Mean Seasonal Cycle Research Articles

Seasonal heat budget in the mixed layer of the southeastern tropical Indian Ocean in a high‐resolution ocean general circulation model

Using results from a high‐resolution ocean general circulation model, this study identifies the impact of ocean dynamics on the mean seasonal cycle of sea surface temperature (SST) in the southeastern tropical Indian Ocean. An important question about the mechanisms of regional SST variation arises in a quick look at satellite‐observed SST and ocean color. Although ocean color indicates a distinctive upwelling season from July to September, the SST depression off Java and Sumatra is small, forming a sharp contrast with other eastern boundary regions. The model results suggest an explanation of the process responsible for the small SST depression. Our analysis indicates that there are three dynamically different regimes within the region. First, near the coast of northwest Australia, surface heat flux controls the seasonal variation of SST, while horizontal advection and vertical entrainment are relatively weak. This result is consistent with previous studies. Second, south of Java and farther to the east, warm horizontal advection of the Indonesian throughflow (ITF) neutralizes the cold upwelling. The transport of the ITF, especially the outflow from the Lombok Strait, reaches its seasonal maximum in July–September, at the same time that the maximum upwelling occurs. Third, west of Sumatra, a thick barrier layer exists, which impedes the cold thermocline water from entering the mixed layer (ML). Although upwelling occurs, it has no significant effect on the SST as a result of the small vertical temperature gradient at the bottom of the ML.

Role of the atmosphere in seasonal phase locking of El Niño

The variations with the seasonal cycle of the atmospheric response to constant SST anomalies in the eastern tropical Pacific are investigated with the atmospheric GCM, HadAM3. The equatorial wind response is weakest in January and February when the warmest SSTs are south of the Equator and strongest in April when the warmest SSTs are on the Equator. This may have consequences for the seasonality of the onset and termination of El Niño. Westerly wind anomalies in the tropical Pacific associated with El Niño have previously been observed to shift south of the Equator, weakening on the Equator, during the northern winter. It has been suggested that this may contribute to the termination of El Niño in spring. These experiments demonstrate that such a shift can arise solely in response to the mean seasonal cycle during El Niño and does not require changes in SST anomalies.

Past and present productivity of the Baltic Sea as inferred from pCO2 data

The spring/summer decrease in the CO2 partial pressure, pCO2, in the surface waters of the Baltic Proper was used to estimate the net biomass production during recent years and at the beginning of the last century. Current pCO2 data were available from 17 cruises in the eastern Gotland Sea during 1995–2001, and provided the mean seasonal pCO2 cycle. Taking into account temperature and salinity, the pCO2 seasonality was used to calculate the changes in the total CO2 (NCT, normalized to total alkalinity) on the basis of the equilibrium relationships of the marine CO2 system. A carbon balance was established for the productive period taking into account the NCT depletion, the CO2 air/sea exchange and the formation of dissolved organic carbon. The closing term of the balance was attributed to the integral net production of particulate organic carbon, PPOC, that was considered to represent the biomass production. The maximum PPOC was 3.6±0.3molm−2 and indicated the termination of net production by the end of July. Historic pCO2 data were available from measurements of alkalinity and pH performed during the spring and summer of 1927–1935 in the eastern Gotland Sea. The carbon balance yielded a PPOC of 1.4molm−2 with an error range of ±0.4molm−2 that was mainly attributed to the uncertainty in the pH measurements. This indicates an increase in the net biomass productivity of the Baltic Proper by a factor of 2.5 (maximum uncertainty: 1.8–3.8) since the beginning of the last century.

Sub-seasonal to interannual variations of sea surface temperature, salinity, oxygen anomaly, and transmissivity in Santa Monica Bay, California from 1987 to 1997

While the basic hydrographic and circulation pattern of Santa Monica Bay (SMB) are relatively well known, the seasonal and interannual variations of these patterns are not well established nor are the mechanisms behind them well understood. We analyzed 10 years of hydrographic observations (1987–1997) in the surface layer of SMB off southern California to establish the mean seasonal cycle and to deduce patterns of sub-seasonal to interannual variability of sea surface temperature, salinity, oxygen anomaly (the difference between oxygen concentration and its solubility), and transmissivity using empirical orthogonal functions and time-lagged cross-correlation methods. All four variables exhibit distinct seasonal variations, whose spatial distributions are a consequence of the interaction of bottom topography, vertical mixing, horizontal advection, freshwater discharge, and biological processes. The seasonal anomalies of all four parameters were estimated by removing the mean seasonal cycle; they exhibit three basic patterns: (1) an inshore/offshore gradient; (2) a balance between horizontal advection of the California Current from the northwest and the Southern California Countercurrent from the south; and (3) freshwater discharge from Ballona Creek. Air/sea heat flux and vertical mixing are the main factors regulating the dynamics of sea surface temperature. Freshwater discharge centered at Ballona Creek is the principal factor creating the salinity pattern. The annual mean oxygen anomaly shows a substantial supersaturation across the entire SMB. The variations of oxygen anomalies are regulated by local biological productivity, vertical mixing, and horizontal transport. Vertical mixing influences water transparency by suspension of sediments and nutrient supply stimulating phytoplankton growth. A remarkably clear pattern of spatio-temporal variations of all parameters is observed in a relatively small basin; these variations are regulated by both local meteorological factors (air temperature, wind, atmospheric precipitation) and remote forcing, including the El Niño–Southern Oscillation cycle.

Sub-seasonal to interannual variations of sea surface temperature, salinity, oxygen anomaly, and transmissivity in Santa Monica Bay, California from 1987 to 1997

While the basic hydrographic and circulation pattern of Santa Monica Bay (SMB) are relatively well known, the seasonal and interannual variations of these patterns are not well established nor are the mechanisms behind them well understood. We analyzed 10 years of hydrographic observations (1987–1997) in the surface layer of SMB off southern California to establish the mean seasonal cycle and to deduce patterns of sub-seasonal to interannual variability of sea surface temperature, salinity, oxygen anomaly (the difference between oxygen concentration and its solubility), and transmissivity using empirical orthogonal functions and time-lagged cross-correlation methods. All four variables exhibit distinct seasonal variations, whose spatial distributions are a consequence of the interaction of bottom topography, vertical mixing, horizontal advection, freshwater discharge, and biological processes. The seasonal anomalies of all four parameters were estimated by removing the mean seasonal cycle; they exhibit three basic patterns: (1) an inshore/offshore gradient; (2) a balance between horizontal advection of the California Current from the northwest and the Southern California Countercurrent from the south; and (3) freshwater discharge from Ballona Creek. Air/sea heat flux and vertical mixing are the main factors regulating the dynamics of sea surface temperature. Freshwater discharge centered at Ballona Creek is the principal factor creating the salinity pattern. The annual mean oxygen anomaly shows a substantial supersaturation across the entire SMB. The variations of oxygen anomalies are regulated by local biological productivity, vertical mixing, and horizontal transport. Vertical mixing influences water transparency by suspension of sediments and nutrient supply stimulating phytoplankton growth. A remarkably clear pattern of spatio-temporal variations of all parameters is observed in a relatively small basin; these variations are regulated by both local meteorological factors (air temperature, wind, atmospheric precipitation) and remote forcing, including the El Niño–Southern Oscillation cycle.

An Intersection of Oceanic Waveguides: Variability in the Indonesian Throughflow Region

Abstract Temperature and sea level variability within the Indonesian seas and southeast Indian Ocean are described based on expendable bathythermograph deployments along volunteer merchant shipping lines under way since 1983. These data resolve variability at time scales ranging from the intraseasonal to the interannual. A lagged partial regression technique reveals that anomalies from a mean seasonal cycle of temperature and sea level for seasonal to interannual time scales can be largely understood in terms of free Kelvin and Rossby waves generated by remote zonal winds along the equator of the Indian and Pacific Oceans, with local wind forcing appearing to play a minor role. About 60%–90% of sea level variability and 70% of thermocline temperature variability can be accounted for in this way. Variations in zonal Pacific equatorial winds force a response along the Arafura/ Australia shelf break through Pacific equatorial Rossby waves exciting coastally trapped waves off the western tip of New Guinea, wh...

Seasonal Variability of the Large-Scale Currents near the Coast of the Philippines*

Abstract The mean seasonal cycle of the western boundary currents in the tropical North Pacific Ocean is studied diagnostically by combining atmospheric climatologies with drifter, satellite altimetry, and hydrographic data in the framework of a simplified numerical model incorporating geostrophy, hydrostatics, continuity, and tracer conservation. The approach enables the authors to diagnose the absolute 3D velocity field and to assess the seasonal cycle of sea surface height (SSH)/total transports. Errors are estimated by considering multiple datasets and averaging over the results of the corresponding diagnostic computations. Analysis shows that bifurcation of the North Equatorial Current (NEC) occurs at 14.3° ± 0.7°N near the Philippine coast. Meridional migration of the NEC bifurcation latitude is accompanied by quantitative changes in the partitioning of the NEC transport between the Kuroshio and Mindanao Current. In February–July, when the NEC transport is 58 ± 3 Sv (Sv ≡ 106 m3 s−1), the Kuroshio t...

Circulation Regimes and SST Forcing: Results from Large GCM Ensembles

Abstract The purpose of this paper is to study the influence of tropical sea surface temperature (SST) on the midlatitude circulation regime behavior in the Pacific–North American region. Toward this end, a cluster analysis has been applied to 55-member ensembles of winter seasonal simulations of the Center for Ocean–Land–Atmosphere Studies general circulation model for 18 winters. The ensemble members for each winter utilize the same prescribed, observed weekly varying SST for that winter. The cluster analysis includes all fluctuations with time scales longer than 10 days except for the ensemble mean seasonal cycle. Using a partitioning algorithm separately applied to each winter's ensemble, clusters are found in the 200-hPa height field that are signicant (vis-a-vis a suitable Gaussian background), reproducible (in half-length datasets), and consistent (with clusters obtained from the 200-hPa u wind) for all winters except the strong El Nino events of 1982/83, 1986/87, and 1997/98. One cluster found con...

Technical note: an interannual inversion method forcontinuous CO<sub>2</sub> data

Abstract. A sequential synthesis inversion method is described to estimate CO2 sources from continuous atmospheric data. The sequential method makes the problem computationally feasible. The method is assessed using four-hourly synthetic concentration data generated from known sources. Multi-year mean sources and seasonal cycles are estimated with comparable quality as those from a traditional inversion of monthly mean data. Interannual variations in the estimated sources are closer to those of the known sources using the four-hourly data rather than monthly data. The computational cost of the basis function simulations can be reduced by generating responses that are only six months long. This does not significantly degrade the inversion results compared to using responses that are 12 months in length.

Seasonal and interannual variability of satellite‐derived chlorophyll pigment, surface height, and temperature off Baja California

Mean fields, seasonal cycles, and interannual variability are examined for fields of satellite‐derived chlorophyll pigment concentrations (CHL), sea surface height (SSH), and sea surface temperature (SST) during 1997–2002. The analyses help to identify three dynamic regions: an upwelling zone next to the coast, the Ensenada Front in the north, and regions of repeated meanders and/or eddy variability west and southwest of Point Eugenia. High values of CHL are found in the upwelling zone, diminishing offshore. The exception is the area north of 31°N (the Ensenada Front), where higher CHL are found about 150 km offshore. South of 31°N, the long‐term mean dynamic topography decreases next to the coast, creating isopleths of height parallel to the coastline, consistent with southward geostrophic flow. North of 31°N the mean flow is toward the east, consistent with the presence of the Ensenada Front. The mean SST reveals a more north‐south gradient, reflecting latitudinal differences in surface heating due to solar radiation. Harmonic analyses and EOFs reveal the seasonal and interannual patterns, including the region of repeated eddy activity to the west and southwest of Point Eugenia. A maximum CHL occurs in spring in most of the inshore regions, reflecting the growth of phytoplankton in response to the seasonal maximum in upwelling‐favorable winds. SST and SSH anomalies are negative in the coastal upwelling zone in spring, also consistent with a response to the seasonal maximum in upwelling. When the seasonal cycle is removed, the strongest signal in the EOF time series is the response to the strong 1997–1998 El Niño, with a weaker signal representing La Niña (1998–1999) conditions. El Niño conditions consist of low chlorophyll, high SSH, and high SST, with opposite conditions during La Niña.

Transcom 3 inversion intercomparison: Model mean results for the estimation of seasonal carbon sources and sinks

The TransCom 3 experiment was begun to explore the estimation of carbon sources and sinks via the inversion of simulated tracer transport. We build upon previous TransCom work by presenting the seasonal inverse results which provide estimates of carbon flux for 11 land and 11 ocean regions using 12 atmospheric transport models. The monthly fluxes represent the mean seasonal cycle for the 1992 to 1996 time period. The spread among the model results is larger than the average of their estimated flux uncertainty in the northern extratropics and vice versa in the tropical regions. In the northern land regions, the model spread is largest during the growing season. Compared to a seasonally balanced biosphere prior flux generated by the CASA model, we find significant changes to the carbon exchange in the European region with greater growing season net uptake which persists into the fall months. Both Boreal North America and Boreal Asia show lessened net uptake at the onset of the growing season with Boreal Asia also exhibiting greater peak growing season net uptake. Temperate Asia shows a dramatic springward shift in the peak timing of growing season net uptake relative to the neutral CASA flux while Temperate North America exhibits a broad flattening of the seasonal cycle. In most of the ocean regions, the inverse fluxes exhibit much greater seasonality than that implied by the ΔpCO2 derived fluxes though this may be due, in part, to misallocation of adjacent land flux. In the Southern Ocean, the austral spring and fall exhibits much less carbon uptake than implied by ΔpCO2 derived fluxes. Sensitivity testing indicates that the inverse estimates are not overly influenced by the prior flux choices. Considerable agreement exists between the model mean, annual mean results of this study and that of the previously published TransCom annual mean inversion. The differences that do exist are in poorly constrained regions and tend to exhibit compensatory fluxes in order to match the global mass constraint. The differences between the estimated fluxes and the prior model over the northern land regions could be due to the prior model respiration response to temperature. Significant phase differences, such as that in the Temperate Asia region, may be due to the limited observations for that region. Finally, differences in the boreal land regions between the prior model and the estimated fluxes may be a reflection of the timing of spring thaw and an imbalance in respiration versus photosynthesis.

Seasonal patterns of sea-surface temperature and ocean color around the Galápagos: regional and local influences

Abstract Monthly climatologies of satellite-derived sea-surface temperature (SST, from AVHRR) and ocean color (from Ocean Color and Temperature Scanner and Sea-viewing Wide-Field-of-view Sensor) around the Galapagos Archipelago are used to estimate the mean seasonal cycle of these properties and their relationship to the equatorial circulation in this oceanographically complex region. Harmonic analysis of the climatological time series indicates a best fit with annual and semi-annual constituents. The annual amplitude is the dominant signal in SST, corresponding to the basin-wide seasonal cycle of warming and cooling associated with the north–south migration of the intertropical convergence zone. Influx of upwelled water from the Panama Bight into the northeastern part of the study area is also consistent with the annual signal. An empirical orthogonal function decomposition identifies two main spatial patterns with amplitude time series representing out-of-phase annual cycles. The dominant mode corresponds to the strengthening of the Equatorial Front and the South Equatorial Current during the second part of the year. This mode explains 92.2% of the SST variance and 82.9% of the ocean-color variance. The second mode is consistent with the topographically induced upwelling of the Equatorial Undercurrent on the western side of the archipelago, and with influx of upwelled Panama Bight water on the eastern side, both reaching their peak during the first part of the year. This mode accounts for 6% of the SST variance and 7.7% of the ocean-color variance. The seasonal evolution of water-column temperature and nitrate (from World Ocean Atlas 1998 climatologies) is consistent with the satellite-derived patterns. A slight tilt aligned with the east–west axis of the Galapagos Platform (outlined by the 2000-m isobath) is evident in all property fields, suggesting that the presence of the archipelago introduces a small but noticeable perturbation to the large-scale currents and property gradients of the eastern equatorial Pacific.

Statistics of calendar month averages of surface temperature: A possible relationship to climate sensitivity

This study makes use of calendar month averages of surface temperature taken from the long control runs of 13 coupled atmosphere/ocean general circulation models (GCMs) as well as from the record of observational surface temperature measurements. After aggregating the data into global averages, we examine the mean seasonal cycle as well as the anomaly statistics. We find a large range of different results for the models in the seasonal cycle of anomaly statistics. There appears to be an “empirical” relationship between the published sensitivity of the models to CO2 doubling and the seasonal cycle of the anomaly statistics. We draw an inference about the sensitivity of the real climate based upon this relationship. The relationship appears to be plausible based upon simple considerations of the land–sea distribution and the wintertime variance of climatic noise forcing. This inference leads us to estimate the sensitivity of climate to a doubling of CO2 to be about 2.7°C with the 95% confidence interval (2.3°C, 3.1°C).

Evaluating the carbon cycle of a coupled atmosphere‐biosphere model

We investigate how well a coupled biosphere‐atmosphere model, CCM3‐IBIS, can simulate the functioning of the terrestrial biosphere and the carbon cycling through it. The simulated climate is compared to observations, while the vegetation cover and the carbon cycle are compared to an off‐line version of the biosphere model IBIS forced with observed climatic variables. The simulated climate presents some local biases that strongly affect the vegetation (e.g., a misrepresentation of the African monsoon). Compared to the off‐line model, the coupled model simulates well the globally averaged carbon fluxes and vegetation pools. The zonal mean carbon fluxes and the zonal mean seasonal cycle are also well represented except between 0° and 20°N due to the misrepresentation of the African monsoon. These results suggest that, despite regional biases in climate and ecosystem simulations, this coupled atmosphere‐biosphere model can be used to explore geographic and temporal variations in the global carbon cycle.

Salinity barrier layer and onset of El Niño in a Pacific coupled model

The importance of the barrier layer during the onset of El Niño is investigated using a coupled ocean‐atmosphere general circulation model. Sensitivity experiments are done by removing or keeping the salinity stratification in the upper layer of the western equatorial Pacific warm pool. The barrier layer favors the maintenance and displacement of the warm pool into the central Pacific by isolating the mixed layer from the entrainment cooling at depth and by confining the response of westerly wind events (WWEs) to a shallow mixed layer. The increased zonal fetch of WWEs through the coupling with sea surface temperature (SST) enhances downwelling equatorial Kelvin waves and thus leads to El Niño. In the absence of salinity stratification, slightly cooler SST and a reduced eastward displacement of the warm pool result in a reduced El Niño or a return to the mean seasonal cycle of the model. The possibility that the barrier layer affects the onset of El Niño pleads for a careful consideration of the salinity stratification in climate forecasts.

Regional ocean climate simulations with a 3D ice-ocean model for the Baltic Sea. Part 2: results for sea ice

The Baltic Sea ice season under changing climate conditions is investigated using a 3D coupled ice–ocean model. Results of multi-year simulations for the period of May 1980 to December 1993 are compared with observations from monitoring stations, ice charts and satellite data. The period 1980–1993 has been selected mainly because of the availability of homogeneous observational data sets for atmospheric variables and river runoff with sufficient quality to force a 3D high resolution Baltic Sea model. The observed seasonal variation of sea ice is well reproduced by the model. Furthermore, two sets of 9-year time slice experiments have been performed using results of an atmospheric regional climate model as forcing, one representing pre-industrial greenhouse conditions (control simulation), and the other a global warming with a 150% increase of equivalent CO2 concentration (scenario simulation). At the lateral boundaries of the regional climate model, results of the global atmosphere–ocean general circulation model HadCM2 have been prescribed. In the control run, the mean seasonal cycle of ice cover and its variability is simulated realistically compared to observations, but the seasonal ice cover maximum is shifted in time by about 18 days and the simulated mean melting date is delayed. Mild winters are missing in the relatively short control run. The decrease of mean ice extent in the scenario, compared to the control run, is dramatic, reducing from 210 · 109 m2 to 82 · 109 m2 (a relative change of 61%). However, in all scenario years, ice is still formed in the northernmost basin of the Baltic Sea, the Bothnian Bay. The minimum ice extent is 16 · 109 m2 (for comparison: the area of the Bothnian Bay is about twice as large). The mean number of ice days decreases significantly. In the fast ice zone of the Bothnian Bay the mean ice season is reduced by 40 days. The ice in the scenario run is thinner with less snow on top. In the central Bothnian Bay, mean maximum annual ice thickness is reduced by 25 cm from 54 to 29 cm. Model dependent uncertainties are discussed.

Subduction of the North Pacific Mode Waters in a Global High-Resolution GCM*

Abstract The annual subduction rate in the North Pacific is estimated using five-day outputs from a high-resolution general circulation model (GCM). Two maxima (>200 m yr−1) are found in the western North Pacific: one is responsible for the formation of the subtropical mode water (STMW) and the other for the formation of the central mode water (CMW). A local maximum (>75 m yr−1) is also found in the formation region of the eastern subtropical mode water (ESMW). These results are compared with a calculation using the winter mixed layer depth and annual mean velocity fields to examine the effect of mesoscale eddies. Although the mesoscale eddies do not markedly affect the general subduction pattern, they enhance the annual subduction rate by up to 100 m yr−1 in the formation region of the STMW/CMW, a 34% increase in a regional average (30°–44°N, 140°E–170°W). Further analysis shows that the effects of the mean seasonal cycle and smaller-scale (<30 days) eddies are generally small. The authors suggest that t...

Seasonal and interannual changes in Intense Benguela Upwelling (1982–1999)

Monthly maps of remotely sensed sea surface temperatures derived from NOAA/AVHRR thermal images are used to describe changes in ‘Intense Benguela Upwelling’ during 1982–1999. The coastal area under investigation lies between 9–34° S and 8–20° E and the total area of cold water between the coast and the course of the 13 °C isotherm is considered to be an index of intense, active upwelling. It exhibits a decreasing trend over the study period of 18 years and some evidence for a quasi-cycle of about 27 months. Seasonal cycles are discussed for the total cold water area as well as for its mean alongshore and offshore extents. The main season of cold surface water was found to occur between July and September during the austral winter. It peaks in August with a mean area of about 30×10 3 km 2 and relaxes drastically during the rest of the year. The underlying process of intense coastal upwelling is regionally trapped in two coastal zones. These are centred around 26 and 29° S and reach a mean offshore extension of 210 and 130 km, respectively, to form giant upwelling filaments. The area of cold water drastically shrinks, roughly by a factor of two, during weak upwelling years but significantly expands by a factor of about 1.5 during strong years. Associated sea level changes along the south-west African coast were derived from measurements at four coastal stations between 23 and 34° S during 1982–1987. The first principal component describes about 63 % of the total sea level variance. The lowest sea levels were found in the prominent Lüderitz cell near 26° S. On both the annual and interannual time scale, decreasing cold water areas are accompanied by increasing sea levels and vice versa. Mean seasonal cycles reveal that variations in the total cold water area lag behind those in the sea level along the entire south-west African coast by about 1 month.

Multivariate stochastic downscaling model for generating daily precipitation series based on atmospheric circulation

The goal of the paper is to present a model for generating daily precipitation time series and its applications to two climatologically different areas. The rainfall is modeled as stochastic process coupled to atmospheric circulation. Rainfall is linked to the circulation patterns using conditional model parameters. Any kind of circulation pattern classification can be used for this purpose. In this study a new fuzzy rule based method of circulation patterns classification was used. The advantage of this classification technique is the fact that in contrast to common circulation patterns classifications its objective is to explain the variability of local precipitation. It means that the circulation patterns explain the relation between large-scale atmospheric circulation and surface climate (precipitation). Therefore the circulation patterns obtained by this classification method are suitable as input for the subsequent precipitation downscaling. The model was successfully applied in two regions with different climate conditions: Central Europe (Germany) and Eastern Mediterranean (Greece). Several tests like comparison of mean seasonal cycles, comparison of mean values and deviations of yearly totals and other standard diagnostics showed that simulated values agree fairly well with historical data.

Mean transport and seasonal cycle of the Kuroshio east of Taiwan with comparison to the Florida Current

Moored observations of Kuroshio current structure and transport variability were made across the channel between northeast Taiwan and the Ryukyu Islands at 24°N 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 aliasing 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.