Terrence M. Joyce

Oceanic transport of subpolar climate signals to mid-depth subtropical waters

The spatial distributions of certain sea-surface properties, such as temperature, fluctuate on timescales from months to decades and in synchrony with the main regional atmospheric patterns comprising the global climate system. Although it has long been assumed that the ocean is submissive to the dictates of the atmosphere, recent studies raise the possibility of an assertive, not merely passive, oceanic role in which water-mass circulation controls the timescales of climate fluctuations. Previously held notions of the immutability of the physical and chemical characteristics of deep water masses are changing as longer time series of ocean measurements indicate that the signatures of varying sea-surface conditions are translated to deep waters,. Here we use such time-series measurements to track signals ‘imprinted’ at the sea surface in the North Atlantic Oceans subpolar Labrador Basin into the deep water of the subtropical basins near Bermuda, and infer an approximately 6-year transit time. We establish a geographic and temporal context for a portion of the long-term warming trend reported for mid-depth subtropical waters over the past 40 or so years,, and we predict that waters at these depths will continue to cool well into the next decade.

Enhanced Warming over the Global Subtropical Western Boundary Currents

Subtropical western boundary currents are warm, fast-flowing currents that form on the western side of ocean basins. They carry warm tropical water to the mid-latitudes and vent large amounts of heat and moisture to the atmosphere along their paths, affecting atmospheric jet streams and mid-latitude storms, as well as ocean carbon uptake1, 2, 3, 4. The possibility that these highly energetic currents might change under greenhouse-gas forcing has raised significant concerns5, 6, 7, but detecting such changes is challenging owing to limited observations. Here, using reconstructed sea surface temperature datasets and century-long ocean and atmosphere reanalysis products, we find that the post-1900 surface ocean warming rate over the path of these currents is two to three times faster than the global mean surface ocean warming rate. The accelerated warming is associated with a synchronous poleward shift and/or intensification of global subtropical western boundary currents in conjunction with a systematic change in winds over both hemispheres. This enhanced warming may reduce the ability of the oceans to absorb anthropogenic carbon dioxide over these regions. However, uncertainties in detection and attribution of these warming trends remain, pointing to a need for a long-term monitoring network of the global western boundary currents and their extensions.

The Relation between Decadal Variability of Subtropical Mode Water and the North Atlantic Oscillation

The Bermuda station ‘‘S’’ time series has been used to define the variability of subtropical mode water (STMW) from 1954 to 1995. This record, which shows decadal variability at a nominal period of about 12‐14 yr, has been used as a baseline for seeking correlation with large-scale atmospheric forcing and with decadal north‐south excursions of the Gulf Stream position defined by the subsurface temperature at 200-m depth. A common time period of 1954‐89 inclusive, defined by the data sources, shows a high degree of correlation among the STMW potential vorticity (PV), Gulf Stream position, and large-scale atmospheric forcing (buoyancy flux, SST, and sea level pressure). Two pentads with anomalously small and large STMW PV were further studied and composites were made to define a revised North Atlantic Oscillation (NAO) index associated with the decadal forcing. During years of low PV at Bermuda, the NAO index is low, the Gulf Stream is in a southerly position, and the zero wind stress curl latitude is shifted south as are the composite extratropical winter storm tracks, in comparison to the period of high PV at Bermuda. Because the NAO, Gulf Stream separation latitude, and STMW PV variations are in phase with maximum annually averaged correlation at zero year time lag, the authors hypothesize that all must be either coupled with one another or with some other phenomenon that determines the covariability. A mechanism is proposed that could link all of the above together. It relies on the fact that during periods of high STMW PV, associated with a northerly Gulf Stream and a high NAO, one finds enhanced production of mode water in the subpolar gyre and Labrador Sea. Export of the enhanced Labrador Sea Water (LSW) component into the North Atlantic via the Deep Western Boundary Current can influence the separation point of the Gulf Stream in the upper ocean once the signal propagates from the source region to the crossover point with the Gulf Stream. If the SST signal produced by the 100-km shift of the Gulf Stream along a substantial (1000 km) length of its path as it leaves the coast can influence the NAO, a negative feedback oscillation may develop with a timescale proportional to the time delay between the change of phase of the air‐ sea forcing in the Labrador Basin and the LSW transient at the crossover point. Both a simple mechanistic model as well as a three-layer numerical model are used to examine this feedback, which could produce decadal oscillations given a moderately strong coupling.

Interannual Variability in the Mid- and Low-Latitude Western North Pacific

Abstract Twenty-two years (1967–88) of hydrographic data collected by the Japan Meteorological Agency along the 137°E meridian and surface wind data compiled by Florida State University (FSU) were analyzed to study the interannual variability in the western North Pacific. In the midlatitude region north of 22°N, the dominant signal in the dynamic height field was the interannual path variations of the Kuroshio. Whereas the eastward transport of the Kuroshio itself had no significant changes between the straight-path and meander-path years, the net transport of the Kuroshio system including recirculations showed a 30% increase during the meander-path years. In the straight-path years when the net transport was small, the Kuroshio tended to take a straight path with a strong recirculation developed to the south. The interannual path variations of the Kuroshio strongly influenced the water-mass movement in the midlatitudes. During the Kuroshio meander years, we found that a significant portion of the North P...

Western Boundary Currents and Frontal Air–Sea Interaction: Gulf Stream and Kuroshio Extension

Abstract In the Northern Hemisphere midlatitude western boundary current (WBC) systems there is a complex interaction between dynamics and thermodynamics and between atmosphere and ocean. Their potential contribution to the climate system motivated major parallel field programs in both the North Pacific [Kuroshio Extension System Study (KESS)] and the North Atlantic [Climate Variability and Predictability (CLIVAR) Mode Water Dynamics Experiment (CLIMODE)], and preliminary observations and analyses from these programs highlight that complexity. The Gulf Stream (GS) in the North Atlantic and the Kuroshio Extension (KE) in the North Pacific have broad similarities, as subtropical gyre WBCs, but they also have significant differences, which affect the regional air–sea exchange processes and their larger-scale interactions. The 15-yr satellite altimeter data record, which provides a rich source of information, is combined here with the longer historical record from in situ data to describe and compare the curr...

Gulf Stream Variability and Ocean-Atmosphere Interactions*

Time series of Gulf Stream position derived from the TOPEX/Poseidon altimeter from October 1992 to November 1998 are used to investigate the lead and lag relation between the Gulf Stream path as it leaves the continental shelf and the changes in sea level pressure, surface wind stress, and sea surface temperature (SST), as given by the NCEP reanalysis. The dominant signal is a northward (southward) displacement of Gulf Stream axis 11 to 18 months after the North Atlantic Oscillation (NAO) reaches positive (negative) extrema. A SST warming (cooling) peaking north of the Gulf Stream is also seen to precede the latitudinal shifts, but it is a part of the large-scale SST anomaly tripole that is generated by the NAO fluctuations. There is no evidence that the Gulf Stream shifts have a direct impact onto the large-scale atmospheric circulation. A fast, passive response of the Gulf Stream to NAO forcing is also suggested by a corresponding analysis of the yearly mean Gulf Stream position estimated from XBT data at 200 m during 1954‐98, where the NAO primarily leads the latitudinal Gulf Stream shifts by 1 yr. The fast Gulf Stream response seems to reflect buoyancy forcing in the recirculation gyres but, as the covariability remains significant when the NAO leads by up to 9 yr, large-scale wind stress forcing may become important after a longer delay. Because of the high NAO index of the last decades, the TOPEX/Poseidon period is one of unprecedented northward excursion of the Gulf Stream in the 45-yr record, with the Gulf Stream 50‐100 km north of its climatological mean position.

The Long-Term Hydrographic Record at Bermuda

Abstract A long, by oceanographic standards, time series of hydrographic observations at Bermuda was begun in 1954 and continues to the present. Analysis of this dataset has shown the temperature and salinity variations on interannual timescales to be largely independent in the surface layer (0–200 dbar), where integral timescales for salinity are 50% longer than for temperature and surface salinity changes are correlated with decadal changes in the 18°C Water production. Temperature/salinity anomalies are highly correlated in the thermocline where the interannual variability at that level is accountable to either vertical oscillations of the thermocline with amplitudes of ±50 m or meridional oscillations of the horizontal gradient set up at the southern edge of the recirculation gyre of ±300 kim. Salinity changes in the deepest layer observed at Bermuda station “S” (1500–2500 dbar) are uncorrelated with temperature, masked by measurement errors in the early years of the time series. Inclusion of earlier ...

A Note on the Lateral Mixing of Water Masses

Abstract The role of medium-scale interleaving of temperature and salinity in frontal regions is investigated and a model is presented in which a statistical equilibrium of the medium scale is achieved. Small-scale diffusion across intrusions, causing an attenuation of their T/S characteristics, is balanced by horizontal advection of heat and salt by the medium-scale motions. The “energy” source for the balance is the lateral variation in the temperature/salinity field associated with water mass transitions. Estimates of the cross frontal heat or sole exchange can he made based upon the intensity of the interleaving T/S fields. The lateral transfer is directly proportional to the vertical transports across intrusion boundaries by microscale processes. The same general principle for the enhancement of the cross frontal heat transfer by interleaving is similar to that achieved in automobile cooling systems by a radiator. The model, in effect, attempts to quantify our ignorance of lateral mixing of water mas...

Velocity and Hydrographic Structure of a Gulf Stream Warm-Core Ring

Abstract The hydrographic and velocity structure of Gulf Stream warm-core ring 81D are investigated using a deep CTD–O2 section through the ring together with horizontal currents at 100 m depth obtained with an acoustic-Doppler system. The pycnocline within the ring is depressed 500 m below that of the surrounding Slope Water and maximum currents at 100 m approach 2 m s−1 at a radius of 70–80 km from the ring center. Water-mass analysis reveals that the waters in the central core of the ring are similar to the Sargasso Sea and distinct from the Slope Water to depths of 1300 m. Near the surface, the central core is rotating as a solid body approximately once every 3.6 days. The azimuthal currents at 100 m were combined with the gradient current relationship to obtain the absolute dynamic topography of the ring. The sea surface in the ring center is 70 dynamic centimeters above that in the Slope Water and ring currents decrease quickly with increasing depth to undetectable levels below 2000 m. The ring is l...

Mean flow and variability in the Kuroshio Extension from Geosat altimetry data

Using altimeter data from the Geosat Exact Repeat Mission (ERM), we investigated the mean flow and temporal and spatial variations of the Kuroshio Extension in the region of 140°–180°E and 30°–40°N. Mean surface height profiles were estimated along individual tracks by assuming the velocity profile of the Kuroshio Extension to be Gaussian-shaped and by successively fitting this synthetic currents height profile to the residual height data. Using the mean profiles from ascending and descending tracks, we derived the mean surface height field by an inverse method and obtained the absolute surface height fields for the first 2.5 years of the Geosat ERM. Both the mean and the instantaneous height fields thus derived compared well with the available hydrographic data and the SST patterns from the NOAA satellites. The mean surface height difference across the Kuroshio Extension attains its maximum around 146°E between the two quasi-stationary meanders, and its decrease thereafter is mainly due to large-scale recirculations on the southern side of the Kuroshio Extension. The ratio of the eddy kinetic energy over the mean kinetic energy has a nearly constant value of 1.5–2.0 along the Kuroshio Extension path. Propagation of mesoscale fluctuations in the height fields is generally westward except for the upstream region of the Kuroshio Extension. Effects of deep mean flow and baroclinic shear are found to be important in explaining the observed propagation speeds. In the upstream region of 141°E and 154°E, annual variations in the surface height difference across the Kuroshio Extension(δh) have a September maximum with an average amplitude of 0.2m. For large-scale interannual fluctuations, anomalies in δh are found to be significantly correlated with those of the current axis positions: a larger surface height difference corresponds to a more northerly position of the Kuroshio Extension. The interannual changes in δh are possibly related to the 86/87 ENSO event in the low-latitude Pacific Ocean.