FALL 2020
Fridays at 9:30 am, RSMAS Auditorium / Virtual Auditorium (unless stated otherwise)
Aug 21: NO SEMINAR
Aug 28: NO SEMINAR (RSMAS Faculty Meeting)
Sep 04: STUDENT SEMINARS (rescheduled from Spring 2020)
Samantha Furtney (OCE)
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)
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 Benjamin 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)
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 (rescheduled from Spring 2020)
Xingchen Yang (MPO)
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 CB cyclonic eddy (CBCE), which consequently impacts the timing of LC eddy (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 LC frontal eddies (LCFEs) is proposed. More analyses are being planned to verify this speculation.
Kelsey Malloy (ATM)
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 (rescheduled from Spring 2020)
Haozhe He (ATM)
Emergent Constraints and Intermodel Spread Analyses of
Combined Lapse Rate – Water Vapor and Cloud Feedbacks
Uncertainties in climate feedbacks are essential sources of spread in simulated surface air temperature responses to anthropogenic forcing. Cloud feedback appears as the main source of disagreement in future projections while lapse rate – water vapor (LR+WV) feedback is often overlooked due to offsets between these two components in both magnitude and uncertainty. Here observation-based emergent constraints are adopted to minimize intermodel spreads in these climate feedbacks. Observed interannual variation provides a useful constraint to narrow down the uncertainty in long-term cloud feedback. Similar regional contributions on both interannual and long-term feedbacks further validate the effectiveness of the constraint on cloud feedback. Additional analysis shows that local contributions to total cloud feedback are dominated by shortwave cloud feedback. Differences in tropical Pacific SST warming patterns are a key to interpret the global-mean cloud feedback spread. Based on observed uncertainty in global-mean LR+WV feedback, however, hardly any model could be excluded. The spread of global-mean LR+WV feedback is dominated by the tropics, where the largest contribution comes from uncertainties of local relative humidity feedback. The local disagreements over the northern and southern extratropics, which are associated with Planck related water vapor feedback under fixed relative humidity conditions, are compensating each other. As a result, the uncertainty of the tropical-mean LR+WV feedback is twice as large as that of the global-mean feedback. Additionally, model divergences in heat uptake over Southern Ocean and Atlantic meridional overturning circulation related downwelling also account for the spread in global-mean LR+WV feedback.
Houraa Daher (OCE)
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 (rescheduled from Spring 2020)
Joseph Anderson (OCE)
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)
On the Complexity of Eddy Diffusivity in the Ocean
Mesoscale eddies profoundly impact the ocean dynamics and climate by redistributing such properties as heat and carbon. The efficiency of eddies in these large-scale tracer transports is usually quantified by eddy diffusivity in most ocean and climate models. Studies show that the simulated state is very sensitive to this parameter, and accurate estimates of eddy diffusivities are clearly needed to improve these simulations. This study aims to examine main properties of the eddy diffusivity, and to explore to what degree the diffusivity is uniquely determined by the flow. We diagnose the two-dimensional lateral diffusivity tensor using a HYCOM-based offline tracer model using an inversion of the conventional flux-gradient relation. The eddy-induced tracer flux is split into three terms: the mean and eddy advection of tracer anomaly, and the eddy advection of mean tracer contour. The dependence of diffusivities from each term is studied. Results show that the diffusivity has a wide range of values among different tracer pairs, indicating that it is not uniquely determined by the flow. However, reduction of the range is observed for certain terms. Our study emphasizes the complexity of eddy parameterization in climate models and the need of a refined diffusion model for eddy-induced transport.
Nektaria Ntaganou (MPO)
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.
Oct 02: Bingkun Luo
Department of Ocean Sciences, RSMAS
(one-hour MPO student seminar)
Saharan Dust Effects on Sea Surface Radiative Fluxes and Satellite Derived SSTs
Determined From Field Data Analysis and Model Simulation
The large-scale Saharan dust outbreaks usually occur over the tropical and subtropical Atlantic Ocean and have been observed from the visible and infrared sensors on satellites for many years. The dust aerosol scattering and absorption effects on the surface radiative fluxes (SRF) are primarily in the shortwave, but the dust aerosols increase the infrared signal attenuation and thus degrade the accuracy of sea-surface temperature (SST) retrievals from measurements of infrared radiometers on satellites. Accurate quantification of the dust perturbation to the surface energy budget is fundamental to understanding critical climate feedbacks and to determine the Earth's surface energy budget; the dust radiative forcing could be determined using satellite-derived variables, but it can be highly uncertain. By exploiting a large set of measurements from many ship campaigns in conjunction with satellite and reanalysis products, this study characterizes the sensitivity of the SRF and satellite derived skin SST to the Saharan dust outflow with field-data match-up analysis and radiative transfer model simulations. The dust aerosol radiative effects are related to the dust layer concentration, altitude and temperature. Greater physical insight into the radiative transfer through an aerosol-burdened atmosphere will substantially improve the predictive capabilities of weather and climate studies on a regional basis.
Oct 09: Dr. Ben Kirtman
Department of Atmospheric Sciences, RSMAS, and
Cooperative Institute for Marine & Atmospheric Studies (CIMAS)
Issues in Sub-Seasonal to Multi-Year Prediction Research
Zoom Recording Available at COMPASS ON DEMAND
This talk summarizes the status of the NOAA funded multi-institutional sub-seasonal prediction experiment (SubX) and the North American Multi-Model Ensemble (NMME) seasonal prediction effort. The focus is on lessons learned and outstanding predictability and prediction research questions. Finally, we describe the background and motivation for an emerging multi-institutional effort in multi-year prediction. Again, emphasizing the most challenging research questions.
Oct 16: STUDENT SEMINARS
Haley Royer (ATM)
Dust Emissions From Dried Out Lakebeds and Their Impacts on Air Quality and Climate
Desiccated saline lakebeds (playas) have recently been growing in spatial extent and number globally due to climate change and land use practices. The growth of these geologic features can in turn impact air quality and climate. For example, reactions between Cl-containing playa dust and dinitrogen pentoxide (N2O5) can lead to the formation of nitryl chloride (ClNO2), a precursor of secondary organic aerosols and tropospheric ozone. Recent research shows that current parameterizations poorly predict ClNO2 formation based on Cl– content from playa dust, suggesting that our understanding of the relationship between aerosol chemical composition and ClNO2 formation is incomplete. To better understand this relationship, we utilized four analytical techniques to determine the chemical composition and mineralogy of playa dust from the southwest United States. Results indicate the presence of highly hygroscopic Cl-containing minerals which can deliquesce at low relative humidity (RH), thereby facilitating ClNO2 formation at low RH. We also detected competing chemical constituents such as clay minerals and organic matter that can impede ClNO2 formation even when Cl– concentrations and RH are favorable. Finally, comparisons of ClNO2 yields and Cl– content from bulk and surface analytical techniques indicate that surface analytical techniques predict ClNO2 formation better than bulk analytical techniques, implying that ClNO2 formation occurs on the surface rather than in the bulk aerosol. These results suggest that current air quality parameterizations must be updated with the additional controlling factors found in this study.
Kayla Besong (ATM)
Greenland Blocking and Influences on Subseasonal to Seasonal
North Atlantic Sea Surface Temperatures
Atmospheric blocking is a subseasonal, quasi-stationary phenomenon that diverts or "blocks" the main westerly winds anywhere from 5-30 days. Persistent diversion of winds leads to consequences on regional weather patterns both local and downstream of their centers, often in the form of temperature and precipitation extremes. This study considers the regional influence of blocking on sea surface temperatures (SSTs), focusing on Greenland blocking events and the North Atlantic over extended winters (DJFM) from 1980-2019 using ERA5 reanalysis at 0.25°×0.25° horizontal resolution. Analysis considers North Atlantic SSTs prior to, during, and post blocking onset at both seasonal and event-based or subseasonal timescales. Results show strong correlations with Greenland blocking and a preferred SST state in the North Atlantic on both timescales. Further considered is if this oceanic state occurs before blocking, providing a more favorable, pre-conditioned state or if blocking events drive these regional changes. Questions remain for a weak pre-blocking SST state, though, some results provide evidence this weak signal is driven by the atmosphere. However, there is a clear post-blocking amplification of SSTs, resembling that of a warm Atlantic tripole pattern. Additional results suggest that Greenland blocking is indeed forcing the oceanic tripole state, influencing the pattern that persists long after blocking decay.
Oct 23: NO SEMINAR
Oct 30: STUDENT SEMINARS
Tyler Fenske (ATM)
No Internal Connection Detected Between North Pacific Climate Modes
and Atlantic Multi-Decadal Variability
We analyze the climatic relationship between the North Atlantic and North Pacific ocean basins by cross-correlating their primary low-frequency climate mode indices, Atlantic Multidecadal Variability for the North Atlantic and the Pacific Decadal Oscillation and the Victoria Mode for the North Pacific. These cross-correlations are then tested to determine if a robust internal connection exists. We use three methods to do so: 1) a bootstrapping statistical significance method testing the maximum value of the observed cross-correlation, 2) sensitivity testing by varying the starting date and period length, and 3) using a multi-model large ensemble to compare nearly 300 unique realizations of this relationship across six models. The role of forcing in this relationship is also evaluated using this multi-model large ensemble, with forcing possibly being a confounding factor in both relationships. Our results all support that the relationship is indistinguishable from noise and thus that no robust internal relationship exists between the North Atlantic and North Pacific ocean basins.
Chelsi Lopez (OCE)
Pacific Hydrographic Fronts Enrich Deep Dissolved Organic Carbon via Sinking Particles
The transfer of organic carbon to the deep ocean is essential for long-term carbon sequestration, ultimately reducing atmospheric CO2 levels. The main mechanism for such transfer is through sinking biogenic particles produced in the upper ocean. An important fraction of those particles sink to great depths and are mainly respired into dissolved inorganic carbon (DIC), thus founding the biological pump. A fraction of these particles solubilize into dissolved organic carbon (DOC), feeding microbes in the deep ocean and perhaps adding modern carbon to the deep DOC reservoir that persists for long periods of time. This analysis indicates that hydrographic fronts in the North Pacific transition zone enhance particle export to the deep ocean, uniquely increasing concentrations of DOC throughout the water column. The waters underlying these frontal systems are likely to be important components of carbon transfer to the deep ocean and ultimately the biological pump.
Nov 06: NO SEMINAR
Nov 12 (Thursday, 3:00 pm): Kurt Hansen
Department of Atmospheric Sciences, RSMAS
(one-hour ATM student seminar)
Use of Vertical Wind Shear in Subseasonal Prediction of
Atlantic Tropical Cyclone Activity
Zoom Recording Available at COMPASS ON DEMAND
This study aims to find additional sources of predictability for subseasonal (about two week) forecasts of Atlantic tropical cyclone (TC) activity. The current state of subseasonal TC forecasts depends on long range prediction of TCs by weather models, which is subject to heavy biases, or from statistical relations with climate oscillations such the El Niño Southern Oscillation (ENSO) or Madden Julian Oscillation (MJO), which may vary on time scales too large to give us subseasonal precision. However, longer term prediction of environmental variables such as vertical wind shear may be easier for weather models to predict and patterns of shear have already been linked to subseasonal TC activity. We create a shear index that captures the influence of environmental factors on subseasonal TC activity. This index's predictive utility is tested using the Navy-ESPC model. We show that the model can capture the subseasonal shear pattern associated with subseasonal TC activity for lead times of about 15 days. This is corroborated by the fact the predicted shear index has a positive correlation with observed TC activity for 15 days. The shear index is then incorporated into a linear regression model with other well-established predictors of seasonal and subseasonal TC activity: ENSO, MJO, and main development region sea surface temperatures. The observed shear index seems to be a significant contributor to this multiple linear regression model, suggesting that it has potential as a useful predictor for Atlantic subseasonal TC activity.
Nov 13: STUDENT SEMINARS
Victoria Schoenwald (ATM)
Understanding Regional Sea Level Rise to Predict Coastal Flooding Risk
Sea level rise (SLR) is accelerating regionally along the East Coast of the United States, threatening coastal communities and ecosystems. Regional sea level (RSL) fluctuations occur from days to decades with amplitudes much greater than global mean SLR. Higher sea levels combined with higher frequency changes in the atmosphere and spring tides can lead to increased localized coastal flooding events. Therefore, it is important to determine what is causing regional SLR in order to inform coastal communities of their flood risk. Many studies have indicated that RSL is linked to large-scale ocean and atmospheric processes such as the North Atlantic Oscillation (NAO) and the Atlantic Meridional Overturning Circulation (AMOC) with a warming Gulf Stream. A thermosteric sea surface height (SSH) tripole in the North Atlantic Ocean has also been found which explains 60-80% of sea level variability South of Cape Hatteras. This study aims to determine how natural climate variability is contributing to regional SLR focusing on Charleston, SC. This region was chosen due to a pronounced acceleration of SLR south of Cape Hatteras since 2010. Using non-tidal residual (NTR) data from the Charleston, SC NOAA tide gauge station correlated with SSH, sea surface temperature (SST), and 850 mb zonal winds, we can see strong correlations with the tropical pacific, signaling an El-Niño Southern Oscillation (ENSO) connection to local sea level at Charleston, SC. Strong correlations are also seen off the coast of California as well as the in the Southern Ocean suggesting a link to the Pacific Decadal Oscillation (PDO) and AMOC, which has been previously proposed.
Amie Dobracki (ATM)
Rethinking the Lifetime of Biomass-Burning Aerosol Over the Southeast Atlantic Ocean
A major challenge and uncertainty that faces the aerosol community is understanding the chemical, physical, and optical properties of long-range transport biomass-burning aerosol. Many field campaigns, chamber studies, and model observations show an early increase in the mass of organic aerosol (OA), and after that initial increase, the aerosol can react and remain in the atmosphere anywhere from a few minutes to a few weeks. Here we present the results from the ORACLES campaign to highlight changes in the chemical and optical properties of biomass-burning aerosol as it ages for up to two weeks in the free troposphere over the southeast Atlantic Ocean. Our analysis allows us to rethink the lifetime of organic aerosol in the free troposphere as we are able to show that lifetime of OA, nitrate, and ammonium is greater than 4 days, a result that has not been observed in previous chamber or in situ studies. The irreversible loss of aerosol due to photodegradation also affects the optical properties of the aerosol such as the single scattering albedo (SSA). One important consequence is that the SSA decreases from 0.875 to 0.84 with age as organic aerosol is lost and black carbon is conserved. Overall, we anticipate the results documented here to lead to model improvements in the depiction of biomass-burning aerosol aging that will also improve climate model predictions of the direct aerosol radiative effect.
Ryder Fox (MPO), 11:00 am
The Impact of Ice Microphysics on Flooding Resulting from Landfalling Hurricanes
At $24 billion in damages, slow-moving Hurricane Florence (2018) became the ninth-most-destructive hurricane to impact the United States. Persisting post-landfall, Florence flooded North Carolina with over 35 inches of rain. Recent advances in dual polarization radar have increased interest in the role of ice processes in tropical cyclone precipitation. In particular, the contribution of graupel to outer rainbands has been explored, where graupel was previously thought to contribute most to the inner rainbands. This study employs the Weather Research and Forecasting model to simulate Hurricane Florence using three double-moment microphysics schemes. Specifically, we seek relationships between frozen hydrometeors and post-landfall accumulated precipitation totals, using rain partitioning algorithms and ground radar mosaics for verification. Early results show that the WDM6 scheme best mirrors observations in size and rainfall distribution. Within the inner rainbands, ice and snow contributed most to the maximum rainfall in the Thompson scheme, while in the outer rainbands, graupel contributed most to the maximum rainfall for the WDM6 and Morrison schemes. The latter suggests confirmation of recent studies, though more work is necessary. Future work will test higher resolution simulations of Hurricane Florence in order to better investigate the microphysical contributions to accumulated rainfall in the outer rainbands.
Nov 20: STUDENT SEMINARS
Marybeth Arcodia (ATM)
Coastal Flooding Today, Tomorrow, and the Next Ten Years in East Coast U.S. Cities
When will a particular city on the Eastern Seaboard have a 70% chance of flooding for at least one hour on at least 70 days of the year? This jam-packed question drives this study, as the frequency of floods that were once categorized as high tide or "nuisance" flooding is rapidly increasing. Many events are escalating to the moderate and major / severe flooding thresholds, putting life and property at risk. In this study, a number of East Coast cities are employed as case studies to analyze current and projected flooding rates. NOAA sea level rise scenario projections are used to determine the probable time period at which a portion of each city will frequently experience flooding for at least one hour per day. We extend the results to analyze six hours of flooding per day, increasing the risk severity of the coastal flooding. This study focuses on flooding events prone to causing property damage and hazardous conditions to make flooding forecasts that are the most impactful.
Sisam Shrestha (ATM)
Tropospheric Water Vapour Trends in CMIP6 Models
Water vapour is a dominant greenhouse gas, accounting for roughly 60 percent of the greenhouse effect and providing the largest feedback for amplifying climate variability and externally-forced climate change. In the tropics, where the tropopause is higher, water vapour absorbs outgoing longwave radiation from hot surface, but emits radiation at a much lower temperature. Understanding how water vapour responds to a warming world is, thus, imperative to climate change studies. Previous works have shown an increase in water vapour in a warming world. In this study, we analyse tropospheric water vapour trends in a multi-model framework and compare them with observations to evaluate their ability to simulate this key feedback process. We use different climate-forcing scenarios from the Coupled Model Intercomparison Project (CMIP6) to analyse and evaluate atmospheric water vapour trends over the past four decades. Preliminary results highlight the moistening of the upper troposphere compared to the lower troposphere, and the hemispheric asymmetry in observed moistening trends.
Paul Wojtal (OCE)
Trophic Dynamics of Particulate Carbon Export
Inferred From Compound Specific Isotope Analysis
Transfer of carbon from the surface ocean to depth, referred to as carbon export, is important for both biogeochemical cycles of the global ocean and for controlling climate. Particulate organic matter (POM) comprises the majority of carbon export, as well as forming the base of pelagic food webs. Understanding the mechanisms and controls on POM export and attenuation is key to being able to more accurately understand and model global ocean biogeochemical processes, to determine the role that the ocean plays in carbon sequestration, and to quantify the degree to which carbon export is changing with further anthropogenic inputs of greenhouse gases. Compound-specific isotope analysis of amino acids (CSIA-AA) allows us to more specifically tease apart different mechanisms within microbial and animal food webs that are involved in enhancing or attenuating export of POM. I present some preliminary results of nitrogen CSIA-AA. These results show that trophic position of small and large particles increases with depth, from a trophic position of 1 at the surface to a trophic position of more than 2 in the mesopelagic, indicating that food webs in the mesopelagic are feeding an entire trophic level higher than similar food webs in the epipelagic. Additionally, source amino acid d15N values indicate microbial heterotrophy. Lastly, I demonstrate that these two variables can begin to separate out processes acting on carbon export and discuss the framework that we will add additional data and variables to for further distinction of mechanisms.
Nov 27: NO SEMINAR (Thanksgiving Recess)
Dec 04: STUDENT SEMINARS
Karen Papazian (ATM)
Tracking Changes in the Intertropical Convergence Zone
Using CESM1 Aquaplanet Simulations
The location and intensity of the Intertropical Convergence Zone, ITCZ, is important especially in tropical regions as local communities can either suffer from droughts or extreme rainfall. It is also an important factor in the Earth's climate as the clusters of clouds contribute to the planetary albedo and therefore the planetary energy budget. In this study we will focus on the ITCZ in the Tropical Pacific. We analyze the precipitation rates and sea surface temperatures, SST, of various runs of the Community Earth System Model 1, CESM1, to further analyze the location and intensity of the ITCZ. We will be using various aquaplanet simulations which are simulations in which land is omitted from the model run. These runs are used in this study to create an idealized environment where we can further understand dynamics of the atmosphere. The prescribed SST differs between these aquaplanet simulations to understand the role that SST has in driving precipitation rates off of the equator.
Hanjing Dai (AMP)
Surface Oil Film Effects on Hydrodynamic Properties of Short Wind-Generated Waves
The hydrodynamics of short waves exert a significant influence on interfacial shear, surface heat fluxes, near-surface turbulent structure, and microscale wave breaking which in turn affect the gas transfer rate. Oil films are known to dampen waves, reduce wave breaking, and decrease wave-induced turbulence thereby dramatically, changing the short wave dynamics in light to moderate winds. A laboratory experiment is taking place at the University of Miami's ASIST wind-wave flume to observe the hydrodynamic properties of short wind-generated waves, near surface currents and momentum fluxes with and without oil films over a wide range of wind speeds up to Category I hurricane equivalent. Based on the results of a series of pre-experiment tests, at low to moderate wind speed, the surface waves were elongated in the along-wind direction with less wave breaking and smaller mean square slope. In high wind speed, the air-water interface was broken down by the intrusion of bubbles and sprays diminishing the viscous damping effect.
Quinton Lawton (ATM)
A Framework to Study the Influence of Convectively Coupled Kelvin Waves (CCKWs)
on the African Easterly Wave (AEW) Pathway to Tropical Cyclogenesis
African Easterly Waves (AEWs) serve as the primary precursor to tropical cyclogenesis in the Atlantic Ocean. It has recently been hypothesized that AEWs can interact with passing Convectively Coupled Kelvin Waves (CCKWs), and that these interactions can influence the pathway to tropical cyclone formation. A long-term research goal is to investigate the mechanisms and predictability of tropical cyclogenesis in the context of CCKW-AEW interactions. To do this, it is necessary to develop a framework such that the influence of CCKWs on AEWs can be isolated from other factors. In this talk I will discuss the first steps towards developing this framework. AEWs are objectively tracked in 40 years of ERA5 reanalysis. Results from this AEW climatology – including wave propagation speed, the evolution of wave intensity, and convective characteristics – compare well to established observations. Focus is then shifted to AEW-CCKW interactions during the 2020 Atlantic hurricane season, with special attention given to waves that developed into tropical cyclones. Here the evolution of the three-dimensional structures of AEWs is explored in the context of CCKWs. I will discuss the implications these early results have on building a robust AEW-CCKW framework, and detail how these methods can be expanded to the entire climatology.
Dec 11: Dr. Kathryn Gunn
Department of Ocean Sciences, RSMAS
Mixing of Subtropical, Central, and Intermediate Waters
Driven by Shifting and Pulsing of the Agulhas Current
Zoom Recording Available at COMPASS ON DEMAND
We found that the position and strength of the Agulhas Current controls mixing. When the current moves away from South Africa, waters towards the coastline become cooler and fresher. Conversely, when the current's strength reduces, waters near the coast become warmer and saltier. The current's shifting position is associated with vertical mixing, whilst the strength of the current is linked to horizontal mixing. Observed salinity changes – used to track water mass mixing – reach up to 10% of the global range of ocean salinities, and are a sign of considerable mixing within the Agulhas Current.
