Remote Winds Research Articles

Amundsen Sea circulation controls bottom upwelling and Antarctic Pine Island and Thwaites ice shelf melting

The Pine Island and Thwaites Ice Shelves (PIIS/TIS) in the Amundsen Sea are melting rapidly and impacting global sea levels. The thermocline depth (TD) variability, the interface between cold Winter Water and warm modified Circumpolar Deep Water (mCDW), at the PIIS/TIS front strongly correlates with basal melt rates, but the drivers of its interannual variability remain uncertain. Here, using an ocean model, we propose that the strength of the eastern Amundsen Sea on-shelf circulation primarily controls TD variability and consequent PIIS/TIS melt rates. The TD variability occurs because the on-shelf circulation meanders following the submarine glacial trough, creating vertical velocity through bottom Ekman dynamics. We suggest that a strong or weak ocean circulation, possibly linked to remote winds in the Bellingshausen Sea, generates corresponding changes in bottom Ekman convergence, which modulates mCDW upwelling and TD variability. We show that interannual variability of off-shelf zonal winds has a minor effect on ocean heat intrusion into PIIS/TIS cavities, contrary to the widely accepted concept.

Impact of local and remote winds on the surface waves in the coastal waters of eastern Arabian Sea

This research aims to provide analysis of wave propagation and the interaction between local and remote winds on surface waves in the coastal waters of the eastern Arabian Sea. The analysis is based on measurements from a buoy deployed at 23 m water depth along with the data from the ERA5 re-analysis. By employing lead-lag correlation analysis techniques, we identified the origin and propagation routes of waves in this region. The analysis identified the western Indian Ocean as the primary source of the waves. However, the impact of northerly systems such as Shamal and Makran winds cannot be ignored, as they play a prominent role during the pre- and post-monsoon seasons and weak southwest monsoon periods. Moreover, the study focuses on event-based analysis of tropical cyclone Tauktae based on cyclone's track, correlation analysis of its impact, and the spatial lag correlation of the associated wave system. The study builds upon existing knowledge concerning the generation and propagation of swells in the Indian Ocean. The findings of this research will contribute to a better understanding of wave dynamics in the eastern Arabian Sea and have implications for coastal engineering and marine operations in the region.

Wind-Driven Mechanisms for the Variations of the Pacific Equatorial Undercurrent Based on Adjoint Sensitivity Analysis

The maintenance and variation of the Pacific Equatorial Undercurrent (EUC) are thought to be controlled by the zonal pressure gradient force (ZPGF). However, a recent study found that a large-scale circulation associated with Rossby waves can also lead to EUC variation, implying that the structures and timing of the influential winds and the underlying wind-driven mechanisms need to be revisited. Here, we use the adjoint-sensitivity method to obtain the crucial winds that can most efficiently influence EUC variations. The obtained adjoint sensitivities (denoting the sensitive winds) are confined to 15°S–15°N and exhibit a funnel-shaped pattern with high symmetry about the equator. The remote winds, which occur 4 to 11 months prior, can lead to EUC variations at the basin scale; in contrast, the near-term winds (occurring not earlier than 4 months) lead only to local EUC variations. Accordingly, we find that wind-initiated equatorial Kelvin waves, equatorial Rossby waves, and the reflected waves at both the western and eastern boundaries superimpose onto each other to result in EUC variations. Specifically, when the travel time is longer than 4 months, the waves can form a negative-positive-negative sea surface height anomaly (SSHA) pattern between 15°S and 15°N in the central-eastern tropical Pacific, indicating the joint effect of both types of waves; they also form a positive SSHA in the western equatorial Pacific, indicating the dominance of the Kelvin wave therein. These mechanisms are complementary to the canonical ZPGF mechanism, which provide a clear theoretical basis for EUC variation studies.

Late 20th Century Indian Ocean Heat Content Gain Masked by Wind Forcing

AbstractRapid increases in upper 700‐m Indian Ocean heat content (IOHC) since the 2000s have focused attention on its role during the recent global surface warming hiatus. Here, we use ocean model simulations to assess distinct multidecadal IOHC variations since the 1960s and explore the relative contributions from wind stress and buoyancy forcing regionally and with depth. Multidecadal wind forcing counteracted IOHC increases due to buoyancy forcing from the 1960s to the 1990s. Wind and buoyancy forcing contribute positively since the mid‐2000s, accounting for the drastic IOHC change. Distinct timing and structure of upper ocean temperature changes in the eastern and western Indian Ocean are linked to the pathway how multidecadal wind forcing associated with the Interdecadal Pacific Oscillation is transmitted and affects IOHC through local and remote winds. Progressive shoaling of the equatorial thermocline—of importance for low‐frequency variations in Indian Ocean Dipole occurrence—appears to be dominated by multidecadal variations in wind forcing.

On the Predictability of Sea Surface Height around Palau

AbstractAs part of the Flow Encountering Abrupt Topography (FLEAT) program, an array of pressure-sensor equipped inverted echo sounders (PIESs) was deployed north of Palau where the westward-flowing North Equatorial Current encounters the southern end of the Kyushu–Palau Ridge in the tropical North Pacific. Capitalizing on concurrent observations from satellite altimetry, FLEAT Spray gliders, and shipboard hydrography, the PIESs’ 10-month duration hourly bottom pressure p and round-trip acoustic travel time τ records are used to examine the magnitude and predictability of sea level and pycnocline depth changes and to track signal propagations through the array. Sea level and pycnocline depth are found to vary in response to a range of ocean processes, with their magnitude and predictability strongly process dependent. Signals characterized here comprise the barotropic tides, semidiurnal and diurnal internal tides, southeastward-propagating superinertial waves, westward-propagating mesoscale eddies, and a strong signature of sea level increase and pycnocline deepening associated with the region’s relaxation from El Niño to La Niña conditions. The presence of a broad band of superinertial waves just above the inertial frequency was unexpected and the FLEAT observations and output from a numerical model suggest that these waves detected near Palau are forced by remote winds east of the Philippines. The PIES-based estimates of pycnocline displacement are found to have large uncertainties relative to overall variability in pycnocline depth, as localized deep current variations arising from interactions of the large-scale currents with the abrupt topography around Palau have significant travel time variability.

Circulation and Thermohaline Variability of the Hanna Shoal Region on the Northeastern Chukchi Sea Shelf

AbstractWe analyzed velocity and hydrographic data from 23 moorings in the northeast Chukchi Sea from 2011 to 2014. In most years the eastern side of Hanna Shoal was strongly stratified year‐round, while weakly stratified regions prevailed on the shelf south and west of the Shoal. Stratification differences cause differential vertical mixing rates, which in conjunction with advection of different bottom water properties resulted in seasonally varying along‐isobath density gradients. In agreement with numerical models, we find that bottom waters flow anticyclonically around the Shoal. Whereas most of the shelf responded barotropically to wind‐forcing, there was a strong baroclinic component to the flow field northeast of Hanna Shoal, resulting in no net vertically integrated transport on average. In contrast there is a net eastward transport from west of the Shoal, which implies convergence north of the Shoal. Convergence and along‐isobath density gradients may foster cross‐shelf exchange north of Hanna Shoal. Modal analyses indicate that the shelf south of the Shoal and Barrow Canyon responded coherently to local and remote winds, whereas the wind‐current response around Hanna Shoal was less coherent. Barotropic topographic waves, of ~3‐day period, were generated episodically northeast of the Shoal and propagate clockwise around Hanna Shoal, but are blocked from entering Barrow Canyon and are possibly scattered by the horizontally sheared flow and converging isobaths on the western side of the Shoal. Analysis of water properties on the western side of Hanna Shoal suggests that these include contributions from the western and southern portions of the Chukchi Sea.

Near-bottom currents at Station M in the abyssal Northeast Pacific

Deep-sea currents in the Northeast Pacific Ocean are examined using observations at Station M, located on the abyssal plain 220 km offshore of central California. Collected near the seabed at ~4000 m depth from October 2014–October 2018, the current meter observations supplement long-term investigations of deep-sea benthic ecology at this site. To investigate mechanisms for variability at different time scales, the observed currents are separated into low-frequency fluctuations with periods of 1.4–60 days, fluctuations that are coherent with astronomical tidal forcing, and other high-frequency fluctuations. These motions contribute similarly to the overall variability at this location. Low-frequency currents exhibit elevated energy at periods of ~30 days, consistent with previous observations elsewhere in the abyssal North Pacific. However, unlike some other observations in the deep ocean, there is no consistent seasonal cycle in the variability of low-frequency motion. The theoretical prediction of Sverdrup balance between transport and local wind stress curl does not explain the timing or the magnitude of low frequency currents, suggesting that they are driven by remote winds or eddy dynamics. The non-tidal high-frequency fluctuations show evidence of a near-inertial peak and clockwise rotary motion, consistent with theoretical expectations for the internal gravity wave spectrum. Like the low-frequency currents, interannual variability of internal wave energy appears to be stronger than seasonal variability. Observations of the vertical structure 2.5–17.5 m above bottom are used to characterize the interactions between currents and the seabed. Observational estimates of diurnal and semidiurnal tidal ellipses are consistent with results from an analytical model that incorporates the effects of the Earth's rotation and tidal acceleration. A numerical model of the time dependent boundary layer provides estimates of bottom roughness and friction velocity that are consistent with previous observations at this site. This study shows that the strength, time scale and polarization of the near-bottom currents at Station M vary significantly between years, demonstrating the value of multi-year records in studies of the lateral transport of organic matter, sediment and organisms at this abyssal site.

Subtidal Water Level and Current Variability in a Bar‐Built Estuary During Cold Front Season: Barataria Bay, Gulf of Mexico

AbstractSubtidal water level and current variability in Barataria Bay—a microtidal bar‐built estuary within the Mississippi Delta—and its relation to local and remote wind forcings were analyzed using three different methods: (1) statistical analysis of in situ observations, (2) an analytical model with idealized wind field, and (3) a barotropic numerical model. Remote wind effects (i.e., the coastal water levels imposed by Ekman transport) are dominant at forcing currents at the bay mouth and thus control estuary‐shelf exchanges. In contrast, subtidal water level variability, which ranges from 0.35 m at the bay mouth to 0.55 m at the bay head, is mainly associated with local wind effects, especially at the head of the bay. This occurs because the local wind‐induced water level tilting within the bay is larger than the coastal sea level pumping, i.e., the spatially uniform variations in water levels forced by remote winds. This result contrasts with (1) most coastal plain, fjord, and tectonic estuaries and (2) with other bar‐built estuaries in the region, in which remote wind effects dominate water level variability within the estuary. These differences are due to geomorphological features unique to Barataria Bay, such as its limited estuarine‐shelf connectivity and its orientation relative to local shoreline.

Saline water intrusion in relation to strong winds during winter cold outbreaks: North Branch of the Yangtze Estuary

The strong saline water intrusion in the North Branch of the Yangtze Estuary threatens the freshwater supply of the region in winter half year. Strong northerly winds have been identified as a factor increasing saline water intrusion. However, there are few studies on this subject, and the mechanisms of winds influencing saline water intrusion are still unclear. In the present contribution, we investigate the variation trend of strong wind events during cold outbreaks in winter half year and their correlation with saline water intrusion in the North Branch, together with the processes and mechanism of strong winds increasing saline water intrusion, based on observations and an analytical salt water intrusion model. The results indicate that the strong northerly and northeasterly wind events and saline water intrusion in the North Branch have similar variation trends in 1994–2008, both being relatively weak in the 1990s but being intensified dramatically after 1999. The significant correlation between these two trends suggests that the increase in strong wind events may be one of the factors inducing the enhanced saline water intrusion. Observations and model output show that the strong northerly and northeasterly winds can induce dramatic water level setup, increase of flood-tide current velocities, decrease of ebb-tide velocities, and decrease of freshwater inflow into the North Branch. These changes in combination cause the enhanced intensity of saline water intrusion. The Ekman transport from remote winds results in water level setup at the estuary mouth pumping more seawater into the North Branch, which should be a dominant mechanism inducing the change in hydrodynamics and increase of saline water intrusion.

Spatial variation of cold front wind-driven circulation and quasi-steady state balance in Lake Pontchartrain Estuary

Circulation in a low-salinity estuary with restricted openings to the ocean is mainly driven by wind and to a lesser extent by water level fluctuations at the open boundaries. Numerical model experiments are conducted using a baroclinic Finite Volume Community Ocean Model (FVCOM) for wind-driven circulations during a period encompassing a total of 16 cold fronts in the Lake Pontchartrain Estuary. The work is focused on the examination of the spatial structure of circulation in response to local and remote winds. A quasi-steady state balance between the surface slope and wind stress is further analyzed. The accuracy of the balance is evaluated by comparing the FVCOM calculated surface slope with that from the steady state balance equation. Results show that this balance is more accurate in the cross-estuary direction than that in the along-estuary direction (R2 ∼ 0.94 vs. 0.60). This difference in the accuracy of the quasi-steady state balance between the cross- and along-estuary directions is caused by the open boundary - a tidally-induced mean slope exists. In addition, even if the tidal effect is removed, the accuracy still decreases toward the open end for slopes in both directions. Remote wind effect and residual flow through the eastern open boundary tend to introduce a departure from the quasi-steady state balance in both along- and cross-estuary directions. Remote wind effect decreases into the interior due to bottom friction. Local wind effect tends to produce downwind flows in coastal, shallow water regions and on the surface, but upwind flows near the bottom, a result consistent with barotropic wind-driven circulations; while the remote wind effect is important mostly near the open boundary. Furthermore, quasi-steady state balance is more accurate in the along-estuary direction before cold front passages then after, because of the relatively higher occurrence of the wind in that direction before the cold fronts than after. In contrast, this quasi-steady state balance is less accurate in the cross estuary direction before cold front passages than after, because of the relatively lower occurrence of the wind in that direction before the cold fronts than after.

Causes of Interannual Variation of Summer Mean Alongshore Current Near the East Coast of Korea Derived From 16‐Year‐Long Observational Data

AbstractInterannual variation in the summer surface current near the east coast of Korea was analyzed using long‐term (~16 years) time series measurements. Significant interannual variations in the summer alongshore current, which have a magnitude comparable to that of the climatological summer mean (equatorward) current, were observed. Positive anomalies (more poleward) occurred during 2001, 2009, 2010, and 2014; negative anomalies (more equatorward) occurred during 2002, 2006, and 2012. A linear combination of equatorward propagating coastal‐trapped waves, forced primarily by remote winds off the Russian coast, and an upwelling/downwelling response to local wind forcing, though contribution of the latter is not as significant as that of the former, successfully reproduced (46%) the observed anomalies in the alongshore current (correlation coefficient of 0.68). The influence of cross‐shore movement of the poleward flowing offshore current explains 35% of the total variance in the interannual anomalies of the alongshore current (correlation coefficient of 0.59), yielding less equatorward or slight poleward current near the coast in 2001 and 2009 with onshore movement of the offshore current. The combined effects of wind and offshore current explain ~72% (correlation coefficient of 0.85) of the anomalies in the alongshore current observed during summer near the east coast of Korea, providing significant implications for interannual current dynamics near the midlatitude western boundary.

Depth‐Dependent Thermal Stress Around Corals in the Tropical Pacific Ocean

AbstractThermally driven bleaching events are a growing concern for reef ecosystems across the tropics. To assess and predict thermal stress impacts on reefs, remotely observed sea surface temperature (SST) commonly is used; however, reef communities typically extend to depths where SST alone may not be an accurate measure of in situ variability. Here nearly two decades of temperature observations (2‐ to 90‐m depth) at three stations around Palau are used to develop an empirical model of temperature variability versus depth based on SST and sea level anomaly (SLA). The technique yields depth‐averaged R2 values >0.88, with SLA predicting fore reef temperatures near the thermocline and SST capturing upper mixed layer temperatures. SLA complements SST by providing a proxy for vertical isotherm displacements driven by local and remote winds on intraseasonal to interannual time scales. Utilizing this concept, thermal stress on corals can be predicted from the surface through the mesophotic zone.

Impact of local and remote winds on the shelf circulation off the central west coast of India

A linear coastal-trapped-wave (CTW) model is used to examine the effects of large-scale winds, with time scale ranging from a few days to a few weeks, on the West India Coastal Current (WICC), particularly on the shelf off the central west coast of India. We show that unlike the seasonal cycle of WICC, which is primarily forced by the winds along the east coast of India, the high-frequency WICC is mostly driven by the west-coast winds. Nevertheless, the influence of winds as far as Sri Lanka and east coast of India cannot be neglected. Simple numerical experiments with the CTW model show that the strong current observed at Goa (15° N) compared to Bhatkal (13° N) and Jaigarh (17° N) is due to two factors: (1) the superposition of local and remote CTWs and (2) the widening of shelf width north of Goa, which decreases the amplitude of the currents poleward of Goa. If the local winds are weak, the amplitude of current decreases poleward due to friction, and the current at the south leads the north. We also note that the observed phase difference between sea level and alongshore current at Goa could be attributed to the propagation of remotely forced higher-order modes of CTWs.

Localized rapid warming of West Antarctic subsurface waters by remote winds

The highest rates of Antarctic glacial ice mass loss are occurring to the west of the Antarctica Peninsula in regions where warming of subsurface continental shelf waters is also largest. However, the physical mechanisms responsible for this warming remain unknown. Here we show how localized changes in coastal winds off East Antarctica can produce significant subsurface temperature anomalies (>2 °C) around much of the continent. We demonstrate how coastal-trapped barotropic Kelvin waves communicate the wind disturbance around the Antarctic coastline. The warming is focused on the western flank of the Antarctic Peninsula because the circulation induced by the coastal-trapped waves is intensified by the steep continental slope there, and because of the presence of pre-existing warm subsurface water offshore. The adjustment to the coastal-trapped waves shoals the subsurface isotherms and brings warm deep water upwards onto the continental shelf and closer to the coast. This result demonstrates the vulnerability of the West Antarctic region to a changing climate. The subsurface waters west of the Antarctic Peninsula are warming rapidly. This study shows that changes in coastal winds in East Antarctica are remotely impacting this region and drive the upwelling of warm deep water.

Rapid response to coastal upwelling in a semienclosed bay

AbstractBays/estuaries forced by local wind show bidirectional exchange flow. When forced by remote wind, they exhibit unidirectional flow adjustment to coastal sea level. Acoustic Doppler Current Profiler observations over 1 year show that the Ria de Vigo (Iberian Upwelling) responds to coastal wind events with bidirectional exchange flow. The duration of the upwelling and downwelling events, estimated from the current variability, was ~3.3 days and ~2.6 days, respectively. Vectorial correlations reveal a rapid response to upwelling/downwelling, in which currents lag local wind by <6 h and remote wind by <14 h, less than the Ekman spinup (17.8 h). This rapidity arises from the ria's narrowness (nonrotational local response), equatorward orientation (additive remote and local wind responses), depth greater than the Ekman depth (penetration of shelf circulation into the interior), and vertical stratification (shear reinforcing shelf circulation). Similar rapid responses are expected in other narrow bays where local and remote winds act together and stratification enhances bidirectional flow.

Remote forcing of subsurface currents and temperatures near the northern limit of the California Current System

AbstractLocal and remote wind forcing of upwelling along continental shelves of coastal upwelling regions play key roles in driving biogeochemical fluxes, including vertical net fluxes of carbon and nutrients. These fluxes are responsible for high primary productivity, which in turn supports a lucrative fishery in these regions. However, the relative contributions of local versus remote wind forcing are not well quantified or understood. We present results of coherence analyses between currents at a single mooring site (48.5°N, 126°W) in the northern portion of the California Current System (CalCS) from 1989 to 2008 and coincident time series of North America Regional Reanalysis (NARR) 10 m wind stress within the CalCS (36–54°N, 120–132°W). The two‐decade‐long current records from the three shallowest depths (35, 100, and 175 m) show a remote response to winds from south as far as 36°N. In contrast, only temperatures at the deepest depth (400 m) show strong coherences with remote winds. Weaker local wind influence is observed in both the currents and 400 m temperatures but is mostly due to the large spatial coherence within the wind field itself. Lack of coherence between distal winds and the 400 m currents suggests that the temperature variations at that depth are driven by vertical motion resulting from poleward travelling coastal trapped waves (CTWs). Understanding the effects of remote forcing in coastal upwelling regions is necessary for determining the occurrence and timing of extreme conditions in coastal oceans, and their subsequent impact on marine ecosystems.

Consequences of inhibition of mixed‐layer deepening by the West India Coastal Current for winter phytoplankton bloom in the northeastern Arabian Sea

AbstractThe intense winter phytoplankton bloom during November–February in the northeastern Arabian Sea (NEAS) was thought, until recently, to be controlled only by a convective deepening of the mixed layer (ML) owing to cool and dry northeasterlies. But a recent study has shown that the deepening of the ML in the southern NEAS is inhibited by the poleward advection of low‐salinity water from the south by the West India Coastal Current (WICC). Using an Ocean General Circulation Model coupled with an ecosystem model, we investigate the consequences of the inhibition of mixed‐layer deepening for winter phytoplankton bloom in the NEAS. We show that, during the winter monsoon, the shallow ML inhibits the entrainment of nutrients in the southern NEAS. Strong (weak) positive nitrate tendency in the northern (southern) NEAS seen in the model during the winter monsoon is maintained by strong (weak) entrainment. As a result, the chlorophyll integrated to 200 m depth from the surface is lower in the southern NEAS than in the northern NEAS. The inhibition of mixed‐layer deepening in the south affects the size‐based distribution of small and large phytoplankton, nutrient limitation terms and growth rate, and their elemental composition. The WICC, which inhibits the deepening of the ML and affects the winter bloom in the NEAS, is driven by coastal Kelvin waves generated by remote winds. This paper demonstrates a mechanism by which remotely forced coastal Kelvin waves impact the biology in the north Indian Ocean.

Coastal upwelling off the southwest coast of India: observations and simulations

ABSTRACTSpatial hydrographic data collected from August 2007 through May 2008 and the Princeton Ocean Model (POM) are utilized to explain and document the upwelling in the south eastern Arabian Sea (SEAS). The decrease in the magnitude of winds towards the coast favors local anti-cyclonic vorticity, resulting in the formation of cyclonic eddy and reversal of coastal currents. The Ekman transport due to alongshore winds, Ekman pumping due to wind stress curl, cyclonic eddy and southward West Indian Coastal Currents play different roles in the SEAS upwelling. In the offshore, wind stress curl leads to the formation of meso-scale eddies, resulting in Ekman pumping at the center and consequent upwelling. The rapid depth variation on the western side of Quilon Mount intensifies upwelling in the offshore. The upwelling Kelvin waves generated at the equator terminates in the Bay of Bengal and do not contribute to the SEAS upwelling. The possible role of local and remote winds, especially around Sri Lanka on the upwelling dynamics of SEAS is described utilizing POM. The Kelvin waves generated off the coast of Sri Lanka during the active phase of the summer monsoon also contribute to the upwelling dynamics off the west coast of India.

Erratum to: Inhibition of mixed-layer deepening during winter in the northeastern Arabian Sea by the West India Coastal Current

Though the deep mixed layers (MLs) that form in the northeastern Arabian Sea (NEAS) during the winter monsoon (November–February) have been attributed to convective mixing driven by dry, cool northeasterly winds from the Indian subcontinent, data show that the deepest MLs occur in the northern NEAS and the maxima of latent-heat and net heat fluxes in the southern NEAS. We use an oceanic general circulation model to show that the deep MLs in the NEAS extend up to ~20°N till the end of December, but are restricted poleward of ~22°N (~23°N) in January (February). This progressive restriction of the deep mixed layers within the NEAS is due to poleward advection of water of lower salinity by the West India Coastal Current (WICC). The deep MLs are sustained till February in the northern NEAS because convective mixing deepens the ML before the waters of lower salinity reach this region and the wind stirring and convective overturning generate sufficient turbulent energy for the ML to maintain the depth attained in January. Though the atmospheric fluxes tend to cool the ML in the southern NEAS, this cooling is countered by the warming due to horizontal advection. Likewise, the cooling due to entrainment, which continues in the southern NEAS even as the ML shallows during January–February, is almost cancelled by the warming caused by a downwelling vertical velocity field. Therefore, the SST changes very little during December–February even as the ML shallows dramatically in the southern NEAS. These deep MLs of the NEAS also preclude a strong intraseasonal response to the intraseasonal variability in the fluxes. This role of horizontal advection implies that the ML depth in the NEAS is determined by an interplay of physical processes that are forced differently. The convective mixing depends on processes that are local to the region, but the advection is due to the WICC, whose seasonal cycle is primarily forced by remote winds. By inhibiting the formation of deep MLs in the southern NEAS, the WICC limits the region of formation of the high-salinity water masses of this region. Since the deep MLs in the NEAS have been linked to the high chlorophyll concentration there, our results imply that the conventional approach of averaging over boxes for studying the impact of physics on biogeochemistry can mask important details that are due to advection because it is the advective component of any budget that is most affected by the averaging process.

Seasonal Variability of the Gulf Stream Kinetic Energy

AbstractThe seasonal variability of the mean kinetic energy (MKE) and eddy kinetic energy (EKE) of the Gulf Stream (GS) is examined using high-resolution regional ocean model simulations. A set of three numerical experiments with different surface wind and buoyancy forcing is analyzed to investigate the mechanisms governing the seasonal cycle of upper ocean energetics. In the GS along-coast region, MKE has a significant seasonal cycle that peaks in summer, while EKE has two comparable peaks in May and September near the surface; the May peak decays rapidly with depth. In the off-coast region, MKE has a weak seasonal cycle that peaks in summer, while EKE has a dominant peak in May and a secondary peak in September near the surface. The May peak also decays with depth leaving the September peak as the only seasonal signal below 100 m. An analysis of the three numerical experiments suggests that the seasonal variability in the local wind forcing significantly impacts the September peak of the along-coast EKE through a local-flow barotropic instability process. Alternatively, the seasonal buoyancy forcing primarily impacts the flow baroclinic instability and is consequently related to the May peak of the upper ocean EKE in both regions. The analysis results indicate that the seasonal cycle of the along-coast MKE is influenced by both local energy generation by wind and the advection of energy from upstream regions. Finally, the MKE cycle and the September peak of EKE in the off-coast region are mainly affected by advection of energy from remote regions, giving rise to correlations with the seasonal cycle of remote winds.