2015 Monday colloquiums at the Bjerknes Centre & GFI

November 2015

 

TUESDAY (1.12) at 13.15, Joel Pedro (NBI, København Universitet) will present ""Unforced Southern Ocean deep convection as a driver ofmillennial-scale Antarctic warming". Joel is a guest of Kerim/Camille.

ABSTRACT

Antarctic Isotope Maxima (AIM) – centennial-to-millennial-scale warming events observed in Antarctic ice cores during the last glacial period – are conventionally explained by changes in northward ocean heat transport during Dansgaard-Oeschger events. We propose an alternative mechanism in which internal atmosphere–sea-ice–ocean processes at southern high latitudes play the central role. Simulations with a free-running coupled climate model show that heat release associated with Southern Ocean deep convection variability can drive Antarctic temperature variations of 0.5–2.0 °C. The mechanism involves three steps: (1) Preconditioning: heat accumulates at depth in the stratified glacial Southern Ocean; (2) Convection onset: wind and/or sea-ice changes (potentially remotely triggered) tip the preconditioned system into the convective state; (3) Antarctic warming: fast sea-ice–albedo feedbacks (on annual–decadal timescales) and slower Southern Ocean frontal and sea-surface temperature adjustments to convective heat release (on multidecadal–centennial timescales), drive an increase in atmospheric heat and moisture transport over Antarctica.

 

 

Monday (30/11) Paola Moffa-Sánchez (Rutgers University) will talk about "Changes in the deep limb of the Atlantic Meridional Overturning Circulation over the last 3000 years". Paola is a guest of Margit.

ABSTRACT

"The Labrador Sea is a key location for the Earth's climate system because the intermediate waters formed here ventilate the intermediate depths of the North Atlantic and beyond. Various studies have revealed considerable spatial and temporal variability, associated with the formation and meridional transport of this water mass, over the past few decades. Yet, crucially, because of the limited temporal extent of the instrumental records, its longer-term history and interactions with the climate at multidecadal to centennial time-scales remains limited. In this talk, I will present New multi-proxy records from decadally resolved marine sediment cores to infer hydrographic changes in the LSW and their relation to centennial climate variability over the late Holocene. These results will then be compared to proxy reconstructions of the strength of the Nordic Seas Overflows over this same period to extract some conclusions on how the deep limb of the Atlantic Meridional overturning Circulation may have varied over the last 3000 years."

 

 

 

On November 9  Michael Gauss, MET Norway, Division of Climate Modelling and Air Pollution will giva the talk "The near-term potential of climate change mitigation through reduction in anthropogenic methane emissions"

 

ABSTRACT

The AMAP (Arctic Monitoring and Assessment Programme) Expert group on methane has been asked to assess the effect of reductions in anthropogenic methane emissions on Arctic climate. MET Norway, together with Environment Canada and Berkeley University, have applied Three Earth System models to calculate the global and Arctic temperature responses. Future scenarios for methane emissions have been provided by IIASA (anthropogenic sources) and by Lund University (terrestrial and marine sources). According to our results, global warming by year 2050 can be reduced by about 0.2 degrees through maximum technically feasible reduction in global anthropogenic methane emissions. The temperature response in the Arctic is larger, but also more uncertain. Part of the reduction in warming may be negated by increases in natural emissions of methane. A technical report summarizing these results is about to be published (before the end of this year) and will also include chapters on Arctic emissions and monitoring of methane. However, during this presentation, focus will be on the climate model results and about the challenge of clearly conveying them to policy.

 

Peter Haugan will give talk on Monday 2 November at 11.15 on "Renewable energy  and energy transition - new opportunities for geophysical Research"

ABSTRACT

The rapid decline in costs of wind and solar energy is having a disruptive influence on the energy sector. In particular solar photovoltaic which can be rolled out very fast and which can be cheap also when deployed at small scale, threatens the entire concept of baseload power supply from coal, gas, nuclear and hydropower. Intermittency, predictability and correlation scales of harvestable solar and wind energy become new geophysical research topics of high interest. These  topics come in addition to the basic issues of resourceassessment for developers, the question of steering and control of Wind turbines and wind farms based on meteorological data and models, and classical energy meteorology issues such as the impact of temperature on energy use, precipitation on hydropower and impacts of hazards, climate change and variability. There are also significant questions concerning regional environmental impacts of large scale deployment and even global geophysical limits which are nontrivial and unresolved. These are scientifically exciting and socially important questions which the geophysical community can and should adress. I will try to give an overview of the present drive and speed of new renewable energy rollout and related research trends and possibilities.

On Tuesday November 3 Von P. Walden will present "The role of clouds on the surface energy balance of Greenland: Results from the ICECAPS experiment at Summit Station". Von is a guest of Tore and Nils Gunnar.

ABSTRACT

The Integrated Characterization of Energy, Clouds, Atmospheric state, and Precipitation over Summit (ICECAPS) experiment has been conducted at Summit Station, Greenland (72.5 N, 38.4 E; 3200 m) since May 2010. A comprehensive suite of instruments has been operated nearly continuously to characterize atmospheric and cloud properties. The instruments include a cloud radar, two cloud lidars, two microwave radiometers, and an infrared spectrometer. Radiosondes are also being launched twice each day. These measurements are providing a better understanding of the effect of clouds on the surface energy budget over Greenland. Many clouds throughout the year contain supercooled liquid water, which have a large impact on the energy budget. The annual average cloud radiative forcing (CRF) is positive, as are all of the monthly means. The positive CRF throughout the summer makes Summit different from other Arctic locations. Overall, the infrared cloud radiative effect (CRE) from individual clouds is larger at Summit than in other Arctic locations.The reason for this is that temperature and humidity control the CRE through competing influences between the mid- and far-infrared spectral regions. The low precipitable water vapor over the high-altitude Greenland plateau results in the increased CRE. At constant relative humidity, CRE is approximately constant for temperatures characteristic of the Arctic. This result also has implications for understanding recent and future changes across the Arctic due to climate change.

 

 

October 2015

Monday October 19, 11:15-12:00: The Maritime Continent and its role on the MJO simulation in ECHAM5-SIT. Wan-Ling Tseng, (Research Centre for Environmental Changes, Academia Sinica).  Wan-Ling Tseng is a guest of Noel Keenlyside.

ABSTRACT

 

The complex topography of the Maritime Continent causes the significant impact on Madden-Julian Oscillation (MJO). Previous study showed the extra lifting and sinking with the large-scale circulation created by topography during MJO passage through the Maritime Continent. Here, we examine the influence of the resolution and the role of the Maritime Continent by ECHAM5-SIT simulation. Two group experiments have been performed, one with the existing distribution of island and a second where the island grid points are replaced by ocean grid points at 10 meter depth. Both groups are separated to T63 and T213 resolutions. The extra sensitivity experiment with T213 resolution is performed where the land grid points were retained but the orography was reduced to zero. The results suggest the Maritime Continent could be an important role to accelerate MJO propagation speed and extend eastward further.

 

 

Observation of the large-scale overturning circulation in the Southern Ocean. Jean Baptiste Sallee is a guest of Elin Darelius. When: Wed Oct 21 (OBS! 14:15-15:00), GFI east wing auditorium.

ABSTRACT

The global ocean overturning circulation plays a central role in climate by transporting heat, freshwater and carbon around the Earth. Cold water that sinks at high latitudes is upwelled along surfaces of constant density connecting the deep ocean to the sea surface in the Southern Hemisphere. Despite its crucial role for climate, the overturning circulation is poorly known since our capacity to measure it has historically been lacking. Indirect measures of the overturning are made using forward models or observationally-constraint inverse models, but all of these estimates are subject to arbitrary choices to represent unresolved and unobserved processes. Most, if not all, of these estimates have to be made in basin-wide averages, lacking to describe the regional structure of the Southern Ocean overturning. In this presentation I will present recent efforts to estimate the intensity of the overturning circulation from direct in situ observations. Both along-density and across-density circulation will be investigated. We show that the isopycnal overturning is strongly constraint to a few regions along the Southern Ocean circumpolar belt. We investigate how water-mass upwelled by the overturning is transformed by air-ice-ocean processes in the surface layer, thereby closing the overturning loop. We also present the first measurements of the cross-density circulation in the Southern Ocean interior and find that 10–25% of the Southern Ocean overturning circulation is redirected through cross-density shortcuts. In agreement with the along-density overturning, we find the cross-density overturning to be strongly intensified regionally.  Our results suggest a disproportionate contribution of those regions to the Southern Ocean overturning circulation, and highlight the importance of reliably representing them in our conceptual and forecast models of the ocean’s role in climate.

 

 

 

On Monday, Oct 12, Hoffman Cheung (University of Hong Kong, China) will give a talk with the title "Observed and Simulated Linkage between Ural blocking and East Asian winter climate" Hoffman Cheung is a guest of Noel Keenlyside.

ABSTRACT

Atmospheric blocking refers to reversal of mid-latitude westerly flow longer than synoptic timescales. The persistence of blocking potentially triggers extreme weather not only near its center, but also its downstream region. The downstream impact of Ural blocking (UB) on severe cold air outbreaks in East Asia is one of the examples. In the past decade, the increasing UB frequency has motivated more research to focus on the role of UB in the East Asian winter climate. In this talk, I will first briefly introduce the major dynamic processes responsible for a typical UB event. Then, I will present the relationship between UB and the East Asian winter climate on interannual and interdecadal timescales, where the role of low-frequency dynamics (such as NAO and ENSO) will be highlighted. Finally, I will discuss the simulations of UB in CMIP5 models, including the cause of biases in the historical scenario and the projection in RCP4.5 and 8.5 runs. Overall, the results suggest that UB is one of the important factors related to the long-term variability of the East Asian winter climate in present and future climate.

 

On Tuesday, October 13, Malte Stucker will talk about "Tropospheric Biennial Oscillation (TBO) indistinguishable from white noise." Malte Stucker is a guest of Mathew.
 

ABSTRACT

Previous studies proposed that the year-to-year variability of seasonal monsoon indices is partly determined by a Tropospheric Biennial Oscillation (TBO). Invoking coupled ocean-atmosphere-land processes and the presence of an annual memory, the TBO mechanism describes how a relatively strong monsoon is followed by a year with weaker monsoon and vice versa. Here we revisit the issue of preferred biennial timescales in tropical monsoon systems, by testing the biennial tendencies in observed and simulated monsoon indices against the white noise null hypothesis. According to an analytical expression for the null hypothesis, we expect the probability for this biennial tendency to be 2/3, which is in close agreement with observations, reanalysis products, and Atmospheric Model Intercomparison Project/ Coupled Model Intercomparison Project general circulation model simulations. Thus, it is concluded that biennial tendencies in these monsoon indices and the associated TBO are fully consistent with a white noise process and do not require the presence of a preferred biennial timescale.

 

Wednesday October 14: "Global ocean warming: using data and climate models to improve our understanding" by Peter Gleckler(PCMDI, LLNL, California). Peter Gleckler is a guest of Detelina.


ABSTRACT

This presentation will describe several studies of large scale ocean temperature changes in the historically forced simulations from the Coupled Model Intercomparison Project (CMIP). The presentation will begin by reviewing the in-situ network of available temperature measurements (circa 1960's to present) that are used to compare with models. A formal detection and attribution study of global ocean warming will then be described. Finally, recent work examining the global ocean uptake of heat during the industrial era will be highlighted, including estimates of changes in the deeper ocean.

 

 

Following up a number of BCCR/UoB seminars dedicated to improving science communication,
next Monday at 11.15, professor Helge Drange (GFI, UoB) will share his experience with media and outreach.

Who: professor Helge Drange (GFI, UoB). "Decadal-scale trends and experience in outreach".When: Monday, October 5 , 11:15.Where: GFI east wing auditorium.

 

ABSTRACT

At a national meeting discussing the first nationally coordinated climate research and modelling project in Norway in the mid 1990’s, I was accused by the a prominent Professor from a major city in Norway to talk as a “car selling person”. The comment was neither meant nor received as a compliment, but the fact is that this comment (or at least the mentioned meeting) - together with other circumstances at the same time - initiated the development of the Bergen Climate Model, the first Norwegian climate model and one of four European climate models taking part in the Fourth Assessment of the Intergovernmental Panel on Climate Change in 2007. Being surrounded by a group of highly qualified colleagues with background in atmospheric and oceanic sciences as well as in modelling/numerics - and at a time when the climate wake-up call was rising on the horizon - the group received requests from media on a near daily basis. Due to a combination of: the alleged “car selling abilities”, that the majority of the colleagues were/still are (?) most confident doing their office-science, that some of the national journalists (and media houses) were genuinely interested in climate, and that I used to have problems saying no, I became a sort of media puppet of the group. And once a name is on a journalist’s notebook, it will hardly be erased… In the presentation I will provide some thoughts - and possibly some advices - based on two decades of experience with media and outreach. A few anecdotes may be revealed as well. Whereas science has hopefully been a common denominator for most of the media and outreach activities, this talk will be as science-free as it possibly could be...

On Friday October 2, at 13:15-14:00 (!! Note time!!), Andrey Gritsun (Institute for Numerical Mathematics, Russian Academy of Science, Moscow) will give a talk about “Chaotic atmospheric systems. Response theory and methods for constructing response operators”. Andrey is guest of Igor Esau.

ABSTRACT

After Lorenz’s discovery of deterministic chaos , dynamical system theory has played an increasing role in the theory of climate. The dynamical system theory studies the asymptotic system behavior in the vicinity of an attractor. Many atmospheric systems have such an attractor. An important property of an attractor is that a stationary probability distribution function could be defined on it, and therefore, it allows averages along system trajectories as well as other statistical characteristics of the system, which gives a strong theoretical basis for climate modelling.

A method for describing chaotic attractors is based on the idea that unstable periodic orbits of the system (UPOs) approximate system trajectory forming “skeleton” of the attractor . In this approach the system measure (or its statistical characteristics) is approximated as a weighted sum over the orbits. The weights are inversely proportional to the orbit instability characteristics so that the least unstable orbits make larger contributions to the PDF. The study of applicability of this approach for typical climate systems is an ongoing research .

Another mathematical tool to be utilized in the climate research is a response theory. In many important cases one can define a linear dependence between the small changes of external forcing acting on the system and its statistical characteristics (the so-called response operator). The knowledge of the response operator gives all the information about system sensitivity with respect to the external perturbations . The major advantage of this approach is that one may calculate system response operator using observational (trajectory) data only. In our studies we demonstrated that this method is very effective for the analysis of atmospheric GCMs .

September 2015

On Monday September 28 at 11.15, Willem van der Bilt (GEO) will present "South Georgia glacier variability over the past millennium in a Southern Ocean climate context”

ABSTRACT

The Southern Ocean has a disproportionate influence on Earth`s climate system, taking up about half of CO2 and ocean heat over the past 50. This global climatic signature stems from the region`s distinct circulation patterns. These encircle the globe and link all major oceans, also interacting with major lower-latitude interannual weather patterns like the ENSO cycle. Yet, the drivers of these teleconnections remain poorly constrained. Proxy data allow us to study these climate interactions beyond instrumental timescales, improving our understanding of their impacts on global climate. But Southern Ocean proxy records remain scarce and far apart, especially terrestrial archives. Also, existing datasets often lack the resolution and chronological control required to capture centennial scale climate shifts. Sediments from distal glacier-fed lakes are considered among the highest-resolution continuous archives of past climate variability, allowing us to address these challenges. Here, we present sediment records from glacier-fed sites on South Georgia, located in the middle of the Southern Ocean and therefore well-situated to reconstruct regional climate. Two sites were investigated to assess the regional representativeness of the ensuing reconstructions. For this purpose, we applied a range of geochemical, magnetic and sedimentological tools, in combination with novel numerical techniques. To constrain the climatic drivers of glacier variability by comparing our findings to other climate records from the surrounding Drake Passage region.

 

On Monday 7 September, John Birks, Department of Biology and BCCR, University of Bergen, and Environmental Change Research Centre, University College London will give the talk "Fifty Years of Fascination, Fun, Fulfilment, Frustration, and Failure in Scientific Research". See the abstract below.

 

ABSTRACT

This seminar is not about current scientific research but is about my life as a research scientist and its inevitable ups and downs, and the major phases in a research scientist's career from the pioneer and building  phases to the mature and senescent phases. I discuss the role of external extrinsic drivers, intrinsic factors, and chance events in my career. I explore what variables best predict, in a statistical sense, 'scientific performance' as assessed by publication citations. I conclude with a discussion of the fascinations, fun, fulfilments, frustrations, and failures in 50 years of research and what one can learn from 'lessons from the past'. Chance events have been the predominant driver in my scientific career.

 

August  2015

On Monday, 17th of August, Elsa Mohino (UCM) will give a talk about: "Impact of SSTs on Sahel rainfall at decadal time scales and implications for short term prediction". The seminar will take place as usual in the east-wing auditorium, at 11:15-12:00. Elsa is a guest of Noel. See the abstract below!

 

ABSTRACT

The Sahel is a semi-arid region in West Africa between the tropical rainforest to the South and the Sahara desert inthe North (approximately 10ºN and 20ºN). It shows climate variability at

a wide range of scales, from intraseasonal to multidecadal timescales. The intense and prolonged drought of the 1970s and 1980s, which was recognized asone of the main recent climate change signals, promoted research as to its causes. The current scientific consensus is that the main factor driving Sahel rainfall at such decadal time scales is the variability of sea surface temperatures. In this talk we will revisit their role and focus our attention on the possibility of predicting Sahel rainfall trends several years in advance. For this sake, we will analyse the skill of decadal predictions of Sahel rainfall with state-of-the-art coupled ocean-atmospheric models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5). We will show that the predictive skill is highly model dependent and mainly related to the initialization of the decadal predictions.

 

 

On Monday, 10th of August, Laurent Bertino (NERSC, Bergen, Norway) will give a talk about: "Assessment of the TOPAZ4 Arctic ice-ocean reanalysis (1991-2013) ". The seminar will take place as usual in the east-wing auditorium, at 11:15-12:00. See the abstract below.

ABSTRACT

We will present a synthesis of the ice-ocean TOPAZ4 reanalysis for the period 1991-2013. TOPAZ4 is a modeling and data assimilation system based on the Nansen Center’s version of the HYCOM model at horizontal resolution of about 12 km and an Ensemble Kalman Filter (EnKF), integrating a dynamical ensemble of 100 members, similarly to the assimilation system NorCPM. The multivariate properties of the EnKF allow the TOPAZ system to assimilate several ocean and sea ice data types simultaneously, both in real-time forecasts applications (exploited operationally at MET Norway) and in reanalysis mode. The TOPAZ system constitutes the Arctic component of the Copernicus Marine Services (http://marine.copernicus.eu).
The results from a 23-years reanalysis show realistic circulation features, a good stability of the assimilation efficiency and its ability to provide physically consistent error estimates for most of the assimilated variables. Rather minor degradations are found compared to independent observations. The application of data assimilation also reveals limitations of the model, notably in terms of sea ice drift and motivates further developments of new sea ice rheology models.

On Monday, 3rd of August, Daniel B. Hewins (University of Alberta) will give a talk about: "Proposal discussion: Integrating observations of biological function into Earth System Models to predict climate feedbacks". Daniel is a guest of Hanna Lee. The seminar will take place as usual in the east-wing auditorium, at 11:15-12. See the abstract below.

 

ABSTRACT

Global climate change, in response to anthropogenic greenhouse gas (GHG) emissions, is expected to increase global temperatures by 2-4°C in the coming century. Temperature increases are associated with the balance of atmospheric GHG concentrations and the uptake by terrestrial ecosystems. In addition to warming, the global change has strong potential to impact terrestrial biogeochemistry by principally reshaping ecosystems and the mechanisms by which they cycle carbon (C). Presently, there is a large uncertainty as to how terrestrial ecosystem structure and functions will respond to increased GHG concentrations (i.e. CO2). However, research suggests potential the strong potential for shifts in the plant communities, which are responsible for recapturing CO2 by photosynthesis, and changes in the rates of microbial functioning, which may lead to further release of CO2. In order to better predict the impact of future climate on the climate system, the proposed work aims to link a global network of sites, which are actively collecting an array of biophysical information with cutting-edge models to investigate the interactions between warming and changes in plant communities. The proposed work aims to investigate the interactions between warming and biological communities and test innovative hypotheses using cutting-edge land based Earth System Models (ESMs), which for the first time integrate biological aspects of ecosystems into land ESMs.

June 2015

Thursday (!), 25th of June, at 13:15 in the east-wing auditorium. Chris Borstad (UNIS), a guest of Kerim, will talk about “Ice shelves are weakening around Antarctica, but can we predict their demise? What the models are getting wrong, and how we might get it right.“

ABSTRACT

Atmospheric and oceanic warming have contributed to the thinning, weakening, and collapse of ice shelves in several sectors of Antarctica. This has led to accelerated discharge of grounded ice into the ocean and thus accelerated sea level rise. To date, the various ice sheet models used to make projections of future ice sheet dynamics for the IPCC have lacked any physics associated with fracturing or rheological weakening of ice shelves. Here I outline recent observational evidence that highlights the importance of ice shelf weakening and its link with accelerated flow of upstream glaciers. By assimilating a time series of remote sensing observations in an ice sheet model, a characteristic pattern of ice weakening through time is revealed for the remnant Larsen B ice shelf. To account for the observed spatial and temporal pattern of weakening, I propose a simple generalization of the constitutive relation for ice creep using a scalar damage variable. This new constitutive framework is straightforward to implement in an ice sheet model, and I demonstrate several applications using the Ice Sheet System Model (ISSM). I discuss the advantages of this new constitutive framework and implications for making better model projections of the fate of the Antarctic ice sheet in a warming climate.

 

 

Monday June 15 at 11:15, Michael Reeder (Monash University) will present "The Dynamics of Australian Monsoon Bursts". Michael is a guest of Thomas Spengler and Mathew Stiller-Reeve.  

ABSTRACT

The wet phase of the Australian monsoon is characteriszed by sub-seasonal periods of excessively wet or dry conditions, commonly know as monsoon bursts and breaks. This study is concerned with the synoptic evolution prior to monsoon bursts, which are defined here by abrupt transitions of the area-averaged rainfall over the tropical parts of the Australian continent. There is large variability in the number of monsoon bursts from year-to-year and in the time interval between consecutive monsoon bursts. Reanalysis data are used to construct a lag composite of the sequence of events prior to a monsoon burst. It is found that a burst in the Australian monsoon is preceded by the development of a well-defined extratropical wave packet in the Indian Ocean, which propagates toward the Australian continent in the few days leading up to the onset of heavy rainfall in the tropics. Consistent with previous studies on the monsoon onset, the extratropical disturbances propagate equatorward over the Australian continent. These extratropical systems are accompanied by lower tropospheric fronts, which also propagate into low latitudes. Ahead of these fronts, relatively warm moist air is advected from the surrounding oceans, locally increasing the convective available potential energy and the likelihood of convection. Commonly employed climate indices shows that monsoon bursts are more likely to occur when the active phase of the Madden-Julian Oscillation is in the vicinity of Australia. Neither the El-Nino Southern Oscillation nor Southern Annular Mode have a significant impact on the occurrence of monsoon bursts.

On Tuesday June 16, at 11:15, Kunihiro Aoki (Faculty of Science, University of Tokyo) will present “A Macro System Based on Thickness Weighted average”. Kunihiro is the guest of Shunya Koseki.

ABSTRACT

How can we describe a time mean circulation in the stratified ocean under the fully turbulent circumstance associated with mesoscale phenomena (eddies)? In terms of the mass transport, the time mean velocity (so called Eulerian mean velocity) commonly-used in oceanography does not describe the time mean distribution of the mass accurately due to the occurrence of the additional transport by eddies, which are basically unobservable variables. We can, however, describe a accurate time mean mass transport by averaging the velocity after weighting isopycnal thickness, which is referred to as thickness weighted average (TWA). The velocities defined by this method (residual mean velocity or Lagrangian mean velocity) are governed by a set of equations formulated based on the primitive equations. The system described by these equations are called the TWA system. This system composed of the equations of momentum, incompressibility, and potential density, like the original primitive equations, except that an eddy momentum fluxes are embedded in the horizontal momentum equation. Supposing that this eddy fluxes are known, the TWA system can be regarded as the one describing a macro fluid motion with the residual mean velocity. The hitherto known TWA system is, however, developed without energetics, which generally gives fundamental of mechanics of the fluid as we have considered in the Navier-Stokes equation. This study, therefore, “reviews” the TWA system in terms of energetics and also gives an interpretation in the view of the variational principle based on the outcomes of the energetics.

On Thursday June 18, at 10:15, Verena Haid (CMCC, Italy) will present “The impact of changes in Antarctic runoff on the Southern Ocean sea ice”. Verena is the guest of Elin Darelius.

ABSTRACT

In recent decades, global warming has been widely observed and discussed. Especially the decreasing Arctic sea ice extent has become one of the most published consequences. It may therefore seem surprising, that contrary to the northern hemisphere, the Southern Ocean sea ice extent exhibits a small, but increasing trend over the same period. This circumpolar trend is the result of larger, but opposed regional trends around the continent. One of several possible explanations for the sea ice increase is the accelerated melting of the Antarctic Ice Sheet. The additional fresh water facilitates sea ice production and inhibits heat transfer from the deeper ocean.

Combining the NEMO ocean model with the LIM2 sea ice model, we investigated the impact of changes in Antarctic runoff volume and distribution on the Southern Ocean sea ice. We simulated and compared four different runoff scenarios over the last decade. As expected, increased runoff leads to more sea ice in the simulations. More interestingly, if the runoff is unevenly distributed around Antarctica, the sea ice dynamics are more important for the regional response than the local thermodynamics. A wide distribution of the runoff (simulating the slow melting of drifting icebergs) strongly increases sea ice thickness. The influence of the changed runoff is much smaller than the interannual variations, but of the order of the observed trends.

All three talks above will take place in the GFI east wing auditorium.

 

 

May 2015

 

 

THURSDAY 28/5

At 9.15 Axel Timmermann (University of Hawaii) will present "Explaining the continuum of Dansgaard-Oeschger variability".Axel is a guest of Noel and Eystein.

ABSTRACT:

Millennial-scale variability associated with Dansgaard Oeschger (DO) and Heinrich events (HE) is one of the most puzzling glacial climate phenomena ever discovered in paleo-climate archives.  The presentation will  describe the results of the first transient global climate hindcast simulation covering the period  50 ka B.P. to 11 ka B.P. The climate model is forced by time-varying external boundary conditions (greenhouse gasses, orbital forcing, and ice-sheet orography and albedo) and anomalous North Atlantic freshwater fluxes, which mimic the effects of changing Northern Hemisphere ice-volume on millennial timescales. Together these forcings generate a realistic global climate trajectory, as demonstrated by an extensive model/paleo data comparison. The analysis presented is consistent with the idea that ice-sheet instabilities and subsequent changes of the Atlantic Meridional Overturning Circulation were the main driver for the continuum of DO and HE variability seen in paleo-records across the globe.

At 11.15, Kelvin Richards  (University of Hawaii) will present "Turbulence in the natural environment”. Kelvin is a guest of Mathew.

ABSTRACT:

Turbulence in both the ocean and atmosphere is an important contributor to the mixing and transport of properties such as momentum, heat, salt and water vapour. In the stably stratified parts of the fluid flow the turbulence is often intermittent and patchy making sampling the process and determining its impact difficult. An exception is the Western Equatorial Pacific which proves to be an ideal natural laboratory to study shear-generated turbulence. Here turbulent production is dominated by the shear associated with relatively long-lived flow structures in the form of small vertical scale inertia-gravity waves and flow instabilities. With enough vertical resolution we can directly measure the characteristics of these flow features. We find a strong relationship between the vertical shear, stratification and the turbulent activity and the implied vertical diffusion coefficient. In addition our observations indicate that the scaling of the vertical mixing length scale is consistent with theory and numerical studies. We will discuss the implications for the large-scale interactions of the tropical ocean and atmosphere such as El Niño, and more generally improvements to the way the effects of turbulence are incorporated into models used to study the Earth’s climate.
 

Both talks will take place in the GFI east wing auditorium

 

FRIDAY 22/5 
“Greenland ice sheet melting, refreezing, runoff and dynamic mass loss: In-situ and remote sensing results from PROMICE and related projects”

Dirk van As from GEUS, CPH
Dirk van is the guest of Henning and Kerim

ABSTRACT

The Programme for Monitoring of the Greenland Ice Sheet was initiated in 2007 and sets out to monitor various aspects of the mass balance of the entire ice sheet. The chief component in PROMICE is the automatic weather station network, which targets the ablation area, a region which is difficult to reach and hard on instrumentation. Station transects provide the means to calculate the regional surface mass balance components. Yet combining PROMICE and GC-Net weather station data with MODIS surface albedo allows for a Greenland-wide observation-based estimate of surface melt. PROMICE weather station observations are also used to calibrate regional climate model output, improving surface mass balance calculations. To obtain the total mass balance from the ice sheet, the dynamic mass loss from ice berg calving is determined from airborne ice thickness measurements and satellite-based surface velocities.

Various related activities target mass balance processes in more detail. Firn coring campaigns discovered the existence of thick ice layers in firn in southwest Greenland, favoring runoff over percolation. Thermistor string measurements confirm that after big melt years refreezing does not occur below thick ice layers, but slush forms at the surface. The resulting surface darkening enhances melt through the melt-albedo feedback. The quantity of meltwater running off regionally can be determined/validated by river discharge measurements.

 

 

 

4/5  Interference between Forced and Unforced Climate Variability in the North Atlantic and the Arctic

Neil Tandon, Department of Physics, UToronto

ABSTRACT
Numerous studies have suggested that variations in the strength of the Atlantic Meridional Overturning Circulation (AMOC) may drive predictable variations in North Atlantic sea surface temperature (NASST). However, two recent studies (Medhaug and Furevik, 2011; Zhang and Wang, 2013) showed results suggesting that coupled models disagree on both the sign and the phasing of the correlation between AMOC and NASST indices. Their analyses were based on linearly detrended output from 20th century historical simulations in CMIP3 and CMIP5. I argue that the apparent disagreement among models arises from a commingling of two processes: 1) a ``bottom-up'' effect in which unforced AMOC changes lead to NASST changes of the same sign, and 2) a ``top-down'' effect in which forced NASST changes lead to AMOC changes of the opposite sign. Linear detrending is not appropriate for separating these two effects because the timescales of forced and unforced variations are not well-separated. I explore the implications of these findings to predictability of NASST, as well as the understanding of Arctic sea ice variations. This ends up raising basic questions about the role of the ocean circulation in Arctic sea ice variability and the relationship between the ocean and atmospheric circulations.

 

FRIDAY 8/5

Climate extremes - what can we learn from global models

Jana Sillmann, Cicero Oslo, 
 

This talk will give an overview on the assessment of climate extremes in the recent IPCC AR5 with focus on global climate models. Common definitions and results from model evaluation and future projections of climate extremes will be shown. The challenges encountered from such an approach with respect to "high impact" events will be addressed and examples for more contextualized analyses of climate extremes will be discussed.

 

11/5 
Surface wave effects in the NEMO ocean model

Øystein Breivik, Meteorologisk institutt, Bergen
 

ABSTRACT
Surface waves affect the ocean mixed layer through at least three distinct mechanisms:

(i) The Coriolis-Stokes effect which adds a forcing term to the momentum equations proportional to the Stokes drift velocity.

(ii) The water-side stress differs from the air-side stress because momentum is absorbed by growing waves and injected into the ocean when waves break.

(iii) Breaking waves modify the level of turbulent kinetic energy (TKE) in the wave-affected upper layer of the ocean mixed layer, thus changing the mixing.

Here we present experiments with NEMO forced with fluxes of momentum and TKE computed from the WAM wave model (archived with the ERA-Interim reanalysis). We also show some early results from coupled experiments where NEMO is coupled to the Integrated Forecast System (IFS) of ECMWF. The momentum flux (stress) and the Coriolis-Stokes forcing change the sea surface temperature by about 0.5 K. The modified TKE has a much greater impact with differences in excess of 2 K in the summer hemisphere.

Since November 2013 (Cycle 40R1) the Integrated Forecast System (IFS) of ECMWF has had a one-degree NEMO model coupled to the atmosphere from day 0 with TKE fluxes and Coriolis-Stokes forcing from the WAM model. Related paper http://dx.doi.org/10.1002/2014JC010565

 

18/5 Will Roberts - "Heinrich Events and other Laurentide ice sheet surges: must they have an external forcing?”

ABSTRACT
The exact cause of Heinrich Events, the periodic surges of icebergs from Hudson Bay during the last ice age, is still unknown, with a lot of the current debate centering on the idea that they require an external forcing. In this talk I will revisit that idea that the Laurentide Ice Sheet is capable of intrinsic oscillations that require no external forcing. I will highlight some of the many deficiencies in the models that have been used to simulate ice sheet surges in the past and propose that we can overcome some of them if we incorporate the basal hydrology of the ice sheet into our trigger mechanism. Using a sliding law that depends upon the depth of water at the base of the ice sheet I will show that we can simulate surges in our model that are unlikely to be a numerical artefact. These surges occur not only in the Hudson Bay area but throughout the whole ice sheet. I will present evidence of past ice streams and surges that corroborates these model results and suggest that the whole Laurentide Ice Sheet underwent numerous growths and collapses. Therefore, unless a new suite of forcing mechanisms is to be found, there's life in the idea of intrinsic ice sheet instabilities yet.

 

FRIDAY 22/5 
“Greenland ice sheet melting, refreezing, runoff and dynamic mass loss: In-situ and remote sensing results from PROMICE and related projects”

Dirk van As from GEUS, CPH
Dirk van is the guest of Henning and Kerim

ABSTRACT

The Programme for Monitoring of the Greenland Ice Sheet was initiated in 2007 and sets out to monitor various aspects of the mass balance of the entire ice sheet. The chief component in PROMICE is the automatic weather station network, which targets the ablation area, a region which is difficult to reach and hard on instrumentation. Station transects provide the means to calculate the regional surface mass balance components. Yet combining PROMICE and GC-Net weather station data with MODIS surface albedo allows for a Greenland-wide observation-based estimate of surface melt. PROMICE weather station observations are also used to calibrate regional climate model output, improving surface mass balance calculations. To obtain the total mass balance from the ice sheet, the dynamic mass loss from ice berg calving is determined from airborne ice thickness measurements and satellite-based surface velocities.

Various related activities target mass balance processes in more detail. Firn coring campaigns discovered the existence of thick ice layers in firn in southwest Greenland, favoring runoff over percolation. Thermistor string measurements confirm that after big melt years refreezing does not occur below thick ice layers, but slush forms at the surface. The resulting surface darkening enhances melt through the melt-albedo feedback. The quantity of meltwater running off regionally can be determined/validated by river discharge measurements.