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Changlin Chen and Guihua Wang

Abstract

The annual cycle of sea surface temperature (SST) in the North Pacific Ocean is examined in terms of its response to global warming based on climate model simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5). As the global ocean warms up, the SST in the North Pacific generally tends to increase and the warming is greater in summer than in winter, leading to a significant intensification of SST annual cycle. The mixed layer temperature equation is used to examine the mechanism of this intensification. Results show that the decrease of mixed layer depth (MLD) in summer is the main reason behind the intensification of SST annual cycle. Because the MLD in summer is much shallower than that in winter, the incoming net heat flux is trapped in a thinner surface layer in summer, causing a warmer summer SST and the amplification of SST annual cycle. The change of the SST annual cycle in the North Pacific may have profound ecological impacts.

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Zheng Ling, Yuqing Wang, and Guihua Wang

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The South China Sea (SCS) is affected by two intraseasonal components in summer: the Madden–Julian oscillation (MJO) and the quasi-biweekly oscillation (QBWO). In the present study, the impacts of the MJO and QBWO on tropical cyclones (TCs) locally formed in the SCS (local TCs) in summer are investigated. The results show that both the MJO and QBWO can affect the genesis frequency, location, and motion of the local TCs. More TCs form in the convectively active phases of the MJO and QBWO in the northern SCS. With the northward propagation of the MJO and QBWO convective signals, the major TC genesis location also shifts northward. Since the western Pacific subtropical high shifts eastward (westward) when convection associated with the MJO and QBWO in the northern SCS is enhanced (suppressed), the steering flow in the major TC genesis region is favorable for the eastward (westward) movement of TCs. Results from the composite analysis of the steering flow indicate that both the MJO and QBWO play an important role in controlling the motion of the eastward-moving TCs.

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Guihua Wang, Chunzai Wang, and Rui Xin Huang

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Based on the Simple Ocean Data Assimilation (SODA) dataset and three types of Sverdrup streamfunction, an interdecadal variability of the eastward current in the middle South China Sea (SCS) during summer is identified. Both the pattern and strength of the summer Asian monsoon wind stress curl over the SCS contribute to the interdecadal variability of this current. From 1960 to 1979, the monsoon intensified and the zero wind stress curl line shifted southward. Both the core of positive wind stress curl in the northern SCS and the negative curl in the southern SCS moved southward and thus induced a southward shift of both the southern anticyclonic and northern cyclonic gyres, resulting in a southward displacement of the eastward current associated with these two gyres. In the meantime, the southern (northern) SCS anticyclonic (cyclonic) ocean gyre weakened (strengthened) and therefore also induced the southward shift of the eastward current near the intergyre boundary. In contrast, the eastward current shifted northward from 1980 to 1998 because the monsoon relaxed and the zero wind stress curl line shifted northward. After 1998, the eastward jet moved southward again as the zero wind stress curl line shifted southward and the SCS monsoon strengthened. The eastward current identified from the baroclinic streamfunction moved about 1.7° more southward than that from the barotropic streamfunction, indicating that the meridional position of the eastward current is depth dependent.

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Rong-Hua Zhang, Dake Chen, and Guihua Wang

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Satellite-based ocean color measurements indicate clear evidence for bioclimate interactions in the tropical Pacific associated with El Niño–Southern Oscillation (ENSO). Recent modeling studies have demonstrated that ocean biology can potentially affect the climate through the penetration depth of solar radiation in the upper ocean (Hp), a primary parameter in coupling biology to physics in the ocean. At present, interannual variability in Hp and its related bioclimate feedback effects have not been adequately represented in coupled ocean–atmosphere models. In this work, chlorophyll (Chl) concentration data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), available since 1997, are used to characterize interannual Hp variability in the tropical Pacific and to quantify its relationships with physical fields, including sea surface temperature (SST) and sea level (SL). It is found that interannual Hp variability is dominated by ENSO signals, with the largest variability located in the central basin near the date line and a coherent relationship with SST. A singular value decomposition (SVD) analysis is adopted to extract interannual covariability patterns between Hp and SST during the period 1997–2007. Their close relationships are then utilized to construct an empirical anomaly model for Hp, allowing for its prognostic estimate in terms of SST anomalies without explicit involvement of a marine ecosystem model. Validation and sensitivity experiments indicate that the empirical model can reasonably well capture interannual Hp responses to SST anomalies in association with ENSO. The derived empirical Hp model offers a simple and an effective way to parameterize and represent the effects of Chl containing biomass on penetrative solar radiation in the tropical Pacific, demonstrating the dynamical implication of remotely sensed Chl data for bioclimate coupling studies. Further improvements and applications of the empirical Hp model to climate modeling are discussed.

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Guihua Wang, Dake Chen, and Jilan Su

Abstract

Generation of mesoscale eddies in the eastern South China Sea (SCS) in winters during August 1999 to July 2002 is studied with a reduced-gravity model. It is found that the orographic wind jets associated with the northeast winter monsoon and the gaps in the mountainous island chain along the eastern boundary of the SCS can spin up cyclonic and anticyclonic eddies over the SCS. Results suggest that direct wind forcing could be an important generation mechanism for the rich eddy activity in the SCS, and that to simulate this mechanism the resolution of the wind forcing has to be high enough to resolve the local wind jets induced by orographic effects.

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Peter C. Chu, Wang Guihua, and Yuchun Chen

Abstract

The autocorrelation functions of temperature and salinity in the three basins (Ulleung, Japan, and Yamato Basins) of the Japan/East Sea are computed using the U.S. Navy's Master Oceanographic Observational Dataset for 1930–97. After quality control the dataset consists of 93 810 temperature and 50 349 salinity profiles. The decorrelation scales of both temperature and salinity were obtained through fitting the autocorrelation function into the Gaussian function. The signal-to-noise ratios of temperature and salinity for the three basins are usually larger than 2. The signal-to-noise ratio of temperature is greater in summer than in winter. There is more noise in salinity than in temperature. This might be caused by fewer salinity than temperature observations. The autocorrelation functions of temperature for the three basins have evident seasonal variability at the surface: less spatial variability in the summer than in the winter. The temporal (spatial) decorrelation scale is shorter (longer) in the summer than in the winter. Such a strong seasonal variability at the surface may be caused by the seasonal variability of the net surface heat flux. The autocorrelation functions of salinity have weaker seasonal variability than those of the temperature field. The temporal and horizontal decorrelation scales obtained in this study are useful for designing an optimal observational network.

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Zhumin Lu, Guihua Wang, and Xiaodong Shang

Abstract

The three-dimensional responses of a cyclonic ocean eddy (COE) for 1–2 months following a typhoon were investigated using altimeter data and numerical experiments. Two significant features were found: 1) the cyclonic eddy was enhanced, and the three-dimensional structure was changed, and 2) the cyclonic eddy underwent two processes: elliptical deformation and reaxisymmetrization in the horizontal plane. These two features are generally associated with typhoon-induced upwelling and the dynamic processes of eddy adjustment. These results imply that the local ocean processes can be affected by a typhoon through low-frequency response.

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Qinbiao Ni, Xiaoming Zhai, Guihua Wang, and David P. Marshall

Abstract

In this study we track and analyze eddy movement in the global ocean using 20 years of altimeter data and show that, in addition to the well-known westward propagation and slight polarity-based meridional deflections, mesoscale eddies also move randomly in all directions at all latitudes as a result of eddy–eddy interaction. The speed of this random eddy movement decreases with latitude and equals the baroclinic Rossby wave speed at about 25° of latitude. The tracked eddies are on average isotropic at mid- and high latitudes, but become noticeably more elongated in the zonal direction at low latitudes. Our analyses suggest a critical latitude of approximately 25° that separates the global ocean into a low-latitude anisotropic wavelike regime and a high-latitude isotropic turbulence regime. One important consequence of random eddy movement is that it results in lateral diffusion of eddy energy. The associated eddy energy diffusivity, estimated using two different methods, is found to be a function of latitude. The zonal-mean eddy energy diffusivity varies from over 1500 m2 s−1 at low latitudes to around 500 m2 s−1 at high latitudes, but significantly larger values are found in the eddy energy hotspots at all latitudes, in excess of 5000 m2 s−1. Results from this study have important implications for recently developed energetically consistent mesoscale eddy parameterization schemes which require solving the eddy energy budget.

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Guihua Wang, Rui Xin Huang, Jilan Su, and Dake Chen

Abstract

The dynamic influence of thermohaline circulation on wind-driven circulation in the South China Sea (SCS) is studied using a simple reduced gravity model, in which the upwelling driven by mixing in the abyssal ocean is treated in terms of an upward pumping distributed at the base of the upper layer.

Because of the strong upwelling of deep water, the cyclonic gyre in the northern SCS is weakened, but the anticyclonic gyre in the southern SCS is intensified in summer, while cyclonic gyres in both the southern and northern SCS are weakened in winter. For all seasons, the dynamic influence of thermohaline circulation on wind-driven circulation is larger in the northern SCS than in the southern SCS. Analysis suggests that the upwelling associated with the thermohaline circulation in the deep ocean plays a crucial role in regulating the wind-driven circulation in the upper ocean.

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Changlin Chen, Guihua Wang, Shang-Ping Xie, and Wei Liu

ABSTRACT

The Kuroshio and Gulf Stream, the subtropical western boundary currents of the North Pacific and North Atlantic, play important roles in meridional heat transport and ocean–atmosphere interaction processes. Using a multimodel ensemble of future projections, we show that a warmer climate intensifies the upper-layer Kuroshio, in contrast to the previously documented slowdown of the Gulf Stream. Our ocean general circulation model experiments show that the sea surface warming, not the wind change, is the dominant forcing that causes the upper-layer Kuroshio to intensify in a warming climate. Forced by the sea surface warming, ocean subduction and advection processes result in a stronger warming to the east of the Kuroshio than to the west, which increases the isopycnal slope across the Kuroshio, and hence intensifies the Kuroshio. In the North Atlantic, the Gulf Stream slows down as part of the Atlantic meridional overturning circulation (AMOC) response to surface salinity decrease in the high latitudes under global warming. The distinct responses of the Gulf Stream and Kuroshio to climate warming are accompanied by different regional patterns of sea level rise. While the sea level rise accelerates along the northeastern U.S. coast as the AMOC weakens, it remains close to the global mean rate along the East Asian coast as the intensifying Kuroshio is associated with the enhanced sea level rise offshore in the North Pacific subtropical gyre.

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