Philip L. Richardson

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.

Two Years in the Life of a Mediterranean Salt Lens

Abstract A lens of Mediterranean water (Meddy) was tracked in the eastern North Atlantic for two years with SOFAR floats. The Meddy was first found between the Canary Islands and the Azores in October 1984. It center moved in an irregular pattern, at speeds of a few cm s−1, and translated 1100 km to the south in two years. This Meddy was surveyed four times by CTD and velocity profilers, and once with the microstructure profiler EPSONDE. When observed during the first two surveys the Meddy had a core that was stably and smoothly stratified in both salinity and temperature, nearly uniform in the horizontal, and was saltier than the surrounding ocean by 0.65 psu. The Meddy was eroded from its edges, top and bottom, and lost salt and hat with an e-folding time of about one year. The salinity at the center remained at its original value during the first year and decreased during the second year. Evidence was seen for mixing by lateral intrusions, double diffusion, and turbulence; the intrusions are thought to...

On the sources of the Florida Current

Abstract In our opinion roughly 13 Sv or 45% of the transport of the Florida Current is of South Atlantic origin, as compensation for the cross-equatorial flow of North Atlantic Deep Water. Of the 8.9 Sv moving through the Straits of Florida with temperatures above 24°C in the upper 100m of the water column, 7.1 Sv is composed of comparatively fresh water coming through the southern Caribbean passages from the tropical South Atlantic. Saltier surface water, 1.8 Sv, enters from the North Atlantic through Windward Passage, as does most of the 18° Water in the Florida Current. A South Atlantic contribution for the uppermost layer is clear-cut because the surface water in the open Atlantic north of the Caribbean is comparatively cold and salty and intrudes south as Subtropical Underwater or Salinity-Maximum Water below a comparatively warm and fresh layer 50-100 m thick, which could hardly he transported from the North Atlantic. Of the 13.8 Sv transported through the Caribbean in the 12-24°C temperature range, 13.0 Sv is of North Atlantic origin, with about 0.8 Sv of comparatively fresh South Atlantic water on the western side of the Florida Straits having entered the Caribbean on the southern side of St. Vincent and St. Lucia Passages. Of the 6 Sv transported by the Florida Current in the 7-12°C temperature range, 5 Sv appears to originate in the South Atlantic. Our estimate of the 13 Sv of South Atlantic and 16 Sv of North Atlantic origin for the total transport of 29 Sv for the Florida Current, along with partitioning in the aforementioned temperature ranges, is approximately consistent with open ocean sections along 24°N and with several previous investigations. We have formed a new estimate of the transport into five key Caribbean passages, yielding 28.8 Sv for the temperature range appropriate to the Straits of Florida off Miami, in close agreement with independent transport measurements for the Florida Current. The five passages and their contributions are: Grenada (7.7 Sv), St. Vincent (7.9 Sv), St. Lucia (3.8 Sv), Dominica (2.6 Sv), and Windward (6.8 Sv). Breakdowns of these passage transport estimates into broad classes by temperature range agree to within about 2 Sv in comparison with similar quantities for the Florida Current. Anegada Passage may transport 0.5 Sv of water that exits through the upper 200 m or so of the Florida Current, and the mid-depth (5-12°C) flow in this passage and in the general vicinity of the Caribbean deserves further examination.

Directly measured mid-depth circulation in the northeastern North Atlantic Ocean

The circulation of water masses in the northeastern North Atlantic Ocean has a strong influence on global climate owing to the northward transport of warm subtropical water to high latitudes. But the ocean circulation at depths below the reach of satellite observations is difficult to measure, and only recently have comprehensive, direct observations of whole ocean basins been possible. Here we present quantitative maps of the absolute velocities at two levels in the northeastern North Atlantic as obtained from acoustically tracked floats. We find that most of the mean flow transported northward by the Gulf Stream system at the thermocline level (about 600 m depth) remains within the subpolar region, and only relatively little enters the Rockall trough or the Nordic seas. Contrary to previous work, our data indicate that warm, saline water from the Mediterranean Sea reaches the high latitudes through a combination of narrow slope currents and mixing processes. At both depths under investigation, currents cross the Mid-Atlantic Ridge preferentially over deep gaps in the ridge, demonstrating that sea-floor topography can constrain even upper-ocean circulation patterns.

A census of Meddies tracked by floats

Recent subsurface float measurements in 27 Mediterranean Water eddies (Meddies) in the Atlantic are grouped together to reveal new information about the pathways of these energetic eddies and how they are often modified and possibly destroyed by collisions with seamounts. Twenty Meddies were tracked in the Iberian Basin west of Portugal, seven in the Canary Basin. During February 1994 14 Meddies were simultaneously observed, 11 of them in the Iberian Basin. Most (69%) of the newly formed Meddies in the Iberian Basin translated southwestward into the vicinity of the Horseshoe Seamounts and probably collided with them. Some Meddies (31%) passed around the northern side of the seamounts and translated southwestward at a typical velocity of 2.0 cm/s into the Canary Basin. Some Meddies observed there were estimated to be up to ∼5 yr old. Four Meddies in the Canary Basin collided with the Great Meteor Seamounts and three Meddies were inferred to have been destroyed by the collision. Overall an estimated 90% of Meddies collided with major seamounts. The mean time from Meddy formation to a collision with a major seamount was estimated to be around 1.7 yr. Combined with the estimated Meddy formation rate of 17 Meddies/yr from previous work, this suggests that around 29 Meddies co-exist in the North Atlantic. Therefore during February 1994 we observed about half of the population of Meddies.

North Brazil Current retroflection eddies

During 1989–1992, six different anticyclonic eddies were observed to translate up the coast of South America between 7°N and 12°N. These eddies, which are similar to those recently observed with coastal zone color scanner images, current meters, and altimetry, are inferred to have formed from pieces of the North Brazil Current, which retroflects or veers offshore near 7°N to flow eastward. The maximum near-surface diameter of the eddies when newly formed is estimated to be 400 km. The retroflection eddies were identified in trajectories of four surface drifters and three SOFAR floats at depths from 900–1200 m that looped as they translated northwestward with a mean velocity of 9 cm/s. The two longest trajectories were (1) by a 900-m float beginning near 7°N in August 1989 that looped 14 times over 151 days with a maximum diameter of 140 km and (2) by a surface drifter beginning near 7°N in October 1990 that looped 9 times over 116 days with a maximum diameter of 250 km and maximum swirl speed of 80 cm/s at that diameter. The mean rotation periods of these two eddies were 11 and 13 days, respectively, with the rotation period of small loops at 900 m around 7 days. Three of the eddies were observed during the period August 1989 to April 1990. Retroflection eddies are considered to provide significant northward volume transport, ∼3 Sv, along the western boundary even during the months when the North Brazil Current has been observed to retroflect into the countercurrent.

Distribution and Movement of Gulf Stream Rings

Abstract The distribution, number and movement of cyclonic Gulf Stream rings were estimated from an analysis of 50 000 temperature records obtained from the National Oceanographic Data Center and Fleet Numerical Weather Central. The data were taken from 1970 through September 1976 in the region bounded by 20–40°N and 50–80°W. Additional ring observations from other sources were also used. Twenty-five ring time series, together with 26 single ring observations were obtained; approximately 11 rings were found to exist at one time. Rings typically moved westward, turned southwest when close to the Gulf Stream and appeared to coalesce with the Stream near Florida. On the average, two rings per year moved down this path with a mean speed of 3 km day−1 and an estimated life span of 2–3 years. Although ring observations were concentrated in the northwestern Sargasso Sea, several were documented east of 60°W. In addition to cold core rings several warm eddies were found south of the Stream; they consisted of at l...

Gulf Stream Rings

Gulf Stream rings are a special type of eddy whose origin has been well documented; they form from cut-off Gulf Stream meanders (Fuglister 1972). Rings are the most energetic eddies in the ocean and their thermocline displacements, swirl speeds and volume transports are nearly equivalent to those of the Gulf Stream.

Deep cross-equatorial flow in the Atlantic measured with SOFAR floats

Neutrally buoyant SOFAR floats at nominal depths of 800, 1800, and 3300 m were tracked for 21 months in the vicinity of tropical boundary currents in the Atlantic near 6°N and at several sites near 11°N as well as along the equator. Trajectories at 1800 m show a swift (>50 cm/s), narrow (100 km wide), southward flowing deep western boundary current (DWBC) extending from 7°N to the equator. The average transport per unit depth in the DWBC was estimated to be 13.8 × 103 m2/s. Coupling this value with mean velocities measured in the DWBC by current meters gave a volume transport of 15 × 106 m3/s between depths of 900 m and 2800 m. Approximately 6 × 106 m3/s recirculated northward between the DWBC and the Mid-Atlantic Ridge, leaving 9 × 106 m3/s as cross-equatorial transport. No obvious DWBC nor swift equatorial current was observed by the 3300-m floats; a low mean velocity at this depth lay between F-11 and higher velocity cores above and below. The 1800-m trajectories also suggest that at times (February-March 1989) the North Atlantic Deep Water in the DWBC turned eastward and flowed along the equator and at other times (August-September 1990) the DWBC crossed the equator and continued southward. The velocity near the equator, calculated by grouping floats in a box along the equator, was eastward at 4.1 cm/s from February 1989 to February 1990 and westward at 4.6 cm/s from March 1990 to November 1990. Thus the amount of cross-equatorial flow in the DWBC appeared to be linked to low-frequency variability of the structure of the equatorial current system. Floats in Antarctic Intermediate Water at 800 m revealed a northwestward western boundary current, although flow patterns were complicated. Three floats that significantly contributed to the northwestward flow looped in anticyclonic eddies that translated up the coast at 8 cm/s. Six 800-m floats drifted eastward along the equator between 5°S and 6°N at a mean velocity of 11 cm/s; one reached 5°W in the Gulf of Guinea, suggesting that the equatorial currents at this depth extended at least 35°–40° along the equator. Three of these floats reversed direction near the end of the tracking period.

Gulf Stream Ring Trajectories

Abstract During the period 1976–78, the movement of 14 Gulf Stream rings, including two anticyclonic and 12 cyclonic rings, was measured with satellite-tracked free-drifting buoys. The buoys in the cyclonic rings showed a tendency to move out toward the high-velocity region of the ring and to remain there circling the center. One buoy stayed in a ring as long as 8 months and completed 86 loops. Periods of rotation ranged from less than 2 days up to 10 days. The movement of the rings was complicated and appears to be related to the Gulf Stream and strong topographic features such as the New England Seamounts. Rings that were not touching the Stream generally moved westward with typical speeds of 5 cm s−1. Rings that were attached to the Stream generally moved downstream in the Stream with speeds up to 75 cm s−1. Frequently rings coalesced with the Gulf Stream and one of the following three things seemed to happen: 1) the ring turned into an open meander of the Stream and was lost; 2) the ring was advected ...