Description: Results from an interannually forced, 0.08 degrees eddy-resolving simulation based on the Hybrid Coordinate Ocean Model, in conjunction with a small but well-determined transport database, are used to investigate the currents and transports associated with the Atlantic meridional overturning circulation (AMOC) in the subpolar North Atlantic (SPNA). The model results yield a consistent warming in the western SPNA since the early 1990s, along with mean transports similar to those observed for the trans-basin AMOC across the World Ocean Circulation Experiment hydrographic section AR19 (16.4 Sv) and boundary currents at the exit of the Labrador Sea near 53 degrees N (39.0 Sv) and east of the Grand Banks near 43 degrees N (15.9 Sv). Over a 34 year integration, the model-determined AMOC across the AR19 section and the western boundary current near 53 degrees N both exhibit no systematic trend but some long-term (interannual and longer) variabilities, including a decadal transport variation of 3-4 Sv from relatively high in the 1990s to low in the 2000s. The decadal variability of the model boundary current transport near 53 degrees N lags the observed winter time North Atlantic Oscillation index by about 2 years and leads the model AMOC across the AR19 section by about 1 year. The model results also show that the long-term variabilities are low compared to those on shorter time scales. Thus, rapid sampling of the current over long time intervals is required to filter out high-frequency variabilities in order to determine the lower frequency variabilities of interest

Description: Intraseasonal signals with periods of 2 to 3 weeks in near-surface alongshore current measurements are detected from four moorings (K1 - K4) deployed from 2000 to 2004 at the 11{degree sign}S section close to the Brazilian coast as part of the German CLIVAR Tropical Atlantic Variability Project. This section crosses the path of the North Brazil Undercurrent, the most powerful western boundary current in the South Atlantic Ocean. We investigate the origin of this intraseasonal variability of the North Brazil Undercurrentby relating the oceanic oscillation of the alongshore currents to its atmospheric counterpart, the meridional wind stress. On average, the results indicate a well-defined lagged (10 days) correlation (~0.6) structure between meridional wind stress and alongshore currents. The oceanic region with the highest cross-correlations is identified as a relatively narrow band along the Brazilian coast, from 22{degree sign}-36{degree sign}S and 40{degree sign}-50{degree sign}W, bounded in the north by an eastward change in coastline orientation. The cross-wavelet transform establishes the common power between the time series of meridional wind stress and alongshore currents, predominantly during austral winter and spring. These signals propagate equatorward with an alongshore speed of 285{plus minus}63 km day-1, consistent with Coastal Trapped Wave theory.

Description: A decade of weak convection in the Labrador Sea associated with decreasing water mass transformation, in combination with advective and eddy fluxes into the convection area, caused significant warming of the deep waters in both the central Labrador Sea and boundary current system along the Labrador shelf break. The connection to the export of Deep Water was studied based on moored current meter stations between 1997 and 2009 at the exit of the Labrador Sea, near the shelf break at 5˚3N. More than 100 year -long current meter records spanning the full water column have been analyzed with respect to high frequency variability, decaying from the surface to the bottom layer, and for the annual mean flow, showing intra- to interannual variability but no detectable decadal trend in the strength of the deep and near-bottom flow out of the Labrador Sea.

Description: Moored observations of Kuroshio current structure and transport variability were made across the channel between northeast Taiwan and the Ryukyu Islands at 24 degreesN from September 19, 1994, to May 27, 1996. This was a cooperative, effort between the United States and Taiwan. The moored array was designated PCM-1, for the World Ocean Circulation Experiment (WOCE) transport resolving array. The dominant current and transport variability occurred on 100-day timescales and is shown by Zhang et al. [2001] to be caused by warm mesoscale eddys merging with the Kuroshio south of the array causing offshore meandering and flow splitting around the Ryukyu Islands. An annual transport cycle could not be resolved from our 20-month moored record because of abasing from the 100-day period events. Sea level difference data were used to extend the transport time series to 7 years giving a variation in the range of the annual transport cycle of 4-10 Sv, with a mean range closer to 4 Sv. The seasonal maximum of 24 Sv occurred in the summer and the seasonal minimum of 20 Sv occurred in the fall. A weaker secondary maximum also occurred in the winter. The cycle of Kuroshio transport appears to result from a combination of local along-channel wind forcing and Sverdrup forcing over the Philippine Sea. Our estimate of the mean transport of the Kuroshio at the entrance to the East China Sea from the moored array is 21.5 +/- 2.5 Sv. The mean transpacific balance of meridional flows forced by winds and thermohaline processes at this latitude requires an additional mean northward flow of 12 Sv with an annual cycle of +/-8 Sv along the eastern boundary of the Ryukyu Islands. The mean transport and annual cycle of the Kuroshio were found to be in reasonable agreement with basin-scale wind-forced models. Remarkable similarities are shown to exist between the mean western boundary currents and their seasonal cycles in the Atlantic (Florida Current and Antilles Current) and Pacific (Kuroshio and boundary current east of Ryukyu Island chain) at the same latitude. However, detailed comparison shows that the mean Kuroshio is weaker and more surface intensified than the mean Florida Current, while the Kuroshio-transport variability is significantly larger.

Description: A new version of SODA, which covers the time period 1958–2005, is used to analyze decadal variability of the Pacific Subtropical Cell (STC) circulation. The analysis is based on transport time series across 9°S and 9°N. At the interannual time scale, STC convergence anomalies decrease during El Niños and increase during La Niñas through Sverdrup transport convergence changes. At decadal time scales, the assimilation shows a reduction of interior STC convergence of about 8 Sv from the 1960s to the 1990s and a subsequent rebound into the early 2000s by a similar amount, in agreement with the STC tendencies reported earlier from geostrophic section analysis, and associated with the occurrence and intensity of ENSO events among the decades analyzed. The results are compared with, and differ significantly from, those obtained by the German ECCO (GECCO) assimilation.

Description: The Deep Western Boundary Current east of the Grand Banks has been observed during 1999–2005 by moored current-meter stations and shipboard current profiling sections. These recent observations can be compared with those of a WOCE moored array deployed during 1993–95 at the same location. Overall, the observations of Deep Water currents east of the Grand Banks reported here do not support suggestions of a basin-wide "slowdown" of the Atlantic Meridional Overturning Circulation.

Description: Over the past 17 years, the western boundary current system of the Labrador Sea has been closely observed by maintaining the 53°N observatory (moorings and shipboard station data) measuring the top-to-bottom flow field offshore from the Labrador shelf break. Volume transports for the North Atlantic Deep Water (NADW) components were calculated using different methods, including gap filling procedures for deployment periods with suboptimal instrument coverage. On average the Deep Western Boundary Current (DWBC) carries 30.2 ± 6.6 Sv of NADW southward, which are almost equally partitioned between Labrador Sea Water (LSW, 14.9 ± 3.9 Sv) and Lower North Atlantic Deep Water (LNADW, 15.3 ± 3.8 Sv). The transport variability ranges from days to decades, with the most prominent multiyear fluctuations at interannual to near decadal time scales (±5 Sv) in the LNADW overflow water mass. These long-term fluctuations appear to be in phase with the NAO-modulated wind fluctuations. The boundary current system off Labrador occurs as a conglomerate of nearly independent components, namely, the shallow Labrador Current, the weakly sheared LSW range, and the deep baroclinic, bottom-intensified current core of the LNADW, all of which are part of the cyclonic Labrador Sea circulation. This structure is relatively stable over time, and the 120 km wide boundary current is constrained seaward by a weak counterflow which reduces the deep water export by 10–15%.

Description: Long term mooring observations show a substantial warming of the Deep Labrador Current (DLC) during the last decade. In this paper we address the question of whether these water mass changes are accompanied by comparable changes in the deep western boundary current. Individual estimates of alongshore current from moored instruments and transports from Lowered ADCP sections indicate a systematic increase of the boundary current strength on the order of 15% of the mean from the period prior to 1999 to the period thereafter. A combination of these measurements allows the indexing of DLC intensity over the last decade.