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Complex interactions between open ocean and nearshore environments pose a predictability problem. Basin-scale ocean models are typically run at grid scales that do not accurately resolve individual islands, and model output is assessed mostly using observations of the open ocean. Thus, model ability to replicate island forereef oceanic variability has gone largely untested. Here, an eight-year regional state estimate covering 2009-2017 is compared to bottom temperature observations at the western Pacific islands of Palau and Pohnpei, and is found to reproduce the observed seasonal to interannual variability. Because of their steep bathymetry, these islands can act as moorings. Sea surface variables, such as temperature (SST) and height (SSH), have been shown to predict upper ocean thermal structure in the region, but the spatial structure of the relationship has gone unexplored. The state estimate was used to examine the multivariate predictability of temperature at depths to about 200 m both at the island boundaries and throughout the domain. The results show that the best multiple linear regression (MLR) skill was found near Palau, but useful skill (>0.6) was available through much of the region within the anticyclonic gyre driven by positive wind-stress curl. Point SSH measurements offered prediction skill for areas extending a few hundred kilometers zonally and perhaps 100 km meridionally. The insights into the additional information contained in surface variables in this region could aid in advancement of ocean modeling as well as predictions of ecological patterns and stressors. [ABSTRACT FROM AUTHOR]

The Republic of Palau, a group of islands in the western tropical Pacific Ocean, is located between the westwardflowing North Equatorial Current (NEC) to the north and the eastward-flowing North Equatorial Countercurrent (NECC) to the south, and the Mindanao Eddy (ME) lies to the west. This unique geographical location may make Palau an oceanographically sensitive region due to strong equatorial zonal flows encountering steep island topography. We investigate the effect Palau has on the regional ocean circulation through numerical model simulations with identical forcing but differing bathymetry: one with Palau and the other without it. The simulations use realistic initial conditions--monthly climatological atmospheric forcing, open-ocean boundary conditions, and runoff fluxes--and were run for up to 37 years to distinguish between model intrinsic variability and deterministic differences. The significant differences between the two solutions show that Palau's effect on circulation is localized and small. The model state differences, quantified as percentage of model variability near Palau from the 37-year solution, are about 20% for sea surface height, 25% for surface velocities, and <1% for surface temperature and salinity. The subsurface velocity fields around Palau show a two-layered flow, as previously reported by other authors, with upper layer flow from the surface to 300 m, and a lower layer flow from 300 m to 3,000 m. The topographic form stress on Palau is <10% of the vertically integrated total form stress in the 5°N-10°N latitudinal band, and when the island is removed from model simulations, the stress is redistributed within the region. Although these results are restricted to model resolution scales and physics, they provide an estimate of the influence of Palau on the large-scale northwestern tropical Pacific Ocean circulation. [ABSTRACT FROM AUTHOR]

The ocean's response to the termination of the major 2015/2016 El Niño event was captured from moorings and gliders deployed near Palau as part of the Office of Naval Research Departmental Research Initiative Flow Encountering Abrupt Topography (FLEAT). As the El Niño transitioned to neutral conditions in spring 2016, pulses of positive (warm) sea surface height anomalies (SSHAs) moved westward, deepening the thermocline and reaching Palau by the end of March. Observations collected nearly two months after the arrival of the warm water revealed intraseasonal oscillations (ISOs) with periods of ~30 days and vertical displacements of isotherms exceeding ~100 m in the deeper part of the thermocline. Arrival of these warm anomalies coincided with the disappearance of the eastward flow associated with the North Equatorial Countercurrent, and anomalously large meridional velocities (0.4 m s-1) and transports (~4-6 Sv) over the Kyushu-Palau Ridge on the northern edge of Palau. The 120-day, high-pass-filtered, satellite-acquired SSHAs showed packets of westwardmoving waves with phase speeds of about 0.2 m s-1 at 8.625°N, with horizontal wavelengths and periods of about 550 km and 30 days, respectively. These waves, which appear to originate in the western Pacific, fall within the characteristics of mode-1 and mode-2 Rossby waves. Similar SSHAs were found near Palau following previous El Niño events, suggesting that formation of intraseasonal oscillations is part of an oceanic response to the termination of El Niño. The transition from El Niño to neutral conditions can affect the coral ecosystem around Palau by creating anomalous circulation and strong thermal anomalies extending from the surface to bottom waters deeper than 150 m, far below the depth limit of coral growth in Palau. [ABSTRACT FROM AUTHOR]

Current structure, transport, and water mass properties of the northward-flowing Kuroshio and the southward-flowing Luzon Undercurrent (LU) were observed for nearly one year, June 8, 2012--June 4, 2013, across the Kuroshio path at 18.75°N. Observations were made from four platforms: an array of six subsurface ADCP moorings, two Seagliders, fivepressure inverted echo sounders (PIES), and five horizontal electric field (HEF) sensors, providing the most detailed time series of the Kuroshio and Luzon Undercurrent water properties to date. Ocean state estimates of the western boundary current system were performed using the MIT general circulation model--four-dimensional variational assimilation (MITgcm-4D-Var) system. Prominent Kuroshio features from observations are simulated well by the numerical model. Annual mean Kuroshio transport, averaged over all platforms, is ~16 Sv with a standard deviation ~4 Sv. Kuroshio and LU transports and water mass pathways east of Luzon are revealed by Seaglider measurements. In a layer above the salinity maximum associated with North Pacific Tropical Water (NPTW), Kuroshio transport is ~7 Sv and contains North Equatorial Current (NEC) and Western Philippine Sea (WPS) waters, with an insignificant amount of South China Sea water on the shallow western flank. In an intermediate layer containing the core of the NPTW, Kuroshio transport is ~10 Sv, consisting mostly of NEC water. In the lower layer of the Kuroshio, transport is ~1.5 Sv of mostly North Pacific Intermediate Water (NPIW) as a part of WPS waters. Annual mean Luzon Undercurrent southward transport integrated to 1,000 m depth is ~2.7 Sv with a standard deviation ~2 Sv, carrying solely WPS waters below the salinity minimum of the NPIW. The transport of the western boundary current integrated over the full ocean depth east of Luzon Island is ~14 ± 4.5 Sv. Sources of the water masses in the Kuroshio and Luzon Undercurrent are confirmed qualitatively by the numerical model. [ABSTRACT FROM AUTHOR]

The Mindanao Current (MC), a low-latitude western boundary current in the Pacific Ocean, plays an important role in heat and freshwater transport to the western Pacific warm pool and the Indian Ocean. However, there have been relatively few comprehensive studies of the structure and variability of the MC and its connectivity to regional circulation. The Origins of the Kuroshio and Mindanao Current (OKMC) initiative combines four years of glider observations of the MC, a historical conductivity-temperature-depth (CTD)/float climatology, and results from a global strongly eddying forward ocean general circulation model simulation and a regional ocean state estimate. The MC is resolved as a strong southward current primarily within the upper 200 m, approaching 1 m s-1, and extending roughly 300 km offshore of Mindanao. Observations and model simulations show a persistent northward Mindanao Undercurrent (MUC) below the thermocline. The MC transports water masses of North Pacific origin southward, while the MUC carries water with South Pacific characteristics northward. The subthermocline transport of the MC and the MUC is connected to other undercurrents in the Philippine Sea. The variability of this transport is a topic of continuing research. [ABSTRACT FROM AUTHOR]

Located at the crossroads of the tropical and subtropical circulations, the westward-flowing North Equatorial Current (NEC) and its subsequent bifurcation off the Philippine coast near 13°N serve as important pathways for heat and water mass exchanges between the mid- and low-latitude North Pacific Ocean. Because the western Pacific warm pool, with sea surface temperatures >28°C, extends poleward of 17°N in the western North Pacific, the bifurcation and transport partitioning of the NEC into the Kuroshio and Mindanao Currents are likely to affect the temporal evolution of the warm pool through lateral advection. In addition to its influence on physical conditions, NEC variability is also important to the regional biological properties and the fisheries along the Philippine coast and in the western Pacific Ocean. This article synthesizes our current understandings of the NEC, especially those garnered through the recent Origins of the Kuroshio and Mindanao Current (OKMC) project. [ABSTRACT FROM AUTHOR]

Numerous observational and modeling studies of ocean circulation surrounding Taiwan have reported occurrences of cold water and doming of isotherms (called the cold dome) that result in the formation of coastal upwelling on the northeastern Taiwan shelf. We use a high-resolution (1/24°) ocean model based on the Massachusetts Institute of Technology general circulation model to study the evolution of this distinct shelf-slope circulation phenomenon. We performed a number of model simulations spanning a five-year period (2004-2008) using realistic atmospheric forcing and initial and open boundary conditions. The model solutions were compared with satellite measurements of sea surface height (SSH), sea surface temperature (SST), and historical temperature and salinity observations. The model showed a realistically shaped cold dome with a diameter of ~100 km and temperature of ~3°C below the ambient shelf waters at 50 m depth. The occurrences of simulated cold dome events appeared to be connected with the seasonal variability of the Kuroshio Current. The model simulations showed more upwelling events during spring and summer when the core of the Kuroshio tends to migrate away from the east coast of Taiwan, compared to fall and winter when the core of the Kuroshio is generally found closer to the east coast of Taiwan. The model also reproduced weak cyclonic circulation associated with the upwelling off northeastern Taiwan. We analyzed the spatio-temporal variability of the cold dome using the model solution as a proxy and designed a "cold dome index" based on the temperature at 50 m depth averaged over a 0.5° x 0.5° box centered at 25.5°N, 122°E. The cold dome index correlates with temperature at 50 m depth in a larger region, suggesting the spatial extent of the cold dome phenomenon. The index had correlation maxima of 0.78 and 0.40 for simulated SSH and SST, respectively, in and around the cold dome box region, and we hypothesize that it is a useful indicator of upwelling off northeastern Taiwan. In addition, both correlation and composite analysis between the temperature at 50 m depth and the East Taiwan Channel transport showed no cold dome events during low-transport events (often in winter) and more frequent cold dome events during high-transport events (often in summer). The simulated cold dome events had time scales of about two weeks, and their centers aligned roughly along a northeastward line starting from the northeastern tip of Taiwan. [ABSTRACT FROM AUTHOR]