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

Abstract

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.

North Atlantic climate variability: phenomena, impacts and mechanisms

“…Subsurface ocean observations provide an even clearer depiction of long-term climate variability because the effect of the annual cycle and month-to-month variability in the atmospheric circulation decays rapidly with depth-see Levitus (1989) and Houghton (1996). Variability in the temperature of the upper subtropical ocean has also been linked with fluctuations in the NAO index (Molinari et al, 1997).…”

“…Curry and McCartney (2001) discuss observed interannual to interdecadal variations in the intensity of ocean gyres and relate them to NAO forcings. A large freshening event (dubbed the Great Salinity Anomaly; GSA) occurred in the subpolar gyre in the early 1970s (Dickson et al, 1988;Levitus, 1989). Observations suggest it may have been linked to coherent changes in the overlying atmospheric circulation and an extreme manifestation of quasi-decadal fluctuations (Kushnir, 1994;Hansen and Bezdek, 1996;Reverdin et al, 1997;Belkin et al, 1998).…”

Variability in sea surface height: A qualitative measure for the meridional overturning in the North Atlantic

“…Decadal variability in the sea surface temperatures (SST) [Deser and Blackman, 1993] and in the subsurface temperatures [Levitus, 1989; Malinari et al, 1997; Curry et al, 1998] in the North Atlantic inspire thoughts as to whether any of this variability could be linked to the changes in the oceanic circulation and especially in the meridional circulation cell. The SST time series extends back to the mid 1800s, and limited subsurface observations are available since the 1960s, but neither of them provide an integrated picture of the upper ocean above the permanent thermocline which is anticipated to participate in the thermohaline circulation.…”

Reduced North Atlantic Central Water formation in response to early Holocene ice‐sheet melting

“…The long-term analytical precision of Mg/Ca ratios is based on three consistency standards of Mg/Ca concentrations of 1.10, 2.40 and 6.10 mmol mol −1 . Over the course of this study the precision for the consistency standards was 1.6, 1.2 and 1.2 % (residual standard [Levitus, 1989;McCartney and Talley, 1982]. The total area of Lake Agassiz/Ojibway is shaded in gray and the general pathway of the lake drainage into the polar North Atlantic is marked by a dark arrow modified after Teller et al [2002].…”

“…Surface densities of SPMW range between s Q = 27.3 to 27.6 [Levitus, 1989;McCartney and Talley, 1982] and their subduction and subsequent southward flow along these density surfaces as Eastern North Atlantic Central Waters (ENACW) along the Northwest African coastline is well established [Keffer, 1985;Levitus, 1989;McCartney and Talley, 1982;McDowell et al, 1982]. After ∼30°N ENACW turn southwestward into the southern STG.…”