Tropical Instability Waves, frequently observed in the tropical regions of the Pacific and Atlantic oceans, usually demonstrate significant air-sea interaction between wave motions in the atmosphere and the ocean. The generation of these waves trapped within the atmospheric boundary layer has been considered a direct response of the atmosphere to the SST variations embedded in the wave pattern near the equator, with wavelengths of about 1000~1200 km and periods between 25 and 40 days. These instability waves not only contribute to climate variability over the Atlantic but are also influenced by it. Previous studies have demonstrated the remote climate impact of these waves near the west coast of Africa and the tropical Atlantic, especially on intraseasonal precipitation variations. Moreover, previous studies have also suggested that the structure of these ocean instability waves is controlled to some extent by external wind forcing, which suggests a feedback from the atmosphere to the corresponding oceanic instability waves. To further explore the air-sea interaction mechanisms of these waves in the atmosphere and ocean, a coupled regional atmosphere-ocean model is developed by coupling a regional climate model to a reduced gravity ocean model.

The regional atmospheric model is the modified version of the fifth-generation Pennsylvania State University (PSU)-NCAR meso-scale model MM5 (V3.6.2) that can be used for seasonal or interannual simulations. The nonhydrostatic dynamics embedded in MM5, along with 24 vertical sigma levels and various convection schemes, enables it to better simulate mesoscale eddies than global models. This model has demonstrated the ability to capture the atmospheric wave response to short-term varying anomalous SST pattern of the tropical instability waves. The ocean model used for coupling is a 2.5-layer reduced gravity ocean model (RGOM), consisting of two active layers: mixed layer on top and thermocline layer below. The deep water below the thermocline layer is assumed to be infinitely deep and motionless. To cover the whole tropical Atlantic, we designed our RCM experiments with a 90-km horizontal resolution for a domain extending from 112°W to 22°E and 31°S to 31°S. To resolve mesoscale ocean eddy activities, a fine horizontal resolution of 0.25° grid spacing is used in RGOM. One noteworthy feature of the RGOM is that it does not include the effect of salinity.

Results from the coupled regional climate model show that it can realistically simulate the annual mean SST distribution and ITCZ precipitation over the tropical Atlantic region and also capture SST anomalies that propagate westwards along the Atlantic cold tongue region from June to November. This suggests that this coupled regional model can be used not only for long-term climate simulations but also for studying other short-term instability waves closely associated with intraseasonal or weather variability. Moreover, it could also serve as a useful tool to investigate other air-sea interaction phenomena, e.g., the formation of tropical cyclones and tropical instability waves near the cold tongue region of SST.