Maisa Rojas

In this paper we assess South American monsoon system (SAMS) variability in the last millennium as depicted by global coupled climate model simulations. High-resolution proxy records for the South American monsoon over this period show a coherent regional picture of a weak monsoon during the Medieval Climate Anomaly and a stronger monsoon during the Little Ice Age (LIA). Due to the small external forcing during the past 1000 years, model simulations do not show very strong temperature anomalies over these two specific periods, which in turn do not translate into clear precipitation anomalies, in contrast with the rainfall reconstructions in South America. Therefore, we used an ad hoc definition of these two periods for each model simulation in order to account for model-specific signals. Thereby, several coherent large-scale atmospheric circulation anomalies are identified. The models feature a stronger monsoon during the LIA associated with (i) an enhancement of the rising motion in the SAMS domain in austral summer; (ii) a stronger monsoon-related upper-tropospheric anticyclone; (iii) activation of the South American dipole, which results in a poleward shift of the South Atlantic Convergence Zone; and (iv) a weaker upper-level subtropical jet over South America. The diagnosed changes provide important insights into the mechanisms of these climate anomalies over South America during the past millennium.

This paper investigates the effect of Last Glacial Maximum (LGM) versus high CO₂ world boundary condition on the Southern Hemisphere atmospheric circulation, in particular on the strength and latitudinal position of the near surface Southern Westerly Winds (SWW). PMIP2 and PMIP3 experiments, as well as the “abrupt 4 × CO2” simulations from CMIP5, were analyzed. Robust findings include poleward expansion of the Mean Meridional Circulation (MMC) and intensified and poleward‐shifted SWW in the 4 × CO2 simulations (consistent with recent observations and 21st century climate change projections); and for the LGM, stronger and southward shifted northern hemisphere MMC, and weakened southern Hadley cell. However, six of the eight LGM simulations show a decrease in the SWW, the other two models simulate the opposite. A critical difference between the models is strong coupling between sea‐ice extent, surface temperature gradients, SWW, and Ferrel cell in the two models with stronger and poleward‐shifted SWW.

The 2‐day wave is a planetary‐scale wave recurrently observed in the summer mesosphere. It is predominantly zonal wave number 3 (though wave numbers 2 and 4 are also observed) and has a period of approximately 2 days. It is generally accepted that the 2‐day wave has characteristics of both global‐scale Rossby‐gravity normal mode and baroclinic/barotropic instability. Here we examine how local instability amplifies the global Rossby‐gravity mode. We perform a two‐dimensional instability calculation to calculate unstable modes. The fastest growing modes have wave number 3, a 35‐hour Rossby mode localized in the summer hemisphere at high latitudes, and a Rossby‐gravity mode with period 42 hours with similar characteristics to the observed 2‐day wave. The Rossby‐gravity mode was examined as the background state was changed from an isothermal atmosphere to realistic conditions. The wave was slowly Doppler shifted toward shorter periods and its spatial structure became distorted. Approaching realistic conditions the mode becomes weakly unstable. Near this point a localized Rossby mode also emerges which exists on the reversed potential vorticity gradients of the summer jet. This has the necessary characteristics to phase lock with the Rossby‐gravity mode. Hence we propose that the 2‐day wave amplifies from the interaction of global‐scale Rossby‐gravity and local modes. A nonlinear model was used to examine the unstable modes as they grow to finite amplitude. It was found that even though the 35‐hour Rossby wave had the largest growth rates, it did not reach large amplitude. It was the slower growing 42‐hour Rossby‐gravity mode which dominated the long time evolution.

Results are reported from two 5-month-long simulations for southern South America using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The periods of simulation correspond to May–September 1997 and 1998, which were anomalously wet and dry winters for central Chile, respectively. The model setup includes triply nested, two-way-interacting domains centered over the eastern South Pacific and the western coast of southern South America, with horizontal grid intervals of 135, 45, and 15 km. Boundary conditions are provided from NCEP–NCAR reanalyzed fields. The analysis focuses on two subregions of central Chile (30°–41°S). Region 1 (32°–35°S), which is where the observed interannual precipitation differences are largest, is topographically very complex, with a mean height of the Andes Cordillera around 4500 m. Region 2 (35°–39°S) has relatively smooth terrain, as the mean height of the Andes drops to 3000 m. Station precipitation and temperature data are used for model ... Abstract Results are reported from two 5-month-long simulations for southern South America using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The periods of simulation correspond to May–September 1997 and 1998, which were anomalously wet and dry winters for central Chile, respectively. The model setup includes triply nested, two-way-interacting domains centered over the eastern South Pacific and the western coast of southern South America, with horizontal grid intervals of 135, 45, and 15 km. Boundary conditions are provided from NCEP–NCAR reanalyzed fields. The analysis focuses on two subregions of central Chile (30°–41°S). Region 1 (32°–35°S), which is where the observed interannual precipitation differences are largest, is topographically very complex, with a mean height of the Andes Cordillera around 4500 m. Region 2 (35°–39°S) has relatively smooth terrain, as the mean height of the Andes drops to 3000 m. Station precipitation and temperature data are used for model ...