Application of a grid-scale lateral discharge model in the BALTEX region

In this study, a hydrological discharge model is presented which may be applied as a tool to validate the simulation of the hydrologic cycle of atmospheric models that are used in climate change studies. It can also be applied in studies of global climate change to investigate how changes in climate may affect the discharge of large rivers. The model was developed for the application with the climate models used at the Max-Planck- Institute for Meteorology. It describes the translation and retention of the lateral waterflows on the global scale as a function of the spatially distributed land surface characteristics which are globally available. Here, global scale refers to the resolution of 0.5° and lower, corresponding to a typical average gridbox area of about 2500 km2. The hydrological discharge model separates between the flow processes of overland flow, baseflow and overflow. The model parameters are mainly functions of the gridbox characteristics of topography and gridbox length. The hydrological discharge model is applied to the BALTEX (Baltic Sea Experiment) region using input from an atmospheric general circulation model (ECHAM4) as well as from a regional climate model (REMO). The simulated inflows into the Baltic Sea and its sub- catchments are compared to observed and naturalized discharges. The results of this comparison are discussed and the simulated values of precipitation, surface air temperature and accumulated snowpack are compared to both observed data and surrogate data

Portrayal of the Indian summer monsoon in the land-ocean-atmosphere system of a coupled GCM

A 150 year-long numerical simulation of present-day climate using the Max-Planck Institute's coupled ocean-atmosphere model ECHAM4-T42-OPYC3 is analysed with regard to the interannual variability of the strength of the Indian summer monsoon and its relation to land- surface and ocean interactions. Individual years are categorised into three classes of monsoons: normal, strong and weak (greater or less than one standard deviation of precipitation over India). The ensembles of anomalous monsoons are then sub-divided into a composite of cases coinciding with sea surface temperature (SST) anomalies in the Pacific related to the El Nifio- Southern Oscillation (ENSO) phenomenon and a second composite of anomalous monsoons occurring, when no SST anomalies are found in the Pacific. The coupled model shows variations of the SST in the Pacific which are as large as and occur at a similar frequency as in observations. Thus it provides the basis for a realistic simulation of the interannual monsoon variability in ENSO-related conditions, but it overemphasizes the biennial component of occurrence. As in observations, about a third of all cases of weak monsoons occur in the summer when an El Nico begins to develop in the Pacific, while strong monsoons are often associated with La NiNa events. This is an improvement from earlier coupled model simulations. In the model simulation, a modulation of the strength of the monsoon is due to a change of the large-scale land/ocean temperature gradient in the Indian Ocean sector in the mid- troposphere. The two composites show different developments during the annual cycle. In ENSO-related strong monsoon cases the atmosphere over land warms up during the spring in association with generally warmer tropics as a remnant from a warm event in the previous winter. During the summer months the warming in the Indian Ocean region is replaced by a cooling in association with the developing La NiNa, while the land remains significantly warmer than normal. Therefore, in the coupled simulation the Indian Ocean only shows very small SST anomalies, while observed SSTs may vary in connection with ENSO events at a time lag of four months. Also for non-ENSO related monsoons a warming occurs over land in the summer, but then neither the Indian Ocean nor the tropical west Pacific exhibit any significant anomalies during the spring. Simulated temperature anomalies responsible for these modulations are relatively small. Independent of the origin of the monsoon anomaly, strong monsoons differ from weak monsoons by a significant precipitation pattern over India and a modification of the 850 hPa zonal wind field over the maritime continent and the West Pacific. Similar to observations, the anomalous monsoons in the coupled model are related to precursors in the 200 hPa zonal wind field in the spring, but no evidence could be found for a significant influence from the Eurasian snow pack in the spring on the subsequent Indian summer monsoon. While the model shows many realistic features, the variation of the monsoon occurs against a background of a deficient regional rainfall pattern in India and too small a range of Indian Ocean SST variations in conjunction with ENSO events

A Normal-Mode Approach to Jovian Atmospheric Dynamics

We propose a nonlinear, quasi-geostrophic, baroclinic model of Jovian atmospheric dynamics, in which vertical variations of velocity are represented by a truncated sum over a complete set of orthogonal functions obtained by a separation of variables of the linearized quasi-geostrophic potential vorticity equation. A set of equations for the time variation of the mode amplitudes in the nonlinear case is then derived. We show that for a planet with a neutrally stable, fluid interior instead of a solid lower boundary, the baroclinic mode represents motions in the interior, and is not affected by the baroclinic modes. One consequence of this is that a normal-mode model with one baroclinic mode is dynamically equivalent to a one layer model with solid lower topography. We also show that for motions in Jupiter's cloudy lower troposphere, the stratosphere behaves nearly as a rigid lid, so that the normal-mode model is applicable to Jupiter. We test the accuracy of the normal-mode model for Jupiter using two simple problem forced, vertically propagating Rossby waves, using two and three baroclinic modes and baroclinic instability, using two baroclinic modes. We find that the normal-road model provide qualitatively correct results, even with only a very limited number of vertical degrees of freedom

Intensified Asian Summer Monsoon and its variability in a coupled model forced by increasing greenhouse gas concentrations

The Asian summer monsoon response to global warming is investigated by a transient green‐house warming integration with the ECHAM4/OPYC3 CGCM. It is demonstrated that increases of greenhouse gas concentrations intensify the Asian summer monsoon and its variability. The intensified monsoon results mainly from an enhanced land‐sea contrast and a northward shift of the convergence zone. A gradual increase of the monsoon variability is simulated from year 2030 onwards. It seems to be connected with the corresponding increase of the sea surface temperature variability over the tropical Pacific