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The ocean regulates the Earth's climate by transporting heat from the equator to the poles. Here, we use satellite‐based sea surface observations of air‐sea heat fluxes and eddy detection to investigate the mesoscale heat transport. "Mesoscale" refers to both the Eulerian perspective as the spatio‐temporal scales of ∼100 km and ∼1 month, as well as the Lagrangian aspect as isolated vortices identified from the dynamic topography. Paradoxically, there are a considerable number of mesoscale eddies inconsistent between their surface thermal and dynamic signals, that is, cold‐core anticyclones and warm‐core cyclones are globally prevalent. On account of such inconsistency, we show that the mesoscale meridional heat transport carried by geostrophic components is 10 times larger than (and opposite in direction to) that of the wind‐driven Ekman components. An offset between SSH‐SST coherent and incoherent eddies in the Ekman heat transport is apparent, whereas the geostrophic heat transport is contained within coherent eddies. Plain Language Summary: Rotating water masses in the ocean, termed as mesoscale eddies, are dynamically important features because they can transport nutrients and displace heat in the Earth's climate system. In the same manner as the atmospheric weather systems that are associated with high or low pressures, mesoscale eddies are accompanied by sea level anomalies and therefore can be observed by satellite images. Conventionally, anticyclonic eddies detected as clockwise vortices in the Northern Hemisphere display warm temperature anomalies at the center. However, recent studies have found that the sea surface temperature signal of an eddy might be uncertain and even contradictory to their dynamical circulation patterns. In this study, we combine sea surface observations of temperature and velocity fields to determine the patterns of global ocean heat transport associated with mesoscale eddies. Key Points: The geostrophic component of meridional heat transport (MHT) at the mesoscale is 10 times larger than the Ekman componentSSH‐SST coherent eddies dominate the spatial patterns of MHT at the mesoscaleThough incomparable in magnitude with the large scale MHT, mesoscale eddies can still transport 30 TW of heat near sea surface [ABSTRACT FROM AUTHOR]

Although El Niño‐Southern Oscillation (ENSO) and its global impacts through teleconnection have been known for decades, if and how the mean currents and mesoscale eddies in the Caribbean Sea are linked to ENSO remains an open question. Here, by analyzing satellite observations and an ocean reanalysis product, we found a close connection between mean currents, eddies in the Caribbean Sea and ENSO on interannual timescales. Strong El Niño events result in enhanced north‐south sea surface height differences and consequently stronger mean currents in the Caribbean Sea, and the opposite happens during La Niña events. The eddy kinetic energy responds to ENSO via eddy‐mean flow interaction, primarily through baroclinic instability, which releases the available potential energy stored in the mean currents to mesoscale eddies. Our results suggest some predictability of the mean currents and eddies in the Caribbean Sea, particularly during strong El Niño and La Niña events. Plain Language Summary: We explored the potential impacts of El Niño‐Southern Oscillation (ENSO) on the circulation and mesoscale eddies in the Caribbean Sea. We found ENSO‐related synchronized changes in mean currents and eddies across the entire Caribbean Sea. The connection between mean currents and ENSO is established through ENSO's impact on the north‐south sea surface height (SSH) difference in the Caribbean Sea, which determines the strength of the geostrophic jet. During strong El Niño events, the easterly wind anomalies will increase the north‐south SSH difference through Ekman transport, and consequently generate stronger mean currents. During strong La Niña events, the opposite happens. Through baroclinic instability, available potential energy stored in the mean currents will be transferred to eddies and results in ENSO‐modified interannual variations of eddy kinetic energy. Our results suggest that interannual variations of mean currents and eddies in the Caribbean Sea might be predictable, particularly during strong El Niño and La Niña events. Key Points: Interannual variations of mean currents and eddies in the Caribbean Sea are linked to El Niño‐Southern Oscillation (ENSO)ENSO‐induced wind pattern changes modulate the north‐south sea surface height differences and hence the mean currents in the Caribbean SeaInterannual variation of eddy kinetic energy in the Caribbean Sea is controlled by baroclinic instability [ABSTRACT FROM AUTHOR]

We assess the contribution of flow over the Northeast Greenland Shelf (NEGS) to the total freshwater transport (FWT) through the Fram Strait. We analyze available observations since 2013 consisting of hydrographic sections, new mooring data, and surface‐geostrophic velocity and ERA5 winds. We provide first seasonal estimates of the FWT over the entire shelf and find a net southward FWT between 47 and 56 mSv (reference salinity 34.9), or ∼40%–45% of the total FWT through the Fram Strait. In summer, the long‐term mean FWT east of 8°W cumulates to 27 mSv southward, there is no trend toward increasing FWT, and new mooring data show a large seasonal cycle. We show that the NEGS contributes significantly to the Arctic freshwater outflow, however, uncertainty is high. To close the Arctic freshwater budget and provide better FWT estimates to the North Atlantic, sustained year‐round observations on the NEGS are essential. Plain Language Summary: We use new observations since 2013 to calculate transport over the Northeaster Greenland Shelf for the first time, and investigate how much it adds to the transport of the East Greenland Current over the shelf break which is monitored by moorings. Over most of the shelf, direct observations of ocean properties are limited to summer. Thus, we use remote‐sensing and wind reanalysis data, with shelf‐wide and year‐round coverage, but higher uncertainty, to estimate the year‐round transport of low‐salinity water (freshwater, less than 34.9) over the entire shelf. We find that on the shelf, freshwater transport is at least 40% of the total in Fram Strait. Still, uncertainty is high and year‐round, direct observations in the western part of the shelf are required to provide accurate estimates for the Arctic freshwater outflow. Key Points: Summer freshwater transport (FWT) over the shelf cumulates to −27 mSv and shows no trend in timeNew mooring data on the shelf at 10.1°W show a very large seasonal cycleYear‐round FWT over the shelf is estimated to ∼−52 mSv or 43% of the total freshwater outflow from the Fram Strait [ABSTRACT FROM AUTHOR]

To investigate the South Equatorial Current (SEC) in the Indian Ocean, 18 drifters were deployed off Sumatra‐Java during boreal spring 2019. These drifter observations revealed a large SEC meander that swung southward about 1,100 km during 40 days along ∼100°E. The southward motion occurred on the eastern edge of the Seychelles‐Chagos thermocline ridge, accompanied by an anticyclonic eddy/Rossby wave from the east, indicated by the geostrophic velocity induced by the zonal pressure gradient. Meanwhile, the ageostrophic velocity induced by the southeasterly wind also contributes to this southward motion. This is verified by a Lagrangian Trajectory Model in which drifter trajectories are well reproduced using Chinese‐French Oceanic SATellite winds products. Further analysis of the historical drifters with similar trajectories reveals that the geostrophic and ageostrophic currents jointly drive drifters to move southward. Ageostrophic component could contribute up to 45% of the southward movement of the SEC meander. Plain Language Summary: The South Equatorial Current (SEC) is westward in the tropical Indian Ocean. During boreal spring 2019, 18 drifters were deployed off Sumatra‐Java, and their trajectories show a large SEC meander, with a typical drifter moved southward over 1,100 km during 40 days along ∼100°E. The geostrophic component, associated with the Seychelles‐Chagos thermocline ridge and an anticyclonic eddy/Rossby wave, and ageostrophic component, induced by southeasterly wind, jointly drive drifters to move southward. Further diagnoses of historical drifters show that the ageostrophic component could take up to 45% contribution in the southward movement in the SEC meander. Key Points: South Equatorial Current in the Indian Ocean advected drifters southward more than 1,100 km within 40 days (∼32 cm s−1) in 2019Combined effects of the zonal pressure gradient and the southeasterly wind contribute to the southward movementSouthward movement usually occurs in spring and summer, contributing from ageostrophic current up to ∼45% [ABSTRACT FROM AUTHOR]

Satellite and in‐situ measurements are used in this study to investigate a possible link between the Florida Current transport (FCT) and temperature changes in the equatorial Pacific. Consistent with the geostrophic nature of the current, the FCT variability shows good correspondence with the changes in sea surface height differences (∆SSH) between the eastern (SSHeast) and western (SSHwest) sides of the Florida Straits. While the variability of SSHwest is mostly associated with seasonal and shorter timescale fluctuations of ∆SSH and FCT, changes in SSHeast are strongly related to the interannual variability of ∆SSH and FCT. A significant correlation is found between the FCT and the Oceanic Niño Index (ONI) on interannual timescales, which explains 21% of the interannual FCT variance. The connection of ONI with FCT is through its impact on SSHeast, associated with the anomalous convergence/divergence in the Caribbean region and the Bahamas forced by ONI‐induced wind stress curl changes. Plain Language Summary: Variations in the Florida Current transport (FCT) have been linked to widespread weather and climate phenomena. Various driving mechanisms for the FCT variability have been proposed, but none can fully explain its wide spectrum variability. In this study, we analyzed in‐situ and satellite observations and found a linear link between the FCT and temperature changes in the equatorial Pacific on interannual timescales. A warming condition in the equatorial Pacific results in low pressure anomalies in the Gulf of Mexico and high pressure anomalies extended into the Caribbean Sea from the tropical Atlantic. This atmospheric pressure pattern is associated with anticyclonic winds over the Caribbean Sea and the Bahamas that drive anomalous oceanic convergence and, therefore, cause higher sea levels in those regions. A higher sea level near the Bahamas often steepens the sea level slope across the Florida Straits, which corresponds to a stronger FCT. A cooling condition in the equatorial Pacific will have the opposite effect, resulting in a reduced FCT. Key Points: Interannual variations in Florida Current transport are highly correlated with sea level at eastern side of the Florida Straits (SSHeast)Changes in SSHeast exhibit a linear link to warming and cooling conditions in the equatorial PacificThis connection is through the wind stress curl changes remotely forced from the equatorial Pacific [ABSTRACT FROM AUTHOR]

Off‐equatorial wind stress curl anomalies that mainly drive geostrophic transport are essential for the phase transition of the El Niño‐Southern Oscillation (ENSO) induced by a recharge‐discharge process. The ENSO‐induced zonal wind stress anomaly also drives surface Ekman currents, which may counteract the geostrophic transport, but its effect on ENSO periodicity remains unclear in the recharge oscillator theory. Here, we extended an ENSO diagnostic framework, called the Bjerknes‐Wyrtki‐Jin index, to evaluate Ekman transport. The extended method applied to reanalysis data sets shows that the ENSO period lengthens by approximately 30%, reconciling the discrepancy between the observed ENSO frequency and estimates from the conventional method. This result demonstrates that the self‐inhibitory process by Ekman transport is an inherent feature of the observed ENSO periodicity. In addition, Ekman transport efficiently works with a wide meridional structure of ENSO, suggesting that the meridional width of individual ENSO events explains the difference in their duration. Plain Language Summary: The dominant interannual climate variability, El Niño‐Southern Oscillation (ENSO), is an oscillatory phenomenon accompanying anomalous sea surface temperature in the eastern equatorial Pacific. ENSO drives extreme weather all over the world; therefore, understanding its periodic nature is important. Previous studies have developed a powerful tool, called the Wyrtki period index, to diagnose ENSO periodicity based on the known ENSO dynamics; however, it has a systematic bias that leads to a shorter ENSO period than those observed. Here, we extended the Wyrtki period index to include the contribution of the surface Ekman transport explicitly, which retards the ENSO phase reversal. Our diagnostics show that the ENSO period is approximately 30% longer than that of the conventional method and reproduces the observed ENSO spectrum well. Key Points: Equatorial wind stress pattern associated with El Niño‐Southern Oscillation (ENSO) drives the Ekman transport, which acts to slow down the ENSO recharge‐discharge processWe extended a theoretical ENSO period index to demonstrate the role of meridional Ekman transport that lengthens the ENSO periodicityEkman transport contributes to lengthening the ENSO period by 30% relative to the estimation by geostrophic transport alone [ABSTRACT FROM AUTHOR]

A submesoscale‐permitting global ocean simulation is used to study the upper ocean turbulence in high kinetic energy (KE) regions. Submesoscale processes peak in winter so that the geostrophic KE spectra tend to be relatively shallow in winter (∼k−2) with steeper spectra in summer (∼k−3). This transition in KE spectral scaling has two phases. In the first phase (late autumn), KE spectra show the presence of two spectral regimes: ∼k−3 power‐law in mesoscales and ∼k−2 power‐law in submesoscales. The first phase appears with the onset of mixed‐layer instabilities, which convert available potential energy into KE, and this process results in a flattening of KE spectra at submesoscales. However, KE spectra at longer wavelengths follow ∼k−3 scaling associated with a forward enstrophy transfer. In the second phase (late winter), KE produced through mixed‐layer instabilities is transferred to larger scales, and k−2 power‐law also develops in mesoscales. Plain Language Summary: Mesoscale and submesoscale motions are important for heat and material transport in the oceans. In the upper ocean, submesoscale turbulence shows seasonal variability and is pronounced in winter. The same distinction is reflected in horizontal wavenumber spectra of kinetic energy (KE) and spectral energy fluxes. In this study, geostrophic KE spectra are analyzed in a submesoscale‐permitting global ocean model to study the seasonal variability in the upper ocean turbulence. We interpret the results in terms of different physical mechanisms and their effects on the evolution of KE spectra over an annual cycle. We find that both mesoscale and submesoscale processes contribute to their characterization. Key Points: Seasonality in the upper ocean turbulence is interpreted in terms of interior baroclinic instabilities and mixed‐layer instabilitiesTemporal variability in the geostrophic kinetic energy spectrum can be subdivided into distinct seasonal phasesBoth a forward enstrophy transfer and an inverse kinetic energy transfer control the spectral slope in the kinetic energy spectrum [ABSTRACT FROM AUTHOR]

Determining the long‐term nature of the Atlantic Meridional Overturning Circulation (AMOC) presents a major step in understanding ocean temperature forcing, and is crucial both for placing the recently observed AMOC slowdown in the context of climate change and for predicting future climate. We present a time series for AMOC strength over the last 120 years which is derived solely from observations. Application of the Bernoulli inverse to hydrography yields the general geostrophic circulation in the North Atlantic without requiring additional sea surface height information, allowing AMOC evaluation in the pre‐satellite era. AMOC varies on a multidecadal timescale, leading the Atlantic Multidecadal Variability (AMV) in sea surface temperature by 2.5 years. We consider the dynamics of the implied AMOC/AMV coupling, and hypothesize that the low‐frequency variability in both parameters is driven by large‐scale density anomalies circulating in the AMOC. Plain Language Summary: The large‐scale ocean currents which influence the Earth's climate system are driven in large part by gradients in the sea surface. However, sea surface height measurements are scarce before 1993, when satellite observations began, limiting our knowledge of ocean currents over climate‐relevant time scales. Here, we use a method for determining sea surface height which does not require satellite data. We can therefore estimate the current strength in the North Atlantic over the last 120 years. We find that changes in the current strength are closely tied to changes in sea surface temperature. Key Points: Atlantic Meridional Overturning Circulation (AMOC) variability leads Atlantic Multidecadal Variability (AMV) by 2.5 yearsAMV is strongly coupled to ocean dynamicsAMOC variability associated with circulation of large‐scale density anomalies [ABSTRACT FROM AUTHOR]

Tropical cyclones (TC), which are among the most destructive natural phenomena on Earth, have significant impacts on the western boundary current (WBC). We quantify the direct impact of TCs on the surface geostrophic velocity of the Kuroshio, the WBC in the western North Pacific, by analyzing satellite‐derived geostrophic current data and using results from theoretical and numerical models. The study reveals that TCs decrease the surface geostrophic velocity by 14 cm s−1 which is 16% of the mean velocity of the Kuroshio (85 cm s−1), with the reduced velocity maintained for a month. The decrease in velocity of the Kuroshio is mainly (87%) due to the effect of TC‐driven upwelling ("Cooling effect"), the effect of ocean heat uptake after the TC ("Warming effect") being minor (13%); these are basically enabled by the strong thermal gradient around the Kuroshio. Plain Language Summary: The tropical cyclone (TC), also known as hurricane or typhoon, has a strong impact on the ocean as well as the land. In this study, we investigate how TCs affect ocean currents, with special focus on the Kuroshio, the western boundary current (WBC) in the North Pacific region. By analyzing satellite‐derived data, and building theoretical and numerical models, we deduce that when a TC hits the Kuroshio, its surface velocity decreases by 16%. This work makes a significant contribution by providing a quantitative understanding of the influence of TCs on the WBC. Further, given that the WBC is the strongest ocean current system on Earth and plays a key role in maintaining global heat balance, this study emphasizes the potential importance of the TC‐induced change on the current for the global distribution of heat energy. Key Points: Tropical cyclones (TC) decrease the surface geostrophic velocity of the Kuroshio by 14 cm s−1 (16% of the mean velocity)"Cooling Effect" of the TC‐driven upwelling explains 87% of the geostrophic velocity decrease immediately after a TC"Warming Effect" of the ocean heat uptake explains 13% of the geostrophic velocity decrease, which is maintained for ∼30 days after a TC [ABSTRACT FROM AUTHOR]

Given the importance of coastal upwelling in the California Current System (CCS), there is a considerable interest in predicting its response to global warming. However, upwelling changes are often treated as synonymous with changes in upwelling‐favorable winds, while the role of geostrophic transport is unaccounted for. Here, we examine the respective roles of Ekman and geostrophic transports using the Community Earth System Model Large Ensemble. In some parts of the CCS, the contribution of geostrophic transport to long‐term changes in upwelling is equal or greater than the contribution from Ekman transport. The combination of the two transports nearly close the momentum budget, and thus reproduce the mean state, interannual variability, and long‐term changes in upwelling. These results highlight the importance of accounting for ocean circulation when quantifying upwelling and its variability and change. Plain Language Summary: The California Current System (CCS) is an eastern boundary current system along the west coast of North America. The CCS hosts a diverse marine ecosystem and a high level of production for commercially valuable fish. Upwelling delivers deep, nutrient‐rich water to the sunlit surface, which provides nutrients that stimulate phytoplankton growth, fueling the marine ecosystem. Thus, potential future changes in coastal upwelling are of concern for the health of the CCS ecosystem, the livelihood of fishers, and food security. To date, projections of upwelling change have focused primarily on modifications of the wind‐driven (Ekman) transport. However, a second component of the ocean circulation, the geostrophic transport, can also impact coastal upwelling. In this study, we investigate the roles of both transport components in coastal upwelling change under global warming. We find that changes in the geostrophic currents, which have been neglected in many previous studies, make a substantial contribution to overall changes in the CCS coastal upwelling. Key Points: Changes in alongshore wind stress are insufficient to obtain overall changes in the California Current System (CCS) upwelling in the CESM1 large ensembleChanges in geostrophic transport make a substantial contribution to changes in the CCS upwellingA combination of changes in Ekman and geostrophic transports reproduces overall changes in the CCS upwelling [ABSTRACT FROM AUTHOR]

In the California Current System (CCS), the nearshore environment experiences natural exposure to low pH and reduced oxygen in response to coastal upwelling. Anthropogenic impacts further decrease pH and oxygen below biological thresholds, making the CCS particularly vulnerable to ocean acidification and hypoxia. Results from a coupled physical‐biogeochemical model reveal a strongly heterogeneous alongshore pattern of nearshore pH and oxygen in the central CCS, both in their long‐term means and trends. This spatial structuring is explained by an interplay between alongshore variability in local upwelling intensity and subsequent primary production, modulated by nearshore advection and regional geostrophic currents. The model solution suggests that the progression of ocean acidification and hypoxia will not be spatially homogeneous, thereby highlighting the need to consider subregional processes when assessing natural and anthropogenic impacts on coastal ecosystems in eastern boundary current upwelling regions. Plain Language Summary: Ocean acidification and hypoxia result from decreasing pH and oxygen levels in the ocean and threaten marine organisms adapted to a specific range of conditions. Along the west coast of the United States, these impacts are particularly severe, because this region exhibits naturally lower pH and oxygen conditions. Our study uses a computer model to show that upwelling, a process in which deeper water (rich in nutrients but also low in pH and oxygen) is brought to the surface, and photosynthesis by phytoplankton, which raises pH and oxygen, are important processes controlling nearshore pH and oxygen at the surface of the ocean. The results also indicate that these processes are not uniform in space or time and are shaped by alongshore changes in the intensity of upwelling and the strength and direction of ocean currents. A key finding from our study is that threats faced by marine organisms from the future progression of ocean acidification and hypoxia along the west coast of the United States will greatly vary by location. Key Points: Alongshore pH and oxygen variability is driven by local upwelling intensity and primary production, modulated by the regional circulationLocal "hotspots" of low pH and oxygen occur directly downstream of enhanced upwelling centersSubregional variability must be accounted for when determining natural and anthropogenic impacts on coastal upwelling ecosystems [ABSTRACT FROM AUTHOR]

Glaciers around the world are shrinking, yet in a region in northwestern High Mountain Asia (HMA), glaciers show growth. A proposed explanation for this anomalous behavior is related to the variability of the "Western Tibetan Vortex" (WTV), which correlates well with near‐surface temperatures in northwestern HMA. Using analytical formulations and ERA5 reanalysis data, we show that the WTV is the change of wind field resulting from changes in near‐surface temperature gradients in geostrophic flow and that it is not unique to northwestern HMA. Instead, we argue that net radiation is likely the main driver of near‐surface temperatures in Western HMA in summer and autumn. The decreasing strength of the WTV during summer in the twentieth century is thus likely the result of decreasing net radiation. We do argue that the WTV is a useful concept that could yield insights in other regions as well. Plain Language Summary: Glaciers are growing in a part of High Mountain Asia (HMA), contrary to the demise of glaciers worldwide. A proposed explanation for this behavior is the decreasing strength of the "Western Tibetan Vortex" (WTV), a circular motion of air in the troposphere around northwestern HMA, which is proposed to drive near‐surface temperatures. Here, we show that the WTV is the change of wind field resulting from changes in near‐surface temperature, and that it is not unique to northwestern HMA, but is generally applicable to large parts of the globe. Instead, we argue that net radiation is likely the main driver of near‐surface temperatures in Western HMA in summer and autumn and that the WTV is the response of the atmosphere to changes in temperature. The decreasing strength of the WTV, as seen during summer in the twentieth century, is thus likely the result of changing net radiation and not the main driver of cooling itself. We do argue that the WTV is a useful concept to understand large‐scale climate variability in the region and that such an approach could yield important insights in other midlatitude regions as well. Key Points: The Western Tibetan Vortex is the result of temperature changes in northwestern High Mountain AsiaIn summer and autumn, the main driver of near‐surface temperatures in northwestern High Mountain Asia is net radiationThe concept behind the Western Tibetan Vortex can also be applied to other regions [ABSTRACT FROM AUTHOR]

The recent dramatic decline of sea ice in the western Arctic Ocean changes the transfer of momentum across the ice‐ocean boundary layer. The surface stress energy input through the surface geostrophic current in the Beaufort Gyre (BG) based on a numerical model is 0.03 mW/m2 in 1992–2004 versus 0.23 mW/m2 in 2005–2017. This energy input is primarily concentrated over the southern Canada Basin and the Chukchi Sea. It is 1.38 × 1016 J in observations versus 4.90 × 1016 J in the model in the BG during 2003–2014. We find that some well‐known freshwater changes in the BG over 1992–2017 resulted from episodic extrema of energy input in 2007, 2012, and 2016. In particular, most of the energy input in 2007 was transformed into potential energy (57%) which resulted in a new state of freshwater budget. Our study suggests that as of 2016, the BG had not yet reached a saturated freshwater state. Our results provide a way to predict the future changes of BG freshwater content. Key Points: 26 years of surface stress energy input (SSE) are evaluated from observations and a model in the western Arctic Ocean for the first timeThe episodic events of SSE in 2007, 2012 and 2016 triggered step changes of freshwater content in 2007–08, 2012–13 and 2016–17, respectivelyEnergy budget analysis reveals that most of the SSE is transformed into potential energy which results in new freshwater state [ABSTRACT FROM AUTHOR]

The Hudson Bay Complex is the outlet for many Canadian rivers, receiving roughly 900 km3/year of river runoff. Historically, studies found a consistent cyclonic flow year‐round in Hudson Bay, due to the geostrophic boundary current induced by river discharge and cyclonic wind forcing that was supported by available observations at that time. Using a high‐resolution ocean general circulation model, we show that in summer, the mean circulation is not cyclonic but consists of multiple small cyclonic and anticyclonic features, with the mean flow directed through the center of the bay. Absolute Dynamic Topography and velocity observations also show this seasonal flow pattern. We find that this summer circulation is driven by geostrophic currents, generated by steric height gradients, which are induced by increased river discharge during the spring freshet, and reinforced by anticyclonic seasonal wind patterns. Plain Language Summary: Knowing the direction of the currents, as well as speed, in Hudson Bay is important for understanding regional ocean flow patterns. Currents in Hudson Bay were historically thought to flow counterclockwise. We use a state‐of‐the‐art computer simulation to see if this flow pattern changes during the year. We found that in May and June, the flow pattern in eastern Hudson Bay reverses and the currents flow in a clockwise direction, while currents in western Hudson Bay still flow counterclockwise. Satellite measurements of ocean currents also show this switch in the current direction in eastern Hudson Bay. This reversal of the current direction occurs because of a change in the wind direction as well as the amount of river water entering the bay during spring (May–June). In spring, river water flowing into the bay increases because of snow melt. More river water enters southeastern Hudson Bay, in and around James Bay, causing higher sea levels in the east compared to the west. This water will flow from high to low (east to west), causing the clockwise flow pattern in summer. Key Points: The spring freshet increases the halosteric height in eastern Hudson Bay, generating a seasonal flowThis anticyclonic seasonal flow pattern is present in observations and the ocean model simulationGeostrophic flow that is reinforced by atmospheric forcing drives this springtime flow pattern [ABSTRACT FROM AUTHOR]

Abstract: A physical‐biogeochemical model is used to produce a retrospective analysis at 3‐km resolution of alongshore phytoplankton variability in the California Current during 1988–2010. The simulation benefits from downscaling a regional circulation reanalysis, which provides improved physical ocean state estimates in the high‐resolution domain. The emerging pattern is one of local upwelling intensification in response to increased alongshore wind stress in the lee of capes, modulated by alongshore meanders in the geostrophic circulation. While stronger upwelling occurs near most major topographic features, substantial increases in phytoplankton biomass only ensue where local circulation patterns are conducive to on‐shelf retention of upwelled nutrients. Locations of peak nutrient delivery and chlorophyll accumulation also exhibit interannual variability and trends noticeably larger than the surrounding shelf regions, thereby suggesting that long‐term planktonic ecosystem response in the California Current exhibits a significant local scale (O(100 km)) alongshore component. [ABSTRACT FROM AUTHOR]

Although various numerical experiments have been carried out to investigate the cascade of internal tide energy associated with parametric subharmonic instability (PSI), most of them assume idealized oceanic situations, ignoring the existence of background relative vorticity (ζ). Here we show, for the first time, that the existence of ζ can significantly modify the behavior of PSI by changing the local effective Coriolis frequency (feff=f+ζ2). A set of numerical experiments for 25.4°N show that the PSI efficiency significantly increases at the position where positive ζ shifts feff close to the M1 critical frequency (ω = ωM1). Furthermore, numerical experiments for 31°N, poleward of the M1 critical latitude, show that significant PSI energy transfer is possible when negative ζ is low enough. The presented results provide important implications for understanding the spatial distribution of diapycnal mixing in the ocean where background geostrophic currents prevail. Plain Language Summary: It is well known that there exists latitude‐dependent mechanism for energy transfer from tides to turbulent processes in the deep ocean. Actually, there is a critical latitude poleward of which this physical mechanism no longer works. By carrying out numerical experiments, we show that the horizontal shear associated with a realistic strong geostrophic current, such as the Kuroshio, can shift this critical latitude of energy transfer process by more than several degrees of latitude. As a result, in the proximity of such strong current, this energy transfer process works even poleward of the M1 critical latitude. This fact is crucial in determining global distribution of deep ocean mixing intensity, which plays a key role in controlling large‐scale ocean circulation and hence global climate change. Key Points: The relative vorticity field caused by the background geostrophic current shear affects parametric subharmonic instability (PSI)It is numerically shown that PSI can occur even poleward of the critical latitudes due to the background negative relative vorticityThe understanding of the modified PSI may facilitate the interpretation of the global distribution of turbulent mixing in the deep ocean [ABSTRACT FROM AUTHOR]