Modulation of Tropical Air-Sea coupling by TIWs: Sources for Tropical Biases in the Pacific Climate System

The El Niño-Southern Oscillation (ENSO) is the most important mode of interannual climate variability. At present, global climate models still suffer from substantial biases in ENSO simulation and prediction, including too-regular ENSO cycles. The cause of the irregularity of ENSO evolution is a topic with an extensive literature; the interactions with the seasonal cycle and stochastic forcing (SF) from the atmosphere are some of the proposed contributors that are still under active research. In particular, numerous modeling studies have demonstrated that stochastic forcing in the atmosphere can modulate ENSO, indicating that it is a leading candidate responsible for ENSO irregularity. 

As an oceanic form of SF, tropical instability waves (TIWs) are a meso-scale phenomenon in the eastern tropical Pacific. Recent high-resolution space-based observations reveal significant two-way air-sea interactions associated with TIWs in the region; their roles in budgets of heat, salt, momentum and biogeochemical fields in the ocean have been demonstrated. At present, realistic simulations of these atmospheric response to TIW-induced sea surface temperature (SSTTIW) anomalies are a great challenge since the details of the response mechanisms in the marine boundary layer and their interactions to the overlying free troposphere are not well-known. In particular, most climate models do not realistically take into account TIW-induced meso-scale atmospheric response to SSTTIW anomalies due to a lack of high resolution in the horizontal and vertical. Therefore, the TIW-induced feedbacks from the atmosphere to the ocean and the corresponding meso-scale coupled air-sea interactions are still missing in largescale climate modeling studies. Due to their large perturbation amplitude, TIWs are expected to have significant impact on ENSO. However, the roles of TIWs in causing ENSO irregularity are not known; their potential roles in causing model biases and improving ENSO simulation and prediction in climate models have not been examined coherently. 

We propose to investigate the role of TIWs in contributing to tropical biases in ENSO simulation and prediction in large-scale climate models. We intend to utilize the feedback signature of TIWs in satellite data to develop an empirical parameterization of their atmospheric response for use in coupled climate models. As such, the effects of TIWs on simulations of mean ocean state, seasonal cycle and interannual variability can be extensively examined using a hybrid coupled model (HCM) which consists of an ocean general circulation model (OGCM) and a statistical atmospheric wind stress anomaly (τ) model over the tropical Pacific domain, a global coupled general circulation model (CGCM), and the NCEP/NOAA CFS, respectively. In support of these studies, an empirical parameterization will be first developed and tested to depict TIW-induced wind stress anomalies (τTIW) in response to SSTTIW variability in the eastern tropical Pacific Ocean using the unprecedented accuracy of satellite observations for sea surface temperature and winds. Next, the empirical parameterization scheme for τTIW will be nested into the HCM to explicitly represent TIW related meso-scale air-sea coupling. Preliminary results have demonstrated that the model can simulate both phenomena (TIWs and El Niño) very well using this innovative nesting approach, and that TIWs in the eastern equatorial Pacific can contribute to systematic biases in mean ocean state, seasonal cycle, and interannual variability through their rectified effect on large-scale air-sea coupling. Therefore, realistic representations of meso-scale processes associated with TIWs in large scale climate oceanatmosphere models will lead to more realistic simulations of the mean state and its interannual variability associated with ENSO. Various experiments will be conducted to quantify the extent to which heat and momentum fluxes related to TIWs can contribute to systematic biases seen in large-scale climate models. The relationship between the strength of TIWs and the level of ENSO irregularity will be a focus. Model simulations will be directly compared with observations from satellite and TOGA-TAO moored buoys. Needless to say that the mechanistic understanding and nested parameterization for TIWs developed in these simplified coupled modeling experiments will be studied carefully such that we can transfer the knowledge to more realistic coupled climate models to reduce tropical biases and to enhance our predictive understanding in the IRI CGCM and the NCEP/NOAA CFS, respectively.