The Monday seminars are held in the GFI East Wing Auditorium at 11.15-12.00

September 2015

On Monday, September 7, 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.

 

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

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.