Implications of climate variability for the detection of multiple equilibria and for rapid transitions in the atmosphere-vegetation system

Paleoclimatic records indicate a decline of vegetation cover in the Western Sahara at the end of the African Humid Period (about 5,500 years before present). Modelling studies have shown that this phenomenon may be interpreted as a critical transition that results from a bifurcation in the atmosphere-vegetation system. However, the stability properties of this system are closely linked to climate variability and depend on the climate model and the methods of analysis. By coupling the Planet Simulator (PlaSim), an atmosphere model of intermediate complexity, with the simple dynamic vegetation model VECODE, we assess previous methods for the detection of multiple equilibria, and demonstrate their limitations. In particular, a stability diagram can yield misleading results because of spatial interactions, and the system's steady state and its dependency on initial conditions are affected by atmospheric variability and nonlinearities. In addition, we analyse the implications of climate variability for the abruptness of a vegetation decline. We find that a vegetation collapse can happen at different locations at different times. These collapses are possible despite large and uncorrelated climate variability. Because of the nonlinear relation between vegetation dynamics and precipitation the green state is initially stabilised by the high variability. When precipitation falls below a critical threshold, the desert state is stabilised as variability is then also decreased. © 2011 The Author(s)

El Niño-related summer precipitation anomalies in Southeast Asia modulated by the Atlantic multidecadal oscillation

AbstractHow the Atlantic Multidecadal Oscillation (AMO) affects El Niño-related signals in Southeast Asia is investigated in this study on a subseasonal scale. Based on observational and reanalysis data, as well as numerical model simulations, El Niño-related precipitation anomalies are analyzed for AMO positive and negative phases, which reveals a time-dependent modulation of the AMO: (i) In May?June, the AMO influences the precipitation in Southern China (SC) and the Indochina peninsula (ICP) by modulating the El Niño-related air-sea interaction over the western North Pacific (WNP). During negative AMO phases, cold sea surface temperature anomalies (SSTAs) over the WNP favor the maintaining of the WNP anomalous anticyclone (WNPAC). The associated southerly (westerly) anomalies on the northwest (southwest) flank of the WNPAC enhance (reduce) the climatological moisture transport to SC (the ICP) and result in wetter (drier) than normal conditions. In contrast, during positive AMO phases, weak SSTAs over the WNP lead to limited influence of El Niño on precipitation in Southeast Asia. (ii) In July?August, the teleconnection impact from the North Atlantic is more manifest than that in May?June. During positive AMO phases, the warmer than normal North Atlantic favors anomalous wave trains, which propagate along the ?great circle route? and result in positive pressure anomalies over SC, consequently suppressing precipitation in SC and the ICP. During negative AMO phases, the anomalous wave trains tend to propagate eastward from Europe to Northeast Asia along the summer Asian jet, exerting limited influence on Southeast Asia

Interdecadal variability of winter precipitation in Southeast China

Interdecadal variability of observed winter precipitation in Southeast China (1961–2010) is characterized by the first empirical orthogonal function of the three-monthly Standardized Precipitation Index (SPI) subjected to a 9-year running mean. For interdecadal time scales the dominating spatial modes represent monopole features involving the Arctic Oscillation (AO) and the sea surface temperature (SST) anomalies. Dynamic composite analysis (based on NCEP/NCAR reanalyzes) reveals the following results: (1) Interdecadal SPI-variations show a trend from a dryer state in the 1970s via an increase during the 1980s towards stabilization on wetter conditions commencing with the 1990s. (2) Increasing wetness in Southeast China is attributed to an abnormal anticyclone over south Japan, with northward transport of warm and humid air from the tropical Pacific to South China. (3) In mid-to-high latitudes the weakened southward flow of polar airmasses induces low-level warming over Eurasia due to stronger AO by warmer zonal temperature advection. This indicates that AO is attributed to the Southeast China precipitation increase influenced by circulation anomalies over the mid-to-high latitudes. (4) The abnormal moisture transport along the southwestern boundary of the abnormal anticyclone over south Japan is related to anomalous south-easterlies modulated by the SST anomalies over Western Pacific Ocean; a positive (negative) SST anomaly will strengthen (weaken) warm and humid air transport, leading to abundant (reduced) precipitation in Southeast China. That is both AO and SST anomalies determine the nonlinear trend observed in winter precipitation over Southeast China

Diagnosing the entropy budget of a climate model

A general circulation model (GCM) of intermediate complexity (Planet Simulator) is subjected to an analysis of the entropy budget and its sensitivity. The entropy production is computed directly based on temperature and temperature tendencies and estimated indirectly based on boundary fluxes. For indirect estimates, the model shows reasonably good agreement with observations. The direct computation indicates deficits of the indirect measures, as they, for example, overestimate the material entropy production (that is, the production by turbulent fluxes). Sensitivity analyses of entropy production are provided, which, depending on changing parameters, hint to a possible applicability of maximum entropy production (MEP) under the constrained dynamics of a complex GC

Multiple steady-states in the terrestrial atmosphere-biosphere system: a result of a discrete vegetation classification?

International audienceMultiple steady states in the atmosphere-biosphere system can arise as a consequence of interactions and positive feedbacks. While atmospheric conditions affect vegetation productivity in terms of available light, water, and heat, different levels of vegetation productivity can result in differing energy- and water partitioning at the land surface, thereby leading to different atmospheric conditions. Here we investigate the emergence of multiple steady states in the terrestrial atmosphere-biosphere system and focus on the role of how vegetation is represented in the model: (i) in terms of a few, discrete vegetation classes, or (ii) a continuous representation. We then conduct sensitivity simulations with respect to initial conditions and to the number of discrete vegetation classes in order to investigate the emergence of multiple steady states. We find that multiple steady states occur in our model only if vegetation is represented by a few vegetation classes. With an increased number of classes, the difference between the number of multiple steady states diminishes, and disappears completely in our model when vegetation is represented by 8 classes or more. Despite the convergence of the multiple steady states into a single one, the resulting climate-vegetation state is nevertheless less productive when compared to the emerging state associated with the continuous vegetation parameterization. We conclude from these results that the representation of vegetation in terms of a few, discrete vegetation classes can result (a) in an artificial emergence of multiple steady states and (b) in a underestimation of vegetation productivity. Both of these aspects are important limitations to be considered when global vegetation-atmosphere models are to be applied to topics of global change

Multiple jets observed in the summer Northern Hemisphere troposphere

Daily observations of the Northern Hemisphere zonal mean zonal wind during July show an intermittent formation of multiple tropospheric jet streams. In particular, a tropospheric westerly, or easterly, jet occurs at latitudes greater than 75◦N: it co-exists with the mid-latitude jet and is characterized by variability on synoptic time scale. Two sample years are here considered, July 1996 and July 1985, when prevailing westerly and easterly jets occur at high latitudes, respectively. Analysis is consistent with a picture where the physical mechanism, which creates and maintains the polar jet in the summer troposphere, is the baroclinic instability process acting on a westerly, or easterly, background zonal flow. Due to the synoptic variability, monthly mean maps of the zonal mean zonal wind for July may show different jet patterns as a function of the year, depending on the occurrences (number and duration) of westerly or easterly polar jets within each month considered. The study of the inter-annual variability of the zonal mean zonal wind through the principal component analysis indicates, in fact, that three tropospheric jet stream patterns can be distinguished. Consistency of observations with the available theories on the double-jet formation is provided and the impact of polar jet occurrences on surface temperature field is evaluated

The large-scale circulations and summer drought and wetness on the Tibetan plateau

Effects of large-scale atmospheric circulation and surface temperatures on extreme dryness and wetness on the Tibetan plateau in summer are analysed using ERA-40 reanalysis and observed precipitation. The extreme cases of drought and wetness can be associated with circulation anomalies in the North Atlantic/European sector and wave trains bridging the Eurasian continent. Drought in Tibet reveals an intense high pressure anomaly over Scandinavia supported by a more south-west to north-east orientated North Atlantic stormtrack. This creates wave trains crossing Eurasia which, on their southward 'great circle route', reach south-eastern Asia where they modulate the flow north and east of the Tibetan plateau by an anticyclone cyclone dipole suppressing moisture supply from the Bay of Bengal. Wetness in Tibet is characterised by a more zonally oriented cross Atlantic stormtrack creating a low pressure anomaly over central Europe and, associated with it, a northward shift of the sub-tropical westerly and tropical easterly jet; wave trains emerging from the North Atlantic on their equatorward route have now a higher chance to reach the sub-tropical jet entrance (instead of propagating further south). Then the wave trains are re-intensified and, passing the Mediterranean Arabian Sea route to India, interact with the monsoon's western branch to lead to ample moisture supply for Tibet. Surface temperatures give indications for positive (negative) El Nino/Southern Oscillation and Indian Ocean Dipole episodes occurring in years of extreme and severe dryness (wetness) on the Tibetan plateau. A pronounced cold surface temperature anomaly in the tropical North Atlantic precedes and accompanies drought on the plateau

Low-frequency variability in idealised GCM experiments with circumpolar and localised storm tracks

International audienceIdealised global circulation model simulations with circumpolar and localised (one and two) storm tracks are re-analysed to determine scaling, intermittency and phase-space structures. In a hundred year experiment with a circumpolar storm track, the spectrum S(f ) of the first principal component of the zonal wind fluctuations shows the following power law regimes: (a) a short-term memory between f- -4 and f -2 up to 50 days and (b) a long-term memory f -1 from 50 to 400 days and f -0.24 beyond 400 days, similar to observed maritime single station near-surface air temperature data. In the presence of localised storm tracks, the wave number two dominates the dynamics and a long-term memory cannot be detected. The recurrence plot is introduced as a novel tool to comprehensively visualise the evolution of the dynamical system in terms of state separations (distances) in phase space. The patterns allow for a qualitative interpretation of the underlying local phenomena in phase space, such as waves, analogs, extremes, and global regimes. Attractor dimensions are, in general, larger than 10, but they appear to be lower in the wave-dominated regimes of the double storm track experiment

Multiple jets observed in the summer Northern Hemisphere troposphere

Daily observations of the Northern Hemisphere zonal mean zonal wind during July show an intermittent formation of multiple tropospheric jet streams. In particular, a tropospheric westerly, or easterly, jet occurs at latitudes greater than 75◦N: it co-exists with the mid-latitude jet and is characterized by variability on synoptic time scale. Two sample years are here considered, July 1996 and July 1985, when prevailing westerly and easterly jets occur at high latitudes, respectively. Analysis is consistent with a picture where the physical mechanism, which creates and maintains the polar jet in the summer troposphere, is the baroclinic instability process acting on a westerly, or easterly, background zonal flow. Due to the synoptic variability, monthly mean maps of the zonal mean zonal wind for July may show different jet patterns as a function of the year, depending on the occurrences (number and duration) of westerly or easterly polar jets within each month considered. The study of the inter-annual variability of the zonal mean zonal wind through the principal component analysis indicates, in fact, that three tropospheric jet stream patterns can be distinguished. Consistency of observations with the available theories on the double-jet formation is provided and the impact of polar jet occurrences on surface temperature field is evaluated

Observed drought and wetness trends in Europe: an update

Abstract. Linear and nonlinear trends of drought and wetness are analysed in terms of the gridded Standardized Precipitation Index (SPI) determined from monthly precipitation in Europe (NCEP/NCAR). In characterizing the meteorological and hydrological aspects, the index is computed on a seasonal and on a bi-annual time scale. Two datasets are compared: one from 1949 to 1997 and the other one includes the update of the last decade (to February 2009). The following results are noted: (i) time series of drought and wetness area coverage (number of grid points above/below the severity threshold) show a remarkable linear trend until about the end of the last century, which is reversed in the last (update) decade. This recent trend reversal is an indication of a nonlinear trend, which is more pronounced on the hydrological time scale. (ii) A nonlinear trend analysis is performed based on the time series of the principal component (PC) associated to the first spatial SPI-eigenvector after embedding it in a time delay coordinate system using a sliding window of 70 months (singular spectrum analysis). Nonlinearity appears as a clear feature on the hydrological time scale. (iii) The first spatial EOF-patterns of the shorter and the longer (updated) SPI time series fields show similar structure. An inspection of the SPI time behaviour at selected grid points illustrates the spatial variability of the detected trends