Amala Mahadevan

Vertical velocities (w) are essential for understanding upper-ocean tracer transport, but remain difficult to observe and quantify directly. Here we exploit the synergy between high-resolution Sea Surface Height observations from the Surface Water and Ocean Topography (SWOT) mission and in-situ glider density profiles to reconstruct the three dimensional density, geostrophic velocity, and quasigeostrophic vertical velocity fields of an intrathermocline eddy in the western Mediterranean Sea. The reconstruction is independently validated against ship-based CTD profiles and shipboard Acoustic Doppler Current Profiler (ADCP) velocities and against an Observing System Simulation Experiment based on a regional numerical model. At 300 m depth, the eddy’s geostrophic flow reaches Rossby numbers of |Ro| ∼ 0.5, and the diagnosed w field exhibits a multipolar pattern of upwelling and downwelling cells at the eddy periphery, up to ±40 m/day. An equivalent reconstruction from DUACS conventional gridded altimetry yields |Ro| ≲ 0.2 and w ∼ ±5 m/day. A natural-coordinate decomposition of the frontogenetic Q-vector forcing identifies curvature-induced frontogenesis as the dominant organizer of the diagnosed vertical motion. Independent glider measurements of dissolved oxygen and spice show an antisymmetric arrangement across the eddy flanks consistent with the diagnosed circulation. Combining SWOT altimetry with targeted in-situ profiling in this way allows us to resolve the three-dimensional dynamics of small-scale ocean features.

Submesoscale dynamics play a crucial role in energy redistribution in the upper ocean but remain challenging to observe due to their small spatial and temporal scales. We use ship‐based observations of an oceanic submesoscale front sampled at high resolution while repeatedly traversed to observe deformation and splitting of an embedded cyclonic eddy. The velocity field from a shipboard acoustic Doppler current profiler was analyzed with spectral Helmholtz techniques and structure functions to examine the partitioning of kinetic energy into rotational and divergent components and energy transfer between mesoscales and submesoscales. During the eddy splitting, intensified mesoscale strain enhances the divergent component of kinetic energy in the mixed layer, where Rossby numbers are . This coincides with a transient forward energy cascade from mesoscales to submesoscales. The wave‐vortex decomposition reveals enhanced wave‐like energy during and after splitting, with subsurface maxima indicative of near‐inertial wave activity and downward energy propagation.

This report describes pilot experiment for the ONR DRI titled CALYPSO, which aims to study three-dimensional pathways from the surface ocean to interior. The study used two ships, the NRV Alliance (100m), and the RV SOCIB (23m) to map the circulation generated by the front between Atlantic and Mediterranean water in the Alboran Sea and to conduct high-resolution frontal surveys. The study employed a large number of drifters and conducted underway profiling from the ship.

Submesoscale dynamics play a crucial role in vertical transport and energy redistribution in the upper ocean but remain challenging to observe due to their small spatial and temporal scales. Spectral Helmholtz decomposition of velocity from a shipboard acoustic Doppler current profiler allows examination of the scale- and depth-dependent partitioning of kinetic energy into rotational and divergent components. This method is applied to repeated high-resolution transects across a submesoscale front, which later splits into a cyclonic eddy in the Mediterranean Sea. We find a transition from balanced, rotational dynamics at scales greater than 10 km to divergent, unbalanced motions at submesoscales below 10 km. Divergent energy is enhanced in the mixed layer during periods of strong frontal strain, eddy deformation, and Rossby numbers of O(1), and appears linked to near-inertial wave interactions.

Earth’s oceans contain large numbers of cyclonic eddies, 10 to 25 kilometers in diameter and unresolved in climate simulations. However, we lack observations of these features due to their small size and fast evolution. Here, we present in situ observations of one such cyclonic eddy with intense chlorophyll at its center as it spontaneously splits into two smaller cyclonic eddies over a few days. This splitting rapidly transports surface waters to depth, with sustained vertical velocities of 60 meters per day, primarily from the center of the eddy where carbon concentrations are largest, facilitating efficient transfer of phytoplankton carbon to depth, below the well-mixed, sunlit surface layer. We reproduce the splitting process in an idealized ocean model and find that splitting is controlled by the initial elliptical eddy shape, size, and intensity. Our observations uncover a mechanism for subduction in the upper ocean and highlight the need for quantifying its global prevalence.

This report describes the data set from the CALYPSO 2022 campaign and the processing and quality-control steps that were taken to produce the data set. The CALYPSO Campaign was conducted from the R/V Pourquoi Pas? (February 17 – March 12, 2022) and the R/V Pelagia (February 20 – March 16, 2022).

This report describes the biogeochemical data set from the CALYPSO 2022 Campaign and the processing and calibration steps taken to quality control the data.

This report summarizes activities on the French research vessel Pourquoi Pas? operating as part of the Office of Naval Research CALYPSO project during its main 2022 field program in the northwest Mediterranean. Other components of this field program included the Netherland’s research vessel R.V. Pelagia, a fleet of underwater gliders operated by the Scripps Institution of Oceanography and the Balearic Islands Coastal Observing and Forecasting System (SOCIB). The goals of the CALYPSO DRI are to observe and predict the three-dimensional pathways by which water from the surface ocean makes its way into the deeper ocean. The focus was on the submesoscale (roughly 10 km and smaller) where geostrophic balance breaks down and ageostrophic flows tend to create strong vertical motion The work involved several US institutions, IMEDEA (CSIC) and SOCIB (Spain) and CNR (Italy). The following cruise report may also be useful. Cruise report R/V Pelagia Cruise No. 64pe497, 18 Feb – 12 Mar 2022, Palma – Palma, Spain: Johnston, S, N. Calafat, B. Casas, E. Cutolo, R. Daniels, F. Falcieri, T. Litchendorf, I. Lizar´an, C. McNeil, A. Pascual, N. Reynard, A. Shcherbina, & J. Tintor´e 2022. doi: 10.17605/OSF.IO/EN7T4

The Indian summer monsoon, which brings heavy precipitation to the densely populated Indian subcontinent, plays an important role in the development of a coastal upwelling circulation that brings colder, nutrient‐rich water to the surface. Although the western shores of the Arabian Sea (AS) and Bay of Bengal (BoB) both experience upwelling‐favorable winds during June‐August, only the AS coastline exhibits significant surface cooling. In contrast, the BoB remains warm and its upwelling is characterized by a transient, weak sea surface temperature (SST) response confined to the east coast of India. A weaker mean alongshore wind stress and coastal circulation do not sufficiently explain the lack of SST response in the BoB. Here, we examine other reasons for the differing behavior of these two coastal margins. Firstly, we show that while winds are persistently upwelling‐favorable in the western AS, intraseasonal wind variability in the BoB induces intermittent upwelling. Secondly, the vertical density stratification is controlled by salinity in the BoB, and upwelled waters are saltier, but only marginally cooler than surface waters. By contrast, the density in the AS is temperature‐controlled, and upwelled waters are substantially colder than the surface. Additionally, satellite‐based SST in the BoB does not adequately resolve the upwelling signal. Using a numerical model, we find that salinity stratification has a greater influence on the mean SST, while wind frequency alters near‐shore SST and its temporal variability. This work has implications for the sensitivity of upwelling regions and their response to wind stress and stratification in a warming climate.

Sea surface temperature anomaly (SSTA) of ocean eddies induces an anomalous air‐sea turbulent heat flux that acts to dampen SSTA. A two‐dimensional SSTA model explores the effect of air‐sea turbulent heat flux, parameterized as SSTA damping, in shaping eddy SSTA patterns. Increased SSTA damping transitions the SSTA pattern from a monopole to dipole, indicating the balance between eddy stirring of the background SST gradient and SSTA damping. The SSTA dipole pattern increases the correlation of eddy velocity and SSTA, but SSTA damping weakens the SSTA, resulting in an optimal damping rate maximizing lateral eddy surface heat transport. Globally, the SSTA damping rate increases toward the equator. In mid‐latitude and high‐latitude regions (e.g., the Kuroshio, the Gulf Stream, and the Southern Ocean), eddy SSTAs are monopoles, while the tropics and subtropics exhibit dipole SSTA patterns due to higher damping rates, facilitating greater lateral eddy heat transport when the SSTA is large.