The low-latitude western boundary current (LLWBC) of the Pacific has been shown to play an important role in the world?s climate. It plays a role in modulating the western Pacific warm pool and ventilating the equatorial thermocline, which are important for phenomena like El Nino/Southern Oscillation and Pacific Decadal Oscillation (PDO). This study combines all available observations with results from the OGCM for the Earth Simulator (OFES) and the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO). The OFES has a higher (0.1deg) resolution and longer (1950-2003) period of integration, and its simulated LLWBC has proven to be one of the best available in a model. The ECCO has a relatively coarse (1deg×1/3deg) resolution and shorter (1982-present) period of integration, but its simulated passive and adjoint tracer provides a unique opportunity to trace the LLWBC. The project goals are to (1) complete the description of 3-dimensional structure and intraseasonal-decadal variability of the LLWBC, (2) examine the energy source of eddies and their impacts on the LLWBC, and (3) identify the origin, pathway, and fate of the LLWBC, with special reference to the intermediate depth off the Philippines. The results will contribute to assessing predictability of the Asia-Australia monsoon system, improving ENSO predictions, and understanding mechanisms of Pacific decadal variability, as well as various impacts of these phenomena.
The project will support a postdoctoral researcher and will enhance infrastructure for research and education by establishing collaborations between different climate research groups in different institutions from different nations.
The low-latitude western boundary current (LLWBC) near the Philippine coast has been shown to play an important role in the world’s climate. A good example of this is its role in modulating the western Pacific warm pool, on which phenomena like El Nino/Southern Oscillation (ENSO) and Pacific Decadal Oscillation critically depend. With the warmest (>30°C) sea surface temperature (SST) in an open ocean, the western Pacific warm pool is recognized as a primary energy source for the global atmosphere, where any small SST changes can result in significant variability in precipitation across the Indo-Pacific basin. As a key element of ocean circulation, the LLWBC system near the Philippine coast consists of the Kuroshio and Mindanao Currents near the sea surface and the Luzon and Mindanao Undercurrents at the subsurface. This current system connects in the south with the New Guinea Coastal Undercurrent and in the north with the South China Sea throughflow. Variability in this current system can affect the heat and freshwater budget of the western Pacific warm pool and consequently modulate the Hadley and Walker Circulations in the atmosphere, playing a role in the world’s climate through atmospheric teleconnections. Due to the lack of observations, earlier studies only focused on one or two aspects of the LLWBC at particular locations and times, and as a consequence a complete description of this current system was lacking. The funding of NSF through grant OCE-1029704 has allowed us to capture a number of new features of the LLWBC near the Philippine coast. Combining recently available satellite and in-situ data with results from high-resolution general circulation models, we were able to provide a 3-dimensional picture of the LLWBC and its intraseasonal-to-decadal variability, to examine the energy source of subthermocline eddies, and to identify the origin and pathway of the LLWBC near the Philippine coast. Our findings include: (1) there is a basin-scale cyclonic circulation in the deep South China Sea, largely forced by the deepwater overflow through the Luzon Strait; (2) the Mindanao Undercurrent exists in the mean but is highly variable, with its dominant time scales ranging between 50 and 100 days; (3) subthermocline eddies, which are largely invisible at the sea surface, are popular and play an important role in the variability of the LLWBC; (5) the South China Sea throughflow has experienced a significant decadal variability, in good correspondence with the Pacific Decadal Oscillation; (6) a large portion of the Luzon Undercurrent originates from the subtropical North Pacific, (7) the deep water overflow through the Luzon Strait varies considerably on time scales from 20 to 100 days; (9) the South China Sea throughflow may have notable impacts on ENSO, reducing its frequency by as much as 1 year. These findings have advanced our knowledge of the LLWBC near the Philippine coast. In particular, they will help us understand the ocean’s role in climate variability and likely result in improved skills for the monsoon and ENSO predictions in the Asia-Pacific sector, on which a large population of people rely. This project has also contributed to training a postdoctoral fellow in the analysis of observational data and model outputs, and to enhancing infrastructure for both research and education by establishing collaborations across different disciplines, institutions, and countries.
