Subtropical Countercurrent Research Articles

Two Types of Intraseasonal Variability With a Vertical Difference in the Currents East of Luzon Island and Their Sources

AbstractIntraseasonal variabilities (ISVs) of the western boundary currents (WBCs) east of Luzon Island were explored using acoustic Doppler current profiler (ADCP) measurements from three moorings at 18°N during 2018–2020. Besides the traditionally known surface‐intensified ISV, subsurface‐intensified ISV with a typical period of approximately 60 days was also detected in the currents, and the strongest signal appeared between 400 and 800 m. Further analysis indicates that they are highly associated with subsurface eddies. Based on their lifespan, subsurface eddies are classified into two categories: short‐lived and medium‐to long‐lived eddies. The short‐lived eddies are primarily generated locally near the eastern coast of Luzon Island, whereas the medium‐to long‐lived eddies are mainly generated away from the western boundary, in the region west of 135°E. Additional energy diagnosis suggests that baroclinic instability induced by the velocity shear of the North Equatorial Current (NEC)/subtropical countercurrent (STCC) system dominates the generation of medium‐to long‐lived subsurface eddies in the interior ocean, while barotropic instability and baroclinic instability play a comparable role in the generation of short‐lived eddies near the eastern coast of Luzon Island.

Observation of Statistical Characteristics and Vertical Structures of Surface Warm Cyclonic Eddies and Cold Anticyclonic Eddies in the North Pacific Subtropical Countercurrent Region

Conventional wisdom about mesoscale eddies is that cyclonic (anticyclonic) eddies are commonly associated with cold(warm) surface cores. Nevertheless, plenties of surface warm cyclonic eddies (WCEs) and cold anticyclonic eddies (CAEs) in the North Pacific Subtropical Countercurrent (STCC) region are observed by a synergistic investigation based on data from satellite altimetry, microwave radiometer, and Argo float profiles in this study. The results indicate that these two types of abnormal eddies (WCEs and CAEs) are prevalent in the STCC region, comprising approximately 30% of all eddies detected via satellite observations. We then analyze their spatial-temporal distribution characteristics and composite vertical structures. A statistical comparison with surface cold cyclonic eddies (CCEs) and warm anticyclonic eddies (WAEs) reveals notable differences between the anomalous and typical eddies. Additionally, we present the composite vertical structures of temperature and salinity anomalies for the anomalous eddies across five delineated subregions within an eddy-coordinate system. Furthermore, the close relationship between these abnormal eddies and subsurface-intensified mesoscale eddies are discussed.

Distribution and stable isotope ratio characteristics of Japanese eel leptocephali in relation to hydrographic structure of their Pacific Ocean spawning area

AbstractTo understand the larval distribution, size variation, and stable isotope ratios of Japanese eel leptocephali in relation to the salinity front and their feeding ecology, larvae from 7 research cruises (2002–2013) in the North Equatorial Current (NEC) spawning area were examined. The smallest early‐stage larvae were distributed south of or near the salinity front, confirming that the salinity front is an important oceanic feature to understand spawning locations of the Japanese eel. Larger size larvae tended to distribute into higher latitudes. Transport to northern latitudes with their growth would facilitate transport into the Kuroshio region, but retention in the Subtropical Countercurrent (STCC) might be detrimental. Preleptocephalus isotope ratios reflected maternal ratios, but feeding‐stage leptocephali (8–56 mm) tended to have higher ∂15N values at lower latitudes typically in areas south of a salinity front. Feeding larvae quickly assimilate isotope ratios from the NEC after spawning and early growth. Large differences of ∂13C values of larvae between the NEC and STCC might vary with spatial baselines in the two currents. However, diel vertical migrations should be considered, because the isotope ratios in particulate organic matter distinctly depend on the depth. Comparisons among Japanese eel larvae and other taxa of leptocephali in the NEC illustrate the need for further studies on the trophic ecology of leptocephali.

ENSO‐Related Interannual Variability of the Kuroshio in the East China Sea Since the Mid‐2000s

AbstractUsing observational and reanalysis data sets, we investigated interannual variability in the Kuroshio in the East China Sea (ECS‐Kuroshio). We exhibited that the surface velocity and position of the ECS‐Kuroshio were synchronized on a quasi‐3‐year interannual timescale during 2005–2016. We further demonstrated that: (a) during 2005–2016, wind stress curl variability related to the El Niño–Southern Oscillation (ENSO) played a leading role in the interannual ECS‐Kuroshio variability by exciting baroclinic Rossby waves along the subtropical countercurrent (STCC) zone east of Taiwan; (b) mesoscale eddy activities in the STCC zone, especially long‐lived (≥150‐day lifetime) cyclonic eddies, probably played a secondary role in the interannual ECS‐Kuroshio variability. In addition, we showed that the occurrence of the quasi‐3‐year interannual variability of the ECS‐Kuroshio since 2005 was likely linked to the following changes in the ENSO‐related atmospheric circulation: (a) The primary ENSO timescale changed from a quasi‐5‐year period in 1993–2004 to a quasi‐3‐year one in 2005–2016; (b) Over the central equatorial Pacific along with the eastern tropical Indian Ocean, the sea surface temperature in 2005–2016 varied with a more intense amplitude than in 1993–2004, which resulted in a different western North Pacific atmospheric response to the ENSO in 2005–2016 from that in 1993–2004.

Diagnostic Analysis of the Response of Volume Transport through the Tsushima Strait to the Eddy-Induced Variations in the Kuroshio Region

Abstract The Tsushima Strait (TS) is the sole passage for volume transport from the East China Sea to the Sea of Japan. To date, the process underlying the interannual variability in volume transport remains unclear. In this study, 27-yr (1993–2019) reanalysis data from the Japan Coastal Ocean Predictability Experiment 2 (JCOPE2M) system and in situ/satellite observations were employed to understand this process. The results suggest that TS transport was generally high in 1999, 2003/04, and 2010 and low in 1995/96, 2005/06, 2008, and 2014/15. The sea level anomaly (SLA) outside the entrance of the TS, that is, the upstream TS forcing, dominates the interannual TS transport variation. A high SLA pumps more water into the Sea of Japan via the TS, and vice versa. By synthesizing JCOPE2M reanalysis data and satellite observations, further analysis revealed that cyclonic mesoscale eddies from the subtropical countercurrent (STCC) could be responsible for this high SLA by reducing Kuroshio transport, enhancing Kuroshio intrusion across the shelf, and increasing the SLA around the upstream TS region. The reverse was true for anticyclonic STCC eddies. Variability in the Kuroshio intrusion southwest of Kyushu induces variations in the TS transport on an interannual time scale.

Effect of western Pacific current uncertainties on the forecasting of eddy shedding from the Kuroshio loop into the South China Sea: A case study

The process of eddy shedding from the Kuroshio loop current (LC) into the South China Sea via the Luzon Strait is highly variable and significantly controlled by the uncertainties in the Western Pacific (WP) currents. This study investigates the forecast of a strong eddy shedding event from the Kuroshio LC during the winter 2016–2017 using the regional ocean modeling system and ensemble Kalman filter assimilation method. Experiments are conducted to explore the sensitivity of forecasts to the assimilation of satellite sea surface height anomaly data from different key regions of the WP, such as the subtropical countercurrent (STCC) and Northern Equatorial Current (NEC) regions. The results indicate that the improved forecast performance of the strong Kuroshio LC eddy shedding event is mainly attributed to assimilation in the STCC region and partly owing to assimilation in the NEC region. Specifically, assimilation in the STCC region reproduces a westward-propagating cyclonic eddy that collides with the Kuroshio in the Luzon Strait, blocking the Kuroshio northward flow and causing Kuroshio LC eddy shedding. Furthermore, assimilation in the NEC region enhances the simulation of the weak Kuroshio background current, which further facilitates the formation of a strong Kuroshio background LC and the occurrence of Kuroshio LC eddy shedding.

Glider Network Observation Unraveling A Cyclonic Eddy in the Northwestern Pacific Subtropical Countercurrent

The North Pacific Subtropical Countercurrent (STCC) is a zonal current consisting of a weak eastward current near the surface and a westward current (north equatorial current, NEC) below. Previous studies have proved that this region is an extremely active region of the North Pacific eddy. The activity of mesoscale eddy in this region can directly or indirectly affect the periodic variations of the Kuroshio and the Chinese coastal waters. In order to study the structure of mesoscale eddies in this region and their impact on the upper ocean, five Petrel-II underwater gliders were conducted as a network to collect hydrographic properties from December 09 2020 to February 05, 2021. It is found that a cyclonic eddy passed through the glider network from east to west during the observation period, and the glider grid almost covers the whole eddy. The radius, amplitude and propagation velocity of the eddy were analyzed. The underwater gliders were equipped with CTD sensors, collected about 1200 full thermohaline profiles. The observation results indicated that the cyclonic eddy can influence over 400 m in the vertical, which caused the isotherm rising up to 40 m. Analysis based on satellite sea surface temperature (SST) and sea level anomaly (SLA) data suggest that the cyclonic eddy is probably locally generated.

Zonally Asymmetric Multidecadal Variability of the North Pacific Subtropical Fronts

Abstract The North Pacific Subtropical Fronts (STFs), accompanied by the eastward-flowing subtropical countercurrent, stretch from the western Pacific Ocean to the north of Hawaii. Previous work has detected different trends of the frontal position and strength between the western STF (WSTF; west of 180°) and the eastern STF (ESTF; east of 180°) in the past 40 years. However, whether the basin-scale STFs have zonally asymmetric variability on multidecadal time scales and what drives that change remain to be quantified. Our recent work has shown that the multidecadal variability of the WSTF is controlled by the Atlantic multidecadal oscillation via the subtropical mode water variability. The present study proposes that the variability of ESTF is modulated by the Pacific decadal oscillation (PDO) via the central mode water (CMW) variability, quasi synchronously on multidecadal time scales. During a PDO positive phase, the enhanced midlatitude westerly winds in the central North Pacific increase the local surface buoyancy loss and deepen the winter mixed layer, which enlarges the CMW formation and thus increases its volume. Meanwhile, accompanied by the southward-migrated outcropping zone, the main body of CMW shifts equatorward. In response to such CMW changes, the ESTF strengthens and shifts equatorward correspondingly. Conversely, during a PDO negative phase, the weakened midlatitude westerly winds in the central North Pacific decrease the local surface buoyancy loss and shallow the winter mixed layer, which reduces the CMW formation and thus decreases its volume. Meanwhile, accompanied by northward-migrated outcropping zone, the main body of CMW shifts poleward. In response to such CMW changes, the ESTF weakens and shifts poleward correspondingly. Our results reveal that the dominant factor controlling the low-frequency variability of the WSTF and ESTF is different, which renews the conventional picture that all of the STFs behave symmetrically, with important implications for the North Pacific climate variability.

Exploring the plankton bacteria diversity and distribution patterns in the surface water of northwest pacific ocean by metagenomic methods

The study of marine microbial communities is crucial for comprehending the distribution patterns, adaptations to the environment, and the functioning of marine microorganisms. Despite being one of the largest biomes on Earth, the bacterioplankton communities in the Northwest Pacific Ocean (NWPO) remain understudied. In this research, we aimed to investigate the structure of the surface bacterioplankton communities in different water masses of the NWPO. We utilized metagenomic sequencing techniques and cited previous 16S rRNA data to explore the distribution patterns of bacterioplankton in different seasons. Our results revealed that Cyanobacteria, Proteobacteria, Bacteroidetes, and Actinobacteria dominated the microbial communities, accounting for over 95% of the total. During spring, we observed significant differentiation in community structure between the different water masses. For instance, Prochlorococcus and Pseudoalteromonas were primarily distributed in the nutrient-deficient subtropical countercurrent zone, while Flavobacteriaceae and Rhodobacteraceae were found in the Kuroshio-Oyashio mixing zone. During summer, the surface planktonic bacteria communities became homogenized across regions, with Cyanobacteria becoming the dominant group (68.6% to 84.9% relative abundance). The metabolic processes of the microorganisms were dominated by carbohydrate metabolism, followed by amino acid transport and metabolism. However, there was a low relative abundance of functional genes involved in carbohydrate metabolism in the Kuroshio-Oyashio mixing zone. The metagenomic data had assembled 37 metagenomic-assembled genomes (MAGs), which belong to Proteobacteria, Bacteroidetes, and Euryarchaeota. In conclusion, our findings highlight the diversity of the surface bacterioplankton community composition in the NWPO, and its distinct geographic distribution characteristics and seasonal variations.

Comparative analysis of four types of mesoscale eddies in the North Pacific Subtropical Countercurrent region - part II seasonal variation

The North Pacific Subtropical Countercurrent area (STCC) is high in mesoscale eddy activities. According to the rotation direction of the eddy flow field and the sign of temperature anomaly within the eddy, they can be divided into four categories: cyclonic cold-core eddy (CCE), anticyclonic warm-core eddy (AWE), cyclonic warm-core eddy (CWE) and anticyclonic cold-core eddy (ACE). CCE and AWE are called normal eddies, and CWE and ACE are named abnormal eddies. Based on the OFES data and vector geometry automatic detection method, we find that at the sea surface, the maximum monthly number of the CCE, AWE, CWE, and ACE occurs in December (765.70 ± 52.05), January (688.20 ± 82.53), August (373.40 ± 43.09) and August (533.00 ± 56.92), respectively. The number of normal eddies is more in winter and spring, and less in summer and autumn, while abnormal eddies have the opposite distribution. The maximum rotation velocity of the four types of eddies appears in June (11.71 ± 0.75 cm/s), June (12.24 ± 0.86 cm/s), May (10.63 ± 0.99 cm/s) and June (9.97 ± 0.91 cm/s), which is fast in winter and spring. The moving speed of the four types of eddies is almost similar (about 10 ~ 11 cm/s). The amplitude of normal and abnormal eddies is both high in summer and autumn, and low in winter and spring, with larger amplitudes in normal than abnormal eddies. The eccentricity (defined as the eccentricity of the ellipse obtained by fitting the eddy boundary) of the four types of eddies is also close to each other, and their variation ranges from 0.7 to 0.8, with no apparent seasonal variation. The vertical penetration depth, which has no significant seasonal difference, is 675.13 ± 67.50 m in cyclonic eddies (CCE and CWE), which is deeper than that 622.32 ± 81.85 m in anticyclonic eddies (ACE and AWE). In addition, increasing the defined temperature threshold for abnormal eddies can significantly reduce their numbers but does not change their seasonal variation trend.

Seasonal Modulation of the Eddy Kinetic Energy and Subtropical Countercurrent Near the Western North Pacific Boundary

AbstractSeasonality of the subtropical countercurrent (STCC) and associated surface eddy kinetic energy (EKE) near the western North Pacific boundary is investigated. Unlike the interior STCC region where the EKE peaks in late spring (May) interpreted by the vertical velocity shear between the STCC and underlying North Equatorial Current in early spring (March), the high EKE near the western boundary lasts from May to August with unclear mechanism. Other studies suggest that the wind, heat flux and mode water could contribute to the EKE seasonality along the STCC band. But all these mechanisms can barely explain the observed EKE seasonality near the western boundary comparing to the interior STCC region, since there are no seasonal phase changes in the above forcings. We found that the eddies provide significant contribution to the EKE and STCC seasonal variation near the western boundary. The westward propagating eddies are intensified by the background shear instability near the western boundary and then further induce the high EKE in summer through the EKE convergence. At the same time, the strong surface current by the enhanced eddy activity changes the seasonality of the STCC. The dynamics is different near the western boundary from that in the interior ocean, with eddies playing more important role in modulating the EKE and STCC seasonal variation. The finding of this study will contribute to a better understanding of the dynamics of eddy‐current interaction and ocean variability near the western boundary.

The Mode‐Water‐Induced Interannual Variation of the North Pacific Subtropical Countercurrent and the Corresponding Winter Atmospheric Anomalies

AbstractCombining several reanalysis and in situ data sets, the low potential vorticity (PV) water masses are Lagrangian tracked in the subsurface ocean southwestwardly from the midlatitude Pacific in winter, which have significant interannual variations. The subsurface low PV water transport is proved to be closely related to the interannual variation of atmospheric North Pacific westerly jet by changing the subtropical countercurrent (STCC) and the sea surface temperature gradient adjacent. Specifically, the enhanced subsurface low PV water mass transported to the west of STCC causes strengthened northeastward warm water transport of STCC branching, and the weakened eastward branching. Therefore, significant negative temperature gradient anomalies of sea surface and lower atmosphere generate in the midlatitude to the east of 170°E. The corresponding local weakened lower atmospheric baroclinity and wave activities then lead to the convergence of anomalous downward atmospheric baroclinic Rossby waves and eventually the upper westerly jet deceleration.

Seasonal variation of atmospheric coupling with oceanic mesoscale eddies in the North Pacific Subtropical Countercurrent

Seasonal variation of atmospheric coupling with oceanic mesoscale eddies in the North Pacific Subtropical Countercurrent

Comparative analysis of four types of mesoscale eddies in the north pacific subtropical countercurrent region – part I spatial characteristics

The North Pacific Subtropical Countercurrent (STCC) region has high mesoscale eddy activities due to its complex circulation structure. This study divides these mesoscale eddies into four types: cyclonic cold-core eddy (CCE), anticyclonic warm-core eddy (AWE), cyclonic warm-core eddy (CWE), and anticyclonic cold-core eddy (ACE) according to the rotation direction of the eddy flow field and the sign of average temperature anomaly within the eddy after spatial high-pass filtering. CCE and ACE are called normal eddies, while CWE and ACE are named abnormal eddies. Using eddy-resolving model data (OFES), this work finds that the abnormal eddy phenomenon mainly occurs in the ocean’s upper layer. The eddy number proportion for CCEs, AWEs, CWEs, and ACEs at the sea surface is 35.60, 32.08, 12.95, and 19.37%. The corresponding average radius is 79.14 ± 3.7, 83.34 ± 3.75, 73.74 ± 4.14, and 79.46 ± 3.89 km, respectively. Each type of eddy’s average amplitude is about 3 cm. Regarding the eddy average eccentricity, the four types of eddies have very close eccentricities, with a range of 0.73 ~ 0.76. If the types of eddies are not distinguished, the eddies generated north of 21°N tend to move southward, while eddies generated south of that latitude tend to move northward. The depth of CCEs, AWEs, CWEs, and ACEs with average eddy nonlinearity larger than one is concentrated in the ocean’s upper layer at 109.0, 116.0, 159.0, and 52.0 m, respectively. This study deepens the understanding of the spatial distribution characteristics of mesoscale eddies in the STCC region.

Weakening of the Kuroshio Upstream by Cyclonic Cold Eddies Enhanced by the Consecutive Passages of Typhoons Danas, Wipha, and Francisco (2013)

An array of five pressure-recording inverted echo sounder (PIES) moorings spanning a distance of 420 km around the subtropical countercurrent and North Equatorial Current regions of the western Pacific detected extraordinary sea level drops from November to December 2013. In October 2013, three typhoons, namely, Danas, Wipha, and Francisco, consecutively passed east of the PIES sites, which significantly strengthened pre-existing cyclonic cold eddies to create the observed sea level drops. The typhoon-strengthened cold eddies propagated westward over approximately 1000 km for approximately 4 months and eventually met the Kuroshio offshore Taiwan. The approaching eddies interacted with the Kuroshio upstream for ~3 months and reduced the Kuroshio intensity by up to 24% in February–May 2014, the lowest record for the last 26 years of satellite measurements. Our results can provide a new mechanism linking typhoon-to-eddy-to-Kuroshio variability through oceanic processes.

Interannual Modulation of Kuroshio in the East China Sea Over the Past Three Decades

Previous studies have suggested that westward-migrating mesoscale eddies are a dominant factor that modulate the interannual Kuroshio intensity in the East China Sea (ECS), indicating a close positive correlation between them. According to the extended record of altimetry-based sea level anomalies (SLAs) until 2020, however, the interannual variation of the Kuroshio intensity no longer has a strong positive correlation with eddy activity in the subtropical countercurrent (STCC) region since the early 2000s. Our observational analyses showed that the Kuroshio intensity in the ECS can be modulated by the combined effect of westward-migrating mesoscale eddies and westward-propagating oceanic planetary waves from the east. Until the early 2000s, the interannual variability of Kuroshio was mainly affected by eddy migration from the STCC region, associated with oceanic instability driven by large-scale wind patterns over the western North Pacific. Since then, oceanic planetary waves propagating westward across the Pacific basin have largely modulated the interannual variability of the ECS-Kuroshio intensity by superimposing the SLAs related to mesoscale eddies that propagated towards the east of Taiwan.

Seasonality of Four Types of Baroclinic Instability in the Global Oceans

AbstractDistinct geographical distributions of four types of mesoscale baroclinic instabilities (BCIs) exist in the global oceans, implying preferences of surface or subsurface mesoscale eddies to specific regions. This study explores seasonal variations of global BCI types and their dependence on varying background ocean states with a focus on three representative regions. First, the regions of the Kuroshio Extension and Gulf Stream are representative for distinctive seasonal transition of BCI types: the Charney surface BCIs prevail in winter, while the Phillips BCIs prevail in summer, which is controlled by weak (strong) upper‐layer stratification in winter (summer) but is less impacted by the seasonal changes in the mean flow‐induced shear. Second, the region covering both the tropics and subtropics of the North Pacific (10°N–35°N) is representative for seasonal meridional migration of the border between the domains of the Phillips and the Charney surface BCIs; their border migrates equatorward (poleward) in spring and summer (autumn and winter), which is primarily caused by the equatorward (poleward) shift of both the North Equatorial Current and the Subtropical Countercurrent in spring and summer (autumn and winter), and is secondarily modulated by the variation in the upper‐layer stratification. Third, the Antarctic Circumpolar Current region is chosen to represent a weak seasonal variation of BCIs. Those seasonal variations in BCI types are further demonstrated as being associated with seasonal variations of baroclinic conversion rate, implying possible generation of corresponding eddies.

Synchronized decadal variabilities in the Kuroshio and Kuroshio Extension system

Using satellite altimetry and wind stress data, the spatially coherent decadal variabilities in current intensity and current path during 1993–2018 were investigated for a western boundary current system, namely, the Kuroshio from the east of Luzon Island to the south of Japan and the Kuroshio Extension (KE). Analyses based on Hovmöller diagrams showing decadal variabilities along the entire current demonstrated three types of synchronic relationship between different regions: 1) an out-of-phase relationship in current intensity between the Kuroshio south of Japan and the KE during 1993–2018; 2) an out-of-phase relationship in current intensity between the Kuroshio from the east of Taiwan (ETW) to the East China Sea (ECS) and the KE during 2006–2014; 3) an out-of-phase relationship in current position between the Kuroshio paths in the ETW to ECS and the Luzon Strait during 1993–2018. These patterns were dynamically regulated by the Pacific Decadal Oscillation (PDO). The synchronized current-intensity variability in 2006–2014, which governed almost the entire current system, was caused by coherent phenomena comprising two kinds of baroclinic Rossby wave propagations along the KE and the subtropical countercurrent (STCC), and a regional sea surface height (SSH) anomaly advection from the South China Sea (SCS) to the ECS via the Taiwan Strait. However, the synchronized current-path variability between 1993 and 2018 was caused by an SSH anomaly migration advected by the Kuroshio from the western North Equatorial Current zone to the ECS and an SSH variability localized in the SCS.In this study, we paid particular attention to that the current intensity of the Kuroshio in the ETW to ECS had a positive no-lag correlation with the SSH-based PDO index in 1996–2005 while a positive 3–4 years lag correlation with that index during 2006–2014. It was emphasized that this transition was due to the following mechanism: the current-intensity variability in 1996–2005 was mainly caused by the variability of the eddy activity in the western STCC zone, while that variability in 2006–2014 was mainly caused by the amplification of baroclinic Rossby waves propagating along the STCC zone; the shift from the former to the latter was generated by a rapid phase shift with a relatively large amplitude for the wind stress curl anomaly and a timescale change of the eddy kinetic energy over the STCC zone around 2006.

Structure of Sea Surface Temperature Anomaly Induced by Mesoscale Eddies in the North Pacific Ocean

AbstractSea surface temperature anomalies (SSTAs) induced by oceanic mesoscale eddies trigger mesoscale air‐sea interactions and modulate large‐scale climate systems. Yet, how eddies drive SSTAs has not been firmly established; particularly, the relative importance of lateral stirring and vertical pumping remains a debated issue. This study investigates characteristics and generation mechanisms of mesoscale eddy SSTAs in three eddy‐enriched domains of the North Pacific Ocean: the Kuroshio Extension (KET), the Subtropical Countercurrent (STCC), and the North Equatorial Countercurrent (NECC). Analysis of satellite observational data reveals quasi‐monopole eddy SSTAs in the KET and NECC and dipole‐like eddy SSTAs in the STCC. By investigating spatial and seasonal variations and performing sensitivity experiments using an idealized model, we demonstrate that lateral stirring plays a more important role than vertical pumping in causing mesoscale eddy SSTAs. The eastward transport by the strong background current UC (∼0.6 and ∼0.4 m s−1 in the KET and the NECC, respectively) and the westward eddy translation UT (∼−0.2 m s−1 in the NECC) can dramatically modify the structure of SSTA. The pure stirring effect of eddy rotation velocities generates strong dipole‐like SSTAs. Owing to the eastward UC and the westward UT, the western SSTA pole tends to approach the eddy center, while the eastern SSTA pole departs from the eddy and scatters along the eddy trajectory. These effects reduce the eddy SSTA amplitude and favor the emergence of quasi‐monopole structure. This work provides a useful benchmark for model simulations of mesoscale SST variability and air‐sea interaction.

Weakening and Poleward Shifting of the North Pacific Subtropical Fronts from 1980 to 2018

Abstract Recent evidence shows that the North Pacific subtropical gyre and the Kuroshio Extension (KE) and Oyashio Extension (OE) fronts have moved poleward in the past few decades. However, changes of the North Pacific Subtropical Fronts (STFs), anchored by the North Pacific subtropical countercurrent in the southern subtropical gyre, remain to be quantified. By synthesizing observations, reanalysis, and eddy-resolving ocean hindcasts, we show that the STFs, especially their eastern part, weakened (20% ± 5%) and moved poleward (1.6° ± 0.4°) from 1980 to 2018. Changes of the STFs are modified by mode waters to the north. We find that the central mode water (CMW) (180°–160°W) shows most significant weakening (18% ± 7%) and poleward shifting (2.4° ± 0.9°) trends, while the eastern part of the subtropical mode water (STMW) (160°E–180°) has similar but moderate changes (10% ± 8%; 0.9° ± 0.4°). Trends of the western part of the STMW (140°–160°E) are not evident. The weakening and poleward shifting of mode waters and STFs are enhanced to the east and are mainly associated with changes of the northern deep mixed layers and outcrop lines—which have a growing northward shift as they elongate to the east. The eastern deep mixed layer shows the largest shallowing trend, where the subduction rate also decreases the most. The mixed layer and outcrop line changes are strongly coupled with the northward migration of the North Pacific subtropical gyre and the KE/OE jets as a result of the poleward expanded Hadley cell, indicating that the KE/OE fronts, mode waters, and STFs change as a whole system.