Subpolar North Pacific Research Articles

Ventilation and transport of thermocline and intermediate waters in the northeast Pacific during recent El Niños

We present time series of tritium (3H) concentrations in seawater from stations in the eastern subpolar (50°N, 145°W) and subtropical (28°N, 122°W) North Pacific. In the eastern subpolar North Pacific the tritium gradient between surface water and North Pacific Intermediate Water is smallest during years that coincide with El Niño events. In the eastern subtropical North Pacific between 200 and 400 m the tritium signature suggests that the water is of subpolar origin during non‐El Niño years. During El Niño years, the water at this location is devoid of tritium. We hypothesize that the El Niño phenomenon alters both the ventilation of thermocline and intermediate waters in the eastern subpolar North Pacific, as well as the transport of this water to the eastern subtropical gyre. Combined with satellite altimeter data, these results offer both a mechanism and a time frame by which subpolar water ventilates the vast reservoir of the subtropical North Pacific.

The barotropic response of the subpolar North Pacific to stochastic wind forcing

In a recent observational study, Koblinsky et al. (1989) attempted to demonstrate that the topographic Sverdrup balance can account for mesoscale current fluctuations with periods between 10 and 100 days in the abyssal North Pacific. However, the topographic Sverdrup balance proved difficult to verify systematically over the North Pacific. In this paper the conditions required to obtain the topographic Sverdrup balance are investigated through numerical experiments with a stochastically forced, barotropic, quasi‐geostrophic numerical model of the subpolar North Pacific. In the numerical experiments the topographic Sverdrup balance emerges from area averages of the terms in the vorticity equation, but only for periods longer than about 40 days when the averages are taken over 4°×4° region. This period increases when averages are taken over progressively smaller regions. An examination of vorticity balances at individual points along sections through the model domain indicates that the topographic Sverdrup balance is not obtained at individual points, even for very long period motions. The possibility of verifying topographic Sverdrup dynamics with oceanic data is considered. It is argued that topographically generated small scales in the velocity field reduce the likelihood that the topographic Sverdrup balance can be systematically observed in velocity measurements from individual current meter moorings. The results suggest that the vorticity balance is complex and that topographic Sverdrup dynamics cannot be expected to provide an adequate explanation for the seasonal modulation of the abyssal eddy kinetic energy that is observed with moored instrumentation in the North Pacific.

Transpacific sections at 47°N and 152°W: distribution of properties

Three CTD/hydrographic sections with closely-spaced stations were occupied between May 1984 and May 1987, primarily in the subpolar North Pacific. Vertical sections of CTD quantities, oxygen and nutrients are presented. Upper water properties suggest that the Subarctic Front is located south of the subtropical/subpolar gyre boundary at 152°W, that there is leakage of North Pacific Intermediate Water from the subtropical to the subpolar gyre in the eastern Pacific, and verify the poleward shift of the subtropical gyre center with depth. At intermediate depths (1000–2000 m), a separation between the western and eastern parts of the subpolar gyre is found at 180° along 47°N. Abyssal waters are oldest in the northeast, with primary sources indicated at the western boundary and north of the Hawaiian Ridge. Properties and geostrophic velocity from detailed crossings of the boundary trenches suggest that flow in the bottom of the Kuril-Kamchatka Trench at the western boundary at 42°N and 47°N is northward. Very narrow boundary layers at intermediate depths are revealed in silica, as well as in the dynamical properties, at both the western and northern boundaries, and probably reflect southward and westward flow.

Seasonal and interannual changes in planktonic foraminiferal production in the North Pacific

It is well established that there are distinct seasonal cycles in the production of plankton, that vary both geographically and in terms of phytoplankton-zooplankton coupling1 . Only recently have attempts been made to measure seasonal variability in the flux of various biogenic components from the ocean surface to the sea-floor2–4 and to relate this flux to local hydrographic conditions5. For example, a simple inverse relationship has been found between sediment flux to the seafloor and sea-surface temperature in the Sargasso Sea5. A three-year sediment trapping project in the subpolar North Pacific has allowed us to document both seasonal and interannual variability in the flux of planktonic foraminifera from the sea surface. We propose a model, based on the temperature preferences of planktonic foraminiferal species, the thermal structure of the upper water column and food availability, to explain the observed changes over time in foraminiferal production and flux.

Neogene Radiolarian Biostratigraphy and Paleo-Oceanography of North Boundary Currents: ABSTRACT

Deep Sea Drilling Project (DSDP) cores from offshore Japan and California, land-based sections from California, and Holocene sediment samples from surrounding areas have been used to develop radiolarian biostratigraphies and to interpret paleo-oceanographic conditions for the Neogene of the Japan Current (a western boundary current) and the California Current (an eastern boundary current). Our studies suggest that (1) the present-day characteristics of boundary currents (western being deeper, narrower, swifter, and less productive than eastern boundary currents) have varied greatly throughout the Neogene, but are tied one to another; and (2) both mid-latitude regions can be correlated biostratigraphically to low latitudes. The Japan region is most correlative to low-latit de radiolarian biostratigraphic zones. The developed biostratigraphy of the region and inferred paleo-oceanographic trends aid in correlation of the California region to low-latitude radiolarian biostratigraphies. Comparison of material from boundary-current regions with equatorial and subpolar North Pacific DSDP material suggests that tectonic events such as the uplift of Panama may be tied to the observed Neogene changes in these boundary-current regions. These studies have also been instrumental in indicating radiolarian taxa that may be used to suggest relative paleodepth of deposition and eutrophic conditions in these boundary-current regions. End_of_Article - Last_Page 514------------