COMPASS Friday

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COMPASS Friday

SPRING 2021
Fridays at 11:00 am, RSMAS Auditorium / Virtual Auditorium
(unless stated otherwise)

Jan 29: NO SEMINAR

Feb 05: NO SEMINAR (Recruitment Weekend)

Feb 12: Dr. Maofeng Liu
Department of Atmospheric Sciences, RSMAS

Enhanced Hydrological Cycle Increases Ocean Heat Uptake
and Moderates Transient Climate Sensitivity
Recording Available at COMPASS ON DEMAND

The large-scale moistening of the atmosphere in response to the greenhouse gas increases tends to amplify the existing patterns of precipitation minus evaporation (P–E) which, in turn, amplifies the spatial contrast in sea surface salinity (SSS). It is proposed that subtropical surface salinification due to the intensified hydrological cycle provides a buoyancy sink that increases the rate of ocean heat uptake and moderates transient climate sensitivity. The impact of these SSS changes is quantified in a series of CO2 doubling experiments using two configurations of a coupled climate model: a standard configuration and a modified one in which SSS is held constant by restoring it back to its seasonally-varying climatology from the control run. In response to CO2-induced warming, dry conditions (P–E < 0) over the subtropical oceans are amplified due to an enhanced hydrological cycle, increasing the SSS in salty regions. There is an increased rate of ocean heat uptake in the standard CO2 doubling experiment relative to the fixed-SSS version. The largest increase in ocean heat content (OHC) for the standard run occurs in the southern subtropical Pacific and the tropical and subtropical Atlantic Ocean, where SSS shows the largest increase, highlighting the role of salinification in accelerating heat uptake. Consistent with a smaller rate of ocean heat uptake, the fixed-SSS version produces a transient climate response approximately 0.4 K greater than the standard run. The weakening of the Atlantic Meridional Overturning Circulation in response to high latitude freshening and warming may also play a role in modulating the OHC, which is explored using a set of partially fixed-SSS experiments.

Feb 19: NO SEMINAR (RSMAS Faculty Meeting)

Feb 26: NO SEMINAR

Mar 05: Dr. Francisco Javier Beron-Vera
Department of Atmospheric Sciences, RSMAS

Nonlinear Saturation of Thermal Instabilities: The Return of the Casimirs

A simple recipe, introduced in as early as at least the late 1960s, to incorporate thermodynamic processes (e.g., those due to heat and freshwater exchanges through the air-sea interface) in a one-layer ocean model consisted in allowing buoyancy to vary with horizontal position and time, while keeping velocity as independent of the vertical coordinate. The simplicity of the resulting inhomogeneous-layer model with fields that do not change with depth – referred to as IL0 to reflect this – promised fundamental understanding of ocean processes which would be difficult to gain by analyzing direct observations or the output from an ocean general circulation model. A number of applications of the IL0, particularly to equatorial dynamics, appeared in the 1980s and 1990s supporting this line of thought. Regrettably, the increase in computational power in the current century has led to emphasize reproducing observations over gaining basic physical insight of the type that ocean modeling based on the IL0 system or variants thereof was expected to provide. A pleasant surprise, however, has been to learn that layer ocean modeling with reduced thermodynamics is regaining momentum. Moreover, the renewed interest in this type of modeling is exceeding a pure oceanographic interest. Indeed, applications of the IL0 have been extended to atmospheric dynamics, both terrestrial and planetary.

Of particular interest to the present work are recent numerical simulations of the IL0 which have shown that it can sustain subinertial (i.e., with frequency smaller than the local Coriolis parameter, twice the local Earth's rotation rate) circulatory motions that resemble quite well submesoscale (1–10 km) features often observed in satellite ocean color images. In this talk I'll show how the conservation laws (energy, zonal momentum, and infinite family of Casimirs!) of the IL0 system constrain such thermal instabilities from growing indefinitely in the absence of a high-wavenumber cutoff in the IL0 model.

Mar 12: STUDENT SEMINARS

Lillian Henderson (OCE)
Does Photosynthesis Drive Vertical Variations in δ13CPOC Values
in a Marine Water Column?

Recent compilation of published particulate organic carbon stable isotope ratios (δ13CPOC) showed a statistically significant, widespread difference in δ13CPOC values between the upper and lower euphotic zone. Particulate organic carbon (POC) makes up the base of marine food webs and is the main driver of carbon export in the ocean. Therefore, understanding what is driving these variations in δ13CPOC over depth in the present-day ocean is critical for interpreting carbon isotopes in the sedimentary record and for applications to food web isotope mixing models. Phytoplankton, or tiny photosynthetic plankton, are a major contributor to POC. Many factors are known to influence the distribution of carbon isotopes between phytoplankton biomass and the carbon they are fixing, and many of these parameters vary significantly between the upper and lower euphotic zones. Therefore, I hypothesize that photosynthetic differences between the upper and lower euphotic zone may be driving these vertical variations in δ13CPOC values. Here, I briefly present the results of the global data analysis, along with a plan to isolate the photosynthetic δ13C signal of POC in order to test this hypothesis.

Yueyang Lu (MPO)
A New Approach to Representing the Lateral Mesoscale Eddy Transport

Mesoscale eddies profoundly impact the ocean dynamics and climate by redistributing such properties as heat and carbon. The eddy transport is usually represented using a flux-gradient relationship with a tensor coefficient for the lateral fluxes. However, recent studies show that this transport tensor is tracer-dependent, implying that it is not uniquely determined by the flow. In addition, the non-zero component of eddy fluxes across horizontal / isopycnal planes makes the representation of lateral fluxes incomplete. To alleviate such issues, we develop a new approach to representing the lateral eddy flux divergence in terms of large‐scale properties. It involves solving a set of pointwise local problems rather than a single global problem that could be sensitive to boundary conditions. The flux divergence is subtracted by a term of the divergent lateral volume transport that aims to reduce the bias caused by the missing representation of the vertical / diapycnal component. We test the skill of our new method by running three simulations in a HYCOM-based offline tracer model: a control simulation, a simulation without eddies and a simulation with eddies substituted by the new scheme. Metrics such as the large-scale tracer distribution, tracer gradient and tracer cloud dispersion are used for comparison. Results show that the new scheme is less tracer-dependent. It is able to reproduce key features that are present in the control run and make an improvement to the "eddy-absent" run. Our study provides the possibility of an alternative type of eddy parameterization.

Kelsey Malloy (ATM)
East Asian Monsoon Forcing and North Atlantic Subtropical High Modulation
of Summer Great Plains Low-Level Jet

Dynamic influences on summertime interannual and intraseasonal United States rainfall variability are not well-understood. The most prominent cause of moisture transport in the summer is the Great Plains low-level jet (LLJ). Using observations and a dry nonlinear atmospheric general circulation model (AGCM), we explored the distinct and combined impacts of two prominent atmospheric teleconnections – the East Asian monsoon (EAM) and North Atlantic subtropical high (NASH) – on the Great Plains LLJ. Separately, a strong EAM and strong western NASH were linked to a strengthened Great Plains LLJ. However, linkages differed when considering their interference. Strong EAM and weak western NASH events are associated with a Rossby wave pattern that amplifies Great Plains LLJ strengthening. In contrast, strong EAM and strong western NASH events are associated with different – even opposite – upper-level wave patterns that weakens or removes the Great Plains LLJ signal. The AGCM unforced experiments simulated comparable dynamic relationships. We also compared unforced and EAM-forced runs in June-July-August (JJA) and July-August-September (JAS) background states to distinguish any subseasonal transitions in these teleconnections and their interference. The experiments revealed that internal atmospheric variability plays an important role. Both the JJA and JAS forced response suggested robust Great Plains LLJ strengthening. Yet, the responses from these experiments had distinct features, especially in the upper levels, suggesting sensitivity to the background state. When differentiating between strong and weak western NASH events in the EAM experiments, we only detected significant differences in the LLJ over the southeast and eastern United States.

Mar 19: Luna Hiron
Department of Ocean Sciences, RSMAS
(one-hour MPO student seminar)

Intensification of Loop Current Frontal Eddies and
Their Interactions With the Loop Current and Surrounding Flow
Recording Available at COMPASS ON DEMAND

Loop Current Frontal Eddies (LCFEs) are cold-core vortices located in the Loop Current (LC) vicinity and are known to intensify and play a role in the LC shedding. LCFE amplification also affects the local circulation; during the 2010 Deepwater Horizon event, part of the oil was entrained inside an intensified LCFE. Based on observations (mooring array and drifters), altimetry, and a 1-km resolution numerical model, we study the effects of the LCFE intensification on the LC and the surrounding flow. First, we show that strong LCFEs strengthen and deepen the LC front, and drive an increase in the horizontal density gradient and local available potential energy in the LC. Additionally, we show that during LC shedding with intensified frontal eddies, the centrifugal force becomes as important as the Coriolis force and the pressure gradient-force: LC meanders are in gradient-wind balance. Finally, we assess the ability of the LCFEs to transport particles without exchange with the exterior (i.e., material coherence). We find that frontal eddies can remain coherent for up to 17 days (23 days) at the surface (deeper layers). The particles inside the frontal eddies were tracked backward in time and showed that the material coherence of the eddies builds up from water not only from the LC but also from the surrounding and the West-Florida shelf, potentially driving cross-shelf exchange of particles, water properties, and nutrients. Thus, LCFE intensification affects the LC flow and the transport of oil and particles in the Eastern Gulf of Mexico.

Mar 26: STUDENT SEMINARS

Jimmy Ge (MPO)
Evolution of Convection and the Early-Stage Tropical Cyclone
in Radiative Convective Equilibrium

The effects of vertical wind shear (hereafter, "shear") on tropical cyclone (TC) structure and intensity have presented a longstanding problem for forecasters and researchers. Shear is generally recognized as an inhibiting factor to storm intensification via vortex misalignment or dry air ventilation. However, some weak storms experience periods of "unexpected" rapid intensification even in moderate shear environments when the low-level circulation reforms near deep convection downshear, with significant implications for storms near landfall. From a theoretical perspective, shear also modifies the location of latent heating relative to the circulation center, invoking asymmetric vortex dynamics in three dimensions. We take a first look at moist thermodynamic processes using an idealized numerical simulation of a developing TC in an atmosphere in radiative-convective equilibrium using the Weather Research and Forecasting (WRF) model. Without a precursor disturbance, in a shear-free environment, we found that convective aggregation occurred in a time span of 40 to 50 days, with a nearly steady-state TC forming toward the end of the simulation period. Future plans for additional simulations with shear, and supplemental analysis with aircraft and satellite observations, are discussed.

Ivenis Pita (MPO)
Analysis of Atlantic Dynamical Sea Level Projections for the 21st Century

Climate models project a mean global sea level increase up to 40 cm in the end of the 21st century, based on optimistic IPCC simulations. However, these projections are subject to strong uncertainty, which include, but are not limited to, ice melting and regional ocean dynamics. Here, the dynamical sea level response to the 21st century forcing is investigated for the Atlantic Ocean, with focus on the South Atlantic, using the Australian Community Climate and Earth System Simulator (ACCESS5). To understand how different drivers influence projections of the dynamical sea level, the forcings are separated into surface heat, freshwater and momentum fluxes. Links with the ocean circulation changes, including the gyre and the Atlantic Meridional Overturning Circulation (AMOC) variability are investigated. Preliminary results show the mean patterns of the sea level in the Atlantic, and how the different forcings influence these patterns via added and redistributed heat across the basin.

Lev Looney (MPO)
The Competing Effects of Ocean Stratification in Hurricane-Induced Ocean Cooling

From 1980 until 2020, the United States had 52 tropical cyclone related billion dollar disasters, totaling over $997 billion in damages. The 2020 Atlantic season alone caused over $51 billion in damages and more than 431 deaths. The best way we can reduce the number of deaths and damages is to better prepare in advance though accurate and timely forecasts. While the forecasts of tropical cyclone tracks have been significantly improving through time, the intensity forecasts have not followed at nearly the same rate. It has been shown that oceanic conditions play a crucial role in the intensification of these storms. As hurricanes primarily get their energy from ocean heat,  they can also cool the surface ocean through vertical mixing, acting as a limitation to their intensity. We aim to further understand the mechanisms that control the amount of ocean cooling under a storm and relate it back to intensity changes. To investigate this, we utilize a one-dimensional mixed layer model, initialized with a variety of temperature and salinity profiles, forced with a simulated cyclone to better forecast the amount of vertical mixing (and thus surface cooling). Temperature stratification has competing effects: stronger stratification leads to cooler water near the surface, yet it also leads to resistance to vertical mixing due to the density gradient. In contrast, salinity stratification almost always acts to reduce mixing and cooling. Initial results have shown that the gradient of temperature plays a much greater role than density stratification in surface cooling.

Apr 02: Kaycie Lanpher
Department of Ocean Sciences, RSMAS
(one-hour OCE student seminar)

Marine Microbial Metabolic Responses to Environmental Conditions:
The Impacts of Light, Temperature, and Oxygen Concentration
Recording Available at COMPASS ON DEMAND

Marine microbes, which include single-celled bacteria, archaea, and eukarya, are the main drivers of carbon and energy transformations in the global ocean and marine food webs, through the microbial loop. Microbial communities are highly sensitive to their physio-chemical environments and change rapidly in response to changing conditions. The adenylate system is the energy currency at the core of cellular metabolic processes and is ubiquitous across microbes. Within this system, energy is chemically stored in the phosphoanhydride bonds of adenosine triphosphate (ATP). The phosphorylation rates, or turnover rates, of ATP are direct measures of microbial metabolic activity. We developed and used a novel high pressure liquid chromatography (HPLC) and radiolabeled phosphate incubation method to quantify the concentration and phosphorylation rates of ATP in microbial communities. We present results from environmental studies quantifying the microbial metabolic activity across a range in oxygen concentrations, light conditions, and temperature to better define the dynamic links between microbial metabolic energy, environmental conditions, and the activity of the microbial loop.

Apr 09: STUDENT SEMINARS

Haozhe He (ATM)
State Dependences of Instantaneous Radiative Forcing from CO2 Perturbation

Instantaneous radiative forcing (IRF) is a fundamental metric for measuring the extent to which anthropogenic activities and natural events perturb the Earth's energy balance. This perturbation initiates all other changes of the climate in response to external forcings. In terms of experiments with abrupt doubling / quadrupling CO2 concentration, we usually think IRF should hold constant for the entire runs. However, our results indicate that IRF from CO2 perturbation increases with surface air temperature, with an IRF sensitivity to global-mean surface warming around 0.07 W m–2 K–1 (0.05 W m–2 K–1) under clear-sky (all-sky) condition, based on evidence from both online and offline double-calls and a theoretical model. In this case, this finding undermines the prevalent forcing-feedback framework (e.g., Gregory et al., 2004). Meanwhile, a further decomposition using the theoretical model suggests the intermodel spread of IRF which has remained high over almost three decades is due to the intermodel uncertainty in stratosphere temperature, especially the stratosphere temperature uncertainty in preindustrial runs.

Manish Devana (MPO)
Iceland Scotland Overflow Variability from the OSNAP Mooring Array
Manish S. Devana, William E. Johns, and Adam Houk

The Iceland Scotland Overflow (ISOW) is one of the critical water masses which make up the Atlantic Meridional Overturning Circulation's deep limb. Moored observations along the Reykjanes Ridge, from the Overturning in the Subpolar North Atlantic Program (OSNAP), show significant variability in the ISOW transport, ranging from 15 Sv to –5 Sv. The observations show a spatially varying structure with three primary cores of flow, each with different temporal variability. EOF decompositions of current meter records show a dominant mode centralized in a rift valley along the mooring array. The second mode shows a second core of variability near the ridge axis. While modes 1 and 2 account for similar fractions of the velocity variance (19.2% and 15.3% respectively), mode 1 recovers nearly 80% of the transport variance. To further investigate ISOW transport variability, the observed transport was compared to geostrophic estimates of transport. Geostrophy was able to recover two thirds of the mean transport (3.4 Sv of 5.3 Sv). The divergence between the two estimates was largely attributed to a failure by geostrophy to recover the flow concentrated in the rift valley. These results combined with the mode decomposition indicate that ISOW transport is dominated by a bottom intensified surging mode. The second mode indicates a meandering variability which is likely associated with upper ocean currents in the region but doesn't significantly affect transport. The ISOW transport variability is likely linked to upstream flow-topography interactions rather than local forcing.

Wei-Ming Tsai (ATM)
Competition Between Shear-Organized and Unorganized Convection
in Large-Domain CRM Simulations

Multicellular organization of deep convection is commonly observed, but ignored in contemporary global circulation model parameterizations. Is it important, and if so, how? In an atmosphere destabilized by homogenous forcing, would a region of more organized convection out-compete regions with spotty convection for the resulting moist convective instability? This study aims to modulate the degree of organization and quantify its effects on both dynamics and thermodynamics. We build a set of idealized experiments to address those questions using large-domain simulations with Cloud Model 1 (CM1), a 3D cloud-resolving model with explicit representations of both convective-scale and domain-scale circulations. A double-periodic domain is uniformly destabilized with a homogeneous cooling of 4K/day and corresponding surface fluxes to maintain the energy balance. A control run is simulated without background wind shear to represent a homogenously convective domain. To generate the organization gradient in the domain, experimental runs are designed by imposing various sheared zonal wind profiles in an east-west strip covering only part of the y-domain. All simulations are run for 10 days to reach their statistical equilibrium states. Preliminary results indicate a domain-averaged drying effect proportional to the organization gradient. The self-generated circulation resulting from competition among two regions supports organized convection. Besides, stronger precipitation is found over the sheared region, implying higher precipitation efficiency associated with the formation of organized systems. 

Apr 16 (9:00): Xingchen Yang
Department of Ocean Sciences, RSMAS
(one-hour MPO student seminar)

Loop Current Frontal Eddies' Response Due to Initial Condition Uncertainties
and
the Teleconnections Between Frontal Eddies
During Loop Current Eddy Shedding Process
Click to Attend via Zoom

An ensemble of simulations of the circulation in the Gulf of Mexico is analyzed to investigate the dynamical interactions between the Loop Current (LC) and the frontal eddies. The ensemble members differ in their initial conditions and more, specifically in the initial strength of the West Florida Cyclonic Eddy (WFCE). Polynomial chaos (PC) approach is applied to link the uncertain model input data with output to empower the dynamical and statistical analysis. The initial condition perturbed the strength and size of the WFCE immediately. The WFCE, with coherent vertical structure, either propagated along the LC eastern edge or penetrated the LC in different extents. The influence of the perturbation over the Campeche Bank Cyclonic Eddy (CBCE) appears later than that of WFCE. For those realizations with strong WFCE and its penetration into the LC, CBCE moved east-northeastward, contributing to the LC necking down, and tending to merge with WFCE. Otherwise, CBCE propagated northwestward, along the western edge of LC. The perturbation of the initial condition on these two LCFEs consequently determined the occurrence and timing of LCE detachment. Positive correlation has been found between the strength of WFCE and CBCE, thus teleconnection between LCFEs is suggested. Comparing with mooring data over eastern Campeche Bank, ensemble simulations did great job at representing CBCE offshore displacement.

Apr 16: Matthew Grossi
Department of Ocean Sciences, RSMAS
(one-hour MPO student seminar)

Ocean Trajectory Prediction Using Machine Learning Tools
Click to Attend via Zoom

Maritime environmental disasters such as the Deepwater Horizon oil spill accentuate the importance of forecasting material transport in the ocean. Yet this remains an arduous task, largely due to the turbulent and chaotic nature of surface flow dynamics. Traditional approaches to ocean forecasting include theory-based circulation models and statistical stochastic models, but predictive performance is often hindered by the need to parameterize sub-grid scale motions and by the sparsity of observational data – which in turn limits the ability to properly initialize models and accurately tune parameters. An alternative approach is to extract information from available observations and use this information to forecast future states, the heuristic basis of machine learning (ML). Here we utilize 300 surface drifters from the Grand Lagrangian Deployment (GLAD) experiment in the Gulf of Mexico to investigate the potential of ML by itself for ocean transport prediction. We start by exploring the learnability of trajectories from increasingly complex simulated flow regimes, trajectories from a physically-based realistic flow scenario, and finally of observed trajectories. ML predictive performance is assessed against traditional autoregressive integrated moving average (ARIMA) models. We also compare the performance of simple artificial neural networks and more complex spatiotemporal graph convolutional neural networks at predicting ocean trajectories on the order of hours and days in order to expose the strengths and weaknesses of these techniques relative to ARIMA.

Apr 23: STUDENT SEMINARS

Leah Chomiak (MPO)

Samantha Furtney (OCE)
Internal Solitary Wave Amplitude and Velocity Retrieval
From Synthetic Aperture Radar Images of the California Inner Shelf Region
Click to Attend via Zoom

The main field campaign of the Inner Shelf Departmental Research Initiative was conducted in the section of coast near Point Sal, California, in September to October 2017 to better understand the dynamic processes that occur along the inner shelf. Internal solitary waves (ISWs) are ubiquitous in the synthetic aperture radar (SAR) satellite images and will be the focus of this presentation. The scale invariant feature transform (SIFT) is used as a tool to track ISWs between pairs of COSMO-SkyMed SAR images acquired about 24 minutes apart. ISW mean velocities are estimated using the SIFT matched keypoints between image pairs. Using a previously developed technique, ISW amplitudes are estimated by relating the distance between the positive (wave peak) and negative (wave trough) peaks of the ISW signatures in radar images. We then calculate the instantaneous ISW velocities in the individual SAR images based on the relationship between ISW amplitude and velocity using the Korteweg-de-Vries equation. Preliminary results show that the SIFT estimated velocities are within 7 cm/s of the amplitude-derived instantaneous ISW speeds. Combining these two techniques, we extend our abilities to study ISWs from space and our understanding of ISW dynamics in the California inner shelf region.

Yu Gao (MPO)

Glorianne Rivera (OCE)

Apr 30: Simge Bilgen
Department of Atmospheric Sciences, RSMAS
(one-hour MPO student seminar)

May 07: Dr. Christine Gommenginger
National Oceanography Centre, Southampton, United Kingdom

SEASTAR: An Innovative Satellite Mission to Measure
Small-Scale Ocean Surface Dynamics and Vertical Ocean Processes
in Coastal, Shelf and Polar Seas
Click to Attend via Zoom