A successful simulation of the western North Pacific summer monsoon needs a regional ocean–atmosphere coupled model(ROAM). How the performance of ROAM relies on the oceanic component model remains unknown. In this study, the authors investigated the effects of different oceanic components on the simulation of western North Pacific(WNP) summer monsoon in a ROAM. Three cases of simulations were performed, viz. the summer of 1998(El Nin o decaying phase), 2004(El Nin o developing phase), and 1993(the non-ENSO phase). Results show that the coupled simulations for different ENSO phases exhibit improvements in the simulation of location of Meiyu rainband and spatial distribution of monsoon low-level flow over WNP, whereas the systemic cold biases of sea surface air temperature are further increased. The coupled simulations with different oceanic components show similar performance, which is not ENSO phase dependent. For the case of the summer of 1998, a slightly stronger western Pacific subtropical high and colder sea surface air temperature are found in the simulation with colder sea surface temperature(SST) biases. The colder SST biases are partly contributed by the ocean dynamics processes because the sea surface net flux favors a warmer SST. This study suggests that the dependence of performance of ROAM over WNP on oceanic models is much weaker than that on atmospheric models.
By applying a global high-resolution （0.1°） OGCM, the influence of the island chains in the Luzon Strait （LS） on the Kuroshio intrusion is studied systematically. The island chains in the LS are separated into three parts： the south island chain, the middle and north island chain, and Babuyan Island. One control and three sensitivity experiments are conducted by adding these three parts of the topography gradually. From comparisons of the circulation, temperature, and salinity structures, it is found that the south island chain decreases the westward bending of the main Kuroshio path, the middle and north island chain increases the westward bending, and Babuyan Island also increases the westward bending. These results are extremely clear in winter. Dynamic diagnoses suggest that the westward bending increases with an increase in the incidence angle of the Kuroshio and an increase in the Kuroshio east branch transport. Moreover, the middle and north island chain can split the Kuroshio into two parts, the Kuroshio west and east branches, which can be seen clearly in the satellite altimeter maps.
A double index （DI）, which is made up of two sub-indices, is proposed to describe the spatial patterns of the Kuroshio intrusion and mesoscale eddies west to the Luzon Strait, based on satellite altimeter data. The area-integrated negative and positive geostrophic vorticities are defined as the Kuroshio warm eddy index （KWI） and the Kuroshio cold eddy index （KCI）, respectively. Three typical spatial patterns are identified by the DI： the Kuroshio warm eddy path （KWEP）, the Kuroshio cold eddy path （KCEP）, and the leaking path. The primary features of the DI and three patterns are further investigated and compared with previous indices. The effects of the integrated area and the algorithm of the integration are investigated in detail. In general, the DI can overcome the problem of previously used indices in which the positive and negative geostrophic vorticities cancel each other out. Thus, the proportions of missing and misjudged events are greatly reduced using the DI. The DI, as compared with previously used indices, can better distinguish the paths of the Kuroshio intrusion and can be used for further research.
The performance of the eddy-resolving LICOM2.0 in simulating the Indonesian Throughflow has been evaluated against the INSTANT data in the present study.The mean vertical structures of the along strait velocities are simulated well in LICOM2.0,but the large velocities at the bottom of the Lifamatola Passage and the Timor Passage cannot be reproduced by LICOM2.0.The causes are considered to be both the errors in the topography and the tidal mixing at the bottom.Despite several biases in the mean velocities,the mean inflow and outflow volume transports in LICOM2.0 are almost identical to the INSTANT data.Compared with the lower resolution LICOM,the most significant improvement is the better simulation of the partitions of the inflow and outflow transports in individual straits.The outflow for low-resolution LICOM is mainly through the Ombai and Lombok Strait,whereas that for LICOM2.0 is mainly through the Timor Passage.The variability of the vertical structure of velocities and the volume transport are also investigated.LICOM2.0 overestimates the magnitude of the upper-layer currents and the amplitude of the variation.We also found that the largest correlation coefficient occurs in the shallowest strait,the Lombok,whereas the lowest occurs in the Timor Passage,especially in the upper layer.The latter may be caused by the unrealistic transport through the Torres Strait in LICOM2.0.
The phenomenon of ENSO asymmetry has been recognized for many years, but most studies have focused on the asymmetry of surface temperature anomalies in the equatorial eastern Pacific. Here, the authors investigate the temperature asymmetry associated with ENSO in the subsurface of the western Pacific through analysis of observations and numerical experiments with an ocean GCM. Both the observation and simulation exhibit significant ENSO asymmetry, characterized by negative temperature skewness in the equatorial western Pacific and positive skewness in the eastern Pacific. Heat budget analysis reveals that nonlinear dynamical heating results in the positive temperature asymmetry in the equatorial eastern Pacific, but tends to weaken the negative temperature asymmetry in the equatorial western Pacific. The climatological meridional current transports the temperature anomalies and corresponding negative asymmetry from the off-equator region to the equator in the subsurface of the western Pacific. Through a sensitivity experiment with reversed wind stress forcing, the authors suggest that the skewness of the wind stress anomalies does not contribute to the negative temperature asymmetry in the western Pacific in the first-order approximation, while the internal nonlinear dynamics does play a key role. The study suggests that, as a result of nonlinear processes, the oceanic responses to anomalous wind stress are nonlinear and asymmetric in the tropical Pacific.
The present study compares the performance of two versions of the LASG/IAP(State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics) Climate System Ocean Model(LICOM) in reproducing the interannual variability associated with El Nio and La Nia events in the tropical Pacific. Both versions are forced with the identical boundary conditions from observed or reanalysis data, in which one version has a finer spatial resolution of(1/10)° in the horizontal domain and 55 vertical layers, and the other version has a coarse resolution of 1° in the horizontal domain and 30 vertical layers. ENSO simulations form the two versions are compared with observations and, in particular, the improvements with regard to ENSO by the finer resolution ocean model are emphasized. As a result of the finer spatial resolution, both the vertical temperature gradient and vertical velocity are better represented in the equatorial Pacific than they are by the coarse resolution model; and thus, the corresponding vertical advections of temperature are more reasonable. Besides the mean climatology, simulated ENSO events and relevant feedbacks are much improved in the finer resolution model. A heat budget analysis suggests that both thermocline feedback and Ekman feedback are mainly responsible for the rapid increase in temperature anomalies during the developing and mature phases of ENSO events.
