Shiqiu Peng

Impacts of nonbreaking wave‐stirring‐induced mixing on the upper ocean thermal structure and typhoon intensity in the South China Sea

To investigate the effect of nonbreaking wave-induced mixing caused by surface wave stirring on the upper ocean thermal structure (UOTS) and the typhoon intensity, a simple nonbreaking wave-stirring-induced mixing parameterization (WMP) scheme is incorporated into a regional coupled atmosphere-ocean model for the South China Sea (SCS), which couples the Princeton Ocean Model (POM) to the Weather Research and Forecasting (WRF) model using the OASIS3 coupler. The results of simulating two selected typhoon cases indicate that the nonbreaking wave-stirring-induced mixing has significant impacts on UOTS and the typhoon intensity, and the incorporation of the simple WMP scheme in the coupled model helps to improve the simulation of UOTS and thus the typhoon intensity. In the case that the typhoon intensity is underestimated by the atmosphere model alone, the improvement of initial UOTS by the ocean model with the WMP included can deepen the initial thermocline depth, reduce the effect of SST cooling, and prevent the typhoon intensity from undesired weakening. In the case that the typhoon intensity is overestimated (with strong winds), including the WMP in the ocean model significantly enhances the total vertical mixing rate in the upper ocean, which in turn enhances the SST cooling and thus reduces the typhoon intensity as desired. The results obtained in this study make a contribution to the ongoing efforts of improving the typhoon intensity forecast using a regional atmosphere-ocean coupled model by worldwide researchers and forecasters, especially for the typhoons in the SCS regions.

Toward a mesoscale hydrological and marine meteorological observation network in the South China Sea

Air-sea interaction in the South China Sea (SCS) has direct impacts on the weather and climate of its surrounding areas at various spatiotemporal scales. In situ observation plays a vital role in exploring the dynamic characteristics of the regional circulation and air-sea interaction. Remote sensing and regional modeling are expected to provide high-resolution data for studies of air-sea coupling; however, careful validation and calibration using in situ observations is necessary to ensure the quality of these data. Through a decade of effort, a marine observation network in the SCS has begun to be established, yielding a regional observatory for the air-sea synoptic system.Earlier observations in the SCS were scarce and narrowly focused. Since 2004, an annual series of scientific open cruises during late summer in the SCS has been organized by the South China Sea Institute of Oceanology (SCSIO), carefully designed based on the dynamic characteristics of the oceanic circulation and air-sea interaction in the SCS region. Since 2006, the cruise carried a radiometer and radiosondes on board, marking a new era of marine meteorological observation in the SCS. Fixed stations have been established for long-term and sustained records. Observations obtained through the network have been used to study regional ocean circulation and processes in the marine atmospheric boundary layer. In the future, a great number of multi-institutional, collaborative scientific cruises and observations at fixed stations will be carried out to establish a mesoscale hydrological and marine meteorological observation network in the SCS.

Tidal Mixing in the South China Sea: An Estimate Based on the Internal Tide Energetics

AbstractBy taking into account the contributions of both locally and remotely generated internal tides, the tidal mixing in the Luzon Strait (LS) and the South China Sea (SCS) is investigated through internal-tide simulation and energetics analysis. A three-dimensional nonhydrostatic high-resolution model driven by four primary tidal constituents (M2, S2, K1, and O1) is used for the internal-tide simulation. The baroclinic energy budget analysis reveals that the internal tides radiated from the LS are the dominant energy source for the tidal dissipation in the SCS. In the LS, the estimated depth-integrated turbulent kinetic energy dissipation exceeds O(1) W m−2 atop the two subsurface ridges, with a dissipation rate of >O(10−7) W kg−1 and diapycnal diffusivity of ~O(10−2) m2 s−1. In the SCS, the most intense turbulence occurs in the deep-water basin with a dissipation rate of O(10−8–10−6) W kg−1 and diapycnal diffusivity of O(10−3–10−1) m2 s−1 within the ~2000-m water column above the seafloor as well as ...

Numerical simulation of the structure and variation of upwelling off the east coast of Hainan Island using QuikSCAT winds

The spatial structure and variation of the upwelling in the waters east and northeast of Hainan Island, China during 2000–2007 were investigated using a nested high-resolution Princeton Ocean Model (POM) forced by QuikSCAT winds. The model produced good simulations of the summer upwelling and the seasonal and annual variability. Strong upwelling occurs from mid-July to mid-August with a peak east of Hainan Island associated with the southwesterly monsoon in the South China Sea. Sensitivity experiments indicated that when the local wind stress controls the variability of the upwelling, the large-scale circulation significantly enhances the upwelling northeast of Hainan Island by inducing a local upwelling and transporting cold water northeast-ward along the island’s east coast. The joint effects of the local wind stress and large-scale circulation result in stronger upwelling northeast of Hainan Island. This implies that the annual variation of the upwelling northeast of Hainan Island is controlled not only by the local alongshore wind stress but also by the large-scale circulation. This result will help us investigate the decadal variation of the upwelling in this region in the future.

Application of Scale-Selective Data Assimilation to Regional Climate Modeling and Prediction

A method referred to as scale-selective data assimilation (SSDA) is designed to inject the large-scale components of the atmospheric circulation from a global model into a regional model to improve regional climate simulations and predictions. The SSDA is implemented through the following procedure: 1) using a low-pass filter to extract the large-scale components of the atmospheric circulation from global analysis or model forecasts; 2) applying the filter to extract the regional-scale and the large-scale components of the atmospheric circulation from the regional model simulations or forecasts; 3) assimilating the large-scale circulation obtained from the global model into the corresponding component simulated by the regional model using the method of three-dimensional variational data assimilation (3DVAR) while maintaining the small-scale components from the regional model during the assimilation cycle; 4) combining the small-scale and the assimilated large-scale components as the adjusted forecasts by the regional climate model and allowing the two components to mutually adjust outside the data assimilation cycle. A case study of summer 2005 seasonal climate hindcasting for the regions of the Atlantic and the eastern United States indicates that the large-scale components from the Global Forecast System (GFS) analysis can be effectively assimilated into the regional model using the scale-selective data assimilation method devised in this study, resulting in an improvement in the overall results from the regional climate model.

Recent advances in regional air-sea coupled models

In this paper, we first briefly review the history of air-sea coupled models, and then introduce the current status and recent advances of regional air-sea coupled models. In particular, we discuss the core technical and scientific issues involved in the development of regional coupled models, including the coupling technique, lateral boundary conditions, the coupling with sea waves (ices), and data assimilation. Furthermore, we introduce the application of regional coupled models in numerical simulation and dynamical downscaling. Finally, we discuss the existing problems and future directions in the development of regional air-sea coupled models.

Retrieving density and velocity fields of the ocean's interior from surface data

Using the “interior + surface quasigeostrophic” (isQG) method, the density and horizontal velocity fields of the oceans interior can be retrieved from surface data. This method was applied to the Simple Ocean Data Assimilation (SODA) and the Hybrid Coordinate Ocean Model (HYCOM)/Navy Coupled Ocean Data Assimilation (NCODA) reanalysis data sets. The input surface data include sea surface height (SSH), sea surface temperature (SST), sea surface salinity (SSS), and a region-averaged stratification. The retrieved subsurface fields are compared with reanalysis data for three tested regions, and the results indicate that the isQG method is robust. The isQG method is particularly successful in the energetic regions like the Gulf Stream region with weak stratification, and the Kuroshio region with strong correlation between sea surface density (SSD) and SSH. It also works, though less satisfactorily, in the Agulhas leakage region. The performance of the isQG method in retrieving subsurface fields varies with season, and peaks in winter when the mixed layer is deeper and stratification is weaker. In addition, higher-resolution data may facilitate the isQG method to achieve a more successful reconstruction for the velocity retrieval. Our results suggested that the isQG method can be used to reconstruct the ocean interior from the satellite-derived SSH, SST, and SSS data in the near future.

Adaptive observation in the South China Sea using CNOP approach based on a 3‐D ocean circulation model and its adjoint model

This study investigates the effect of adaptive (or targeted) observation on improving the midrange (30 days) forecast skill of ocean state of the South China Sea (SCS). A region associated with the South China Sea Western Boundary Current (SCSWBC) is chosen as the “target” of the adaptive observation. The Conditional Nonlinear Optimal Perturbation (CNOP) approach is applied to a three-dimensional ocean model and its adjoint model for determining the sensitive region. Results show that the initial errors in the sensitive region determined by the CNOP approach have significant impacts on the forecast of ocean state in the target region; thus, reducing these initial errors through adaptive observation can lead to a better 30 day prediction of ocean state in the target region. Our results suggest that implementing adaptive observation is an effective and cost-saving way to improve an ocean models forecast skill over the SCS.

Characteristics of the Near-Surface Currents in the Indian Ocean as Deduced from Satellite-Tracked Surface Drifters. Part I: Pseudo-Eulerian Statistics

Using the 1985‐2013 record of near-surface currents from satellite-tracked drifters, the pseudo-Eulerian statistics of the near-surface circulation in the Indian Ocean (IO) are analyzed. It is found that the distributions of the current velocities and mean kinetic energy (MKE) in the IO are extremely inhomogeneous in space and nonstationary in time. The most energetic regions with climatologic mean velocity over 50cms 21 and MKE over 500cm 2 s 22 are found off the eastern coast of Somalia (with maxima of over 100cms 21 and 1500cm 2 s 22 ) and the equatorial IO, associated with the strong, annually reversing Somalia Current and the twice-a-year eastward equatorial jets. High eddy kinetic energy (EKE) is found in regions of the equatorial IO, western boundary currents, and Agulhas Return Current, with a maximum of over 3000cm 2 s 22 off the eastern coast of Somalia. The lowest EKE (,500cm 2 s 22 ) occurs in the south subtropical gyre between 308 and 408S and the central-eastern Arabian Sea. Annual and semiannual variability is a significant fraction of the total EKE off the eastern coast of Somalia and in the central-eastern equatorial IO. In general, both the MKE and EKE estimated in the present study are qualitatively in agreement with, but quantitatively larger than, estimates from previous studies. These pseudo-Eulerian MKE and EKE fields, based on the most extensivedrifterdatasettodate,arethemostpreciseinsituestimatestodateandcanbeusedtovalidatesatellite and numerical results.

East Hainan upwelling fronts detected by remote sensing and modelled in summer

Using the Belkin and O’Reilly algorithm and high-resolution (1 km) satellite sea surface temperature (SST) and chlorophyll-a (chl-a) data from 2002 to 2011, fronts were detected off the east/northeast coast of Hainan Island, South China Sea. These fronts were mainly produced by upwelling off eastern Hainan Island, through which cold, high-salinity, high-density, and nutrient-rich bottom water was brought to the surface and subsurface and then transported to the northeast of Hainan Island by the along-shore currents. The fronts are anisotropic, with a dominant orientation SSW–NNE. A three-dimensional ocean model forced by the Quick Scatterometer (QuikSCAT) winds was employed to study the three-dimensional structure of these fronts as well as the relationship between the fronts and upwelling or summer monsoon. The results show that the front intensity (cross-frontal gradient) is strongly correlated with the along-shore local winds, and has a strong seasonal and a weak inter-annual variation with a maximum of about 0.5°C km–1 at the subsurface (about 15 m) rather than the surface.