Seasonal Variation Of Volume Transport Research Articles

Combining in situ measurements and altimetry to estimate volume, heat and salt transport variability through the Faroe–Shetland Channel

Abstract. From 1994 to 2011, instruments measuring ocean currents (Acoustic Doppler Current Profilers; ADCPs) have been moored on a section crossing the Faroe–Shetland Channel. Together with CTD (Conductivity Temperature Depth) measurements from regular research vessel occupations, they describe the flow field and water mass structure in the channel. Here, we use these data to calculate the average volume transport and properties of the flow of warm water through the channel from the Atlantic towards the Arctic, termed the Atlantic inflow. We find the average volume transport of this flow to be 2.7 ± 0.5 Sv (1 Sv = 106 m3 s–1) between the shelf edge on the Faroe side and the 150 m isobath on the Shetland side. The average heat transport (relative to 0 °C) was estimated to be 107 ± 21 TW (1 TW = 1012 W) and the average salt import to be 98 ± 20 × 106 kg s−1. Transport values for individual months, based on the ADCP data, include a large level of variability, but can be used to calibrate sea level height data from satellite altimetry. In this way, a time series of volume transport has been generated back to the beginning of satellite altimetry in December 1992. The Atlantic inflow has a seasonal variation in volume transport that peaks around the turn of the year and has an amplitude of 0.7 Sv. The Atlantic inflow has become warmer and more saline since 1994, but no equivalent trend in volume transport was observed.

Seasonal Variation of Volume Transport through the Straits of the East/Japan Sea Viewed from the Island Rule

Among others, a question that has long been unanswered is why the seasonal variation of volume transport is larger in the Soya and Korea/Tsushima Straits than in the Tsugaru Strait. An attempt is made to answer this question in terms of the island rule with friction being taken into account. The problem is idealized as a simple model. The model results indicate that volume transport through a channel is determined not only by the circulation created around the adjacent island but also by those created around the neighboring islands farther away. The latter is due to the presence of bottom friction in the channels. The volume transports through the Korea/Tsushima, Tsugaru and Soya Straits estimated from the model using observed wind data show the general pattern of observed seasonality, although they contain large errors associated with the uncertain frictional parameter employed in the model. The model indicates that the observed seasonality arises essentially from the fact that wind stress curl changes its sign, from negative in the summer to positive in winter, following a large fluctuation of zero-stress curl latitude east of Hokkaido.

The seasonal circulation and volume transport on the northwest European continental shelf: A fine‐resolution model study

In this paper the circulation of the northwest European continental shelf is investigated using the first year‐long density‐evolving simulation at shelf wide scales and sub‐Rossby Radius resolution (∼1.8 km). A series of numerical experiments are conducted to distinguish between the wind‐, density‐, and oceanic‐driven components of the flow. These demonstrate that, while all components have a role throughout the year, the density‐driven component is particularly important during the summer and autumn months. The time evolution of the density field makes a significant contribution to the seasonal variation of volume transport on shelf wide scales and is persistent in direction; whereas the wind‐driven volume transport acts on much shorter timescales and is more variable in direction. The importance of the oceanic forcing is demonstrated, representing tidal residuals and large‐scale oceanic sea level (pressure) variation; this forcing drives a substantial component of the circulation throughout the year. Twenty six satellite tracked drifters deployed in the summer of 2001 are used for a direct validation of the model currents. The model current speeds are found to be accurate to ∼46% when averaged over ∼40 d, but tend to be too slow. The summer volume fluxes are compared with estimates in the literature showing good agreement, although there is a suggestion that the North Sea inflows are overestimated. Comparisons with the coarser resolution model used for boundary conditions demonstrate the importance of fine‐resolution to the details of the frontal currents with consequences for modeling exchange processes and biological activity in these regions.

Seasonal variation of volume transport in the major inflow region of the Taiwan Strait: the Penghu Channel

Abstract Eight cruises of current measurements along a zonal transect (∼31.84 km) across the major inflow region of the Taiwan Strait, the Penghu Channel, were carried out using the shipboard Acoustic Doppler Current Profiler during 1999–2001. On each cruise, the measurement was repeated twice along the transect with a time lag of 6 h and 12 min, and the repeated data were averaged to eliminate the dominant semidiurnal tidal currents. Velocities, after removing semidiurnal tides, suggest a strong northward flow in the channel, with a speed of about 100 cm/s in the upper 50 m in summer. The northward flow becomes much weaker in winter. The calculated throughflow transports vary seasonally and are correlated with the change of the East Asia monsoon. The estimated transport is around 0 during the peak northeast monsoon in winter, increases from 0.5 to 1 Sv (1 Sv=106 m3/s) as the northeast monsoon weakens in spring, peaks to 1.5 Sv at the end of southwest monsoon in summer, and decreases rapidly from 1.5 to 0 Sv when the northeast monsoon intensifies in fall. The error, mostly induced by unfiltered diurnal tidal currents, is estimated to be±0.20 Sv in the transport calculation.

Variability of Sea Surface Circulation in the Japan Sea

Composite sea surface dynamic heights (CSSDH) are calculated from both sea surface dynamic heights that are derived from altimetric data of ERS-2 and mean sea surface that is calculated by a numerical model. The CSSDH are consistent with sea surface temperature obtained by satellite and observed water temperature. Assuming the geostrophic balance, sea surface current velocities are calculated. It is found that temporal and spatial variations of sea surface circulation are considerably strong. In order to examine the characteristics of temporal and spatial variation of current pattern, EOF analysis is carried out with use of the CSSDH for 3.5 years. The spatial and temporal variations of mode 1 indicate the strength or weakness of sea surface circulation over the entire Japan Sea associated with seasonal variation of volume transport through the Tsushima Strait. The spatial and temporal variations of mode 2 mostly indicate the temporal variation of the second branch of the Tsushima Warm Current and the East Korean Warm Current. It is suggested that this variation is possibly associated with the seasonal variation of volume transport through the west channel of the Tsushima Strait. Variations of mode 3 indicate the interannual variability in the Yamato Basin.

Mechanism of the seasonal transport variation through the Tokara Strait

To investigate an mechanism of the seasonal variation of transport through the Tokara Strait, two numerical experiments with real geometry and wind forcing were carried out. The models are linear barotropic models which are a North Pacific Ocean model and a limited-area model with a fine grid. The seasonal variation of volume transport with a maximum in the summer and a minimum in the autumn could be well reproduced by both models. The results demonstrate the wind stress component normal to a gradient vector of bottom topography is crucial for determining the seasonal variation. The similar seasonal variation widely covers the East China Sea and has a large amplitude near the Tokara Strait. Finally, it can be concluded that winds north of 35°N have little influence on the seasonal response of our model at the Tokara Strait.

Seasonal Variation in Volume Transport of the Kuroshio South of Japan

Abstract A two-layer numerical model driven by the observed wind stress compiled by K. Kutsuwada and T. Teramoto is used to study the seasonal variation in volume transport of the Kuroshio south of Japan. The authors focus on the observational evidence that the geostrophic volume transport of the Kuroshio is maximum in summer and minimum in winter; however, Sverdrup transport estimated by the observed wind stress indicates a maximum in winter and a minimum in summer. It is shown from a flat-bottom model that the western boundary current has a maximum transport of 90 Sv (Sv ≡ 106 m3 s−1) in late winter and a minimum transport of 40 Sv from summer to early autumn. The time lag of the calculated transport from Sverdrup transport is about one month, which shows that the barotropic response is dominant for the seasonal change in wind stress. However, the vertical velocity difference at the western boundary region reaches a maximum in summer and a minimum in autumn, of which the summer maximum coincides with th...