Seasonal Transport Variation Research Articles

Seasonal and Interannual Movement of the Mindanao Current Retroflection at the Pacific Entrance of the Indonesian Seas

Abstract The seasonal and interannual variations of the Mindanao Current (MC) retroflection are studied using surface geostrophic currents of satellite altimeters covering January 1993–December 2019. The results show that the Mindanao Current mainstream retroflects back to the Pacific Ocean north of the Talaud Islands in boreal summer and intrudes into the northern Maluku Sea in boreal winter. The variation of the MC retroflection has resulted in the seasonal movement of the sea surface color fronts at the entrance of the Indonesian seas, both of which are highly correlated to the seasonal transport variations of the North Equatorial Countercurrent, lagging the latter due to the westward propagation of the seasonal Rossby waves. The MC retroflection and sea surface color fronts are found to move synchronously on interannual time scales at the Pacific entrance of the Indonesian seas, with the Niño-3.4 index lagging by about 2 months. The MC retroflection intrudes anomalously deeper than the seasonal cycle into the northern Maluku Sea in El Niño winters, while it tends to take a leaping path in La Niña winters. During El Niño summers, the leaping path of the MC is changed into a penetrating path sometimes. Significance Statement The Mindanao Current is the western boundary current (WBC) of the North Pacific Ocean tropical gyre and is much stronger than the ocean interior gyre circulation. This equatorward WBC forces strong exchanges between the equatorial Pacific and the marginal seas as it turns eastward at the entrance of the Indonesian seas. The path of the Mindanao Current retroflection is very important for Rossby wave reflection and for Indo-Pacific interbasin exchange leading to global repercussions, both of which are thought to be controlled by linear dynamics in the past. Here, we disclose the seasonal and interannual movement of the retroflection path, showing strong nonlinear dynamics important for ENSO and interocean exchange.

Sensitivity of stratospheric water vapour to variability in tropical tropopause temperatures and large-scale transport

Abstract. Concentrations of water vapour entering the tropical lower stratosphere are primarily determined by conditions that air parcels encounter as they are transported through the tropical tropopause layer (TTL). Here we quantify the relative roles of variations in TTL temperatures and transport in determining seasonal and interannual variations of stratospheric water vapour. Following previous studies, we use trajectory calculations with the water vapour concentration set by the Lagrangian dry point (LDP) along trajectories. To assess the separate roles of transport and temperatures, the LDP calculations are modified by replacing either the winds or the temperatures with those from different years to investigate the wind or temperature sensitivity of water vapour to interannual variations and, correspondingly, with those from different months to investigate the wind or temperature sensitivity to seasonal variations. Both ERA-Interim reanalysis data for the 1999–2009 period and data generated by a chemistry–climate model (UM-UKCA) are investigated. Variations in temperatures, rather than transport, dominate interannual variability, typically explaining more than 70 % of variability, including individual events such as the 2000 stratospheric water vapour drop. Similarly seasonal variation of temperatures, rather than transport, is shown to be the dominant driver of the annual cycle in lower stratospheric water vapour concentrations in both the model and reanalysis, but it is also shown that seasonal variation of transport plays an important role in reducing the seasonal cycle maximum (reducing the annual range by about 30 %). The quantitative role in dehydration of sub-seasonal and sub-monthly Eulerian temperature variability is also examined by using time-filtered temperature fields in the trajectory calculations. Sub-monthly temperature variability reduces annual mean water vapour concentrations by 40 % in the reanalysis calculation and 30 % in the model calculation. As with other aspects of dehydration, simple Eulerian measures of variability are not sufficient to quantify the implications for dehydration, and the Lagrangian sampling of the variability must be taken into account. These results indicate that, whilst capturing seasonal and interannual variation of temperature is a major factor in modelling realistic stratospheric water vapour concentrations, simulation of seasonal variation of transport and of sub-seasonal and sub-monthly temperature variability are also important and cannot be ignored.

Seasonal Variation in Transport of Zooplankton Into the Arctic Basin Through the Atlantic Gateway, Fram Strait

The largest contribution of oceanic heat to the Arctic Ocean is the warm Atlantic Water (AW) inflow through the deep Fram Strait. The AW current also carries Atlantic plankton into the Arctic Basin and this inflow of zooplankton biomass through the Atlantic-Arctic gateway far exceeds the inflow through the shallow Pacific-Arctic gateway. However, because this transport has not yet been adequately quantified based on observational data, so that the present contribution is poorly defined, and future changes in Arctic zooplankton communities are difficult to project and observe. Our objective was to quantify the inflow of zooplankton biomass through the Fram Strait during different seasons, including winter. We collected data with high spatial resolution covering hydrography (CTD), currents (ADCP and LADCP) and zooplankton distributions (LOPC and MultiNet) from surface to 1000 m depth along two transects crossing the AW inflow during three cruises in January, May and August 2014. Long-term variations (1997-2016) in the AW inflow were analyzed based on moored current meters. Water transport across the inflow region was of the same order of magnitude during all months (January 2.2 Sv, May 1.9 Sv, August 1.7 Sv). We found a higher variability in zooplankton transport between the months (January 51 kg C s-1, May 34 kg C s-1, August 50 kg C s-1), related to seasonal changes in the vertical distribution of zooplankton. However, high abundances of carbon-rich copepods were observed in the AW inflow during all months. Surface patches with high abundances of C. finmarchicus, Microcalanus spp., Pseudocalanus spp. and Oithona similis clearly contributed to the advected biomass, also in winter. The data reveal that the phenology of species is important for the amount of advected biomass, and that the advective input of zooplankton carbon into the Arctic Basin is important during all seasons. The advective zooplankton input might be especially important for mesopelagic planktivorous predators that were recently observed in the region, particularly during winter. The inflow of C. finmarchicus with AW was estimated to be in the order of 500 000 metric tons C y-1, which compares well to modeled estimates.

Meridional Overturning Circulation Transport Variability at 34.5°S During 2009–2017: Baroclinic and Barotropic Flows and the Dueling Influence of the Boundaries

AbstractSix years of simultaneous moored observations near the western and eastern boundaries of the South Atlantic are combined with satellite winds to produce a daily time series of the basin‐wide meridional overturning circulation (MOC) volume transport at 34.5°S. The results demonstrate that barotropic and baroclinic signals at both boundaries cause significant transport variations, and as such must be concurrently observed. The data, spanning ~20 months during 2009–2010 and ~4 years during 2013–2017, reveal a highly energetic MOC record with a temporal standard deviation of 8.3 Sv, and strong variations at time scales ranging from a few days to years (peak‐to‐peak range = 54.6 Sv). Seasonal transport variations are found to have both semiannual (baroclinic) and annual (Ekman and barotropic) timescales. Interannual MOC variations result from both barotropic and baroclinic changes, with density profile changes at the eastern boundary having the largest impact on the year‐to‐year variations.

A novel experimental study of aeolian snow transport in Adelie Land (Antarctica)

None of the previous aeolian snow transport campaigns in Antarctica meet the requirements in terms of temporal resolution, long-term series and qualified instruments for evaluations of meteorological and climate models including parameterization for aeolian snow transport. Consequently, determining the quantity of snow transported remains a challenge. A field campaign was therefore launched in January 2009, in Adélie Land, Antarctica, to acquire new model-evaluation-oriented observations within the European ICE2SEA project, with the logistical support of the French polar Institute (IPEV). The available aeolian snow transport sensors are reviewed and the sensor that best suited our specific needs was chosen: FlowCapt™ acoustic sensors. Three automatic weather stations were deployed with FlowCapts™ close to the coast. The stations' locations are distinct, ranging from 1 to 100km inland, one of them with a 7-m mast with six levels of anemometers and thermohygrometers. The fluid and impact threshold friction velocities recorded were 0.48±0.09ms−1 and 0.4±0.09ms−1, respectively, with a high standard deviation of 0.12±0.03ms−1 and 0.13±0.03ms−1, respectively. The aeolian snow transport frequency in Adélie Land was very high with seasonal variation of transport occurring with minima during the austral summer. Seven percent of the aeolian snow transport events were drifting snow (maximum particle's height, <1m above the surface). The snow quantity transported was above 1kiloton per year in the first meter above the surface.

Transport Variability in the Korea/Tsushima Strait: Characteristics and Relationship to Synoptic Atmospheric Forcing

The characteristics of high-frequency transport variation through the Korea/Tsushima Strait and its response to atmospheric synoptic forcing are investigated by using acoustic velocity measurements and a barotropic model. The first principal mode for the current data in the high-frequency band (3–50 days) is mostly coherent to transport variation through the strait. As the dominant mode, the cross-sectional pattern of the transport variability is clarified: strong axes exist in the centers of the western and eastern channels of the strait. Seasonal change in the high-frequency variability is also elucidated: it is strong from November to January, peaks in April and September, and is weak from May to August. Both data analysis and numerical experiments clarify that the transport variation is mainly attributable to wind and pressure forcing, especially in a synoptic time band (3–9 days). The amplitude of the high-frequency variability is even stronger than that of seasonal transport variation. The lagged cross-covariance reveals a spatial pattern that causes the transport variation to be a synoptic weather system passing eastward along the mid-latitudes with a period of around 5 days. The transport response to this synoptic change is investigated through numerical experiments with idealized atmospheric forcing. The experiments reveal that about 40–50% of the total transport fluctuations in the synoptic time band are explained by the idealized forcing, with a contribution from wind stress higher than that from pressure. Sensitivity experiments on the forcing area show that the pressure-driven transport fluctuation is mainly determined by forcing over the East/Japan Sea. Additional experiments indicate that the strait transport fluctuation responds coherently to the local and remote winds of the synoptic atmospheric forcing.

Seasonal transport variations in the straits connecting Prince William Sound to the Gulf of Alaska

Exchange of water between Prince William Sound and the Gulf of Alaska has a significant impact on its circulation and biological productivity. Current meter records from moored instruments in the two major straits connecting Prince William Sound to the Gulf of Alaska are analyzed to characterize the seasonal variations in water exchange. Eight individual deployments, each lasting for about 6 months, were made during the years 2005–2010. Two moorings were placed across each passage to account for horizontal flow variability. Monthly averaged, depth-integrated transport in winter is characterized by a strong barotropic inflow through Hinchinbrook Entrance and outflow through Montague Strait. The transport through each passage can reach 0.2Sv, which could replenish the volume of Prince William Sound in as little as 3 months. Depth-integrated transport is weaker and more variable in direction in summer than in winter, implying that Prince William sound is not always a simple flow-through system. Monthly transports range between −0.05 and 0.08Sv in each passage, and the corresponding flushing times exceed 1 year. The flow through both passages is highly baroclinic in the summer, so that the layer transport can be significant. For example, the deep inflow through Hinchinbrook Entrance can reach 0.05Sv, which would flush the deep regions of Prince William Sound (>400m) in only 23 days. The transport imbalance between Montague Strait and Hinchinbrook Entrance cannot be accounted for by considering other terms in a volume budget such as local freshwater input, meaning the imbalance is mostly a result of under-resolving the cross-strait flow variability. The magnitude of the monthly mean depth-integrated transport through Montague Strait and Hinchinbrook Entrance depends non-linearly on the shelf winds. Strong downwelling conditions, characteristic of the winter, drive inflow through Hinchinbrook Entrance, which is balanced by outflow through Montague Strait. Weak downwelling or upwelling conditions, characteristic of the summer, allow deep water from below the shelf break to flow in through Hinchinbrook Entrance.

Circulation and transport of water masses in the Lazarev Sea, Antarctica, during summer and winter 2006

The distribution and circulation of water masses in the region between 6°W and 3°E and between the Antarctic continental shelf and 60°S are analyzed using hydrographic and shipboard acoustic Doppler current profiler (ADCP) data taken during austral summer 2005/2006 and austral winter 2006. In both seasons two gateways are apparent where Warm Deep Water (WDW) and other water masses enter the Weddell Gyre through the Lazarev Sea: (a) a probably topographically trapped westward, then southwestward circulation around the northwestern edge of Maud Rise with maximum velocities of about 20 cm s −1 and (b) the Antarctic Coastal Current (AntCC), which is confined to the Antarctic continental shelf slope and is associated with maximum velocities of about 25 cm s −1. Along two meridional sections that run close to the top of Maud Rise along 3°E, geostrophic velocity shears were calculated from CTD measurements and referenced to velocity profiles recorded by an ADCP in the upper 300 m. The mean accuracy of the absolute geostrophic velocity is estimated at ±2 cm s −1. The net baroclinic transport across the 3°E section amounts to 20 and 17 Sv westward for the summer and winter season, respectively. The majority of the baroclinic transport, which accounts for ∼60% of the total baroclinic transport during both surveys, occurs north of Maud Rise between 65° and 60°S. However, the comparison between geostrophic estimates and direct velocity measurements shows that the circulation within the study area has a strong barotropic component, so that calculations based on the dynamic method underestimate the transport considerably. Estimation of the net absolute volume transports across 3°E suggests a westward flow of 23.9±19.9 Sv in austral summer and 93.6±20.1 Sv in austral winter. Part of this large seasonal transport variation can be explained by differences in the gyre-scale forcing through wind stress curl.

Numerical simulation on seasonal transport variations and mechanisms of suspended sediment discharged from the Yellow River to the Bohai Sea

Based on sediment and discharge flux data for the Yellow River, realistic forcing fields and bathymetry of the Bohai Sea, a suspended sediment transport module is driven by a wave-current coupled model to research seasonal variations and mechanisms of suspended load transport to the Bohai Sea. It could be concluded that surface sediment concentration indicates a distinct spatial distribution characteristic that varies seasonally in the Bohai Sea. Sediment concentration is rather high near the Yellow River estuary, seasonal variations of which are controlled by quantity of sediment from the Yellow River, suspended sediment concentration reaches its maximum during summer and fall. Furthermore, sediment concentration decreases rapidly in other seas far from the Yellow River estuary and maintains a very low level in the center of the Bohai Sea, and is dominated by seasonal variations of climatology wind field in the Bohai Sea. Only a small amount of sediments imported from the Yellow River are delivered northwestward to the southern coast of the Bohai Bay. Majority of sediments are transported southeastward to the Laizhou Bay, where sediments are continuously delivered into the center of the Bohai Sea in a northeastward direction, and part of them are transported eastward alongshore through the Bohai Strait. 69% of sediments from the Yellow River are deposited near the river delta, 31% conveyed seaward, within which, 4% exported to the northern Yellow Sea through the Bohai Strait. Wind wave is the most essential contributor to seasonal variations of sediment concentration in the Bohai Sea, and the contribution of tidal currents is also significant in shallow waters when wind speed is low.

Evaluation of emissions and transport of CFCs using surface observations and their seasonal cycles and the GEOS CCM simulation with emissions‐based forcing

Levels of ozone depleting substances (ODSs) in our atmosphere are determined by production, emission, and loss processes. However, atmospheric models are forced by the specified mixing ratios of these ODSs rather than the more fundamental emissions‐based forcing. To more accurately represent the physics and chemistry of climate change on atmospheric circulation and ODSs, and therefore future ozone recovery, it is desirable to switch from the current highly constrained mixing‐ratio‐based forcing to emissions‐based forcing in general circulation models (GCMs). As a first step of this model transition, we have conducted a 45‐year (1960–2005) emissions‐based simulation of the three primary chlorofluorocarbons (CFC‐11, ‐12, ‐113) using the GEOS coupled chemistry‐climate model (CCM). The simulated CFC concentrations and their seasonal cycles are compared with AGAGE and NOAA‐GMD observations to evaluate emissions and atmospheric transport. The simulated CFC‐12 agrees well with the observations, indicating a good estimate of emission and atmospheric loss. The simulated CFC‐11 and CFC‐113 shows high biases due to overestimate of emissions. Using tagged CFC tracers to track recent surface emissions and aged air masses transported downward from the stratosphere separately, we quantify the relative contribution of stratosphere‐troposphere exchange (STE) and tropospheric transport to the seasonal cycles of CFCs in the lower troposphere. The seasonal cycles of CFCs in the lower troposphere are dominated by tropospheric transport of recent emissions during 1985–1994. The 1995–1999 period marks the transition period when variations due to fresh emissions and STE become equally important. Seasonal cycles of CFCs at most surface sites in the 2000–2004 period are dominated by STE. Seasonal cycles of CFCs due to STE show a late winter/early spring maximum and a summer/fall minimum. Seasonality of the tropospheric transport component at individual stations is governed by seasonal transport variations of fresh emissions from the polluted regions.

Seasonal energy cycle of wind-driven ocean circulation with particular emphasis on the role of bottom topography

We investigate how the bottom slope affects the time-dependent global energy balance of a two-layer subtropical gyre driven by seasonal winds in order to reformulate the concept joint effect of baroclinicity and bottom relief (JEBAR) based on energetics rather than vorticity dynamics. It is shown that the role of JEBAR in this situation is to transfer energy between the barotropic and baroclinic fields. Since a deep current tends to flow in meridional directions along a meridional ridge, the geostrophically balanced pressure-gradient forces can perform work on the zonal barotropic flow over the ridge. The direction of the deep motion, and hence the sign of the work is reversed seasonally because the pressure field in the lower layer exhibits an anticyclonic tendency in winter and a cyclonic tendency in summer. The topographic beta effect strengthens the work on the northwest and southeast sides of the ridge, so that the net contribution from the ridge region is negative in winter and positive in summer. On the other hand, this work must be canceled by enhancing the energy conversion to satisfy the energy equation. As a result, the ridge not only accelerates but also seasonally reverses the sign of the rate of energy conversion. With some modification, a meridional trench and a western continental slope turn out to have qualitatively the same effect on the seasonal transport variation. Therefore, the annual range in the barotropic transport of the gyre is, to varying degrees, reduced irrespective of the details of the large-scale bottom topography.

Effect of air-sea heat exchange on seasonal transport variation of the Kuroshio

Effect of air-sea heat exchange on seasonal transport variation of the Kuroshio

Interannual and seasonal variations in transport to a measuring site in western Siberia and their impact on the observed atmospheric CO2 mixing ratio

Interannual and seasonal variations in atmospheric transport to a measuring site in western Siberia were studied using three‐dimensional 5‐day back‐trajectories. We identified large differences in transport between summer and winter, but also some differences between the years. Westerly flow was dominating during winter, whereas the transport in summer also had a contribution from the high Arctic and central Siberia. Cluster analysis was applied to the trajectory data to determine systematically to what degree different atmospheric flow patterns influence the variability of the atmospheric CO2. We found that during July, the short‐term variability in CO2 was primarily determined by the interplay between planetary boundary layer dynamics and surface fluxes in the region within tens to one hundred kilometers from the measuring site. The weaker biogenic fluxes during winter, resulted in mixing ratios more clearly influenced by long‐range transport of CO2. However, the highest CO2 concentrations were not associated with westerly winds reflecting transport of polluted air from Europe but periods characterized by stagnant flow conditions. It was suggested that these high mixing ratios were due to respired CO2 accumulated in the lowest one hundred meters of the atmosphere. The mean duration of the identified flow patterns was about two days, implying a sampling frequency not less than every other day in order to monitor variations in atmospheric CO2 due to air mass changes. This study demonstrates that the flux footprint of a site is variable which must be considered when making comparisons between different seasons or years for a measurement location.

Annual cycle of the Brazil‐Malvinas confluence region in the National Center for Atmospheric Research Climate System Model

The objective of this study is to compare the mean and seasonal variability of the circulation in the southwest Atlantic with observations. The results used in the comparison are from the last 200 years of a 300 year control integration of the Climate System Model (CSM). The area of study includes the confluence region between the subtropical and subpolar waters represented by the Brazil and Malvinas Currents. The seasonal variation of transport and its relationship to changes in the wind stress forcing and in the sea surface temperature are examined and compared to available oceanographic observations. This study shows that a coarse resolution climate model, such as the CSM, can successfully reproduce major characteristics of the Brazil‐Malvinas confluence seasonally, although the mesoscale features involving recirculation and meander dynamics are not resolved. The CSM transport values in the region of 38°S are consistent with hydrographically derived values. The transport of the CSM Brazil Current is higher during austral summer and smaller during austral winter. Conversely, the Malvinas Current transport is weaker during austral summer and stronger during austral winter. This is also consistent with observations. The CSM seasonal cycle in transport associated with both the Brazil and Malvinas Currents and its meridional displacement is closely linked to the seasonal variations in the local wind stress curl. However, the displacement is much smaller in the model than in observations. The CSM results show that the latitudinal displacement of the 24°C and 17°C at the South American coast beween austral summer and winter is 20° and 12°, respectively. This is very similar to the displacement seen in observations.

Models of the Southeast Asian Seas

Abstract The mean and seasonal variations in transport through and within the Southeast Asian seas are investigated using a series of simple models. The results are compared with results from a fine-resolution, 3D, numerical simulation of the global circulation from 1987 to 1995 [Parallel Ocean Climate Model (POCM)]. For the mean circulation, the models are based on Sverdrup dynamics with the circulation around each island calculated according to the island rule with overlapping islands taken into account. Assuming all of the passages are wide and deep yields an archipelago circulation vastly at odds with observations. A large westward transport through Torres Strait provides the throughflow between the Pacific and Indian Oceans. A large westward transport through Luzon Strait passes southward through the South China Sea into the Sulu Sea and exits into the Pacific Ocean through the Celebes Sea. There is a northwestward transport through the remainder of the archipelago. By successively blocking straits u...

西岸境界流の流量は風応力から推定できるのか

The question whether the transport of the western boundary currents in the subtropics can be accounted for only by the wind stress is examined on the basis of previous works. The seasonal transport variation of the Florida Current in the North Atlantic is probably able to be estimated from the wind stress, while its mean transport cannot be estimated because the thermohaline effect as well as the wind effect is important. In the case of the Kuroshio in the North Pacific, the total eastward transport and its seasonal variation cannot be evaluated only from the wind stress due to eddy activities around the Kuroshio, but the net transport including recirculation and its seasonal variation can be evaluated.

Seasonal variations of the Indonesian throughflow in a general ocean circulation model

Seasonal variability of the depth‐integrated transport in the Indonesian Seas is investigated in detail, together with its driving mechanism, by use of an Indo‐Pacific ocean general circulation model that allows the circulation around Australasia. Modeled net throughflow shows an annual mean transport of 9.5 Sv from the Pacific to the Indian Ocean with a maximum (minimum) transport of 11.6 Sv (6.0 Sv) in August (January). Seventy‐five percent of this transport flows through the Lombok Strait in the present model. Comparison with the result of Masumoto and Yamagata [1993], in which the circulation around Australasia was not permitted, clarifies that the magnitude of the net interbasin throughflow does not affect the seasonal variations in the depth‐integrated transport through the Lombok Strait. Several additional experiments are also carried out by masking winds over particular regions to identify which part of winds is actually effective in the seasonal variations in the depth‐integrated transport through the Timor Sea as well as the Lombok Strait. It turns out that the seasonal variability of the transport through the Lombok Strait is almost perfectly governed by the winds over the equatorial Indian and Pacific Oceans. On the other hand, the coastally trapped waves generated by the winds along the western and southern coast of Australia play an important role in the seasonal transport variation through the Timor Sea.

Seasonal transport variations of the wind-driven ocean circulation in a two-layer planetary geostrophic model with a continental slope

Using a simple two-layer planetary geostrophic model with a continental slope, the observed seasonal variation of the total transport of the Kuroshio is explained qualitatively for the first time in a quite concise way. During the period of weak winds in summer, the total transport is much larger than the nontopographic Sverdrup transport. This is because the joint effect of baroclinicity and bottom relief (so-called JEBAR) drives the barotropic flow with releasing the available potential energy accumulated in winter. In winter, on the other hand, the transport is much smaller than the nontopographic Sverdrup transport. This is mainly due to the topographic beta-effect. In addition to the available potential energy supplied directly by winds, the barotropic flow excited by the strong winds in winter impinges on the bottom slope to induce the baroclinic flow so that the wind-driven barotropic flow is converted into the available potential energy to be released in summer. Thus, the role of JEBAR is to make the total transport of the Kuroshio relatively insensitive to seasonal changes of winds as observed. This mechanism may be called JEBAR rectification for simplicity.

Study of seasonal transport variations in the Indonesian seas

Seasonal transport variations between the Pacific and Indian Oceans via the Indonesian seas were studied by the Euler‐Lagrangian method. The velocity field was calculated with a fairly high resolution robust diagnostic model. The model well reproduces the features of seasonal variations in the Indonesian seas. The total volume transport of the Indonesian throughflow is 20±3 Sv (1 Sv = 106 m3 s−1), the maximum being from boreal spring to boreal summer and the minimum in boreal winter. The values are similar to those of previous general circulation models with a wide Indonesian passage despite resolution of the presence of the many small islands in the Indonesian seas. Although a large portion of the net transport is contained in the upper layer, deep transport below 1000‐m depth is about 5 Sv. This value corresponds to approximately 25% of the total transport, which means that disregard of the deep transport leads to underestimation of the volume transport of the throughflow. Tracking of numerous labeled particles in the calculated velocity field clarified the sources and pathways of the Indonesian throughflow. The major route is a western one through both the Makassar and Lombok Straits. Most of the North Pacific water supplied from the Mindanao Current passes along this route, entering the Indian Ocean within several months with almost no loss of its properties (intense vertical mixing around the Lombok sill reported by observations could not be reproduced in our model). In contrast, South Pacific water takes the eastern route into the eastern Indonesian seas and subsequently mixes with waters from the North Pacific and Indian Oceans in the Banda Sea, which means that it has a long travel time (at least a few years). Water taking the eastern route therefore loses its original properties before arriving in the Indian Ocean. The transport processes also are significantly affected by seasonal variations in equatorial circulation in the western Pacific. In the surface layer, North Pacific water is vigorously supplied to the western route only from boreal spring to summer in association with the linkage between the current flowing through the Makassar Strait and the Mindanao Current. In other seasons, because the Mindanao Current is strongly linked with the North Equatorial Countercurrent and the New Guinea Coastal Current primarily by northeasterly monsoonal winds, its upper water flows back to the Pacific Ocean. In the subsurface layer, a pronounced inflow of Mindanao Current water into the western route occurs from boreal winter to spring, when the subsurface link between that current and the Equatorial Undercurrent tends to weaken. In the deep, the quasi‐steady transport of Pacific water into the Indian Ocean via the eastern route is fed by the westward deep current in the equatorial Pacific.

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.