XXII.—Report on Fossil Plants, collected by the Geological Survey of Scotland in Eskdale and Liddesdale

n/

Determination of the Head-discharge Reiationship for an Existing Highway Culvert to Enabie Its Use for Measuring Runoff

Agricultural Engineerin

On the Dynamical Mechanism of the Southern Annular Mode, Including Seasonality: Inter-Annual Variability: and Trends

This thesis considers the dynamics of the leading mode of extratropical atmospheric variability, the so-called annular modes, with a focus on the Southern Hemisphere (SH). Various aspects of the annular modes are addressed, from the underlying mechanism, to variability at progressively longer time-scales; ranging from the seasonality; to inter-annual variability; to the observed and predicted trends. The underlying mechanism of the annular modes is approached in the context of the recent theory that eddy-driven jets may be self-maintaining. We show that the leading mode of variability is associated with changes in the eddy source latitude, and that the latitude of the eddy source region is organised by the mean flow. This is consistent with the idea that the annular modes should be thought of as the meridional wandering of a self-maintaining jet, and that a positive baroclinic feedback prolongs these vacillations. Further, the degree to which the eddy-driven flow is self-maintaining determines the time-scale of the leading mode in a simplified general circulation model (GCM). Preliminary results indicate that the same dynamics are important in the real atmosphere. Secondly the seasonality of the southern annular mode (SAM) is investigated. As with previous studies, during summer the SAM is found to be largely zonally symmetric, whereas during winter it exhibits increased zonal wave number 2-3 variability. This is consistent with seasonal variations in the mean-state, and it is argued that the seasonal cycle of near-surface temperature over the Australian continent plays an important role, making the eddy driven jet, and hence the SAM, more zonally symmetric during summer than winter. During winter, the SAM exhibits little variability over the South Pacific and southeast of Australia. Dynamical reasons for this behaviour are discussed. This seasonality is discussed in the context of New Zealand climate, where it is shown that the variability in rainfall and temperature data are impacted by the large-scale seasonality of the SAM. Thirdly the zonally symmetric response of the SH to the El Nino Southern Oscillation (ENSO) is examined. Such a response is only observed in the mid-latitudes during austral summer and autumn, the same period when the climatological mean flow and storm-track is most zonally symmetric. During all seasons the ENSO stationary wave, or Pacific South American mode affects the baroclinicity at 850 hPa in the South Pacific region, so that during La Nina (El Nino) events the baroclinicity is increased (reduced). During summer La Nina events the anomalous transient eddy activity is increased over the entire meridional extent of the storm-track in the South Pacific region, whereas down-stream, over the Atlantic and Indian Oceans, the storm track moves poleward. It is suggested that during La Nina events, more vigorous eddy activity in the South Pacific leads to a poleward shift of the storm-track immediately down-stream, in the East Pacific. During summer and autumn the location of the storm-track in the Pacific region may be communicated around the hemisphere because there is a single climatological storm track, and so eddies can propagate from the Pacific region to the Atlantic region. There is some evidence of these dynamics in that the anomalous eddy activity associated with La Nina events begins in the South Pacific region and subsequently propagates zonally. Finally the cause of the poleward shift of the mid-latitude eddy-driven jet streams under global warming is considered. GCMs indicate that the recent poleward shift of the eddy-driven jet streams will continue throughout the 21st Century. Here it is shown that the shift is associated with an increase in the eddy length-scale. The cause of the increase in eddy length-scale is discussed. Larger eddies are shown to propagate preferentially poleward, and it is argued that this may induce a corresponding shift in the mean flow that they maintain. The mechanism is investigated using a simplified GCM

Evaluating the land and ocean components of the global carbon cycle in the CMIP5 earth system models

PublishedJournal ArticleThe authors assess the ability of 18 Earth system models to simulate the land and ocean carbon cycle for the present climate. These models will be used in the next Intergovernmental Panel on Climate Change (IPCC) Fifth AssessmentReport (AR5) for climate projections, and such evaluation allows identification of the strengths and weaknesses of individual coupled carbon-climate models as well as identification of systematic biases of themodels. Results show thatmodels correctly reproduce the main climatic variables controlling the spatial and temporal characteristics of the carbon cycle. The seasonal evolution of the variables under examination is well captured. However, weaknesses appear when reproducing specific fields: in particular, considering the land carbon cycle, a general overestimation of photosynthesis and leaf area index is found for most of the models, while the ocean evaluation shows that quite a few models underestimate the primary production. The authors also propose climate and carbon cycle performance metrics in order to assess whether there is a set of consistently better models for reproducing the carbon cycle. Averaged seasonal cycles and probability density functions (PDFs) calculated from model simulations are compared with the corresponding seasonal cycles and PDFs from different observed datasets. Although the metrics used in this study allow identification of somemodels as better or worse than the average, the ranking of this study is partially subjective because of the choice of the variables under examination and also can be sensitive to the choice of reference data. In addition, it was found that the model performances show significant regional variations. © 2013 American Meteorological Society.This work was supported by the European Commission's 7th Framework Programme under Grant Agreements 238366 (GREENCYCLESII) and 282672 (EMBRACE), while Dr. Jones was supported by the Joint DECC/Defra Met Office Hadley Centre Climate Program (GA01101)

The impact of the state of the troposphere on the response to stratospheric heating in a simplified GCM

Previous studies have made use of simplified general circulation models (sGCMs) to investigate the atmospheric response to various forcings. In particular, several studies have investigated the tropospheric response to changes in stratospheric temperature. This is potentially relevant for many climate forcings. Here the impact of changing the tropospheric climatology on the modeled response to perturbations in stratospheric temperature is investigated by the introduction of topography into the model and altering the tropospheric jet structure. The results highlight the need for very long integrations so as to determine accurately the magnitude of response. It is found that introducing topography into the model and thus removing the zonally symmetric nature of the model’s boundary conditions reduces the magnitude of response to stratospheric heating. However, this reduction is of comparable size to the variability in the magnitude of response between different ensemble members of the same 5000-day experiment. Investigations into the impact of varying tropospheric jet structure reveal a trend with lower-latitude/narrower jets having a much larger magnitude response to stratospheric heating than higher-latitude/wider jets. The jet structures that respond more strongly to stratospheric heating also exhibit longer time scale variability in their control run simulations, consistent with the idea that a feedback between the eddies and the mean flow is both responsible for the persistence of the control run variability and important in producing the tropospheric response to stratospheric temperature perturbations

Is climate sensitivity related to dynamical sensitivity? A Southern Hemisphere perspective

This study examines whether the spread in the climate sensitivity of Coupled Model Intercomparison Project Phase 5 (CMIP5) models also captures the spread in the Southern Hemisphere dynamical response to greenhouse gas forcing. Three metrics are proposed to quantify the “dynamical sensitivity” of the Southern Hemisphere: the poleward expansion of the Hadley circulation, the poleward expansion of the subtropical dry zone, and the poleward shift of the midlatitude jet. In the CMIP5 abrupt 4 × CO₂ integrations, the expansion of the Hadley circulation is well correlated with climate sensitivity in all seasons; in contrast, the shifts in the subtropical dry zone and midlatitude jet are significantly correlated with climate sensitivity only in summer and fall. In winter, those responses are more strongly linked to the control climatology in each model. Thus, a narrow focus on traditional climate sensitivity alone might miss out on important features of the atmospheric circulation's response to increasing greenhouse gases, particularly in the extratropics

Response of the large-scale structure of the atmosphere to global warming

This is the peer reviewed version of the following article: Vallis, G. K., Zurita-Gotor, P., Cairns, C. and Kidston, J. (2015), Response of the large-scale structure of the atmosphere to global warming. Q.J.R. Meteorol. Soc., 141: 1479–1501, which has been published in final form at 10.1002/qj.2456. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving: http://olabout.wiley.com/WileyCDA/Section/id-820227.html#termsThis article discusses the possible response of the large-scale atmospheric structure to a warmer climate. Using integrations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) in conjunction with physical arguments, we try to identify what changes are likely to be robust and what the underlying mechanisms might be. We focus on the large-scale zonally averaged circulation, in particular on height of the tropopause, the strength and position of the surface westerlies and the strength and extent of the Hadley Cell. We present analytic arguments and numerical calculations which suggest that under global warming the height of the tropopause will increase in both the transient response and final equilibrium state, and an increase is clearly found in all the comprehensive models in CMIP5. Upper stratospheric cooling is also found in the comprehensive models, and this too can be explained by a radiative argument. Regarding the circulation, most models show a slight expansion and weakening of the Hadley Cell, depending on season and hemisphere. The expansion is small and largely confined to winter but with some expansion in Southern Hemisphere summer. The weakening occurs principally in the Northern Hemisphere but the intermodel scatter is large. There is also a general polewards shift in surface westerlies, but the changes are small and again are little larger than the intermodel variability in the change. This shift is positively correlated with the Hadley Cell expansion to a degree that depends somewhat on the metric chosen for the latter. There is a robust strengthening in the Southern Hemisphere surface winds across seasons. In the Northern Hemisphere there is a slight strengthening in the westerlies in most models in winter but a consistent weakening of the westerlies in summer. We present various physical arguments concerning these circulation changes but none that are both demonstrably correct and that account for the model results. We conclude that the above-mentioned large-scale thermodynamic/radiative changes in the large-scale atmospheric structure are generally robust, in the sense of being both well understood and consistently reproduced by comprehensive models. In that sense the dynamical changes are less robust given the current state of knowledge and simulation, although one cannot conclude that they are, in principle, unknowable or less predictable.National Science FoundationRoyal SocietyWolfson FoundationMarie Curie fellowshipMinistry of Science and Innovation of Spai