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Shota Katsura, Janet Sprintall, J. Thomas Farrar, Dongxiao Zhang, and Meghan F. Cronin

Abstract

Formation and evolution of barrier layers (BLs) and associated temperature inversions (TIs) were investigated using a 1-yr time series of oceanic and air–sea surface observations from three moorings deployed in the eastern Pacific fresh pool. BL thickness and TI amplitude showed a seasonality with maxima in boreal summer and autumn when BLs were persistently present. Mixed layer salinity (MLS) and mixed layer temperature (MLT) budgets were constructed to investigate the formation mechanism of BLs and TIs. The MLS budget showed that BLs were initially formed in response to horizontal advection of freshwater in boreal summer and then primarily maintained by precipitation. The MLT budget revealed that penetration of shortwave radiation through the mixed layer base is the dominant contributor to TI formation through subsurface warming. Geostrophic advection is a secondary contributor to TI formation through surface cooling. When the BL exists, the cooling effect from entrainment and the warming effect from detrainment are both significantly reduced. In addition, when the BL is associated with the presence of a TI, entrainment works to warm the mixed layer. The presence of BLs makes the shallower mixed layer more sensitive to surface heat and freshwater fluxes, acting to enhance the formation of TIs that increase the subsurface warming via shortwave penetration.

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Samuel M. Kelly and Sebastine Ogbuka

Abstract

Coastal trapped waves (CTWs) transport energy along coastlines and drive coastal currents and upwelling. CTW modes are nonorthogonal when frequency is treated as the eigenvalue, preventing the separation of modal energy fluxes and quantification of longshore topographic scattering. Here, CTW modes are shown to be orthogonal with respect to energy flux (but not energy) when the longshore wavenumber is the eigenvalue. The modal evolution equation is a simple harmonic oscillator forced by longshore bathymetric variability, where downstream distance is treated like time. The energy equation includes an expression for modal topographic scattering. The eigenvalue problem is carefully discretized to produce numerically orthogonal modes, allowing CTW amplitudes, energy fluxes, and generation to be precisely quantified in numerical simulations. First, a spatially uniform K1 longshore velocity is applied to a continental slope with a Gaussian bump in the coastline. Mode-1 CTW generation increases quadratically with the amplitude of the bump and is maximum when the bump’s length of coastline matches the natural wavelength of the CTW mode, as predicted by theory. Next, a realistic K1 barotropic tide is applied to the Oregon coast. The forcing generates mode-1 and mode-2 CTWs with energy fluxes of 6 and 2 MW, respectively, which are much smaller than the 80 MW of M2 internal-tide generation in this region. CTWs also produce 1-cm sea surface displacements along the coast, potentially complicating the interpretation of future satellite altimetry. Prospects and challenges for quantifying the global geography of CTWs are discussed.

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Lisa Maillard, Julien Boucharel, and Lionel Renault

Abstract

Tropical instability waves (TIWs) are oceanic features propagating westward along the northern front of the Pacific cold tongue. Observational and modeling studies suggest that TIWs may have a large impact on the eastern tropical Pacific background state from seasonal to interannual time scales through heat advection and mixing. However, observations are coarse or limited to surface data, and modeling studies are often based on the comparison of low- versus high-resolution simulations. In this study, we perform a set of regional high-resolution ocean simulations (CROCO 1/12°) in which we strongly damp (NOTIWs-RUN) or not (TIWs-RUN) TIW propagation, by nudging meridional current velocities in the TIW region toward their monthly climatological values. This approach, while effectively removing TIW mesoscale activity, does not alter the model internal physics in particular related to the equatorial Kelvin wave dynamics. The impact of TIWs on the oceanic mean state is then assessed by comparing the two simulations. While the well-known direct effect of TIW heat advection is to weaken the meridional temperature gradient by warming up the cold tongue (0.34°C month−1), the rectified effect of TIWs onto the mean state attenuates this direct effect by cooling down the cold tongue (−0.10°C month−1). This rectified effect occurs through the TIW-induced deepening and weakening of the Equatorial Undercurrent, which subsequently modulates the mean zonal advection and counterbalances the TIWs’ direct effect. This approach allows quantifying the rectified effect of TIWs without degrading the model horizontal resolution and may lead to a better characterization of the eastern tropical Pacific mean state and to the development of TIW parameterizations in Earth system models.

Significance Statement

Tropical instability waves (TIWs), meandering features at the surface of the equatorial Pacific Ocean, have long been recognized as a key component of the climate system that can even impact marine ecosystems. Yet, they are still hardly simulated in coupled global climate models. Here, we introduce a new framework to isolate and quantify their complex influence on the tropical Pacific background climate. This approach allows revealing a so far overlooked effect of TIWs on the mean circulation and heat transport in this region that should be accounted for in the next generation of global coupled climate models through parameterization or increased resolution.

Open access
Qi Li, Zhaohui Chen, Shoude Guan, Haiyuan Yang, Zhao Jing, Yongzheng Liu, Bingrong Sun, and Lixin Wu

Abstract

Shipboard observations of upper-ocean current, temperature–salinity, and turbulent dissipation rate were used to study near-inertial waves (NIWs) and turbulent diapycnal mixing in a cold-core eddy (CE) and warm-core eddy (WE) in the Kuroshio Extension (KE) region. The two eddies shed from the KE were energetic, with the maximum velocity exceeding 1 m s−1 and relative vorticity magnitude as high as 0.6f. The mode regression method was proposed to extract NIWs from the shipboard ADCP velocities. The NIW amplitudes were 0.15 and 0.3 m s−1 in the CE and WE, respectively, and their constant phase lines were nearly slanted along the heaving isopycnals. In the WE, the NIWs were trapped in the negative vorticity core and amplified at the eddy base (at 350–650 m), which was consistent with the “inertial chimney” effect documented in existing literature. Outstanding NIWs in the background wavefield were also observed inside the positive vorticity core of the CE, despite their lower strength and shallower residence (above 350 m) compared to the counterparts in the WE. Particularly, the near-inertial kinetic energy efficiently propagated downward and amplified below the surface layer in both eddies, leading to an elevated turbulent dissipation rate of up to 10−7 W kg−1. In addition, bidirectional energy exchanges between the NIWs and mesoscale balanced flow occurred during NIWs’ downward propagation. The present study provides observational evidence for the enhanced downward NIW propagation by mesoscale eddies, which has significant implications for parameterizing the wind-driven diapycnal mixing in the eddying ocean.

Significance Statement

We provide observational evidence for the downward propagation of near-inertial waves enhanced by mesoscale eddies. This is significant because the down-taking of wind energy by the near-inertial waves is an important energy source for turbulent mixing in the interior ocean, which is essential to the shaping of ocean circulation and climate. The anticyclonic eddies are widely regarded as a conduit for the downward near-inertial energy propagation, while the cyclonic eddies activity influencing the near-inertial waves propagation lacks clear cognition. In this study, enhanced near-inertial waves and turbulent dissipation were observed inside both cyclonic and anticyclonic eddies in the Kuroshio Extension region, which has significant implications for improving the parameterization of turbulent mixing in ocean circulation and climate models.

Open access
Yiming Guo, Stuart Bishop, Frank Bryan, and Scott Bachman

Abstract

We use an interannually forced version of the Parallel Ocean Program, configured to resolve mesoscale eddies, to close the global eddy potential energy (EPE) budget associated with temperature variability. By closing the EPE budget, we are able to properly investigate the role of diabatic processes in modulating mesoscale energetics in the context of other processes driving eddy–mean flow interactions. A Helmholtz decomposition of the eddy heat flux field into divergent and rotational components is applied to estimate the baroclinic conversion from mean to eddy potential energy. In doing so, an approximate two-way balance between the “divergent” baroclinic conversion and upgradient vertical eddy heat fluxes in the ocean interior is revealed, in accordance with baroclinic instability and the relaxation of isopycnal slopes. However, in the mixed layer, the EPE budget is greatly modulated by diabatic mixing, with air–sea interactions and interior diffusion playing comparable roles. Globally, this accounts for ∼60% of EPE converted to EKE (eddy kinetic energy), with the remainder being dissipated by air–sea interactions and interior mixing. A seasonal composite of baroclinic energy conversions shows that the strongest EPE to EKE conversion occurs during the summer in both hemispheres. The seasonally varying diabatic processes in the upper ocean are further shown to be closely linked to this EPE–EKE conversion seasonality, but with a lead. The peak energy dissipation through vertical mixing occurs ahead of the minimum EKE generation by 1–2 months.

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Jiabi Du, Weifeng (Gordon) Zhang, and Yizhen Li

Abstract

Intruding slope water is a major source of nutrients to sustain the high biological productivity in the Gulf of Maine (GoM). Slope water intrusion into the GoM is affected by Gulf Stream warm-core rings (WCRs) impinging onto the nearby shelf edge. This study combines long-term mooring measurements, satellite remote sensing data, an idealized numerical ocean model, and a linear coastal-trapped wave (CTW) model to examine the impact of WCRs on slope water intrusion into the GoM through the Northeast Channel. Analysis of satellite sea surface height and temperature data shows that the slope sea region off the GoM is a hotspot of ring activities. A significant linear relationship is found between interannual variations of ring activities in the slope sea region off the GoM and bottom salinity at the Northeast Channel, suggesting the importance of WCRs in modulating variability of intruding slope water. Analysis of the mooring data reveals enhanced slope water intrusion through bottom-intensified along-channel flow following impingements of WCRs on the nearby shelf edge. Numerical simulations qualitatively reproduce the observed WCR impingement processes and associated episodic enhancement of slope water intrusion in the Northeast Channel. Diagnosis of the model result indicates that baroclinic CTWs excited by the ring–topography interaction are responsible for the episodically intensified subsurface along-channel inflow, which carries more slope water into the GoM. A WCR that impinges onto the shelf edge to the northeast of the Northeast Channel tends to generate stronger CTWs and cause stronger enhancement of the slope water intrusion into the GoM.

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Igor Kamenkovich and Zulema Garraffo

Abstract

The Atlantic meridional overturning circulation (AMOC) plays a key role in climate due to uptake and redistribution of heat and carbon anomalies. This redistribution takes place along several main pathways that link the high-latitude North Atlantic with midlatitudes and the Southern Ocean and involves currents on a wide range of spatial scales. This numerical study examines the importance of mesoscale currents (“eddies”) in these AMOC pathways and associated time scales, using a highly efficient offline tracer model. The study uses two boundary impulse response (BIR) tracers, which can quantify the importance of the Atlantic tracer exchanges with the high-latitude atmosphere in the north and with the Southern Ocean in the south. The results demonstrate that mesoscale advection leads to an increase in the overall BIR inventory during the first 100 years and results in a more efficient and spatially uniform ventilation of the deep Atlantic. Mesoscale currents also facilitate meridional spreading of the BIR tracer and thus assist the large-scale advection. The results point toward the importance of spatial inhomogeneity and anisotropy of the eddy-induced mixing in several mixing “hotspots,” as revealed by an eddy diffusivity tensor. Conclusions can be expected to assist evaluations of eddy-permitting simulations that stop short of full resolution of mesoscale, as well as development of eddy parameterization schemes.

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Lauren Hoffman, Matthew R. Mazloff, Sarah T. Gille, Donata Giglio, and Aniruddh Varadarajan

Abstract

Atmospheric rivers (ARs) result in precipitation over land and ocean. Rainfall on the ocean can generate a buoyant layer of freshwater that impacts exchanges between the surface and the mixed layer. These “fresh lenses” are important for weather and climate because they may impact the ocean stratification at all time scales. Here we use in situ ocean data, collocated with AR events, and a one-dimensional configuration of a general circulation model, to investigate the impact of AR precipitation on surface ocean salinity in the California Current System (CCS) on seasonal and event-based time scales. We find that at coastal and onshore locations the CCS freshens through the rainy season due to AR events, and years with higher AR activity are associated with a stronger freshening signal. On shorter time scales, model simulations suggest that events characteristic of CCS ARs can produce salinity changes that are detectable by ocean instruments (≥0.01 psu). Here, the surface salinity change depends linearly on rain rate and inversely on wind speed. Higher wind speeds (U > 8 m s−1) induce mixing, distributing freshwater inputs to depths greater than 20 m. Lower wind speeds (U ≤ 8 m s−1) allow freshwater lenses to remain at the surface. Results suggest that local precipitation is important in setting the freshwater seasonal cycle of the CCS and that the formation of freshwater lenses should be considered for identifying impacts of atmospheric variability on the upper ocean in the CCS on weather event time scales.

Significance Statement

Atmospheric rivers produce large amounts of rainfall. The purpose of this study is to understand how this rain impacts the surface ocean in the California Current System on seasonal and event time scales. Our results show that a greater precipitation over the rainy season leads to a larger decrease in salinity over time. On shorter time scales, these atmospheric river precipitation events commonly produce a surface salinity response that is detectable by ocean instruments. This salinity response depends on the amount of rainfall and the wind speed. In general, higher wind speeds will cause the freshwater input from rain to mix deeper, while lower wind speeds will have reduced mixing, allowing a layer of freshwater to persist at the surface.

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Larry T. Gulliver and Timour Radko

Abstract

This study explores the dynamics of intense coherent vortices in large-scale vertically sheared flows. We develop an analytical theory for vortex propagation and validate it by a series of numerical simulations. Simulations are conducted using both stable and baroclinically unstable zonal background flows. We find that vortices in stable westward currents tend to adjust to an equilibrium state characterized by quasi-uniform zonal propagation. These vortices persist for long periods, during which they propagate thousands of kilometers from their points of origin. The adjustment tendency is realized to a much lesser extent in eastward background flows. These findings may help to explain the longevity of the observed oceanic vortices embedded in predominantly westward flows. Finally, we examine the influence of background mesoscale variability induced by baroclinic instability of large-scale flows on the propagation and persistence of isolated vortices.

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Jacob M. Steinberg, Sylvia T. Cole, Kyla Drushka, and Ryan P. Abernathey

Abstract

Oceanic mesoscale motions including eddies, meanders, fronts, and filaments comprise a dominant fraction of oceanic kinetic energy and contribute to the redistribution of tracers in the ocean such as heat, salt, and nutrients. This reservoir of mesoscale energy is regulated by the conversion of potential energy and transfers of kinetic energy across spatial scales. Whether and under what circumstances mesoscale turbulence precipitates forward or inverse cascades, and the rates of these cascades, remain difficult to directly observe and quantify despite their impacts on physical and biological processes. Here we use global observations to investigate the seasonality of surface kinetic energy and upper-ocean potential energy. We apply spatial filters to along-track satellite measurements of sea surface height to diagnose surface eddy kinetic energy across 60–300-km scales. A geographic and scale-dependent seasonal cycle appears throughout much of the midlatitudes, with eddy kinetic energy at scales less than 60 km peaking 1–4 months before that at 60–300-km scales. Spatial patterns in this lag align with geographic regions where an Argo-derived estimate of the conversion of potential to kinetic energy is seasonally varying. In midlatitudes, the conversion rate peaks 0–2 months prior to kinetic energy at scales less than 60 km. The consistent geographic patterns between the seasonality of potential energy conversion and kinetic energy across spatial scale provide observational evidence for the inverse cascade and demonstrate that some component of it is seasonally modulated. Implications for mesoscale parameterizations and numerical modeling are discussed.

Significance Statement

This study investigates the seasonality of upper-ocean potential and kinetic energy in the context of an inverse cascade, consisting of energy transfers to and through the mesoscale. Observations show a scale-dependent cycle in kinetic energy that coincides with temporal variability in mixed layer potential energy and progresses seasonally from smaller to larger scales. This pattern appears dominant over large regions of the ocean. Results are relevant to ocean and climate models, where a large fraction of ocean energy is often parameterized. A customizable code repository and dataset are provided to enable comparisons of model-based resolved and unresolved kinetic energy to observational equivalents. Implications result for a range of processes including mixed layer stratification and vertical structure of ocean currents.

Open access