Moored Array Research Articles

Variability of sea surface salinity in the tropical Indian Ocean as inferred from Aquarius and in situ data sets

Sea surface salinity (SSS) is one of the key components of the Earth’s global water cycle. Reliable information on SSS is very important for ocean modelling, data assimilation, and ocean and climate research applications. In this study, SSS variability in the tropical Indian Ocean (TIO) was analysed using the Aquarius instrument on board the SAC-D satellite and in situ observations from the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA) buoys and Array for Real-Time Geostrophic Oceanography (ARGO) data sets for the period 2012–2013. Comparison of two recent versions (V2 and V3) of Aquarius-based SSS estimates to nine RAMA buoys on a daily timescale showed excellent mutual agreement. The systematic underestimation of SSS by satellite-based V2 products over the TIO shows a clear advantage for the new version product (V3). A larger root-mean-square error of the order of 0.50 psu in the satellite-based SSS was observed over the highly variable (larger standard deviation) Bay of Bengal region as compared with ARGO data sets. In the eastern equatorial Indian Ocean region, satellite-based SSS overestimated SSS below 34 psu and underestimated SSS of 34–35 psu as compared with ARGO data. However, the V3 SSS from Aquarius showed marginal improvement over V2 SSS. Monthly variation and fast Fourier analysis of the satellite-based SSS estimates are in reasonably good agreement with in situ observations which suggest that Aquarius is able to capture SSS variability in the TIO. The Aquarius-based V3 SSS showed a temporal autocorrelation of 0.6 over most parts of the TIO up to day 10, and decreased gradually with time. Overall analysis suggests that Aquarius-derived V3 SSS can detect variability in SSS satisfactorily in the TIO and is in reasonably good agreement with in situ observations.

On the origin and propagation of Denmark Strait overflow water anomalies in the Irminger Basin

AbstractDenmark Strait Overflow Water (DSOW) supplies the densest contribution to North Atlantic Deep Water and is monitored at several locations in the subpolar North Atlantic. Hydrographic (temperature and salinity) and velocity time series from three multiple‐mooring arrays at the Denmark Strait sill, at 180 km downstream (south of Dohrn Bank) and at a further 320 km downstream on the east Greenland continental slope near Tasiilaq (formerly Angmagssalik), were analyzed to quantify the variability and track anomalies in DSOW in the period 2007–2012. No long‐term trends were detected in the time series, while variability on time scales from interannual to weekly was present at all moorings. The hydrographic time series from different moorings within each mooring array showed coherent signals, while the velocity fluctuations were only weakly correlated. Lagged correlations of anomalies between the arrays revealed a propagation from the sill of Denmark Strait to the Angmagssalik array in potential temperature with an average propagation time of 13 days, while the correlations in salinity were low. Entrainment of warm and saline Atlantic Water and fresher water from the East Greenland Current (via the East Greenland Spill Jet) can explain the whole range of hydrographic changes in the DSOW measured downstream of the sill. Changes in the entrained water masses and in the mixing ratio can thus strongly influence the salinity variability of DSOW. Fresh anomalies found in downstream measurements of DSOW within the Deep Western Boundary Current can therefore not be attributed to Arctic climate variability in a straightforward way.

Observed sub-inertial current variability and volume transport over the continental shelf in the northern South China Sea

Sub-inertial currents (SICs) over the continental shelf of the northern South China Sea (NSCS) are investigated using the in-situ current observations of acoustic Doppler current profiler (ADCP) mooring arrays off the Pearl River Estuary in 2006 and 2007. The survey was carried out in four separate time periods: summer, winter, spring (before the onset of the southwesterly monsoon), and fall (after the establishment of the northeasterly monsoon). The observations showed that the current directions were generally along the shelf, consistent with the directions of monsoonal winds. The currents were also affected by a few of mesoscale eddy events. In summer 2006, the volume transport was northeastward with a mean magnitude of 1.4 Sv through a cross-shelf section from the site of the depth of 135 m to the coast; in winter 2006/2007, spring 2007 and fall 2007, the volume transports were all southwestward with magnitudes of 2.0, 2.1, and 0.9 Sv, respectively, through a cross-shelf section from the site of the depth of 290 m to the coast. The standard deviations of the SICs were generally smaller than the velocities of the mean currents, and the variability of SICs showed significant correlation with the local sea surface winds. No persistent counter-wind currents were observed in the study area during the fall and winter observational periods.

Ocean Response to CINDY/DYNAMO MJOs in Air-Sea-Coupled COAMPS

The response of the ocean to three Madden-Julian Oscillation (MJO) events during the fall of 2011 is simulated by the Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) in a fully coupled mode with high resolution in the atmosphere and ocean. The model simulates the cooler sea surface temperature (SST) and disappearance of the diurnal cycle in SST during the active phase of the MJO and it compares well with the observed SST from Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) buoys. The most striking direct response to the westerly zonal wind stress associated with the onset of the MJO is a rapidly accelerating Yoshida jet in the ocean mixed layer with equatorial zonal currents exceeding 1 m s-1. These jets are found in the model as well as the RAMA buoy observations. In the model, the Yoshida jet is superimposed on the seasonal Wyrtki jet that has a subsurface local maximum between 50 and 150 m. A shear layer separates the subsurface seasonal jet and the surface jet forced by the MJO. The sea surface elevation response and upper ocean heat content show wind generated westward propagating Rossby waves symmetric around the equator and an associated eastward propagating equatorial Kelvin wave response. After the third MJO event, the Yoshida jet spans most of the equatorial Indian Ocean. Upon reaching the Indonesian coast, the associated equatorial Kelvin wave reflects and generates additional westward propagating equatorial Rossby waves. The volume transport associated with these waves causes the westward advection of low-salinity north and south of the equator, impacting the tropical ocean circulation.

Analysis of MJO Wind-Flux Feedbacks in the Indian Ocean Using RAMA Buoy Observations

Observations spanning 2004-2012 from two Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) buoys along the equator in the Indian Ocean are used in conjunction with Tropical Rainfall Measuring Mission (TRMM) to assess the relative importance of surface latent heat fluxes to intraseasonal convection. This work is motivated by previous observational and modeling studies that have suggested the importance of wind-induced surface fluxes to the dynamics of the Madden-Julian Oscillation (MJO). Intraseasonal variability is isolated in two ways: 1) 20-100-day bandpass filtering and 2) using the global real time multivariate MJO index. Linear regression shows latent heat flux anomalies to be between 4 % and 8 % of precipitation anomalies when the two variables are compared using similar Wm-2 energy units. From a moist static energy budget viewpoint, these results confirm the potential of wind-induced latent heat fluxes to aid destabilization of MJO convection. Results derived from using both simple intraseasonal filtering and global MJO indices indicate that precipitation leads latent heat flux on the order of a few days, indicating surface fluxes may be more important for maintenance of deep MJO convection in addition to helping set the MJO’s propagation speed. Sensitivity tests using smoothed wind speed or thermodynamics (i.e., air temperature, relative humidity, and sea surface temperature) to compute latent heat flux show wind speed variability explains most of the latent heat flux variability on intraseasonal timescales. A similar conclusion is found via linearization of the latent heat flux formula. Additional analysis shows mesoscale and synoptic scale wind variability have negligible impact on intraseasonal latent heat flux anomalies.

Dynamics of the surface layer diurnal cycle in the equatorial Atlantic Ocean (0°, 23°W)

AbstractA 15 year time series (1999–2014) from the 0°, 23°W Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) mooring, which includes an 8 month record (October 2008 to June 2009) of high‐resolution near‐surface velocity data, is used to analyze the diurnal variability of sea surface temperature, shear, and stratification in the central equatorial Atlantic. The ocean diurnal cycle exhibits pronounced seasonality that is linked to seasonal variations in the surface wind field. In boreal summer and fall, steady trade winds and clear skies dominate, with limited diurnal variability in sea surface temperature. Diurnal shear layers, with reduced Richardson numbers, are regularly observed descending into the marginally unstable equatorial undercurrent below the mixed layer, conditions favorable for the generation of deep‐cycle turbulence. In contrast, in boreal winter and spring, winds are lighter and more variable, mixed layers are shallow, and diurnal variability of sea surface temperature is large. During these conditions, diurnal shear layers are less prominent, and the stability of the undercurrent increases, suggesting seasonal covariance between diurnal near‐surface shear and deep‐cycle turbulence. Modulation of the ocean diurnal cycle by tropical instability waves is also identified. This work provides the first observational assessment of the diurnal cycle of near‐surface shear, stratification, and marginal instability in the equatorial Atlantic, confirming previous modeling results and offering a complementary perspective on similar work in the equatorial Pacific.

Near-inertial waves in the wake of 2011 Typhoon Nesat in the northern South China Sea

In September 2011, Typhoon Nesat passed over a moored array of instruments recording current and temperature in the northern South China Sea (SCS). Awake of baroclinic near-inertial waves (NIWs) commenced after Nesat passed the array. The associated near-inertial currents are surface-intensified and clockwise-polarized. The vertical range of NIWs reached 300 m, where the vertical range is defined as the maximum depth of the horizontal near-inertial velocity 5 cm/s. The current oscillations have a frequency of 0.7099 cycles per day (cpd), which is 0.025f higher than the local inertial frequency. The NIWs have an e-folding time-scale of 10 d based on the evolution of the near-inertial kinetic energy. The depth-leading phase of near-inertial currents indicates downward group velocity and energy flux. The estimated vertical phase velocity and group velocity are 0.27 and 0.08 cm/s respectively, corresponding to a vertical wavelength of 329 m. A spectral analysis reveals that NIWs act as a crucial process to redistribute the energy injected by Typhoon Nesat. A normal mode and an empirical orthogonal function analysis indicate that the second mode has a dominant variance contribution of 81%, and the corresponding horizontal phase velocity and wavelength are 3.50 m/s and 420 km respectively. The remarkable large horizontal phase velocity is relevant to the rotation of the earth, and a quantitative analysis suggests that the phase velocity of the NIWs with a blue-shift of 0.025f overwhelms that of internal gravity waves by a factor of 4.6.

Variability of zonal currents in the eastern equatorial Indian Ocean on seasonal to interannual time scales

This study examines equatorial zonal current variations in the upper layers of eastern Indian Ocean in relation to variations in the Indian Ocean Dipole (IOD). The analysis utilizes data from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) and the European Centre for Medium-Range Weather Forecasts-Ocean Reanalysis System 4 (ECMWF-ORAS4). Surface currents are characterized by semiannual eastward flowing Wyrtki jets along the equator in boreal spring and fall, forced by westerly monsoon transition winds. The fall jet intensifies during negative IOD (NIOD) events when westerlies are anomalously strong but significantly weakens during positive IOD (PIOD) events when westerlies are anomalously weak. As zonal wind stress weakens during PIOD events, sea surface height becomes unusually low in the eastern basin and high in the west, setting up an anomalous pressure force that drives increased eastward transport in the thermocline. Opposite tendencies are evident during NIOD events in response to intensified equatorial westerlies. Current transport adjustments to anomalous zonal wind forcing during IOD events extend into the following year, consistent with the cycling of equatorial wave energy around the basin. A surface layer mass budget calculation for the eastern sea surface temperature (SST) pole of the IOD indicates upwelling of ∼2.9±0.7 Sv during normal periods, increasing by 40–50% during PIOD events and reducing effectively to zero during NIOD events. IOD-related variations in Wyrtki jet and thermocline transports are major influences on these upwelling rates and associated water mass transformations, which vary consistently with SST changes.

Tropical Atlantic variability and coupled model climate biases: results from the Tropical Atlantic Climate Experiment (TACE)

(SST) has a strong influence on regional climate variations, including the strength and onset date of the West African Monsoon (WAM). However, the predictability of the WAM is strongly limited by large biases in coupled climate models. The observational studies presented here are largely based on enhanced observations taken during the TACE period, but also include studies based on historical and remote sensing data and data from the global in situ observing system. The enhanced observations have provided tremendous new insight into physical processes at work shaping the Atlantic climate system. However, while significant progress has been made towards improving coupled climate simulations of the tropical Atlantic, a strong bias in the southeastern tropical Atlantic is a common feature of current generation climate models. As this region still remains underrepresented in the available in situ observational datasets, new observational programs have been developed in the eastern tropical Atlantic, with a particular focus on the coastal upwelling regions. At the same time, the essential ocean observations obtained during TACE, including subsurface moorings along the equatorial wave guide, are being continued. The articles contributed to this volume represent new advances in observing, modeling, understanding and predicting TAV. They encompass a range of topics, including tropical sea surface temperature, salinity, and subsurface oxygen variability, mixed layer heat and freshwater budgets, equatorial circulation and its seasonal to interannual variability, modes of tropical Atlantic variability, tropical Atlantic biases in coupled climate models and their potential causes, and connections between Atlantic and Pacific climate variability. This special issue is coordinated by William Johns, Peter Brandt, and Ping Chang, representatives of the TACE Observations and TACE Modeling and Synthesis working groups. Tropical Atlantic variability (TAV) and coupled model climate biases have been the focus of the recently completed Tropical Atlantic Climate Experiment (TACE, 2006–2011), an international CLIVAR program (http://www.clivar.org/ organization/atlantic/tace). TACE with its backbone, the Prediction and Research Moored Array in the Tropical Atlantic (PIRATA), was closely linked to other initiatives in the tropical Atlantic, such as the French EGEE (Etude de la circulation oceanique et de sa variabilite dans le Golfe de Guinee) program and the African Monsoon Multidisciplinary Analyses (AMMA). One of the main goals of TACE was to improve the observational database in the equatorial Atlantic and to carry out dedicated process studies enhancing our understanding of the tropical Atlantic climate system. The regional focus of TACE was on the central and eastern equatorial Atlantic, characterized by the development of the Atlantic cold tongue (ACT) during boreal summer. The year-to-year variability of the ACT sea surface temperature

Sustaining observations of the unsteady ocean circulation.

Sustained observations of ocean properties reveal a global warming trend and rising sea levels. These changes have been documented by traditional ship-based measurements of ocean properties, whereas more recent Argo profiling floats and satellite records permit estimates of ocean changes on a near real-time basis. Through these and newer methods of observing the oceans, scientists are moving from quantifying the 'state of the ocean' to monitoring its variability, and distinguishing the physical processes bringing signals of change. In this paper, I give a brief overview of the UK contributions to the physical oceanographic observations, and the role they have played in the wider global observing systems. While temperature and salinity are the primary measurements of physical oceanography, new transbasin mooring arrays also resolve changes in ocean circulation on daily timescales. Emerging technologies permit routine observations at higher-than-ever spatial resolutions. Following this, I then give a personal perspective on the future of sustained observations. New measurement techniques promise exciting discoveries concerning the role of smaller scales and boundary processes in setting the large-scale ocean circulation and the ocean's role in climate. The challenges now facing the scientific community include sustaining critical observations in the case of funding system changes or shifts in government priorities. These long records will enable a determination of the role and response of the ocean to climate change.

Unexpected impacts of the Tropical Pacific array on reanalysis surface meteorology and heat fluxes

AbstractThe Tropical Pacific mooring array has been a key component of the climate observing system since the early 1990s. We identify a pattern of strong near surface humidity anomalies, colocated with the array, in the widely used European Center for Medium Range Weather Forecasting Interim atmospheric reanalysis. The pattern generates large, previously unrecognized latent and net air‐sea heat flux anomalies, up to 50 Wm−2 in the annual mean, in reanalysis derived data sets employed for climate studies (TropFlux) and ocean model forcing (the Drakkar Forcing Set). As a consequence, uncertainty in Tropical Pacific ocean heat uptake between the 1990s and early 2000s at the mooring sites is significant with mooring colocated differences in decadally averaged ocean heat uptake as large as 20 Wm−2. Furthermore, these results have major implications for the dual use of air‐sea flux buoys as reference sites and sources of assimilation data that are discussed.

Ocean Data Impacts in Global HYCOM*

Abstract The impact of the assimilation of ocean observations on reducing global Hybrid Coordinate Ocean Model (HYCOM) 48-h forecast errors is presented. The assessment uses an adjoint-based data impact procedure that characterizes the forecast impact of every observation assimilated, and it allows the observation impacts to be partitioned by data type, geographic region, and vertical level. The impact cost function is the difference between HYCOM 48- and 72-h forecast errors computed for temperature and salinity at all model levels and grid points. It is shown that routine assimilation of large numbers of observations consistently reduces global HYCOM 48-h forecast errors for both temperature and salinity. The largest error reduction is due to the assimilation of temperature and salinity profiles from the tropical fixed mooring arrays, followed by Argo, expendable bathythermograph (XBT), and animal sensor data. On a per-observation basis, the most important global observing system is Argo. The beneficial impact of assimilating Argo temperature and salinity profiles extends to all depths sampled, with salinity impacts maximum at the surface and temperature impacts showing a subsurface maximum in the 100–200-m-depth range. The reduced impact of near-surface Argo temperature profile levels is due to the vertical covariances in the assimilation that extend the influence of the large number of sea surface temperature (SST) observations to the base of the mixed layer. Application of the adjoint-based data impact system to identify a data quality problem in a geostationary satellite SST observing system is also provided.

Investigation of the Relationship Between the Yucatan Channel Transport and the Loop Current Area in a Multidecadal Numerical Simulation

Abstract A hypothesis by Maul (1977), stating the rate of change of loop current (LC) volume is related to deep Yucatan Channel (YC) transport, is tested with a continuous 54-year simulation of the Gulf of Mexico (GoM) using a regional 1/25° resolution Hybrid Coordinate Ocean Model (HYCOM) configuration. The hypothesis states that the imbalance of transport between the upper YC and the Florida Straits controls the rate of change of the LC volume and that the imbalance is compensated by transport through the deep YC. Bunge et al. (2002) found a strong relationship between the deep YC transport and the LC area using 7.5 months of data from a mooring array in the YC, but the observational record length was relatively short compared to the time scale of LC variability. The 54-year HYCOM simulation provides a much longer and spatially complete data set to study the LC variability. Results show that the time evolution of the LC between two shedding events can be viewed as a combination of relatively high-frequency fluctuations superimposed on a low-frequency trend. The high-frequency variability of the LC area time derivative and the deep YC transport are related. The low-frequency variability is examined by comparing the LC area time series with time-integrated transport in the deep YC, and statistically similar trends are identified, supporting the Maul (1977) theory.

Impact of recirculation on the East Greenland Current in Fram Strait: Results from moored current meter measurements between 1997 and 2009

Transports of total volume and water masses obtained from a mooring array in the East Greenland Current (EGC) in Fram Strait are presented for the period 1997–2009. The array in the EGC was moved along isobaths from 79°N to 78°50′N in 2002 to line up with moorings in the eastern Fram Strait. Analysis of the time series at the two latitudes shows that associated with the southward move, the annual mean volume transport of the EGC increased from 5.8±1.8Sv to 8.7±2.5Sv, mostly related with an increase in barotropic flow. This suggests a recirculation of close to 3Sv at 78°50′N as a consequence of the large-scale wind-driven cyclonic gyre in the Nordic Seas. In addition, the volume transport at 78°50′N showed a clear seasonal cycle which was absent at 79°N. Estimates of the wind-driven Sverdrup transport at two different latitudes show that the difference in total volume transport and seasonality can largely be explained by the wind-stress curl. However, weak transport in 2003 was only partially related with weak Sverdrup transport and coincided also with anomalously weak northerly winds. The stronger recirculation at 78°50′N has also consequences for the observed Atlantic Water: there is significantly more Atlantic derived water present at the southerly latitude. In addition, the warm anomaly in Fram Strait between 2005 and 2007 doubled the amount of Recirculated Atlantic Water temporarily. Finally, we estimate that close to 2.7Sv, or 50%, of Atlantic derived water recirculates in Fram Strait.

A Model Analysis for the Design and Deployment of an Eulerian Sediment Trap Mooring Array in a Western Boundary Upwelling System from Southeast Brazil

This work addresses the design and configuration of a Eulerian sediment trap mooring array, which was deployed at the shelf edge (zm ≈ 140 m) 80 km off Cabo Frio, SE- Brazil (23° S). The site was subject to interplay between the Tropical Waters (TW) of the Brazil Current (BC), intrusions from the South Atlantic Central Waters (SACW), which are the source of upwelling in the region, and other oceanographic processes. Detailed computations were used to optimize the total weight, buoyancy balance, and maximum acceptable tilt to avoid hydrodynamic bias in the trapping efficiency and array adaptation to the local oceanographic conditions with the assistance of Matlab and Muringa programs and Modular Ocean Model 4.0 (MOM; i.e., to assert the vertical distribution of the meridional current component). The velocity range of the current component was determined by short term measurements to be between 0.1 and 0.5 m/s. Projections led to a resulting minimum anchor weight of 456 kg. The necessary line tension was ascertained by using the appropriate distribution of a series of buoys along the array, which finally attained a high vertical load of 350 kg because of the attached oceanographic equipment. Additional flotation devices resulted in a stable mooring array as reflected by their low calculated tilt (2.6° ± 0.6°). A low drag of 16 N was computed for the maximum surface current velocity of 0.5 m/s. The Reynolds number values ranged from 4 × 104 to 2 × 105 and a cone-trap aspect ratio of 1.75 was used to assess the trap sampling efficiency upon exposure to different current velocities.

Flows in the Tasman Front south of Norfolk Island

The Tasman Front is a narrow band of eastward flowing subtropical water crossing the Tasman Sea from Australia to North Cape, New Zealand. It is the link between the two subtropical western boundary currents of the South Pacific, the East Australian Current (EAC) off eastern Australia, and the East Auckland Current (EAUC) off northeastern New Zealand. Here we report the first direct measurements of flow in the Tasman Front from a moored array deployed across gaps in the submarine ridges south of Norfolk Island and hydrographic and ADCP measurements during the deployment and recovery voyages. The mean flow through the array over July 2003 to August 2004 was found to be eastward only in the upper 800 m with a transport of ∼6 Sv. Below 800 m a weak westward mean flow (∼1.5 Sv) was measured, associated with Antarctic Intermediate Water (AAIW). Using sea surface height to account for additional transport south of the moored array results in a total mean eastward transport between Norfolk Island and North Cape, New Zealand of ∼8 Sv, varying between −4 and 18 Sv. The measurements show that the Tasman Front is much shallower than either the EAC or EAUC, both of which extend below 2000 m depth, has less transport than either the EAC or EAUC and has instances of flow reversal. Thus, the Tasman Front is a weaker connection between the EAC and EAUC than the paradigm of a contiguous South Pacific western boundary current system would suggest.

Zonal structure and seasonal variability of the Atlantic Equatorial Undercurrent

Simultaneous mooring arrays were maintained along the path of the Equatorial Undercurrent (EUC) at three longitudes (23°W, 10°W, and 0°E), from October 2007 to June 2011, as part of the CLIVAR Tropical Atlantic Climate Experiment. The measurements allow for the first time a description of the seasonal cycle and interannual variability of the EUC across the Atlantic basin. The mean transport of the EUC at 23°W is 14.3 ± 0.6 Sv, decreasing to 12.1 ± 0.9 and 9.4 ± 0.6 Sv at 10°W and 0°E, respectively. The EUC shows a changing seasonal cycle across the basin: at 23°W, the strongest EUC transport occurs in boreal fall in association with maximum easterly wind stress, at 10°W the EUC transport shows a semiannual cycle with a maximum in boreal spring and fall, while at 0°E the EUC has a single spring maximum. At all locations the EUC core exhibits a similar seasonal vertical migration, with shallowest core depths occurring in boreal spring and deepest core depths in boreal fall. The maximum core intensity occurs in boreal spring all across the basin, when the EUC is shallow, during the annual wind relaxation. The weakest EUC core intensity occurs during the boreal summer cold tongue phase, especially in the eastern part of the basin. At both 23°W and 10°W, a deep extension of the EUC occurs in boreal summer, which increases the transport in the lower thermocline and partially offsets the weaker upper EUC transport during boreal summer. No clear linkage could be established between the interannual variability of the EUC in the eastern part of the basin and the intensity of the summer cold tongue, despite evidence for such a linkage in the western part of the basin.

Multiyear Volume, Liquid Freshwater, and Sea Ice Transports through Davis Strait, 2004–10*

Abstract Davis Strait is a primary gateway for freshwater exchange between the Arctic and North Atlantic Oceans including freshwater contributions from west Greenland and Canadian Arctic Archipelago glacial melt. Data from six years (2004–10) of continuous measurements collected by a full-strait moored array and concurrent high-resolution Seaglider surveys are used to estimate volume and liquid freshwater transports through Davis Strait, with respective annual averages of −1.6 ± 0.5 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) and −93 ± 6 mSv (negative sign indicates southward transport). Sea ice export contributes an additional −10 ± 1 mSv of freshwater transport, estimated using satellite ice area transport and moored upward-looking sonar ice thickness measurements. Interannual and annual variability of the net transports are large, with average annual volume and liquid freshwater transport standard deviations of 0.7 Sv and 17 mSv and with interannual standard deviations of 0.3 Sv and 15 mSv. Moreover, there are no clear trends in the net transports over the 6-yr period. However, salinity in the upper 250 m between Baffin Island and midstrait decreases starting in September 2009 and remains below average through August 2010, but appears to return to normal by the end of 2010. This freshening event, likely caused by changes in arctic freshwater storage, is not apparent in the liquid freshwater transport time series due to a reduction in southward volume transport in 2009–10. Reanalysis of Davis Strait mooring data from the period 1987–90, compared to the 2004–10 measurements, reveals less arctic outflow and warmer, more saline North Atlantic inflow during the most recent period.

Summer circulation dynamics within the Perth coastal waters of southwestern Australia

The dynamics of the summer circulation in the coastal waters off Perth in Western Australia were investigated during a two-month field experiment. The study included the deployment of an array of moorings spanning the outer shelf, the inner shelf, within the inshore Perth coastal lagoon, and in the large coastal embayment of Cockburn Sound. The results revealed highly transient coastal circulation patterns that responded to variability in both the locally- and remotely-generated forcing. Local wind forcing played a primary role in driving much of the alongshore current variability at the shallower (<20m depth) inshore sites, with a well-defined peak wind forcing time scale of ~1 week that fell within the synoptic weather band in the region. Due to the mean northward wind stress that persisted during this summer period, a mean northward current of 0.05–0.1ms−1 was observed at these inshore sites. Large-scale variations in alongshore water level (pressure) gradients also episodically generated strong along- and cross-shore current oscillations throughout the region. Major events were associated with the propagation of coastally-trapped waves generated by a tropical low pressure system far (~1000km) to the north of Perth, which propagated down the Western Australia coast. On the outer shelf, local wind forcing played a minor (but still not a negligible) role in driving alongshore current variability, with this momentum balance instead dominated by the alongshore pressure gradient variability. Due to the unusually large alongshore pressure gradient that persists year round along the Western Australia coast, currents on the shelf were on average southward. However, large-scale northward reversals of the shelf flow were also observed when northward wind stresses were sufficiently large and/or the local alongshore pressure gradient became episodically weak.

Coastal-Trapped Waves in the East China Sea Observed by a Mooring Array in Winter 2006

Abstract Using five mooring array observations in the coastal water of the East China Sea (ECS) in winter 2006, the authors identify three kinds of low-frequency waves using the ensemble empirical mode decomposition (EEMD) method. The analysis indicates that the periods of the waves varied from 2 to 10 days, which are consistent with coastal-trapped wave (CTW) modes: the Kelvin wave (KW) mode, the first shelf wave (SW1) mode, and the second shelf wave (SW2) mode. An analytical model is established and the dispersion relation of the waves from the analytical method agrees well with the observations. The wind-forced, coastal-trapped wave theory is then applied. The calculation shows that over a wide shelf, the forcing term of wind stress curl plays an important role in shaping the CTW. Numerical solutions reproduce the sea level variation and the alongshore current. The results show that the sea level variation mainly resulted from the KW mode, but the alongshore current resulted from both the KW and SW1 modes.