, applications to meteorological and hydrological situations are always hampered by the brevity of the available datasets, as the required return levels exceed the record lengths by a factor of 10 to 100. In order to overcome this problem, we use archived data from all past seasonal forecast ensemble runs of the European Centre for Medium-Range Weather Forecasts (ECMWF) since 1987 as input for extreme-value statistics analysis. We make use of the fact that the seasonal forecast has little seasonal skill for the Netherlands, which implies that the ensembles can be regarded as independent sets that cumulate to over 1500 years.We investigate the hydraulic response in the Netherlands to extreme synoptic-scale weather systems by studying the extreme-value distributions of sea storm surge levels, waves and river discharges. The application is detailed in four practical examples originating from coastal protection, river flooding protection, and water management problems. The long record length of the ECMWF data reduces the uncertainty in the 10 3 -year and the 10 4 -year return values considerably with respect to the results based on observational time series. The ECMWF dataset gives the opportunity to explore the distribution of events that depend on several kinds of extreme.

Ensemble simulations with a total length of 7540 years are generated with a climate model, and coupled to a simple surge model to transform the wind field over the North Sea to the skew surge level at Delfzijl, The Netherlands. The 65 constructed surge records, each with a record length of 116 years, are analysed with the generalized extreme value (GEV) and the generalized Pareto distribution (GPD) to study both the model and sample uncertainty in surge level estimates with a return period of 104 years, as derived from 116-year records. The optimal choice of the threshold, needed for an unbiased GPD estimate from peak over threshold (POT) values, cannot be determined objectively from a 100-year dataset. This fact, in combination with the sensitivity of the GPD estimate to the threshold, and its tendency towards too low estimates, leaves the application of the GEV distribution to storm-season maxima as the best approach. If the GPD analysis is applied, then the exceedance rate, lambda, chosen should not be larger than 4. The climate model hints at the existence of a second population of very intense storms. As the existence of such a second population can never be excluded from a 100-year record, the estimated 104-year wind-speed from such records has always to be interpreted as a lower limit.

No abstract

No abstract