Exploring the upper ocean

Tandon Laboratory at UMass Dartmouth

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“Effects of the seasonality of mesoscale eddies on the planktonic dynamics off eastern Brazil” published in DAO

A paper by Caique Luko (an M.S student from University of São Paulo co-advised by Prof. Tandon)  titled  “Effects of the seasonality of mesoscale eddies on the planktonic dynamics off eastern Brazil” has been published in Dynamics of Atmosphere and Ocean with Prof. Tandon and Filipe Pereira (a PhD candidate from the USP-UMassD dual-degree program) as some of the co-authors of the paper. This study was aimed to understand if the seasonality of these mesoscale eddies affects the regional phytoplankton annual cycle. To achieve that, the authors analyzed chlorophyll-a satellite observations and performed two experiments using a Nutrients-Phytoplankton-Zooplankton (NPZ) model coupled to a 1 and a 1/2 -layer Quasi-Geostrophic model.

The results reveal that the phytoplankton annual cycle off eastern Brazil is mainly controlled by the seasonally varying advection of material offshore caused by the mesoscale eddies. Such mechanism may represent an important source of material to the tropical oligotrophic ocean. More on this study could be found here.

Congratulations Caique, Filipe and Prof Tandon!!

Horizontal distributions of the streamlines and the Chl-a obtained in: (1 st row) Satellite observations; (2 nd row) Experiment with no advection of enriched coastal material; and (3 rd row) Experiment with advection of enriched coastal material. Average scenarios of: (1 st column) summer; (2 nd column) fall; (3 rd column) winter and; (4 th column) spring. The concentration of chlorophyll-a is on a logarithmic scale. Dashed lines represent negative streamlines, and solid lines represent positive streamlines. The white shaded area masks regions shallower than 100 m.

Highlights on the UMassD IMS Symposium 2022: Adriano wins the best poster

Inter-campus Marine Science (IMS) program at University of Massachusetts organized their annual symposium at UMass Dartmouth in March 2022. This event marked the return of the in-person gathering at this symposium. Researchers from the five UMass campuses (Amherst, Boston, Dartmouth, Lowell and Worcester) gather to discuss their work in marine sciences.

Iury Simoes-Sousa was invited to present a plenary talk on “Atmospheric cold pools in the Bay of Bengal”, while Adriano Giangiardi Filho, Filipe Pereira, Sid Kerhalkar, Patrick Pasteris and Ersen’S Joseph presented posters. Their posters can be found here . Sid was also one of the student organizers of the IMS symposium.

Iury presenting a plenary talk at IMS symposium 2022

Adriano won the best poster award in the Oceanography section. Congratulations Adriano!

Members of Tandon Lab present at Ocean Sciences Meeting 2022

Prof Amit Tandon and the rest of the lab had strong participation in the recently concluded Ocean Sciences Meeting-2022. Originally planned to be in Hawaii, the pandemic forced the organizers (AGU, ASLO, TOS) to make the event virtual. Spread across 7 days in February and March, it was definitely a good experience for the group to be a part of this event by presenting their published/on-going work as well as attending lots of other talks from a plethora of topics being discussed. Following are the details of the talks either given by the lab members or with Prof Tandon as a co-author.

1) Topographically-Generated Submesoscale Shear Instabilities associated with Brazil Current Meanders: Caique Luko

2) Diurnal Warm Layers in the Bay of Bengal during Monsoon 2019: Siddhant Kerhalkar

3) Atmospheric Cold Pools in the Bay of Bengal: Iury T.Simoes Sousa

4) Dynamics of a Baroclinically-unstable Meander and its Ecological Impacts: Filipe Pereira

5) Validation of a Hybrid Co-ordinate Ocean Model (HYCOM) in the Eastern Tropical North Pacific Oxygen Deficient Zone: Valentina Guinta

6) Next steps toward understanding Arabian Sea dynamics and ecology: Amit Tandon

7) Energy Exchange Between Internal Gravity Waves and Balanced Flow: Wave Action Conservation and a pathway to Dissipation:  Eric Kunze (with Amit Tandon)

8) Reabsorption of Lee-Wave Energy in Bottom-Intensified Currents: Yue Wu (with Amit Tandon)

9) Incorporating irreversible fluxes in K-e mixing model for Bay of Bengal: Shikha Singh (with Amit Tandon)

10) Characteristics and Variability of Air-sea fluxes in Bay of Bengal from OMNI Buoy Measurements: Jossia Joseph (with Amit Tandon)

11) Wave Induced Stokes Drift from a decade of Moored Buoy Observations in the Bay of Bengal: Kalyani M (with Amit Tandon).

Along with these 11 talks, Prof Tandon was also a session chair for the session “PL04- Indian Ocean circulation and its impact on air-sea interactions, biogeochemistry and ecology”.





JGR-Ocean Paper published on submesoscale coherent vortices in the South Atlantic Ocean acting as a pathway for energy dissipation

A paper by Dr Caue Lazaneo (a former PhD student in Tandon Lab) has just been published in JGR-Ocean titled “Submesoscale Coherent Vortices in the South Atlantic Ocean: A Pathway for Energy Dissipation” with Prof Amit Tandon as one of the co-authors. This paper is motivated by observations during summer 2017 aboard R/V Alpha-Crucis as a part of “Islands” experiment. This paper shows the role of seamounts in the generation of submesoscale features due to the interaction of the rich mesoscale eddy field with the local topography. The paper could be found here.

Congratulations Caue and Prof Tandon!

A plain language summary of the paper appears below:
The interaction between topography and oceanic flows results in the development of small-scale turbulent phenomena. The occurrence of such phenomena is significant for the ocean energy balance due to energy dissipation, which occurs on spatial scales smaller than a centimeter. In this study, we describe, for the first time in the western South Atlantic Ocean, the physics of two adjacent submesoscale coherent vortices in the vicinity of the Vitória-Trindade Ridge. These vortices have radii of 12 and 16 km and a subsurface signature with intensified velocity and weak stratification. Since this type of vortex has no surface expression, it is challenging to detect it due to its small horizontal scale. Microstructure measurements collected by a ship inside and outside the vortex enable us to evaluate, for the first time, its influence on energy dissipation and ocean mixing.

JAOT paper published on Longwave Radiation corrections in the OMNI Buoy Network

A paper by Ms Jossia Joseph (Scientist-E at National Institute of Ocean Technology) has just been published in Journal of Atmospheric and Oceanic Technology titled “Longwave Radiation Corrections for the OMNI Buoy Network” with Prof Amit Tandon as one of the co-authors. This paper describes corrections performed in the OMNI Buoy network when there was an over-estimation of Longwave Radiation in OMNI Buoy network found when compared with a nearby mooring from Woods Hole Oceanographic Institute (WHOI) during 2015 ASIRI experiment. The paper can be found here.

Congratulations Jossia and Prof Tandon!

OMNI Buoy Network (figure-1 of paper)

A major result showing corrections in Longwave radiation calculations (figure-10 of the paper).

The abstract of this study appears below:
The inception of a moored buoy network in the northern Indian Ocean in 1997 paved the way for systematic collection of longterm time series observations of meteorological and oceanographic parameters. This buoy network was revamped in 2011 with OMNI (Ocean Moored buoy Network for north Indian Ocean) buoys fitted with additional sensors to better quantify the air-sea fluxes. An inter-comparison of OMNI buoy measurements with the nearby WHOI mooring during the year 2015 revealed an overestimation of downwelling longwave radiation (LWR↓). Analysis of the OMNI and WHOI radiation sensors at a test station at NIOT during 2019 revealed that the accurate and stable amplification of the thermopile voltage records along with the customized data logger in the WHOI system results in better estimations of LWR↓. The offset in NIOT measured LWR↓ is estimated firstly by segregating the LWR↓ during clear sky conditions identified using the downwelling shortwave radiation measurements from the same test station, and secondly, finding the offset by taking the difference with expected theoretical clear sky LWR↓. The corrected LWR↓ exhibited good agreement with that of collocated WHOI measurements, with a correlation of 0.93. This method is applied to the OMNI field measurements and again compared with the nearby WHOI mooring measurements, exhibiting a better correlation of 0.95. This work has led to the revamping of radiation measurements in OMNI buoys and provides a reliable method to correct past measurements and improve estimation of air-sea fluxes in the Indian Ocean.

Prof Tandon and group awarded the 2021 OTCV grant

The University of Massachusetts System Office of Technology Commercialization and Ventures (OTCV) recently announced the recipients of their 2021 grant awards for their innovative research proposals which include UMass Dartmouth faculty member, Prof Amit Tandon. The OTCV awards invest in faculty research and development projects that lead to further investment, workforce training, and job creation opportunities within the Commonwealth.

Professor Amit Tandon (Mechanical Engineering) and his project “Aurelia: Low-cost user-friendly depth changing vehicle for ocean sensors” dive into the issue surrounding a growing need for inexpensive devices that detect important sub-surface ocean data (e.g., temperature, salinity, currents) in the upper ocean. The Aurelia is a unique simple-to-use low-cost design advancing work done by a senior design team at UMass Dartmouth, led by Mr. Pasteris, mentored, and guided by Dr. Amit Tandon. Aurelia features a low-cost depth control system to allow several dive/surface trips, an Android graphical interface for Bluetooth programming, and a deployable antenna for retrieval after completing its mission.

More on this here

Prof Tandon and Patrick Pasteris with Aurelia profiler

JGR-Oceans paper published on “Revisiting the Atlantic South Equatorial Current”

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?”

To read more about this research access: click here

High-resolution turbulence measurements reveal a large-scale mixing hotspot in the Kuroshio flowing seamounts south of Kyushu Japan

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 10th 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 10th 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 10th of meters) and mesoscale (a few 10th 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


Siddhant (Sid) Kerhalkar is now a PhD Candidate

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.

Iury Simoes-Sousa is now a PhD Candidate

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.

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