The effect of wind on long-term summer water temperature trends in Tokyo Bay, Japan

The effect of wind on long-term summer water temperature trends in Tokyo Bay, Japan Li-Feng Lu 0 Ryo Onishi 0 Keiko Takahashi 0 Responsible Editor: Leo Oey 0 0 Center for Earth Information Science and Technology, Japan Agency for Marine-Earth Science and Technology , Yokohama, Kanagawa 236-0001 , Japan The effect of wind on summer water temperature trends in a semi-closed bay (Tokyo Bay, Japan) is examined through several numerical experiments using a highresolution three-dimensional ocean model. The model is executed under no-wind and uniform southerly/northerly wind conditions, and monthly mean currents and temperature distributions and heat transport in Tokyo Bay for July are calculated. The model results show that wind has a significant effect on heat transport and temperature distribution in the bay. (1) When a southerly wind prevails northward cool water transport intensifies while southward warm water transport declines, thus decreasing the water temperature in the central bay area while increasing temperature at the bay head. (2) A northerly wind has an opposing effect and decreases the water temperature in coastal bay head area while increase the temperature along the southwest coast. The results also suggest that the trend of increasing southerly wind amplitude may have affected water temperature trends in Tokyo Bay from 1979 to 1997. The model results demonstrated that the an intensified southerly wind lowers water temperatures in most areas of the bay by enhancing upwelling and open oceanwater intrusion near the bay mouth while increases Water temperature trends; Wind; Tokyo Bay; Heat transport - * Li-Feng Lu temperatures in the bottom layer of the bay head by suppressing southward warm water transport. 1 Introduction Trends of increasing coastal water temperature with significant impacts on local ecosystems have been recently reported (e.g., Nixon et al 2004; Presten 2004; Shearman and Lentz 2010). However, in Tokyo Bay, a semi-closed bay located along the east coast of Japan (Fig. 1), water temperatures exhibited a descending trend in the surface layer and deep region of the bottom layer in July from 1976 to 1997 (Figs. 3 and 4 in Ando et al. 2003). Similar trends have been observed in Fukuoka Bay, another semi-closed bay in Japan (Aoki and Isobe 2006). The continuation of this decreasing summer water temperature trend will greatly impact not only the local environment and ecosystem but also the coastal urban climate (Oda and Kanda 2009). Therefore, the identification of mechanisms underlying these trends, particularly for the summer season, is ecologically and environmentally important. The previous studies have suggested that descending summer water temperature trends in these coastal bays are associated with increased cool water intrusion from the open ocean. Yagi et al. (2004) found a negative correlation between longterm summer surface temperature trends and salinity near the bay mouth in Tokyo Bay. Nomura (1996) observed that the number of certain species of zooplankton that reproduce in the outer bay increased in the central bay from 1981 to 1990. To explore the causes of the increased ocean water intrusion, Yanagi (2008) simulated the residual flows for August in Fig. 1 The study area location and the topography of Tokyo Bay. Solid lines denote topographic contours, and dotted lines demarcate sections S1, S2 and S3. The black triangle denotes the wind observation station, and black circles denote the surface water temperature observation stations Tokyo Bay using a three-dimensional prognostic numerical model. The model results indicated that water exchange between the bay and open ocean was enhanced by intensified gravitational circulation at the bay mouth induced by increased river discharge and decreased tidal amplitude due to reclamation. However, the mechanisms behind some processes remain unclear. For example, near the bay mouth, there is a negative correlation between water temperature and salinity, suggesting that intrusion by cool ocean water is a major cooling factor, although not for the bay head. Moreover, bay head water temperatures exhibit opposing trends in the surface and bottom layers. To further elucidate the mechanisms that control water temperature trends in Tokyo Bay during strong summer stratification periods, the detailed heat transport pathways in the bay and their dominant processes must be identified. The heat content in Tokyo Bay is mainly dominated by advective heat transfer related to residual currents (Hinata et al. 2001). Observations and modeling have demonstrated that the residual currents in Tokyo Bay are characterized by strong cyclonic and weak anti-cyclonic circulation at the bay head and bay center, respectively; these circulations are strongly influenced by wind forcing (Unoki et al. 1980; Guo and Yanagi 1996). When southerly/northerly winds prevail, upwelling occurs along the southwest/east coast, inducing internal Kelvin waves propagating cyclonically along the coast (Suzuki and Matsuyama 2000); consequently, bay head water temperatures increase/decrease (Tabeta and Fujino 1996; Hinata et al. 2001). Magome et al. (2012)s observation and modeling results demonstrated that southerly winds suppress water exchange processes at the bay mouth and trap bay water in the head region. Nakayama et al. (2014) suggested that Futtsu Cape B linear/nonlinear Ekman layers induced by wind curl produce anti-cyclonic/cyclonic circulation immediately below the surface mixed layer, producing material transport convergence/ divergence in the bay head region. In addition, wind forcing has considerable effects on the water temperature structure and heat transport processes in other bays, including Fukuoka Bay (Aoki and Isobe 2006) and the Gulf of Mexico (Chang and Oey 2010). However, few studies have examined the effect of wind on long-term heat transport and water temperature trends in Tokyo Bay. In this study, we use a high-resolution three-dimensional ocean model to examine how wind forcing affects Tokyo Bay heat transport and water temperature trends during the summer. The paper is organized as follows. The model and its qualitative verification regime are described in section 2. In section 3, the effects of southerly/northerly wind on heat transport are presented. Section 4 discusses the effect of wind on longterm summer water temperature trends in Tokyo Bay and section 5 presents the conclusions. 2 Model description and implementation 2.1 Model description The Multi-Scale Simulator for the Geoenvironment (MSSG) model is employed in this study. The model is based on incompressible NavierStokes equations with the Boussinesq approximation and is designed for parallel computers (Marshall et al. 1997a, b). In a rectangular coordinate system, the following equations govern the evolution of currents, temperature, salinity, and pressure fields (Marshall et al. 1997a, b). Motion equation Continuity equation St (...truncated)