Curtis A. Collins

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.

Biological and chemical consequences of the 1997–1998 El Niño in central California waters

Abstract The physical, chemical and biological perturbations in central California waters associated with the strong 1997–1998 El Nino are described and explained on the basis of time series collected from ships, moorings, tide gauges and satellites. The evolution of El Nino off California closely followed the pattern observed in the tropical Pacific. In June 1997 an anomalous influx of warm southerly waters, with weak signatures on coastal sea level and thermocline depth, marked the onset of El Nino in central California. The timing was consistent with propagation from the tropics via the equatorial and coastal wave-guide. By late 1997, the classical stratified ocean condition with a deep thermocline, high sea level, and warm sea surface temperature (SST) commonly associated with El Nino dominated the coastal zone. During the first half of 1998 the core of the California Current, which is normally detected several hundred kilometers from shore as a river of low salinity, low nutrient water, was hugging the coast. High nutrient, productive waters that occur in a north–south band from the coast to approximately 200 km offshore during cool years disappeared during El Nino. The nitrate in surface waters was less than 20% of normal and new production was reduced by close to 70%. The La Nina recovery phase began in the fall of 1998 when SSTs dropped below normal, and ocean productivity rebounded to higher than normal levels. The reduction in coastal California primary productivity associated with El Nino was estimated to be 50 million metric tons of carbon (5 × 10 13 g C). This reduction certainly had deleterious effects on zooplankton, fish, and marine mammals. The 1992–1993 El Nino was more moderate than the 1997–1998 event, but because its duration was longer, its overall chemical and biological impact may have been comparable. How strongly the ecosystem responds to El Nino appears related to the longer-term background climatic state of the Pacific Ocean. The 1982–1983 and 1992–1993 El Ninos occurred during the warm phase of the Pacific Decadal Oscillation (PDO). The PDO may have changed sign during the 1997–1998 El Nino, resulting in weaker ecological effects than would otherwise have been predicted based on the strength of the temperature anomaly.

Lagrangian Exploration of the California Undercurrent, 1992–95

Abstract During the period 1992–95, nineteen isobaric RAFOS floats, placed in the California Undercurrent at intermediate depths (150–600 m) off Monterey and San Francisco, California, reveal a region of varying width of subsurface, poleward flow adjacent to the continental margin. The float trajectories exhibit three patterns: poleward flow in the undercurrent; reversing, but predominately alongshore, flow adjacent to the continental margin;and, farther offshore, anticyclonic motion accompanied by slow westward drift. Flow continuity of the undercurrent exists between Pt. Reyes and at least Cape Mendocino with an average speed dependent on the float depth. Speeds were variable, but common features were acceleration occurring to the south of Pt. Arena and deceleration to the north of Cape Mendocino. An important mechanism for floats, and water, to enter the ocean interior from the undercurrent is through the formation of submesoscale coherent vortices.

The California Current system off Monterey, California: physical and biological coupling

Repeated hydrobiological surveys over the period 1988–2002 perpendicular to the central California coast indicate strong coupling between physical circulation and biological production. An equatorward-flowing jet about 100–200 km from shore marked the inshore edge of the California Current (CC). This ‘‘CC Jet’’ had its highest velocities during late winter and spring. The jet divided inshore, biologicallyproductive waters from offshore, low-production waters. Mean flow in the inshore waters is poleward. However, this flow is interrupted in late spring and summer bya surfaceenhanced, equatorward-flowing, coastal upwelling jet. The upwelling jet coincides with maxima of nutrients, chlorophyll-a and primaryproduction. Annual variabilityin the inshore zone is related to (1) vertical py cnocline movements associated with geostrophic adjustments to accelerations of the California Current system, and (2) coastal upwelling. In offshore waters, the annual cycle accounted for a small fraction of the variability, indicating the dominance of eddies and meanders in this zone (J. Geophys. Res. 92 (1987) 12 947). The offshore regime was mesotrophic to oligotrophic, with a subsurface chlorophyll-a maximum above the nutricline. Considerable subduction mayoccur under the California Current jet and be an important process in the export of biogenic material to the deep sea. Published byElsevier Ltd.

Observations and modeling of the 1991–1992 El Niño signal off central California

Five research cruises were conducted over the continental shelf and slope near the Farallon Islands, California, in February, May, August, and October/November 1991 and February 1992. The observations consisted of shipboard hydrographic and acoustic Doppler current profiler data and moored current meter measurements. Water mass anomalies were calculated for each cruise by subtracting seasonal means based on historical data. In general, the maximum anomalies were observed subsurface in the 100-to 150-m range. In May 1991, equatorward, upwelling favorable winds elevated the thermocline resulting in cold, salty anomalies nearshore, with cold, fresh anomalies offshore associated with the advection of Pacific Subarctic Water into the region from the north. Warm, fresh anomalies and a strongly depressed thermocline were observed during the February 1992 cruise. A combination of coastal sea level and wind stress data and output from the Los Alamos National Laboratory parallel ocean program model was used to explain the cause of these anomalies. The February 1992 anomalies were shown to be due to both the deepening of the Aleutian low in the North Pacific associated with the 1991–1993 El Nino/Southern Oscillation event in the equatorial Pacific and poleward propagating intraseasonal coastal trapped Kelvin waves also arising from this event. The anomalous poleward wind forcing produced onshore flow, deepening of the thermocline, and downwelling at progressively southward locations. The “downwelling” Kelvin waves propagated northward with the two signals meeting somewhere near the cruise region. Both the model and the coastal sea level data showed the phase speed of the waves to slow by about 50% after passing the Gulf of California. This may be due to the scattering of energy from the fastest baroclinic mode into a slower mode. The strongest wave signal in the equatorial Pacific did not necessarily produce the strongest anomalies off central California.

High resolution modeling and data assimilation in the Monterey Bay area

Abstract A high resolution, data assimilating ocean model of the Monterey Bay area (ICON model) is under development within the framework of the project “An Innovative Coastal-Ocean Observing Network” (ICON) sponsored by the National Oceanographic Partnership Program. The main objective of the ICON model development is demonstration of the capability of a high resolution model to track the major mesoscale ocean features in the Monterey Bay area when constrained by the measurements and nested within a regional larger-scale model. This paper focuses on the development of the major ICON model components, including grid generation and open boundary conditions, coupling with a larger scale, Pacific West Coast (PWC) model, atmospheric forcing etc. Impact of these components on the Models predictive skills in reproducing major hydrographic conditions in the Monterey Bay area are analyzed. Comparisons between observations and the ICON model predictions with and without coupling to the PWC model, show that coupling with the regional model improves significantly both the correlation between the ICON model and observed ADCP currents, and the ICON models skill in predicting the location and intensity of observed upwelling events. Analysis of the ICON model mixed layer depth predictions show that the ICON model tends to develop a thicker than observed mixed layer during the summer time, and while assimilation of sea surface temperature data is enough for development of observed thin mixed layer in the regional larger-scale model, the fine-resolution ICON model needs variable heat fluxes as surface boundary conditions for the accurate prediction of the vertical thermal structure. The paper targets researchers involved in high-resolution numerical modeling of coastal areas in which the dynamics are determined by the complex geometry of a coastline, variable bathymetry and by the influence of complex water masses from a complicated hydrographic system (such as the California Current system).

Evidence of a turbidity current in Monterey Submarine Canyon associated with the 1989 Loma Prieta earthquake

Abstract Evidence of a turbidity current sweeping through the Monterey Submarine Canyon following the October 1989 Loma Prieta earthquake was documented by the movement of bottom-deployed acoustic transponders used to navigate free-falling oceanographic instrumentation. Measuring sites located along the Canyon at distances of 55, 130 and 190 km from the Canyon head off Moss Landing, CA, all showed evidence of tectonically induced sediment transport. At the site 55 km from the Canyon head, one transponder, located in the axis of the Canyon, was carried 1.9 km down the axis and was deposited among a field of rocks. The other three transponders had been deployed on the sides of the canyon and showed evidence of sediment slumping toward the Canyon axis. Circumstantial evidence from the site 130 km down the canyon suggests that sediment deposition occurred outside the channel axis. Sediment slumping or erosional cutting moved one transponder deeper at the site 190 km from the canyon head.

Ocean currents across the entrance to the Gulf of California

Observations of transport and currents in April, May, and December 1992 and January 1993 were made across the entrance to the Gulf of California with an acoustically tracked dropsonde. Flow was into the gulf along Sinaloa and out of the gulf along Baja California. The transports were 5–10 Sv and the currents were deep, with 10 cm s−1 flow extending to depths greater than 1000 m. The currents were intensified in the upper 300 m. Geostrophic flows compared well with observed currents when smoothed to 2–3 times the Rossby radius. Salinity in the upper 300 m was higher on the Baja California side of the gulf, indicating modification of Subtropical Subsurface waters within the Gulf as well as the presence of surface and near-surface gulf waters. The salinity minimum associated with California Current waters at 50 m had narrow cores that can be resolved only with closely spaced conductivity-temperature-depth casts. Mass and heat fluxes for the upper 300 m were estimated as 280 t s−1 and −0.1×1012 W in May and 170 t s−1 and 2.0×1012 W in December.

Changes in the hydrography of Central California waters associated with the 1997-98 El Nino

Abstract Oceanographic conditions off Central California were monitored by means of a series of 13 hydrographic cruises between February 1997 and January 1999, which measured water properties along an oceanographic section perpendicular to the California Coast. The 1997–98 El Nino event was defined by higher than normal sea levels at Monterey, which began in June 1997, peaked in November 1997, and returned to normal in March 1998. The warming took place in two distinct periods. During June and July 1997, the sea level increased as a result of stronger than normal coastal warming below 200 dbars and within 100 km of the coast, which was associated with poleward flow of saltier waters. During this period, deeper (400–1000 dbar) waters between 150–200 km from shore were also warmed and became more saline. Subsequently, sea level continued to rise through January 1998, mostly as a result of the warming above 200 dbars although, after a brief period of cooling in September 1997, waters below 200 dbar were also warmer than normal during this period. This winter warm anomaly was also coastally trapped, extending 200 km from shore and was accompanied by cooler and fresher water in the offshore California current. In March and April 1998, sea level dropped quickly to normal levels and inshore waters were fresher and warmer than the previous spring and flowed southward. The warming was consistent with equatorial forcing of Central California waters via propagation of Kelvin or coastally-trapped waves. The observed change in heat content associated with the 1997–98 El Nino was the same as that observed during the previous seasonal cycle. The warming and freshening events were similar to events observed during the 1957–58 and 1982–83 El Ninos.

Observations of the geostrophic current and water mass characteristics off Point Sur, California, from May 1988 through November 1989

The Point Sur transect (POST) has been occupied 6–8 times per year since 1988 to resolve the flow in the California Current system at seasonal and interannual time scales. The POST extends offshore, normal to bottom topography, along 36°20′N, to 123°01.7′W where it doglegs southwest along the California Cooperative Fisheries Investigation (CalCOFI) line 67. Hydrographic observations from seven cruises over 2 years have been used to study variations of alongshore geostrophic velocities and water mass characteristics within these time scales. The California Undercurrent was a prominent feature in six of the seven sections analyzed and was very weak during a period of strong equatorward wind stress. The position of the undercurrent core varied from 12 to 42 km from shore while its strength varied from less than 5 cm s−1 to 35 cm s−1, with the maximum flow occurring in winter. The undercurrent (core) over the continental slope was found from 70 to 460 m depth throughout these seven cruises. The nature of the alongshore geostrophic velocities and the location and spatial extent of the California Undercurrent appear strongly related to specific wind events, both local and remote. Remote wind forcing from the south was believed to cause anomalous, strong poleward flow throughout the entire water column above 1000 dbar during a period of local equatorward wind stress.