Extreme weather and climate events, such as hurricanes, heat waves, and intense rainfall, usually occur on local and regional scales. However, the frequency and intensity of their occurrence are modulated by planetary scale modes of climate variability, such as the El Niño-Southern Oscillation (ENSO) and the Atlantic Multidecadal Oscillation (AMO). For example, the frequency of hurricanes tends to be lower than normal during an El Niño event, because of increased vertical shear over the Atlantic associated with a weakening of the Walker circulation. The modes of climate variability involve interactions among different components of the climate system, including the atmosphere, ocean, land, and sea-ice. The current generation of global Coupled General Circulations Models (CGCMs) has demonstrated the capability to simulate these modes of climate variability with some degree of fidelity. However, the coarse spatial resolution of these models, i.e., horizontal grid size of the order of 100 km, is incapable of fully resolving the small-scale processes that play a vital role in the occurrence of extreme events. Time-slice experiments and regional climate modeling are two approaches that have been used to address the lack of adequate horizontal resolution in CGCMs used for climate projections. However, these approaches are often based on uncoupled integrations using an atmospheric only model, which ignores the role of atmosphere-ocean interaction. Our proposed research uses a new approach, utilizing a coupled regional climate model (CRCM) to assess the effect of fine spatial resolution, teleconnections, and air-sea coupling on extreme events in the Atlantic region. Our CRCM consists of a state-of-the-art regional atmospheric model coupled to a state-of-the art regional ocean model, with lateral boundary conditions derived from either observations or global CGCM simulations. Our research is motivated by the following two hypotheses:

1. Increased horizontal resolution can significantly alter the spatial and temporal statistics of extreme events, especially those associated with orographic features and moist convective processes that are poorly resolved in global CGCMs
2. Allowing air-sea interaction in regional modeling experiments can enhance the fidelity of regional model simulations and predictions of extreme events originating over the ocean, such as tropical cyclones and droughts associated with displacements of the marine ITCZ.

To understand the role of ENSO and AMO, as well as anthropogenic climate change, in modulating extreme events in the Atlantic region, we propose to carry out a number of numerical experiments using the high-resolution CRCM. Some of these experiments will be similar to dynamical downscaling experiments, using “realistic” lateral boundary conditions from global CGCM simulations, for the current climate as well as future climate scenarios. Other experiments will be of a mechanistic nature, using “idealized” boundary conditions designed specifically to isolate processes such as the remote influence of ENSO and the air-sea interaction associated with AMO. The findings of the proposed research will lead to a better understanding of scale interactions between modes of climate variability and extreme events and help improve model projections of climate change at regional scales, thus furthering the scientific goals of DOE/BER climate science activities.