Youfang Yan

Effect of subtropical mode water on the decadal variability of the subsurface transport through the Luzon Strait in the western Pacific Ocean

Analysis of the 62 year hindcast outputs from an eddy-resolving ocean general circulation model shows a good correspondence of the Luzon Strait subsurface transport to the Pacific Decadal Oscillation (PDO) index on a decadal time scale, with the latter leading by about 5 years. The backward particle tracing experiments indicate that the part of the subsurface water in the Luzon Strait generated by the subduction processes comes from the Subtropical Mode Water (STMW). The model results also show a strong PDO signal in the subduction rate, as well as the subsurface low potential vorticity (PV), in the formation region of the STMW, and these decadal signals can be traced all the way to the Luzon Strait as PV anomalies follow the subtropical gyre circulation in about 5 years. The PV anomalies from the STMW affect the subsurface net transport in the Luzon Strait through changing the subsurface density structure and then zonal velocity.

Entropy budget of the ocean system

An entropy balance equation including the transfers of heat and mass, work of external force of the ocean is presented and discussed. Entropy flux through the sea surface are calculated, and the results show that entropy flux due to heat transfer is about - 555.6 mWm(-2) K-1, entropy flux connected with the work of wind stress is relatively smaller at about - 0.09 mWm(-2) K-1, entropy flux due to mass transfer is - 0.02 mWm(-2) K-1, the sum of all the entropy flux is - 555.7 mWm(-2) K-1, the mean rate of generation of entropy inside the ocean must be 555.7 mWm(-2)K(-1) when the ocean is in a climatic steady condition.

The impact of mean dynamic topography on a sea-level anomaly assimilation in the South China Sea based on an eddy-resolving model

The sea-level anomaly (SLA) from a satellite altimeter has a high accuracy and can be used to improve ocean state estimation by assimilation techniques. However, the lack of an accurate mean dynamic topography (MDT) is still a bothersome issue in an ocean data assimilation. The previous studies showed that the errors in MDT have significant impacts on assimilation results, especially on the time-mean components of ocean states and on the time variant parts of states via nonlinear ocean dynamics. The temporal-spatial differences of three MDTs and their impacts on the SLA analysis are focused on in the South China Sea (SCS). The theoretical analysis shows that even for linear models, the errors in MDT have impacts on the SLA analysis using a sequential data assimilation scheme. Assimilation experiments, based on EnOI scheme and HYCOM, with three MDTs from July 2003 to June 2004 also show that the SLA assimilation is very sensitive to the choice of different MDTs in the SCS with obvious differences between the experimental results and observations in the centre of the SCS and in the vicinity of the Philippine Islands. A new MDT for assimilation of SLA data in the SCS was proposed. The results from the assimilation experiment with this new MDT show a marked reduction (increase) in the RMSEs (correlation coefficient) between the experimental and observed SLA. Furthermore, the subsurface temperature field is also improved with this new MDT in the SCS.

The effects of oceanic barrier layer on the upper ocean response to tropical cyclones

Previous studies of the barrier layer (BL) and tropical cyclone (TC) intensification show controversial results. Some studies suggest that the BL is favorable for TC intensification, whereas others demonstrate that the BL has little impact. The present paper reconciles these previous results by showing that the influence of the BL on TC intensification is complex and dependent on the stages, TC intensity, TC forcing time, and ocean stratification. The influence can be divided into three different stages. First, when a TC is weak or the TC-induced forcing cannot break through the mixed layer (ML), the presence of the BL leads to a thin ML, results in a large ML cooling, and reduces the effective TC heat potential (ETCHP) by distributing the air-sea heat loss to the thin ML, thus suppressing TC growth. Second, when TC strengthens or wind forcing increases, the forcing becomes strong enough to break through the ML and penetrates into the BL. The entrainment of BL water, which is warmer than the cooled ML, compensates the surface heat loss and leads to an increase of the ML temperature, supporting TC growth. Third, when the TC-induced forcing is strong enough to break through the BL base and penetrates to the thermocline, the BL reduces the magnitudes of cooling temperature and of decreasing ETCHP compared to those without the BL, which is also favorable for TC intensification. Thus, whether the BL intensifies TCs depends on the conditions associated with the upper layer ocean and TCs.

Performances of Seven Datasets in Presenting the Upper Ocean Heat Content in the South China Sea

In this study, the upper ocean heat content (OHC) variations in the South China Sea (SCS) during 1993–2006 were investigated by examining ocean temperatures in seven datasets, including World Ocean Atlas 2009 (WOA09) (climatology), Ishii datasets, Ocean General Circulation Model for the Earth Simulator (OFES), Simple Ocean Data Assimilation system (SODA), Global Ocean Data Assimilation System (GODAS), China Oceanic ReAnalysis system (CORA), and an ocean reanalysis dataset for the joining area of Asia and Indian-Pacific Ocean (AIPO1.0). Among these datasets, two were independent of any numerical model, four relied on data assimilation, and one was generated without any data assimilation. The annual cycles revealed by the seven datasets were similar, but the interannual variations were different. Vertical structures of temperatures along the 18°N, 12.75°N, and 120°E sections were compared with data collected during open cruises in 1998 and 2005–08. The results indicated that Ishii, OFES, CORA, and AIPO1.0 were more consistent with the observations. Through systematic comparisons, we found that each dataset had its own shortcomings and advantages in presenting the upper OHC in the SCS.

Entropy budget of the earth, atmosphere and ocean system

The energy budget in the system of the earth, atmosphere and ocean conforms to the first law of thermodynamics, namely the law of conservation of energy, and it is balanced when the system is in a steady-state condition. However, the entropy budget following the second law of thermodynamics is unbalanced. In this paper, we deduce the expressions of entropy flux and re-estimate the earth, atmosphere and ocean annual mean entropy budget with the updated climatologically global mean energy budget and the climatologically air-sea flux data. The calculated results show that the earth system obtains a net influx of negative entropy (-1179. 3 mWm(-2)K(-1)) from its surroundings, and the atmosphere and the ocean systems obtain a net input of negative entropy at about -537.4 mWm(-2)K(-1) and -555.6 mWm(-2)K(-1), respectively. Calculations of the entropy budget can provide some guidance for further understanding the spatial-temporal change of the local entropy flux, and the entropy production resulting from all kinds of irreversible processes inside these systems.