CASPO Wednesday Seminar

Current Schedule

Unless noted otherwise, seminars are given in Nierenberg 101 and begin at 3:30 pm.

January 20, 2016
Mark Merrifield

January 27, 2016
Gregory Johnson

February 3, 2016
Please Volunteer!

February 10, 2016
Navid Constantinou (SIO)
A theory for large-scale structure formation in atmospheric/oceanic turbulence: Is jet formation a phase transition phenomenon?

Abstract (click to expand)

Planetary turbulence is anisotropic and inhomogeneous with the prominent presence of large-scale coherent structures in the form of zonal jets and vortices. These structures exhibit great constancy in form on time scales long compared to dissipation or advection, despite being embedded in strong turbulence. The emergence of jets and vortices from turbulence is not associated with an instability of the mean flow, and their equilibration and stability at finite amplitude does not arise solely from the linear or nonlinear dynamics of these structures in isolation from the turbulence surrounding them.

A new theory for the statistical state dynamics of planetary turbulence is presented. This theory, called Stochastic Structural Stability Theory (S3T), studies the dynamics of the first two cumulants of the turbulent flow. It is able to account for the transition from homogeneousto inhomogeneous turbulent state and the constancy of the large-scale structure. The success of S3T implies that the dominant dynamics in planetary turbulence is the direct interaction between the large-scale flow with the smaller-scale turbulent eddies. The results presented provide a constructive counterexample to the classical inverse energy cascade phenomenology regarding large-scale structure formation.

S3T predicts that the transition from homogeneous to inhomogeneous state is a bifurcation phenomenon, in the same manner as phase transitions. Moreover, it allows the study of the structural stability of the homogeneous or inhomogeneous turbulent state, providing thus a first step towards developing a theory for the sensitivity of the climate.

February 17, 2016
Liam Brannigan (Oxford)
Intense submesoscale upwelling in anticyclonic eddies - a new pathway to the ocean interior

Abstract (click to expand)

Numerous observations show the presence of anomalously high biological activity in anticyclonic mesoscale eddies in the ocean. This high productivity suggests that upwelling of nutrient-rich water from the thermocline occurs inside anti-cyclonic eddies. However, this upwelling is not captured by existing climate models and so hinders efforts to close the carbon budget. The upwelling process is considered for the first time from the fundamental potential vorticity viewpoint. High resolution numerical simulations show intense upwelling of high potential vorticity filaments from the thermocline in anticyclonic eddies that can drive simulated phytoplankton blooms. The dynamical cause of this upwelling is symmetric instability. As well as being a new upwelling pathway for global nutrient budgets, numerical and observational evidence is presented to show that symmetric instability in mesoscale eddies also leads to strong downwelling of fluid from the mixed layer to the thermocline.

March 2, 2016
Enrique Vivoni (CICESE)
Terrestrial Sources of Evaporation in the Southwest U.S. and Northwest Mexico and the Impacts of Regional Land Cover Change from Deforestation and Irrigated Agriculture

Abstract (click to expand)

Despite its importance in the hydrological cycle, the fluxes of water vapor derived from the terrestrial land surface are difficult to estimate. In this talk, I will present recent estimates of evapotranspiration and its abiotic and biotic components obtained from a macroscale hydrologic model that has been modified to properly account for natural ecosystem structure and agricultural areas in the region. Daily, seasonal and annual evapotranspiration estimates over 2000-2012 are compared to a network of eddy covariance towers, to water balance estimates and to remotely-sensed and model-derived gridded products. The confidence built on the macroscale model estimates provides a means to assess the major sources of evapotranspiration during winter and summer seasons. In addition, an assessment of the impact of two types of land cover change - agricultural extensification and deforestation for pasture establishment – on regional evapotranspiration is discussed.

*Special time* 11:00 am, FRIDAY, March 4, 2016
Kate Adams (Plymouth University)

Abstract (click to expand)

Investigations in various large-scale current systems have shown the importance of submesoscale frontal dynamics on the vertical exchange between the atmosphere and the mixed layer. There is a dearth of observations, however, which resolve submesoscale variability along the strongly strained, eddy-rich frontal regions in the Southern Ocean. We present here observations from the Surface Mixed Layer Evolution at Submesoscales (SMILES) cruise conducted in May 2015 that constitute the first in-situ, submesoscale-resolving measurements of the formation and evolution of a closed core eddy within the ACC. Our observations, primarily consisting of towed CTD (Seasoar), vessel-mounted ADCP, and drogued drifters, are concentrated on a prominent northward meander of the ACC within the frontal region east of Drakes Passage where the Subantarctic and Polar fronts converge. The Seasoar survey began at precisely the moment that the meander separated from the ACC and formed a closed, cold-core mesoscale eddy. The eddy crossed the Scotia Ridge through a narrow gap to the east of Burwood Bank and moved northwards towards the Falkland Island shelf sea. In conjunction with the towed CTD survey, a drifter triplet was released within a narrow (<5 km) cold water filament embedded within the front. The drifters completed three revolutions before being ejected from the eddy in a streamer that was clearly visible in a rare cloud-free SST image to emanate from the south-eastern sector of the eddy. Additional drifters released at various positions within the eddy also exited at a similar position suggesting a localized degradation in frontal integrity. The ship-based measurements ultimately encompassed the entire eddy with a horizontal resolution of O(2 km) down to 200-m water depth and thus, along with the drifter trajectories, provide insight into the mixed-layer variations in lateral frontal structure in different sectors. The northern sector where we began the survey was composed of the frontal region associated with the original meander and was defined by a strong cyclonic circulation and well-defined frontal system. The newly formed southern portion of the eddy was distinctly different, with decreased along-front velocities and a wider frontal zone composed of several filaments that further complicate the frontal dynamics within this sector of the eddy. The eddy was observed in altimetry fields for approximately 3 months after it formed. To give context to the trajectory of the observed eddy, an eddy tracking algorithm was run on 20+ years of altimetry data. The location of the observed eddy death is found to be a local hotspot of eddy activity, or an eddy graveyard within the Falkland Chasm.

March 9, 2016
Shota Katsura (AORI)
Structure and Variation of Upper Ocean Salinity in the subtropical Pacific: North Pacific Tropical Water and Barrier Layer

Abstract (click to expand)

Formation and subduction of North Pacific Tropical Water (NPTW) and formation mechanism of barrier layer (BL) in the subtropical Pacific were investigated by using raw and gridded Argo profiling float data and various surface flux data. The sea surface salinity (SSS) maximum in the subtropical North Pacific, which corresponds to the formation region of NPTW, had a zonally bimodal structure. Mixed layer salinity variations in the western and eastern parts of NPTW formation region were significantly different. While seasonal variation was dominant in the eastern part controlled by evaporation, precipitation and entrainment of fresher water below mixed layer, interannual variation was dominant in the western part controlled by evaporation, precipitation and the Pacific Decadal Oscillation-related eddy diffusivity. After subduction, while denser NPTW formed in the eastern part dissipated quickly, lighter NPTW formed in the western part was advected westward as far as the Philippine Sea, transmitting the interannual variation of salinity away from its formation region. BLs exist within the SSS front located on the equator side of tropical water formation regions. Analysis of raw Argo profiling float data showed that the temporal scale of BLs is shorter than 10 days. Subsurface equatorward intrusion of tropical water was too deep to produce BLs in the subtropical Pacific, which is formed in winter within the SSS front located on the equator side of tropical water formation regions. Poleward Ekman advection of fresher water was dominant as the surface freshening, but cannot explain the observed seasonal variations of BL. These results strongly suggest that BLs in the subtropical Pacific are formed mainly through tilting of the SSS front. This idea is supported by dominant contribution of the meridional SSS gradient to the meridional sea surface density gradient within the SSS front and the correspondence between the seasonal variations of BL and isothermal layer depth.

March 16, 2016
Shijian Hu (SIO)
Tropical North Pacific western boundary currents and Indonesian Throughflow: from observations to dynamics

Abstract (click to expand)

The North Pacific Ocean western boundary currents (NPWBC) and the Indonesian Throughflow (ITF) constitute a complicated current system that is of widely accepted importance in the climate system. During the past five years, more than 7 subsurface moorings have been deployed within the Indonesian seas and western Pacific Ocean under the international program Northwestern Pacific Ocean Circulation and Climate Experiment (NPOCE). Spatial and temporal characteristics of the NPWBC system including the Mindanao Current (MC) and Kuroshio feeding by the North Equatorial Current (NEC) and the underlying Mindanao Undercurrent (MUC)/Luzon Undercurrent (LUC) have been investigated based on the mooring and Argo profiling observations, as well as other historical observations. The NPWBC system shows strong intraseasonal variability that is related to the surface and subthermocline meso-scale eddies. Wind forcing associated with the El Niño Southern Oscillation (ENSO) cycle plays a key role in the interannual variability of the NPWBC and ITF system through Rossby waves and local Ekman pumping. Numerical experiments using a 2.5-layer reduced gravity model show that the western Pacific Ocean wind forcing contributes 54% and 72% of the total interannual variability of the MC and MUC, respectively. Rossby waves triggered by western Pacific Ocean wind forcing propagate through the Indonesian waveguide and explain most of the ITF variability as well. But wind forcing is not the only factor, especially within the Indonesian seas where the freshwater input and mixing are very strong. By separating the salinity and temperature effects on the ITF transport, we find that the halosteric variability contributes about (36±7)% to the interannual variability of the ITF. At decadal time scales, the NEC transport seems to be increased by the enhanced trade winds, and the ITF gets stronger due to the intensified rainfall over the Maritime Continent during the past one to two decades. The multi-decadal trend of the NPWBC-ITF system is found to be essential in the heat and freshwater redistribution in the Indo-Pacific region and global climate system.

March 23, 2016
Spring Break

March 30, 2016
Please Volunteer!

April 6, 2016
Matthew Mazloff (SIO)
Rapid variability of Antarctic Bottom Water transport into the Pacific Ocean inferred from GRACE

Abstract (click to expand)

Abyssal ventilation represents an important sequestration mechanisms and must be quantified, but monitoring abyssal transport has proven challenging. The Gravity Recovery and Climate Experiment (GRACE) mission is providing a time-series of ocean mass redistribution. Here we use the GRACE measurements to infer a 2003-2014 time-series of bottom water export into the South Pacific. This transport is highly variable, with a standard deviation of 1.87 Sv and a decorrelation timescale of less than one month. A significant trend is undetectable. This work confirms the hypothesis that it is possible to monitor some bottom water transport pathways using GRACE.

April 13, 2016
Please Volunteer

April 20, 2016
Ata Suanda (SIO)
Wind relaxation and a buoyant plume north of Pt. Conception, CA: observations, simulations, and scalings

Abstract (click to expand)

The coastal ocean response to a wind relaxation event around Pt. Conception, CA is simulated with a multi-nested Regional Ocean Model (ROMS) forced by realistic surface, lateral boundary, and tidal processes. This talk presents results of a hindcast simulation from the year 2000, when both outer shelf (SIO) and inner shelf (UCSB) moored observations were available for comparison. The model reproduces well the statistics of observed temperature and velocity across the continental shelf. An approximately 5oC warming of the water column associated with poleward-propagating Southern California Bight water is also reproduced. A theoretical scaling for buoyant plumes with both surface-trapped and bottom slope-controlled dynamics predicts well the propagation speed, length scales, and velocity structure of the modeled plume. Momentum balances are distinct between the offshore (> 30 m depth) region where the plume is surface-trapped, and the nearshore (within 5 km from shore, < 30 m depth) where the plume water mass extends to the bottom and is slope-controlled. In the nearshore, bottom stress is important in the alongshore momentum equation and generates vertical vorticity an order of magnitude larger than the vorticity due to geostrophic shear in the plume core. Numerical experiments without tidal forcing show that modeled surface temperatures are additionally biased 0.5 oC high, potentially affecting plume propagation distance and persistence. This work is funded by the ONR Departmental Research Initiative on inner-shelf processes.

April 27, 2016
Please Volunteer!

May 4, 2016
Jacqueline McSweeney (Rutgers)
Spatiotemporal Variability of Sediment Transport Processes in Delaware Estuary

Abstract (click to expand)

The Estuarine Turbidity Maximum (ETM) zone is classically thought to be an area of intense sediment deposition that is generated by near-bottom convergence due to an along-channel baroclinic pressure gradient. Within this framework, estuarine sediment transport has typically been described in terms of along-channel processes. However, recent studies have shown that lateral circulation contributes significantly to transport processes and that the ETM zone has important 3-dimensional structure, with differing processes dominant on the flanks and channel.

This study utilizes a Regional Ocean Modeling System (ROMS) coupled hydrodynamic and sediment model of Delaware Estuary to explore the importance of lateral sediment processes. Along with observations from a mooring array deployed in 2011 within the ETM, the model offers insight into the mechanisms that move sediment between the channel and flanks. A decomposition analysis isolates the tidal pumping and mean advection terms, revealing the importance of tidal asymmetries of stratification and resuspension. Since there is a known mechanism of sediment export on the DE flank, these findings have particularly important implications for the trapping efficiency the ETM.

May 11, 2016
Please Volunteer!

May 18, 2016
Veronica Tamsitt (SIO)
Three-dimensional pathways of deep upwelling through the Southern Ocean in observations and models (SIO)

Abstract (click to expand)

Analysis of lagrangian particle pathways in the Southern Ocean State Estimate, GFDL CM2.6 and the Community Earth System Model illuminate the 3-dimensional pathways of deep water masses from the Indian, Pacific and Atlantic oceans to the surface of the Southern Ocean. The pathways show a spiralling of deep waters southward and upward, supporting the patterns observed in water mass property distributions from hydrographic data. The lagrangian pathways show that topography strongly controls the location of upwelling of deep waters and transformation of water masses along the upwelling pathways show the importance of diabatic processes in upwelling. The interaction of upwelled deep waters with the Antarctic continental shelf is explored. These upwelling pathways are fundamentally important to understanding the 3-dimensional structure of the Southern Ocean overturning circulation and the supply of carbon and nutrient-rich waters to the surface of the Southern Ocean.

May 25, 2016
Patrice Klein (IFREMER/CNRS)
Seasonal modulation of mesoscale eddy turbulence by mixed-layer instabilities in the North Pacific Ocean

June 1, 2016
Jeff Severinghaus, Professor of Geosciences, Oceans and Atmospheres Section, Bernhard Bereiter, Sarah Shackleton (SIO)
Antarctic deep water formation processes dominate the volume-averaged temperature variability of the deep ocean: evidence from ice cores

Abstract (click to expand)

A decades-long puzzle of oceanography has been the observed predominance of carbon-14 in the deep ocean that originated in the North Atlantic surface, despite abundant physical-oceanographic evidence that most of the volume of deep ocean is flooded with Antarctic-sourced waters. This paradox is usually explained by invoking the fact that surface water carbon-14 takes about 10 years to equilibrate with the atmosphere, yet water only spends a few weeks at the surface around Antarctica, so the carbon-14 does not have enough time to get “reset” at the surface. North Atlantic Deep Water, in contrast, is formed from waters that have typically had several decades to equilibrate with the atmosphere. Here we confirm the view that Antarctic-sourced water dominates the deep, by showing that millennial-scale variability of the mean ocean temperature follows Antarctic climate in lockstep, in stark opposition to North Atlantic climate, which varies in antiphase by the so-called “bipolar see-saw".

June 8, 2016
Sachihiko Itoh (AORI, UTokyo)
Mesoscale eddies in the Kuroshio-Oyashio Extension Region

Abstract (click to expand)

The ocean is full of mesoscale eddies but their distribution is not uniform. The Kuroshio-Oyashio Extension region (KOER) in the northwestern Pacific is one of the hotspots where various types of eddies occur and interact. In this talk, I will present the results of recent studies of our group on these eddies. One of unique characteristics of the eddies in this region is that some of warm anticyclonic eddies (warm-core rings) detached from the Kuroshio Extension propagate poleward (northward), which is different from similar eddies such as Gulf Stream warm-core rings that usually propagate southwestward. We also found that cold-core anticyclonic eddies originating from the Sea of Okhotsk vigorously interact with warm-core anticyclonic eddies from the Kuroshio Extension, which causes a warm-cold double core structure within anticyclonic eddies, or sometimes regenerate the eddies from warm to cold. It is suggested that propagation and mixing of these eddies play important roles in heat/salt transport, the water mass formation, and biological productivity in various trophic levels.

August 10, 2016
Martin Hoecker-Martinez (University of Michigan)
Air-sea fluxes using a synthesis of boundary layer measurements and model extrapolation

Abstract (click to expand)

Frameworks for inferring vertical fluxes in the oceanic and atmospheric boundary layers are presented in the context of two large scale observation campaigns in the Indian Ocean and the Southern Ocean. Vertical cross sections of momentum and buoyancy fluxes during a strong wind event in the Indian Ocean are extrapolated from observations taken as part of the DYNAMO observational campaign using a Large Eddy Simulation. Model dynamics are used to diagnose the mechanisms which drive the growth of the ocean mixed layer. Measurement thresholds for atmospheric fluxes of CO2 and O2 over the Southern Ocean for the ORCAS campaign are estimated using climatologies. How the modeling system was included in flight planning described highlighting both the benefits and limitations. Particular attention is given to flights which span the Palmer Long Term Environmental Research Network and the track of the RV Gould.

October 5, 2016
Masatoshi Miyamoto (AORI, UTokyo)

October 12, 2016

October 26, 2016
Yasuhide Kobayashi (University of Hokkaido)

November 2, 2016
Marine Tort (SIO)
Modeling and dynamics of the non-traditional Coriolis force in geophysical flows

Abstract (click to expand)

Large-scale atmospheric and oceanic motion are often well described using hydrostatic primitive equations (HPE) based on (i) the shallow-atmosphere approximation, in which the thickness of the atmosphere is taken to be small in comparison with the planetary radius and (ii) the traditional approximation, in which the horizontal component of the earth's rotation is neglected. The two approximations are usually made together in order to preserve absolute angular momentum. We show however that it is possible to retain the non-traditional (NT) component of the Coriolis force under the shallow-atmosphere approximation while still satisfying all conservation laws, both in the 3D compressible equations [3] and in the spherical shallow-water (SW) equations [4], significantly extending previous work done on the plane. The derivation invokes Hamilton's principle of least action with an approximate Lagrangian capturing the small increase with height of the solid-body entrainment velocity due to planetary rotation. For flows with a small aspect ratio, the new NT model is both consistent and accurate. In this limit, two analysis of idealized oceanic flow (1) and (2) have been undertaken using continuously stratified (Boussinesq) models taking into account the complete Coriolis force:

(1) Mid-latitude inertial instability of an ageostrophic zonal jet. We show that NT effects significantly increase the growth rate of the instability at small enough Burger numbers (weak stratifications) for realistic aspect ratios of the jet [2].

(2) Propagation of near-inertial waves (NIWs) beneath atmospheric storm tracks. While the inclusion of NT effects permits sub-inertial wave propagation, fluctuating zonal winds over an eddying ocean do not excite these motions very strongly. However a poleward energy flux, carried by super-inertial waves, is found to be as strong as the equatorward flux [1].

November 9, 2016
Anthony Kirincich (WHOI)