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Fridays at 11:00 am, RSMAS Auditorium (unless stated otherwise)

Jan 10 (SLAB 103): Dr. Rui Ni
Johns Hopkins University, Baltimore, Maryland

Bubbles and Spray in Turbulence
Recording Available at COMPASS ON DEMAND

Air-sea interactions are often associated with strong turbulence and two-phase mixtures, including spray in the air and bubbles entrained in water. Although attempts have been made to simplify the two-phase flow problems with single-phase-fluid-like parameterizations, there is a fundamental difference between these two systems. In this talk, I will leverage some advanced diagnostic tools developed in fluid dynamics community to address some key two-phase flow problems. In particular, I will show a simple example to extract the momentum transfer between two phases and illustrate some key parameters that have been elusive in turbulent two-phase flow. The goal of developing these small-scale laboratory experiments is to compartmentalize the complex air-sea interaction problems into several simpler questions that will hopefully lead to better parameterizations and large-scale modelling.

Dr. Ni recently joined the Johns Hopkins University as Assistant Professor of Mechanical Engineering in 2018. Before this position, he was the endowed Kenneth K. Kuo Early Career Professor at Penn State since 2015. He received his Ph.D. in Physics in 2011 from the Chinese University of Hong Kong and worked as a postdoctoral scholar at Yale and Wesleyan University. He won the NSF CAREER award in fluid dynamics and ACS-PRF New Investigator Award in 2017. His primary research focus is the development of advanced experimental methods for understanding gas-liquid and gas-solid multiphase flow as well as two-phase heat transfer problem.


Manish Devana (MPO)
Rapid Entrainment-Forced Freshening of the Iceland Scotland Overflow
Manish Devana, William E. Johns, and Sijia Zou

The Iceland Scotland Overflow (ISOW) is a major component of the Atlantic Meridional Overturning Circulation's deep limbs. Newly available mooring observations from the Overturning in the Subpolar North Atlantic Program (OSNAP) show an abrupt decline in ISOW salinity. ISOW salinity, and its variability, is governed by the combination of two distinct pathways: convection in the Nordic Seas, and entrainment along the Iceland Faroe Ridge. Previous ISOW salinity anomalies have been attributed primarily to the convective pathway acting on decadal, and longer, timescales. However, we show that entrainment allowed an upper ocean anomaly to bypass the convective pathway to drive the overflow salinity decline. This is shown by tracking propagation of the upper ocean salinity anomaly into ISOW along the entrainment pathway. We tracked the anomaly using a combination of mooring and Argo observations, surface drifter trajectories, and the FLAME numerical model. The upper ocean segment of the pathway advected the anomaly in the North Atlantic Current to the Iceland Faroe Ridge and mixed downwards to depths of active entrainment. The total upper ocean advection time was ~6 months. After being entrained into the overflow, the anomaly took 1–1.5 years to flow southwards back to the OSNAP array in the ISOW layer. A 2-year transit time from the upper ocean into the ISOW layer was found, which is significantly faster than the convective pathway involved with ISOW formation. This shows that entrainment allows interannual to sub-decadal scale upper ocean variability to directly modify the abyssal ISOW.

Simge Bilgen (MPO)
Understanding the Delayed Warming of the Southern Ocean

Here, a fully coupled model run at multiple resolutions from coarse to eddy resolving, driven by observed historical and fixed CO2 concentration is used to investigate the delayed warming of the Southern Ocean (SO). We analyze the 1941-2014 SO sea surface temperature (SST) and ocean potential temperature for the first 1 km trends simulated in the coupled general circulation model and evaluate possible causes of the model's inability to reproduce the observed 1941-2014 SO cooling. We used NCAR's Community Climate System Model version 4 (CCSM4) at both eddy resolving (0.1 degree ocean model) and eddy parameterized resolutions (1 degree ocean model) to explore the mesoscale atmosphere-ocean interactions in the SO in a fully coupled regime and to understand the role of ocean dynamics in modulating temperature response. At both resolutions the models successfully reproduce the observed warming response for the northern flank of the Antarctic Circumpolar Current (ACC). The eddy resolving simulations are able to reproduce the observed SO cooling, however in the eddy parameterized simulations, the SO SST response is inconsistent with the observations for the south of the ACC. The results presented here provide support for the hypothesis that the cooling around the Antarctic is intimately connected with ocean meso-scale processes that cannot be captured by ocean eddy parameterized models typically used for IPCC simulations.

Jan 24: NO SEMINAR (Auditorium in use for Miami Climate Symposium)

Jan 31: John Lodise
Department of Ocean Sciences, RSMAS
(one-hour MPO student seminar)

Measuring Surface Ocean Currents Using Massive Arrays of CARTHE Drifters
Recording Available at COMPASS ON DEMAND

Very near surface currents are vital to the transport and aggregation of biogeochemical materials naturally found in the ocean, as well as the fate of buoyant pollutants, like oil and plastics. In this presentation we investigate and report on very near surface currents using the massive array of CARTHE drifters deployed during the LAgrangian Submesoscale ExpeRiment (LASER) that took place from January to March of 2016 in the Northern Gulf of Mexico. Surface currents are especially complex, due to the wide array of forcing mechanisms that drive the surface flow on many different spatial and temporal scales. Specifically, very near surface currents can be easily dominated by wind and wave forcing during moderate to severe winds associated with atmospheric fronts. However, surface currents under mild wind conditions are mainly forced by pressure gradient-driven flows set up by density fronts and varying stratification, which often develop into smaller scale, ageostrophic flows. Given the difficult task of studying these varying dynamics, the objectives of this work were to: (1) deconstruct near surface currents to isolate and describe the vertical structure of wind- and wave-driven surface flows under high wind conditions and (2) investigate the interactions between mesoscale and submesoscale structures in order to observe the processes involved with surface convergence and the vertical exchange of surface and interior waters. The major findings of the work, shed new light on the vertical structure of wind-driven currents through the use of drogued and undrogued drifters, as well as the connection between mesoscale and submesoscale flows, using a Gaussian process regression based interpolation method to calculate Eulerian estimates of divergence and relative vorticity from Lagrangian drifter data.

Feb 07: NO SEMINAR (Recruitment Weekend)


Matthew Grossi (MPO)
Predicting Particle Trajectories Using Artificial Neural Networks
Matthew D. Grossi1, Miroslav Kubat2, and Tamay M. Özgökmen1
1 University of Miami Rosenstiel School of Marine and Atmospheric Science, Miami, FL, USA
2 University of Miami College of Electrical and Computer Science, Miami, FL, USA

Artificial neural networks (ANNs) may be futuristic tools for predicting maritime oil dispersion, but only if they are capable of learning realistic particle trajectories in a turbulent ocean. We explore the predictability of 2D trajectories from a variety of flow regimes. After conducting proof-of-concept experiments consisting of simulated flows of increasing complexity, we generate realistic particle trajectories using modeled flow fields from a regional ocean general circulation model for the Gulf of Mexico. We choose as a test case of interacting scales of motion a mesoscale eddy surrounded by submesoscale dynamics. ANNs are developed to predict particles' future velocities based on their past observations. A rolling window training approach enables the ANNs to be continuously updated according to the most recent available data. ANNs are trained in two ways to predict future velocities: first, a so-called "one-to-one ANN" uses only a particle's most recently observed velocity as input, and second, a "time series ANN" uses the past 24 hours' worth of velocity observations. We compare ANN output to rudimentary persistence predictions within a 24-hour forecast window and find that, for realistic trajectories, one-to-one networks offer little to no improvement over persistence while time series ANN forecast errors are at least half those of persistence, implying that realistic trajectories do contain some inherent learnability. By always testing the simplest possible ANN, our networks have much room for further development and performance enhancement. Our results suggest that ANNs are a promising new data-driven approach to forecasting material transport in the ocean.

Kayla Besong (ATM)
Atmospheric Blocking, Forecast Model Resolution,
and Winter Weather Conditions in the U.S.

An atmospheric block is defined as a large-scale obstruction of zonal flow in the form of 500 mb quasi-stationary cyclones and anticyclones lasting a minimum of five days. Their persistent displacement of the jetstream coincides with a shift in storm tracks, influencing regional weather patterns, often in the form of temperature and precipitation extremes. With resulting impacts from extremes on human and natural systems at large, the significance in predictability of blocks is highlighted. Climate models are notorious for lack of skill in accurately capturing atmospheric blocking, primarily with strong underestimations of wintertime blocking frequencies over the North Atlantic basin. Suggestions to decrease model biases relating to blocking include increasing horizontal resolution and the use of a fully coupled ocean-atmosphere model. Therefore both 1.0º×1.0º and 0.5º×0.5º retrospective forecasts of the Community Climate System Model, version 4 (CCSM4) have been evaluated in their ability to capture January-March blocking frequency, duration, and consequential up- and downstream impacts on the mean flow with associated regional precipitation and temperature extremes. Duration of blocking events, mean blocked flow and resulting regional impacts were well represented by the model. However, blocking frequencies were poorly captured for both higher and lower versions of CCSM4, with a strong underestimation over the North Atlantic. Differences between resolutions are minimal for all analysis, suggesting that increasing horizontal resolution does not improve blocking frequency bias nor does it increase confidence in accurately predicting impacts caused by blocking events.

Tyler Fenske (ATM)
The Relationship Between the Pacific Decadal Oscillation
and the Atlantic Multi-Decadal Oscillation in a Multi-Ensemble

We explore the potential relationship between the Atlantic Multi-decadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO), the leading climate modes in their respective basins. Their drivers are generally not well understood; current leading theories suggest that the PDO is primarily driven by internal variability, while the AMO is primarily driven by externally forced variability. Both modes have downstream teleconnections that can affect weather patterns in North America and Europe. These teleconnections can also reach other ocean basins, thus allowing for the possibility of these modes being linked by their teleconnections. Our observational results suggest that statistically significant correlations exist between the two modes when the PDO leads by 14 years and the AMO leads by 24 years. Previous studies also find statistically significant correlations, but with shorter time lags where the AMO leads by 12 years and the PDO leads by 1 year. We further analyze this link with a novel approach by using CLIVAR's "ensemble of ensembles", a set of six large-ensemble climate model runs. These results show a nearly perfect negative correlation with no time lag between the two in the ensemble means for each model. Additionally, they all show a negative trend in the PDO and a positive trend in the AMO over the last 30 years. Both of these findings suggest that a forced connection between these modes may be present. Future work will determine whether this connection is indeed forced, as well as determining a mechanism for this connection.

Feb 21: NO SEMINAR (week of AGU Ocean Sciences)



Kaycie Lanpher (OCE)
Assessing Marine Microbial Metabolic Strategies Through
Quantification of Their Energy Stocks and Turnover Rates

Microbial productivity controls the role of the ocean as a global carbon sink. The energy required for microbial metabolic activity is obtained from light or organic carbon for phototrophs and heterotrophs, respectively. Variations in these metabolic functions can be identified via changes in metabolic energy. The primary biochemical used for metabolic energy within cells is adenosine triphosphate (ATP).  ATP is part of the adenylate system, which includes ATP, adenosine diphosphate (ADP), and adenosine monophosphate (AMP). These molecules are ubiquitous in microbes, necessary for performing metabolic processes, and required for growth and maintaining life. Primarily, energy is transferred within cells through the production of ATP from inorganic phosphate and ADP. We have adapted and optimized a high throughput quantitative method for the purification and quantification of ATP, ADP, and AMP in marine microbes using high pressure liquid chromatography (HPLC). Coupling this HPLC purification with radiolabeled phosphate incubations enabled additional measurements of the turnover rate of ATP, ADP, and AMP in marine microbes. Measuring the turnover rate of ATP is a proxy for measuring the turnover rate of energy in the cells, whereas the turnover rates of AMP are correlated with net growth and generation time. We present our results tracking changes in these energy stocks across an ocean basin and using that to identify different metabolic strategies used by marine microbes. Additionally, we present preliminary results for how these energy stocks and turnover rates change in response to nutrient additions in a coastal environment.

Rebecca Evans (MPO)
Diurnal Oscillations in Tropical Cyclone Outflow, Structure, and Intensity
in Three Full-Physics Hurricane Simulations

Diurnal oscillations in the outflow, structure, and intensity of Tropical Cyclones have been qualitatively explored in recent observations and modeling studies. However, the exact timings and magnitudes of diurnal oscillations remain unclear, rendering it difficult to prove the importance of the diurnal cycle, as well as test any underlying mechanisms for its cause. This study aims to provide a quantitative evaluation of the diurnal cycle using a hurricane nature run as well as two idealized simulations of Category 3 and 5 intensity. Fourier filtering and Empirical Orthogonal Function analysis are used to identify modes of spatiotemporal variability. A sizeable diurnal oscillation is found in the strength of the TC outflow and radial mass flux, where outflow is enhanced during the day. This may enhance the filtering of vertically-propagating gravity waves during the day, resulting in a diurnal variation in vertical momentum fluxes into the stratosphere. In addition, there is a diurnal expansion and contraction of the storm in the vertical which accordingly modifies the upper troposphere - lower stratosphere environment. The diurnal cycles in TC structure and intensity are consistent across the three simulations, and consistent with recent studies.

Shannon Doherty (OCE)
Zoop Poop: Estimating the Contribution of Zooplankton Fecal Pellets
to Marine Organic Particle Pools

Zooplankton fecal pellets (FP) are a major contributor to the vertical flux of particulate organic matter (POM) through the marine water column and into sediments. However, the contribution of FP to POM is difficult to quantify. Current estimates and models often rely on visual identification of FP to determine their relative input to POM pools, but these methods only capture FP that are intact or recognizable. Both microbial alteration and coprohexy redistribute FP carbon into small and suspended particle pools, where particles are too small to visually identify. Lipid biomarkers are also used to quantify detrital FP contributions, but FP are not reliably biochemically distinct from zooplankton food sources. We used compound-specific stable isotope analysis of amino acids (CSIA-AA) to characterize zooplankton FP and propose that these analyses can provide an isotopic "fingerprint" of the zooplankton FP end-member in POM deriving from mixed sources. CSIA-AA can capture metabolic signatures in organic matter, allowing for the separation of FP from zooplankton biomass and food sources. We present a preliminary quantitative model to estimate FP contributions to POM based on these data and estimate the contribution of zooplankton FP to suspended and sinking particles from the subtropical and subarctic North Pacific.

Mar 13: NO SEMINAR (Spring Recess)


Mar 27 (Virtual Auditorium): Tiago Bilo
Department of Ocean Sciences, RSMAS
(one-hour MPO student seminar)

Pathways of the North Atlantic Deep Water in the North Atlantic Subtropics:
Structure and Dynamics

The structure and dynamical processes controlling the North Atlantic Deep Water (NADW) interior pathways and recirculation in the North Atlantic subtropics (15-50°N) are investigated using different observational datasets and eddy-resolving numerical experiments. Combining 12 years of Argo profiles and subsurface Argo drift data, pathways and transports of the upper-NADW (1000-2000 m) were studied. The results show clear evidence for interior pathways of upper-NADW that separate from the western boundary near the Grand Banks and flow within a large-scale deep anticyclonic gyre in the northern subtropical Atlantic (30-50°N) that extends to the eastern side of the Mid Atlantic Ridge. Between 15-30°N, our Argo-based circulation and observations from oceanographic cruises show that the mean NADW pathways are characterized by the DWBC flowing southward along the continental slope and multiple localized cyclonic recirculation cells embedded in a larger scale gyre. An assessment of the modeled mean potential vorticity (PV) budget shows that the convergence of mean eddy-PV fluxes is responsible for forcing the boundary flow to recirculate locally below 1000 m. Studying the dominant eddies in the region, the PV fluxes appear to be generated by two main types of variability: (1) DWBC meanders with periods of 100-250 days that propagate southward with the current, and (2) energetic anticyclonic oscillations with periods of ~500-550 days that occur sporadically. These large eddies slowly propagate northwestward along the continental slope, counter to the direction of the DWBC. Moored current meter records at 26.5°N from 2004-2018 suggest that similar eddies exist in the real ocean and are directly responsible for the DBWC transport variability.

Apr 03 (Virtual Auditorium): Jeremy Klavans
Department of Atmospheric Sciences, RSMAS
(one-hour MPO student seminar)

Towards a Conceptual Model of Atlantic Multidecadal Variability

The North Atlantic experiences basin-wide, multidecadal changes in sea-surface temperature (SST), and this SST variability is linked with regional- to continental-scale impacts. Despite the value in predicting these impacts, there is still scientific disagreement on the causes of Atlantic Multidecadal Variability (AMV). In this talk, I will show that the phasing of the AMV in climate models is guided by variability in external forcing with little influence from the large-scale atmospheric or oceanic circulation. Prior to the industrial revolution, much of the variability in external forcing is associated with volcanic eruptions. After 1850, the observed AMV appears to respond to a mix of variability in volcanic aerosols, anthropogenic aerosols, and greenhouse gases. Using summertime Florida rainfall as a case study, I demonstrate that when climate models include the time history of external forcing they are more likely to reproduce observed AMV impacts. I will extend this analysis to other known AMV impacts including European summer temperature and Sahel rainfall. Next, I will offer preliminary evidence that AMV magnitude in climate models is likely a balance between a model's representation of vertical ocean processes and a model's response to external forcing. Finally, I will conclude with a discussion of how these results influence our understanding of the future AMV.





Di Sang (OCE)
Case Study of Marine Atmospheric Boundary Layer Features in the Gulf of Alaska
Using Sentinel-1 SAR Images

The marine atmospheric boundary layer (MABL) is where the ocean and the atmosphere exchange large amounts of heat, moisture, and momentum, mainly via turbulent transport. Since cells and roll vortices inside the atmospheric boundary layer can change the sea surface wind field, imprints of such features can be seen in high-resolution synthetic aperture radar (SAR) images from remote sensing satellites. Changes in the wind speed and wind direction modulate the small-scale roughness spectrum of the sea surface, and therefore the intensity of the backscattered radar signals, by air-sea interaction. Since the two European SAR satellites Sentinel-1A and Sentinel-1B were launched in 2014 and 2016, thousands of high-quality SAR images have been acquired and made available free of charge to let us monitor the environment and to help us to understand phenomena such as changes in the global climate through radar remote sensing. The work presented here uses Interferometric Wide Swath (IW) images of the Gulf of Alaska region with a swath width of 250 km, provided as high spatial resolution level-1 data with a pixel size of 5 m × 20 m. Through an initial analysis of 196 images from a 3-year period (2017-2019), numerous interesting signatures of marine atmospheric boundary layer cells, roll vortices, and other interesting features have been identified. We will present examples of different kinds of features and discuss what quantitative information can be extracted from the SAR signatures by further analysis, based on methods described in recent publications in this field.



Aug 28: NO SEMINAR (RSMAS faculty meeting)

Sep 04: STUDENT SEMINARS (originally scheduled for April)

Samantha Shawver (OCE) new date confirmed
Internal Solitary Wave Amplitude and Velocity Retrieval in the
California Inner Shelf Region from Synthetic Aperture Radar Images

The Inner Shelf Departmental Research Initiative is an Office of Naval Research funded project developed to study dynamic processes in the inner shelf region off the coast of California. The inner shelf is a relatively shallow region where a wide range of processes can affect the vertical structure of the water column. During the main field experiment in September and October 2017, scientists from institutions across the U.S. performed field measurements from a variety of moorings and research vessels. The University of Miami's Center for Southeastern Tropical Advanced Remote Sensing (CSTARS) acquired 54 SAR images, including about 20 COSMO-SkyMed images with signatures of internal solitary waves (ISWs). The research discussed in this seminar focuses on using such images to better understand ISW dynamics and the role they play in this region. ISWs travel in the interior of the ocean with much larger amplitudes and propagation speeds than surface gravity waves. Echosounder ship data show ISWs with amplitudes greater than 10 m; therefore, the higher order Korteweg-de Vries (KdV) equation for a two-layer system is needed to describe the ISW dynamics. ISW signatures become visible in SAR images by wave-current interaction. We will use a simple theoretical radar imaging model to estimate large ISW amplitudes from the peak-trough distance in the SAR signatures. We will then estimate ISW velocities using pairs of SAR images from COSMO-SkyMed acquired 24 minutes apart. We believe this is the first study in which ISW velocity retrievals have been performed on pairs of COSMO-SkyMed images. We hope to then combine these two techniques to look at the relationship between ISW amplitudes and velocities.

Yu Gao (MPO) new date confirmed
The Role of Mesoscale Ocean Currents in the Mixed-Layer Heat Budget
and Air-Sea Coupling in the Southern Ocean
Yu Gao, Igor Kamenkovich, Natalie Perlin, and Ben Kirtman

Ocean mixed layer (OML) modulates the air-sea heat exchange by changing the effective heat capacity of the surface water and responding to sea surface temperature anomalies (SSTAs). Cooler SSTs generally lead to deeper OML, which is manifested by the negative correlation between the OML depth and SSTAs observed in most of the ocean. However, this simple relationship is broken in several parts of the Antarctic Circumpolar Current, where the mean currents and eddy activity are both strong and heat advection plays a big role in the mixed-layer heat balance. This property is observed in both comprehensive climate-model simulations and high-resolution regional simulations of a sector of the Southern Ocean. In this study, the importance of mesoscale advection in the OML heat budget and air-sea coupling is explored using a regional high-resolution atmosphere-ocean coupled model with a realistic atmospheric component and an oceanic model of a zonal flow. The results show that the OML heat budget is dominated by the heat advection by both the large-scale and mesoscale currents and by the heat exchanges at the base of OML. On average, the OML-integrated mesoscale heat advection is shown to induce SSTAs, while the large-scale heat advection acts to weaken them. The negative correlation between SSTAs and air-sea heat flux anomalies further demonstrates that these mesoscale current-induced SSTAs drive the anomalous air-sea heat exchange, with the warmer SSTAs releasing heat to the atmosphere, and vice versa. Therefore, neglecting the mesoscale currents in low-resolution climate model may lead to errors in the simulated SST variability and air-sea heat exchange.

Glorianne Rivera (OCE) new date confirmed
The Effects of Sea Spray on Oil Transport During High Wind Conditions

Sea spray in the ocean environment is generated by breaking waves (creating spume) as well as collapse of bubbles rising to the surface (jet drops). Sea spray impacts local air-sea fluxes and may affect the transport of heat and momentum, as well as materials such as petroleum. Likewise, the presence of slicks may impact sea spray concentrations. The behavior of sea spray in high wind conditions is still poorly understood, and the interaction of spray with oil is a new area of research. This study is performed using the Air-Sea Interaction Saltwater Tank (ASIST) at the SUSTAIN Lab at RSMAS. This experiment is conducted in two parts (1) without oil, and (2) with crude oil added to the surface. Wind speeds up to 25 m/s with fresh and salt water are studied. Images of the particles are taken with a high speed camera. Properties of sea spray including concentration, radius, and centroid are recorded and analyzed using Particle Image Velocimetry (PIV). Wave phase, local slope and height are also measured using optical imaging methods and conductivity wave probes. The analysis provides a way to understand the characteristics of these sea spray particles, primarily their distribution as function of height, size, and wave phase. Results from this experiment will help in establishing an advanced simulation tool for the modeling and prediction of oil transport in both water and air under a variety of wind and wave conditions.

Sep 11: STUDENT SEMINARS (originally scheduled for April)

Wei-Ming Tsai (ATM) new date confirmed
Composite Views of Tropical Convective Events Based On Cloud Scale

Moisture, convection, and precipitation are tightly connected in the hydrological cycle, and deep convection exhibits complex organization over a wide range of scales. Connections between the spatial size of convective systems and the surrounding environment, however, remain unclear, which is especially critical in improving the representation of multi-scale convection in weather and climate models. The above raises questions: (1) Do certain time scales and vertical structures of moistening / heating correspond to different spatial scales of convection development? In other words, what are key factors controlling the size of convection? (2) How does convection of various scales modulate the environment which is essential to successive convective events? Five-year gridded multiple datasets of 3-hourly temporal resolution are used to obtain composite time series of atmospheric states around defined convective events within 5°×5° boxes over deep tropics. Composite time series of vertical moisture indicate a 6-hour lead of low-level moisture peak prior to the precipitation maximum in larger convective events with scales above 100 km (cloud coverage 0.2). The moisture budget analysis shows the importance of convergence in convection formation of all scales and consistent drying after the formation of larger systems. A tight coupling between moisture convergence and precipitation is observed during the development of convective systems, indicating strong interactions between large-scale dynamics and convection, especially for large (high coverage) scenes. A vertical mode analysis with cloud observations reveals that deep convection corresponds to the first baroclinic mode and stratiform clouds to the second mode with a 6-hour lag to rainfall maximum, reconfirming the robust relationship between decomposed structures of large-scale motion and cloud types.

Xingchen Yang (MPO) new date confirmed
Response of the Loop Current Frontal Eddies (LCFEs) to the
Initial Condition Perturbation in the Loop Current Eddy (LCE) Shedding Forecast

An analysis of the Potential Vorticity (PV) evolution in the Loop Current (LC) region is presented for a 49-member HYCOM ensemble. The first two leading Empirical Orthogonal Functions (EOF) modes obtained from a multivariate EOF analysis of two weeks of daily outputs of HYCOM simulation are used to perturb the initial conditions. The EOF modes perturb the strength and paths of west Florida cyclonic eddy (WFCE), which presents vertical coherence during the evolution. Ensembles with positive EOF amplitude perturbations yield higher PV in the WFCE and further intrusion into the LC than those with negative perturbations. Comparing to the immediate differences shown in WFCE, the differences in PV among ensemble members occur much later over the Campeche Bank (CB). In the ensemble members with positive perturbation, the high PV over the east CB tends to bend northeastwards and develop the Campeche Bank cyclonic eddy (CBCE), which consequently impacts the timing of LCE shedding. Covariance analysis shows the averaged PV value over the east CB is positively related to the strength of WFCE front and the distance between them. Hence the speculation that teleconnection exists between the LCFEs is proposed. More analyses are being planned to verify this speculation.

Kelsey Malloy (ATM) new date confirmed
Predictability of Great Plains Low-Level Jet and Summer Hydroclimate:
Assessing Extratropical Teleconnections

Extreme warm-season precipitation in the United States dominates in the north-central and Midwest regions. This has significant socioeconomic implications in the U.S. "Corn Belt", ranging from agricultural production to human and property loss from associated flooding. Unfortunately, current subseasonal-to-seasonal forecasts for summer precipitation have very low skill compared to winter. The main reason for this is that there is little to no consensus about the fundamental cause of interannual and intraseasonal variability of continental U.S. (CONUS) summer rainfall. It is proposed that much of CONUS rainfall variability can be explained by the fluctuations in strength and orientation of southerly moisture transport by the Great Plains low-level jet (LLJ). Given that El Niño-Southern Oscillation (ENSO) forcing of the upper-level geopotential height pattern is fairly weak in the summer, other remote influences of the Great Plains LLJ are explored during the June-July-August-September (JJAS) season using an idealized dry baroclinic model. Results indicate that there are distinctive and interfering roles of forcing from the East Asian monsoon and North Atlantic Subtropical High (NASH) in influencing Great Plains LLJ magnitude and variability. In addition, there is an intraseasonal transition (JJA versus JAS) in prevalence of remote influences. This study quantifies the relative contribution of ENSO, the East Asian monsoon, and NASH, which facilitates greater understanding of Great Plains and Midwest hydroclimate.

Sep 18: STUDENT SEMINARS (originally scheduled for April)

Haozhe He (ATM) new date confirmed
CMIP Radiative Forcing Evolution

Since radiative forcing is rarely computed separately in climate models, a comprehensive analysis of instantaneous radiative forcing (IRF) and effective radiative forcing (ERF) is conducted by adopting radiative kernel-regression technique to coupled model simulations from recent three CMIP phases. The results show that the IRFs under historical, 1pctCO2 and abrupt 4×CO2 scenarios all have increased substantially in CMIP6 collection. Less negative shortwave IRF leads to the historical IRF increase while increase in longwave IRF is attributable to the IRF increase to CO2 quadrupling. Decomposition indicates that spread in the ERF, which accounts on the IRF and rapid adjustments to this forcing, receives equally substantial contributions (roughly 50%) from intermodel differences in the IRF and cloud adjustment, respectively. Nearly all of the spread in ERF can be explained by differences in the IRF, cloud adjustment, and stratospheric adjustment, implying that tropospheric non-cloud adjustments to CO2 forcing play only a secondary role. Moreover, a significant positive correlation between the IRF and IRF-initiated cloud adjustment is identified. This further highlights the effects of IRF intermodel uncertainty exerted on substantial differences in the ERF and projected detailed climate responses, and once again underscores the need to archive double-call radiative transfer calculations of the IRF as a routine diagnostic.

Luna Hiron (MPO) new date confirmed
Ageostrophic Flow in the Loop Current System

The Loop Current (LC) system is well known to be energetic and dynamic, and due to its irregularity, the forecast of this system remains a challenge. Aiming to elucidate the dominant forcings involved, a momentum budget analysis was conducted using a high-resolution, free-run model. To complement this analysis, the ageostrophic flow was quantified using in-situ temperature and total velocity from a mooring array, and temperature and salinity from an oceanographic survey mission on a NOAA WP-3D hurricane hunter aircraft. The results indicate that the pressure gradient and the Coriolis forces have the same magnitude and are in balance, except for some specific cases. During periods of interaction between the LC and the Loop Current Frontal Eddies (LCFEs), the nonlinear term becomes larger and appears to be equally important as the pressure gradient and the Coriolis force. During these periods, the flow becomes more ageostrophic as the LCFE grows, and the geostrophic balance no longer holds. These results show that the ageostrophic component of the flow also plays a fundamental role in the Loop Current dynamics, in particular during LCFE intensification and LC shedding, and therefore cannot be neglected when studying the LC system. It also highlights the importance of measuring ocean current structures, especially during the last stages of the LC.

Houraa Daher (OCE) new date confirmed
How Will Agulhas Leakage Respond as the Ozone Recovers?

The stratospheric ozone is expected to recover within the next 50 years as a result of the implementation of the Montreal Protocol. The recovery of the ozone hole over Antarctica has large implications for the Southern Hemisphere (SH) climate as the ozone and greenhouse gases (GHGs) will no longer work together to increase the positive phase of the Southern Annular Mode (SAM) and the poleward shift in the westerly jet, but rather the two forces will begin to oppose each other. The ozone recovery will lead to a negative phase of the SAM while the GHGs will continue to cause the SAM to be in its positive phase. Therefore, the ozone recovery will work to weaken or reverse the effects on the SH circulation caused by ozone depletion and the increase in GHGs over the past few decades. Previous studies are all in agreement that Agulhas leakage is strongly correlated with the Southern Hemisphere westerlies and the SAM, with the intensification of the westerly jet and positive phase of the SAM leading to an increase in leakage. However, how will Agulhas leakage change as the ozone recovers over the next century? Using CCSM4, a coupled climate model, we will examine the Agulhas leakage response to the ozone recovery for the first time. One might expect Agulhas leakage to decrease as previous studies show that nearly all climate changes in the Southern Hemisphere have essentially been reversed as the ozone recovers, however, we expect that how much the GHG forcing increases over the next century to play a large role in determining the behavior of Agulhas leakage during the ozone recovery period.

Sep 25: STUDENT SEMINARS (originally scheduled for March)

Joseph Anderson (OCE) new date confirmed
Segmentation of Sea Ice in Synthetic Aperture Radar Imagery

Sea ice concentration (SIC) and floe size distribution (FSD) are variables derived from sea ice imagery used in climate models. Satellite observations of SIC and FSD in the visible spectrum are somewhat rare, due to the Arctic's inclement weather and the total darkness in the winter. Synthetic Aperture Radar (SAR) operates independently of sunlight and clouds are transparent to it, making it an effective tool for studying the Arctic. One drawback of working with SAR is that images may be difficult to interpret due to the effects of wind and changing incidence angles on returned brightness temperature, making automatic determination of FSD and SIC difficult. Current state-of-the-art methods to determine FSD and SIC use machine learning and computer vision algorithms such as k-means clustering and the watershed algorithm, requiring human inspection at several steps in the process. Preliminary results suggest that current computer vision algorithms are not well suited for fully automated segmentation of sea ice in SAR imagery. To make use of the large number of SAR images available, a deep learning approach is suggested, where a fully convolutional network (FCN) is used to segment images without any human input after model training.

Yueyang Lu (MPO) new date confirmed
Estimate of Eulerian Eddy Diffusivity Using Lagrangian Particles

Eddy diffusivity quantifies the efficiency of eddies in large-scale tracer transport and is formulated by assuming a linear relationship between eddy flux and tracer gradient. The traditional way to calculate eddy diffusivity needs simulations of at least two passive tracers, which are transported by the same velocity field. However, physically speaking, pathways of fluid parcels are the same for different tracers. Here a new method to derive Eulerian eddy diffusivity from Lagrangian particle data is proposed and is used to study the eddy-induced diffusion in the Gulf Stream region by using data from the Hybrid Coordinate Ocean Model (HYCOM). We assume that particle trajectories record the pathways of fluid parcels. Thus the large-scale tracer and eddy flux fields are readily constructed by prescribing each particle a certain amount of concentration. This method is computationally more efficient than the traditional one because only one integration of the velocity field is needed. We will show that the eddy-induced transport is highly dependent on direction and location. The dependence of eddy diffusivity on the number of particles will be also discussed, with a focus on the convergence to a "true" diffusivity.

Nektaria Ntaganou (MPO) new date confirmed
The impact of the West Florida Shelf Topography on the Loop Current System Variability
Nektaria Ntaganou, Vassiliki Kourafalou, Matthieu Le Hénaff, and Yannis Androulidakis

The effects of the West Florida Shelf topography on the Loop Current system have been studied in the past but remain a matter under investigation. The goal of this study is to improve our understanding of the controlling factors of Loop Current intrusion, as well as the impact of the shelf's topography on the eddies associated to the Loop Current. We address the issue by conducting numerical experiments that only differ with respect to the topography of the West Florida Shelf, using the Hybrid Coordinate Ocean Model (HYCOM) applied in the Gulf of Mexico (GoM-HYCOM). This approach allows to isolate the topographic controls and investigate their sole impact on the Loop Current System evolution. Our results show that the topography of the West Florida Shelf controls not only the intrusion of the Loop Current into the Gulf of Mexico, but also its minimum latitude and axis tilt. In the modified topography experiments, the Loop Current, regardless of its initial position, does not retract below 26°N and has a predominant westward tilt, whereas in the realistic case, the minimum latitude can be as south as 24°N and the mean axis angle is eastward. A predominant westward axis tilt promotes the LCE shedding and the shifting of the eddy activity towards the deeper Gulf of Mexico waters. Thus, the depth and steepness of the shelf control to a great extent both the variability of the current and the distribution of the eddy kinetic energy. Finally, we find that positive potential vorticity anomaly advection from the southwestern tip of the Florida Shelf into the Florida Straits seems to be one of the controlling factors of the LC intrusion into the Gulf of Mexico.