Universities Global Atmospheric Modelling Programme Research Articles

Ten-year climatological features and air origin of midlatitude double tropopauses

The 10-year climatological features related to midlatitude double tropopause events (DTs) are examined using ERAInterim data from 2003 to 2012. The analysis is based on tropopauses defined by lapse rate. Results show that DTs are permanent or semi-permanent in the midlatitudes, and high DT frequency bands move poleward in winter and equatorward in summer, which is consistent with the seasonal movement of the subtropical jet. Based on our statistics, the second tropopause is found at about 100 hPa in the subtropics and at slightly lower altitudes in sub-polar regions. The thickness between the first and second tropopause is smaller in the subtropics and increases with latitude. Next, the origin of air sandwiched between the first and second tropopause of DTs is studied with a revised version of the UK Universities Global Atmospheric Modelling Programme Offline Trajectory Code (Version 3) diabatic trajectory model. The results show that, in the lower or middle troposphere, air is transported into the DTs from lower latitudes, mainly in the tropics. The dominant source regions are mainly areas of deep convection and steep orography, e.g., the western Pacific and Himalayan Mountains, and they show strong seasonality following the seasonal shift of these strong upwelling regions.

Eastern boreal North American wildfire risk of the past 7000 years: A model‐data comparison

We present here a 7000‐year wildfire reconstruction based on sedimentary charcoal series from five lakes located south of Hudson Bay in eastern boreal North America. The reconstruction shows a significant downward trend in the frequency of large fires from 0.0061 fire·yr−1ca. 5000 cal yr BP to 0.0033 fire·yr−1 at present. Simulations of fire‐season climate based on UK Universities Global Atmospheric Modelling Programme output and reconstructions based on proxy data both indicate a shift toward increasing available moisture in the region between the mid‐Holocene and today. We infer that the diminishing trend in wildfire activity was ultimately caused by the steady orbitally driven reduction in summer insolation. Future higher temperatures not compensated for by significant precipitation increases will bring fire frequency back toward its upper limit, recorded between 6000 and 2000 cal yr BP.

Atmospheric multiple equilibria and non‐Gaussian behaviour in model simulations

AbstractMultiple equilibria from general circulation models (GCMs) are investigated via Gaussian mixture models. The cross‐validation technique based on bootstrap, used to find the possible number of clusters, is shown to have a tendency to display a bias in the number of clusters when the sample is not statistically independent. The method is shown, however, to give satisfactory results when applied with a strategy based on statistical independence. The mixture model is successfully applied to a few dynamical systems and data from the UK Universities Global Atmospheric Modelling Programme (UGAMP) GCM. The same analysis is then carried out and applied to the wintertime 500 mb geopotential height field taken from a 50‐year run of the Hadley Centre HadAM3 GCM. Three regions, the North Pacific‐American, the North Atlantic, and the northern hemisphere are investigated for nonlinear circulation regimes. It is shown that the model displays robust regime behaviour over the North Pacific‐American sector, while only marginal regime behaviour is observed over the North Atlantic, and no trace of hemispheric regimes.

South American palaeoclimate model simulations: how reliable are the models?

Modelling climate change over South America represents a special challenge for computer climate models. The coarse resolution of most general circulation models (GCM) means that the high but narrow relief of the Andes is poorly represented. In addition, changes in the tropical lowlands will be strongly dependent on the ability of the models to accurately simulate convection, cloud cover and land surface processes. Both marine and terrestrial data also will be relatively sparse compared with northern latitudes. This paper examines the modelling uncertainties by discussing results from the Palaeoclimate Model Intercomparison Project (PMIP), which simulated the mid-Holocene (6 kyr BP) and the Last Glacial Maximum (LGM, 21 kyr BP). The PMIP mid-Holocene experiments show a reasonable amount of agreement, especially for surface air temperature changes. The PMIP LGM simulations show a greater degree of scatter, which is partly associated with uncertainties in the treatment of the ocean surface. This suggests that uncertainties in the boundary conditions can be as important or more important than uncertainties due to the internal physics within the GCMs. In addition, we consider the results from a new set of simulations for 15 kyr BP, 12 kyr BP and 9 kyr BP, using the climate model of the UK Universities Global Atmospheric Modelling Programme (UGAMP). We compare these results with previous simulations by COHMAP. The higher resolution of the UGAMP model allows us to examine mid-latitude storm-track changes. The changes in the storm track are largest upstream of South America and reach their largest at around 15 kyr. The model also predicts a positive snow mass balance in the Southern Andes up until 12 kyr BP. The associated changes in mid-latitude temperatures and precipitation over South America, however, are generally small and are broadly in agreement with previous studies. All of these results suggest that the basic elements of the GCM predictions of climate change are relatively robust, and the most important uncertainties probably arise from lack of detailed knowledge of the required boundary conditions. The next generation of climate models will alleviate many of these problems. Copyright © 2000 John Wiley & Sons, Ltd.

Simulations of the Last Glacial Maximum climates using a general circulation model: prescribed versus computed sea surface temperatures

The climate during the Last Glacial Maximum (LGM) has been simulated using the UK Universities Global Atmospheric Modelling Programme (UGAMP) general circulation model (GCM) with both prescribed sea surface temperatures (SSTs) based on the CLIMAP reconstruction and computed SSTs with a simple thermodynamic slab ocean. Consistent with the Paleoclimate Modelling Intercomparison Project (PMIP), the other boundary conditions include the large changes in ice-sheet topography and geography, a lower sea level, a lower concentration of CO2 in the atmosphere, and a slightly different insolation pattern at the top of the atmosphere. The results are analysed in terms of changes in atmospheric circulation. Emphasis is given to the changes in surface temperatures, planetary waves, storm tracks and the associated changes in distribution of precipitation. The model responds in a similar manner to the changes in boundary conditions to previous studies in global mean statistics, but differs in its treatment of regional climates. Results also suggest that both the land ice sheets and sea ice introduce significant changes in planetary waves and transient eddy activity, which in turn affect regional climates. The computed SST simulations predict less sea ice and cooler tropical temperatures than those based on CLIMAP SSTs. It is unclear as to whether this is a model and/or a data problem, but the resulting changes in land temperatures and precipitation can be large. Snow mass budget analysis suggests that there is net ice loss along the southern edges of the Laurentide and Fennoscandian ice sheets and net ice gain over other parts of the two ice sheets. The net accumulation is mainly due to the decrease in ablation in the cold climate rather than to the changes in snowfall. The characteristics of the Greenland ice-sheet mass balance in the LGM simulations is also quite different from those in the present-day (PD) simulations. The ablation in the LGM simulations is negligible while it is a very important process in the ice mass budget in the PD simulations.

The influence of Carboniferous palaeoatmospheres on plant function: an experimental and modelling assessment

Geochemical models of atmospheric evolution predict that during the late Carboniferous, ca . 300 Ma, atmospheric oxygen and carbon dioxide concentrations were 35% and 0.03%, respectively. Both gases compete with each other for ribulose–1,5–bisphosphate carboxylase/oxygenase–the primary C–fixing enzyme in C 3 land plants: and the absolute concentrations and the ratio of the two in the atmosphere have the potential to strongly influence land–plant function. The Carboniferous therefore represents an era of potentially strong feedback between atmospheric composition and plant function. We assessed some implications of this ratio of atmospheric gases on plant function using experimental and modelling approaches. After six weeks growth at 35% O 2 and 0.03% carbon dioxide, no photosynthetic acclimation was observed in the woody species Betula pubescens and Hedera helix relative to those plants grown at 21% O 2 . Leaf photosynthetic rates were 29% lower in the high O 2 environment compared to the controls. A global–scale analysis of the impact of the late Carboniferous climate and atmospheric composition on vegetation function was determined by driving a process–based vegetation–biogeochemistry model with a Carboniferous global palaeoclimate simulated by the Universities Global Atmospheric Modelling Programme General Circulation Model. Global patterns of net primary productivity, leaf area index and soil carbon concentration for the equilibrium model solutions showed generally low values everywhere, compared with the present day, except for a central band in the northern land mass extension of Gondwana, where high values were predicted. The areas of high soil carbon accumulation closely match the known distribution of late Carboniferous coals. Sensitivity analysis with the model indicated that the increase in O 2 concentration from 21% to 35% reduced global net primary productivity by 18.7% or by 6.3 GtC yr –1 . Further work is required to collate and map at the global scale the distribution of vegetation types, and evidence for wildfires, for the late Carboniferous to test our predictions.

Contour lengthening rates near the tropopause

We present the results of contour advection calculations using winds derived from the UK Universities Global Atmospheric Modelling Programme (UGAMP) model. Contours are initialised using PV values on a set of potential temperature surfaces which span the stratosphere and troposphere in order to examine the mixing properties of stratospheric and tropospheric air on a time‐scale of a few days. We show that the contour stretching rates do not convey any information about the location of the tropopause, although the lower layers of the stratosphere are vigorously mixed with the troposphere over this time. The results suggest that the stratosphere becomes more isolated from the troposphere with increasing potential temperature, with mixing confined to a narrow region in the subtropics at the higher levels (greater than 330K).

Temperature dependence of the ClO concentration near the stratopause

From an analysis of Microwave Limb Sounder (MLS) ClO data near the stratopause where it is reasonable to assume that ClO is in photochemical equilibrium, it is found that there is a strong temperature dependence of the order of 1000 K(±200 K) including the effects of scaling and the bias uncertainties in the retrieved MLS data. Results from a three‐dimensional model (extended UK Universities Global Atmospheric Modelling Programme (UGAMP) general circulation model (EUGCM)) agree well with the MLS results; we obtain a temperature dependence of 950 K in the three‐dimensional model. The temperature dependencies obtained from the MLS data and the EUGCM agree quite well with results from a purely photochemical box model which yields a temperature dependence of the order of 900 K. The generally good agreement between the temperature dependence of ClO obtained from the MLS measurements, from both the photochemical model and the three‐dimensional general circulation model does not suggest that there are major uncertainties in our understanding of the ClO chemistry in the upper stratosphere.

Sensitivity of the Asian summer monsoon to aspects of sea-surface-temperature anomalies in the tropical Pacific Ocean

The response of both the time of onset, and the strength of the Asian summer monsoon, to regional aspects of the sea surface temperature (SST) anomalies in the tropical Pacific Ocean associated with El Niño/Southern Oscillation (ENSO) has been investigated through a series of sensitivity experiments with the Universities Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (UGCM). This paper builds on the work of Ju and Slingo (1995) and on their hypothesis that the relationship between the Asian summer monsoon and ENSO involved the latitudinal position and strength of the tropical convective maximum (TCM) over Indonesia and the west Pacific in the preceding spring. the inference from their results was that the modulation of the TCM might be associated either with changes in the Walker circulation through the influence of the east Pacific SST anomalies, or with changes in the local Hadley circulation associated with the in situ SST anomalies in the west Pacific. the investigation has focused on the particular contrasting years of 1983 and 1984. the experiments described in this paper are designed to isolate the effects of the principal SST anomalies in the east and central Pacific, associated with El Niño/La Niño, from those of opposite sign which develop in the west Pacific as a complementary pattern during the mature phase of El Niño/La Niño. The results of the experimentation suggest that, at least for the test cases of 1983 and 1984, the modulation of the Walker circulation, with implied additional subsidence over the eastern hemisphere, is the dominant mechanism whereby the Asian summer monsoon is weakened during El Niñño years. However, the late onset during El Niño years may also be associated with the complementary cold SST anomalies in the west Pacific which delay the northwards transition of the TCM. During La Niño, the modulation of the Walker circulation appears not to be the controlling factor which determines the stronger monsoons. the UGCM results suggest that the complementary warm SST anomalies in the west Pacific enhance the TCM, and it is this in situ response by the TCM that leads to an early onset and stronger monsoon. the importance of warm anomalies in the west Pacific in the development of a strong monsoon has been investigated further through a case-study of the 1994 season. the year 1994 was an El Niño year in which the monsoon was unexpectedly active, but which was also marked by warmer than normal SSTs in the west Pacific. The sensitivity experiments have also elucidated the role of El Niño in influencing the precursory signature of stronger subtropical westerlies over India and south-east Asia during the winter and spring preceding weak monsoons. the results suggested that the equatorwards shift of the subtropical jet is a remote response to the warm SST anomalies in the central and east Pacific associated with El Niño.

The climate of antarctica in the UGAMP GCM: Sensitivity to topography

Most spectral general circulation models (GCMs) use an envelope topography to set on land surface elevations. The UK Universities Global Atmospheric Modelling Programme General Circulation Model (UGAMP GCM) uses such a formulation for Antarctica, based on the US Navy 10 are minute charts of the region. In the marginal regions of the continent, an envelope topography consistently overestimates the elevation leading to lower-than-observed surface temperatures. Furthermore, errors in excess of 1000 m exist in the US Navy data, and the UGAMP GCM treats the major ice shelves as sea ice, introducing a 12% reduction in the snow-covered area of the continent. Here, we use a new high-resolution, high-accuracy digital elevation model to improve the representation of Antarctica in the UGAMP GCM. The effect of changing the land-sea mask and the topography on the surface temperature, precipitation and wind held is investigated for both summer and winter runs. Changing the land�sea mask had a dramatic effect on temperature, producing a reduction of 13.3°C for the sector west of the Ross Ice Shelf. Using the new mean topography also introduces substantial differences in temperature, wind speed and precipitation for summer and winter.

Sensitivity of the asian summer monsoon to aspects of sea‐surface‐temperature anomalies in the tropical pacific ocean

AbstractThe response of both the time of onset, and the strength of the Asian summer monsoon, to regional aspects of the sea surface temperature (SST) anomalies in the tropical Pacific Ocean associated with El Niño/Southern Oscillation (ENSO) has been investigated through a series of sensitivity experiments with the Universities Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (UGCM). This paper builds on the work of Ju and Slingo (1995) and on their hypothesis that the relationship between the Asian summer monsoon and ENSO involved the latitudinal position and strength of the tropical convective maximum (TCM) over Indonesia and the west Pacific in the preceding spring. the inference from their results was that the modulation of the TCM might be associated either with changes in the Walker circulation through the influence of the east Pacific SST anomalies, or with changes in the local Hadley circulation associated with the in situ SST anomalies in the west Pacific. the investigation has focused on the particular contrasting years of 1983 and 1984. the experiments described in this paper are designed to isolate the effects of the principal SST anomalies in the east and central Pacific, associated with El Niño/La Niño, from those of opposite sign which develop in the west Pacific as a complementary pattern during the mature phase of El Niño/La Niño.The results of the experimentation suggest that, at least for the test cases of 1983 and 1984, the modulation of the Walker circulation, with implied additional subsidence over the eastern hemisphere, is the dominant mechanism whereby the Asian summer monsoon is weakened during El Niñño years. However, the late onset during El Niño years may also be associated with the complementary cold SST anomalies in the west Pacific which delay the northwards transition of the TCM. During La Niño, the modulation of the Walker circulation appears not to be the controlling factor which determines the stronger monsoons. the UGCM results suggest that the complementary warm SST anomalies in the west Pacific enhance the TCM, and it is this in situ response by the TCM that leads to an early onset and stronger monsoon. the importance of warm anomalies in the west Pacific in the development of a strong monsoon has been investigated further through a case‐study of the 1994 season. the year 1994 was an El Niño year in which the monsoon was unexpectedly active, but which was also marked by warmer than normal SSTs in the west Pacific.The sensitivity experiments have also elucidated the role of El Niño in influencing the precursory signature of stronger subtropical westerlies over India and south‐east Asia during the winter and spring preceding weak monsoons. the results suggested that the equatorwards shift of the subtropical jet is a remote response to the warm SST anomalies in the central and east Pacific associated with El Niño.

Ice-sheets’ surface mass-balance evaluation in the UGAMP GCM: the climate of Antarctica

General Circulation Models (GCMs) will be more and more used for coupled climatic simulations involving ice sheets. It is therefore of prime importance to evaluate the performance of these models in simulating the mass balance and climate over ice sheets. The Antarctic climate simulated with the U.K. Universities Global Atmospheric Modelling Programme General Circulation Model (UGAMP GCM, hereafter referred to as the UGCM) is in good agreement with the available observations. In particular, the accumulation pattern appears fairly reasonable. Some imperfections are related to the surface temperature and energy budget but without severe consequences for the atmosphere behaviour. Refining the snow-related parameterizations could improve the results of the model in high latitudes.

Ice-sheets’ surface mass-balance evaluation in the UGAMP GCM: the climate of Antarctica

General Circulation Models (GCMs) will be more and more used for coupled climatic simulations involving ice sheets. It is therefore of prime importance to evaluate the performance of these models in simulating the mass balance and climate over ice sheets. The Antarctic climate simulated with the U.K. Universities Global Atmospheric Modelling Programme General Circulation Model (UGAMP GCM, hereafter referred to as the UGCM) is in good agreement with the available observations. In particular, the accumulation pattern appears fairly reasonable. Some imperfections are related to the surface temperature and energy budget but without severe consequences for the atmosphere behaviour. Refining the snow-related parameterizations could improve the results of the model in high latitudes.

Tides in the Extended UGAMP General Circulation Model

AbstractIn this paper the effect of tides on the zonal mean flow in the Extended UK Universities Global Atmospheric Modelling Programme General Circulation Model (EUGCM) is examined. Previous modelling studies have shown that omitting the diurnal cycle of solar radiation gives rise to changes in the zonal wind of the order of 10–20 m s−1 near the mesopause, and to larger changes in the lower thermosphere. In the EUGCM, which has a top level near the mesopause, turning off the diurnal cycle gives rise to model zonal winds in July that are up to 65 m s−1 stronger near the equatorial mesopause. It is shown that such zonal wind changes are reduced by up to 20 m s−1 when the model horizontal diffusion is reduced by a factor of 100, but are increased by up to 10 m s−1 when the model vertical diffusion is switched off above the lower stratosphere. Such large differences in zonal wind between diurnal and non‐diurnal runs cannot be attributed to excessive model tidal amplitudes, since they are found to be in good general agreement with both observations and other modelling studies. An exception is the diurnal tide in the subtropics, which decreases with height above about 0.01 mb instead of increasing to the model top. It is shown that this behaviour is insensitive to changes in the model diffusion, but it is suggested that it could be caused by spurious reflections at the model top.

Mean climate and transience in the tropics of the UGAMP GCM: Sensitivity to convective parametrization

AbstractThe sensitivity of the UK Universities Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (UGCM) to two very different approaches to convective parametrization is described. Comparison is made between a Kuo scheme, which is constrained by large‐scale moisture convergence, and a convective‐adjustment scheme, which relaxes to observed thermodynamic states. Results from 360‐day integrations with perpetual January conditions are used to describe the model's tropical time‐mean climate and its variability. Both convection schemes give reasonable simulations of the time‐mean climate, but the representation of the main modes of tropical variability is markedly different. The Kuo scheme has much weaker variance, confined to synoptic frequencies near 4 days, and a poor simulation of intraseasonal variability. In contrast, the convective‐adjustment scheme has much more transient activity at ail time‐scales. The various aspects of the two schemes which might explain this difference are discussed. The particular closure on moisture convergence used in this version of the Kuo scheme is identified as being inappropriate.

Three dimensional modelling of chlorine activation in the Arctic stratosphere

The UK Universities Global Atmospheric Modelling Programme (UGAMP) General Circulation Model has been used to study the chemical evolution of the northern hemisphere polar vortex. A run that includes a parametrisation of the heterogeneous reactions occurring on polar stratospheric clouds (PSCs) has been initialised with the ECMWF analyses of the 5th of January 1992. This run produces high values of active chlorine within the polar vortex, for example mixing ratios of ClOx (=Cl+ClO+2×Cl2O2) of 1.8ppbv are calculated after six days of integration, with correspondingly low values of HCl and ClONO2. During warm periods of this model run, when the PSC reactions are not active, the ClOx is converted slowly back into ClONO2. Chemical loss of ozone in January in the model is small.