Freshwater flux (FWF) is an important atmospheric forcing to the ocean, involving interactions among the Earth's water cycle, ocean circulation, and climate. Recent modeling studies have shown that the FWF-related ocean salinity in the tropical Pacific is important to the tropical dynamics and El Niño-Southern Oscillation (ENSO), and also has important implications for the use and interpretation of sea level, and for ocean data assimilation. However, FWF forcing and its related salinity effects in the tropical Pacific have not been realistically represented in many state-of-the-art coupled climate models; the FWF forcing has not been even included in most simplified coupled models used for ENSO forecasts.
This project will investigate the roles of FWF forcing in modulating interannual variability and predictability for the tropical Pacific climate system using observational data and a hybrid coupled model (HCM). The investigators will test a hypothesis that FWF forcing in the tropical Pacific induces a positive climate feedback, which presents a new mechanism for the modulation of ENSO and significant tropical bias sources for ENSO simulation and prediction. They will develop an empirical model for interannual FWF variability from historical precipitation and evaporation data. This Sea Surface Temperature (SST)-dependent, prognostic representation of anomalous FWF forcing allows for an interactive feedback between FWF and SST during ENSO cycles. Then, this FWF model will be incorporated into an HCM of the tropical Pacific to take into account FWF forcing and to represent FWF-induced climate feedback. The realistic inclusion of FWF forcing in a coupled ocean-atmosphere model is expected to lead to better ENSO simulations and predictions. Various numerical experiments will be conducted to quantify the extent to which FWF forcing can contribute to seasonal-to-interannual climate variability and predictability in the region. In particular, the effect of FWF forcing on the ENSO modulation will be a focus.
The broader impacts of this project include (1) the potential for direct improvements in ENSO simulations and predictions in coupled ocean-atmosphere models by taking into account the FWF-induced climate feedback; (2) important implication for understanding global water cycle in a changing climate due to global warming; (3) interdisciplinary learning and training experience for undergraduate and graduate students, and postdoctoral researchers; (4) wide dissemination of the results through publications and meeting/workshop presentations.
climate system using observational data and simplidied coupled models (Intermediate coupled model (ICM) and hybrid coupled model (HCM)). The working hypothesis is that FWF forcing in the tropical Pacific induces a new positive climate feedback, which presents a new mechanism for the modulation of ENSO; taking into account this atmospheric forcing component adequately in coupled ocean-atmosphere models of the tropical Pacific acts to reduce tropical biases, leading to improvements in ENSO simulation and prediction. Observations of FWF and salinity fields are used to characterize FWF/salinity variability and quantify their coherent relationships. In particular, interannual FWF variability is derived from remotely sensed precipitation data (GPCP). On coupled modeling side, an empirical model for FWF variability has been derived first from a singular value decomposition (SVD) analysis of historical data. This SST-dependent, prognostic representation of anomalous FWF forcing allows for an interactive feedback between FWF and SST during ENSO cycles. Then, this FWF model is incorporated into the ICM and HCM of the tropical Pacific to take into account FWF forcing and to represent FWF-induced climate feedback. Various numerical experiments have been conducted to quantify the extent to which FWF forcing can contribute to seasonal-to-interannual climate variability and predictability in the region. The proposed research is expected to have new scientific results, wide applications and benefits to others. For example, understanding ENSO modulation remains an outstanding issue in the climate community. A new mechanism for ENSO modulation has been identified that is ascribed to FWF forcing in the tropical Pacific. The proposed research develops a simple parameterization scheme for representing FWF forcing, which can be used in other coupled models that at present are unable to explicitly represent FWF forcing. Improved predictions are achieved by adequately taking into account this forcing in coupled models used for ENSO forecasts. Needless to say that the mechanistic understanding developed in this proposal allows for transferring the knowledge gained from simplified coupled model to more realistic coupled climate models to enhance our predictive understanding of FWF forcing such as at NASA/NSIPP and NOAA/NCEP. In addition, education and outreach have remained an important component of our research activities at AOS and ESSIC; this research provides learning and training experience for undergraduate and graduate students, and postdoctoral researchers. In particular, a visiting scientist is involved directly in this interdisciplinary project. Also, the results have been used in teaching activities at the Department of Atmosphere and Ocean Science/UMD, including (1) in the spring semester 2013, a course entitled Dynamics of the Atmosphere and Ocean at the Department of Atmosphere and Ocean Sciences (Totally there are 15 students attending the class); (2) in the spring semester 2014 for 3 lectures for a course entitles Dynamics of the Atmosphere and Ocean at the Department of Atmosphere and Ocean Sciences. Some publications supported by this project Zheng F., R.-H. Zhang, and J. Zhu, 2014: Effects of Interannual Salinity Variability on the Barrier Layer in the Western-Central Equatorial Pacific: A Diagnostic analysis from Argo. Adv. Atmos. Sci., 31:532-542. 10.1007/s00376-013-3061-8 Zhang, R.-H., 2014: Effects of tropical instability wave (TIW)-induced surface wind feedback in the tropical Pacific Ocean. Climate Dynamics, 42, 467-485. 10.1007/s00382-013-1878-6 Zhang, R.-H., 2014: Structure and Effect of Ocean Biology-induced Heating (OBH) in the Tropical Pacific, Diagnosed from a Hybrid Coupled Model Simulation, Climate Dynamics, in press. DOI: 10.1007/s00382-014-2231-4 Zhang, R.-H, Zhongxian Li, Jieshun Zhu, Xianbiao Kang, and Jinzhong Min, 2014: Impact of Tropical Instability Waves (TIWs)-induced SST Forcing on the Atmosphere in the Tropical Pacific: A Clean Evaluation Using CAM5.1, Atmos. Sci. Lett., 15: 186–194. DOI: 10.1002/asl2.488 Zhang, R.-H., F. Zheng, J. Zhu, and Z. G. Wang, 2013: A successful real-time forecast of the 2010–11 La Nina event. Sci. Rep. 3, 1108; DOI:10.1038/srep01108. Zhang, R.-H., 2013: Using satellite data to represent tropical instability waves (TIWs)-induced wind feedbacks in the Pacific Ocean, Remote Sensing, 5, 2660-2687; doi:10.3390/rs5062660. Zhang, R.-H., et al. 2013: Remote Effects of Tropical Cyclone (TC) Wind Forcing over the Western Pacific on the Eastern Equatorial Ocean, Adv. Atmos. Sci., 30: 1507-1525. Doi: 10.1007/s00376-013-2283-0 Zhang R.-H. and Zhanggui Wang, 2013: Model evidence for interdecadal pathway changes in the subtropics and tropics of the South Pacific Ocean. Adv. Atmos. Sci., 30: 1-9, doi: 10.1007/s00376-012-2048-1. Zhang, R.-H., F. Zheng, J. Zhu, Yuhua Pei, Quanan Zheng, and Zhanggui Wang 2012: Modulation of El Niño-Southern Oscillation by Freshwater Flux and Salinity Variability in the Tropical Pacific, Adv. Atmos. Sci., 29, 647-660, doi: 10.1007/s00376-012-1235-4. Zheng F, and R.-H. Zhang, 2012: Effects of Interannual Salinity Variability and Freshwater Flux Forcing on the Development of the 2007/08 La Niña Event Diagnosed from Argo, Dyn. Atmos. Oceans, 57, 45-57, DOI: 10.1016/j.bbr.2011.03.031. Zhu, J., B. Huang, L. Marx, J. L. Kinter III, M. A. Balmaseda, R.-H. Zhang, and Z. Z. Hu, 2012: Ensemble ENSO Hindcasts Initialized from Multiple Ocean Analyses, Geophys. Res. Lett., 39, L09602, doi:10.1029/2012GL051503. Zhu, J., G. Zhou, R.-H. Zhang, and Z. Sun, 2012: Improving ENSO prediction in a hybrid coupled Model with an embedded entrainment temperature parameterization. Int. J. Climatol., DOI: 10.1002/joc.3426. Zhu, J., B. Huang, L. Marx, J. L. Kinter III, M. A. Balmaseda, R.-H. Zhang, and Z. Z. Hu, 2012: Ensemble ENSO Hindcasts Initialized from Multiple Ocean Analyses, Geophys. Res. Lett., 39, L09602, 7 PP., 2012, doi:10.1029/2012GL051503. Hackert, E., J. Ballabrera-Poy, A. J. Busalacchi, R.-H. Zhang, and R. G. Murtugudde, 2011: Impact of Sea Surface Salinity Assimilation on Coupled Forecasts in the tropical Pacific. J. Geophys. Res., 116, C05009, doi:10.1029/2010JC006708. Zhang, R.-H., D. Chen, and G. Wang, 2011: Using satellite ocean color data to derive an empirical model for the penetration depth of solar radiation (Hp) in the tropical Pacific ocean, J. Atmos. Oceanic Technol., 28 (No. 7), 944-965. Zhu, J., G. Zhou, R.-H. Zhang, and Z. Sun, 2011: On the Role of Ocean Entrainment Temperature (Te) in Decadal Changes of El Niño/Southern Oscillation, Ann. Geophys., 29, 529-540, doi:10.5194/angeo-29-529-2011. Zheng, Q., C.-K. Tai, J. Hu, H. Lin, R.-H. Zhang, F.-C. Su, and X. Yang, 2011: Satellite altimeter observations of nonlinear Rossby eddy-Kuroshio interaction at the Luzon Strait. J. Oceanography, 67, No. 4, 365-376, DOI: 10.1007/s10872-011-0035-2. Zhang, R.-H., G. Wang, D. Chen, A. J. Busalacchi, and E. C. Hackert, 2010: Interannual biases induced by freshwater flux and coupled feedback in the tropical Pacific, Mon. Wea. Rev., 138, No. 5, 1715–1737.
