Sack Lunch Seminars

Sack Lunch Seminar 2009 - 2010

Sack Lunch STAFF — Thu, 15 Oct 2009 00:04:42 +0000

October 16 - Mark Merrifield

Sack Lunch STAFF — Wed, 14 Oct 2009 20:42:16 +0000

October 16

Mark Merrifield

University Hawaii

Evidence for an Anomalous Recent Increase in the Rate of Global Sea Level Rise

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Tide gauge data are used to estimate trends in global sea level for the period 1955 to 2007. Linear trends over 15-year segments are computed for each tide gauge record, averaged over latitude bands, and combined to form an area-weighted global mean trend. The uncertainty of the global trend is specified as a sampling error plus a random vertical land motion component, but land motion corrections do not change the results. The average global sea level trend for the time segments centered on 1962 through 1990 is 1.5±0.5 mm/yr (standard error), in agreement with previous estimates of late 20th century sea level rise. After 1990, the global trend increases to the most recent rate of 3.2±0.4 mm/yr, matching estimates obtained from satellite altimetry. The acceleration is distinct from decadal variations in global sea level that have been reported in previous studies. Increased rates in the tropical and southern oceans primarily account for the acceleration.

October 14 - Marko Scholze

Sack Lunch STAFF — Fri, 09 Oct 2009 12:43:27 +0000

October 14

Marko Scholze

University of Bristol

A Global Carbon Cycle Data Assimilation System (CCDAS) to Infer Terrestrial CO2 Fluxes and Their Uncertainties

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Atmospheric inversion studies have become an important tool for identifying terrestrial sources and sinks of CO2 at the interannual time scale. Such traditional top-down studies have so far delivered important insights into the atmosphere-biosphere and atmosphere-ocean CO2 exchanges fluxes. However, they suffer from the inverse problem being seriously under determined and they do not have any prognostic power, i.e. they cannot predict the evolution of future CO2 fluxes. These inverse methods are usually contrasted with bottom-up approaches using process-based terrestrial or oceanic models capable of predicting CO2 fluxes. These models, however, cannot take into account the information contained in CO2 measurements from the extensive flask sampling network. I will present results from a carbon cycle data assimilation system (CCDAS) in which atmospheric CO2 concentration data are assimilated into a terrestrial biosphere model. CCDAS calibrates values of the parameters controlling the function of the processes within the terrestrial biosphere model against these observations. The model is run forward using these optimized parameters and calculates quantities of interest such as the net carbon flux. This can be done in a diagnostic mode, calculating fluxes for the same period as the assimilation consistent both with observations and model dynamics. If, however, the model is prognostic one can run it for other periods, either the future or the past. The assimilation procedure also allows to calculate uncertainties on the parameters, which can then be propagated onto diagnostic/prognostic quantities. I will report on results from a hindcasting experiment for prognostically calculated net CO2 fluxes plus uncertainties for the years 2000 to 2003 as well as on the most recent developments to include more observational constraints such as remotely sensed vegetation activity.

October 7 - Wallace S. Broecker

Sack Lunch STAFF — Fri, 02 Oct 2009 13:32:42 +0000

October 7

Wallace S. Broecker

Columbia University

A Failed Search for a 14C-Depleted Glacial-Age Isolated Abyssal Reservoir

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Fundamental to the field of radiocarbon dating is not only the establishment of the temporal record of the calendar age-radiocarbon age offsets but also the development of an understanding of their cause. Although part of the decline in the magnitude of this offset over the past 40,000 can be explained by a drop in 14C production rate associated with a progressive increase in the strength of the Earth’s magnetic shielding, it is clear that changes in the distribution of 14C among the Earth’s active carbon reservoirs are also required. In particular, the steep 15% decline in the 14C to C ratio in atmospheric CO2 and surface ocean CO2, which occurred in a 3 kyr-duration interval marking the onset of the last deglaciation, appears to require that a very large amount (at least 5000 gigatons) of 14C-deficient carbon was transferred to or within the ocean during this time interval. As no chemical or stable isotope anomaly associated with this injection appears in either the marine sediment or polar ice records, this injection must involve a transfer within the ocean (i.e. a mixing of 2 ocean reservoirs, one depleted in 14C and the other enriched in 14C). Although evidence for the existence of a salt-stabilized glacial-age abyssal ocean reservoir exists, a search based on benthic-planktic age differences and 13C measurements appears to place a limit on its size well below that required to account for the steep 14C decline.

September 30 - Glenn R. Flierl

Sack Lunch STAFF — Wed, 23 Sep 2009 18:31:38 +0000

September 30

Glenn R. Flierl

MIT

Adaptive dynamics and effect of circulation and mixing

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As climate and therefore the ocean changes, the organisms will also evolve to survive. Most large-scale biogeochemistry models work with a small set of plants and animals all having fixed traits; thus they cannot see this kind of change. In contrast, adaptive or trait-based models explicitly incorporate the possibility that different populations may appear and become dominant. We shall discuss the formulation and properties of such models. In the ocean, the “fitness” of an organism depends also on the physical circulation and mixing, since these induce temporal changes in the environment following a fluid parcel as well as bringing in potential competitors. We examine some of these effects to understand the interplay between the physics and the adaptive dynamics.

September 23 - Christoph Stegert

Sack Lunch STAFF — Mon, 21 Sep 2009 12:14:54 +0000

September 23

Christoph Stegert

MIT/WHOI

Congener Copepods in the northern Atlantic Ocean: an inter-regional analysis on the response to changes in environmental conditions
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As the investigation of key copepod species gained a special interest within GLOBEC several population models have been used within different regional projects. Copepods of the genera Pseudocalanus, Centropages and Calanus are widespread in the northern Atlantic Ocean and are stockforming in different areas. While some species are present in the whole North Atlantic Pseudocalanus is separated into habitats of congener species of which P. newmani and P. moultoni are found in the Gulf of Maine whereas P. elongatus inhabits the European Continental Shelf. Population dynamics of these copepods are investigated within three-dimensional ecosystem models using different stage grouped population models. Within the U.S. GLOBEC program a population model was developed and applied to the Gulf of Maine. The model estimates temperature dependent developmental rates calculating the abundance of four stage groups where development is controlled by their mean age. Using temperature data from climate models potential biogeographical boundaries are determined for different species. As food concentration can limit growth rates in certain species, the uptake of food was considered in another model, in which individual growth is controlled by the mean mass. Investigations in the North Sea within the German GLOBEC project provide the interaction with other trophic levels. Using both approaches developmental rates under changing conditions are compared to time series at single stations. The population life history, its structure, growth and distribution are examined in the context of seasonal and inter-annual climate variability.

September 16 - Jessica Benthuysen

Sack Lunch STAFF — Thu, 10 Sep 2009 12:39:40 +0000

September 16

Jessica Benthuysen

MIT/WHOI

Nonlinear stratified spindown over a sloping bottom
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On a stratified sloping bottom, the nonlinear spindown of an along-isobath flow gives rise to asymmetry in Ekman pumping and suction. We show that nonlinear advection of buoyancy weakens Ekman pumping to a greater extent than Ekman suction. This mechanism for asymmetry contrasts with previous studies of nonlinear homogeneous spindown over a flat bottom, in which nonlinear advection of momentum weakens Ekman pumping and strengthens Ekman suction. Theoretical and numerical calculations are performed for increasing Rossby number to examine the relative roles of nonlinear advection of buoyancy and momentum on the asymmetry in Ekman pumping and suction and its subsequent feedback into the vertical vorticity field. The results of this study are applied to flows on the continental slope.

Fall 2009 Schedule

Sack Lunch STAFF — Tue, 01 Sep 2009 18:32:26 +0000

September 16

Jessica Benthuysen

MIT/WHOI
Nonlinear Stratified Spindown Over A Sloping Bottom

September 23

Christoph Stegert

MIT/WHOI
Congener Copepods in the northern Atlantic Ocean: an inter-regional analysis on the response to changes in environmental conditions

September 30

Glenn R. Flierl

MIT
Adaptive Dynamics and Effect of Circulation and Mixing

October 7

Wally Broecker

Columbia University
A Failed Search for a 14C-Depleted Glacial-Age Isolated Abyssal Reservoir

October 14

Marko Scholze

University of Bristol
A Global Carbon Cycle Data Assimilation System (CCDAS) to Infer Terrestrial CO2 Fluxes and Their Uncertainties

*October 16

Mark Merrifield

University Hawaii
Evidence for an Anomalous Recent Increase in the Rate of Global Sea Level Rise

October 21

Roman Stocker

MIT
Marine Microbes in Motion

*October 23

Thierry Penduff

Florida State University & CNRS-LEGI, Grenoble, France
Assessing the Forced and Intrinsic Ocean Variability in Multi-resolution Global DRAKKAR Simulations

October 28

Larry Pratt

WHOI
The Strait of Gibraltar: Maximal or Submaximal Exchange?

November 4

Sonya Legg

Princeton University
Improved Overflow Representation in Ocean Climate Models

November 18

Sukyoung Lee

Penn State
Finite-Amplitude Equilibration of Baroclinic Waves on a Jet

December 2

Georgi Sutyrin

University of Rhode Island
Coupling of Deep Eddies with Baroclinic Rings

June 3 - Andrew Thompson

Sack Lunch STAFF — Mon, 01 Jun 2009 14:51:11 +0000

June 3

Andrew Thompson

DAMTP - University of Cambridge
Vacillating jets: baroclinic turbulence and topography

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Observations from satellite altimetry and output from high-resolution ocean models indicate that the Southern Ocean is characterized by an intricate web of narrow, meandering, filamentary jets. These jets undergo spontaneous formation, merger and splitting events, and rapid latitude shifts over periods of weeks to months. The role of topography in controlling jet variability is explored using a doubly-periodic, forced-dissipative, two-layer quasi-geostrophic model. The system is forced by a baroclinically-unstable, vertically-sheared mean flow in a domain that is large enough to accommodate multiple jets. The dependence of (i) meridional jet spacing, (ii) time scales of jet variability and (iii) large-scale, domain-averaged transport properties on changes in the length scale and steepness of simple sinusoidal topographical features is analyzed. Unsteady jet behavior is found to occur when the characteristic length scale of the eddies is comparable to or smaller than the scale of the topography. In particular, specific cases are considered where jets may drift across mean potential vorticity gradients or undergo periodic transitions between topographically-steered and zonal structures. The potential for these processes to play a role in the dynamic nature of Southern Ocean jets is discussed.

May 27 - Kjetil Vage

Sack Lunch STAFF — Wed, 27 May 2009 15:37:13 +0000

May 27

Kjetil Våge

MIT/WHOI Joint Program
Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007-2008

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Deep open-ocean convection in the subpolar North Atlantic Ocean, which impacts the meridional overturning circulation and oceanic heat flux, has been largely absent since the mid-1990s. In the winter of 2007-2008 convection returned suddenly in both the Labrador and Irminger Seas. Here we document this return, and address the reasons why it happened. Profiling float data from the Argo program are used to characterize the deep mixing. Analysis of a variety of in situ, satellite, and reanalysis data shows that, contrary to expectations, the transition to a convective state took place abruptly, without going through a phase of preconditioning. Changes in hemispheric air temperature, storm tracks, the flux of fresh water to the Labrador Sea, and the distribution of pack ice all contributed to an enhanced flux of heat from the sea to the air, making the surface water sufficiently cold and dense to initiate deep convection. Given this complexity, we conclude that it will be difficult to predict when deep mixing may occur again.