Sloyan, Bernadette M.
Wijffels, Susan E.
The tropical Pacific and Indian Oceans are connected via a complex system of currents known as the Indonesian Throughflow (ITF). More than 30% of the variability in the ITF is linked to the seasonal cycle, influenced by the Monsoon winds. Despite previous efforts, a detailed knowledge of the ITF response to the components of the seasonal forcing is still lacking. Here, we describe the seasonal cycle of the ITF based on new observations of velocity and properties in Timor Passage, satellite altimetry and a high-resolution regional model. These new observations reveal a complex mean and seasonally varying flow field. The amplitude of the seasonal cycle in volume transport is approximately 6 Sv. The timing of the seasonal cycle, with semi-annual maxima (minima) in May and December (February and September), is controlled by the flow below 600 m associated with semi-annual Kelvin waves. The transport of thermocline waters (<300 m) is less variable than the deep flow but larger in magnitude. This top layer is modulated remotely by cycles of divergence in the Banda Sea, and locally through Ekman transport, coastal upwelling, and non-linearities of the flow. The latter manifests through the formation of eddies that reduce the throughflow during the Southeast Monsoon, when is expected to be maximum. While the reduction in transport associated with the eddies is small, its impact on heat transport is large. These non-linear dynamics develop over small scales (<10 km), and without high enough resolution, both observations and models will fail to capture them adequately.
Joyce, Terrence M.
Sea Surface Temperature (SST) and Sea Surface Height (SSH) data for 1993–2007 for the North Atlantic are combined with hydrographic data at 69W to investigate the relation between the Gulf Stream (GS) position and the Slope Water properties. SST anomalies north of the GS are correlated with changes in its path. The lag of this correlation is such that positive temperature anomalies precede northward shifts of the GS, and suggests that SST anomalies move westward with speeds of several cm/s. EOF analysis of the SST and SSH fields shows that cooling and strengthening of the SW flow are in phase over the Slope Water, which is mirrored in the vertical structure of these fields at 69W, indicating larger transports in the Deep Western Boundary Current lead to southward shifts of the mean GS path. This relation between the Slope Water and the GS path provides some predictability for the latter.
McCartney, Michael S.
Rintoul, Stephen R.
The Antarctic Slope Current (ASC), defined here as the region of westward flow along the continental slope off Antarctica, forms the southern limb of the subpolar gyres. It regulates the exchange of water across the shelf break and provides a path for interbasin westward transport. Despite its significance, the ASC remains largely unobserved around most of the Antarctic continent. Here we present direct velocity observations from a 17 month current meter moored array deployed across the continental slope between the 1000 and the 4200 m isobaths, in the southeastern Indian Ocean near 113°E. The observed time-mean flow consists of a surface-intensified jet associated with the Antarctic Slope Front (ASF) and a broader bottom-intensified westward flow that extends out to approximately the 4000 m isobath and is strongest along the upper slope. The time-mean transport of the ASC is −29.2 Sv. Fluctuations in the transport are large, typically exceeding the mean by a factor of 2. They are mainly due to changes in the northward extent of the current over the lower slope. However, seasonal changes in the wind also drive variations in the transport of the ASF and the flow in the upper slope. Both mean and variability are largely barotropic, thus invisible to traditional geostrophic methods
Joyce, Terrence M.
Toole, John M.
Horizontal velocity, temperature and salinity measurements from the Line W array for the period 2004–2008 show large changes in the water mass structure and circulation of the Deep Western Boundary Current (DWBC). Fluctuations in the flow with periods from 10 to 60 days are bottom intensified: signals most likely associated with topographic Rossby waves (TRW). A fraction (∼15%) of the DWBC transport variability is caused by Gulf Stream rings and meanders. These flow anomalies are surface intensified and fluctuate at frequencies lower than the TRW. Interannual variability in the velocity field appears to be related to changes in the hydrographic properties. The dominant mode of variability is characterized by an overall freshening, cooling, a potential vorticity (PV) increase in the deep Labrador Sea Water (dLSW) and a PV decrease in the Overflow Water (OW). The variability in the flow associated with these property changes is not spatially homogeneous. Offshore (water depths larger than 3500 m) changes in the velocity are in phase with PV changes in the OW: a decrease in the OW PV is accompanied by an increase in the southward (negative) transport. Conversely, variations of the inshore flow are in phase with changes in the dLSW PV (increasing PV and decreasing transport). This trend, true for most of the record, reverses after the winter of 2007–2008. A sudden decrease of the dLSW PV is observed, with a corresponding intensification of the flow in the inner DWBC as well as a northward shift in the Gulf Stream axis.
Joyce, Terrence M.
Toole, John M.
Water properties measured by the central mooring in the Line W mooring
array southeast of Cape Cod document a large character shift during the
period of November 2001 to April 2008. The observed temperature, salinity
and planetary potential vorticity (PPV) anomalies manifest changes in
the formation region of the water masses present at Station W, specifically
upper Labrador Sea Water (uLSW), deep Labrador Sea Water (dLSW) and
Overflow Water (OW). During the observation period, the minimum in the
PPV anomaly field relative to the record mean PPV profile migrated from
1500m, where it was originally found, to 700m. Temporal changes in the vertical
distribution of temperature and salinity were correlated with the PPV
changes. This suggests a dLSW-dominated first half of the record, versus an
uLSW-dominated second half. The structure of these anomalies is consistent
with observations within the Labrador Sea, and their transit time to Line W agrees well with tracer-derived times for signals spreading along the western
boundary. In that context, the observed water properties at Line W in the
early 2000s reflected the intense deep convection in the Labrador Sea in the
mid 1990s, with less intense convection subsequently affecting lighter isopycnals.
The observed velocity field is dominated by high-frequency (periods of
days to months) fluctuations, however, a fraction of the velocity variability is
correlated with changes in water mass properties, and indicate a gradual acceleration
of the southwestward flow, with a corresponding increase in Deep
Western Boundary Current transport.
The variability in the DWBC, its connection to the forcing in the northern North
Atlantic and interaction with the Gulf Stream were explored from a combination of
remote sensing and in-situ measurements in the western North Atlantic.
Using satellite altimetry and Sea Surface Temperature (SST) we found evidence
of the relation between changes in the Gulf Stream path and the variability in the
temperature and velocity fields in the Slope Water. This relation was such that southward
shifts of the main axis of the Gulf Stream were preceded by cold temperature
anomalies and intensification of the southwestward flow.
The analysis of 5.5 years of moored CTD and horizontal velocity data in the
DWBC at 69°W recorded during the period 2002-2008, showed that the variability
along the DWBC is linked to changes in the dense water formation regions. The
evolution of potential vorticity (PV) at the mooring site, characterized by a transition
from deep to upper Labrador Sea Water (LSW), was similar to that observed in the
Labrador Sea 6 to 9 years earlier, and imply spreading rates for the LSW that varied
over time from 1.5 to 2.5cm/s. The time dependence of the spreading rates was in
good agreement with changes in the strength of the DWBC at the mooring site.
The evolution of the DWBC transport was explored in more detail from a 5-element moored array, also at 69°W. The results, for the period of 2004-2008, were
consistent with the single mooring analysis. The variability measured from the array
showed that upper, intermediate and deep water mass layers expand and contract
at each other’s expense, leading to alternating positive and negative PV anomalies
at the upper-LSW, deep-LSW and Overflow Water (OW). Larger DWBC transports
were associated with enhanced presence of recently ventilated upper-LSW and OW,
rather than deep-LSW. The relative contribution of the different water masses to the
observed circulation was investigated by inverting individual PV anomalies isolated
from the observations. We found that changes in the depth-integrated circulation
were mostly driven by changes in the OW.