Hongli Fu

Improvement of short-term forecasting in the northwest Pacific through assimilating Argo data into initial fields

The impact of assimilating Argo data into an initial field on the short-term forecasting accuracy of temperature and salinity is quantitatively estimated by using a forecasting system of the western North Pacific, on the base of the Princeton ocean model with a generalized coordinate system (POMgcs). This system uses a sequential multigrid three-dimensional variational (3DVAR) analysis scheme to assimilate observation data. Two numerical experiments were conducted with and without Argo temperature and salinity profile data besides conventional temperature and salinity profile data and sea surface height anomaly (SSHa) and sea surface temperature (SST) in the process of assimilating data into the initial fields. The forecast errors are estimated by using independent temperature and salinity profiles during the forecasting period, including the vertical distributions of the horizontally averaged root mean square errors (H-RMSEs) and the horizontal distributions of the vertically averaged mean errors (MEs) and the temporal variation of spatially averaged root mean square errors (S-RMSEs). Comparison between the two experiments shows that the assimilation of Argo data significantly improves the forecast accuracy, with 24% reduction of H-RMSE maximum for the temperature, and the salinity forecasts are improved more obviously, averagely dropping of 50% for H-RMSEs in depth shallower than 300 m. Such improvement is caused by relatively uniform sampling of both temperature and salinity from the Argo drifters in time and space.

The Reanalysis of Currents and Throughflow Volume Transport in the Taiwan Strait

Currents and throughflow transport in the Taiwan Strait (TS) are systematically analyzed using a new regional ocean reanalysis dataset (China Ocean ReAnalysis, CORA in brief), recently released by National Marine Data and Information Service (NMDIS) in 2009, covering Chinas coastal waters and adjacent seas. The total throughflow volume transport through TS presents a seasonal variability with the annual-mean of 1.49Sv. It reaches maximum in summer with mean value of 2.33Sv, while in winter the mean transport falls to 0.78Sv on average. The estimated throughflow transport is in good agreement with the observational results. The comparisons of the results with historical observations and numerical simulations also show that CORA performs well on reproducing the currents and temperature field in TS region. In addition, CORA not only successfully reproduced the known oceanic phenomena in TS such as temperature front, up-wind flow in fall, winter northward flow blocking in Penghu Channel, and the winter anticyclonic eddy located at north of Changyun Rise (CYR), but it also found a new cyclonic eddy in north TS in winter.

Impact of sea spray on upper ocean temperature during typhoon passage: simulation with a 1-D turbulent model

At the interface between the lower atmosphere and sea surface, sea spray might significantly influence air-sea heat fluxes and subsequently, modulate upper ocean temperature during a typhoon passage. The effects of sea spray were introduced into the parameterization of sea surface roughness in a 1-D turbulent model, to investigate the effects of sea spray on upper ocean temperature in the Kuroshio Extension area, for the cases of two real typhoons from 2006, Yagi and Soulik. Model output was compared with data from the Kuroshio Extension Observatory (KEO), and Reynolds and AMSRE satellite remote sensing sea surface temperatures. The results indicate drag coefficients that include the spray effect are closer to observations than those without, and that sea spray can enhance the heat fluxes (especially latent heat flux) considerably during a typhoon passage. Consequently, the model results with heat fluxes enhanced by sea spray simulate better the cooling process of the SST and upper-layer temperature profiles. Additionally, results from the simulation of the passage of typhoon Soulik (that passed KEO quickly), which included the sea spray effect, were better than for the simulated passage of typhoon Yagi (that crossed KEO slowly). These promising 1-D results could provide insight into the application of sea spray in general circulation models for typhoon studies.

A Study of Transport and Impact Strength of Fukushima Nuclear Pollutants in the North Pacific Surface

Based on the statistics of surface drifter data of 1979–2011 and the simulation of nuclear pollutant particulate movements simulated using high quality ocean reanalysis surface current dataset, the transport pathways and impact strength of Fukushima nuclear pollutants in the North Pacific have been estimated. The particulates are used to increase the sampling size and enhance the representativeness of statistical results. The trajectories of the drifters and particulates are first examined to identify typical drifting pathways. The results show that there are three types of transport paths for nuclear pollutants at the surface: 1) most pollutant particles move eastward and are carried by the Kuroshio and Kuroshio-extension currents and reach the east side of the North Pacific after about 3.2–3.9 years; 2) some particles travel with the subtropical circulation branch and reach the east coast of China after about 1.6 years according to one drifter trajectory and about 3.6 years according to particulate trajectories; 3) a little of them travel with local, small scale circulations and reach the east coast of China after about 1.3–1.8 years. Based on the particulates, the impact strength of nuclear pollutants at these time scales can be estimated according to the temporal variations of relative concentration combined with the radioactive decay rate. For example, Cesium-137, carried by the strong North Pacific current, mainly accumulates in the eastern North Pacific and its impact strength is 4% of the initial level at the originating Fukushima area after 4 years. Due to local eddies, Cesium-137 in the western North Pacific is 1% of the initial pollutant level after 1.5 years and continuously increases to 3% after 4 years. The vertical movement of radioactive pollutants is not taken into account in the present study, and the estimation accuracy would be improved by considering three-dimensional flows.

Tropical cyclone footprint in the ocean mixed layer observed by Argo in the Northwest Pacific

This study systematically investigated the ocean mixed layer responses to tropical cyclone (TC) using available Argo profiles during the period of 1998–2011 in the northwest Pacific. Results reveal that isothermal layer (IL) deepening and isothermal layer (IL) cooling with evident rightward biases induced by strong TCs are clearer compared to the weak TCs. Likewise, the rightward biases of IL deepening and cooling induced by fast TCs are more obvious than that induced by slow TCs. The upwelling within TCs eye is much stronger for the strong (slow) TCs than weak (fast) TCs. For the strong and slow TCs, the TC-induced rainfall reduces deepening of constant density layer (with its depth called the mixed layer depth, MLD), and in turn increases the barrier layer thickness (BLT). The initial BL prior to TC can restrict IL cooling more markedly under the weak and fast TCs than under the strong and slow TCs. The inertial oscillation is stronger induced by the strong (fast) TCs than by the weak (slow) TCs. In addition, the most pronounced TC-induced mixed layer deepening and IL cooling in July to October climatology occur in the subtropical gyre of the northwest Pacific with enhanced vertical diffusivity. The maximum increase of isothermal layer depth (ILD) and MLD is up to 5 m, with IL cooling up to 0.4°C.

Climate modulation on sea surface height in China seas

The climate modulation on the sea surface height (SSH) in China seas is investigated using a China Ocean Reanalysis (CORA) dataset from 1958–2008. The dataset is constructed by assimilating the temperature/salinity profiles derived from the satellite altimetry data and historical observational temperature/salinity profiles. Based on the Empirical Orthogonal Function (EOF), the CORA sea surface height anomaly (SSHa) is decomposed, and the interannual and decadal variability of the first three leading modes are analyzed. On the interannual timescale, the first principal component (PC1) is significant positively correlated with the El Niño/Southern Oscillation (ENSO). On the decadal timescale, North Pacific Gyre Oscillation (NPGO) has significant negative correlation with PC1 whereas Pacific Decadal Oscillation (PDO) is in phase with PC3. Analysis shows that the decadal variability of SSH is mainly modulated by the wind stress curl variability related to the NPGO and PDO. In addition, the effect of net heat flux associated to the NPGO and PDO on SSH is also investigated, with net heat flux variability in the Luzon strait and tropic Pacific found to influence the decadal variability of SSH.

An ensemble estimation of impact times and strength of Fukushima nuclear pollution to the east coast of China and the west coast of America

Based on the statistics of all surface drifting buoys of 1978–2011 and Lagrangian tracers simulated from high quality ocean reanalysis currents, the impact times and strength of Fukushima nuclear pollution to the east coast of China and the west coast of America have been estimated. Under the circumstances of the radioactive pollutants drifting in the ocean surface, preliminary research results show that while the tracers took about 4 years to reach the west coast of USA, there are two types of tracers to carry out Fukushima nuclear pollutants to reach the east coast of China, corresponding to 1.5-year recirculation gyre transport and 3.5-year subtropical circulation transport. The distributions of the impact strength at these time scales are given according to the variation of relative number concentration with time combined with the decaying rate of radioactive matter. For example, starting from 1% at 1.5-year, of the initial level at the originating area of Fukushima nuclear pollution, the impact strength of Cesium-137 in the South China Sea continuously increases up to 3% by 4 years, while the impact strength of Cesium-137 in the west coast of America is as high as 4% due to the role of strong Kuroshio-extension currents as a major transport mechanism of nuclear pollutants for that area.

Impact of sea spray on the Yellow and East China Seas thermal structure during the passage of Typhoon Rammasun (2002)

Strong winds lead to large amounts of sea spray in the lowest part of the atmospheric boundary layer. The spray droplets affect the air-sea heat fluxes due to their evaporation and the momentum due to the change of sea surface, and in turn change the upper ocean thermal structure. In this study, impact of sea spray on upper ocean temperatures in the Yellow and East China Seas (YES) during typhoon Rammasuns passage is investigated using the POMgcs ocean model with a sea spray parameterization scheme, in which the sea spray-induced heat fluxes are based on an improved Fairalls sea spray heat fluxes algorithm, and the sea spray-induced momentum fluxes are derived from an improved COARE version 2.6 bulk model. The distribution of the sea spray mediated turbulent fluxes was primarily located at Rammasun eye-wall region, in accord with the maximal wind speeds regions. When Rammasun enters the Yellow sea, the sea spray mediated latent (sensible) heat flux maximum is enhanced by 26% (13.5%) compared to that of the interfacial latent (sensible) heat flux. The maximum of the total air-sea momentum fluxes is enhanced by 43% compared to the counterpart of the interfacial momentum flux. Furthermore, the sea spray plays a key role in enhancing the intensity of the typhoon-induced “cold suction” and “heat pump” processes. When the effect of sea spray is considered, the maximum of the sea surface cooling in the right side of Rammasuns track is increased by 0.5°C, which is closer to the available satellite observations.

Reconstruction of Typhoon Structure Using 3-Dimensional Doppler Radar Radial Velocity Data with the Multigrid Analysis: A Case Study in an Idealized Simulation Context

Extracting multiple-scale observational information is critical for accurately reconstructing the structure of mesoscale circulation systems such as typhoon. The Space and Time Mesoscale Analysis System (STMAS) with multigrid data assimilation developed in Earth System Research Laboratory (ESRL) in National Oceanic and Atmospheric Administration (NOAA) has addressed this issue. Previous studies have shown the capability of STMAS to retrieve multiscale information in 2-dimensional Doppler radar radial velocity observations. This study explores the application of 3-dimensional (3D) Doppler radar radial velocities with STMAS for reconstructing a 3D typhoon structure. As for the first step, here, we use an idealized simulation framework. A two-scale simulated “typhoon” field is constructed and referred to as “truth,” from which randomly distributed conventional wind data and 3D Doppler radar radial wind data are generated. These data are used to reconstruct the synthetic 3D “typhoon” structure by the STMAS and the traditional 3D variational (3D-Var) analysis. The degree by which the “truth” 3D typhoon structure is recovered is an assessment of the impact of the data type or analysis scheme being evaluated. We also examine the effects of weak constraint and strong constraint on STMAS analyses. Results show that while the STMAS is superior to the traditional 3D-Var for reconstructing the 3D typhoon structure, the strong constraint STMAS can produce better analyses on both horizontal and vertical velocities.

Assessment of intraseasonal variabilities in China Ocean Reanalysis (CORA)

A regional reanalysis product—China Ocean Reanalysis (CORA)—has been developed for the China’s seas and the adjacent areas. In this study, the intraseasonal variabilities (ISVs) in CORA are assessed by comparing with observations and two other reanalysis products (ECCO2 and SODA). CORA shows a better performance in capturing the intraseasonal sea surface temperatures (SSTs) and the intraseasonal sea surface heights (SSHs) than ECCO2 and SODA do, probably due to its high resolution, stronger response to the intraseasonal forcing in the atmosphere (especially the Madden-Julian Oscillation), and more available regional data for assimilation. But at the subsurface, the ISVs in CORA are likely to be weaker than reality, which is probably attributed to rare observational data for assimilation and weak diapycnal eddy diffusivity in the CORA model. According to the comparison results, CORA is a good choice for the study related to variabilities at the surface, but cares have to be taken for the study focusing on the subsurface processes.