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The Atlantic meridional overturning circulation (AMOC) is continually monitored along 26°N by the RAPID-MOCHA array. Measurements from this array show a 6.7 Sv seasonal cycle for the AMOC, with a 5.9 Sv contribution from the upper-mid-ocean. Recent studies argue that the dynamics of the eastern Atlantic is the main driver for this seasonal cycle; specifically, Rossby waves excited south of the Canary Islands. Using inverse modelling, hydrographic, mooring and altimetry data, we describe the seasonal cycle of the ocean mass transport around the Canary Islands and at the eastern boundary, under the influence of the African slope, where eastern component of the RAPID-MOCHA array is situated. We find a seasonal cycle of -4.1±0.5 Sv for the oceanic region of the Canary Current, and +3.7±0.4 Sv at the eastern boundary. This seasonal cycle along the eastern boundary is in agreement with the seasonal cycle of the AMOC, that requires the lowest contribution to the transport in the upper-mid-ocean to occur in fall. However, we demonstrate that the linear Rossby wave model used previously to explain the seasonal cycle of the AMOC is not robust, since it is extremely sensitive to the choice of the zonal range of the wind stress curl and produces the same results with a Rossby wave speed of zero. We demonstrate that the seasonal cycle of the eastern boundary is due to the recirculation of the Canary Current and to the seasonal cycle of the poleward flow that characterizes the eastern boundaries of the oceans.

Global warming seems leveling off somewhat during 1993-2013 despite increasing atmospheric greenhouse gases. What has happened to the Kuroshio Current system concurrently? Available independent datasets from 1993 to 2013 point to a single answer. Here we show a systemwide weakened Kuroshio during the period despite enhanced warming along its path. The Pacific warm pool upstream of the Kuroshio is still becoming warmer during the period. It injects more heat into the Current despite the weakened Kuroshio, which is associated with weakened westerlies and cyclonic trends of basin-scale wind stress curl. The weakened Kuroshio will modulate heat and mass exchanges between the tropics and extratropics, impacting the energy balance of climate system. It will also significantly influence mass, heat, salinity, and nutrient exchanges between the Pacific and adjacent marginal seas, which in turn impacts the regional weather, fisheries, and environments.

Owing to lack of observational data and accurate definition, it is difficult to distinguish the Kuroshio intrusion water from the Pacific Ocean into the South China Sea (SCS). By using a passive tracer to identify the Kuroshio water based on an observation-validated three-dimensional numerical model MITgcm, the spatio-temporal variation of the Kuroshio intrusion water into the SCS has been investigated. Our result shows the Kuroshio intrusion is of distinct seasonal variation in both horizontal and vertical directions. In winter, the intruding Kuroshio water reaches the farthest, almost occupying the area from 18°N to 23°N and 114°E to 121°E, with a small branch flowing towards the Taiwan Strait. The intrusion region of the Kuroshio water decreases with depth gradually. However, in summer, the Kuroshio water is confined to the east of 118°E without any branch reaching the Taiwan Strait; meanwhile the intrusion region of the Kuroshio water increases from the surface to the depth about 205 m, then it decreases with depth. The estimated annual mean of Kuroshio Intrusion Transport (KIT) via the Luzon Strait is westward to the SCS in an amount of–3.86×106 m3/s, which is larger than the annual mean of Luzon Strait Transport (LST) of–3.15×106 m3/s. The KIT above 250 m accounts for 60%–80% of the LST throughout the entire water column. By analyzing interannual variation of the Kuroshio intrusion from the year 2003 to 2012, we find that the Kuroshio branch flowing into the Taiwan Strait is the weaker in winter of La Nina years than those in El Nino and normal years, which may be attributed to the wind stress curl off the southeast China then. Furthermore, the KIT correlates the Nino 3.4 index from 2003 to 2012 with a correlation coefficient of 0.41, which is lower than that of the LST with the Nino 3.4 index, i.e., 0.78.

Based on the Simple Ocean Data Assimilation (SODA) products, we study the mean properties and variations of the Southern Hemisphere subpolar gyres (SHSGs) in this paper. The results show that the gyre strengths in the SODA estimates are (55.9 +/- 9.8)x10(6) m(3)/s for the Weddell Gyre (WG), (37.0 +/- 6.4)x10(6) m(3)/s for the Ross Gyre (RG), and (27.5 +/- 8.2)x10(6) m(3)/s for the Australian-Antarctic Gyre (AG), respectively. There exists distinct connectivity between the adjacent gyres and then forms an oceanic super gyre structure in the southern subpolar oceans. And the interior exchanges are about (8.0 +/- 3.2)x10(6) m(3)/s at around 70A degrees E and (4.3 +/- 3.1)x10(6) m(3)/s at around 140A degrees E. The most pronounced variation for all three SHSGs occurs on the seasonal time scale, with generally stronger (weaker) SHSGs during austral winter (summer). And the seasonal changes of the gyre structures show that the eastern boundary of the WG and AG extends considerably further east during winter and the interior exchange in the super gyre structure increases accordingly. The WG and RG also show significant semi-annual changes. The correlation analyses confirm that the variations of the gyre strengths are strongly correlated with the changes in the local wind forcing on the semi-annual and seasonal time scales.

Statistically different precursory air–sea signals between a super and a regular El Nino group are investigated, using observed SST and rainfall data, and oceanic and atmospheric reanalysis data. The El Nino events during 1958–2008 are first separated into two groups: a super El Nino group (S-group) and a regular El Nino group (R-group). Composite analysis shows that a significantly larger SST anomaly (SSTA) tendency appears in S-group than in R-group during the onset phase [April–May(0)], when the positive SSTA is very small. A mixed-layer heat budget analysis indicates that the tendency difference arises primarily from the difference in zonal advective feedback and the associated zonal current anomaly (u′). This is attributed to the difference in the thermocline depth anomaly (D′) over the off-equatorial western Pacific prior to the onset phase, as revealed by three ocean assimilation products. Such a difference in D′ is caused by the difference in the wind stress curl anomaly in situ, which is mainly regulated by the anomalous SST and precipitation over the Maritime Continent and equatorial Pacific.

Phet super cyclone (31 May–7 June 2010) was the most intense and also the rarest of the rare track in Arabian Sea as per the recorded history during 1877–2009. The present study focuses on the ocean physical responses to Phet cyclone using satellite and Argo observations. The sea surface temperature is decreased to 6 °C with an approximately 350 km long and 100 km width area in the Arabian Sea after the cyclone passage. The translation speed of cyclone is 3.86 m/s, the mixed layer is 79 m, and thermocline displacement is 13 m at the cooling area. With the relationship of wind stress curl and Ekman pumping velocity (EPV), the author found that the speed of EPV was increased after the passage of cyclone. So the extent of the SST drop was probably due to the moving speed of cyclone and the depth of the mixed layer.

The regional sea level variation in the East China Sea (ECS) was influenced not only by local factors but also by remote wind from adjoining ocean with the oceanic connectivity influenced by upper-ocean circulation. The satellite altimeter observations showed that from 1993 to 2008, the inter-annual sea level variation in the ECS was negatively related to the strength of Kuroshio. To investigate the relative role of local and remote wind, two sensitive experiments were carried out using the POP model. Model experiments revealed that wind-induced redistributions of water played a significant role in the sea level variation of the ECS. The seasonal variations were induced by both local winds and remote Pacific wind stress with approximately equal contribution. However, on the inter-annual sea level variations, the remote wind forcing over the North Pacific could contribute substantially more than that of local wind which modulated sea level immediately. Remote wind influenced the China Sea in forms of changing of wind stress curl and ocean currents, which influenced the intensity of the Kuroshio, especially during El Nino episodes. (C) 2015 Elsevier B.V. All rights reserved.

The Pacific climate regime has anomalous warm and cool periods every decade associated with atmospheric circulation changes, which are known to have modulated the tropical and subtropical Pacific during the recent Pacific hiatus regime (1999–2013). However, the influence of the hiatus regime on the Kuroshio Extension (KE) remains unclear. Here, we show that the KE jet underwent enhanced warming (increased 1–1.5 °C), intensification (8–19%) and northward migration (0.5–1°). The KE jet became more perturbed in the upstream region (increased by 70%, west of 146°E) but became stable downstream (perturbation decreased 5–11%, east of 146°E). A poleward shift of the mid-latitude jet stream and weakened Aleutian Low (AL) contributed to the northward migration and intensification of the KE jet, respectively. The weakened AL was associated with negative wind stress curl (WSC) in the eastern Pacific, and this WSC generated an underlying positive sea surface height anomaly that propagated westward, intensifying the KE jet when it reached the KE region. Since the recent Pacific hiatus regime ended after 2013, these changes of the KE jet may reverse during the ongoing warming regime.

The variability of the Atlantic Meridional Overturning Circulation (AMOC) may play a role in sea surface temperature predictions on seasonal to decadal time scales. Therefore, AMOC seasonal cycles are a potential baseline for interpreting predictions. Here, we present estimates for the seasonal cycle of transports of volume, temperature, and freshwater associated with the upper limb of the AMOC in the eastern subpolar North Atlantic on the Extended Ellett Line hydrographic section between Scotland and Iceland. Due to weather, ship-based observations are primarily in summer. Recent glider observations during other seasons present an opportunity to investigate the seasonal variability in the upper layer of the AMOC. First, we document a new method to quality control and merge ship, float, and glider hydrographic observations. This method accounts for the different spatial sampling rates of the three platforms. The merged observations are used to compute seasonal cycles of volume, temperature, and freshwater transports in the Rockall Trough. These estimates are similar to the seasonal cycles in two eddy-resolving ocean models. Volume transport appears to be the primary factor modulating other Rockall Trough transports. Finally, we show that the weakest transports occur in summer, consistent with seasonal changes in the regional-scale wind stress curl. Although the seasonal cycle is weak compared to other variability in this region, the amplitude of the seasonal cycle in the Rockall Trough, roughly 0.5 to 1 Sv about a mean of 3.4 Sv, may account for up to 7 to 14% of the heat flux between Scotland and Greenland.

Interannual variability of thermal front west of Luzon Island during the winter of 1993–2013 is examined with the method of singular value decomposition (SVD) and a suite of satellite measurements in this paper. It is found that both the area and intensity of the thermal front west of Luzon Island show apparent interannual variability. Further study based on SVD shows that the interannual variability of the thermal front is highly associated with El Nino and Southern Oscillation (ENSO), and the correlation coefficient between Nino3.4 index and the first Principal Component (PC1) of thermal front can reach–0.65. The mechanism can be described as follows. In El Nino (La Nina) years, the East Asian winter monsoon (EAWM) is weakened (enhanced), inducing weaker (stronger) local wind stress curl (WSC) west of Luzon Island, and resulting in weakened (enhanced) Luzon cold eddy, which finally leads to the weakening (enhancement) of the thermal front.

A variable-grid global ocean circulation model (MITgcm) was employed to study the effects of wind and Kuroshio intrusion on the Luzon cold eddies (LCEs) in the northern South China Sea (SCS). Study findings showed that there are two cold eddies northwest of the Luzon Island. The cold eddies above 300 m and below 500 m depth are referred to as the upper and lower layer LCE, respectively, based on their different generation sites, properties and dynamic mechanisms in these layers. In this study, the upper layer LCE was mainly induced by the winter wind stress curl in the SCS and generated northwest of the Luzon Island in late November, but it may have been weakened by the Kuroshio intrusion. It moved northwestward and decayed at about (115 degrees E, 20 degrees N) in late June. The center of the upper layer LCE tilted westward with increasing depth until March. The vorticity intensity of the upper layer LCE peaked in December to January and generally decreased with depth. The contributions of both wind stress curl and Kuroshio intrusion to the generation of the lower layer LCE were roughly equal. The lower layer LCE originated from a cold eddy, which moved westward from the Luzon Strait in August through October and then intensified by wind stress curl. Subsequently, it moved westward with a slight southward migration and finally decayed near the Xisha Islands in April to May. Its amplitude and radius reached a maximum around December, while the vorticity intensity peaked in January to February. (C) 2015 Elsevier Ltd. All rights reserved.

Based on the analyses of 59 year (1950–2008) surface geostrophic velocities, the multidecadal changes of Kuroshio from the eastern Luzon to the southern Japan are investigated. Result shows that the upstream Kuroshio from the east of Luzon to southern East China Sea suffers much more obvious multidecadal changes. Except for the decade of 1980, the Kuroshio east of Luzon possesses a multidecadal tendency opposite to that east of Taiwan. Besides 1980s, the multidecadal change of Kuroshio bordering Taiwan is mainly governed by the eddy activity off the eastern Taiwan, while the wind stress curl plays a major role in the Kuroshio in the east of Luzon and in the Luzon Strait. The wind-stress-induced Ekman transport plays a secondary role in regulating the Kuroshio east of Luzon. In addition, the multidecadal fluctuation of Kuroshio east of Luzon also modulates the westward intrusion in the Luzon Strait. Instead, the Ekman transport dominated the whole upstream Kuroshio area from the eastern Luzon to the vicinity of Taiwan during the exceptional decade of 1980. Associated changes of water properties in the northern South China Sea and southern East China Sea are also ascribed to the multidecadal changes of surface Kuroshio in the upstream area.

The Agulhas Current plays a significant role in both local and global ocean circulation and climate regulation, yet the mechanisms that determine the seasonal cycle of the current remain unclear, with discrepancies between ocean models and observations. Observations from moorings across the current and a 22 year proxy of Agulhas Current volume transport reveal that the current is over 25% stronger in austral summer than in winter. We hypothesize that winds over the Southern Indian Ocean play a critical role in determining this seasonal phasing through barotropic and first baroclinic mode adjustments and communication to the western boundary via Rossby waves. Our hypothesis is explored using single layer and one‐and‐a‐half layer models. We find that the barotropic contribution to seasonal phasing is small, with the majority of the seasonal signal deflected offshore and along the Mozambique Ridge. The summertime maximum and wintertime minimum can, however, be reproduced by a one‐and‐a‐half layer reduced gravity model in which adjustment time to wind forcing via Rossby waves is in line with observations from satellite altimetry. Additionally, near‐field winds (to the west of 35° E) are shown to have a controlling influence on the seasonal phasing, as signals from farther afield dissipate through destructive interference with overlying winds before reaching the western boundary. These results suggest a critical role for a baroclinic adjustment to near‐field winds in setting the summertime maximum in Agulhas Current transport. Plain Language Summary The Agulhas Current flows along the east coast of South Africa, transporting warm Indian Ocean water southwards, acting as a vital limb of the global ocean conveyor‐belt and influencing local rainfall and climate. This study looks at the seasonal cycle of the Agulhas Current and uses idealized models to explore how winds over the Southern Indian Ocean may influence this seasonality. The current is 25% stronger in summertime, yet we do not have existing knowledge regarding the principal drivers that set this observed seasonality. In this study, we find that baroclinic processes communicating the wind stress curl variability from near‐field winds have a dominant contribution to the seasonality. Wind signals from further afield are found to die out during their journey west and so have little effect on the seasonal cycle of the Agulhas Current. Furthermore, correctly capturing the adjustment time to wind forcing is found to be very important when endeavoring to simulate the Agulhas Current seasonal phasing using a reduced gravity model. This study exposes a link between the seasonality of Agulhas Current and propagation of first baroclinic mode Rossby waves communicating the near‐field wind stress signal over across the western portion of the Southern Indian Ocean.

An update on the Eastern South Pacific Intermediate Water (ESPIW) distribution has been possible based on ARGO (Array for Real-Time Geostrophic Oceanography) data. This study comprised over 46,000 profiles obtained during 2007-2012 from an area between the western coast of South America and 150°W, and 5° and 40°S. This information was complemented with data from the World Ocean Circulation Experiment (WOCE) P06 transects conducted in 1992 and 2010 at 32°30’S in the South Pacific Ocean. Based on a comparison of this update and data from 1990-2001, it was established that the ESPIW volume has increased by approximately 53%. Moreover, the area occupied by ESPIW in the 2010 WOCE P06 transect was 48% higher than in 1992. Ekman pumping velocity, calculated from the wind for the time and region of ESPIW formation, showed a significant increase in Ekman pumping between 2000 and 2012. The increase in Ekman pumping from 2000 onward could explain the observed changes in ESPIW distribution (the water mass had extended zonally) and properties (salinity showed a decrease of 0.1). These changes could be explained as the consequence of atmospheric climate variability over the interior ocean. RESUMEN Una actualización de la distribución del Agua Intermedia del Pacífico Sur Oriental (AIPSO) fue realizada con datos ARGO (Array for Real-Time Geostrophic Oceanography). Este estudio comprendió más de 46.000 perfiles obtenidos durante2007-2012 del área entre la costa occidental de Sudamérica y 150° O, y 5°S y 40°S. Esta información se complementó con datos de las secciones P06 del World Ocean Circulation Experiment (WOCE) realizadas en 1992 y 2010 en 32° 30’S en el Océano Pacífico Sur. Comparando ésta actualización y los datos de 1990-2001 se estableció que el volumen del AIPSO ha aumentado aproximadamente un 53%. Además, el área ocupada por AIPSO en el transecto WOCE P06 de 2010 fue 48% más grande que en 1992. La velocidad de bombeo Ekman, calculada a partir del viento para el tiempo y la región de la formación del AIPSO, mostró un aumento significativo en el bombeo de Ekman entre 2000 y 2012. El aumento del bombeo de Ekman desde el 2000 podría explicar los cambios observados en la distribución del AIPSO (la masa de agua se extendió zonalmente) y las propiedades (la salinidad disminuyó 0.1). Estos cambios podrían explicarse como la consecuencia de la variabilidad climática atmosférica sobre el océano interior.

Despite attempts in the literature to link large-scale wind to long-term variations of the Kuroshio in East China Sea (ECS), the driving mechanism(s) are unknown. Here we use satellite altimetry data, wind, surface heat fluxes and sea-surface temperatures (SST) to explain the low-frequency fluctuations of Kuroshio path (KP) in ECS. The dominant fluctuations occur northeast of Taiwan. The KP correlates best with the PTO index of Chang and Oey (2012), less with the PDO index and a Kuroshio transport index, and poorly with other climate indices. The forcing are wind stress curl and surface heat flux northeast of Taiwan, which produce a thermocline tilt along the Kuroshio. Shelf's SST warms and cools in response to onshore and offshore KP, but prominent change occurs at a localized coastal zone shoreward of the above dominant KP-fluctuations. Over the past 2 decades, the KP has shifted onshore, coincident with a coastal warming trend.

Cross shelf hydrographic and hydrochemical conditions were investigated during the seasonal maximum of upwelling in the northern Benguela upwelling system. The study combines in situ observations, remotely sensed data and results of a regional 3-dimensional numerical model. In situ observations were recorded along a cross shelf transect off Namibia starting at 20°S 13°E, repeated five times during 16 August 2011 until 19 September 2011. Comparison of wind forcing and sea surface temperatures during the time of the expedition with long-term climatological data as well as the index of intensity of the Benguela upwelling indicates “normal” upwelling conditions in austral winter 2011 in the northern Benguela. Small scale temporal (days) and spatial (km) variability is high during the upwelling season, primarily caused by highly variable wind forcing and dynamics of mesoscale structures like eddies and filaments as found in remotely sensed data. This mesoscale dynamics impact the applicability of a conceptual 2-dimensional circulation model, i.e. a linear succession along the cross-shelf transect. Therefore, an age proxy for surface water was constructed based on oxygen and heat fluxes during the first aging period and on salinity and heat fluxes during the second phase. The application of an age proxy instead of distance to shore successfully validates the succession concept. Furthermore, the investigation of the upwelling strengths by analytical and circulation models verified their dependence on coastal- and curl driven upwelling processes with the onshore dominance of coastal upwelling. In the investigated time period, offshore, curl driven upwelling dominated with a maximum located on the shelf.

A high-resolution ocean general circulation model (OGCM) is used to investigate the Kuroshio path variations south of Japan. The model reproduces many important features of the Kuroshio system including its interannual bimodal variability south of Japan. A decreasing trend of the spatial averaged relative vorticity is detected when the Kuroshio takes the non-large meander (NLM) path, and during the transition period from the NLM to the large meander (LM), a sudden release of velocity shear corresponds well to the weakening of the Shikoku recirculation gyre (SRG), which plays a key role in modulating the Kuroshio path variations. Analysis of eddy energetics indicates that baroclinic instability is mainly responsible for the formation of the LM. In addition, further analysis shows that the strength of the SRG could be largely influenced by the baroclinic Rossby wave adjustment process, forced by the wind stress curl anomalies in the North Pacific basin, based on the model investigation. It is suggested that the cyclonic disturbances might account for the weakening of the SRG, and act as a remote trigger for the baroclinic instability of the Kuroshio south of Japan.

The paper demonstrates that the wind stress curl as an external vorticity source plays an important role in shaping large scale shallow lake circulations. The analysis of purpose-oriented simultaneous wind and current measurements data from the Hungarian part of Lake Neusiedl reasonably fits well the internal boundary layer development theory over the lake surface. A 2-D vorticity formulation of wind-induced flows is used to demonstrate mathematically the IBL-related large scale circulation generation mechanism well reflected in the measured data. Further validation of the findings is carried out by means of simple 2-D numerical flow modelling, in which details on the flow pattern besides the measurement sites could be also revealed. Wind-induced lake circulations linked to IBL development shows a novelty to be implemented in up-to-date numerical flow models.

The Southern Ocean plays a leading oceanic role in Earth’s changing climate, accommodating over two thirds of the excess anthropogenic heat to date. In spite of this, climate and ocean models exhibit substantial variability in Southern Ocean heat uptake and storage, reflecting the modelling challenges of representing the responsible mechanisms. Here, Southern Ocean heat uptake is investigated in a suite of global ocean–sea-ice simulations run over a range of vertical and horizontal resolutions and forced with an idealized thermal perturbation. It is found that the ocean surface speed is sensitive to the vertical resolution at the ocean surface. Subsequently, the wind stress and its curl, which are important factors for Southern Ocean heat uptake, are also sensitive to the vertical resolution at the ocean surface, tending to decrease with refined surface resolution. It follows that the Southern Ocean heat uptake itself is sensitive to the vertical resolution at the surface; indeed, the simulations here with relatively coarse surface resolution (10m) exhibit heat content changes at rates nearly double that of simulations with finer surface resolution (1m). There is relatively little sensitivity of Southern Ocean heat uptake to the horizontal resolutions examined here (1°and 0.25°), although the higher resolution simulations exhibit increased wind-driven upwelling. Importantly, all simulations demonstrate that the location of the zero wind stress curl determines the location of the maximum heat uptake. These findings offer guidance for examining Southern Ocean heat uptake and storage in future modelling studies and observations.

The purpose of this study is to identify trends in the long-term variability of the wind stress curl in the Black Sea and to analyze its response to changes in the direction of the prevailing wind flow over the sea. NCEP/NCAR reanalysis (1948–2016) and ERA Interim reanalysis (1979–2016) wind data at 10-m height are used. The temporal and spatial characteristics of the wind stress curl are compared by using these reanalyses with various spatial resolutions. Interannual changes in the wind stress curl are examined with long-term time series data of NCEP/NCAR reanalysis. The wind stress curl seasonal cycle, the dependence of its magnitude on wind direction, and the frequency of wind direction are in good agreement for the two reanalyses. There are multi-year periods of predominant positive and negative anomalies of the curl according to annual mean and winter data. The cyclonic wind stress curl increases in the late 1960s and early 1970s and weakens at the end of the 1990s. There is a weak positive trend in the curl time series in summer. The long-term variations of the wind stress curl are related to changes in the wind frequency of certain directions. High values of positive correlation coefficients have been obtained between the time series of the basin-averaged wind stress curl and the frequency of northeastern, eastern winds, and negative coefficients for the frequency of southwestern, western winds.

Kang, S.K.; Kim, K.O.; Kim, E.J.; Seung, Y.H., and So, J.K., 2019. Satellite-driven observational and numerical modeling evidence of negative surge by typhoon Talim 2017 over the shelf of East China Sea. In: Lee, J.L.; Yoon, J.-S.; Cho, W.C.; Muin, M., and Lee, J. (eds.), The 3rd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 91, pp. 171-175. Coconut Creek (Florida), ISSN 0749-0208.Typhoon Talim was generated in the Northwest Pacific (NWP), entering into the East China Sea (ECS) off the Northeastern Taiwan after propagating northwestward, and then slowly turning northeastward before finally crossing the Kyushu, Japan, from Sep. 9 to 17, 2017. Satellite altimeter data extraordinarily revealed the presence of negative surge in the ECS, while it was greatly masked by warm and cold eddies dominant in the open ocean of the NWP, south of the Ryukyu Islands. This fact was confirmed by a numerical modeling which shows that the negative surge appears in the ECS off the northeast Taiwan as the typhoon Talim approaches the shelf. It seems that the negative surge is driven by typhoon Talim associated with the Ekman divergence forced by strong positive wind stress curl formed around the typhoon center.

Local air–sea interactions over the high sea surface temperature (SST) band along the Hawaiian Lee Countercurrent (HLCC) are examined with a focus on dynamical feedback of SST-induced wind stress to the ocean using the atmosphere–ocean coupled general circulation model (CGCM). A pair of ensemble CGCM simulations are compared to extract the air–sea interactions associated with HLCC: the control simulations and other simulations, the latter purposely eliminating influences of the high SST band on the sea surface flux computations in the CGCM. The comparison reveals that oceanic response to surface wind convergence and positive wind stress curl induced by the high SST band increases (decreases) the HLCC speed in the southern (northern) flank of the HLCC. The HLCC speed changes are driven by the Ekman suction associated with positive wind stress curl over the warm HLCC via the thermal wind balance. The HLCC speed increase is more significant than its decrease. This dynamical feedback is likely to be important to sustain the extension of the HLCC far to the west. The heat budget analysis confirms that advection of warm water from the west associated with this significant current speed increase plays a role in the southward shift of the HLCC axis. The dynamical feedback with the HLCC speed increase can potentially amplify the seasonal and interannual variations of HLCC.

The seasonal and interannual variability of mesoscale circulation along the eastern coast of the Sakhalin Island in the Okhotsk Sea is investigated using the AVISO velocity field and oceanographic data for the period from 1993 to 2016. It is found that mesoscale cyclones with the horizontal dimension of about 100 km occur there predominantly during summer, whereas anticyclones occur predominantly during fall and winter. The cyclones are generated due to a coastal upwelling forced by northward winds and the positive wind stress curl along the Sakhalin coast. The anticyclones are formed due to an inflow of low-salinity Amur River waters from the Sakhalin Gulf intensified by southward winds and the negative wind stress curl in the cold season. The mesoscale cyclones support the high biological productivity at the eastern Sakhalin shelf in July– August.

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.

Outputs based on the National Centers for Environmental Prediction (NCEP) Global Ocean Data Assimilation System (GODAS) are adopted to contrast the current variations in the equatorial Pacific between two types of El Nino. The model fully resolves the equatorial currents. We found that the central Pacific El Nino (CP-El Nino) corresponds well with previous El Nino studies in that both the eastward Equatorial Undercurrent (EUC) and westward South Equatorial Current (SEC) weaken. On the other hand, the eastern Pacific El Nino (EP-El Nino) displays a distinct circulation pattern. The North Equatorial Countercurrent (NECC) strengthens in the developing phase and persists into the peak of the warm event, whereas the northern branch of the SEC (SECn) also intensifies during the mature phase and lasts for about six months. The South Equatorial Countercurrent (SECC) strengthens during the decaying phase of the EP-El Nino. The shifting of the wind stress curl associated with the thermocline variability is chiefly ...

Branches of the submerged Atlantic Water (AW) slope-current in the Nansen Basin enter the Barents Sea from the north between Svalbard and Franz Josef Land. Using hydrographic observations from annual surveys during 1970–2009, the mean state, variability and trend of the AW in the northern Barents Sea were documented, and the dominant driving forces were identified. The AW temperature has a strong positive trend over the last 40 years that accelerated in the late 1990s. The most important driving factor is the upstream temperature in the West Spitsbergen Current, which influences the entire region occupied by AW. This driving factor has pronounced multiannual variability and has a significant increasing trend, although it cannot account for the accelerated increase since the late 1990s. The secondary forcing is associated with the wind stress curl/Ekman pumping on the shelf-break towards the Arctic Ocean, causing cross-shelf exchange between the Barents Sea and the Arctic Ocean. This process increases the penetration of AW onto the shelf and is mostly confined to the northern shelf. The signal is dominated by multidecadal variability with a notable shift in the late 1990s/early 2000s, thereby amplifying the AW temperature increase compared with the upstream conditions. Additionally, coastal upwelling along northern Svalbard and the winter-mean surface air temperature were found to impact the AW temperature variability, although they were of less importance than the wind stress curl. Variability in the sea ice cover does not appear to influence the subsurface AW temperature. Variability in the AW temperature is transferred to the Arctic Water (ArW), and the vertical extent of the ArW varies considerably. Before the early 2000s, the ArW temperature was stable and low; afterwards, both the variability and the temperature increased. Our results indicate that the ArW in the northern Barents Sea is mainly heated from below.

An eddy-permitting state estimate and its adjoint are used to analyze the influence of wind stress perturbations on the transport of the Antarctic Circumpolar Current (ACC) system through Drake Passage. The transport is found to be sensitive to wind stress perturbations both along the ACC path and also in remote regions. The time scale of influence of wind stress perturbations is on the order of 100 days. Regarding spatial scales, the sensitivity of transport to wind stress is relatively smooth in regions of flat topography. In boundary regions and regions with complex topography, however, the sensitivity is enhanced and characterized by shorter length scales of order 100 km. Positive perturbations to the zonal wind stress usually increase the ACC transport, though the wind stress curl is of primary influence where the currents are steered by topography. Highlighting locations where the ACC is especially responsive to air–sea momentum fluxes reveals where an accurate determination of atmospheric winds may best enhance ocean modeling efforts.

A 1.5-layer reduced-gravity model forced by wind stress is used to study the bifurcations of the North Equatorial Current (NEC). The authors found that after removing the Ekman drift, the modelled circulations can serve well as a proxy of the SODA circulations on the σθ=25.0 kg m−3 potential density surface based on available long-term reanalysis wind stress data. The modelled results show that the location of the western boundary bifurcation of the NEC depends on both zonal averaged and local zero wind stress curl latitude. The effects of the anomalous wind stress curl added in different areas are also investigated and it is found that they can change the strength of the Mindanao Eddy (ME), and then influence the interior pathway.

Recent changes of Mixed Layer Depth (MLD) in the East/Japan Sea (EJS) is investigated by using ocean reanalysis data. There is a significant shoaling of MLD in the southern EJS for 1994-2007, showing the change rate of - 1.24 m yr−1. The present study demonstrates that the wind stress curl mostly attributes to the MLD changes. The long-term variation of wind stress curl with a positive trend in the southern EJS is generally well consistent with the MLD variation. In addition, upward displacement estimated from the wind stress curl reaches 14.6 m during 14 years, which is comparable to the shoaling of MLD (i.e., 17.4 m). Thus, the wind stress curl with positive trend would induce the increase of upward Ekman pumping which in turn lead to the shoaling of MLD. The change of temperature structure in the subsurface layer, that is shrinking and upward displacement of thermal layer from the reanalysis and observational data, also supports this conclusion.

Tropical cyclones (TCs) that affect the South China Sea (SCS) can be generated in either the SCS or the northwestern Pacific (NWP). Using satellite measurements, the Sverdrup theory and a 1.5-layer nonlinear reduced gravity model, the present paper investigates the effects of SCS and NWP TCs on the summer SCS upper layer ocean circulation. Both SCS and NWP TCs enhance the summer mean circulation pattern of the cyclonic gyre in the northern SCS and the anti-cyclonic gyre in the southern SCS. However, the effect of SCS TCs is much larger than that of NWP TCs, although the number of SCS TCs is smaller than NWP TCs. This is because the SCS TCs-induced wind stress curl pattern is favorable for enhancing the summer SCS mean circulation.

The interannual variability of upper ocean heat content (OHC) in the South China Sea (SCS) for the period of 1987-2006 and its response to ENSO events are investigated. It is found that the variability has a good correspondence with ENSO events, but with opposite phase. Negative OHC anomalies appear during ENSO warm phases, while positive OHC anomalies occur during ENSO cool phases. In addition, negative (positive) OHC anomalies propagate westward obviously during ENSO warm (cool) phases in the northern SCS. In contrast. OHC anomalies in the southern SCS do not exhibit distinct westward propagation during ENSO events. To explore why the OHC anomalies cannot propagate westward in the southern SCS, the interannual variability of oceanic and atmospheric anomaly fields including wind stress curl (WSC), horizontal wind stress, latent heat flux (LHF) and sea level pressure (SLP) is investigated. The results show that after a mature phase of ENSO warm (cool) event, negative (positive) OHC anomalies first appear in the northern SCS, which comes from the western Pacific through Luzon Strait. Then cyclonic (anticyclonic) wind stress anomalies occur in the northern SCS, which leads to positive (negative) WSC anomalies. Meanwhile, positive (negative) LHF anomalies which correspond to oceanic heat loss (gain) occur in this region. The effects of WSC and LHF, combined with the westward propagating negative (positive) OHC anomalies from the western Pacific, may contribute to rapid growth and propagation of the OHC anomalies in the northern SCS. On the contrary, the negative (positive) WSC and LHF anomalies associated with positive (negative) SLP in the southern SCS seem to be the important processes responsible for the weakening and non-propagation of the OHC anomalies in the southern SCS after a mature phase of ENSO warm (cool) event. (C) 2010 Elsevier B.V. All rights reserved.

The intra-seasonal variation of the Kuroshio southeast of Taiwan has been studied using satellite data and a numerical model. Superimposed with the main stream of the Kuroshio, two intra-seasonal signals are revealed in the study region. The fluctuation with a period of 1–6 months results from offshore eddies. The westward propagating cyclonic eddies can reduce or reverse the northward flow east of the Kuroshio between 121° and 123° E, but only slightly touch the core velocity of the Kuroshio. The fluctuation with a period of 2–4 weeks is only significant between Taiwan and the Lan-Yu Island (the low-velocity region). Different mechanisms are responsible for the fluctuation in the low-velocity region in different seasons. In winter, the change of negative wind stress curl in the northeastern South China Sea modulates the circulation southeast of Taiwan, while the typhoon-induced intense wind is responsible for the current fluctuation in summer.

The main features, interannual variability of the eddy pair (an anti-cyclonic southern eddy and a cyclonic northern eddy) off eastern Vietnam and its impact on local thermohaline structure are analyzed using altimeter data, the Simple Ocean Data Assimilation (SODA) data, and the Medium-Range Weather Forecasts (ECWMF) data. The results show that the eddy pair is a seasonal phenomenon usually found in the summer to fall. It can reach downward to about 400 m depth but is strongest at the upper 100 m depth. In addition, the eddy pair displays a remarkable interannual variability with periods of 5.6 and 3.6 years. The local wind stress curl plays an important role in the interannual variability of eddy pair. Moreover, the eddy pair has an obvious impact on the thermohaline structure of the local upper ocean. The eddy pair can lead to stronger temperature variability in the upper 380 m but stronger salinity variability only in the upper 150 m depth. When the eddy pair is strong, the anticyclonic eddy deepens the thermocline, while the cyclonic eddy shoals the thermocline. Taking the depth of 20 degrees C isotherm as the thermocline depth, we find that in September, the thermocline drops downward to 110-140 m depth in the anti-cyclonic eddy but rises up to 50-80 in depth in the cyclonic eddy. (C) 2009 Elsevier Ltd. All rights reserved.

Fifteen years of merged altimetric data were used to acquire the seasonal to interanual variations of eddy kinetic energy (EKE) in the South China Sea (SCS). The results show that climatological mean EKE in the SCS ranges from 50 cm(2)/s(2) to 1,400 cm(2)/s(2), with high values in the regions southeast of Vietnam and southwest of Taiwan Island. The amplitude of the annual harmonic of the EKE is characterized by high values to the southeast of Vietnam where the maximum exceeds 800 cm(2)/s(2). The EKE in the northern SCS reaches its maximum in August-February, while it peaks in September-December in the southern SCS. Besides the seasonal variation, the EKE also shows strong interannual variation, which has a negative (positive) anomaly in boreal winter during El Nino (La Nina) events. The interannual variation of local wind stress curl associated with El Nino-Southern Oscillation events may be the cause of the interannual variation of the EKE in the SCS.

Based on the Pathfinder sea surface temperature (PFSST), the surface axis and its pattern of the Yellow Sea Warm Current (YSWC) are discussed. A structure of double-warm-tongue is found in February and it varies in different years. Two indexes are calculated to represent the westward shift (WSI) and northward extension (NEI) of the warm water in the Yellow Sea (YS). Wavelet analysis illustrates that the WSI and NEI have prominent periods of 3–6 years and 3–4 years, respectively. The Empirical Orthogonal Function (EOF) decomposition is applied to the winter wind stress curl and the Kuroshio Current (KC) transport, which are believed to play important roles in forcing the variability of the YSWC surface axis. Statistics shows that the WSI is significantly related with the second EOF mode of the wind stress curl in February, which may force the YSWC surface axis moving westward and maintaining the double warm tongues because of its opposite curl in the YSWC domain. The first EOF mode of wind stress curl in January is propitious for inducing the warm tongue in the YS to advance more northward. Hence, the wind stress curls both in January and in February could force variations of the YSWC surface axis; however, the effect of the January wind stress curl is relatively weaker than that of the February. The relationship between the NEI and the KC transport is remarkable, and it seems that the stronger KC supplies more power to push the YSWC northward against the southward wind.

The grid altimetry data between 1993 and 2006 near the Philippines were analyzed by the method of Empirical Orthogonal Function (EOF) to study the variation of bifurcation of the North Equatorial Current at the surface of the Pacific. The relatively short-term signals with periods of about 6 months, 4 months, 3 months and 2 months are found besides seasonal and interannual variations mentioned in previous studies. Local wind stress curl plays an important role in controlling variation of bifurcation latitude except in the interannual timescale. The bifurcation latitude is about 13.3°N in annual mean state and it lies at the northernmost position (14.0°N) in January, at the southernmost position (12.5°N) in July. The amplitude of variation of bifurcation latitude in a year is 1.5°, which can mainly be explained as the contributions of the signals with periods of about 1 year (1.2°) and 0.5 year (0.3°).

Data from a subsurface mooring deployed in the western South China Sea shows clear intra-seasonal oscillations (ISO) at the period of 40~70 days. Analysis of remotelysensed sea surface height (SSH) anomalies in the same area indicates that these ISO signals propagate both eastward and westward. Time-longitude diagrams of ISO signals in SSH anomalies and wind-stress curl indicate that the eastward propagating SSH anomalies is forced by wind-stress curl. This is also confirmed by lag correlation between SSH anomalies and the wind-stress-curl index (wind stress curl averaged over 109.5ºE -115ºE and 12ºN -13.5ºN). Lag correlation of SSH anomaly suggests that the westward propagating signals are free Rossby waves.

The dynamics of the wind-driven circulations and surface transport processes in Suruga Bay have been examined by performing numerical experiments. While strong winds exist outside the bay, the winds inside the bays are greatly reduced, which generates a strong wind stress curl in winter and autumn. In particular, in winter, a strong positive curl region is located across the bay mouth, and a strong surface circulation with counterclockwise rotation is generated beneath it. The circulation is nearly geostrophic, but is not affected by the bottom topography in the deep bay. It is suggested that intense surface water exchange through the bay mouth occurs in winter, whereas it is not active in the other seasons when no significant vorticity is supplied on the bay mouth from the atmosphere. Moreover, we propose a hypothesis that the atmospheric wind stress curl will cause the frequent appearance of the counterclockwise circulation in winter in the real ocean.

A comparative box analysis based on a multi-decadal physical–biogeochemical hindcast simulation (1980–2009) was conducted to characterize the drivers of the spatial distribution of phytoplankton biomass and production in the north-west (NW) African upwelling system. Alongshore geostrophic flow related to large-scale circulation patterns associated with the influence of coastal topography is suggested to modulate the coastal divergence, and then the response of nutrient upwelling to wind forcing. In our simulation, this translates into a coastal upwelling of nitrate being significant in all regions but the Cape Blanc (CB) area. However, upwelling is found to be the dominant supplier of nitrate only in the northern Saharan Bank (NSB) and the Senegalo-Mauritanian (SM) regions. Elsewhere, nitrate supply is dominated by meridional advection, especially off Cape Blanc. Phytoplankton displays a similar behaviour with a supply by lateral advection which equals the net coastal phytoplankton growth in all coastal regions except the Senegalo-Mauritanian area. Noticeably, in the Cape Blanc area, the net coastal phytoplankton growth is mostly sustained by high levels of regenerated production exceeding new production by more than twofold, which is in agreement with the locally weak input of nitrate by coastal upwelling. Further offshore, the distribution of nutrients and phytoplankton is explained by the coastal circulation. Indeed, in the northern part of our domain (i.e. Saharan Bank), the coastal circulation is mainly alongshore, resulting in low offshore lateral advection of nutrients and phytoplankton. Conversely, lateral advection transports coastal nutrients and phytoplankton towards offshore areas in the latitudinal band off the Senegalo-Mauritanian region. Moreover, this latter offshore region benefits from transient southern intrusions of nutrient-rich waters from the Guinean upwelling.

We have investigated interannual-scale variations of oceanic and atmospheric anomaly fields, such as upper ocean heat content (OHC), sea surface temperature (SST), latent heat flux (LHF) through the sea surface, sea level pressure (SLP) and wind stress curl (WSC) in the tropical Pacific and their relationships to El Nino/Southern Oscillation (ENSO) events. The results reported here show that the OHC and SST anomalies are almost in phase and lead LHF anomalies in the western tropical Pacific (WTP) region, which are preferable to the generation of subsequent atmospheric anomalies in the WTP. We also describe linear relationships between the amplitudes of these variables in the WTP. In addition, the results show that the both WSC and LHF anomalies are in phase with the temporal trend of OHC anomalies in the WTP, and suggest a combined effect of the local WSC and LHF anomaly in the WTP and ENSO-related, off-equatorial, westward propagating OHC anomaly to generate a large OHC anomaly in the WTP. In contrast to the WTP, OHC and SST anomalies are not in phase to the east of the WTP. The results also indicate that OHC anomalies in the WTP have a potential effect on the generation of an equatorial OHC anomaly via both a reflection of waves at the western boundary and atmospheric variations, which force the enhancement of western equatorial OHC anomaly. Therefore, the WTP is a key region where ENSO events are significantly modulated, and OHC anomalies in the WTP play an important role in the subsequent ENSO event.

The warming Nordic seas potentially tend to decrease the overflow across the Greenland–Iceland–Scotland Ridge (GISR) system. Recent observations by Macrander et al. document a significant drop in the intensity of outflowing Denmark Strait Overflow Water of more than 20% over 3 yr and a simultaneous increase in the temperature of the bottom layers of more than 0.4°C. A simulation of the exchange across the GISR with a regional ocean circulation model is used here to identify possible mechanisms that control changes in the Denmark Strait overflow and its relations to changed forcing condition. On seasonal and longer time scales, the authors establish links of the overflow anomalies to a decreasing capacity of the dense water reservoir caused by a change of circulation pattern north of the sill. On annual and shorter time scales, the wind stress curl around Iceland determines the barotropic circulation around the island and thus the barotropic flow through Denmark Strait. For the overlapping time scales, the barotropic and overflow component interactively determine transport variations. Last, a relation between sea surface height and reservoir height changes upstream of the sill is used to predict the overflow variability from altimeter data. Estimated changes are in agreement with other recent transport estimates based on current-meter arrays.

Discharge and recharge of the warm water volume (WWV) above the 20°C isotherm in an equatorial Pacific Ocean box extending across the Pacific from 156°E to the eastern ocean boundary between latitudes 5°S and 5°N are key variables in ENSO dynamics. A formula linking WWV anomalies, zonally integrated wind stress curl anomalies along the northern and southern edges of the box, and flow into the western end of the box is derived and tested using monthly data since 1993. Consistent with previous work, a WWV balance can only be achieved if the 20°C isotherm surface is not a material surface; that is, warm water can pass through it. For example, during El Niño, part of the WWV anomaly entering the box is cooled so that it is less than 20°C and therefore passes out of the bottom of the box, the 20°C isotherm surface. The observations suggest that the anomalous volume passing through the 20°C isotherm is approximately the same as T ′W, the anomalous WWV entering the western end of the box. Therefore the observed WWV anomaly can be regarded as being driven by the anomalous wind stress curl along the northern and southern edges of the box. The curl anomaly changes the WWV both by divergent meridional flow at the edges of the box and vortex stretching; that is, the Sverdrup balance does not hold in the upper ocean. A typical amplitude for the rate of change of WWV for the 5°S–5°N box is 9.6 Sv (Sv ≡ 106 m3 s−1). The wind stress curl anomaly and the transport anomaly into the western end of the box are highly correlated with the El Niño index Niño-3.4 [the average sea surface temperature anomaly (SSTA) over the region 5°S–5°N, 170°–120°W] and Niño-3.4 leads minus the WWV anomaly by one-quarter of a cycle. Based on the preceding results, a simple discharge/recharge coupled ENSO model is derived. Only water warmer than about 27.5°–28°C can give rise to deep atmospheric convection, so, unlike past discharge/recharge oscillator models, the west-central rather than eastern equatorial SSTAs are emphasized. The model consists of two variables: T ′, the SSTA averaged over the region of strong ENSO air–sea interaction in the west-central Pacific equatorial strip 5°S–5°N, 156°E–140°W and D′, the 20°C isotherm depth anomaly averaged over the same region. As in the observations, T ′ lags D′ by one-quarter of a cycle; that is, ∂T ′/∂t = νD′ for some positive constant ν. Physically, when D′ > 0, the thermocline is deeper and warmer water is entrained through the base of the mixed layer, the anomalous heat flux causing ∂T ′/∂t > 0. Also, when D′ > 0, the eastward current anomaly is greater than zero and warm water is advected into the region, again causing ∂T ′/∂t > 0. Opposite effects occur for D′ < 0. A second relationship between T ′ and D′ results because the water is warm enough that T ′ causes deep atmospheric convection anomalies that drive the wind stress curl anomalies that change the heat storage ∂D′/∂t. The atmosphere responds essentially instantly to the T ′ forcing and the curl causes a discharge of WWV during El Niño (T ′ > 0) and recharge during La Niña (T ′ < 0), so ∂D′/∂t = −μT ′ for some positive constant μ. The two relationships between T ′ and D′ result in a harmonic oscillator with period 2π/νμ ≈ 51 months.