Recirculation Gyre Research Articles

Major Driver Leading to Winter SST Variability in the Kuroshio Recirculation Gyre Region and Its Decadal Changes: Refreshening Versus Spring‐Initiated Reemergence Processes

AbstractThe analyses using reanalysis data and one‐dimensional ocean model during the past decades (1963–2012) revealed that the epoch‐dependent refreshening and reemergence mechanisms alternatively influenced the winter sea surface temperature (SST) in the Kuroshio recirculation gyre region. During 1971–1985, the refreshening mechanism due to the strong East Asian winter monsoon significantly contributed to the interannual winter SST variability, consistent with previous works. In contrast, the spring‐initiated reemergence process better explains the winter SST variability for 1991–2005, a period corresponding to a declined winter monsoon. The classical winter‐to‐winter reemergence did not occur during either studied period; thus, the observed spring‐to‐winter reemergence is an original feature in the Kuroshio recirculation gyre region. The winter monsoon‐related strong atmospheric forcing and prominent year‐to‐year variability of spring SST are important prerequisites for the regional refreshening and reemergence of the winter SST anomaly, respectively.

Upgradient and Downgradient Potential Vorticity Fluxes Produced by Forced Rossby Waves. Part I: Basic Experiments

AbstractThe mean flow and potential vorticity (PV) flux produced by Rossby waves are examined, particularly by focusing on the effects of stratification and nonlinearity on upgradient and downgradient PV fluxes. Rossby waves are excited by an external forcing confined near the surface and produce a northward (upgradient) PV flux in the surface layer. While the meridional PV flux is considerably weak in the deep layer in the weakly nonlinear case, the southward (downgradient) PV flux is produced as nonlinearity increases. In both the surface and deep layers, the distribution of the PV flux and mean flow is qualitatively similar to that in recirculation gyres obtained in an eddy-resolving model of the wind-driven circulation. A perturbation analysis shows that the primary and harmonic waves are excited by external forcing and wave–wave interaction between the primary waves, respectively. The meridional PV fluxes in the surface and deep layers are mostly produced by the primary and harmonic waves, respectively. The southward PV flux in the deep layer is produced by the interaction between the barotropic harmonic wave and the first baroclinic component of nonlinear forcing by the primary waves. The irreversible processes that are implicitly assumed in the PV homogenization theory (Rhines and Young) do not substantially affect the southward PV flux. The qualitative features of the PV flux remain unchanged even when nonlinearity is increased beyond the range in which the perturbation theory is exactly applicable.

On the influence of Subtropical Mode Water on the South Atlantic Ocean

Abstract We investigate the spatiotemporal variability of the source water masses (i.e., varieties of Subtropical Mode Water – STMW) that contribute to the South Atlantic Central Water (SACW) in the South Atlantic Ocean. Thus, the composition of the SACW layer is updated. For this investigation, we applied an optimum multiparameter (OMP) analysis and used the conservative and semi-conservative parameters available from the World Ocean Database and Argo floats for the South Atlantic Ocean. The STMW18 (at upper levels) sourced in the central and eastern regions of the South Atlantic and the STMW12 (at lower levels) sourced at the boundaries of the South Atlantic Subtropical Front are the main contributors to the SACW. Although also important, the contribution of STMW14 (sourced in the Brazil-Malvinas Confluence zone) is regionally confined by the Brazil Current recirculation gyre. The contributions from Subtropical Indian Mode Water (SIMW) increased westward along the Agulhas Corridor, while the contribution from STMW12 decreased. The relatively low contribution from SIMW matches the results of previous studies regarding the influence of these waters in the climatology of the South Atlantic Ocean. However, it cannot be ignored, since the results bring new light to further investigations of the mixing processes in the ocean interior of the South Atlantic Ocean.

The Interaction of Recirculation Gyres and a Deep Boundary Current

AbstractMotivated by the proximity of the Northern Recirculation Gyre and the deep western boundary current in the North Atlantic, an idealized model is used to investigate how recirculation gyres and a deep flow along a topographic slope interact. In this two-layer quasigeostrophic model, an unstable jet imposed in the upper layer generates barotropic recirculation gyres. These are maintained by an eddy-mean balance of potential vorticity (PV) in steady state. The authors show that the topographic slope can constrain the northern recirculation gyre meridionally and that the gyre’s adjustment to the slope leads to increased eddy PV fluxes at the base of the slope. When a deep current is present along the topographic slope in the lower layer, these eddy PV fluxes stir the deep current and recirculation gyre waters. Increased proximity to the slope dampens the eddy growth rate within the unstable jet, altering the geometry of recirculation gyre forcing and leading to a decrease in overall eddy PV fluxes. These mechanisms may shape the circulation in the western North Atlantic, with potential feedbacks on the climate system.

A Reconstruction of Subtropical Western North Pacific SST Variability Back to 1578, Based on a Porites Coral Sr/Ca Record from the Northern Ryukyus, Japan

AbstractWe present a seasonal reconstruction of sea surface temperature (SST) from 1578 to 2008, based on a Porites coral Sr/Ca record from the northern Ryukyus, within the Kuroshio southern recirculation gyre. Interannual SST anomalies are generally ~0.5°C, making Sr/Ca‐derived SST reconstructions a challenging task. Replicate measurements along adjacent coral growth axes, enabled by the laser ablation inductively coupled plasma mass spectrometry technique used here, give evidence of rather large uncertainties. Nonetheless, derived winter SST anomalies are significantly correlated with the Western Pacific atmospheric pattern which has a dominant influence on winter temperature in East Asia. Annual mean SSTs show interdecadal variations, notably cold intervals between 1670 and 1700 during the Maunder Minimum (MM) and between 1766 and 1788 characterized by a negative phase of the North Atlantic Oscillation. Cold summers in 1783 and 1784 coincide with the long‐lasting Laki eruption that had a profound impact on the Northern Hemisphere climate, including the severe “Tenmei” famine in Japan. The decades between 1855 and 1900 are significantly cooler than the first half of the twentieth century, while those between 1700 and 1765, following the MM, are warmer than average. SST variability in the Ryukyus is only marginally influenced by the Pacific Decadal Oscillation, so that external forcing remains the main driver of low‐frequency temperature changes. However, the close connection between the Kuroshio extension (KE) and its recirculation gyre suggests that decadal SST anomalies associated with the KE front also impact the Ryukyus, and there is a possible additional role for feedback of the Kuroshio‐Oyashio variability to the large‐scale atmosphere at decadal timescale.

Climatology and decadal variations in multicore structure of the North Pacific subtropical mode water

AbstractThe pycnostad of the North Pacific subtropical mode water (STMW) often displays multiple vertical minima in the potential vorticity profile. Argo profile data from 2004 to 2015 are used to investigate interannual to decadal variations of the multicore structure. Climatologically, about 24% pycostads of STMW have multicore structure, and most of them distribute in the region west of 150°E. STMW cores are classified into three submodes—the cold, middle, and warm ones with potential temperatures of 16.0–17°C, 17–18°C, and 18–19.5°C, respectively. The Kuroshio Extension (KE) varies between stable and unstable states. The unstable KE with large meanders increases the subsurface stratification and shoals the winter mixed layer east of 150°E with warmer temperatures. There, the dominant STMW type varies from the cold single type in stable KE years (making up 72% of the profiles with STMW) to the middle single one (53%) in unstable years. The variation of the dominant STMW type in the region east of 150°E subsequently affects the multicore structure of STMW west of 150°E. In a broad region between 130°E and 180°E, profiles with STMW are fewer in unstable years but the proportion of multicore profiles increases among STMW profiles. This might be due to the split recirculation gyre with a chaotic KE.

The annual cycle of surface eddy kinetic energy and its influence on eddy momentum fluxes as inferred from altimeter data

The annual cycle of surface eddy kinetic energy (EKE) and its influence on eddy momentum fluxes are investigated using an updated record of satellite altimeter data. It is found that there is a phase difference between the annual cycles of EKE in the western boundary current regions and EKE in the interior of the subtropical gyres, suggesting that different mechanisms may be at work in different parts of the subtropical gyres. The annual cycles of EKE averaged in the two hemispheres are found to be of similar magnitude but in opposite phase. As a result, the globally-averaged EKE shows little seasonal variability. The longer record of altimeter data used in this study has brought out a clearer and simpler picture of eddy momentum fluxes in the Gulf Stream and Kuroshio Extensions: eddies in both regions systematically flux westerly momentum into the western boundary current jets. Considerable seasonal variations in eddy momentum fluxes are found in the western boundary current regions, which potentially play an important role in modulating the strength of the western boundary currents and their associated recirculation gyres on the seasonal time scale.

Response of theKuroshioExtension path state to near‐term global warming in CMIP5 experiments with MIROC4h

AbstractIn this study, responses of the Kuroshio Extension (KE) path state to near‐term (2006–2035) global warming are investigated using a Kuroshio‐resolving atmosphere‐ocean coupled model. Under the representative concentration pathway 4.5 (RCP4.5) forcing, the KE system is intensified and its path state tends to move northward and becomes more stable. It is suggested that the local anticyclonic wind stress anomalies in the KE region favor the spin‐up of the southern recirculation gyre, and the remote effect induced by the anticyclonic wind stress anomalies over the central and eastern midlatitude North Pacific also contributes to the stabilization of the KE system substantially. The dominant role of wind stress forcing on KE variability under near‐term global warming is further confirmed by adopting a linear 1.5 layer reduced‐gravity model forced by wind stress curl field from the present climate model. It is also found that the main contributing longitudinal band for KE index (KEI) moves westward in response to the warmed climate. This results from the northwestward expansion of the large‐scale sea level pressure (SLP) field.

Comparison Study of Subtropical Mode Waters in the World Ocean

Subtropical Mode Water (STMW) is a distinctive upper ocean structure in the western part of subtropical gyres in the world ocean. This paper proposes a common criterion of STMW to quantify and compare spatial structures of STMWs in different basins. STMW can be defined as a thermostad (a layer weakly stratified in temperature) by applying a criterion of averaged core layer temperature (CLT)±1oC with its layer thickness greater than 100 m. Two features are highlighted when comparing the STMWs in different basins. Firstly, the North Atlantic hosts the thickest STMW in the world ocean. Secondly, the South Atlantic STMW has an unique vertical structure of density compensating temperature and salinity stratification. By comparing the thickness of STMW against the strength of winter cooling and the volume transport of associated western boundary current (WBC) in different basins, it is shown that thicker STMW tends to be accompanied with stronger WBC. From a view point of vorticity dynamics, we suggest that the North Atlantic may have the most efficient condition to host the thickest STMW and the strongest recirculation gyre in the world ocean.

Warming of the Global Ocean: Spatial Structure and Water-Mass Trends

Abstract This study investigates the multidecadal warming and interannual-to-decadal heat content changes in the upper ocean (0–700 m), focusing on vertical and horizontal patterns of variability. These results support a nearly monotonic warming over much of the World Ocean, with a shift toward Southern Hemisphere warming during the well-observed past decade. This is based on objectively analyzed gridded observational datasets and on a modeled state estimate. Besides the surface warming, a warming climate also has a subsurface effect manifesting as a strong deepening of the midthermocline isopycnals, which can be diagnosed directly from hydrographic data. This deepening appears to be a result of heat entering via subduction and spreading laterally from the high-latitude ventilation regions of subtropical mode waters. The basin-average multidecadal warming mainly expands the subtropical mode water volume, with weak changes in the temperature–salinity (θ–S) relationship (known as “spice” variability). However, the spice contribution to the heat content can be locally large, for example in Southern Hemisphere. Multidecadal isopycnal sinking has been strongest over the southern basins and weaker elsewhere with the exception of the Gulf Stream/North Atlantic Current/subtropical recirculation gyre. At interannual to decadal time scales, wind-driven sinking and shoaling of density surfaces still dominate ocean heat content changes, while the contribution from temperature changes along density surfaces tends to decrease as time scales shorten.

Mean Structure of the North Atlantic Subtropical Permanent Pycnocline from In Situ Observations

AbstractA new objective algorithm for the characterization of the permanent pycnocline (OAC-P) in subtropical gyres is proposed. OAC-P is based on a pragmatic analysis of vertical density gradient features to identify the permanent pycnocline: OAC-P identifies the permanent pycnocline as the stratified layer found below surface mode waters. OAC-P provides the permanent pycnocline depth, unequivocally associated with a local maximum in the stratification, and top and bottom thicknesses, associated with upward and downward decreases in stratification, respectively. OAC-P uses half Gaussian curves as asymmetric nonlinear analytical models of the stratification peak. It is the first time that an algorithm is proposed to characterize objectively the permanent pycnocline for a region where handling the stronger stratification peak of the seasonal pycnocline is complex. A guideline for how to implement the OAC-P is given, with application to the North Atlantic Ocean Argo data as an example. OAC-P provides a detailed description of the mean structure of the North Atlantic subtropical permanent pycnocline. OAC-P detects a permanent pycnocline throughout the subtropical gyre north of the North Equatorial Current. The large-scale description of the permanent pycnocline depth structure as a classic bowl shape is captured however with much more detail. New regional information is provided. In particular, (i) there is only one region—the southern recirculation gyre of the Gulf Stream extension—where the permanent pycnocline is along an isopycnal surface and (ii) vertical asymmetries clearly discriminate one region from another.

The Instabilities and Multiscale Energetics Underlying the Mean–Interannual–Eddy Interactions in the Kuroshio Extension Region

AbstractUsing a recently developed energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), this study investigates the intricate nonlinear mutual interactions among the decadally modulating mean flow, the interannual fluctuations, and the transient eddies in the Kuroshio Extension region. It is found that the mean kinetic energy maximizes immediately east of the Izu–Ogasawara Ridge, while the transient eddy kinetic energy does not peak until 400 km away downstream. The interannual variabilities, which are dominated by a jet-trapped Rossby wave mode, provide an energy reservoir comparable to the other counterparts. In the upstream, strong localized barotropic and baroclinic transfers from the mean flow to the eddies are observed, whereas those from the interannual variabilities are not significant. Besides fueling the eddies, the unstable mean jet also releases energy to the interannual-scale processes. Between 144° and 154°E, both transfers from the mean flow and the interannual variabilities are important for the eddy development. Farther downstream, eddies are found to drive the mean flow on both the kinetic energy (KE) and available potential energy (APE) maps. They also provide KE to the interannual variabilities but obtain APE from the latter. The gained eddy APE is then converted to eddy KE through buoyancy conversion. Upscale energy transfers are observed in the northern and southern recirculation gyre (RG) regions. In these regions, the interannual–eddy interaction exhibits different scenarios: the eddies lose KE to the interannual processes in the northern RG region, while gaining KE in the southern RG region.

The Icelandic Low as a Predictor of the Gulf Stream North Wall Position

AbstractThe Gulf Stream’s north wall east of Cape Hatteras marks the abrupt change in velocity and water properties between the slope sea to the north and the Gulf Stream itself. An index of the north wall position constructed by Taylor and Stephens, called Gulf Stream north wall (GSNW), is analyzed in terms of interannual changes in the Icelandic low (IL) pressure anomaly and longitudinal displacement. Sea surface temperature (SST) composites suggest that when IL pressure is anomalously low, there are lower temperatures in the Labrador Sea and south of the Grand Banks. Two years later, warm SST anomalies are seen over the Northern Recirculation Gyre and a northward shift in the GSNW occurs. Similar changes in SSTs occur during winters in which the IL is anomalously west, resulting in a northward displacement of the GSNW 3 years later. Although time lags of 2 and 3 years between the IL and the GSNW are used in the calculations, it is shown that lags with respect to each atmospheric variable are statistically significant at the 5% level over a range of years. Utilizing the appropriate time lags between the GSNW index and the IL pressure and longitude, as well as the Southern Oscillation index, a regression prediction scheme is developed for forecasting the GSNW with a lead time of 1 year. This scheme, which uses only prior information, was used to forecast the GSNW from 1994 to 2015. The correlation between the observed and forecasted values for 1994–2014 was 0.60, significant at the 1% level. The predicted value for 2015 indicates a small northward shift of the GSNW from its 2014 position.

Multicore structure of the North Pacific subtropical mode water from enhanced Argo observations

AbstractSeventeen Argo profiling floats with enhanced vertical and temporal sampling were deployed in the Kuroshio recirculation gyre in the western North Pacific in late March 2014. The Subtropical Mode Water (STMW) observed in many profiles displays a “multicore structure” with more than one minima in potential vorticity (PV), corroborated by vertical covariations in apparent oxygen utilization (AOU). These cores are classified into four submodes according to density and AOU. The submode waters are typically 100 m thick, in which PV varies by 1 × 10−10 m−1 s−1 and AOU by 10 µmole/kg. The STMW multicore structure is most frequently observed in spring, gradually taken over by single‐core profiles into summer. The seasonal evolution is suggestive of vertical mixing, especially in STMW of lower density.

The current system east of the Ryukyu Islands as revealed by a global ocean reanalysis

The structure and variability of the Ryukyu Current System (RCS), which forms the western boundary current along the eastern slope of the Ryukyu Islands, are studied using results from a 32-layer, 1/12.5° global HYbrid Coordinate Ocean Model (HYCOM) and Navy Coupled Ocean Data Assimilation (NCODA) reanalysis for the period 1993–2012. It is confirmed that the reanalysis realistically reproduces salient features of the observed currents at three sections southeast of Miyakojima, Okinawa and Amami-Ohshima. The mean velocity sections show well-developed subsurface velocity maxima between 700 and 900m. The current core southeast of Amami-Ohshima shows year-to-year variations with cyclonic (anticyclonic) circulation east of Amami-Ohshima generating weak (strong) velocity cores. Interaction of the RCS with an anticyclonic eddy often produces a two-core velocity structure, with a surface core in the upper 300m and a deeper core near 700–900m. The horizontal structure of the RCS at 15m depth shows a well-developed northeastward current northeast of Okinawa, which is partly fed by the southwestward extension of the anticyclonic recirculation gyre. The RCS forms a continuous northeastward current from Miyakojima to Amami-Ohshima below 500m with shoreward intensification. The circulation at 2000m shows a seasonal flow reversal, which is northeastward from December to June and southwestward from August to October with July and November being the transition months. The volume transports across these three sections have respective mean values of 0.6, 6.2 and 12.4Sv (1Sv≡106m3s−1) and standard deviations of 10.2, 7.1 and 11.3Sv. They have dominant seasonal variations with the maximum in winter and spring and the minimum in summer. The interannual variation of the transport anomaly, which co-varies with the RCS core, results from westward propagating mesoscale eddies arriving from the Pacific interior.

The impact of global warming on Kuroshio Extension and its southern recirculation using CMIP5 experiments with a high-resolution climate model MIROC4h

Responses of the Kuroshio Extension (KE) and its southern recirculation gyre (SRG) to global warming are investigated using CMIP5 experiments with a high-resolution climate model MIROC4h. The results show that MIROC4h well reproduces the essential features of the KE system and its low-frequency variations. In three-member-ensemble future climate experiments (with a medium mitigation emissions scenario RCP4.5), the strengths of the KE and its SRG increase, relative to the prescribed historical run with natural and anthropogenic forcing. By investigating the mechanism resulting in these variations of the KE and its SRG, it turns out that wind stress changes and ocean stratification changes both contribute to the enhancement of the KE and its SRG. Specifically, the wind stress changes increase upper ocean momentum in the SRG region. Meanwhile, the increased stratifications hinder the transfer of momentum from the upper ocean to the deeper ocean. Besides, the strengthened ocean stratification could enhance the eddy kinetic energy (EKE) in the downstream KE region, which can feedback to intensify the SRG. As a result, the strength of the SRG increases under global warming condition. Then the intensification of the SRG leads to large acceleration of the KE. Eventually, both the KE and its SRG intensify.

Estimating diffusivity from the mixed layer heat and salt balances in the North Pacific

AbstractData from two National Oceanographic and Atmospheric Administration (NOAA) surface moorings in the North Pacific, in combination with data from satellite, Argo floats and glider (when available), are used to evaluate the residual diffusive flux of heat across the base of the mixed layer from the surface mixed layer heat budget. The diffusion coefficient (i.e., diffusivity) is then computed by dividing the diffusive flux by the temperature gradient in the 20 m transition layer just below the base of the mixed layer. At Station Papa in the NE Pacific subpolar gyre, this diffusivity is 1 × 10−4 m2/s during summer, increasing to ∼3 × 10−4 m2/s during fall. During late winter and early spring, diffusivity has large errors. At other times, diffusivity computed from the mixed layer salt budget at Papa correlate with those from the heat budget, giving confidence that the results are robust for all seasons except late winter‐early spring and can be used for other tracers. In comparison, at the Kuroshio Extension Observatory (KEO) in the NW Pacific subtropical recirculation gyre, somewhat larger diffusivities are found based upon the mixed layer heat budget: ∼ 3 × 10−4 m2/s during the warm season and more than an order of magnitude larger during the winter, although again, wintertime errors are large. These larger values at KEO appear to be due to the increased turbulence associated with the summertime typhoons, and weaker wintertime stratification.

Decadal variations in mixed layer salinity in the Kuroshio Extension recirculation gyre region: influence of precipitation during the warm season

The salinity of the mixed layer in the Kuroshio Extension recirculation gyre (KERG) region (28°–35°N, 141°–160°E) was examined over a period of 20 years (1993–2012), and was found to show low-frequency variations on a decadal (~10 years) timescale: salinity decreased during the periods 1995–2001 and 2006–2009, but increased over the periods 2002–2005 and 2010–2012. Salinity anomalies at the sea surface during the warm season (June–October) influenced salinity in the following winter. These warm-season anomalies were caused by precipitation changes related to the number of low-pressure systems passing over the KERG associated with a westward elongation/eastward shrinkage of the summertime North Pacific subtropical high, which is driven by the Pacific–Japan (PJ) teleconnection pattern.

Decadal variability of Subtropical Mode Water subduction and its impact on biogeochemistry

Temperature and salinity data from Argo profiling floats during 2005–2014 were analyzed to examine the decadal variability of the North Pacific Subtropical Mode Water (STMW) in relation to that of the Kuroshio Extension (KE) system. The formation volume of STMW in the southern recirculation gyre of KE in the cooling season was larger during the stable KE period after 2010 than the unstable KE period of 2006–2009 by 50 %. As a result, the volume and spatial extent of STMW increased (decreased) in the formation region during the stable (unstable) KE period, as well as in the southern, downstream region with a time lag of 1–2 years. The decadal expansion and contraction of STMW were also detected by shipboard observations conducted routinely in the most downstream region near the western boundary, in terms of not only physical, but also biogeochemical parameters. After 2010, enhanced subduction of STMW consistently increased dissolved oxygen, pH, and aragonite saturation state and decreased potential vorticity, apparent oxygen utilization, nitrate, and dissolved inorganic carbon, among which changes of dissolved inorganic carbon, pH, and aragonite saturation state were against their long-term trends. These results indicate a new mechanism consisting of westward sea surface height anomaly propagation, the KE state transition, and the STMW formation and subduction, by which the climate variability affects physical and biogeochemical structures in the ocean’s interior and potentially impacts the surface ocean acidification trend and biological production.

Effects of Eddy Vorticity Forcing on the Mean State of the Kuroshio Extension

AbstractEddy–mean flow interactions along the Kuroshio Extension (KE) jet are investigated using a vorticity budget of a high-resolution ocean model simulation, averaged over a 13-yr period. The simulation explicitly resolves mesoscale eddies in the KE and is forced with air–sea fluxes representing the years 1995–2007. A mean-eddy decomposition in a jet-following coordinate system removes the variability of the jet path from the eddy components of velocity; thus, eddy kinetic energy in the jet reference frame is substantially lower than in geographic coordinates and exhibits a cross-jet asymmetry that is consistent with the baroclinic instability criterion of the long-term mean field. The vorticity budget is computed in both geographic (i.e., Eulerian) and jet reference frames; the jet frame budget reveals several patterns of eddy forcing that are largely attributed to varicose modes of variability. Eddies tend to diffuse the relative vorticity minima/maxima that flank the jet, removing momentum from the fast-moving jet core and reinforcing the quasi-permanent meridional meanders in the mean jet. A pattern associated with the vertical stretching of relative vorticity in eddies indicates a deceleration (acceleration) of the jet coincident with northward (southward) quasi-permanent meanders. Eddy relative vorticity advection outside of the eastward jet core is balanced mostly by vertical stretching of the mean flow, which through baroclinic adjustment helps to drive the flanking recirculation gyres. The jet frame vorticity budget presents a well-defined picture of eddy activity, illustrating along-jet variations in eddy–mean flow interaction that may have implications for the jet’s dynamics and cross-frontal tracer fluxes.