A paper published on June 17, 2021 by Caique Luko, an oceanographer at the University of São Paulo, Brazil and colleagues including Iury Simoes-Sousa, PhD Candidate at UMass Dartmouth and Prof Tandon (Caique’s co-advisor in his Master’s thesis) in JGR-Oceans reveal that the southern branch of the South Equatorial Current is a multi-banded flow with surface signatures arising from different subsurface cores (Figure 1).

Figure 1: (a) Horizontal distribution of the geostrophic velocities mean field derived from altimetry data. Gray shaded area masks regions shallower than 100 m, and background gray shading highlights regions shallower than 4,000 m. (b) Meridional section of the zonal velocities at 30°W (blue line in panel a). (c) Horizontal distribution of the geostrophic velocities mean field zoomed in to the region of the Brazil Current origin site (blue dashed box in panel a)

“The SSEC is an ocean current that flows from Africa to South America. It consists on the northern limit of the South Atlantic Subtropical Gyre. This current is the main pathway that connects waters from the Pacific and Indian Oceans to the tropical Atlantic western boundary currents supply (Brazil Current and North Brazil Undercurrent). By feeding both currents, the SSEC can affect the global climate since the SSEC-North Brazil Undercurrent-North Brazil Current system is an element of the upper portion of the Meridional Overturning Circulation. In fact, the SSEC constitutes an important source of warm water to the North Atlantic, which explicits its importance to global climate.”

Figure 2: 3-D depiction of the complexity of incoming SSECjets that form the WBCs off
the coast of Brazil. Extracted from Soutelino et. al (2013). SEC: South Equatorial Current; BC: Brazil Current; NBUC: North Brazil Undercurrent; NBC: North Brazil Current; IWBC: Intermediate Western Boundary Current; DWBC: Deep Western Boundary Current; TW: Tropical Water; SACW: South Atlantic Central Water; AAIW: Antarctic Intermediate Water; NADW: North Atlantic Deep Water.

Originally, the southern branch of the South Equatorial Current (SSEC) was thought to be a broad sluggish jet that narrowed and shifted its bifurcation point poleward with increasing depth (Figure 2). However, by analyzing satellite data and reanalyses outputs, Caique and the co-authors showed that, actually, the SSEC is a multi-banded flow that extends its vertical extension and deepens its core depth with increasing latitudes (Figures 1 and 3).

Figure 3: Mean zonal velocity vertical section at 32°W derived from: (a) GLORYS12V1; (b) FOAM-GloSea5; (c) ORAS5; and (d) C-GLORS05. Red dashed boxes delimit the specified SSEC sectors. Solid (dashed) contours represent positive (negative) velocities, and contour intervals are of 0.005 m s ⁻¹ .

“This work gives more detail to the SSEC mean state and annual cycle, as well as its impacts on the western boundary circulation off Eastern Brazil. However, there is still a lot of work to be done. We still do not know what causes this distinct multi-banded pattern to occur. Furthermore, as the SSEC is a key component to global climate, we must ask: Does this flow change its structure in a changing climate, and what would be the consequences of these possible alterations? Finally, we observed that the SSEC seasonality modulates the annual cycle of semi-permanent mesoscale eddies off Brazil. A question that must be posed is: Does this observed variability impact the regional planktonic dynamics?”

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A recent paper published August 23, 2021, in the journal, Nature: Communication, Earth & Environment by Takeyoshi Nagai, an oceanographer at Tokyo University of Marine Science and Technology, Japan and colleagues including Prof. Amit Tandon at UMassD, who was Takeyoshi’s mentor 17 years ago, reveal an unprecedented long-lasting vigorous turbulent streak over 100 km along the Kuroshio Current in the North Pacific, flowing over the seamounts in the Tokara Strait south of Kyushu Japan. This long-lasting turbulent streak cannot be explained by previous turbulence theories for ocean interior that consider only velocity vertical gradient of internal waves as the major source of turbulence. By conducting a series of intensive turbulence observations and numerical simulations, the authors find submesoscale (a few hundred meters – a few kilometers) instability caused by the Kuroshio flowing over steep seamounts can extract energy of the Kuroshio through its velocity gradients along horizontal as well as vertical directions and inject it to microscale (a few millimeters – a few 10^th of meters) turbulence.

Sunset over islands in south of Kyushu (Kagoshima). There are many volcanic islands and seamounts where the Kuroshio Current flows.

“Our finding may have big implications for understanding how the wind-driven ocean general circulation is maintained and how the interior of the ocean is mixed. Both remain uncertain for decades, and the latter is very important for nutrient supply for phytoplankton” says Tandon.

Ocean general circulation, that consists of numerous mesoscale eddies and large scale major ocean currents, is driven by about 1 TW power input by wind. To achieve its equilibrium, energy has to be dissipated at the same rate. The energy dissipation can occur by fluid friction only at the smallest scale, microscale (a few millimeters – a few 10^th of meters), while large scale flows tend to transfer energy to larger scale. Therefore, it has been a puzzle on how the energy of the general circulation reach to the scale to the dissipation. One of the candidates is the lee waves. Lee waves can be generated by ocean currents flowing over the bottom topography. Several studies pointed out that lee waves can extract energy from general circulation at a rate about 0.2 TW. However, a previous study suggested that a large fraction of the lee wave energy is not dissipating near the generation sites and radiates away. These radiated waves can be reabsorbed to the large scale ocean current and do not necessarily dissipate their energy.

Takeyoshi continues “The missing pieces could be hidden in the submesoscale”. The submesoscale processes have spatial scale that ranges from a few hundred meters to a few kilometers, which is in between microscale (a few millimeters – a few 10^th of meters) and mesoscale (a few 10^th of kilometers – a few hundred meters), both are relatively well-explored. “We are now very exciting era, in which observationally and numerically resolvable scales are emerging both from microscale and mesoscale toward the submesoscale.  Tandon sensei said this when I joined him as a Postdoctoral fellow at UMassD, and it got me very interested. I have been working on the submesoscale observationally and numerically since then” said by Takeyoshi. The word “sensei” means teacher in Japanese.

Their paper finds that when the Kuroshio flows over the steep seamounts in the Tokara Strait, clockwise spinning flow, in opposite sense to the Earth’s spinning, is generated from one side of the slopes, producing a streak of water spinning clockwise that triggers submesoscale instability, called inertial and symmetric instabilities and associated strong microscale turbulence. Although these instabilities have been reported along the continental slopes in the abyssal Southern Ocean, there has been no observational evidence that this mechanism provides a large scale mixing hotspot in the major ocean currents flowing over seamounts in relatively shallow layers until this study.

Picture 1: Tow-yo turbulence profiler, Underway-VMP, which consists of VMP250 (Rockland Scientific International) and UCTD Winch (Teledyne Ocean Science)

Turbulence is intermittent. That’s why scientists need to measure it at high spatial resolution to know where mixing occurs. But measuring turbulence is not easy, because turbulence probes are very sensitive and easily detect unwanted signals caused by instrument motions by rope’s tension. “In fact, it can detect noises of your steps when it placed at the lab” says Takeyoshi. His team developed the method to measure turbulence at 1-2 km horizontal resolution using a tow-yo microstructure profiler (Picture 1) without stopping a ship. They use 1.5 mm diameter Dyneema rope to let it sink quasi-freely without severe unwanted contamination from the rope’s tension and recover it to the surface repeatedly while a ship is steaming at a slow speed, 1-2 m/s. While yo-yo profiling is to keep deploying and recovering the instrument vertically at one location like a yo-yo, this method is called tow-yo profiling. Using this new technique and Training vessel Kagoshima-Maru (Kagoshima University, Picture 2), the authors and participants of cruises (Picture 3) find the enhanced turbulence by 100-1000-fold compared to typical interior ones, that spreads over 100 km scale along the Kuroshio behind the seamounts.

Picture 2: Training vessel Kagoshima-Maru (Kagoshima University)

Picture 3: Group photos of the Kagoshima-Maru cruises

Takeyoshi continues “Our finding may have very important implications in nutrient supply to the Kuroshio, and this could be a key to solve the Kuroshio Paradox”. The Kuroshio surface water is known to be nutrient poor and yet many fish species spawn in the regions southwest of Kyushu where their eggs and larvae are advected from by the nutrient poor Kuroshio Current and utilize the Kuroshio to migrate north. Nutrient depletion usually means less food availability for higher trophic levels. Why many fish species spawn there, risking their larva’s lives under food depleting environment? Why are there so many fish in the Kuroshio regions, one of the major fishing sites of the North Pacific, even with less food? This has been called the “Kuroshio Paradox”. The observed strong turbulence can inject nutrients, which are abundant in the deeper layers of the Kuroshio, toward the sunlit surface, where phytoplankton can photosynthesize. Then, zooplankton eat phytoplankton and are eaten by fish in the downstream. But the question remains why the Kuroshio surface water remains nutrient poor even with the observed large nutrient injection. “This is probably because these biological responses are happening in the subsurface layers. Also, another recent study pointed out that these trophic transfers may occur very rapidly in nutrient depleted environment. In other words, as everybody is so hungry, they eat as fast as possible so that nobody can see it. We need to have more observations to figure this out” Takeyoshi said.

This is not the end of the story, because the Kuroshio and the Kuroshio Extension regions are found to be one of the major net CO₂ sinks for the Earth’s atmosphere. The diffused nutrients toward the shallower layers and associated phytoplankton growth are very important for the CO₂ uptake in these regions. “However, climate model predicts the upper layers of these regions will be warmed leading to more strongly stratified layers, which will become more difficult to mix by turbulence within several decades. If this is the case, then phytoplankton will decrease and CO₂ uptake in these regions will also decline. Furthermore, dependent higher trophic levels including commercially valuable fish are expected to decrease too” said by Takeyoshi.

ONR (N0014-18-1-2799) supported Prof. Amit Tandon for this work. Also, check out the videos which explain this particular phenomenon visually. The paper can be found here

Congratulations are in order to Sid, who defended his PhD dissertation proposal titled “On the Diurnal Warm Layers in the Bay of Bengal” on August 23, 2021 (in the aftermath of Hurricane Henri) and thus, has transitioned to a PhD candidate position in Tandon laboratory. This session involved him giving a public talk for about 50 minutes which was followed by a closed room discussion session with his advisor Prof Amit Tandon (UMass Dartmouth) and committee members Prof Miles Sundermeyer (UMass Dartmouth), Dr Tom Farrar (WHOI) and Dr Ken Hughes (Oregon State Univ).  

Prior to this session, he also took a written comprehensive exam in June 2021 which comprised of two sessions: an open book and a closed book session, each lasting 4 hours. The exams consisted of questions from his formal coursework in UMass Dartmouth and questions related to his research.

Congratulations are in order to Iury, who defended his PhD dissertation proposal titled “On Three Sub-Grid Scale Processes and Their Influence on Larger Scales in the Ocean-Atmosphere System” on August 16, 2021, and thus has transitioned to a PhD candidate position in Tandon laboratory. This session involved him giving a public talk for about 50 minutes which was followed by a closed room discussion session with his advisor, Prof Amit Tandon (UMass Dartmouth) and committee members Prof Geoff Cowles (UMass Dartmouth), Prof Dan MacDonald (UMass Dartmouth) and Prof Sam Kelly (University of Minnesota Duluth).  
 
Prior to this session, he also took a written comprehensive exam in December 2019.

Congratulations to Dr. Caue Lazaneo! 

Caue defended his PhD dissertation titled “Mixing and Submesoscale Dynamics on the Western South Atlantic Ocean” on July 29, 2021. This session involved him giving a public talk for about 50 minutes which was followed by a closed room Q&A session with his advisors Prof Ilson da Silveira (USP), Prof Amit Tandon (UMass Dartmouth) and committee members Prof Joseph Harari (USP), Prof Dan MacDonald (UMass Dartmouth) and Prof Paulo Calil (Helmholtz-Zentrum Hereon). 

Caue was enrolled in the UMass Dartmouth- USP Dual Degree PhD program.

July 08. 2021: We now have two hydrophones and a weather station deployed on our pier at the School for Marine Science and Technology (SMAST), just in time for Tropical Storm Elsa, to get some good pilot data this weekend. Alan and CJ did a great job getting everything together for the deployment before Alan heads to San Diego to work in Dr Luca Centurioni’s lab. Forrest and Jen, the SMAST divers were super helpful working with us.

The hydrophones are approximately 1m and 2m below the low tide mark, and the tide is roughly 1 m here, so we’ll get a range of depths over time.

Since the COVID-19 was in full force in Massachusetts for the past year, the members of the Tandon Lab were forced to stay at home (as per the local policies) and work remotely. But the rapid vaccination campaigns across the state lead to the university allowing the research to be conducted in person. This lead to a very unique setting of a hybrid meeting on June 10, 2021 where the local members of the group attended the meeting in-person at the SMAST East conference room while the international members attended this meeting remotely (via zoom). This is because Prof Tandon co-advises a lot of students from international collaborations (including the dual degree PhD students from the University of Sao Paolo) who could not join us to meet in person due to the current travel restrictions.

Sitting: Patrick, Standing (from left): Prof Tandon, Alan, Iury, Ersen’S and Sid at SMAST-East conference room

Local participants included Prof Amit Tandon, Iury (PhD candidate), Sid (PhD student), Patrick (Masters student), Alan (Former REU and incoming Masters student) and Ersen’S (Summer REU student) while international members included Shikha (Scientist at IITM Pune), Filipe (PhD dual degree candidate) and Caique (Masters student at USP co-advised with Prof Ilson). The unique settings were a challenge initially but the members of the group were resilient during the meeting. Tandon Lab is now functioning in person.

Local Members of Tandon Lab with the glorious SMAST-E building in the background. From left: Sid, Iury, Prof Tandon, Patrick, Alan, Ersen’S and CJ

On Tuesday, April 13^th 2021, Alan Andonian, a Mechanical Engineering senior and an undergraduate researcher presented his research on Fluid Motion Over Topography on a Rotating Earth at the National Conference of Undergraduate Research (NCURS). On Friday, April 23^rd he will be presenting this study at the Massachusetts Undergraduate Research Conference (MasssURC). Alan’s mentors were Engineering and Applied Science (EAS) PhD Student Iury Simoes-Sousa and Prof. Amit Tandon from the Department of Mechanical Engineering. Alan’s presentation shows a numerical simulation of the ocean phenomena known as the Taylor Column. This counterintuitive phenomenon occurs at large-scales affected by Earth’s rotation. It can also be simulated at smaller scales, as in a rotating tank in the laboratory, which was made difficult by the pandemic. Therefore to demonstrate the effects of rotation in geophysical fluids, a MITgcm numerical model was created (Figure below). From this image, we can see that the flow goes around the topography even towards the surface. It is as if there is an invisible obstacle blocking the flow trajectory, the Taylor Column.

Alan Andonian

Iury Simoes-Sousa

A paper by Dante and Prof Amit Tandon has just been published in JGR-Ocean titled “Submesoscale phenomena due to the Brazil Current crossing of the Vitória‐Trindade Ridge”. The paper can be found here.

Congratulations Dante and Prof Tandon!

Plain language summary of this paper appears below:
“At 20.5°S, strong currents interact with a submarine chain, the Vitória‐Trindade Ridge. In this study, we use new observations and a 2 km‐resolution regional numerical model to analyze how the interaction between the Brazil Current and the Vitória‐Trindade Ridge seamounts give rise to submesoscale instabilities. We present new high‐resolution velocity and density observations that capture submesoscale features associated with the flow, with patches of unstable flow associated with the Brazil Current interacting with the seamounts. In the same transects of the cruise, our simulation shows that submesoscale activity follows a typical seasonal cycle. But this seasonality is masked in regions where the flow intercepts topography. A Spatio-temporal analysis of the vertical fluxes points to flow‐topography interactions as the main source for these recurrent, deeper instabilities. As the Vitória‐Trindade Ridge emerges as a submesoscale hotspot in the oligotrophic South Atlantic, the lack of observations still remains the main obstacle to better understand the submesoscale processes in the region.”

A Paper by Sanjiv and Prof Amit Tandon has just been published in JGR-Ocean titled “Generation of Submesoscale Temperature Inversions Below Salinity Fronts in the Bay of Bengal” . This process study paper is motivated by our winter BoB observations of sub-surface warm layers.

Link to the paper can be found here.

Congratulations Sanjiv and Prof Tandon !

A plain language summary of the paper appears below:

“The ocean and atmosphere communicate at the air-sea interface. This communication occurs through exchange of heat, momentum, and other properties. The exchange of heat, in particular, shapes the coupled interplay of the ocean and atmosphere over periods ranging from hours to years. The change in ocean temperature versus depth crucially impacts how much heat is available for exchange with the overlying atmosphere. Typically, temperature decreases with depth but in regions like the Bay of Bengal (BoB), it can increase with depth for some distance before continuing to decrease at greater depths. Such increases in temperature are called “inversions.” In this study, we use high-resolution numerical modeling to explain the formation of inversions in the BoB that have a thickness of 10–30 m and a horizontal size of 1–10 km (submesoscale). Observations show frequent occurrence of such inversions in this region. We identify mechanisms illustrating how such inversions might be formed. Our results have potential implications for climate models where the grid spacing is too coarse to capture such mechanisms. The study demonstrates the value of high-resolution modeling in identifying new processes missing in today’s climate models.”