Eric Firing

Source waters of the Pacific Equatorial Undercurrent

Abstract Hydrographic and direct current measurements were made north and east Papua New Guinea in June–August 1985 and January–February 1986 as part of the Western Equatorial Pacific Ocean Circulation Study (WEPOCS). Analyses of the data indicate that the major portion of the water in the Equatorial Undercurrent at its beginning north of Papua New Guinea is supplied from the south by a narrow western boundary undercurrent (New Guinea Coastal Undercurrent) transporting high-salinity, low-tritium, high-oxygen, low-nutrient water from the Solomon Sea northwestward along the north coast of Papua New Guinea through the Vitiaz Strait. The New Guinea Coastal Undercurrent has a maximum speed of 40–70 cm s −1 at a depth of about 200 m. It is a permanent feature despite the reversals of the wind and the surface current during the period of the northwest monsoon in austral summer. Its transport through the Vitiaz Strait is as high as 8 × 10 6 m 3 s −1 , which is of the same magnitude as the Equatorial Undercurrent transport at 143°E. The New Guinea Coastal Undercurrent revealed by the WEPOCS data is consistent with the low-latitude equatorward western boundary current implied in a calculation of the Sverdrup transport based on the observed wind-stress distribution for the tropical Pacific Ocean. High-salinity, low-tritium, low -oxygen, high -nutrient water which flows westward into the Bismarck Sea passing north of the Solomon Islands is entrained into the Equatorial Undercurrent north of New Ireland and returns to the east, resulting in a down-stream increae in the Undercurrent transport. Low-salinity, high-tritium, high-nutrient water of eastern North Pacific origin also contributes to the Equatorial Undercurrent in its source area west of the WEPOCS region. However, there is no evidence that northern waters are being continuously entrained into the Undercurrent in the WEPOCS region.

The geostrophic balance of the Pacific Equatorial Undercurrent

Abstract The geostrophic balance of the mean Pacific Equatorial Undercurrent (EUC) has been demonstrated by using current and density measurements obtained during the 16-month NORPAX Hawaii-to-Tahiti Shuttle Experiment (February 1979 to June 1980). The computed core depth, maximum speed, and boundaries of the EUC agree remarkably with those measured by profiling current meters.

Changes in Ocean Heat, Carbon Content, and Ventilation: A Review of the First Decade of GO-SHIP Global Repeat Hydrography.

Global ship-based programs, with highly accurate, full water column physical and biogeochemical observations repeated decadally since the 1970s, provide a crucial resource for documenting ocean change. The ocean, a central component of Earths climate system, is taking up most of Earths excess anthropogenic heat, with about 19% of this excess in the abyssal ocean beneath 2,000 m, dominated by Southern Ocean warming. The ocean also has taken up about 27% of anthropogenic carbon, resulting in acidification of the upper ocean. Increased stratification has resulted in a decline in oxygen and increase in nutrients in the Northern Hemisphere thermocline and an expansion of tropical oxygen minimum zones. Southern Hemisphere thermocline oxygen increased in the 2000s owing to stronger wind forcing and ventilation. The most recent decade of global hydrography has mapped dissolved organic carbon, a large, bioactive reservoir, for the first time and quantified its contribution to export production (∼20%) and deep-ocean oxygen utilization. Ship-based measurements also show that vertical diffusivity increases from a minimum in the thermocline to a maximum within the bottom 1,500 m, shifting our physical paradigm of the oceans overturning circulation.

Comparison of three velocity sections of the Agulhas Current and Agulhas Undercurrent

Lowered acoustic Doppler current profiles (LADCP) from an early March 1995 cruise across the Agulhas Current show a swift, narrow undercurrent flowing northeast along the continental slope. Neither this Agulhas Undercurrent nor the adjacent deep extension of the Agulhas Current are evident from measurements of water properties alone, and their absence from the conventional referencing of geostrophic current estimates biases net southward transport estimates high by several sverdrups. Here we refine the original transport calculation by removing barotropic tides and by estimating instrumental and sampling errors. Two additional LADCP sections, from cruises in late March and June 1995, also show the undercurrent and the deep extension of the Agulhas. Differences in the current structure are evident. The Agulhas Current extends throughout the water column in March, but extends only to 2300 m depth in June. Additionally, the current extends further offshore in March. Of the three available LADCP sections, only those from early March and June have sufficient sampling to calculate the net southward transport of the Agulhas Current and Undercurrent. The two estimates, 78±3 and 76±2 Sv, are nearly identical. Consideration of water properties on density surfaces shows that although the undercurrent carries intermediate water with Red Sea Water influence northward, the bulk of this water mass is flowing southward, away from its source, in the Agulhas Current.

Mixing in the western equatorial Pacific and its modulation by ENSO

[1]xa0High vertical resolution measurements of the flow in the western equatorial Pacific reveal the vertical shear to be dominated by flow features that have a small vertical scale (O(10 m)). This is true for all the measurements we have taken over a 3 year period and differing ENSO states. Much of the measured turbulent activity is found to be associated with these small scale features, with the suggestion that mixing in the region is heavily influenced by the presence of this small scale activity. The level of mixing within and above the thermocline is strongly modulated by ENSO events, with the level of mixing being significantly greater during the observed La Nina event. Changes to the stratification above the thermocline during ENSO events play a major role in both changes to the level of turbulent activity and the effective vertical diffusion coefficient.

Currents in the Aleutian Basin and subarctic North Pacific near the dateline in summer 1993

[1]xa0The currents in the Aleutian Basin of the Bering Sea and in the subarctic North Pacific near the dateline in summer 1993 are described, based on Shipboard Acoustic Doppler Current Profiler and Lowered Acoustic Doppler Current Profiler velocity measurements and on conductivity-temperature-depth profiles. The strongest flow is the Alaskan Stream with velocities up to 90 cm s−1 and with a subsurface core as narrow as 30 km at about 150 m depth. Velocity at the bottom below the Alaskan Stream could not be distinguished from zero. The inflow of the Alaskan Stream through Amchitka Pass accounted for about 70% of the Aleutian North Slope Current in the Aleutian Basin. Two anticyclonic eddies occupied most of the Aleutian Basin along the cruise track. Both eddies extended to the bottom.

Shear‐generated turbulence in the equatorial Pacific produced by small vertical scale flow features

We investigate the characteristics of shear-generated turbulence in the natural environment by considering data from a number of cruises in the western equatorial Pacific. In this region, the vertical shear of the flow is dominated by flow structures that have a relatively small vertical scale of O(10 m). Combining data from all cruises, we find a strong relationship between the turbulent dissipation rate, ϵ, vertical shear, S, and buoyancy frequency, N. Examination of ϵ at a fixed value of Richardson number, Riu2009=u2009N2∕S2, shows that ϵ∝ut2N for a wide range of values of N, where ut is an appropriate velocity scale which we assume to be the horizontal velocity scale of the turbulence. The implied vertical length scale, lvu2009=u2009ut∕N, is consistent with theoretical and numerical studies of stratified turbulence. Such behavior is found for Riu2009<u20090.4. The vertical diffusion coefficient then scales as κv∝ut2/N at a fixed value of Richardson number. The amplitude of ϵ is found to increase with decreasing Ri, but only modestly, and certainly less dramatically than suggested by some parameterization schemes. Provided the shear generating the turbulence is resolved, our results point to a way to parameterize the unresolved turbulence.