Monday, November 20 |
07:00 - 08:45 |
Breakfast ↓ Breakfast is served daily between 7 and 9am in the Vistas Dining Room, the top floor of the Sally Borden Building. (Vistas Dining Room) |
08:45 - 09:00 |
Introduction and Welcome by BIRS Station Manager (TCPL 201) |
09:00 - 17:30 |
Monday Chair: Adam Manahan, Hai Lin (TCPL 201) |
09:00 - 09:30 |
Timothy Delsole: Understanding the role of ocean dynamics in multi-year predictability ↓ Recent studies have questioned the degree to which interactive ocean circulations are important for making useful predictions of the next decade. We investigate this question by identifying the most predictable patterns of global sea surface temperature in coupled atmosphere-ocean models. Remarkably, the most predictable patterns in models that include interactive ocean circulation are very similar to predictable patterns in models without interactive ocean circulations (i.e., models whose ocean is represented by a 50m-deep slab ocean mixed layer with no interactive currents). In addition, these patterns can be skillfully predicted in observational data using empirical models trained on simulations from either type of climate model. These results suggest that interactive ocean circulation is not essential for the spatial structure of multi-year predictability previously identified in coupled models and observations. However, the time scale of predictability, and the relation of these predictable patterns to other climate variables, is sensitive to whether the model supports interactive ocean circulations or not, especially over the North Atlantic. To understand this sensitivity, a hierarchy of ocean models coupled to stochastic atmospheric models are examined, ranging from slab mixed-layer models to a stochastically forced Stommel box model. The box model is able to reproduce many statistical characteristics of sea surface temperatures that are relevant to predictability. This model is then used to suggest hypotheses that can be tested about the role of ocean dynamics in multi-year predictability. (TCPL 201) |
09:30 - 10:00 |
Entcho Demirov: North Atlantic atmospheric and ocean decadal climate variability – dominant patterns and abrupt climate shifts ↓ The atmosphere and ocean of the North Atlantic have undergone significant changes in the past century. To understand these changes, their mechanisms, and their regional implications requires a quantitative understanding of processes in the coupled ocean and atmosphere system. Central to this understanding is the role played by the dominant patterns of ocean and atmospheric variability which define coherent variations in physical characteristics over large areas.
Four dominant subseasonal weather regimes are defined using Bayesian Gaussian mixture models. All correlation patterns of the Sea Level Pressure (SLP) anomalies with the membership probability timeseries for the weather regimes show similarities with the dipole structure typical for the North Atlantic Oscillation (NAO). The SLP patterns of two of the regimes represent the opposite phases NAO+ and
NAO-. The two other weather regimes, the Atlantic Ridge (AR) and Scandinavian-Greenland dipole (SG), have dipole spatial structures with the northern and southern centres of action shifted with respect to the NAO pattern. These two patterns define blocking structures over Scandinavia and near the southern tip of Greenland, respectively. The storm tracks typical for the four regimes resemble the well known paths for positive/negative phases of NAO for the NAO+/NAO- weather regimes, and paths influenced by blocking off the south Greenland tip for AR and over Scandinavia for SG. The correlation patterns of momentum and heat fluxes to the ocean for the four regimes have tripole structures with positive (warm) downward heat flux anomalies over the Subpolar North Atlantic (SPNA) for the NAO- and the AR and negative heat flux anomalies over the SPNA for the NAO+. The downward heat flux anomalies associated with the SG are negative over the Labrador Sea and positive over the eastern SPNA.
The long-term impact of the weather regimes on the regional climate is characterized by their distribution; i.e. the frequency of occurrence and persistence in time of each of them. Four typical distributions of the weather regimes are identified in this study which are associated with four dominant spatial interannual patterns representing the phases of two asymmetrical ``modes''. The first two patterns have the spatial structures of positive and negative phases of the North Atlantic Oscillation (NAO). The third and fourth patterns, here referred to as G+ and G-, define the opposite phases of a mode, that has a spatial structure defined by three centers found over Florida, south of Greenland and over Scandinavia. The NAO+ interannual patterns are associated with negative anomalies of the surface downward heat flux and ocean heat content over the SPNA. The NAO- and G+ are associated with positive anomalies of heat flux and ocean heat content.
In the 1960s the dominant NAO- and G+ interannual patterns favored warmer than normal atmospheric and ocean temperatures over the SPNA. The winters in the late 1980s and early 1990s over the SPNA were colder than normal. This decadal shift in the atmospheric state between the 1970s and 1980s was
associated with a change in the dominant interannual patterns towards NAO+~and~G- in the late 1980s and early 1990s. The recent warming of the SPNA since the mid-1990s was related to the dominance of the G+/G- interannual patterns in the distribution of interannual patterns probability membership. Our analysis suggests that this decadal variability was associated with a long-term shifts in atmospheric behavior over the SPNA that can be described by a change in the 1980s of the distribution of membership probabilities for interannual patterns. In the phase space of the interannual patterns, this transition is characterized with a shift from the NAO-/G+/G- subspace subspace in the 1950 and 1960s, towards NAO+/G+/G- since the mid 1980s..
Based on this analysis we developed a a computationally efficient stochastic weather generator for analysis and prediction of the Subpolar North Atlantic amospheric decadal variability. The method is tested by the stochastic simulation of sea level pressure over the sub-polar North Atlantic. The weather generator includes a hidden Markov model, which propagates regional circulation patterns identified by a self organising map analysis, conditioned on the state of large-scale interannual patterns. The remaining residual effects are propagated by a regression model with added noise components. The regression step is performed by one of two methods, a linear model or artificial neural networks and the performance of these two methods is assessed and compared. (TCPL 201) |
10:00 - 10:30 |
Coffee Break (TCPL Foyer) |
10:30 - 11:00 |
Adam Monahan: Enhancement of sea surface wind skewness by filtering ↓ In many locations, sea surface wind components (particularly the zonal) display a pronounced skewness such that where the mean component is positive the skewness is negative (and vice versa). Several years ago, I proposed an idealized model for this behaviour in which the skewness arises because of the nonlinear dependence of surface momentum fluxes on near-surface wind speed.
A recent study (Proistoescu et al., GRL, 2016) considered the skewness of tropospheric temperature measurements from radiosondes, and showed that the skewness of the time series is generally reduced by digital filtering. They put forward a geometrical argument for the general nature of this result, through consideration of the bispectrum of the process. This argument, however, assumes that the time series are serially independent. They then used simulations to show that serially-dependent Correlated Multiplicative-Additive (CAM) noise also shows this behaviour. In such processes, which have been suggested as generic models for non-Gaussian variability in the atmosphere and ocean, the skewness arises because of the presence of multiplicative noise.
In this talk, I will show that lowpass filtering of sea surface wind actually enhances the skewness if the cutoff frequency is not too low. Extrema of skewness are found when the cutoff frequency corresponds to the synoptic timescale in the midlatitudes, and for timescales associated with subseasonal to seasonal variability in the equatorial band. I will show that this behaviour is captured by an idealized stochastic model of sea surface winds, such that the optimal lowpass cutoff frequency is set by a nonlinear dynamical timescale. These results suggest an approach to determining if non-Gaussianity results from dynamical nonlinearity or multiplicative noise, and indicate in particular that for sea-surface winds it is the former rather than the latter. (TCPL 201) |
11:30 - 13:00 |
Lunch ↓ Lunch is served daily between 11:30am and 1:30pm in the Vistas Dining Room, the top floor of the Sally Borden Building. (Vistas Dining Room) |
13:00 - 14:00 |
Guided Tour of The Banff Centre ↓ Meet in the Corbett Hall Lounge for a guided tour of The Banff Centre campus. (Corbett Hall Lounge (CH 2110)) |
14:00 - 14:20 |
Group Photo ↓ Meet in foyer of TCPL to participate in the BIRS group photo. The photograph will be taken outdoors, so dress appropriately for the weather. Please don't be late, or you might not be in the official group photo! (TCPL Foyer) |
14:30 - 15:00 |
Hai Lin: Nonlinearity of atmospheric response to ENSO and MJO ↓ Ensemble integrations using a primitive equation atmospheric model were performed to investigate the atmospheric transient response to tropical thermal forcings that resemble El Nino and La Nina. The response develops in the North Pacific within one week after the integration. The signal in the North Atlantic and Europe is established by the end of the second week. Significant asymmetry was found between the responses in the El Nino and the La Nina that is similar to the observations, i.e., one feature is that the 500 hPa positive height response in the North Pacific of the La Nina run is located about 30 degrees west of the negative response of the El Nino run; and another feature is that the responses in the North Atlantic and Europe for the La Nina and El Nino cases have similar patterns with the same polarity.
Numerical experiments are also performed with anomalous tropical thermal forcings that resemble positive MJO (a dipole tropical diabatic heating similar to phase 2) and negative MJO (similar to phase 6). The response in the first week is largely linear. After that, significant asymmetry is found between the response in positive MJO and negative MJO. This simulated nonlinearity is in agreement with the observations.
Several factors contribute to this nonlinearity of the response. In the Tropics, the shape of the Rossby wave response and the zonal extent of the Kelvin wave are not symmetric in the positive and negative forcing experiments, which is associated with the dependence of the wave property on the modified zonal mean flow. This is especially important in the equatorial region to the west of the forcing, which is likely responsible for the phase shift of the major extratropical response in the North Pacific. The transient eddy activity in the extratropics feeds back to the response and helps to maintain the nonlinearity. (TCPL 201) |
15:00 - 15:30 |
Coffee Break (TCPL Foyer) |
15:30 - 16:00 |
Shouhong Wang: Dynamic transitions in geophysical fluid dynamics ↓ I will present an overview of dynamic transition theory and its applications to various geophysical fluid flows. (TCPL 201) |
16:00 - 16:30 |
Corentin Herbert: Kinetic theory for geophysical flows ↓ A prominent feature of geophysical flows is the formation of large scale structures such as vortices and jets. Understanding the dynamics of these structures (e.g. fluctuations of the
Jet Stream, abrupt transitions between different flow regimes,...) is a crucial point for weather and climate. Unlike other turbulent flows, geophysical flows may also exhibit a clear timescale separation between the large-scale structures and the small-scale turbulent fluctuations.
I will discuss how this property can be used to construct a perturbative theory which describes the large-scale flow, relying on a technique known as stochastic averaging. Using direct numerical simulations in an idealized situation (vortices on a plane), I will study the validity and limitations of this approach by testing analytical predictions for the structure of the large-scale flow, such as the velocity profile or the eddy momentum flux profile. Finally, I will discuss the long term dynamics of large-scale geophysical flows and the abrupt transitions they undergo, using tools from large deviation theory. (TCPL 201) |
16:30 - 17:00 |
David Straub: Freely decaying turbulence near a model tropopause ↓ Kinetic energy in the atmospheric mesoscale closely follows a -5/3 power law. Most theories addressing this assume unbalanced dynamics but ignore the tropopause (where the bulk of the data was collected). Instead, the debate centers around the extent to which the mesoscale spectrum might be thought of as quasi-linear waves versus fully nonlinear turbulence. Conversely, it has also been proposed that the shallow spectrum is related to tropopause-induced modificatons of quasigeostrophic dynamics. Here we consider these various points of view by examining simulations of freely decaying turbulence in a non-hydrostatic Boussinesq model for which the base state stratification contains a tropopause. For weak flow, results are consistent with quasigeostrophic dynamics: the flow is characterized by secondary roll-ups of filaments near the tropopause, but not elsewhere. For flow strengths more typical of the atmosphere, the shallow part of the spectrum is unbalanced. Similar to previous results ignoring the tropopause, the transition to an approximate -5/3 power law can be thought of as the crossing point of a shallow spectrum of unbalanced energy with a steeper spectrum of (geostrophically) balanced energy. Various diagnostics suggest that, although results are consistent with weakly nonlinear theories at large (synoptic) scales, the (model) mesoscale itself is turbulent. Implications and limitations of these findings are discussed, including implications for stimulated loss of balance, or SLOB. (TCPL 201) |
17:00 - 17:30 |
Discussion (TCPL 201) |
17:30 - 19:30 |
Dinner ↓ A buffet dinner is served daily between 5:30pm and 7:30pm in the Vistas Dining Room, the top floor of the Sally Borden Building. (Vistas Dining Room) |