Dynamical changes in the ENSO system in the last 11,000 years

A thorough analysis of a proxy El Nino/Southern Oscillation (ENSO) record indicates that a bifurcation occurred in the ENSO system sometime around 5,000 years b.p. As a result of this bifurcation the attractor became higher dimensional and a new mechanism of instability was introduced. As a consequence of these changes the system switched from a dynamics where the normal condition (La Nina) was dominant to a dynamics characterized by more frequent and stronger El Nino events.     

Constraining ocean diffusivity from the 8.2 ka event

Greenland ice-core data containing the 8.2 ka event are utilized by a model-data intercomparison within the Earth system model of intermediate complexity, CLIMBER-2.3 to investigate their potential for constraining the range of uncertain ocean diffusivity properties. Within a stochastic version of the model (Bauer et al. in Paleoceanography 19:PA3014, 2004) it has been possible to mimic the pronounced cooling of the 8.2 ka event with relatively good accuracy considering the timing of the event in comparison to other modelling exercises. When statistically inferring from the 8.2 ka event on diffusivity the technical difficulty arises to establish the related likelihood numerically per realisation of the uncertain model parameters: while mainstream uncertainty analyses can assume a quasi-Gaussian shape of likelihood, with weather fluctuating around a long term mean, the 8.2 ka event as a highly nonlinear effect precludes such an a priori assumption. As a result of this study the Bayesian Analysis leads to a sharp single-mode likelihood for ocean diffusivity parameters within CLIMBER-2.3. Depending on the prior distribution this likelihood leads to a reduction of uncertainty in ocean diffusivity parameters (e.g. for flat prior uncertainty in the vertical ocean diffusivity parameter is reduced by factor 2). These results highlight the potential of paleo data to constrain uncertain system properties and strongly suggest to make further steps with more complex models and richer data sets to harvest this potential.     

Abrupt temperature changes during the last 1,500 years

We investigate the occurrence of abrupt changes in a total of 35 different proxy records from the extra-tropical Northern Hemisphere for the last ~1,500 years. The proxy records include ice-core δ¹⁸O, speleothem, tree ring width/density, marine sediment and lake sediment records with annual, sub-decadal or decadal resolutions. The aim is to explore the spatio–temporal distribution of abrupt climate changes using a kink point analysis technique. A clustering of warm kink points (the kink points with the highest temperatures) around AD 1000 appears corresponding to the Medieval Warm Period and indicates a geographically widespread temperature peak at that time. Kink points around AD 1000 are somewhat more numerous on higher latitudes than on lower latitudes. There are some tendencies for the coldest kink points (the kink points with the lowest temperatures) to be clustered in the ninetenth century, but they are generally more unevenly spaced in time than the warm peaks around AD 1000. The relative lack of kink points detected during the 1500 s–1700 s, likely the coldest part of the Little Ice Age, implies that this cold period was relatively stable and without abrupt events. A possible cluster of kink points on lower latitudes in the early ninth century is also found. No clear difference in the timing of kink points between the different proxy types can be observed.     

Non-uniform interhemispheric temperature trends over the past 550 years

The warming trend over the last century in the northern hemisphere (NH) was interrupted by cooling from ad 1940 to 1975, a period during which the southern hemisphere experienced pronounced warming. The cause of these departures from steady warming at multidecadal timescales are unclear; the prevailing explanation is that they are driven by non-uniformity in external forcings but recent models suggest internal climate drivers may play a key role. Paleoclimate datasets can help provide a long-term perspective. Here we use tree-rings to reconstruct New Zealand mean annual temperature over the last 550 years and demonstrate that this has frequently cycled out-of-phase with NH mean annual temperature at a periodicity of around 30–60 years. Hence, observed multidecadal fluctuations around the recent warming trend have precedents in the past, strongly implicating natural climate variation as their cause. We consider the implications of these changes in understanding and modelling future climate change.     

The rainfall regime in Lisbon in the last 150 years

The first decades of the rainfall series of Lisbon have been digitized recently allowing a long-term assessment of the rainfall regime for 150 years of uninterrupted, i.e., the first assessment for the longest continuous precipitation time series in western Iberia. This data has been monitored continuously at the D. Luís observatory having started to be published in 1864 in the Observatory's log books (Annals). We use an approach based on different characteristics of rain spells that has been proved to be satisfactory for the analysis of the different parameters related to the rainfall regime in that part of the world. Thus, a rain spell is defined as a series of consecutive days with a measured daily rainfall equal or higher than 1.0 mm. Each rain spell is preceded and followed by at least one dry day. For each rain spell, its duration, its yield (RSY), and its average intensity (RSI) was calculated. Additionally, the total number of rain spells in each year was also considered. Dryness was analyzed using the dry days since last rain approach. Besides the evaluation over the entire 150-year period available, we have also looked into three equally spaced sub-periods. Lisbon reveals large inter-annual and intra-annual variability and both have increased considerably in the last decades. The large intra-annual variability is demonstrated by both; a very large range of annual rainfall percentage accumulated at any given date and by a very large range of dates on which a certain rainfall percentage was accumulated. Again, both metrics increased in the last decades. Parallel to the increase in the uncertainty, a very significant net increase is noticed in the annual totals since the 1960s compared to the first half of the previous century. The increase is mainly due to more intense events which are reflected by higher RSY and RSI values in the last 50 years.     

A role for icebergs in the 8.2 ka climate event

We investigate the potential role of icebergs in the 8.2 ka climate event, using a coupled climate model equipped with an iceberg component. First, we evaluate the effect of a large iceberg discharge originating from the decaying Laurentide ice sheet on ocean circulation, compared to a release of an identical volume of freshwater alone. Our results show that, on top of the freshwater effect, a large iceberg discharge facilitates sea-ice growth as a result of lower sea-surface temperatures induced by latent heat of melting. This causes an 8% increased sea-ice cover, 5% stronger reduction in North Atlantic Deep Water production and 1°C lower temperature in Greenland. Second, we use the model to investigate the effect of a hypothetical two-stage lake drainage, which is suggested by several investigators to have triggered the 8.2 ka climate event. To account for the final collapse of the ice-dam holding the Laurentide Lakes we accompany the secondary freshwater pulse in one scenario with a fast 5-year iceberg discharge and in a second scenario with a slow 100-year iceberg discharge. Our experiments show that a two-stage lake drainage accompanied by the collapsing ice-dam could explain the anomalies observed around the 8.2 ka climate event in various climate records. In addition, they advocate a potential role for icebergs in the 8.2 ka climate event and illustrate the importance of latent heat of melting in the simulation of climate events that involve icebergs. Our two-stage lake drainage experiments provide a framework in the discussion of two-stage lake drainage and ice sheet collapse.     

Initialisation and predictability of the AMOC over the last 50 years in a climate model

The mechanisms involved in Atlantic meridional overturning circulation (AMOC) decadal variability and predictability over the last 50 years are analysed in the IPSL–CM5A–LR model using historical and initialised simulations. The initialisation procedure only uses nudging towards sea surface temperature anomalies with a physically based restoring coefficient. When compared to two independent AMOC reconstructions, both the historical and nudged ensemble simulations exhibit skill at reproducing AMOC variations from 1977 onwards, and in particular two maxima occurring respectively around 1978 and 1997. We argue that one source of skill is related to the large Mount Agung volcanic eruption starting in 1963, which reset an internal 20-year variability cycle in the North Atlantic in the model. This cycle involves the East Greenland Current intensity, and advection of active tracers along the subpolar gyre, which leads to an AMOC maximum around 15 years after the Mount Agung eruption. The 1997 maximum occurs approximately 20 years after the former one. The nudged simulations better reproduce this second maximum than the historical simulations. This is due to the initialisation of a cooling of the convection sites in the 1980s under the effect of a persistent North Atlantic oscillation (NAO) positive phase, a feature not captured in the historical simulations. Hence we argue that the 20-year cycle excited by the 1963 Mount Agung eruption together with the NAO forcing both contributed to the 1990s AMOC maximum. These results support the existence of a 20-year cycle in the North Atlantic in the observations. Hindcasts following the CMIP5 protocol are launched from a nudged simulation every 5 years for the 1960–2005 period. They exhibit significant correlation skill score as compared to an independent reconstruction of the AMOC from 4-year lead-time average. This encouraging result is accompanied by increased correlation skills in reproducing the observed 2-m air temperature in the bordering regions of the North Atlantic as compared to non-initialized simulations. To a lesser extent, predicted precipitation tends to correlate with the nudged simulation in the tropical Atlantic. We argue that this skill is due to the initialisation and predictability of the AMOC in the present prediction system. The mechanisms evidenced here support the idea of volcanic eruptions as a pacemaker for internal variability of the AMOC. Together with the existence of a 20-year cycle in the North Atlantic they propose a novel and complementary explanation for the AMOC variations over the last 50 years.     

Western Mediterranean precipitation over the last 300 years from instrumental observations

The paper reports the results of the analysis of the 14 longest precipitation instrumental series, covering the last 300 years, that have been recovered in six subareas of the Western Mediterranean basin, i.e., Portugal, Northern and Southern Spain, Southern France, Northern and Southern Italy. This study extends back by one century our knowledge about the instrumental precipitation over the Western Mediterranean, and by two centuries in some specific subareas. All the time series show repeated swings. No specific trends have been found over the whole period, except in a few cases, but with modest time changes and sometimes having opposite tendency. The same can be said for the most recent decades although with some more marked departures from the average. The correlation between the various Mediterranean subareas is generally not significant, or almost uncorrelated. The Wavelet Spectral Analysis applied to the precipitation identifies only a minor 56-year cycle in autumn, i.e., the same return period that has been found in literature for the Sea Surface Temperature over North Atlantic. A comparison with a gridded dataset reconstruction based on mixed multiproxy and instrumental observations, shows that the grid reconstruction is in good agreement with the observed data for the period after 1900, less for the previous period.     

The spectral description of climate change including the 100 ky energy

Core records, both ice and deep-sea, suggest that the dominant character is that of a red-noise process or random walk. Examination of a few typical records supports the inference that the contribution of the Milankovitch frequencies to climate change at most represents only a small fraction of total climate variance. Most spectral densities are sufficiently "flat" that rates of change will be dominated by the highest frequencies present in the forcing. A broad maximum near 100 ky period can be readily rationalized without invoking an oscillator. One need only suppose that there is an approximate threshold beyond which the climate system collapses. The quasiperiodicity is then governed by a combination of the collapse threshold, the system memory time scale, and the intensity of the stochastic forcing. Changes in the forcing intensity would lead to a shift in the dominant time scale. Some inferred spectral power laws may be inaccurate owing to undersampling of the records.     

Pairwise comparisons to reconstruct mean temperature in the Arctic Atlantic Region over the last 2,000 years

Existing multi-proxy climate reconstruction methods assume the suitably transformed proxy time series are linearly related to the target climate variable, which is likely a simplifying assumption for many proxy records. Furthermore, with a single exception, these methods face problems with varying temporal resolutions of the proxy data. Here we introduce a new reconstruction method that uses the ordering of all pairs of proxy observations within each record to arrive at a consensus time series that best agrees with all proxy records. The resulting unitless composite time series is subsequently calibrated to the instrumental record to provide an estimate of past climate. By considering only pairwise comparisons, this method, which we call PaiCo, facilitates the inclusion of records with differing temporal resolutions, and relaxes the assumption of linearity to the more general assumption of a monotonically increasing relationship between each proxy series and the target climate variable. We apply PaiCo to a newly assembled collection of high-quality proxy data to reconstruct the mean temperature of the Northernmost Atlantic region, which we call Arctic Atlantic, over the last 2,000 years. The Arctic Atlantic is a dynamically important region known to feature substantial temperature variability over recent millennia, and PaiCo allows for a more thorough investigation of the Arctic Atlantic regional climate as we include a diverse array of terrestrial and marine proxies with annual to multidecadal temporal resolutions. Comparisons of the PaiCo reconstruction to recent reconstructions covering larger areas indicate greater climatic variability in the Arctic Atlantic than for the Arctic as a whole. The Arctic Atlantic reconstruction features temperatures during the Roman Warm Period and Medieval Climate Anomaly that are comparable or even warmer than those of the twentieth century, and coldest temperatures in the middle of the nineteenth century, just prior to the onset of the recent warming trend.     

Model support for forcing of the 8.2 ka event by meltwater from the Hudson Bay ice dome

Previous model experiments of the 8.2 ka event forced by the drainage of Lake Agassiz often do not produce climate anomalies as long as those inferred from proxies. In addition to the Agassiz forcing, there is new evidence for significant amounts of freshwater entering the ocean at 8.2 ka from the disintegration of the Laurentide ice sheet (LIS). We use the Community Climate System Model version 3 (CCSM3) to test the contribution of this additional meltwater flux. Similar to previous model experiments, we find that the estimated freshwater forcing from Lake Agassiz is capable of sustaining ocean and climate anomalies for only two to three decades, much shorter than the event duration of ~150 years in proxies. Using new estimates of the LIS freshwater flux (~0.13 Sv for 100 years) from the collapse of the Hudson Bay ice dome in addition to the Agassiz drainage, the CCSM3 generates climate anomalies with a magnitude and duration that match within error those from proxies. This result is insensitive to the duration of freshwater release, a major uncertainty, if the total volume remains the same. An analysis of the modeled North Atlantic freshwater budget indicates that the Agassiz drainage is rapidly transported out of the North Atlantic while the LIS contribution generates longer-lasting freshwater anomalies that are also subject to recirculation by the subtropical gyre back into the North Atlantic. Thus, the meltwater flux originating from the LIS appears to be more important than the Agassiz drainage in generating 8.2 ka climate anomalies and is one way to reconcile some model-data discrepancies.     

Thermal comfort and tourism climate changes in the Qinghai–Tibet Plateau in the last 50 years

In this paper, the thermal comfort and its changes in the Qinghai–Tibet Plateau over the last 50 years have been evaluated by using the physiological equivalent temperature (PET), and a more complete tourism climate picture is presented by the Climate–Tourism–Information Scheme (CTIS). The results show that PET classes in the Qinghai–Tibet Plateau cover six out of the nine-point thermal sensation scale — very cold, cold, cool, slightly cool, neutral and slightly warm — and cold stress is prevailing throughout the year. A small number of slightly cool/warm and neutral days occur in summer months. There occur no warm, hot and very hot days. The frequency of PET classes varies among regions, depending on their altitude/latitude conditions. Xining, Lhasa and Yushu are the top three cities in terms of thermal favorability. With global warming, annual cumulative number of thermally favorable days has been increasing, and that of cold stress has been reducing. The change is more obvious in lower elevation than that in higher elevation regions. The improving thermal comfort in the Qinghai–Tibet Plateau might be a glad tiding for local communities and tourists. Besides PET, CTIS can provide a number of additional bioclimatic information related to tourism and recreational activities. CTIS for Lhasa and Xining shows that sunshine is plentiful all the year round, and windy days occur frequently from late January to early May. This is a useful bioclimatic information for tourism authorities, travel agencies, resorts and tourists.     

Atmospheric circulation changes and neoglacial conditions in the Southern Hemisphere mid-latitudes: insights from PMIP2 simulations at 6 kyr

Glacial geologic studies in the Southern Hemisphere (SH) mid-latitudes (40–54°S) indicate renewed glacial activity in southern South America (Patagonia) and New Zealand’s (NZ) South Island starting at ～7 kyr, the so-called neoglaciation. Available data indicate that neoglacial advances in these regions occurred during a rising trend in atmospheric CO₂ and CH₄ concentrations, lower-than-present but increasing summer insolation and seasonality contrasts. In this paper we examine the climatological context in which neoglaciations occurred through analysis of the complete Paleoclimate Modelling Inter-comparison Project (PMIP2) database of simulations at 6 kyr for the SH. We observe that the amplitude of the annual insolation cycle in the SH did not change significantly at 6 kyr compared to the pre-industrial values, the largest difference occurring in autumn (MAM, negative anomalies) and spring (SON, positive anomalies). The simulated changes in temperatures over the SH respond to the insolation changes, with a 1–2 month delay over the oceans. This results in a reduced amplitude of the annual cycle of temperature and precipitation over most continental regions, except over Patagonia and NZ, that show a slight increase. In contrast, large-scale circulation features, such as the low and upper level winds and the subtropical anticyclones show an amplified annual cycle, as a direct response to the increased/decreased insolation during the transitional seasons SON/MAM. In the annual mean, there is a small but consistent equatorward shift of the latitude of maximum wind speed of 1–3° over the entire SH, which results in a small increase of wind speed over the South Pacific and Atlantic Oceans north of ～50°S and a widespread decline south of 50°S. PMIP2 simulations for 6 kyr, indicate that in the annual mean, the SH mid-latitudes were colder, wetter and with stronger winds north of about 50°S. These conditions are consistent with the observed neoglacial advances in the region, as well as with terrestrial paleoclimate records from Patagonia that indicate cooling and a multi-millennial rising trend in Southern Westerly Wind intensity starting at ～7.8 kyr.     

Simulation of the evolutionary response of global summer monsoons to orbital forcing over the past 280,000 years

We describe the evolutionary response of northern and southern hemisphere summer monsoons to orbital forcing over the past 280,000 years using a fully coupled general circulation ocean-atmosphere model in which the orbital forcing is accelerated by a factor of 100. We find a strong and positive response of northern (southern) summer monsoon precipitation to northern (southern) summer insolation forcing. On average, July (January) precipitation maxima and JJA (DJF) precipitation maxima have high coherence and are approximately in phase with June (December) insolation maxima, implying an average lag between forcing and response of about 30° of phase at the precession period. The average lag increases to over 40° for 4-month precipitation averages, JJAS (DJFM). The phase varies from region to region. The average JJA (DJF) land temperature maxima also lag the June orbital forcing maxima by about 30° of phase, whereas ocean temperature maxima exhibit a lag of about 60° of phase at the precession period. Using generalized measures of the thermal and hydrologic processes that produce monsoons, we find that the summer monsoon precipitation indices for the six regions all fall within the phase limits of the process indices for the respective hemispheres. Selected observational studies from four of the six monsoon regions report approximate in-phase relations of summer monsoon proxies to summer insolation. However other observational studies report substantial phase lags of monsoon proxies and a strong component of forcing associated with glacial-age boundary conditions or other factors. An important next step will be to include glacial-age boundary condition forcing in long, transient paleoclimate simulations, along with orbital forcing.     

Attribution of SST variability in global oceans and the role of ENSO

Based on a novel design of coupled model simulations where sea surface temperature (SST) variability in the equatorial tropical Pacific was constrained to follow the observed El Niño—Southern Oscillation (ENSO) variability, while rest of the global oceans were free to evolve, the ENSO response in SSTs over the other ocean basins was analyzed. Conceptually the experimental setup was similar to discerning the contribution of ENSO variability to interannual variations in atmospheric anomalies. A unique feature of the analysis was that it was not constrained by a priori assumptions on the nature of the teleconnected response in SSTs. The analysis demonstrated that the time lag between ENSO SST and SSTs in other ocean basins was about 6 months. A signal-to-noise analysis indicated that between 25 and 50 % of monthly mean SST variance over certain ocean basins can be attributed to SST variability over the equatorial tropical Pacific. The experimental setup provides a basis for (a) attribution of SST variability in global oceans to ENSO variability, (b) a method for separating the ENSO influence in SST variations, and (c) understanding the contribution from other external factors responsible for variations in SSTs, for example, changes in atmospheric composition, volcanic aerosols, etc.     

Mediterranean drought fluctuation during the last 500 years based on tree-ring data

A 2.5 × 2.5° gridded summer (April–September) drought reconstruction over the larger Mediterranean land area (32.5°/47.5°N, 10°W/50°E; 152 grid points) is described, based on a network of 165 tree-ring series. The drought index used is the self-calibrated Palmer Drought Severity Index, and the period considered is 1500–2000. The reconstruction technique combines an analogue technique for the estimation of missing tree-ring data with an artificial neural network for optimal non-linear calibration, including a bootstrap error assessment. Tests were carried out on the various sources of error in the reconstructions. Errors related to the temporal variations of the number of proxies were tested by comparing four reconstructions calibrated with four different sized regressor datasets, representing the decrease in the number of available proxies over time. Errors related to the heterogeneous spatial density of predictors were tested using pseudo-proxies, provided by the global climate model ECHO-G. Finally the errors related to the imperfect climate signal recorded by tree-ring series were tested by adding white noise to the pseudo-proxies. Reconstructions pass standard cross-validation tests. Nevertheless tests using pseudo-proxies show that the reconstructions are less good in areas where proxies are rare, but that the average reconstruction curve is robust. Finally, the noise added to proxies, which is by definition a high frequency component, has a major effect on the low frequency signal, but not on the medium frequencies. The comparison of the low frequency trends of our mean reconstruction and the GCM simulation indicates that the detrending method used is able to preserve the long-term variations of reconstructed PDSI. The results also highlight similar multi-decadal PDSI variations in the central and western parts of the Mediterranean basin and less clear low frequency changes in the east. The sixteenth and the first part of the seventeenth centuries are characterized by marked dry episodes in the west similar to those observed in the end of the twentieth century. In contrast, the eighteenth and nineteenth centuries (Little Ice Age) are characterized by dominant wet periods. In the eastern part of the Mediterranean basin the observed strong drought period of the end of the twentieth century seems to be the strongest of the last 500 years.     

A critical overview of pan evaporation trends over the last 50 years

Despite the observed increases in global average temperature, observations across the world show that the rate of pan evaporation at a regional scale has been steadily decreasing over the past 50 years. This is known as the pan evaporation paradox. This paper reviews current reported pan evaporation trends, examines available theoretical explanations about this “paradox”, and discusses current research gaps and priorities. It concludes that: (1) three major potential causes of pan evaporation, solar radiation, vapour pressure deficit (VPD) and wind speed, have been changing in the last 50 years. The magnitude of changes and importance of each of these three causes varies from region to region, as does the pan evaporation trend, although overall there is a decreasing trend. (2) Currently two existing theories explaining the pan evaporation trends have limits and are only valid in some specific regions and seasons. Neither of them provides a fundamental physical-based theory that could be applied everywhere. (3) Further investigations are needed before we can fully understand the global evapotranspiration trend in global warming scenarios.     

Comparing the patterns of 20–70 days intraseasonal oscillations over Central Africa during the last three decades

In this paper, we used the Outgoing Longwave Radiation (OLR) data to compare the intraseasonal atmospheric variability patterns over Central Africa, during the last three decades. The spectral analysis indicates that for the three decades, the intraseasonal variability is dominated by 20–80 days periods band with the center near 40–50 days. The results of Empirical Orthogonal Functions (EOFs) analysis have shown that the amount of variance explained by the three retained EOFs are 41.6 % for 1981–1990, 44.2 % for 1991–2000 and 42.6 % for 2001–2010. For the three decades, the three leading EOFs retained exhibit high spatial loadings over Northern Congo, Southern Ethiopia, and Southwestern Tanzania. The power spectra of the leading principal components have their peaks near 40 days for the three decades, indicating MJO signal. The PCs time series revealed that the amplitude of intraseasonal oscillations (ISO) globally decreases from decade to another. The plot of ISO and El Niño-Southern Oscillation (ENSO) indices revealed that the lowest values of ISO strength generally correspond to the relatively large values of ENSO indices and inversely. The mean ISO strength and ISO fluctuations were highest during 1981–1990, and this period also corresponds to the highest fluctuations of ENSO signal.