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1.
A comparison of PMIP2 model simulations and the MARGO proxy reconstruction for tropical sea surface temperatures at last glacial maximum 总被引:2,自引:1,他引:1
Bette L. Otto-Bliesner Ralph Schneider E. C. Brady M. Kucera A. Abe-Ouchi E. Bard P. Braconnot M. Crucifix C. D. Hewitt M. Kageyama O. Marti A. Paul A. Rosell-Melé C. Waelbroeck S. L. Weber M. Weinelt Y. Yu 《Climate Dynamics》2009,32(6):799-815
Results from multiple model simulations are used to understand the tropical sea surface temperature (SST) response to the
reduced greenhouse gas concentrations and large continental ice sheets of the last glacial maximum (LGM). We present LGM simulations
from the Paleoclimate Modelling Intercomparison Project, Phase 2 (PMIP2) and compare these simulations to proxy data collated
and harmonized within the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface Project (MARGO). Five atmosphere–ocean
coupled climate models (AOGCMs) and one coupled model of intermediate complexity have PMIP2 ocean results available for LGM.
The models give a range of tropical (defined for this paper as 15°S–15°N) SST cooling of 1.0–2.4°C, comparable to the MARGO
estimate of annual cooling of 1.7 ± 1°C. The models simulate greater SST cooling in the tropical Atlantic than tropical Pacific,
but interbasin and intrabasin variations of cooling are much smaller than those found in the MARGO reconstruction. The simulated
tropical coolings are relatively insensitive to season, a feature also present in the MARGO transferred-based estimates calculated
from planktonic foraminiferal assemblages for the Indian and Pacific Oceans. These assemblages indicate seasonality in cooling
in the Atlantic basin, with greater cooling in northern summer than northern winter, not captured by the model simulations.
Biases in the simulations of the tropical upwelling and thermocline found in the preindustrial control simulations remain
for the LGM simulations and are partly responsible for the more homogeneous spatial and temporal LGM tropical cooling simulated
by the models. The PMIP2 LGM simulations give estimates for the climate sensitivity parameter of 0.67°–0.83°C per Wm−2, which translates to equilibrium climate sensitivity for doubling of atmospheric CO2 of 2.6–3.1°C. 相似文献
2.
Om Prakash Tripathi Sophie Godin-Beekmann Franck Lefèvre Marion Marchand Andrea Pazmiño Alain Hauchecorne Florence Goutail Hans Schlager C. Michael Volk B. Johnson G. König-Langlo Stefano Balestri Fred Stroh T. P. Bui H. J. Jost T. Deshler Peter von der Gathen 《Journal of Atmospheric Chemistry》2006,55(3):205-226
Simulations of polar ozone losses were performed using the three-dimensional high-resolution (1∘ × 1∘) chemical transport model MIMOSA-CHIM. Three Arctic winters 1999–2000, 2001–2002, 2002–2003 and three Antarctic winters 2001, 2002, and 2003 were considered for the study. The cumulative ozone loss in the Arctic winter 2002–2003 reached around 35% at 475 K inside the vortex, as compared to more than 60% in 1999–2000. During 1999–2000, denitrification induces a maximum of about 23% extra ozone loss at 475 K as compared to 17% in 2002–2003. Unlike these two colder Arctic winters, the 2001–2002 Arctic was warmer and did not experience much ozone loss. Sensitivity tests showed that the chosen resolution of 1∘ × 1∘ provides a better evaluation of ozone loss at the edge of the polar vortex in high solar zenith angle conditions. The simulation results for ozone, ClO, HNO3, N2O, and NO
y
for winters 1999–2000 and 2002–2003 were compared with measurements on board ER-2 and Geophysica aircraft respectively. Sensitivity tests showed that increasing heating rates calculated by the model by 50% and doubling the PSC (Polar Stratospheric Clouds) particle density (from 5 × 10−3 to 10−2 cm−3) refines the agreement with in situ ozone, N2O and NO
y
levels. In this configuration, simulated ClO levels are increased and are in better agreement with observations in January but are overestimated by about 20% in March. The use of the Burkholder et al. (1990) Cl2O2 absorption cross-sections slightly increases further ClO levels especially in high solar zenith angle conditions. Comparisons of the modelled ozone values with ozonesonde measurement in the Antarctic winter 2003 and with Polar Ozone and Aerosol Measurement III (POAM III) measurements in the Antarctic winters 2001 and 2002, shows that the simulations underestimate the ozone loss rate at the end of the ozone destruction period. A slightly better agreement is obtained with the use of Burkholder et al. (1990) Cl2O2 absorption cross-sections. 相似文献
3.
Tropical climates at the Last Glacial Maximum: a new synthesis of terrestrial palaeoclimate data. I. Vegetation, lake-levels and geochemistry 总被引:1,自引:1,他引:0
I. Farrera S. P. Harrison I. C. Prentice G. Ramstein J. Guiot P. J. Bartlein R. Bonnefille M. Bush W. Cramer U. von Grafenstein K. Holmgren H. Hooghiemstra G. Hope D. Jolly S.-E. Lauritzen Y. Ono S. Pinot M. Stute G. Yu 《Climate Dynamics》1999,15(11):823-856
Palaeodata in synthesis form are needed as benchmarks for the Palaeoclimate Modelling Intercomparison Project (PMIP). Advances
since the last synthesis of terrestrial palaeodata from the last glacial maximum (LGM) call for a new evaluation, especially
of data from the tropics. Here pollen, plant-macrofossil, lake-level, noble gas (from groundwater) and δ18O (from speleothems) data are compiled for 18±2 ka (14C), 32 °N–33 °S. The reliability of the data was evaluated using explicit criteria and some types of data were re-analysed
using consistent methods in order to derive a set of mutually consistent palaeoclimate estimates of mean temperature of the
coldest month (MTCO), mean annual temperature (MAT), plant available moisture (PAM) and runoff (P-E). Cold-month temperature
(MAT) anomalies from plant data range from −1 to −2 K near sea level in Indonesia and the S Pacific, through −6 to −8 K at
many high-elevation sites to −8 to −15 K in S China and the SE USA. MAT anomalies from groundwater or speleothems seem more
uniform (−4 to −6 K), but the data are as yet sparse; a clear divergence between MAT and cold-month estimates from the same
region is seen only in the SE USA, where cold-air advection is expected to have enhanced cooling in winter. Regression of
all cold-month anomalies against site elevation yielded an estimated average cooling of −2.5 to −3 K at modern sea level,
increasing to ≈−6 K by 3000 m. However, Neotropical sites showed larger than the average sea-level cooling (−5 to −6 K) and
a non-significant elevation effect, whereas W and S Pacific sites showed much less sea-level cooling (−1 K) and a stronger
elevation effect. These findings support the inference that tropical sea-surface temperatures (SSTs) were lower than the CLIMAP
estimates, but they limit the plausible average tropical sea-surface cooling, and they support the existence of CLIMAP-like
geographic patterns in SST anomalies. Trends of PAM and lake levels indicate wet LGM conditions in the W USA, and at the highest
elevations, with generally dry conditions elsewhere. These results suggest a colder-than-present ocean surface producing a
weaker hydrological cycle, more arid continents, and arguably steeper-than-present terrestrial lapse rates. Such linkages
are supported by recent observations on freezing-level height and tropical SSTs; moreover, simulations of “greenhouse” and
LGM climates point to several possible feedback processes by which low-level temperature anomalies might be amplified aloft.
Received: 7 September 1998 / Accepted: 18 March 1999 相似文献
4.
Timothy M. Lenton 《Climatic change》2006,76(1-2):7-29
Anthropogenic climate change will continue long after anthropogenic CO2 emissions cease. Atmospheric CO2, global warming and ocean circulation will approach equilibrium on the millennial timescale, whereas thermal expansion of
the ocean, ice sheet melt and their contributions to sea level rise are unlikely to be complete. Atmospheric CO2 in year 3000 depends non-linearly on the total amount of CO2 emitted and is very likely to exceed the present level of ∼380 ppmv. CO2 is doubled for ∼2500 GtC emitted, quadrupled if all ∼5000 GtC of conventional fossil fuel resources are emitted, and increases
by a factor of ∼32 if a further 20,000 GtC of exotic fossil fuel resources are emitted. Global warming in year 3000 will also
depend on climate sensitivity to doubling CO2, which is most probably ∼3 ∘C but highly uncertain. Thermal expansion will contribute 0.5–2 m to millennial sea level rise for each doubling of CO2. The Greenland ice sheet could melt completely within the millennium under > 8×CO2, adding a further ∼7 m to sea level. The rate of melt depends on the magnitude of forcing above a regional warming threshold
of 1–3 ∘C. The West Antarctic ice sheet could be threatened by 4–10 ∘C local warming, and its potential contribution to millennial sea level rise exceeds current maximum estimates of ∼1 m. The
fate of the ocean thermohaline circulation may depend on the rate as well as the magnitude of forcing. 相似文献
5.
Tropical cooling and the isotopic composition of precipitation in general circulation model simulations of the ice age climate 总被引:2,自引:0,他引:2
We test the climate effects of changes in the tropical ocean by imposing three different patterns of tropical SSTs in ice
age general circulation model simulations that include water source tracers and water isotope tracers. The continental air
temperature and hydrological cycle response in these simulations is substantial and should be directly comparable to the paleoclimatic
record. With tropical cooling imposed, there is a strong temperature response in mid- to high-latitudes resulting from changes
in sea ice and disturbance of the planetary waves; the results suggest that tropical/subtropical ocean cooling leads to significant
dynamical and radiative feedbacks that might amplify ice age cycles. The isotopes in precipitation generally follow the temperature
response at higher latitudes, but regional δ18O/air temperature scaling factors differ greatly among the experiments. In low-latitudes, continental surface temperatures
decrease congruently with the adjacent SSTs in the cooling experiments. Assuming CLIMAP SSTs, 18O/16O ratios in low-latitude precipitation show no change from modern values. However, the experiments with additional cooling
of SSTs produce much lower tropical continental δ18O values, and these low values result primarily from an enhanced recycling of continental moisture (as marine evaporation
is reduced). The water isotopes are especially sensitive to continental aridity, suggesting that they represent an effective
tracer of the extent of tropical cooling and drying. Only one of the tropical cooling simulations produces generalized low-latitude
aridity. These results demonstrate that the geographic pattern of cooling is most critical for promoting much drier continents,
and they underscore the need for accurate reconstructions of SST gradients in the ice age ocean.
Received: 26 July 1999 / Accepted: 10 July 2000 相似文献
6.
Besides sea surface temperature (SST), soil moisture (SM) exhibits a significant memory and is likely to contribute to atmospheric
predictability at the seasonal timescale. In this respect, West Africa was recently highlighted as a “hot spot” where the
land–atmosphere coupling could play an important role, through the recycling of precipitation and the modulation of the meridional
gradient of moist static energy. Particularly intriguing is the observed relationship between summer monsoon rainfall over
Sahel and the previous second rainy season over the Guinean Coast, suggesting the possibility of a soil moisture memory beyond
the seasonal timescale. The present study is aimed at revisiting this question through a detailed analysis of the instrumental
record and a set of numerical sensitivity experiments. Three ensembles of global atmospheric simulations have been designed
to assess the relative influence of SST and SM boundary conditions on the West African monsoon predictability over the 1986–1995
period. On the one hand, the results indicate that SM contributes to rainfall predictability at the end and just after the
rainy season over the Sahel, through a positive soil-precipitation feedback that is consistent with the “hot spot” hypothesis.
On the other hand, SM memory decreases very rapidly during the dry season and does not contribute to the predictability of
the all-summer monsoon rainfall. Though possibly model dependent, this conclusion is reinforced by the statistical analysis
of the summer monsoon rainfall variability over the Sahel and its link with tropical SSTs. Our results indeed suggest that
the apparent relationship with the previous second rainy season over the Guinean Coast is mainly an artefact of rainfall teleconnections
with tropical modes of SST variability both at interannual and multi-decadal timescales. 相似文献
7.
The Possible Climatic Impact in China of Iceland's Eldgjá Eruption Inferred from Historical Sources 总被引:1,自引:0,他引:1
Based on Chinese historical sources, the possible climatic impact in China of the prolonged Eldgjá eruption starting around 934 AD was investigated. An extremely hot summer was reported in 934 AD; hundreds of people died of the intense heat of this summer in Luoyang, the capital of the Later Tang Empire (923–936 AD). Snowless (and possibly also mild) winters probably occurred successively following the Eldgjá eruption until 938 AD. In 939 AD, cold weather set in abruptly and lasted for about 3 years; whereas peak cooling occurred in 939AD. In the summer of 939 AD, it snowed in the southeast of the Inner Mongolia Plateau (about 40–44∘N, 113–123∘E). From 939AD to 941 AD, hard winters occurred successively in China. Worse, unprecedented drought and plague of locusts broke out in 942 AD and persisted in 943 AD. More than several hundred thousand people were starved to death. This catastrophe was at least partly responsible for the collapse of the Later Jin Dynasty in China. By comparison with the tree-ring evidence and uncovered European historical evidence, the spatial response to the Eldgjá eruption appeared to be complex, whereas hemispheric or global cooling occurred in 939–942 AD. 相似文献
8.
T. M. Lenton M. S. Williamson N. R. Edwards R. Marsh A. R. Price A. J. Ridgwell J. G. Shepherd S. J. Cox 《Climate Dynamics》2006,26(7-8):687-711
A new Earth system model, GENIE-1, is presented which comprises a 3-D frictional geostrophic ocean, phosphate-restoring marine biogeochemistry, dynamic and thermodynamic sea-ice, land surface physics and carbon cycling, and a seasonal 2-D energy-moisture balance atmosphere. Three sets of model climate parameters are used to explore the robustness of the results and for traceability to earlier work. The model versions have climate sensitivity of 2.8–3.3°C and predict atmospheric CO2 close to present observations. Six idealized total fossil fuel CO2 emissions scenarios are used to explore a range of 1,100–15,000 GtC total emissions and the effect of rate of emissions. Atmospheric CO2 approaches equilibrium in year 3000 at 420–5,660 ppmv, giving 1.5–12.5°C global warming. The ocean is a robust carbon sink of up to 6.5 GtC year−1. Under ‘business as usual’, the land becomes a carbon source around year 2100 which peaks at up to 2.5 GtC year−1. Soil carbon is lost globally, boreal vegetation generally increases, whilst under extreme forcing, dieback of some tropical and sub-tropical vegetation occurs. Average ocean surface pH drops by up to 1.15 units. A Greenland ice sheet melt threshold of 2.6°C local warming is only briefly exceeded if total emissions are limited to 1,100 GtC, whilst 15,000 GtC emissions cause complete Greenland melt by year 3000, contributing 7 m to sea level rise. Total sea-level rise, including thermal expansion, is 0.4–10 m in year 3000 and ongoing. The Atlantic meridional overturning circulation shuts down in two out of three model versions, but only under extreme emissions including exotic fossil fuel resources. 相似文献
9.
Article 2 of the UN Framework Convention on Climate Change (UNFCCC), which states the treaty's long-term objective, is the
subject of a growing literature that examines means to interpret and implement this provision. Here we provide context for
these studies by exploring the intertwined scientific, legal, economic, and political history of Article 2. We review proposed
definitions for “dangerous anthropogenic interference” and frameworks that have been proposed for implementing these definitions.
Specific examples of dangerous climate changes suggest limits on global warming ranging from 1 to 4 ∘C and on concentrations ranging from 450 to 700 ppm CO2 equivalents. The implications of Article 2 for near term restrictions on greenhouse-gas emissions, e.g., the Kyoto Protocol,
are also discussed. 相似文献
10.
Polar amplification of surface warming has previously been displayed by one of the authors in a simplified climate system
model with no ice-albedo feedbacks. A physical mechanism responsible for this pattern is presented and tested in an energy
balance model and two different GCMs through a series of fixed-SST and “ghost forcing” experiments. In the first ghost forcing
experiment, 4 W/m2 is added uniformly to the mixed layer heat budget and in the second and third, the same forcing is confined to the tropics
and extra-tropics, respectively. The result of the uniform forcing is a polar amplified response much like that resulting
from a doubling of CO2. Due to an observed linearity this response can be interpreted as the sum of the essentially uniform response to the tropical-only
forcing and a more localized response to the extra-tropical-only forcing. The flat response to the tropical forcing comes
about due to increased meridional heat transports leading to a warming and moistening of the high-latitude atmosphere. This
produces a longwave forcing on the high-latitude surface budget which also has been observed by other investigators. Moreover,
the tropical surface budget is found to be more sensitive to SST changes than the extra-tropical surface budget. This strengthens
the tendency for the above mechanism to produce polar amplification, since the tropics need to warm less to counter an imposed
forcing. 相似文献
11.
The 4-year data sets (1998–2001) of PM10 and other gaseous pollutants at four rural and urban monitoring sites provided by Environmental Protection Department of
Hong Kong have been analyzed for days of extremely high and low PM10 levels. The annual means of PM10 concentrations are between 37 and 57 μg/m3. The level of high PM10 concentration is defined from the comparison of local and international standards. Episode days are mainly controlled by
different meteorological conditions: the continental outflow, the land-sea breeze/weak synoptic forcings and the approaching
tropical cyclones. Integrated approaches have been used to distinguish between the predominantly “territory wide” and “long-range
transport” (LRT) episode days. Case studies of these types of episodes are presented and the number of episode days per year
for each type has been estimated. It is found that the LRT contributions are significant and account for ∼66% of the PM10 episode days. Very high correlations between CO and PM10 concentrations, and between SO2 and PM10 concentrations, can be found during the “territory wide” episode days which implies the important contributions of fossil
fuel combustion to the PM10 episodes. The number of “low level” PM10 days per year has decreased by a factor near 3 from 1998 to 2001. Precipitation scavenging is the major process causing low
levels of PM10, irrespective of the associated weather systems. The regional annual background level is ∼9 μg/m3. With the exception of seasalt components, the average elemental concentrations of major inorganic species are similar for
all of the sites during LRT events and constitute representative compositions of PM10 during such events. 相似文献
12.
Data collected during the SHEBA and CASES-99 field programs are employed to examine the flux–gradient relationship for wind
speed and temperature in the stably stratified boundary layer. The gradient-based and flux-based similarity functions are
assessed in terms of the Richardson number Ri and the stability parameter z/Λ*, z being height and Λ* the local Obukhov length. The resulting functions are expressed in an analytical form, which is essentially unaffected by
self-correlation, when thermal stratification is strong. Turbulence within the stably stratified boundary layer is classified
into four regimes: “nearly-neutral” (0 < z/Λ* < 0.02), “weakly-stable” (0.02 < z/Λ* < 0.6), “very-stable” (0.6 < z/Λ* < 50), and “extremely-stable” (z/Λ* > 50). The flux-based similarity functions for gradients are constant in “nearly-neutral” conditions. In the “very-stable”
regime, the dimensionless gradients are exponential, and proportional to (z/Λ*)3/5. The existence of scaling laws in “extremely-stable” conditions is doubtful. The Prandtl number Pr decreases from 0.9 in nearly-neutral conditions and to about 0.7 in the very-stable regime. The necessary condition for the
presence of steady-state turbulence is Ri < 0.7. 相似文献
13.
Philip B. Holden N. R. Edwards K. I. C. Oliver T. M. Lenton R. D. Wilkinson 《Climate Dynamics》2010,35(5):785-806
In order to investigate Last Glacial Maximum and future climate, we “precalibrate” the intermediate complexity model GENIE-1
by applying a rejection sampling approach to deterministic emulations of the model. We develop ~1,000 parameter sets which
reproduce the main features of modern climate, but not precise observations. This allows a wide range of large-scale feedback
response strengths which generally encompass the range of GCM behaviour. We build a deterministic emulator of climate sensitivity
and quantify the contributions of atmospheric (±0.93°C, 1σ) vegetation (±0.32°C), ocean (±0.24°C) and sea–ice (±0.14°C) parameterisations to the total uncertainty. We then perform
an LGM-constrained Bayesian calibration, incorporating data-driven priors and formally accounting for structural error. We
estimate climate sensitivity as likely (66% confidence) to lie in the range 2.6–4.4°C, with a peak probability at 3.6°C. We estimate LGM cooling likely to lie in
the range 5.3–7.5°C, with a peak probability at 6.2°C. In addition to estimates of global temperature change, we apply our
ensembles to derive LGM and 2xCO2 probability distributions for land carbon storage, Atlantic overturning and sea–ice coverage. Notably, under 2xCO2 we calculate a probability of 37% that equilibrium terrestrial carbon storage is reduced from modern values, so the land
sink has become a net source of atmospheric CO2. 相似文献
14.
A comparison of estimates of the root-mean-square error (RMSE) and potential predictability index (PPI) is carried out between
experiments with observed and “persistent” anomalies of sea surface temperature (SST). The results obtained point to a possible
significant bias of seasonal forecasting results in some regions when boundary conditions are introduced by a “persistence”
procedure, particularly for summer T
850. Indirect evidence of the influence of extratropical SST anomalies points to their possible role in seasonal forecasts, which
is more substantial in the summer season. Although the conclusions should rather be regarded as preliminary ones because of
a limited size of the sample, it is nonetheless certain that the influence of boundary conditions governing the signal becomes
more significant in summer because of a decrease in the instability of the internal atmospheric dynamics. 相似文献
15.
Generally, it is assumed that UV-light, high temperature or reactive molecules like O3 and OH are needed to activate gas reactions in air. In consequence, the catalytic activity on natural materials such as sand and soil on the earth's surface is assumed to be insignificant. We have measured O2-dissociation rates on natural quartz sand at 40˚C and compared these with O2-dissociation rates near 500˚C on materials with well-known catalytic activity. In terms of probabilities for dissociation of impinging O2-molecules the measured rates are in the 10−12–10−4 range. We have also measured dissociation rates of H2 and N2, water-formation from H2 and O2 mixtures, exchange of N between N2, NO
x
and a breakdown of HNO3, NO2 and CH4 on natural quartz sand at 40˚C. The measured rates together with an effective global land area have been used to estimate the impact of thermodynamically driven reactions on the earth's surface on the global atmospheric budgets of H2, NO2 and CH4. The experimental data on natural quartz sand together with data from equilibrium calculations of air suggest that an expected increase in anthropogenic supply of air pollutants, such as NO
x
or other “reactive” nitrogen compounds, hydrogen and methane, will be counter-acted by catalysis on the earth's surface. On the other hand, at Polar Regions and boreal forests where the “reactive” nitrogen concentration is below equilibrium, the same catalytic effect activates formation of bio-available nitrogen compounds from N2, O2 and H2O. 相似文献
16.
Summary ?Thirty years (1958–1987) of daily rainfall data for Kenya and north eastern Tanzania are analysed with the aim to characterize
the interannual variability of the onset and cessation of the East African “long rains” (boreal spring). The leading principal
component (PC1) depicts consistent rainfall variations over much of the region. Cumulative PC1 scores for each year serve
to identify onset and cessation dates. The robustness of the dates derived from this method is demonstrated through the use
of an independent sample of stations. Their spatial representativity is assessed by daily rainfall composites. Average onset
occurs on March 25th, and cessation on May 21st. The interannual variability of the onset (standard deviation of 14.5 days) is larger than that of the withdrawal (10.3 days),
but the onset is also spatially much more consistent. Mean dates and dates in selected anomalous years agree well with previous
studies.
The relationship between onset time-series and large-scale atmospheric and oceanic fields is analysed. On a monthly time-scale,
interannual variations in “long rains” onset are associated with sea-surface temperature (SST) and sea-level pressure (SLP)
patterns that have a different sign for the Atlantic and Indian Oceans. A warm South Atlantic and a cool Indian Ocean are
associated with low and high SLP anomalies, respectively. These patterns are conducive to enhanced equatorial easterlies and
surface divergence over East Africa. This maintains the meridional branch (north–south orientated) of the Intertropical Convergence
Zone (ITCZ) further west, and the net result is a delayed onset of the “long rains”. Some of the South Atlantic features are
already present during January–February, suggesting some potential for monitoring interannual variations in the wet season
onset, based on SST and SLP patterns. Additional signals are found over Europe and the Mediterranean Sea in terms of the interaction
between the Northern Hemisphere extratropics and equatorial eastern Africa. A surge in the mid-tropospheric northerlies at
this time induces instability that may lead to an early onset event.
Received July 3, 2002; revised November 28, 2002; accepted December 7, 2002
Published online March 17, 2003 相似文献
17.
Forecasting the equatorial Pacific sea surface temperatures by neural network models 总被引:2,自引:0,他引:2
We used neural network models to seasonally forecast the tropical Pacific sea surface temperature anomalies (SSTA) in the
Ni?o 3.4 region (6 °S–6 °N, 120 °W–170 °W). The inputs to the neural networks (i.e., the predictors) were the first seven wind stress empirical orthogonal function
(EOF) modes of the tropical Pacific (20 °S–20 °N, 120 °E–70 °W) for four seasons and the Ni?o 3.4 SSTA itself for the final season. The period of 1952–1981 was used for training the neural
network models, and the period 1982–1992 for forecast validation. At 6-month lead time, neural networks attained forecast
skills comparable to the other El Ni?o-Southern Oscillation (ENSO) models. Our results suggested that neural network models
were viable for ENSO forecasting even at longer lead times of 9 to 12 months. We hypothesized that at these longer leads,
the underlying relationship between the wind stress and Ni?o 3.4 SSTA became increasingly nonlinear. The neural network results
were interpreted in light of current theories, e.g., the role of the “off-equatorial” Rossby waves in triggering the onset
of an ENSO event and the delayed-oscillator theory in the development and termination of an ENSO event.
Received: 31 October 1995 / Accepted: 25 July 1996 相似文献
18.
The spectral characteristics of the δ18O isotopic ratio time series of the Quelccaya ice cap summit core are investigated with the multi taper method (MTM), the
singular spectrum analysis (SSA) and the wavelet transform (WT) techniques for the 500 y long 1485–1984 period. The most significant
(at the 99.8% level) cycle according to the MTM F-test has a period centered at 14.4 y while the largest variance explaining
oscillation according to the SSA technique has a period centered at 12.9 y. The stability over time of these periods is investigated
by performing evolutive MTM and SSA on the 500 y long δ18O series with a 100 y wide moving window. It is shown that the cycles with largest amplitude and that the oscillations with
largest extracting variance have corresponding periods aggregated around 13.5 y that are very stable over the period between
1485 and 1984. The WT of the same isotopic time series reveals the existence of a main oscillation around 12 y which are also
very stable in time. The relation between the isotopic data at Quelccaya and the annual sea surface temperature (SST) field
anomalies is then evaluated for the overlapping 1919–1984 period. Significant global correlation and significant coherency
at 12.1 y are found between the isotopic series and the annual global sea surface temperature (GSST) series. Moreover, the
correlation between the low (over 8 y) frequency component of the isotopic time series and the annual SST field point out
significant values in the tropical North Atlantic. This region is characterized by a main SST variability at 12.8 y. The Quelccaya
δ18O isotopic ratio series may therefore be considered as a good recorder of the tropical North Atlantic SSTs. This may be explained
by the following mechanism: the water vapor amount evaporated by the tropical North Atlantic is function of the SST. So is
the water vapor δ18O isotopic ratio. This water vapor is advected during the rainy season by northeast winds and precipitates at the Quelccaya
summit with its tropical North Atlantic isotopic signature. It is also suggested from this described stability of the decadal
time scale variability observed in the Quelccaya isotopic series, that the decadal time scale GSST variability was also stable
during the last five centuries.
Received: 12 February 1997 / Accepted: 9 September 1997 相似文献
19.
Climatic impacts of historical wetland drainage in Switzerland 总被引:1,自引:0,他引:1
The effects of historical land-use and land-cover changes on the climate of the Swiss Plateau in the different seasons were
investigated. In the 19th century, a civil engineering project was initiated to reshape the lake and river system on the Swiss
Plateau in order to ban the frequent flooding during extreme weather events. The landscape modifications consisted primarily
of a conversion of wetlands with extended peat soils into a highly productive agricultural landscape. Historical maps (1800–1850)
served as a basis for the reconstruction of the past land use. The “Lokal-Modell” of the Consortium for Small-Scale Modelling
was used to conduct eight one-month long high-resolution simulations (1.5 × 1.5 km2) with present and past landscape conditions. The modified soil and surface properties led to distinctly altered energy and
moisture exchanges at the surface and as a consequence affected the local and regional climate. The climatic changes show
different characteristics and magnitudes in the cold (October – March) as compared to the warm season (April – September).
The landscape modifications led to an average daytime cooling between −0.12 °C (January) and −0.61 °C (April) and a night-time
warming of 0.19 °C−0.34 °C. The differences in the mean monthly temperatures show a warming of 0.1 °C−0.2 °C in the cold season
and a cooling of similar magnitude in most of the study area in the warm season. The modification of the radiation budget
and the surface energy balance distinctly affected the convective activity in the study area in the warm season, but had only
a weak effect on convectivity in the cold season. The cloud coverage in the warm season is therefore distinctly reduced compared
to the past. 相似文献
20.
Spring Phenophases in Recent Decades Over Eastern China and Its Possible Link to Climate Changes 总被引:7,自引:0,他引:7
In light of the observed climate changes in recent decades over eastern China, we studied the changes in spring phenophases of woody plants observed at 16-stations during 1963–1996, and explored the possible link between the spring phenophases changes and climate changes before the phenophase onset. It is found that, in the region north of 33∘N (including Northeast, North China and the lower reaches of the Huaihe River), the phenophase advanced 1.1–4.3 days per decade for early spring and 1.4–5.4 days per decade for late spring, but in the eastern part of the southwest China it was dealyed by 2.9–6.9 days per decade in early spring and 2.4–6.2 days per decade in late spring. One outstanding feature is identified in Guangzhou in south China, where significant advance of 7.5 days per decade in early spring and delay of 4.6 days per decade in late spring were detected. Statistically siginficant correlation was found between the changes of spring phenophase and the temperatures of one or several months before the phenophase onset. The relationship between the trend of phenophase change and temperature change was highly non-linear (more sensitivity to cooling than to warming) and reached an asymptote 0.5∘C per decade, which may have implication in the responses of the ecosystem in a future global warming scenario. In addition, we also examined the link between the spring phenophase, and length and mean temperature of the growing season, and the analyses suggested that they were highly correlated as well. 相似文献