Zhao Jian

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  • Article
    Author correction : Meridional heat transport variability induced by mesoscale processes in the subpolar North Atlantic
    (Nature Publishing Group, 2018-06-14) Zhao, Jian ; Bower, Amy S. ; Yang, Jiayan ; Lin, Xiaopei
  • Article
    Overturning in the Subpolar North Atlantic Program : a new international ocean observing system
    (American Meteorological Society, 2017-04-24) Lozier, M. Susan ; Bacon, Sheldon ; Bower, Amy S. ; Cunningham, Stuart A. ; de Jong, Marieke Femke ; de Steur, Laura ; deYoung, Brad ; Fischer, Jürgen ; Gary, Stefan F. ; Greenan, Blair J. W. ; Heimbach, Patrick ; Holliday, Naomi Penny ; Houpert, Loïc ; Inall, Mark E. ; Johns, William E. ; Johnson, Helen L. ; Karstensen, Johannes ; Li, Feili ; Lin, Xiaopei ; Mackay, Neill ; Marshall, David P. ; Mercier, Herlé ; Myers, Paul G. ; Pickart, Robert S. ; Pillar, Helen R. ; Straneo, Fiamma ; Thierry, Virginie ; Weller, Robert A. ; Williams, Richard G. ; Wilson, Christopher G. ; Yang, Jiayan ; Zhao, Jian ; Zika, Jan D.
    For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.
  • Article
    Changes in bottom water physical properties above the mid-Atlantic ridge flank in the Brazil Basin
    (John Wiley & Sons, 2018-01-30) Zhao, Jian ; Thurnherr, Andreas M.
    Warming of abyssal waters in recent decades has been widely documented around the global ocean. Here repeat hydrographic data collected in 1997 and 2014 near a deep fracture zone canyon in the eastern Brazil Basin are used to quantify the long-term change. Significant changes are found in the Antarctic Bottom Water (AABW) within the canyon. The AABW in 2014 was warmer (0.08 ± 0.06 inline image), saltier (0.01 ± 0.005), and less dense (0.005 ± 0.004 inline image) than in 1997. In contrast, the change in the North Atlantic Deep Water has complicated spatial structure and is almost indistinguishable from zero at 95% confidence. The resulting divergence in vertical displacement of the isopycnals modifies the local density stratification. At its peak, the local squared buoyancy frequency ( inline image) near the canyon is reduced by about 20% from 1997 to 2014. Similar reduction is found in the basinwide averaged profiles over the Mid-Atlantic Ridge flank along 25 inline imageW in years 1989, 2005, and 2014. The observed changes in density stratification have important implications for internal tide generation and dissipation.
  • Article
    Structure and formation of anticyclonic eddies in the Iceland Basin
    (John Wiley & Sons, 2018-08-08) Zhao, Jian ; Bower, Amy S. ; Yang, Jiayan ; Lin, Xiaopei ; Zhou, Chun
    The Iceland Basin has the most energetic eddy activities in the subpolar North Atlantic. This study documents the structure for an anticyclonic eddy in the Iceland Basin using high‐resolution hydrographic and velocity observations. The eddy core waters have lens‐like structure with warm and salty features in the upper 1,000 m. The eddy distorts the density surface by doming the upper isopycnals and deepening the ones near the permanent pycnocline. The eddy has a diameter of about 120 km with substantial barotropic component in the velocity profiles. One branch of the North Atlantic Current in the central Iceland Basin is superimposed onto the eddy, leading to asymmetric velocity structure. Satellite maps show that eddy first shows up over the western slope of the Hatton Bank and moves westward to the central Iceland Basin. The waters enclosed in the eddy core share the same properties with Subpolar Mode Waters. Similar anticyclonic eddies are also found in high‐resolution numerical model simulations, which is used to explore eddy formation. The model results reveal that the potential vorticity gradient prior to the eddy event change signs in both horizontal and vertical directions. This potential vorticity gradient structure meets the necessary condition for the barotropic and baroclinic instabilities. Further calculation of the energy conversions suggests that eddies extract mean potential energy from the large‐scale isopycnal slope and gain the mean kinetic energy in the upper ocean. Therefore, both barotropic and baroclinic instabilities are involved to support the eddy growth.
  • Article
    A numerical study of interannual variability in the North Icelandic Irminger Current
    (American Geophysical Union, 2018-10-11) Zhao, Jian ; Yang, Jiayan ; Semper, Stefanie ; Pickart, Robert S. ; Våge, Kjetil ; Valdimarsson, Héðinn ; Jónsson, Steingrímur
    The North Icelandic Irminger Current (NIIC) is an important component of the Atlantic Water (AW) inflow to the Nordic Seas. In this study, both observations and a high‐resolution (1/12°) numerical model are used to investigate the seasonal to interannual variability of the NIIC and its forcing mechanisms. The model‐simulated velocity and hydrographic fields compare well with the available observations. The water mass over the entire north Icelandic shelf exhibits strong seasonal variations in both temperature and salinity, and such variations are closely tied to the AW seasonality in the NIIC. In addition to seasonal variability, there is considerable variation on interannual time scales, including a prominent event in 2003 when the AW volume transport increased by about 0.5 Sv. To identify and examine key forcing mechanisms for this event, we analyzed outputs from two additional numerical experiments: using only the seasonal climatology for buoyancy flux (the momentum case) and using only the seasonal climatology for wind stress (the buoyancy case). It is found that changes in the wind stress are predominantly responsible for the interannual variations in the AW volume transport, AW fraction in the NIIC water, and salinity. Temperature changes on the shelf, however, are equally attributable to the buoyancy flux and wind forcing. Correlational analyses indicate that the AW volume transport is most sensitive to the wind stress southwest of Iceland.
  • Article
    Meridional heat transport variability induced by mesoscale processes in the subpolar North Atlantic
    (Nature Publishing Group, 2018-03-19) Zhao, Jian ; Bower, Amy S. ; Yang, Jiayan ; Lin, Xiaopei
    The ocean’s role in global climate change largely depends on its heat transport. Therefore, understanding the oceanic meridional heat transport (MHT) variability is a fundamental issue. Prevailing observational and modeling evidence suggests that MHT variability is primarily determined by the large-scale ocean circulation. Here, using new in situ observations in the eastern subpolar North Atlantic Ocean and an eddy-resolving numerical model, we show that energetic mesoscale eddies with horizontal scales of about 10–100 km profoundly modulate MHT variability on time scales from intra-seasonal to interannual. Our results reveal that the velocity changes due to mesoscale processes produce substantial variability for the MHT regionally (within sub-basins) and the subpolar North Atlantic as a whole. The findings have important implications for understanding the mechanisms for poleward heat transport variability in the subpolar North Atlantic Ocean, a key region for heat and carbon sequestration, ice–ocean interaction, and biological productivity.