Historical Hydrographic Observations Research Articles

Human-induced changes in the global meridional overturning circulation are emerging from the Southern Ocean

In a warming climate, the Global Meridional Overturning Circulation (GMOC) is expected to change significantly with a risk of disrupting the global redistribution of ocean properties that sustains marine ecosystems, carbon cycle, and others. Here we make a novel attempt to utilize a diagnostic ocean & sea-ice model to estimate the GMOC and its interdecadal changes since the mid-1950s that are consistent with historical hydrographic observations. We find that significant changes in the GMOC have already occurred, most notably in the upper and lower overturning cells in the Southern Ocean. The former has expanded poleward and into denser water and strengthened by 3–4 Sv since the mid-1970s, while the latter has contracted and weakened by a similar rate during the same period. These changes are driven by the increasing Southern Hemisphere (SH) Ferrel cell and associated increases in the westerlies and the surface buoyancy loss over its sinking branch, and the increasing Antarctic meltwater discharge, in response to ozone depletion in the SH stratosphere and increasing atmospheric CO2. A large-scale readjustment of the GMOC seems to be underway in the South Atlantic and Indo-Pacific Oceans since the mid-2000s in response to the Southern Ocean changes.

Observations of the warm-tongue circulation in the northern East China Sea

A subsurface thermohaline front semi-permanently formed in association with near-bottom cyclonic circulation in the northern East China Sea was newly found from detailed hydrographic data collected during two cruises in February 2017 (winter) and April 2018 (spring) along with supplementary satellite remote sensing and historical hydrographic data. An alternate intruding frontal structure in water properties was observed across the cyclonic circulation in both seasons as formed by two contrasting water masses—low-temperature and low-salinity (i.e., low spiciness) water transported by the East China Sea Current and high-temperature and high-salinity (i.e., high spiciness) water transported by the Tsushima Warm Current. Consistent structures were confirmed from current observations during the two cruises, historical hydrographic observations, and satellite altimetry-derived sea surface height and surface frontal structure, indicative of retroflection of the Cheju Warm Current as deemed by the seasonal development of thermal stratification in spring. Our results reveal significant heat and material exchanges between the open Pacific and the broad shelf, particularly via diapycnal mixing and cross-front transports associated with across-front flow and cyclonic circulation, in the northern East China Sea.

Pacific Abyssal Transport and Mixing: Through the Samoan Passage versus around the Manihiki Plateau

AbstractThe main source feeding the abyssal circulation of the North Pacific is the deep, northward flow of 5–6 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) through the Samoan Passage. A recent field campaign has shown that this flow is hydraulically controlled and that it experiences hydraulic jumps accompanied by strong mixing and dissipation concentrated near several deep sills. By our estimates, the diapycnal density flux associated with this mixing is considerably larger than the diapycnal flux across a typical isopycnal surface extending over the abyssal North Pacific. According to historical hydrographic observations, a second source of abyssal water for the North Pacific is 2.3–2.8 Sv of the dense flow that is diverted around the Manihiki Plateau to the east, bypassing the Samoan Passage. This bypass flow is not confined to a channel and is therefore less likely to experience the strong mixing that is associated with hydraulic transitions. The partitioning of flux between the two branches of the deep flow could therefore be relevant to the distribution of Pacific abyssal mixing. To gain insight into the factors that control the partitioning between these two branches, we develop an abyssal and equator-proximal extension of the “island rule.” Novel features include provisions for the presence of hydraulic jumps as well as identification of an appropriate integration circuit for an abyssal layer to the east of the island. Evaluation of the corresponding circulation integral leads to a prediction of 0.4–2.4 Sv of bypass flow. The circulation integral clearly identifies dissipation and frictional drag effects within the Samoan Passage as crucial elements in partitioning the flow.

Wind modulation of upwelling at the shelf‐break front off Patagonia: Observational evidence

AbstractThe South‐Atlantic Patagonian shelf is the largest chlorophyll‐a (Chl‐a) hot spot in Southern Ocean color images. While a persistent 1500 km long band of high Chl‐a along the shelf‐break front (SBF) is indicative of upwelling, the mechanisms that drive it are not entirely known. Along‐front wind oscillations can enhance upwelling and provide a nutrient pumping mechanism at shelf‐break fronts of western boundary currents. Here we assess wind‐induced upwelling at the SBF off Patagonia from daily satellite Chl‐a and winds, historical hydrographic observations, cross‐shelf Chl‐a fluorescence transects from two cruises, and in situ winds and water column structure from a mooring site. Satellite Chl‐a composites segregated by along‐front wind direction indicate that surface Chl‐a is enhanced at the SBF with southerly winds and suppressed with northerly winds. Northerly winds also result in enhanced Chl‐a further offshore (∼25–50 km). Synoptic transects as well as mean hydrographic sections segregated by along‐front winds show isopycnals tilted upward for southerly winds. Spring observations from the mooring also suggest that southerly winds destratify the water column and northerly winds restratify, in agreement with Ekman transport interacting with the front. Moreover, changes in water column temperature lag along‐front wind forcing by 2–4 days. Our results suggest that oscillations in along‐front winds, on timescales typical of atmospheric storms (2–10 days), can significantly modulate the upwelling and Chl‐a concentrations at the SBF off Patagonia, revealing the importance of wind‐induced upwelling for shelf‐slope exchange at shelf‐break fronts of western boundary currents.

New developments in mode-water research: an introduction

Two landmark papers appeared in 1969, laying the foundation for this special section in Journal of Oceanography. Masuzawa (1969) coined the term ‘‘subtropical mode water’’ (STMW) in the North Pacific, characterizing it as a thick, vertically uniform layer of rh 25.2–25.6 that forms a distinct mode in water volume censuses as a function of temperature, salinity or density. By carefully studying historical hydrographic observations, Uda and Hasunuma (1969) discovered an eastward subtropical countercurrent (STCC) in the southern subtropical gyre where the Sverdrup theory predicts a broad westward current. This countercurrent is peculiar in its direction against both the prevailing northeast trade winds and the westward current underneath. Uda and Hasunuma noted in passing that the STMW resides north of the STCC, splitting the thermocline into a northward-shoaling upper branch and a downward-sloping lower thermocline. Since then, studies of mode water and STCC went their separate ways. The STMW is a favorite subject for water-mass analysts, with other mode waters found in the 1990s in the central and eastern North Pacific. The strong deviation of STCC from the barotropic Sverdrup streamfunction, on the other hand, captured the imagination of dynamists, who proposed a flurry of theories that attempt to explain this peculiar current. A self-consistent theory of STCC had to wait for 30 years, identifying none other than mode waters as the culprit. As often happens in history, seemingly distinct phenomena—mode water and STCC in this case—turn out to be closely related from a fresh perspective under the light of new theories. Based on a ventilated thermocline model, Kubokawa (1999) showed that mode waters of different densities, formed in the deep winter mixed layer around the Kuroshio Extension east of Japan, will cross their paths on their way south in the subtropical gyre, stack up in the vertical, and shoal the upper thermocline, creating a surface-intensified STCC on the south flank of the modewater pool. Subsequent observational and modeling studies lend support for this theory of STCC. Thus, mode waters are far more than just thick water masses moving around passively in the subtropical gyre but assume the role of a dynamical agent that causes surface currents to deviate substantially from the Sverdrup dynamics. In retrospect, the dynamical effect of mode water may not be too surprising given that mode waters are characterized by their low potential vorticity, and that potential vorticity is a dynamical tracer conservative following a parcel’s trajectory on an isopycnal. S.-P. Xie (&) International Pacific Research Center and Department of Meteorology, SOEST, University of Hawaii at Manoa, Honolulu, HI 96822, USA e-mail: xie@hawaii.edu

Regional distribution of steric and mass contributions to sea level changes

The contributing factors to regional sea level variability have been explored for the period 2004–2008 based on altimetry observations, hydrographic data and GRACE measurements. The regional averaged annual cycle of the mass contribution to sea level is shown to be highly unsteady. When compared with steric-corrected altimetry, both signals are coherent, though in some regions the coherence analysis is limited by the use of interpolated hydrographic data and in the equatorial regions it is limited by the low signal-to-noise ratio of GRACE data. The closure of regional sea level budgets depends mainly on the GIA correction chosen. A reconstructed global sea level field (with the atmospheric signal eliminated) spanning the second half of the 20th century together with historical hydrographic observations are used to infer the regional mass contribution to sea level rise for the last decades. Results indicate that mass addition from continental ice is the major contributor to regional mean sea level rise for the last decades. In addition, the spatial patterns of mass rates of change point at Greenland as the main source of fresh water input.

Cold deep water in the South China Sea

Two deep channels that cut through the Luzon Strait facilitate deep (>2000 m) water exchange between the western Pacific Ocean and the South China Sea. Our observations rule out the northern channel as a major exchange conduit. Rather, the southern channel funnels deep water from the western Pacific to the South China Sea at the rate of 1.06 ± 0.44 Sv (1 Sv = 106 m3s−1). The residence time estimated from the observed inflow from the southern channel, about 30 to 71 years, is comparable to previous estimates. The observation-based estimate of upwelling velocity at 2000 m depth is (1.10 ± 0.33) × 10−6 ms−1, which is of the same order as Ekman pumping plus upwelling induced by the geostrophic current. Historical hydrographic observations suggest that the deep inflow is primarily a mixture of the Circumpolar Deep Water and Pacific Subarctic Intermediate Water. The cold inflow through the southern channel offsets about 40% of the net surface heat gain over the South China Sea. Balancing vertical advection with vertical diffusion, the estimated mean vertical eddy diffusivity of heat is about 1.21 × 10−3 m2s−1. The cold water inflow from the southern channel maintains the shallow thermocline, which in turn could breed internal wave activities in the South China Sea.

Interdecadal water mass changes in the Southern Ocean between 30°E and 160°E

Interdecadal water mass changes in the Indian ‐ Western Pacific sectors of the Southern Ocean were investigated using the Japanese Antarctic Research Expeditions and historical hydrographic observations from the 1950s to 1990s. Freshening and cooling occurred on the neutral density surfaces of 27.0 kg·m−3 equatorward of Sub‐Antarctic Front. Results for the area south of the Polar Front show warm and saline anomalies and oxygen decreases on the surfaces around 27.9 kg·m−3, which correspond to the Upper Circumpolar Deep Water. These latter anomalies are most simply explained by the mixing of these shallow waters with warmer and fresher surface waters. Steric sea level has also increased with an average change of 1mm·yr−1 from the 1970s to 1990s. The changes are larger north of the Sub‐Antarctic Front, implying a strengthening of the Antarctic Circumpolar Current. It appears that the observed changes are consistent with the results from coupled climate model results for a similar period.

Errata: Intermediate Circulation in the Northwestern North Pacific Derived from Subsurface Floats

In order to examine the formation, distribution and synoptic scale circulation structure of North Pacific Intermediate Water (NPIW), 21 subsurface floats were deployed in the sea east of Japan. A Eulerian image of the intermediate layer (density range: 26.6–27.0σθ) circulation in the northwestern North Pacific was obtained by the combined analysis of the movements of the subsurface floats in the period from May 1998 to November 2002 and historical hydrographic observations. The intermediate flow field derived from the floats showed stronger flow speeds in general than that of geostrophic flow field calculated from historical hydrographic observations. In the intermediate layer, 8 Sv (1 Sv ≡ 106 m3s−1) Oyashio and Kuroshio waters are found flowing into the sea east of Japan. Three strong eastward flows are seen in the region from 150°E to 170°E, the first two flows are considered as the Subarctic Current and the Kuroshio Extension or the North Pacific Current. Both volume transports are estimated as 5.5 Sv. The third one flows along the Subarctic Boundary with a volume transport of 5 Sv. Water mass analysis indicates that the intermediate flow of the Subarctic Current consists of 4 Sv Oyashio water and 1.5 Sv Kuroshio water. The intermediate North Pacific Current consists of 2 Sv Oyashio water and 3.5 Sv Kuroshio water. The intermediate flow along the Subarctic Boundary contains 2 Sv Oyashio water and 3 Sv Kuroshio water.

Interpentadal variability of steric sea level and geopotential thickness of the north Atlantic Ocean, 1970–1974 versus 1955–1959

We have composited and objectively analyzed historical hydrographic observations for the North Atlantic Ocean for two periods, 1955–1959 and 1970–1974. Difference fields of steric sea level and geopotential thickness for the North Atlantic Ocean in the 0‐ to 1500‐m depth range between the 1955–1959 and 1970–1974 pentads were computed. We find that in the central portion of the subtropical gyre, steric sea level decreased by up to 17.5 dyn cm, whereas in the western subarctic gyre (north of the Gulf Stream) steric sea level increased by an amount up to 7.5 dyn cm from the earlier to the later pentad. A decrease in steric sea level (∼5 dyn cm) occurred along the eastern boundary of the North Atlantic.

Interpentadal variability of temperature and salinity in the deep North Atlantic, 1970–1974 versus 1955–1959

Historical hydrographic observations have been composited and objectively analyzed for the North Atlantic Ocean for two pentads, 1955–59 and 1970–74, for the purpose of studying the temporal variability of the thermohaline structure of the deep North Atlantic Ocean. Statistically significant differences between the two pentads are found in the deep ocean. At 1750 m depth most of the North Atlantic increased in temperature (approximately 0.1°C) and salinity (0.025‰) from the earlier to the later period. An exception to this was a region in the eastern Atlantic between 33°N and 50°N where a cooling (0.1°C) and freshening (0.025‰) occurred. Changes at other depths as evidenced by difference fields of temperature along 24.5°N and 36.5°N are in agreement with differences found by Roemmich and Wunsch (1984) who described changes at these latitudes between sections taken in the late 1950's and sections taken in 1981.

Interpentadal variability of salinity in the upper 150 m of the North Atlantic Ocean, 1970–1974 versus 1955–1959

We have composited and objectively analyzed historical hydrographic observations for the North Atlantic Ocean for two pentads, 1955–1959 and 1970–1974. Our purpose is the study of the temporal variability of salinity in the upper layers of the North Atlantic Ocean. Upper ocean (0–150 m) salinity differences between the two pentads exhibit statistically significant changes. The subarctic gyre east and north of Newfoundland freshened by as much as 0.5‰. The northern portion of the subtropical gyre increased in salinity by as much as 0.4‰, while the interior of this gyre decreased in salinity.

Interpentadal variability of temperature and salinity at intermediate depths of the north Atlantic Ocean, 1970–1974 versus 1955–1959

We have composited historical hydrographic observations for the world ocean for two pentads, 1955–1959 and 1970–1974, and have objectively analyzed these data for the purpose of studying the temporal variability of temperature and salinity in the North Atlantic Ocean. The subtropical gyre (500–1300 m) of the North Atlantic was colder and fresher during 1970–1974 compared to 1955–1959. The relatively large changes in temperature and salinity on constant‐depth surfaces in the subtropical gyre were due to upward displacements of isopycnal surfaces. At intermediate depths the eastern portion of the subarctic gyre was also colder and fresher during the 1970–1974 pentad. In contrast, the western portion of the subarctic gyre exhibited higher temperatures and salinities during 1970–1974 compared to 1955–1959. Characteristic differences in temperature and salinity for both gyres were 0.5°C and 0.025‰, respectively. Analysis of changes in potential vorticity indicates that in the subtropical gyre potential vorticity increased in all density layers above the 26.5 surface, suggesting that convection in this region may have been shallower in 1970–1974 compared to 1955–1959. Below the 26.5 surface, potential vorticity decreased through the 27.3 surface, indicating that deeper than normal convection may have formed the water masses in these layers. At 250‐m depth, relatively large changes in temperature and salinity occurred along the line of maximum climatological mixed layer depth that extends northeast to southwest across the North Atlantic Ocean.