Eastward Currents Research Articles

High frequency radar error classification and prediction based on K-means methods

This study aims to characterize the high frequency radar and numerically simulated low-frequency filtered currents in the south-eastern Bay of Biscay (study area) using a K-means classification algorithm based on an improved Euclidean Distance calculation method that does not take missing values. The errors between observations and simulations was estimated and predicted based on this classification method. Results indicate that predominantly eastward (northward) currents over the Spanish (French) continental shelf/slope in winter and more variable currents in the west and south-west in summer. The model classification results for circulation characteristics are in relatively good agreement with HF radar results, especially for currents on the Spanish (French) shelf/slope. In addition, the probabilistic relationship between observed and modeled currents was explored, obtaining the probability of occurrence of modeled current groups when each group of observed currents occurs. Finally, predictions of model and observed current errors were made based on the classification results, and it was found that the predictions based on the classification of all data had the smallest errors, with a 17% improvement over the unclassified control experiment. This study provides a foundation for subsequent model error testing, forecast product improvement and data assimilation.

Observation of deep currents around a seamount between the Japan and Kuril trenches

Hydrographic observations and two-year current measurements revealed the flow around the Erimo Seamount between the Japan and Kuril trenches. Northward currents were observed to flow both around the seamount and on the eastern flank of the trench. Since the dissolved oxygen concentration of the former is higher than that of the latter, these currents correspond to the western and eastern branch currents of the deep western boundary current in the North Pacific, respectively. The western branch current forms a Taylor cap over the seamount, although during certain periods, it joins the eastern branch current. During such periods, a strong jet forms around the seamount and the Taylor cap is washed away. Such alternately intensified currents are consistent with previous observations of the Japan Trench. On the western flank of the trench, a southward current flows near the seafloor, while an eastward current is formed at a depth of 3360 m due to the confluence of the currents from north and south.

Anomalous ocean currents and European anchovy dispersal in the Iberian ecosystem

Unlike other upwelling areas where sardine and anchovy species dominate the pelagic ecosystems, the Western Iberian ecosystem has been consistently dominated by European sardine Engraulis encrasicolus, while anchovy had a residual presence from the start of acoustic surveys, in 1989, to 2014. Since 2015, the abundance of anchovy in the Western Iberian margins has sharply increased and continues to show an increasing trend as of 2023. It is unclear if this increase is a result of dispersal from nearby recruitment areas, higher survival rates of early life stages due to favorable environmental conditions, or both. We used a set of different models to simulate the dispersion and survival of anchovy early life stages in the Iberian region for the years preceding the increase in anchovy abundance. An ocean model simulation with the model CROCO provided the fields used as background for Lagrangian simulations coupled to an individual-based model of anchovy eggs and larvae. We simulated the years 2013-2015, and the results show that in 2014 and 2015, anomalous upper-ocean circulation patterns with strong and persistent westward currents transported a large number of eggs and larvae from the Bay of Biscay (BoB) westward along the Northern Iberian margin. The maximum transport occurred in June and July 2015, when 8 and 4%, respectively, of the eggs spawned in the BoB potentially reached the Iberian west coast as larvae. This process might explain the increase in anchovy abundance in the Western Iberian ecosystem. The results of the study show that episodes of anomalous intense ocean currents, when coincident with high presence of eggs, can lead to the colonization of new areas, and connectivity between areas varies dramatically with time.

Spring circulation characteristics and formation mechanism in the Beibu Gulf

Spring circulation in the Beibu Gulf remains an underinvestigated research topic. Model results validated by observations reveal that the northern and southern parts of the Beibu Gulf are controlled by an enclosed cyclonic gyre and a cyclonic gyre, respectively, while in the vertical, the circulation at the surface differs from those in the middle and deep layers. Contrary to that of the classical view, the modeled Qiongzhou Strait (QS) current flows eastward during some days in spring. This eastward current has an occurrence proportion of 29.3–55.4%, and its longest duration period is 7~18 d. The average current in the QS in spring is approximately westward except in some typical years. In spring, the northern Beibu Gulf is affected by the cold water mass (CWM), which is depicted by the 12°C and 10°C isolines in strong and weak years, respectively. Sensitivity experiments show that the heat flux is dominant in the northern Beibu Gulf via the effect of the CWM, and the monsoon wind favors the formation of the cyclonic gyre. Conversely, the cyclonic gyre in the southern Beibu Gulf is jointly driven by the monsoon wind and heat flux, with the effect of the monsoon wind being more significant. The current off the northern coast of Vietnam (VNC) can be either northeastward or southwestward in spring in different years. The VNC is mainly controlled by the elevation gradient along the northern coast of Vietnam, and the Coriolis force, horizontal advection, and vertical diffusion also play significant roles. Additionally, the CWM is mainly controlled by the heat flux whereas the monsoon wind plays a secondary role. And the VNC, current in the QS, CWM, and cyclonic gyre jointly affect nutrient distributions in the northern Beibu Gulf.

Causes of Diverse Impacts of ENSO on the Southeast Asian Summer Monsoon among CMIP6 Models

Abstract This study examines the fidelity of 47 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6) in representing the influence of El Niño–Southern Oscillation (ENSO) on the Southeast Asian summer monsoon (SEASM) during the ENSO decaying summer. The response of the SEASM to ENSO shows a large model spread among the models, some of which even simulate opposite signs of SEASM anomalies compared to the observed values. The bad-performance models (BPMs) are therefore selected to be compared with both the good-performance models (GPMs) and observations to explore the possible causes of the deficiency. Results show that in the BPMs, the ENSO-related warm sea surface temperature (SST) anomalies extend too far westward in the western equatorial Pacific (WEP) and they do not dissipate in the El Niño decaying summer in comparison with those in the GPMs and observations, interfering with the effect of ENSO on the SEASM. The slow decay of WEP SST anomalies from the El Niño mature winter to the decaying summer in the BPMs is mainly caused by a weak negative shortwave radiation feedback due to a low sensitivity of convection to local SST anomalies, which is related to the cold bias in climatological SST over this region. On the other hand, from the mature winter to the decaying summer of El Niño, the El Niño–related anomalous eastward current does not reverse to a westward current in the BPMs, which also contributes to the slow decay of WEP SST anomalies via inducing excessively persistent warm zonal advection. Significance Statement We investigate the possible causes of the diverse impacts of El Niño–Southern Oscillation (ENSO) on the Southeast Asian summer monsoon (SEASM) among 47 CMIP6 models. We find that a plausible reason for the deficiency of some models in simulating the influence of ENSO on the monsoon is that the sea surface temperature (SST) anomalies associated with ENSO are unrealistic in the western equatorial Pacific (WEP) in these models. Further diagnoses indicate that the unrealistic WEP SST anomalies are related to the cold bias of the climatological SST, which could lead to a weak negative shortwave radiation feedback and excessively persistent warm zonal advection. The information provided in this study is useful for improving the skill of the climate models in representing the ENSO–SEASM relationship.

RETRIEVAL OF SURFACE CURRENT FROM HIMAWARI-8 SEA SURFACE TEMPERATURE DATA AND PARTICLE IMAGE VELOCIMETRY METHOD

As human activity occurs at the surface, which is influenced by surface currents, understanding surface currents is crucial. Ocean currents are not solely caused by wind, but also of other factors, such as sea surface temperature. To obtain surface current data, BMKG used HF radar installed in several places, including Flores Sea. This location is significant because it has large ports and heavy crossing traffic. This study aims to analyse surface current data retrieve from Himawari-8 SST data by applied PIV algorithm. Using the PIV algorithm, a cross-correlation plane is generated by comparing two identically sized interrogation windows obtained from successive images. On 5 December 2022 in the Flores Sea, Himawari-8 SST data was employed to estimate sea surface currents every 10 minutes. Surface current motion varies widely from image to image. Western areas are dominated by westward currents, while central and eastern areas are dominated by eastward currents during daylight hours, and westward currents during the night. Synoptic wind affects surface current movement during those hours. Validation with observational data from the Labuan Bajo HF radar shows that the V component current estimation is better than the U component current estimation. This is particularly prevalent in the south and in areas close to the KAWA radar HF location.
 Keywords: Flores Sea; SST; Himawari-8; PIV; surface current

Average Ionospheric Mid‐ and Low‐Latitude Currents From E‐ and F‐Region Altitudes, Derived by the Swarm Constellation

AbstractBy utilizing magnetic field measurements from the Swarm constellation during the years 2014–2019, average characteristics of the height‐integrated ionospheric zonal and meridional currents at middle and low latitudes are investigated. We provide for the first time the detailed dependencies of these currents on location, local time, and season. The main results obtained are: (a) Large parts of the ionospheric currents flow in the F‐region, particularly on the nightside and in the winter hemisphere. (b) Clearly outstanding zonal current is the equatorial electrojet (EEJ) around noon, accompanied by currents in opposite direction at middle latitudes. At night, eastward currents are prevailing in the summer hemisphere, and westward in winter. During equinox the nighttime zonal currents show a lot of hemispheric similarity, with westward currents around pre‐midnight and eastward currents post‐midnight at mid latitudes. These features can be attributed to the meridional thermospheric wind effects. (c) The meridional currents are weak at night, exhibiting a preferred direction toward the summer hemisphere. At daytime the meridional currents flow toward equator in the pre‐noon sector and opposite current directions prevail in the afternoon, which is consistent with the closing of the EEJ current circuit. Large hemispheric differences in meridional currents are found during December solstice, suggesting that most of the EEJ current is closed through the winter, northern hemisphere. (d) The zonal and meridional currents at middle latitude exhibit clear signatures of solar tides at daytime. Most prominent are the non‐migrating tidal components DE3 in the zonal currents and SW5 in the meridional.

Quasi-permanent meanders of the Kuroshio Extension: Laboratory experiments with altimetry and numerical simulations

This is a study of gyres bounded by a barrier in the west and forced by a localized upwelling. It is motivated by observations of quasi-permanent meanders of the Kuroshio Extension in the North Pacific Subtropical Gyre. Laboratory experiments are performed in a rotating tank where a parabolic shape of the surface of water is used to model the latitudinal variation of the Coriolis parameter. Optical altimetry provides accurate records of circulations and allows us to observe the evolution of β-plumes (Rossby wave envelopes), including their reflection from the barrier. Experiments are supplemented by numerical simulations which extend the range of the control parameters. We demonstrate that meanders occur in the northern branch of the gyres, within an eastward current similar to those in the Kuroshio Extension. The meanders are due to the Rossby wave reflection from the boundary. The wavelength of the meanders is selected by the condition of stationarity of the westward propagating Rossby-wave phase within an eastward current.

Contrasting eddy-driven transport in baroclinically unstable eastward currents and subtropical return flows

We identify and explore the fundamental differences in the dynamics of mesoscale vortices in eastward background (EB) parts in mid-latitude ocean gyres and in westward background (WB) return flows. In contrast to eddy behavior in EB flow, a systematic meridional drift of eddies in WB flow results in poleward expulsion of cold-core cyclones and equatorward expulsion of warm-core anticyclones from the unstable zone with a negative potential vorticity gradient (PVG). Consequently, heat can be transferred further by upper ocean vortices intrinsically coupled with deep opposite sign partners. Such structures can drift through the stable zone with positive PVG in both layers. This mechanism of lateral transfer is not captured by local models of homogeneous turbulence. The crossflow drift is related to the coupling of the upper vortices with opposite sign deep eddies shifted eastward. The abyssal vortices can be viewed as lee Rossby waves induced by their upper-layer partners and described analytically in the vicinity of latitude of marginal stability. Here, we show how such self-amplifying hetons, emerging in homogeneous turbulence, saturate when they approach locally stable regions of inhomogeneous currents. The presented results indicate that subtropical regions with return WB flows in the upper layer favor long-distance heat transport by spatially coherent eddies in accordance with observations and motivate the development of non-local parameterizations of eddy fluxes.

Subinertial frequency variations in the axis of the Tsugaru Warm Current east of the Tsugaru Strait

The eastern part of the Tsugaru Strait (located at ~ 41.5° N) is a remarkable region of the coastal flow system around Japan due to the Tsugaru Warm Current (TgWC), an intensive eastward current that flows over its complex topography and exhibits marked tidal variation. The TgWC has a jetlike structure and demonstrates drastic seasonal changes in its path and vertical current structure. Using a high-resolution dataset obtained from a high-frequency radar system (HFR) and a coastal data assimilation experiment (JCOPE-T DA), we focused on the frontal variation, which is shorter than the seasonal timescale, along the axis of the TgWC in relation to the meanders of the jet. We found variations in the axis with multiple timescales between 1 and 14 days, especially in summer and autumn. The ~ 14-day variation, a remarkable occurrence on the west side of the eastern Tsugaru Strait, was consistent with the tidal variation in the strait. The zonal scale of the axis meander estimated by the HFR matched a scale several times larger than the internal deformation radius, especially in summer and autumn. JCOPE-T DA outputs showed that the crest of such a meander at greater depths progresses in relation to that at the surface. The outputs also suggested instabilities, indicating potential and kinetic energy conversion from the mean to the eddy field. The propagation speed of the several-day variation in the axis estimated from the HFR data was consistent with that of the two-layer baroclinic instability. The several-day variation also showed relatively high coherence with wind variation from summer to autumn, during which time the seasonal wind over the strait switched from easterly to westerly. These variations with multiple timescales imply an effect on north–south water mass mixing in the eastern part of the strait, including rapid acidification in the strait and intensification of stratification.

Atlantic Water Circulation and Properties Northwest of Svalbard During Anomalous Southerly Winds

AbstractAtlantic Water (AW), the main source of heat and salt for the Arctic Ocean, undergoes large transformations (cooling and freshening) north of Svalbard as it flows near the surface above the Yermak Plateau (YP). In September 2017, a SeaExplorer ocean glider deployed in the West Spitsbergen Current (WSC) and recovered north of Svalbard documented the circulation and properties of the AW crossing the YP. The glider sampled the different branches of the AW flowing into the Arctic around the YP: the WSC, the Svalbard Branch (SB), the Yermak Pass Branch, and the Yermak Branch. Unusual southerly winds prevailed in summer 2017 impacting AW circulation in the region. Cold and fresh lenses of shelf‐origin waters detached from the slope in the WSC to reach their density level below the core of the AW. This resulted in cooling and freshening of the AW inflow from below. The eastward current associated with the SB was found to be weak at its usual location above the 400 m isobath, likely the result of the adjustment of the flow influenced by anomalous southerly wind conditions.

Mixed Layer Salinity Balance in the Eastern Tropical Indian Ocean

AbstractThe mixed layer salinity (MLS) balance in the eastern tropical Indian Ocean is investigated using observations and output from the Estimating the Circulation and Climate of the Ocean model. The results suggest that the annual mean MLS is determined by the balance between the surface freshwater forcing and ocean advection. The seasonal MLS variation in the eastern equatorial Indian Ocean is driven strongly by salt advection associated with the Wyrtki Jet in spring and fall despite weak seasonal cycles of precipitation and evaporation over this region. Off the south Java coast, the MLS is the lowest in summer, due to the advection of freshwater southward associated with the meridional currents, whereas it reaches the maximum in fall, due to strong evaporation. During the negative Indian Ocean Dipole (IOD), the anomalous eastward currents transported high‐salinity water to the east, generating positive MLS anomalies in the eastern tropical Indian Ocean. The regional evaporation minus precipitation anomalies were negative, which reduced the MLS. The MLS balance during the positive IOD is found to be nearly a mirror reflection of that during the negative IOD.

An Eastward Current Encircling Mercury

AbstractMercury has a terrestrial‐like magnetosphere which is usually taken as a scaled‐down‐version of Earth's magnetosphere with a similar current system. We examine Mercury's magnetospheric current system based on a survey of Mercury's magnetic field measured by the Mercury Surface, Space Environment, Geochemistry, and Ranging spacecraft as well as computer simulations. We show that there is no significant Earth‐like ring current flowing westward around Mercury, instead, we find, for the first time, an eastward current (EC) encircling the planet near the night‐side magnetic equator with an altitude of ∼500–1,000 km. The EC is closed with the dayside magnetopause current and could be driven by the gradient of plasma pressure as a diamagnetic current. Thus, Mercury's magnetosphere is not a scaled‐down Earth magnetosphere, but a unique natural space plasma laboratory. Our findings offer fresh insights to analyze data from the BepiColombo mission, which is expected to orbit Mercury in 2025.

Abyssal current and water mass in the Main Gap and an adjacent Small Gap of the Emperor Seamount Chain

From 2016 to 2017, we conducted 1-year mooring current observation at the widest, deepest gap, widely known as the Main Gap, and a narrower, shallower gap just south of the Main Gap, called the Small Gap in this study, in the Emperor Seamount Chain, which divides the Northwest Pacific Basin (NWPB) from the Northeast Pacific Basin. We also conducted two hydrographic sections with a conductivity–temperature–depth sensor and a lowered acoustic Doppler current profiler around the two gaps in 2012 and 2016. At the Main Gap, the abyssal current flowed east–northeastward at a statistically significant mean velocity of 1.3–2.4 cm s−1, while the current was dominated by tidal and inertial variability with a magnitude of 3–4 cm s−1. At the Small Gap, the abyssal current had an eastward mean velocity of 9.4 cm s−1, accompanied by mesoscale variability with a magnitude of 7 cm s−1. These eastward currents carried abyssal water below a depth of 5000 m, which came from the northern part of the NWPB, with a volume transport of 1.6 × 106 m3 s−1 through the Main Gap and 0.5 × 106 m3 s−1 through the Small Gap. Cold, saline, oxygen-rich Lower Circumpolar Deep Water, which occupied the south of the zonal seamount range at 37°N, called the S–E Seamounts in this study, was found near the seafloor north of the S–E Seamounts to the west of the Small Gap; however, it did not extend to the Main Gap or to the east of the Small Gap.

Effects of Subauroral Polarization Streams on the Equatorial Electrojet During the Geomagnetic Storm on 1 June 2013: 2. The Temporal Variations

AbstractUsing ground‐based magnetic field measurements and numerical simulations from the Thermosphere‐Ionosphere Electrodynamic General Circulation Model (TIEGCM), a first paper (Zhang, Yamazaki, et al., 2021, doi: https://doi.org/10.1029/2021JA029681; under review) introduced the potential roles of disturbance dynamo electric field due to subauroral polarization streams (SAPS) on the equatorial electrojet (EEJ) during a moderate geomagnetic storm on 1 June 2013. Our second study investigated the temporal responses of equatorial electrojet to SAPS. At noon, the residual EEJ (ΔEEJ) owing to SAPS flows westward, that is, counter equatorial electrojet (CEJ). The temporal variation of CEJ excited by the dynamo electric field was basically consistent with that by SAPS, and the effects of zonal wind were larger than those of meridional wind. The relative time delay of CEJ and SAPS was related to the propagation time of disturbance wind from mid‐latitudes to low‐latitudes. It took 2–3 hr for SAPS‐related disturbance wind to propagate to the equatorial region and change the polarity of EEJ. The influence of meridional winds on the temporal variations of ΔEEJ is related to the generation of eastward currents at mid‐latitudes, which can accumulate the positive charges at dusk terminator and then generate a westward electric field at lower latitudes.

Evidences and drivers of ocean deoxygenation off Peru over recent past decades

Deoxygenation is a major threat to the coastal ocean health as it impacts marine life and key biogeochemical cycles. Understanding its drivers is crucial in the thriving and highly exploited Peru upwelling system, where naturally low-oxygenated subsurface waters form the so-called oxygen minimum zone (OMZ), and a slight vertical shift in its upper limit may have a huge impact. Here we investigate the long-term deoxygenation trends in the upper part of the nearshore OMZ off Peru over the period 1970–2008. We use a unique set of dissolved oxygen in situ observations and several high-resolution regional dynamical-biogeochemical coupled model simulations. Both observation and model present a nearshore deoxygenation above 150 m depth, with a maximum trend of – 10 µmol kg−1 decade1, and a shoaling of the oxycline depth (− 6.4 m decade−1). Model sensitivity analysis shows that the modeled oxycline depth presents a non-significant (+ 0.9 m decade−1) trend when remote forcing is suppressed, while a significant oxycline shoaling (− 3 m decade−1) is obtained when the wind variability is suppressed. This indicates that the nearshore deoxygenation can be attributed to the slowdown of the near-equatorial eastward currents, which transport oxygen-rich waters towards the Peruvian shores. The large uncertainties in the estimation of this ventilation flux and the consequences for more recent and future deoxygenation trends are discussed.

Impacts of Precipitation–Evaporation–Salinity coupling on upper ocean stratification and momentum over the tropical pacific prior to onset of the 2018 El Niño

Precipitation affects upper ocean salinity and temperature over a wide range of temporal and spatial scales. Although the effects of freshwater on the diurnal mixing and formation of barrier layer have been shown in previous studies, the impacts of freshwater from the large-scale precipitation associated with the Madden–Julian Oscillation (MJO) on the ocean from the western to central-eastern Pacific are not well understood. This study investigates the coupling of precipitation, evaporation, and salinity (P–E–S) and its impact on the upper ocean stratification and momentum on time scales from days to weeks (intraseasonal) and beyond. The Unified Wave INterface – Coupled Model (UWIN-CM) is used to simulate multiscale air–sea interactions during January–July 2018, including multiple MJO events leading up to the onset of the 2018–2019 El Niño. The atmospheric component of the UWIN-CM is constrained by reanalysis and reinitialized daily, while the ocean component evolves continuously and is fully coupled with the atmosphere. The initial and lateral boundary conditions for the atmosphere and ocean are from the ECMWF ERA5 and HYCOM global analysis, respectively. The UWIN-CM simulation captures the surface winds, rainfall, and the evolution of the upper ocean compared with in situ and satellite observations. To determine the effect of P–E–S coupling, the UWIN-CM simulation with full P–E–S coupling is compared with a model experiment in which the P–E input to the ocean is set to zero (NoPE). The difference between the two increases with time and becomes particularly large during the 3rd MJO event in April 2018. Two key results emerge. First, heavy precipitation and strong winds from the MJO events result in a locally fresher, warmer western Pacific warm pool with a shallower mixed layer and thicker barrier layer. Second, the fresh, warm water mass extends into the central-eastern Pacific from mid-April to late May with up to 0.5 m s−1 eastward currents. A well-defined salinity/density front, propagating eastward at about 0.3 m s−1, is located at the leading edge of the warm pool. The P–E–S coupling plays an important role in the eastward extension of the warm pool prior to the onset of El Niño in July 2018.

The features and mechanisms of the North Shandong Coastal Current: a case study in 2014

The North Shandong Coastal Current (NSCC) is an important transporting route for sediment, drifting algae, and spilled oil from the Bohai Sea to the Yellow Sea. This study investigated the features and formation mechanism of the NSCC using observational data of current velocity and a numerical coastal ocean model. Our results confirmed the existence of the NSCC in the sense of climatological current in winter when northerly wind prevails in the Bohai Sea and Yellow Sea. The magnitude of the NSCC in the monthly time scale ranged 0.07–0.12 m/s and the current direction was parallel to the coastline. The detided residual current on the pathway of the NSCC was unstable, but variable with the local wind speed. The residual current was well correlated with northerly wind speed and tended to be parallel to coastline with the increase of wind speed. Strong wind plays key roles in the formation of eastward mean flow on the pathway of the NSCC in winter. We found that strong wind can generate a stronger eastward current in the southern side of the northern Yellow Sea, but a smaller westward return flow during the strong wind relaxation period. The asymmetry of wind-related residual current during and after strong wind events accounts for the formation of the eastward NSCC. A momentum analysis was performed using the numerical model results during strong wind events. We found that the barotropic pressure gradient was the dominant driving force of the residual current both during and after a strong wind event.

Anomalous Upper-Ocean Circulation of the Western Equatorial Pacific following El Niño Events

AbstractMooring measurements at ~140°E in the western equatorial Pacific Ocean documented greatly intensified eastward subsurface currents, which largely represent the nascent Equatorial Undercurrent, to ~67 cm s−1 in boreal summer of 2016. The eastward currents occupied the entire upper 500 m while the westward surface currents nearly disappeared. Historical in situ data observed similar variations after most El Niño events. Further analysis combining satellite and reanalysis data reveals that the eastward currents observed at ~140°E are a component of an anomalous counterclockwise circulation straddling the equator, with westward current anomalies retroflecting near the western boundary and feeding southeastward current anomalies along the New Guinea coast. A 1.5-layer reduced-gravity ocean model is able to crudely reproduce these variations, and a hierarchy of sensitivity experiments is performed to understand the underlying dynamics. The anomalous circulation is largely the delayed ocean response to equatorial wind anomalies over the central-to-eastern Pacific basin emerging in the mature stage of El Niño. Downwelling Rossby waves are generated by the reflection of equatorial Kelvin waves and easterly winds in the eastern Pacific. Upon reaching the western Pacific, the southern lobes of Rossby waves encounter the slanted New Guinea island and deflect to the equator, establishing a local sea surface height maximum and leading to the detour of westward currents flowing from the Pacific interior. Additional experiments with edited western boundary geometry confirm the importance of topography in regulating the structure of this cross-equatorial anomalous circulation.

Steady flow in a frictionless homogenous ocean

A mathematical model is developed to study the free (frictionless) steady horizontal flow which can occur in a homogeneous ocean of constant depth. The flow satisfies the dynamic constraint that the vertical component of absolute vorticity is constant along a streamline. The conclusion is reached that in an enclosed ocean a free steady circulation cannot have any slow broad eastward currents. The eastward currents must occur as narrow streams of high velocity and high relative vorticity. Intensified currents are present along the eastern and western coasts. The theory which is developed for the homogeneous ocean of constant depth can be applied to the two-layer ocean if the horizontal divergence of flow is negligible. If the horizontal divergence is not negligible, then the intensification of poleward currents is more pronounced and that of equatorward currents less pronounced as compared with the homogeneous ocean.