State analysis using the Local Ensemble Transform Kalman Filter (LETKF) and the three-layer circulation structure of the Luzon Strait and the South China Sea

Abstract

A new circulation model of the western North Pacific Ocean based on the parallelized version of the Princeton Ocean Model and incorporating the Local Ensemble Transform Kalman Filter (LETKF) data assimilation scheme has been developed. The new model assimilates satellite data and is tested for the period January 1 to April 3, 2012 initialized from a 24-year simulation to estimate the ocean state focusing in the South China Sea (SCS). Model results are compared against estimates based on the optimum interpolation (OI) assimilation scheme and are validated against independent Argo float and transport data to assess model skills. LETKF provides improved estimates of the western North Pacific Ocean state including transports through various straits in the SCS. In the Luzon Strait, the model confirms, for the first time, the three-layer transport structure previously deduced in the literature from sparse observations: westward in the upper and lower layers and eastward in the middle layer. This structure is shown to be robust, and the related dynamics are analyzed using the results of a long-term (18 years) unassimilated North Pacific Ocean model. Potential vorticity and mass conservations suggest a basin-wide cyclonic circulation in the upper layer of the SCS (z>−570 m), an anticyclonic circulation in the middle layer (−570 m ≥ z > −2,000 m), and, in the abyssal basin (< −2,000 m), the circulation is cyclonic in the north and anticyclonic in the south. The cyclone-anticyclone abyssal circulation is confirmed and explained using a deep-layer reducedgravity model as being caused by overflow over the deep sill of the Luzon Strait, coupled with intense, localized upwelling west of the strait.

On the near-inertial variations of meridional overturning circulation in the South China Sea

“…Based on field observations, studies confirm the hypothesis that the Luzon Strait transport (LST) has a sandwiched vertical structure, which shows a westward flow in the upper layer (< 500 m) and in the deeper layer (> 1500 m), and an eastward flow in the intermediate layer (500-1500 m; Tian et al, 2006;Yang et al, 2010). The corresponding circulation in the SCS is consistent with the potential vorticity constraint Price, 2000, 2007), which suggests that the mixing-induced circulation inside the SCS should be cyclonic gyres at the surface and at the bottom (Chao et al, 1996;Li and Qu, 2006;Wang et al, 2011;Lan et al, 2013;Xu and Oey, 2014), and an anti-cyclonic gyre at an intermediate depth (Isobe and Namba, 2001;Yuan, 2002). Note that the upper-layer SCS circulation is also affected by the seasonally reversing monsoon, exhibiting a cyclonic circulation over the whole SCS basin in winter, and in summer a strong anti-cyclonic circulation in the southern SCS and a weak cyclonic circulation in the northern SCS (Wrytki, 1961;Chu et al, 1999;Chu and Li, 2000;Qu, 2000;Hu et al, 2000;Liu et al, 2001;Wang et al, 2003;Su, 2004).…”

Editorial—the 7th International Workshop on Modeling the Ocean (IWMO 2015)

“…Combining surface observations with vertical profile data of temperature and salinity can lead to improved representation of mesoscale eddies in the region. Zheng et al (2016) presented an interesting modification of the 4DVAR data assimilation scheme in which the linear tangent equation is approximated by introducing perturbations to the parameters to be estimated and then running the ensemble models as in the ensemble Kalman filter method (Evensen 1994;Xu et al 2013;Xu and Oey 2014). In this way the adjoint equation is not required.…”

Editorial—the 5th International workshop on modeling the ocean (IWMO 2013)

“…Although the assimilation model does not beat persistence (from AVISO), the level of EKE is in better agreement with the drifter data. It will be interesting in the future to compare their results with those obtained using the Princeton Ocean Model (POM) and different assimilation schemes (e.g., Miyazawa et al 2012;Xu et al 2013;Xu and Oey 2014). Chang and Oey (2014) used a parallelized version of the POM to analyze eddies that are generated by the instability of the North Pacific Subtropical Counter Current (STCC).…”

“…There are seven papers on analysis of model outputs, data-assimilation or numerical methods (Backeberg et al 2014;Chang and Oey 2014;Dominicis et al 2014;Hjelmervik and Hjelmervik 2014;Miyama and Miyazawa 2014;Xu and Oey 2014;Wang et al 2014a). Three of the papers address air-sea interactions and upper layer physics (Broström et al 2014;Esau 2014;Troitskaya et al 2014).…”

“…Moreover, the flow near the seamount seems to be quite sensitive to the choice of the time stepping method. Xu and Oey (2014) used the local ensemble transformation Kalman Filter (LETKF) combined with the parallelized POM to the state estimation of the South China Sea (SCS) circulation. LETKF reasonably outperforms the optimum interpolation data assimilation scheme in representation of the observed features in the western North Pacific Ocean state including transports through various straits in SCS.…”