Atmospheric Sciences

COMPASS (Combined OCE MPO ATM Seminar Series) FRIDAY


(11:00 in the Auditorium)


Jan 26: NO SEMINAR (OCE Faculty Retreat)

Feb 02 (2:00 pm): Dr. Jie He (Princeton University / Georgia Tech)

"Quantifying Tropical Air-Sea Interactions"

It is well accepted that tropical atmospheric variability is largely regulated by variations in sea surface temperature (SST). On the other hand, the atmosphere is able to generate variability internally, which in turn influences the SST. As a result, the coupled climate system reflects a complex combination of oceanic and atmospheric forcing. This makes it challenging to understand either process directly from coupled air-sea relationships. For example, observations show a very weak correlation between convection and SST variability at high background SSTs. While this led some early studies to counterintuitively suggest that the SST forcing of convection might be weak in warm pool regions, the weak correlation could also result from a large atmospheric intrinsic variability. In this study, I will show that the uncoupled atmosphere-only simulations are a perfect stepping stone to understanding coupled air-sea relationships due to their ability to isolate SST forcing. Using these atmosphere-only simulations, I will discuss precipitation and evaporation sensitivity to tropical SST variability and will attempt to clarify some previous misunderstandings on this subject. I will finally present a linear framework derived from atmosphere-only simulations as an effective tool for the quantitative understanding of tropical air-sea relationships.

Feb 09: NO SEMINAR (Recruitment Day)

Feb 16: NO SEMINAR (Ocean Sciences Meeting)

Feb 23: Student Seminars

James Hlywiak (MPO): "Coupled 3d Numerical Simulations of the Effects of Ocean Salinity on TC Intensity"

Warm surface ocean waters are a necessary ingredient for the maintenance and intensification of tropical cyclones (TCs). Ocean heat content (OHC) is a valuable tool for quantifying the amount of upper ocean energy that may be available to a passing TC, and is based on the upper ocean temperature profile. However, observations from the past couple decades have suggested that strong salinity gradients in the upper ocean may play a significant role in suppressing entrainment mixing at the base of the oceanic mixed layer. Freshwater input at the ocean surface leads to the formation of oceanic barrier layers (BLs) between the mixed layer and isothermal depths, which are common features of summertime tropical oceans in regions where TCs regularly generate over. Previous studies have indicated that TC-induced SST cooling is suppressed in the presence of BLs, thus limiting the negative feedback between ocean cooling and TC intensity. Here, idealized 3D ocean-atmosphere model simulations initialized with various vertical salinity profiles are carried out with the goal of finding a correlation between salinity stratification and TC intensity.

Jeremy Klavans (MPO): "Estimating the Magnitude of the Lagged SST Response to the North Atlantic Oscillation"

In the presence of an active ocean, North Atlantic sea-surface temperatures (SST) exhibit a lagged response to atmospheric forcing, in both models and observations. However, variable ocean currents are not necessary to reproduce the pattern and statistics of the Atlantic Multidecadal Oscillation (AMO). We examine these seemingly contradictory claims by estimating the magnitude and contribution of this ocean dynamics mechanism to multidecadal North Atlantic SST variability, in observations, observational products, and climate models. When testing a null hypothesis that accounts for spurious signals introduced by low-pass filtering, we find that the ocean response to the NAO is limited to the sub-polar gyre. Further, the lagged SST response to the NAO is small in magnitude and offers a ¬limited contribution to the AMO pattern, statistics, or predictability. We conclude that it is not necessary to invoke the full overturning of the Atlantic Ocean to explain this ocean dynamical response to the NAO.

Bosong Zhang (MPO): "Quasi-Biennial Oscillation - Madden-Julian Oscillation Connection"

Activities of the Madden-Julian Oscillation (MJO) in boreal winter has recently been found to be stronger in easterly phases of the stratospheric quasi-biennial oscillation (QBO) than its westerly phases. This QBO-MJO connection was investigated in this study using a method that identifies individual MJO events by tracking their eastward propagating signals in precipitation. Stronger MJO activities in QBO easterly phases are a consequence of more MJO days, not larger amplitudes of individual MJO events as previously thought. More MJO days come from more MJO events initiated over the Indian Ocean and their longer duration because of a weaker barrier effect of the Maritime Continent on MJO propagation. Zonal heterogeneity exists in the connection between QBO, MJO, and tropical total precipitation in general. This poses a challenge to our current understanding of the MJO dynamics, which has yet to fully include upper-tropospheric and stratospheric processes.

Mar 02: Student Seminars

Kaycie Lanpher (OCE): "Deciphering the Relationships Between Phosphorus, Metabolic Energy Potential, and Microbes Across the South Pacific"

Important factors for ocean productivity are the relative availability of nutrients and the generation and storage of metabolic energy to convert these nutrients into biomass. Adenosine triphosphate (ATP) is a primary energy trafficking molecule in cells and plays a key role in providing intracellular energy for metabolism. We are investigating ATP, as a measure for metabolic energy potential, relative to other biogeochemical parameters: biomass, community composition, and nutrient resources in two dissolved phosphorus pools. These comparisons will address the relationship between availability of nutrient resources, metabolic energy, and biomass. We will be testing the hypothesis that the allocation of energy compounds as a fraction of biomass will have an inverse relationship with nutrient concentrations. We collected data across the P06 transect in the Southern Pacific Ocean, crossing several different ocean regimes, with depth profiles of the upper 200 m for concentrations of dissolved organic phosphorus (DOP), dissolved inorganic phosphorus (DIP), particulate phosphorus (PP), particulate ATP (p-ATP), dissolved ATP (d-ATP), and cell counts of phytoplankton and heterotrophic bacteria. I will be presenting a preliminary dataset investigating the cross correlations of these data in different ocean regimes to explore the variation in microbial allocation to energy storage and the relationship between microbial abundance and the dissolved phosphorus pools.

Molly Martin (MAC): "Decadal Changes in Oxygen Parameters in the Subtropical South Pacific"

The ocean provides about half of the oxygen that living beings breathe. Climate models suggest that warming associated with greenhouse gases will lead to increased stratification and decreased solubility of gases in the ocean. Both increased stratification and decreased solubility will negatively impact the global ocean oxygen reservoir. We use data from GEOTRACES (2013), and the WOCE (1990s), CLIVAR (2000s), and GO-SHIP (2010s) repeat hydrography programs along ~12⁰S ± 3⁰. Analyzed sections span the subtropical eastern South Pacific (P19C, P17S, P16C, and GEOTRACES EPZT). Practically at least 3 stations of data were averaged together to diminish the effects of bias from eddies (radius of deformation along 12⁰S is 1⁰) and fronts. A new tracer O2*b, which is based on the O2* semi-conservative tracer is used to identify changes in biological activity. In the OMZ between 1993 and 2013, there is a decrease in oxygen concentration of ~5 μmol/kg or 0.25 μmol/kg. Of this decrease, most of it is appears to be due to physical processes and not biological. The decrease is ~2 times greater than Stramma et al. (2008) found over 50 years for 5⁰S-5⁰N in the eastern Pacific. It is not surprising that our increase is greater, as we expect the effect of climate change to be more for recent years. There are no discernable changes in oxygen parameters in the gyre regions.

Mingming Shao (AMP): "Enhanced Heat Fluxes Observed Near Sub-Mesoscale Fronts"

Sub-mesoscale fronts (SF) may affect local air-sea interaction but limited observations have been done at such scales. To address this gap an investigation of air-sea interaction at SF was conducted in the northern part of Gulf of Mexico, during the LAgrangian Sub-mesoscale Experiment (LASER). A pair of flux towers were mounted on the twins bows of the catamaran RV Walton Smith. The flux data was accompanied with marine X-band radar measurements, shipboard flow through sampling, a towed CTD and drifter observations. Surface wind acceleration (deceleration) in several cross-frontal transects was observed, which was considered as mesoscale atmosphere features across. Momentum and heat fluxes were calculated from eddy covariance and the bulk method. The comparison between them indicates that momentum flux was closely related with local SST and structure near the front. Both sensible heat fluxes and latent heat flux were generally 1.5-2 times larger than a commonly used bulk algorithm, which indicated SF may significantly accelerate the energy exchange between the ocean and atmosphere. In particular cases, positive and negative heat flux appeared in different side of SF. Secondary circulation may account for that. The phenomena could have significant implications for air-sea flux parameterization and the sub-mesoscale energy cascade. Future work about symmetry instability phenomena observed near SF will also be discussed.

Mar 02 (5:00 pm, Wetlab): POSTER SESSION

Mar 09: Dr. Claire Paris (RSMAS)

"From Crude Oil to Live Oil: The Story of the Deepwater Horizon Blowout"

The Deepwater Horizon spill was unlike any other oil spill. It was the first uncontrolled release of gas and oil in the deep ocean, at 1522 m from the sea surface of the Gulf of Mexico. The spill was also unique since it was the first time that chemical dispersantS were applied at a subsea wellhead to keep oil submerged. This novel response posed a unique challenge for reliable predictions of oil transport and fate. Not only was there no operational three dimensional model to track the oil, but gas-saturated oil spewing under very high hydrostatic pressure has characteristics remarkably distinct from oil released in shallow water —one of the most sought after parameters was, and still is, the droplet size distribution (DSD) that controls how much oil reaches the surface.

To address these challenges, we have taken a two-prong approach,  developing (1) a DSD model based on turbulent dissipation rate TDR validated with laboratory high-pressure and field experiments, and (2) a Lagrangian-inertial blowout modeling tool, whose algorithm and configuration has improved over the years with findings of fundamental processes.  A key finding is the behavior  of live oil versus crude oil and how it regulates the blowout characteristics. The blowout model resolves the complex thermodynamic processes occurring in the near field, meters above the wellhead, and the hydrodynamic processes in the far field, up to kilometers away —including he decay rate of hydrocarbons untreated and treated with dispersants. We demonstrate that the model provides accurate predictions of oil concentration at the sea surface and of its mass partition in the water column. Additional discoveries of live oil processes, such as degasing in the oil droplet, are being incorporated in the model. We briefly discuss the use of the model in advancing ongoing impact studies of the DWH disaster and for improved first response in the future.

Mar 16: NO SEMINAR (Spring Recess)

Mar 23: Student Seminars

John Lodise (MPO): "Vertical Structure of Wind Driven Currents at the Very Near Surface"

Observations of wind and wave driven currents at two depths within the first meter of the surface are made utilizing trajectory data from both drogued and undrogued drifters during the LAgrangian Submesoscale ExpeRiment (LASER) that took place from January to March of 2016 in the Northern Gulf of Mexico. Examination of dense, collocated populations of drifters during periods of high wind speeds (greater than 12 m/s) reveal that the surface currents captured by both drogued and undrogued drifters are dominantly wind and wave driven. A Lagrangian variational method is used in order to create hourly Eulerian velocity fields from trajectory data of both drifter types, separately. To quantify the direct effect of the wind on the surface flow, we use wind and Stokes drift output data from the Unified Wave INterface-Coupled Model (UWIN-CM), that were stored during the experiment for later analysis. We calculate the purely wind-driven surface current by subtracting the Stokes drift velocity fields from the full velocity fields constructed from the drifter data, thus removing the wave component from the flow. We then calculate the deflection angle and velocity magnitude of the calculated wind driven surface currents with respect to the wind.

Joshua Wadler (MPO):  "Downdrafts and the Evolution of Boundary Layer Thermodynamics in Hurricane Earl (2010) Before and During Rapid Intensification"

Using a combination of NOAA P-3 aircraft Doppler radar, NOAA and NASA dropsondes, and buoy and drifter based sea surface temperature data, different types of downdrafts and their influence on boundary layer (BL) thermodynamics are examined in Hurricane Earl (2010) during periods right prior to rapid intensification (RI; a 30 knot increase in intensity over 24 hours) and during RI. Before RI, the BL was generally warm and moist. Convectively driven downdrafts inside the radius of maximum winds (RMW) and upshear-right quadrant, and vortex-tilt induced downdrafts outside the RMW in the upshear-left quadrant were the largest hindrances for intensification. Possible mechanisms for overcoming the low entropy (θe) air induced by these downdrafts are BL recovery through air-sea enthalpy fluxes and turbulent mixing by atmospheric eddies.

During RI, convective downdrafts of varying strengths in the upshear-left quadrant had differing effects on the low-level entropy and surface heat fluxes. Interestingly, the stronger downdrafts corresponded with maximums in 10-m θe. It is hypothesized that the large amount of evaporation in a strong (>2 m s-1) downdraft underneath a precipitation core can lead to high amounts of near-surface specific humidity. By contrast, weaker downdrafts corresponded with minimums in 10-m θe, likely because they contained lower evaporation rates. Since weak and dry downdrafts require more surface fluxes to recover the low entropy air than strong and moist downdrafts, they are greater hindrances to storm intensity. This study emphasizes different types of downdrafts and their role in air-sea interaction which is linked to hurricane intensity change.

Jianhao Zhang (MPO): "Boundary Layer Characteristics Above Ascension During Biomass Burning Seasons Inferred from LASIC Field Campaign Observations"

The areal extent of the absorbing aerosol above low clouds in the Southeast Atlantic is large enough for the coupled cloud-aerosol-ocean system to have a regional climate impact, but the cloud adjustments to the presence of the aerosol are still poorly understood. Cloud responses to the aerosol presence can be both radiative and microphysical, depending on the relative location of the aerosol to the cloud. This study focuses on observations collected by DOE ARM Mobile Facility during the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign from June 2016 to October 2017, at Ascension Island (8°S, 14°W), located ~2,000 km offshore of continental Africa in the trade-wind regime. Micropulse-lidar-derived extinction profiles suggest that aerosol is almost always present in the atmospheric column during July-October, if in varying amounts and within multiple layers. A two-layer cloud structure is representative of a boundary layer that is usually decoupled. The July-August observations reveal that, when absorbing aerosol is present in the cloudy boundary layer, the diurnal cycle of the potential temperature vertical profile is more prominent, the boundary layer is deeper and more well-mixed, the cloud top inversion is weaker, and the atmosphere is less stable, despite the presence of absorbing aerosol aloft. The weaker inversion under smoky conditions co-occurs with higher cloud bases at both levels. The near-surface cloud condensation nuclei concentration increase, correlating well with black carbon mass concentration. Drizzle suppression and reduced drop sizes near the cloud base are also apparent in the radar reflectivity composites under smoky conditions.

Mar 30: Luna Hiron, Steven Simon, Rachel Sodowsky

Apr 06: Marybeth Arcodia, Xingchen Yang, Wei Zhang

Apr 13: Simge Bilgen, Yi Dai, Yu Gao, Matthew Grossi

Apr 20: Alessandro Cresci (OCE), Houraa Daher (OCE), Shannon Doherty (OCE)

Apr 27Romain Chaput (OCE), Valeria Donets, Mampi Sarkar