Peter Hacker

Observations of the Mindanao Current during the western equatorial Pacific Ocean circulation study

The Western Equatorial Pacific Ocean Circulation Study (WEPOCS) III expedition was conducted from June 18 through July 31, 1988, in the far western equatorial Pacific Ocean to observe the low-latitude western boundary circulation there, with emphasis on the Mindanao Current. This survey provides the first quasi-synoptic set of current measurements which resolve all of the important upper-ocean currents in the western tropical Pacific. Observations were made of the temperature, salinity, dissolved oxygen, and current profiles with depth; of water mass properties including transient tracers; and of evolving surface flows with a dense array of Lagrangian drifters. This paper provides a summary of the measurements and a preliminary description of the results. The Mindanao Current was found to be a narrow, southward-flowing current along the eastward side of the southern Philippine Islands, extending from 14°N to the south end of Mindanao near 6°N, where it then separates from the coast and penetrates into the Celebes Sea. The current strengthens to the south and is narrowest at 10°N. Direct current measurements reveal transports in the upper 300 m increasing from 13 Sv to 33 Sv (1 Sverdrup = 1 × 106 m3 s−1) between 10°N and 5.5°N. A portion of the Mindanao Current appears to recurve cyclonically in the Celebes Sea to feed the North Equatorial Countercurrent, merging with waters from the South Equatorial Current and the New Guinea Coastal Undercurrent. Another portion of the Mindanao Current appears to flow directly into the NECC without entering the Celebes Sea. The turning of the currents into the NECC is associated with the Mindanao and Halmahera eddies.

Equatorial subthermocline currents across the Pacific

Recent sections crossing the equator at nine longitudes from 146°E to 86°W show zonal currents below 400 m similar to those found in a 16-month mean section on 159°W from the Pacific Equatorial Ocean Dynamics (PEQUOD) program in 1982–1983. Most of the new measurements were made with a lowered acoustic Doppler current profiler. Eastward current extrema (North and South Intermediate Countercurrents) are found in all sections about 2° from the equator in the depth range 500–1500 m, with the possible exceptions of 146°E, where topography complicates the picture south of the equator, and 150°E, where the sections extend only from 1°S to 2°N. Poleward of the intermediate countercurrents, westward flow is found near 3° from the equator in most of the sections. On the equator, from 250–500 m depth, a westward Equatorial Intermediate Current is found in many but not all of the sections. Similarly, it is present in the PEQUOD mean but not in all synoptic sections. Below the intermediate currents and countercurrents, an eastward current near 3000 m was found at and south of the equator in the PEQUOD mean and in the new sections from 150°E to 110°W. Near 4000 m there is westward flow south of the equator in the PEQUOD mean and near the equator in the new sections from 179°E to 135°W. Geostrophic currents calculated from an 8-year mean hydrographic section at 165°E also resemble the PEQUOD mean. Although not conclusive, the evidence presented here indicates that these currents are basin-scale components of the general circulation, perhaps involving vigorous horizontal recirculation in a set of basin-wide elongated gyres within a few degrees of the equator.

Advection and diffusion of Indonesian Throughflow Water within the Indian Ocean South Equatorial Current

Warm, low salinity Pacific water weaves through the Indonesian Seas into the eastern boundary of the Indian Ocean. The Indonesian Throughflow Water (ITW) adds freshwater into the Indian Ocean as it spreads by the advection and diffusion within the Indian Oceans South Equatorial Current (SEC). The low salinity throughflow trace, centered along 12oS, stretches across the Indian Ocean, separating the monsoon dominated regime of the northern Indian Ocean from the more typical subtropical stratification to the south. ITW is well represented within the SEC thermocline, extending with concentrations above 80% of initial characteristics from the sea surface to 300-m within the eastern half of the Indian Ocean, with 60% concentration reaching well into the western Indian Ocean. The ITW transport within the SEC varies from 4 to 12 x 10 6 m3sec -1, partly in response to variations of the injection rate at the eastern boundary and to the likelihood of a zonally elongated recirculation cell between the Equatorial Counter Current and the SEC within the Indian Ocean. Lateral mixing disperses the ITW plume meridionally with an effective isopycnal mixing coefficient of 1.1 to 1.6 x 10 4 m2sec -1 .

Effect of Mesoscale Eddies on Subtropical Mode Water Variability from the Kuroshio Extension System Study (KESS)

Abstract Forty-eight profiling floats have been deployed in the Kuroshio Extension (KE) region since May 2004 as part of the Kuroshio Extension System Study (KESS) project. By combining the float temperature–salinity measurements with satellite altimetry data, this study investigates the role played by mesoscale eddies in controlling the property changes in North Pacific Subtropical Mode Water (STMW). Following a 3-yr period of low eddy activity in 2002–04, the KE showed a transition to a high eddy kinetic energy state in 2005. This transition is the result of delayed oceanic response to the 2002 shift in the basin-scale surface wind forcing in connection with the Pacific decadal oscillation. The high eddy kinetic energy state of the KE is characterized by successive shedding of strong cold-core rings into the recirculation gyre, resulting from the interaction of the KE jet with the Shatsky Rise or the preexisting cutoff rings. By transporting northern-origin, high-potential-vorticity (PV) KE water into t...

Observations of the Subtropical Mode Water Evolution from the Kuroshio Extension System Study

Abstract Properties and seasonal evolution of North Pacific Ocean subtropical mode water (STMW) within and south of the Kuroshio Extension recirculation gyre are analyzed from profiling float data and additional hydrographic and shipboard ADCP measurements taken during 2004. The presence of an enhanced recirculation gyre and relatively low mesoscale eddy variability rendered this year favorable for the formation of STMW. Within the recirculation gyre, STMW formed from late-winter convection that reached depths greater than 450 m near the center of the gyre. The lower boundary of STMW, corresponding to σθ ≃ 25.5 kg m−3, was set by the maximum depth of the late-winter mixed layer. Properties within the deep portions of the STMW layer remained largely unchanged as the season progressed. In contrast, the upper boundary of the STMW layer eroded steadily as the seasonal thermocline deepened from late April to August. Vertical eddy diffusivity responsible for this erosion was estimated from a budget analysis of ...

Multiple deep gyres of the western North Pacific: A WOCE section along 149°E

The top to bottom large-scale ocean circulation in the northwest Pacific is described using a World Ocean Circulation Experiment (WOCE) onetime hydrographic section along 149°E between Papua New Guinea and Japan. The circulation is quantified using a combination of geostrophic and lowered acoustic Doppler current profiler velocity estimates. At the northern end of the section the flow regime is distinct in that the deep flow largely reflects that at the surface: the Kuroshio jet and its northern and southern recirculations have deep expressions. South of 25°N, the deep and bottom water flows do not mirror the surface flows, and the circulation assumes a highly baroclinic structure. Below the depth of local North Pacific ventilation the flow in the upper deep waters (800-2500 m) alternates in sign roughly every 10° of latitude revealing a set of deep clockwise gyres with significant transports of 40 Sv (1 Sv = 10 6 m 3 s -1 ) for a tropical gyre (south of 6°N) and 20 Sv in a subtropical gyre (6° - 24°N). These gyres provide a pathway for South Pacific influences to reach 22°N (the location of a strong water mass front) through exchange along the western boundary. Maps of properties on density surfaces suggest that the zonal extent of the upper deep water gyres found along 149°E is basin wide. Below 2500 m, flow across the section is isolated from the Philippine Sea by the Izu-Ogasawara-Mariana Ridge and the flow regime and property distribution reflect this: Lower Circumpolar Water flows west in a deep western boundary current near 10°N and coalesces at the Izu-Ogasawara-Mariana Ridge with a tongue of North Pacific Deep Water also flowing west near 15°N. About 4 Sv of a mixture of these waters flows east again near 25°N, associated with an abyssal water mass front. North of the front, the water properties are laterally homogeneous on density surfaces in the strongly recirculating gyres associated with the deep Kuroshio system.

Upper ocean heat and salt balances in response to a westerly wind burst in the western equatorial Pacific during TOGA COARE

Two volume control methods are used to analyze the upper ocean heat and salt balances in response to a westerly wind burst event in the western equatorial Pacific during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. One method uses a fixed-thickness surface layer, and the other uses an isopycnal depth as the lower boundary. Horizontal advection terms in the budget calculations are estimated using the R/V Wecoma repeat hydrographic survey data within a 133 km x 133 km region. In both methods, the upper ocean heat budget is balanced within 10 W m(-2) of the surface air-sea flux observations during a 19-day time period, which covers the December 1992 westerly wind burst and a low-wind recovery period in early January 1993. The standard error in the estimation of heat advection is 11 W m(-2). The salt budget yields a rain rate estimate of 15.4 mm d(-1) with an error bar of 4 mm d(-1). This estimate is within 20% of the optical rain gauge measurements. The advection terms are important in both the heat and salt balances. Meridional advection dominates over zonal and vertical advection, acting to decrease temperature and increase salinity in the surface layer. From the isopycnal boundary method, the diapycnal turbulent flux transports a mean heat flux of 17 W m(-2) into the thermocline. Diapycnal advection is almost equally important, so that the total heat flux into the thermocline is estimated to be more than 30 W m(-2) during the study time period. Both terms are also important in the salt budget.

Impact of atmospheric intraseasonal variability in the Indian Ocean: Low-frequency rectification in equatorial surface current and transport

An ocean general circulation model (OGCM) is used to investigate the low-frequency (period longer than 90 days) rectification of atmospheric intraseasonal variability (10‐90-day periods) in zonal surface current and transport of the equatorial Indian Ocean. A hierarchy of OGCM solutions is found in an actual tropical Indian Ocean basin for the period of 1988‐2001. To help to identify and isolate nonlinear processes, a linear continuously stratified model and a 4 -layer intermediate ocean model are also used. Results from the OGCM solutions suggest 1 2 that intraseasonal atmospheric forcing acts to weaken the equatorial seasonal surface currents. Amplitudes of the spring and autumn eastward surface jets, the Wyrtki jets (WJ), and the westward surface current during January‐March are reduced by as much as 15‐25 cm s21 by intraseasonal atmospheric forcing, and strengths of the rectification exhibit a significant interannual variability. Important processes that cause the low-frequency rectification are asymmetric response of mixed layer depth to easterly and westerly winds, entrainment, and upwelling of momentum. During spring and autumn, the westerly (easterly) phase of an intraseasonal event enhances (weakens or even reverses) the seasonal westerly winds, increases (decreases) equatorial convergence and entrainment, and thus deepens (thins) the mixed layer. A net, westward current is generated over an event mean because easterly wind acts on a thinner surface mixed layer whereas westerly wind acts on a thicker one. In contrast, during January‐March when the seasonal winds are equatorial easterlies, surface currents are westward and equatorial undercurrents (EUC) develop. The rectified surface currents are eastward, which reduces the westward surface flow. This eastward rectification results largely from the vertical advection and entrainment of the EUC. The seasonal-to-interannual variability of the rectified surface flow is determined primarily from the seasonal cycle and interannual variability of the background state. Seasonal-to-interannual variability of the intraseasonal wind forcing also contributes. The rectified low-frequency zonal volume (heat) transports integrated over the entire water column along the Indian Ocean equator are persistently eastward with an amplitude of 0‐ 15 3 106 m3 s 21 (0‐1.2 pW). This is because westerly winds generate equatorial downwelling, advecting the surface eastward momentum downward and giving an eastward subsurface current. Easterly winds cause equatorial upwelling and produce an eastward pressure gradient force that drives an eastward subsurface current. This eastward subsurface current is advected upward by upwelling. The mean effect over an intraseasonal event at the equator is to increase the eastward transport in the water column. In the layers above the thermocline, the rectified zonal volume (heat) transports are in the same direction as the rectified surface currents. Results from this paper may have important implications for understanding climate variability because modification of WJ strength and transports can affect the SST and heat storage in the equatorial Indian Ocean warm pool.

Synoptic-Scale Air–Sea Flux Forcing in the Western North Pacific: Observations and Their Impact on SST and the Mixed Layer

Abstract Decade-long surface meteorological measurements from a Japan Meteorological Agency buoy at 29°N, 135°E are analyzed to elucidate the surface air–sea flux forcing in the western North Pacific Ocean. Besides the well-defined annual cycles, the observed heat and momentum fluxes are dominated by signals related to synoptic-scale weather disturbances. The synoptic-scale heat flux signals have a dominant time scale of 3–14 days, whereas the wind stress signals have a scale of 2–8 days. A comparison between the heat fluxes estimated using the buoy measurements and those from the NCEP reanalysis reveals that the daily NCEP product overestimates both the incoming solar radiation at sea surface and the turbulent heat flux amplitude associated with the individual weather events. The rms amplitude of the synoptic-scale net heat flux of the NCEP product is found to be positively biased by 23%. Despite this amplitude bias, the NCEP product captures the timing and relative strength of the synoptic-scale net hea...

Upper-ocean heat and salt balances in the Western Equatorial Pacific in response to the Intraseasonal Oscillation during TOGA COARE

During the TOGA COARE Intensive Observing Period (IOP) from November 1992 through February 1993, temperature, salinity, and velocity profiles were repeatedly obtained within a 130 km 3 130 km region near the center of the Intensive Flux Array (IFA) in the western equatorial Pacific warm pool. Together with high quality measurements of air‐sea heat flux, rain rate, upper-ocean microstructure, and penetrating solar radiation, they make up a unique dataset for upper-ocean heat and freshwater budget studies. Three survey cruises sampled different phases of the Intraseasonal Oscillation (ISO) during the IOP. Temporal evolution and advective terms in the heat and salt balance equations, on timescales of 3 days and longer, are estimated using the survey data. The upper-ocean (0‐50 m) heat and salt budgets at the center of the IFA were estimated and are closed to within 10 Wm 22 of observed air‐sea heat fluxes and to within approximately 20% of observed rain rates during each of the three cruises. Generally, advection in the upper ocean cannot be neglected during the IOP. Zonal advection alternates sign but had a net warming and freshening tendency. Meridional advection decreased temperature and increased salinity in the surface layer, while vertical advection warmed and freshened the surface layer because of the general downwelling trend. Heat advection is as important as the net air‐sea flux during the westerly wind burst time periods. The sub-ISO timescale upper-ocean dynamics, such as the strong meridional advection caused by inertial motions, are found to have important contributions to the upper-ocean heat and freshwater balances.