Dongliang Yuan

Surface Kuroshio path in the Luzon Strait area derived from satellite remote sensing data

[ 1] Satellite ocean color, sea surface temperature, and altimeter data are used to study the surface Kuroshio path in the Luzon Strait area. The results suggest that the dominant path of surface Kuroshio intrusion in winter is a direct route from northeast of Luzon to southwest of Taiwan and then westward along the continental slope of northern South China Sea. Anticyclonic intrusions of the Kuroshio in the Luzon Strait area are observed during less than 30% of the time on average and in all four seasons of the year. Winter is the most favorable season for the formation of the anticyclonic intrusions. However, the Kuroshio is observed to deviate from the dominant path during only a little over one third of the wintertime on average. The loop currents of the Kuroshio, which feature prominent inflow-outflow currents in the Luzon Strait during the anticyclonic intrusions, are observed only occasionally, with more episodes in summer than in winter. The observation of more frequent loop currents of the Kuroshio in summer than in winter is a revision to the existing conclusion. These results demonstrate that the anticyclonic intrusion of the Kuroshio is a transient phenomenon rather than a persistent circulation pattern in the Luzon Strait area as suggested by some of the existing numerical model simulations. The growth and decay of the anticyclonic intrusions of the Kuroshio are closely related to the passages and evolution of mesoscale eddies in the Luzon Strait area. Each anticyclonic intrusion event lasts for a few weeks. Its termination sometimes results in a detached anticyclonic eddy propagating to the western basin along the continental slope of the northern South China Sea.

Anti-cyclonic eddies northwest of Luzon in summer-fall observed by satellite altimeters

Anti-cyclonic eddies northwest of Luzon of the Philippines in summer-fall are identified in the merged data products of satellite altimeters of Topex/Poseidon, Jason-1 and European Research Satellites. The generation and propagation of the anti-cyclonic eddies, which are confirmed by satellite ocean color data, are found to be a seasonal phenomenon that is phase-locked to the onset of the southwesterly monsoon and the relaxation of the cyclonic wind curl in the northeastern South China Sea. The eddies originate from northwest of Luzon in summer, move across the northeastern South China Sea to reach the China continental slope in fall, and propagate southwestward along the continental slope in fall-winter, inducing shelfbreak current variations in the western South China Sea in fall-winter. The anti-cyclonic eddy discovered by Li et al. (1998) in the northern South China Sea is found to originate from northwest of Luzon and carry primarily the South China Sea waters. It does not appear to be an eddy shed from the Kuroshio in the Luzon Strait area as alluded by Li et al. (1998) and others.

Intraseasonal variability of Indian Ocean sea surface temperature during boreal winter: Madden‐Julian Oscillation versus submonthly forcing and processes

[ 1] Intraseasonal variability of Indian Ocean sea surface temperature (SST) during boreal winter is investigated by analyzing available data and a suite of solutions to an ocean general circulation model for 1998 - 2004. This period covers the QuikSCAT and Tropical Rainfall Measuring Mission (TRMM) observations. Impacts of the 30 - 90 day and 10 - 30 day atmospheric intraseasonal oscillations (ISOs) are examined separately, with the former dominated by the Madden-Julian Oscillation (MJO) and the latter dominated by convectively coupled Rossby and Kelvin waves. The maximum variation of intraseasonal SST occurs at 10 degrees S - 2 degrees S in the wintertime Intertropical Convergence Zone (ITCZ), where the mixed layer is thin and intraseasonal wind speed reaches its maximum. The observed maximum warming ( cooling) averaged over ( 60 degrees E - 85 degrees E, 10 degrees S - 3 degrees S) is 1.13 degrees C ( - 0.97 degrees C) for the period of interest, with a standard deviation of 0.39 degrees C in winter. This SST change is forced predominantly by the MJO. While the MJO causes a basin-wide cooling ( warming) in the ITCZ region, submonthly ISOs cause a more complex SST structure that propagates southwestward in the western-central basin and southeastward in the eastern ocean. On both the MJO and submonthly timescales, winds are the deterministic factor for the SST variability. Short-wave radiation generally plays a secondary role, and effects of precipitation are negligible. The dominant role of winds results roughly equally from wind speed and stress forcing. Wind speed affects SST by altering turbulent heat fluxes and entrainment cooling. Wind stress affects SST via several local and remote oceanic processes.

Roles of equatorial waves and western boundary reflection in the seasonal circulation of the equatorial indian ocean

An ocean general circulation model (OGCM) is used to study the roles of equatorial waves and western boundary reflection in the seasonal circulation of the equatorial Indian Ocean. The western boundary reflection is defined as the total Kelvin waves leaving the western boundary, which include the reflection of the equatorial Rossby waves as well as the effects of alongshore winds, off-equatorial Rossby waves, and nonlinear processes near the western boundary. The evaluation of the reflection is based on a wave decomposition of the OGCM results and experiments with linear models. It is found that the alongshore winds along the east coast of Africa and the Rossby waves in the off-equatorial areas contribute significantly to the annual harmonics of the equatorial Kelvin waves at the western boundary. The semiannual harmonics of the Kelvin waves, on the other hand, originate primarily from a linear reflection of the equatorial Rossby waves. The dynamics of a dominant annual oscillation of sea level coexisting with the dominant semiannual oscillations of surface zonal currents in the central equatorial Indian Ocean are investigated. These sea level and zonal current patterns are found to be closely related to the linear reflections of the semiannual harmonics at the meridional boundaries. Because of the reflections, the second baroclinic mode resonates with the semiannual wind forcing; that is, the semiannual zonal currents carried by the reflected waves enhance the wind-forced currents at the central basin. Because of the different behavior of the zonal current and sea level during the reflections, the semiannual sea levels of the directly forced and reflected waves cancel each other significantly at the central basin. In the meantime, the annual harmonic of the sea level remains large, producing a dominant annual oscillation of sea level in the central equatorial Indian Ocean. The linear reflection causes the semiannual harmonics of the incoming and reflected sea levels to enhance each other at the meridional boundaries. In addition, the weak annual harmonics of sea level in the western basin, resulting from a combined effect of the western boundary reflection and the equatorial zonal wind forcing, facilitate the dominance by the semiannual harmonics near the western boundary despite the strong local wind forcing at the annual period. The Rossby waves are found to have a much larger contribution to the observed equatorial semiannual oscillations of surface zonal currents than the Kelvin waves. The westward progressive reversal of seasonal surface zonal currents along the equator in the observations is primarily due to the Rossby wave propagation.

Dynamics of Intraseasonal Sea Level and Thermocline Variability in the Equatorial Atlantic during 2002–03

Abstract Satellite and in situ observations in the equatorial Atlantic Ocean during 2002–03 show dominant spectral peaks at 40–60 days and secondary peaks at 10–40 days in sea level and thermocline within the intraseasonal period band (10–80 days). A detailed investigation of the dynamics of the intraseasonal variations is carried out using an ocean general circulation model, namely, the Hybrid Coordinate Ocean Model (HYCOM). Two parallel experiments are performed in the tropical Atlantic Ocean basin for the period 2000–03: one is forced by daily scatterometer winds from the Quick Scatterometer (QuikSCAT) satellite together with other forcing fields, and the other is forced by the low-passed 80-day version of the above fields. To help in understanding the role played by the wind-driven equatorial waves, a linear continuously stratified ocean model is also used. Within 3°S–3°N of the equatorial region, the strong 40–60-day sea surface height anomaly (SSHA) and thermocline variability result mainly from the...

Forcing of the Indian Ocean Dipole on the Interannual Variations of the Tropical Pacific Ocean: Roles of the Indonesian Throughflow

Controlled numerical experiments using ocean-only and ocean-atmosphere coupled general circulation models show that interannual sea level depression in the eastern Indian Ocean during the Indian Ocean dipole (IOD) events forces enhanced Indonesian Throughflow (ITF) to transport warm water from the upper-equatorial Pacific Ocean to the Indian Ocean. The enhanced transport produces elevation of the thermocline and cold subsurface temperature anomalies in the western equatorial Pacific Ocean, which propagate to the eastern equatorial Pacific to induce significant coupled evolution of the tropical Pacific oceanic and atmospheric circulation. Analyses suggest that the IOD-forced ITF transport anomalies are about the same amplitudes as those induced by the Pacific ENSO. Results of the coupled model experiments suggest that the anomalies induced by the IOD persist in the equatorial Pacific until the year following the IOD event, suggesting the importance of the oceanic channel in modulating the interannual climate variations of the tropical Pacific Ocean at the time lag beyond one year.

Calculation of Pressure in Ocean Simulations

Abstract Many state-of-the-art numerical ocean models calculate pressure using the hydrostatic balance, or an equation derived from it. The proper form of this deceptively simple-looking equation, ∂p/∂z = −gρ(S, T, p) (where notation is standard), is nonlinear in the pressure p. In contrast, most numerical models solve the linear equation ∂p/∂z = −gρ(S, T, z). This modification essentially replaces the total pressure, which includes a time-dependent signal, with an approximate time-independent pressure associated with the depth of a model grid point. In this paper, the authors argue that the inclusion of the total pressure when solving the hydrostatic equation can generate a depth-dependent baroclinic pressure gradient equivalent to a geostrophic velocity of several centimeters per second. Further, this effective velocity can increase with depth and is largest in dynamically important areas like western boundary currents. These points suggest that the full feedback of pressure on density should be include...

Direct Measurements of the Luzon Undercurrent

AbstractThe Luzon Undercurrent (LUC) was discovered about 20 years ago by geostrophic calculation from conductivity–temperature–depth (CTD) data. But it was not directly measured until 2010. From November 2010 to July 2011, the LUC was first directly measured by acoustic Doppler current profiler (ADCP) from a subsurface mooring at 18.0°N, 122.7°E to the east of Luzon Island. A number of new features of the LUC were identified from the measurements of the current. Its depth covers a range from 400 m to deeper than 700 m. The observed maximum velocity of the LUC, centered at about 650 m, could exceed 27.5 cm s−1, four times stronger than the one derived from previous geostrophic calculation with hydrographic data. According to the time series available, the seasonality of the LUC strength is in winter > summer > spring. Significant intraseasonal variability (ISV; 70–80 days) of the LUC is exposed. Evidence exists to suggest that a large portion of the intraseasonal variability in the LUC is related to the w...

Geostrophic Circulation in the Tropical North Pacific Ocean Based on Argo Profiles

Absolute geostrophic currents in the North Pacific Ocean are calculated from the newly gridded Argo profiling float data using the P-vector method for the period of 2004-11. The zonal geostrophic currents based on the Argo profile data are found to be stronger than those based on the traditional World Ocean Atlas 2009 (WOA09) data. A westward mean geostrophic flow underneath the North Equatorial Countercurrent is identified using the Argo data, which is evidenced by sporadic direct current measurements and geostrophic calculations in history. This current originates east of the date line and transports more than 4 x 10(6) m(3) s(-1) of water westward in the subsurface northwestern tropical Pacific Ocean. The authors name this current the North Equatorial Subsurface Current. The transport in the geostrophic currents is compared with the Sverdrup theory and found to differ significantly in several locations. Analyses have shown that errors of wind stress estimation cannot account for all of the differences. The largest differences are found in the area immediately north and south of the bifurcation latitude of the North Equatorial Current west of the date line and in the recirculation area of the Kuroshio and its extension, where nonlinear activities are vigorous. It is, therefore, suggested that the linear dynamics of the Sverdrup theory is deficient in explaining the geostrophic transport of the tropical northwestern Pacific Ocean.

A numerical study of barotropicly forced intrusion in DeSoto Canyon

[1] The intrusion of open ocean water across the head of DeSoto Canyon offshore of Pensacola, Florida, is investigated using a version of the Bryan-Cox model of the entire Gulf of Mexico. The Loop Current-forced model circulation in DeSoto Canyon features an eastward, nearly along-isobath, current with a weak cross-isobath velocity component. In contrast, the southward wind-driven currents over the west Florida shelf in response to northerly winds induce strong cross-isobath flow at the head of DeSoto Canyon. The flow is diagnosed to be the bottom Ekman flow associated with the strong along-isobath currents. In addition, transient responses of sea levels to the northerly wind bursts give rise to pressure gradient-forced currents up the canyon during the collapse phase of the winds. A case study in November 1997 is conducted, and the above arguments are substantiated with the simulated and the observed currents.