AbstractMixing along isopycnals plays an important role in the transport and uptake of oceanic tracers. Isopycnal mixing is commonly quantified by a tracer diffusivity. Previous studies have estimated the tracer diffusivity using the rate of dispersion of surface drifters, subsurface floats, or numerical particles advected by satellite‐derived velocity fields. This study shows that the diffusivity can be more efficiently estimated from the dispersion of coherent mesoscale eddies. Coherent eddies are identified and tracked as the persistent sea surface height extrema in both a two‐layer quasigeostrophic (QG) model and an idealized primitive equation (PE) model. The Lagrangian diffusivity is estimated using the tracks of these coherent eddies and compared to the diagnosed Eulerian diffusivity. It is found that the meridional coherent eddy diffusivity approaches a stable value within about 20–40 days in both models. In the QG model, the coherent eddy diffusivity is a good approximation to the upper‐layer tracer diffusivity in a broad range of flow regimes, except for small values of bottom friction or planetary vorticity gradient, where the motions of same‐sign eddies are correlated over long distances. In the PE model, the tracer diffusivity has a complicated vertical structure and the coherent eddy diffusivity is correlated with the tracer diffusivity at the e‐folding depth of the energy‐containing eddies where the intrinsic speed of the coherent eddies matches the rms eddy velocity. These results suggest that the oceanic tracer diffusivity at depth can be estimated from the movements of coherent mesoscale eddies, which are routinely tracked from satellite observations.

The vertical structure of ocean eddies is generally surface-intensified, commonly attributed to the dominant baroclinic modes arising from the boundary conditions (BCs). Conventional BC considerations mostly focus on either flat- or rough-bottom conditions. The impact of surface buoyancy anomalies—often represented by surface potential vorticity (PV) anomalies—has not been fully explored. Here, we study the role of the surface PV in setting the vertical distribution of eddy kinetic energy (EKE) in an idealized adiabatic ocean model driven by wind stress. The simulated EKE profile in the extra-tropical ocean tends to peak at the surface and have an e -folding depth typically smaller than half of the ocean depth. This vertical structure can be reasonably represented by a single surface quasi-geostrophic (SQG) mode at the energy-containing scale resulting from the large-scale PV structure. Due to isopycnal outcropping and interior PV homogenization, the surface meridional PV gradient is substantially stronger than the interior PV gradient, yielding surface-trapped baroclinically unstable modes with horizontal scales comparable to or smaller than the deformation radius. These surface-trapped eddies then grow in size both horizontally and vertically through an inverse energy cascade up to the energy-containing scale, which dominates the vertical distribution of EKE. As for smaller horizontal scales, the EKE distribution decays faster with depth. Guided by this interpretation, an SQG-based scale-aware parameterization of the EKE profile is proposed. Preliminary offline diagnosis of a high-resolution simulation shows the proposed scheme successfully reproducing the dependence of the vertical structure of EKE on the horizontal grid resolution.

In a recent paper [Chu (2023; Chu23)], the author formulated the equations governing atmospheric motion in a spheroidal coordinate system. Since the mass distribution of the Earth is not exactly spheroidal, the true gravity is not vertical in that coordinate system. Chu23 compared the magnitude of the static horizontal component of gravity in that system to those of the dynamically active forces and concluded that the horizontal components of gravity should not be neglected. In recent papers by the authors [Chang and Wolfe (2022; CW22) and Stewart and McWilliams (2022; CW22)], we explained that the actual interpretation of the approximation made in atmospheric and oceanic modeling is not neglecting the horizontal component of the true gravity, but is a geometrical approximation, approximating nearly spheroidal geopotential surfaces with bumps on which the true gravity is vertical by exactly spheroidal surfaces. We showed that under such an interpretation, the errors due to the geometrical approximation are small. Chu23 claimed that CW22 and SM22 erroneously neglected the gravity perturbations in their analyses. Here, we explain further the differences between these approaches, in the process showing that the criticisms of Chu23 on CW22 and SM22 are invalid, further supporting our conclusion that the horizontal component of the true gravity is not relevant in ocean and atmospheric dynamics. Physically, the reason why horizontal gravity is irrelevant in the coordinate system used by Chu23 is that it is balanced by a static horizontal pressure gradient force.

The relationship between Gulf Stream (GS) transport and coastal sea level is investigated using monthly GS transport between 1993 and 2019 at Florida Straits and 10 altimeter tracks. The results show that GS transport decorrelates quickly along its path, indicating it is misleading to assume that transport at a particular location represents strength of the GS as a whole. GS transport south of Cape Hatteras is significantly correlated with coastal sea level in South Atlantic Bight from both altimetry and tide gauges. North of Cape Hatteras, sea level changes associated with GS transport decay rapidly away from GS on the onshore side and become negligible approximately 300 km northwest of GS axis. In this region, the correlations between GS transport and sea level are primarily in the deep ocean and rarely on the shelf, indicating that coastal sea level is unlikely to be driven by geostrophic adjustment to changes in GS transport.

Abstract Isopycnal mixing of tracers is important for ocean dynamics and biogeochemistry. Previous studies have primarily focused on the horizontal structure of mixing, but what controls its vertical structure is still unclear. This study investigates the vertical structure of the isopycnal tracer diffusivity diagnosed by a multiple‐tracer inversion method in an idealized basin circulation model. The first two eigenvalues of the symmetric part of the 3D diffusivity tensor are approximately tangent to isopycnal surfaces. The isopycnal mixing is anisotropic, with principal directions of the large and small diffusivities generally oriented along and across the mean flow direction. The cross‐stream diffusivity can be reconstructed from the along‐stream diffusivity after accounting for suppression of mixing by the mean flow. In the circumpolar channel and the upper ocean in the gyres, the vertical structure of the along‐stream diffusivity follows that of the rms eddy velocity times a depth‐independent local energy‐containing scale estimated from the sea surface height. The diffusivity in the deep ocean in the gyres instead follows the profile of the eddy kinetic energy times a depth‐independent mixing time scale. The transition between the two mixing regimes is attributed to the dominance of nonlinear interactions and linear waves in the upper and deep ocean, respectively, distinguished by a nonlinearity parameter. A formula is proposed that accounts for both regimes and captures the vertical variation of diffusivities better than extant theories. These results inform efforts to parameterize the vertical structure of isopycnal mixing in coarse‐resolution ocean models.

The Gulf Stream (GS) is expected to slow and shift poleward over the next century due to climate change. We investigate whether such changes are already observable in the altimetric record (1993–2018) using along‐track altimetry. Decadal trends in latitude, speed, transport, and width are calculated in stream‐following coordinates to avoid spurious signals due to changes in higher‐frequency GS variability. Statistically significant trends are few and apparently randomly distributed. Further, small changes to the length of the record lead to large changes in the trends and their significance. These results suggest that the current observations are insufficient to detect significant trends in these metrics. If the trends continue at the current rate, detection of trends at more than half of the altimetry tracks would require 22–23 additional years of observations for latitude and transport and 44 additional years for speed.

Lagrangian methods have been used to estimate the lateral eddy diffusivity in the ocean using surface drifter and subsurface float tracks and using the numerical particles advected by satellite-derived velocity fields. The diffusivity is estimated from the rate of dispersion of these particles. Accurate point-wise estimates of diffusivity generally require averages over a large number of drifters or floats, but the distribution of drifters and floats is generally sparse and many tracks of drifters are contaminated by winds. On the other hand, the convergence time for the particle-based diffusivity is on the order of a month for both in situ and numerical particles, which makes the estimates inefficient and allows for the accumulation of measurement error. Studies of vortex-dominated 2D turbulence have found that particle dispersion is dominated by the movement of coherent eddies, and that the dispersion rate of coherent eddies themselves can provide accurate estimates of the Lagrangian diffusivity. We found that the potential vorticity diffusivity in two-layer quasigeostrophic turbulence can also be accurately estimated by the rate of dispersion of coherent eddies, and this estimate converges more than four times faster than the diffusivity estimated from particles inserted uniformly in the flow. If this result also holds for oceanic mesoscale turbulence, it can form the basis for a potentially useful technique for diagnosing mesoscale diffusivity based on the tracks of coherent mesoscale eddies.This presentation examines the relation between the dispersion of coherent eddies and tracer diffusivity in an idealized configuration of Massachusetts Institute of Technology general circulation model which contains multiple gyres, boundary currents, and a zonally reentrant channel flow analogous to the Antarctic Circumpolar Current. The coherent eddies are identified and tracked from the sea surface height snapshots, and the diffusivity estimated from coherent eddies is compared to the tracer diffusivity diagnosed by a tracer inversion method. The diffusivity inferred from dispersion of coherent eddies generally converges within 15 days. Direct comparison of two diffusivity estimates is not straightforward, since the tracer-based diffusivity varies vertically. Approaches for reconciling the two estimates are discussed. This study shows the possibility of relating the Lagrangian movement of coherent eddies to the Eulerian tracer diffusivity.

Previous work to find an association between variations of annually averaged Florida Current transport and the North Atlantic Oscillation (NAO) have yielded negative results (Meinen et al. 2010). Here we show that Florida current in winter is impacted by displacements in the positions of the Azores High and the Icelandic Low, the constituent pressure centers of the NAO. As a one-dimensional representation of North Atlantic atmospheric circulation, the NAO index does not distinguish displacements of the pressure centers from fluctuations in their intensity. Florida Current transport is significantly correlated with Icelandic Low longitude with a lag of less than one season. We carried out perturbation experiments in the ECCOv4 model to investigate these correlations. These experiments reveal that east-west shifts of the Icelandic Low perturb the wind stress in mid-latitudes adjacent to the American coast, driving downwelling (through longshore winds) and offshore sea level anomalies (through wind stress curl) which travel to the Florida Straits within the same season. Florida Current transport is also correlated with the latitude variations of both the Icelandic Low and the Azores High with a lag of four years. Regression analysis shows that latitude variations of the Icelandic Low and the Azores High are associated with positive wind stress curl anomalies over extended regions in the ocean east of Florida. Rossby wave propagation from this region to the Florida Straits has been suggested as a mechanism for perturbing FCT transport in several previous studies (DiNezio et al. 2009; Czeschel et al. 2012; Frajka-Williams et al. 2013; Domingues et al. 2016, 2019).

The zonal and meridional overturning circulations of buoyancy-forced basins are studied in an eddy-resolving model. The zonal overturning circulation (ZOC) is driven by the meridional gradient of buoyancy at the surface and stratification at the southern boundary. The ZOC, in turn, produces zonal buoyancy gradients through upwelling and downwelling at the western and eastern boundaries, respectively. The meridional overturning circulation (MOC) is driven by these zonal gradients rather than being directly driven by meridional gradients. Eddies lead to a broadening of the upwelling and downwelling limbs of the ZOC, as well as a decoupling of the locations of vertical and diapycnal transport. This broadening is more prominent on the eastern boundary, where westward-moving eddies transport warm water away from a poleward-flowing eastern boundary current. Most of the diapycnal downwelling occurs in the "swash zone"-the region where the isopycnals intermittently come in contact with the surface and lose buoyancy to the atmosphere. A scaling for the overturning circulations, which depends on the background stratification and the surface buoyancy gradient, is derived and found to be an excellent fit to the numerical experiments.

Abstract Idealized models can reveal insights into Earth’s climate system by reducing its complexities. However, their potential is undermined by the scarcity of fully coupled idealized models with components comparable to contemporary, comprehensive Earth System Models. To fill this gap, we compare and contrast the climates of two idealized planets which build on the Simpler Models initiative of the Community Earth System Model (CESM). Using the fully coupled CESM, the Aqua configuration is ocean‐covered except for two polar land caps, and the Ridge configuration has an additional pole‐to‐pole grid‐cell‐wide continent. Contrary to most sea surface temperature profiles assumed for atmosphere‐only aquaplanet experiments with the thermal maximum on the equator, the coupled Aqua configuration is characterized by a global cold belt of wind‐driven equatorial upwelling, analogous to the eastern Pacific cold tongue. The presence of the meridional boundary on Ridge introduces zonal asymmetry in thermal and circulation features, similar to the contrast between western and eastern Pacific. This zonal asymmetry leads to a distinct climate state from Aqua, cooled by ∼2°C via the radiative feedback of clouds and water vapor. The meridional boundary of Ridge is also crucial for producing a more Earth‐like climate state compared to Aqua, including features of atmospheric and ocean circulation, the seasonal cycle of the Intertropical Convergence Zone, and the meridional heat transport. The mean climates of these two basic configurations provide a baseline for exploring other idealized ocean geometries, and their application for investigating various features and scale interactions in the coupled climate system.

Tropical modes of variability, including the Madden‐Julian Oscillation (MJO) and the El Niño‐Southern Oscillation (ENSO), are challenging to represent in climate models. Previous studies suggest their fundamental dependence on zonal asymmetry, but such dependence is rarely addressed with fully coupled ocean dynamics. This study fills the gap by using fully coupled, idealized Community Earth System Model (CESM) and comparing two nominally ocean‐covered configurations with and without a meridional boundary. For the MJO‐like intraseasonal mode, its separation from equatorial Kelvin waves and the eastward propagation of its convective and dynamic signals depend on the zonal gradient of the mean state. For the ENSO‐like interannual mode, in the absence of the ocean's meridional boundary, a circum‐equatorial dominant mode emerges with distinct ocean dynamics. The interpretation of the dependence of these modes on zonal asymmetry is relevant to their representation in realistic climate models.

Short-finned pilot whales (Globicephala macrorhynchus) experience dramatic changes in temperature during deep dives, but studies of pilot whale habitat use typically rely solely on surface temperature measurements. We quantified vertically integrated thermal habitat for short-finned pilot whales using a novel metric, degree-hours, developed using data from digital acoustic recording tags (DTAGs) deployed off Cape Hatteras along with interpolated temperature profiles at depth from the Met Office Hadley Centre EN4 oceanographic data set. We then compared estimates of thermal habitat calculated from surface waters with estimates of vertically integrated thermal habitat calculated using EN4 data collected along the eastern seaboard of the U.S. to understand how available thermal habitat is influenced by seasonal and spatial variability in water temperature. Estimates of vertically integrated thermal habitat were typically lower than estimates produced using surface temperatures, and the difference was greatest at intermediate latitudes and in warmer seasons, where and when there is a high degree of variability between surface and bottom temperatures. Our work highlights the importance of considering temperature at depth to accurately assess the thermal habitat of deep-diving marine vertebrates, and presents a means of quantifying thermal habitat that will be useful for understanding the thermal ranges of these species.

Earth and Space Science Open Archive PosterOpen AccessYou are viewing the latest version by default [v1]A Simplified Climate Model for Understanding Tropical Cyclones and Ocean Heat TransportAuthors Xiaoning Wu iD Kevin Reed Scott Bachman iD Frank Bryan Christopher Wolfe iD Gustavo Marques iDSee all authors Xiaoning WuiDCorresponding AuthorStony Brook UniversityiDhttps://orcid.org/0000-0002-4951-5668view email addressThe email was not providedcopy email addressKevin ReedStony Brook Universityview email addressThe email was not providedcopy email addressScott BachmaniDNCARiDhttps://orcid.org/0000-0002-6479-4300view email addressThe email was not providedcopy email addressFrank BryanNCARview email addressThe email was not providedcopy email addressChristopher WolfeiDStony Brook UniversityiDhttps://orcid.org/0000-0002-7062-2070view email addressThe email was not providedcopy email addressGustavo MarquesiDNCARiDhttps://orcid.org/0000-0001-7238-0290view email addressThe email was not providedcopy email address

The transport by materially coherent eddies is studied in a two-layer quasigeostrophic model of geophysical turbulence. The coherent eddies are identified by closed contours of the Lagrangian-averaged vorticity deviation obtained from Lagrangian particles advected by the flow. A series of flow regimes with different bottom friction strengths are considered---it is found that coherent eddies become more prevalent and longer-lasting as the strength of the bottom drag increases. These coherent eddies, with average core radius close to the deformation radius, propagate zonally with speeds close to the long baroclinic Rossby wave speed and meridionally with a preference for cyclones to propagate poleward and anticyclones to propagate equatorward. The meridional propagation preference of the coherent eddies gives rise to a systematic upgradient potential vorticity (PV) transport, which is in the opposite direction as the background PV transport and not captured by standard Lagrangian diffusivity estimates. The upgradient PV transport by coherent eddy cores is less than 10% of the total PV transport, but the PV transport by the periphery flow induced by the PV inside coherent eddies is significant and downgradient. This clarifies the distinct roles of the trapping and stirring effect of coherent eddies in PV transport in geophysical turbulence.

Abstract Downstream of Cape Hatteras, the Gulf Stream (GS) is bounded to the north by a sharp temperature front known as the North Wall (NW). Previous studies have generally assumed that variations of the NW and GS are equivalent. Using satellite sea surface height to identify the GS and the 15 °C isotherm at 200‐m depth to represent the NW, this paper examines their similarities and differences during 1993–2016. The NW and GS are geographically close and vary similarly only to the west of 71°W. Downstream of that, they rapidly diverge—and the variances of their latitudes increase by more than a factor of 2—as the GS flows past the New England Seamounts. Evidence is presented to show that the difference in properties of the NW and the GS is related to the presence of mesoscale eddies in the region separating them.

The time-mean zonal and meridional overturning circulations of the entire Mediterranean Sea are studied in both the Eulerian and residual frameworks. The overturning is characterized by cells in the vertical and either zonal or meridional planes with clockwise circulations in the upper water column and counterclockwise circulations in the deep and abyssal regions. The zonal overturning is composed of an upper clockwise cell in the top 600 m of the water column related to the classical Wüst cell and two additional deep clockwise cells, one corresponding to the outflow of the dense Aegean water during the Eastern Mediterranean Transient (EMT) and the other associated with dense water formation in the Rhodes Gyre. The variability of the zonal overturning before, during, and after the EMT is discussed. The meridional basinwide overturning is composed of clockwise, multicentered cells connected with the four northern deep ocean formation areas, located in the Eastern and Western Mediterranean basins. The connection between the Wüst cell and the meridional overturning is visualized through the horizontal velocities vertically integrated across two layers above 600 m. The component of the horizontal velocity associated with the overturning is isolated by computing the divergent components of the vertically integrated velocities forced by the inflow/outflow at the Strait of Gibraltar.

In recent years, significant progress has been made in the development of high-resolution ocean reanalysis products. This paper compares aspects of the Gulf Stream (GS) from the Florida Straits to south of the Grand Banks-particularly Florida Strait transport, separation of the GS near Cape Hatteras, GS properties along the Oleander Line (from New Jersey to Bermuda), GS path, and the GS north wall positions-in 13 widely used global reanalysis products of various resolutions, including two unconstrained products. A large spread across reanalysis products is found. HYCOM and GLORYS2v4 stand out for their superior performance by most metrics. Some common biases are found in all discussed models; for example, the velocity structure of the GS near the Oleander Line is too symmetrical and the maximum velocity is too weak compared with observations. Less than half of the reanalysis products show significant correlations (at the 95% confidence level) with observations for the GS separation latitude at Cape Hatteras, the GS transport, and net transport across Oleander Line. The cross-stream velocity structure is further discussed by a theoretical model idealizing GS as a smoothed PV front.

An eastern boundary current (EBC) system driven by a large-scale meridional buoyancy gradient is simulated using an idealized eddy-resolving model. The EBC system consists of a pair of stacked meridional currents that flow poleward near the surface and equatorward at intermediate depths. Buoyancy advection in the EBC is primarily balanced by the shedding of eddies, with anticyclonic, warm-core eddies dominating near the surface and cyclonic, cold-core eddies found at intermediate depths. These boundary eddies play a significant role in both the eastern boundary circulation—by helping to trap the EBC near the coast—and the large-scale circulation through their effect on the downwelling limb of the overturning circulation. Momentum and thickness budgets analyzed using the thickness-weighted average framework highlight the role of eddy form drag in shaping and maintaining the EBC. The efficiency of the form drag increases dramatically at the offshore flank of the EBC. This zonal variation of the form drag is essential for maintaining a swift, narrow EBC. The essential physics of the EBC are illustrated using a simple, semianalytical model.

The path of the Gulf Stream as it leaves the continental shelf near Cape Hatteras is marked by a sharp gradient in ocean temperature known as the North Wall. Previous work in the literature has considered processes related to the North Atlantic Oscillation (NAO) in triggering latitudinal displacements of the North Wall position. This paper presents evidence that the Atlantic meridional mode (AMM) also impacts interannual variations of the North Wall position. The AMM signal from the tropics propagates to the Gulf Stream near the 200-m depth, and there are two time scales for this interaction. Anomalous Ekman suction induced by AMM cools the tropical Atlantic. The cold water in the Caribbean Sea is entrained into the currents feeding the Gulf Stream, and this cooling signal reaches the North Wall within a year. A second mechanism involves cold anomalies in the western tropical Atlantic, which initially propagate westward as baroclinic planetary waves, reaching the Gulf Stream and resulting in a southward shift in the North Wall position after a delay of about one year. In an analysis for the period 1961–2015, AMM’s signal dominates North Wall fluctuations in the upper 300 m, while NAO is the major influence below ~500 m; the influence of both the teleconnections is seen between 300 and 500 m. The relationship between the Atlantic meridional overturning circulation (AMOC) and the North Wall is investigated for the 2005–15 period and found to be statistically significant only at the sea surface in one of the three North Wall indices used.