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1.
Influence of vegetation changes during the Last Glacial Maximum using the BMRC atmospheric general circulation model 总被引:3,自引:0,他引:3
The influence of different vegetation distributions on the atmospheric circulation during the Last Glacial Maximum (LGM,
21 000 years before present) is investigated. The atmospheric general circulation model of the Bureau of Meteorology Research
Center was run using a modern vegetation and in a second experiment with a vegetation reconstruction for the LGM. It is found
that a change from conifer to desert and tundra causes an additional LGM cooling of 1–2 °C in Western Europe, up to −4 °C
in North America and −6 °C in Siberia. An expansion of dryland vegetation causes an additional annual cooling of 1–2 °C for
Australia and northern Africa. On the other hand, an increase of temperature (2 °C) is found in Alaska due to changes in circulation.
In the equatorial region the LGM vegetation leads to an increased modelled temperature of 0.5–1.5 °C and decreased precipitation
(30%) over land due to a reduction of the tropical rainforest, mainly in Indonesia, where the reduction of precipitation over
land is associated with an increase of precipitation of 30% over the western Pacific.
Received: 15 December 1999 / Accepted: 10 January 2001 相似文献
2.
Tropical paleoclimates at the Last Glacial Maximum: comparison of Paleoclimate Modeling Intercomparison Project (PMIP) simulations and paleodata 总被引:12,自引:2,他引:10
S. Pinot G. Ramstein S. P. Harrison I. C. Prentice J. Guiot M. Stute S. Joussaume 《Climate Dynamics》1999,15(11):857-874
Seventeen simulations of the Last Glacial Maximum (LGM) climate have been performed using atmospheric general circulation
models (AGCM) in the framework of the Paleoclimate Modeling Intercomparison Project (PMIP). These simulations use the boundary
conditions for CO2, insolation and ice-sheets; surface temperatures (SSTs) are either (a) prescribed using CLIMAP data set (eight models) or
(b) computed by coupling the AGCM with a slab ocean (nine models). The present-day (PD) tropical climate is correctly depicted
by all the models, except the coarser resolution models, and the simulated geographical distribution of annual mean temperature
is in good agreement with climatology. Tropical cooling at the LGM is less than at middle and high latitudes, but greatly
exceeds the PD temperature variability. The LGM simulations with prescribed SSTs underestimate the observed temperature changes
except over equatorial Africa where the models produce a temperature decrease consistent with the data. Our results confirm
previous analyses showing that CLIMAP (1981) SSTs only produce a weak terrestrial cooling. When SSTs are computed, the models
depict a cooling over the Pacific and Indian oceans in contrast with CLIMAP and most models produce cooler temperatures over
land. Moreover four of the nine simulations, produce a cooling in good agreement with terrestrial data. Two of these model
results over ocean are consistent with new SST reconstructions whereas two models simulate a homogeneous cooling. Finally,
the LGM aridity inferred for most of the tropics from the data, is globally reproduced by the models with a strong underestimation
for models using computed SSTs.
Received: 9 September 1998 / Accepted: 18 March 1999 相似文献
3.
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
4.
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
5.
Last Glacial Maximum climate of the former Soviet Union and Mongolia reconstructed from pollen and plant macrofossil data 总被引:2,自引:2,他引:0
P. E. Tarasov O. Peyron J. Guiot S. Brewer V. S. Volkova L. G. Bezusko N. I. Dorofeyuk E. V. Kvavadze I. M. Osipova N. K. Panova 《Climate Dynamics》1999,15(3):227-240
An improved concept of the best analogues method was used to reconstruct the Last Glacial Maximum (LGM) climate from a set
of botanical records from the former Soviet Union and Mongolia. Terrestrial pollen and macrofossil taxa were grouped into
broad classes – plant functional types (PFTs), defined by the ecological and climatic parameters used in the BIOME1 model.
PFT scores were then calibrated in terms of modern climate using 1245 surface pollen spectra from Eurasia and North America.
In contrast to individual taxa, which exhibit great variability and may not be present in the palaeoassemblages, even in suitable
climates, PFTs are more characteristic of the vegetation types. The modified method thus allows climate reconstruction at
time intervals with partial direct analogues of modern vegetation (e.g. the LGM). At 18 kBP, mean temperatures were 20–29 °C
colder than today in winter and 5–11 °C colder in summer in European Russia and Ukraine. Sites from western Georgia show negative,
but moderate temperature anomalies compared to today: 8–11 °C in January and 5–7 °C in July. LGM winters were 7–15 °C colder
and summers were 1–7 °C colder in Siberia and Mongolia. Annual precipitation sums were 50–750 mm lower than today across northern
Eurasia, suggesting a weakening of the Atlantic and Pacific influences. Reconstructed drought index shows much drier LGM conditions
in northern and mid-latitude Russia, but similar to or slightly wetter than today around the Black Sea and in Mongolia, suggesting
compensation of precipitation losses by lower-than-present evaporation.
Received: 11 May 1998 / Accepted: 25 September 1998 相似文献
6.
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
7.
Coupled ocean-atmosphere surface variability and its climate impacts in the tropical Atlantic region
This study examines time evolution and statistical relationships involving the two leading ocean-atmosphere coupled modes
of variability in the tropical Atlantic and some climate anomalies over the tropical 120 °W–60 °W region using selected historical
files (75-y near global SSTs and precipitation over land), more recent observed data (30-y SST and pseudo wind stress in the
tropical Atlantic) and reanalyses from the US National Centers for Environmental Prediction (NCEP/NCAR) reanalysis System
on the period 1968–1997: surface air temperature, sea level pressure, moist static energy content at 850 hPa, precipitable
water and precipitation. The first coupled mode detected through singular value decomposition of the SST and pseudo wind-stress
data over the tropical Atlantic (30 °N–20 °S) expresses a modulation in the thermal transequatorial gradient of SST anomalies
conducted by one month leading wind-stress anomalies mainly in the tropical north Atlantic during northern winter and fall.
It features a slight dipole structure in the meridional plane. Its time variability is dominated by a quasi-decadal signal
well observed in the last 20–30 ys and, when projected over longer-term SST data, in the 1920s and 1930s but with shorter
periods. The second coupled mode is more confined to the south-equatorial tropical Atlantic in the northern summer and explains
considerably less wind-stress/SST cross-covariance. Its time series features an interannual variability dominated by shorter
frequencies with increased variance in the 1960s and 1970s before 1977. Correlations between these modes and the ENSO-like
Nino3 index lead to decreasing amplitude of thermal anomalies in the tropical Atlantic during warm episodes in the Pacific.
This could explain the nonstationarity of meridional anomaly gradients on seasonal and interannual time scales. Overall the
relationships between the oceanic component of the coupled modes and the climate anomaly patterns denote thermodynamical processes
at the ocean/atmosphere interface that create anomaly gradients in the meridional plane in a way which tends to alter the
north–south movement of the seasonal cycle. This appears to be consistent with the intrinsic non-dipole character of the tropical
Atlantic surface variability at the interannual time step and over the recent period, but produces abnormal amplitude and/or
delayed excursions of the intertropical convergence zone (ITCZ). Connections with continental rainfall are approached through
three (NCEP/NCAR and observed) rainfall indexes over the Nordeste region in Brazil, and the Guinea and Sahel zones in West
Africa. These indices appear to be significantly linked to the SST component of the coupled modes only when the two Atlantic
modes+the ENSO-like Nino3 index are taken into account in the regressions. This suggests that thermal forcing of continental
rainfall is particularly sensitive to the linear combinations of some basic SST patterns, in particular to those that create
meridional thermal gradients. The first mode in the Atlantic is associated with transequatorial pressure, moist static energy
and precipitable water anomaly patterns which can explain abnormal location of the ITCZ particularly in northern winter, and
hence rainfall variations in Nordeste. The second mode is more associated with in-phase variations of the same variables near
the southern edge of the ITCZ, particularly in the Gulf of Guinea during the northern spring and winter. It is primarily linked
to the amplitude and annual phase of the ITCZ excursions and thus to rainfall variations in Guinea. Connections with Sahel
rainfall are less clear due to the difficulty for the model to correctly capture interannual variability over that region
but the second Atlantic mode and the ENSO-like Pacific variability are clearly involved in the Sahel climate interannual fluctuations:
anomalous dry (wet) situations tend to occur when warmer (cooler) waters are present in the eastern Pacific and the gulf of
Guinea in northern summer which contribute to create a northward (southward) transequatorial anomaly gradient in sea level
pressure over West Africa.
Received: 14 April 1998 / Accepted: 24 December 1998 相似文献
8.
CLIMAP SSTs re-revisited 总被引:1,自引:1,他引:0
T. J. Crowley 《Climate Dynamics》2000,16(4):241-255
Since the 1976 publication of the CLIMAP ice age sea surface temperature (SST) reconstruction showing a 1–2 ∘C tropical cooling a substantial debate has arisen as to whether tropical SSTs may instead have been 4–5∘ colder than present. Herein I review the arguments for large SST variations and question a number of key findings, particularly
the validity of ice-age coral SST estimates and “down-projecting” tropical snowline changes to the surface. GCM results indicate
that an intermediate solution requiring ∼2.5 ∘C warm pool cooling is consistent with most quantitative low elevation surface land data and is small enough to allow the
persistence of tropical biota in the ocean during glacial times. The proposal reduces estimated ice-age climate sensitivity
(for a doubling of CO2) from a “high-end” sensitivity of about 4.5 ∘C (for a 5–6 ∘C tropical cooling) to a “mid-range” sensitivity of about 3.0 ∘C for a 2.5 ∘C warm-pool decrease.
Received: 28 July 1999 /Accepted: 12 August 1999 相似文献
9.
S. H. Sajjad Babar Hussain M. Ahmed Khan Asif Raza B. Zaman Ijaz Ahmed 《Climatic change》2009,96(4):539-547
Karachi is the largest city of Pakistan. The temperature change in Karachi is studied in this research by analyzing the time
series data of mean maximum temperature (MMxT), mean minimum temperature (MMiT) and mean annual temperature (MAT) from 1947
to 2005 (59 years). Data is analyzed in three parts by running linear regression and by taking anomalies of all time periods:
(a) whole period from 1947–2005; (b) phase one 1947–1975 and (c) phase two 1976–2005. During 1947 to 2005 MMxT has increased
about 4.6°C, MMiT has no change and MAT has increased 2.25°C. During 1947–1975, MMxT increased 1.9°C, in this period there
is − 1.3°C decrease in MMiT and MAT has raised upto 0.3°C. During 1976–2005, the MMxT, MMiT and MAT increased 2.7°C, 1.2°C
and 1.95°C, respectively. The analysis shows significantly the role of extreme vulnerability of MMxT in rising the temperature
of Karachi than the MMiT. 相似文献
10.
Reanalysis datasets potentially offer the opportunity to examine the tropical quasi-biennial oscillation (QBO) in greater
detail than in the past, including the associated meridional circulation and the links with other parts of the atmosphere.
For such studies to be useful, the QBO represented by the reanalyses should be realistic. In this work, the QBO in the ERA
and NCEP reanalyses is validated against rawinsonde observations from Singapore. Monthly mean data are used. In the lower
stratosphere (at 50 hPa and 30 hPa) the ERA QBO is reasonable, although the wind extrema in both phases are too weak and the
vertical shear and the temperature anomalies are too small. The NCEP QBO is weaker still. At 10 hPa neither reanalysis system
performs well, both systems failing to reproduce the westerlies, possibly because of the proximity of the upper boundary.
The Singapore wind is representative of the zonal means in the reanalyses. The weak wind extrema in the reanalyses would not
support a wave-mean flow interaction theory of the QBO, because a large portion of the gravity wave spectrum which would be
absorbed in reality would be transmitted beyond 10 hPa. The stronger shear zones captured in the ERA data are associated with
larger, more realistic temperature perturbations near 30 hPa. The northward velocities in the NCEP data show a more realistic
structure than in the ERA reanalysis, where they are dominated by a vertical “gridpoint wave” structure in the lowermost stratosphere.
Despite the shortcomings of the reanalyses, the high correlations of the wind at 30 hPa and 50 hPa with the observations at
Singapore mean that the reanalyses could potentially be used to examine the effects of the QBO away from the tropical stratosphere.
Future reanalyses need to take full account of the wind shears evident in the rawinsonde observations and use models with
an adequate resolution to capture these vertical scales.
Received: 23 June 1997/Accepted 17 December 1998 相似文献
11.
Easterly wave regimes and associated convection over West Africa and tropical Atlantic: results from the NCEP/NCAR and ECMWF reanalyses 总被引:1,自引:0,他引:1
NCEP/NCAR and ECMWF daily reanalyses are used to investigate the synoptic variability of easterly waves over West Africa
and tropical Atlantic at 700 hPa in northern summer between 1979–1995 (1979–1993 for ECMWF). Spectral analysis of the meridional
wind component at 700 hPa highlighted two main periodicity bands, between 3 and 5 days, and 6 and 9 days. The 3–5-day easterly
wave regime has already been widely investigated, but only on shorter datasets. These waves grow both north and south of the
African Easterly Jet (AEJ). The two main tracks, noted over West Africa at 5 °N and 15 °N, converge over the Atlantic on latitude
17.5 °N. These waves are more active in August–September than in June–July. Their average wavelength/phase speed varies from
about 3000 km/8 m s-1 north of the jet to 5000 km/12 m s-1 south of the jet. Rainfall, convection and monsoon flux are significantly modulated by these waves, convection in the Inter-Tropical
Convergence Zone (ITCZ) being enhanced in the trough and ahead of it, with a wide meridional extension. Compared to the 3–5-day
waves, the 6–9-day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics.
The only main track is located north of the AEJ along 17.5 °N both over West Africa and the Atlantic. The mean wavelength
is higher, about 5000 km long, and the average phase speed is about 7 m s-1. Then the wind field perturbation is mostly evident at the AEJ latitude and north of it. The perturbation structure is similar
to that of 3–5-days in the north except that the more developed circulation centers, moving more to the north, lead to a large
modulation of the jet zonal wind component. South of the AEJ, the wind field perturbation is weaker and quite different. The
zonal wind core of the jet appears to be an almost symmetric axis in the 6–9-day wind field pattern, a clockwise circulation
north of the AEJ being associated with a counter-clockwise circulation south of the jet, and vice versa. These 6–9-day easterly
waves also affect significantly rainfall, convection and monsoon flux but in a different way, inducing large zonal convective
bands in the ITCZ, mostly in the trough and behind it. As opposed to the 3–5-day wave regime, these rainfall anomalies are
associated with anomalies of opposite sign over the Guinea coast and the Sahelian regions. Over the continent, these waves
are more active in June–July, and in August–September over the ocean. GATE phase I gave an example of such an active 6–9-day
wave pattern. Considered as a sequence of weak easterly wave activity, this phase was also a sequence of high 6–9-day easterly
wave activity. We suggest that the 6–9-day regime results from an interaction between the 3–5-day easterly wave regime (maintained
by the barotropic/baroclinic instability of the AEJ), and the development of strong anticyclonic circulations, north of the
jet over West Africa, and both north and south of the jet over the Atlantic, significantly affecting the jet zonal wind component.
The permanent subtropical anticyclones (Azores, Libya, St Helena) could help initiation and maintenance of such regime over
West Africa and tropical Atlantic. Based on an a priori period-band criterion, our synoptic classification has enabled us
to point out two statistical and meteorological easterly wave regimes over West Africa and tropical Atlantic. NCEP/NCAR and
ECMWF reanalyses are in good agreement, the main difference being a more developed easterly wave activity in the NCEP/NCAR
reanalyses, especially for the 3–5-day regime over the Atlantic.
Received: 28 May 1998 / Accepted: 2 May 1999 相似文献
12.
A general circulation model is used to examine the effects of reduced atmospheric CO2, insolation changes and an updated reconstruction of the continental ice sheets at the Last Glacial Maximum (LGM). A set
of experiments is performed to estimate the radiative forcing from each of the boundary conditions. These calculations are
used to estimate a total radiative forcing for the climate of the LGM. The response of the general circulation model to the
forcing from each of the changed boundary conditions is then investigated. About two-thirds of the simulated glacial cooling
is due to the presence of the continental ice sheets. The effect of the cloud feedback is substantially modified where there
are large changes to surface albedo. Finally, the climate sensitivity is estimated based on the global mean LGM radiative
forcing and temperature response, and is compared to the climate sensitivity calculated from equilibrium experiments with
atmospheric CO2 doubled from present day concentration. The calculations here using the model and palaeodata support a climate sensitivity
of about 1 Wm-2 K-1 which is within the conventional range.
Received: 8 February 1997 / Accepted: 4 June 1997 相似文献
13.
Summary We have examined station data from around the world to study the separate effects of the latitude (between 60° N–40° S),
elevation and distance inland, on the annual-mean screen temperature. In the first 200–400 km from some west coasts, screen
temperatures (after adjustment for elevation) rise inland, reaching a maximum called the ‘thermal-ridge temperature’ Tr. The
rise of temperature within this littoral fringe (of width F) depends mainly on the difference between the sea-surface temperature
off the west coast and the zonal mean. Further inland than such a fringe, adjusted temperatures generally decline eastwards,
approximately linearly, at a rate C. The rate is related to hemisphere and latitude.
Empirical relationships between latitude and the observed coastal sea-surface temperature, the near-shore screen temperature,
Tr, C and F for each continent are used to estimate annual mean temperatures on land. Independent estimates of this kind for
48 places, using a look-up table, differ overall by only 0.7 K from the actual long-term average annual mean temperatures.
This is less than half the error resulting from an assumption of zonal-mean temperatures. Basing estimates on coastal sea-surface
temperatures, instead of the look-up table, results in an average error of 1.0 K for the 48 places. The errors are comparable
with the standard deviation of annual mean temperatures during 30 years or so.
Received March 6, 2001 Revised July 30, 2001 相似文献
14.
Summary In this study, we perform experiments with a coupled atmosphere-ocean general circulation model (CGCM) to examine ENSO’s influence
on the interannual sea-surface temperature (SST) variability of the tropical Indian Ocean. The control experiment includes
both the Indian and Pacific Oceans in the ocean model component of the CGCM (the Indo-Pacific Run). The anomaly experiment
excludes ENSO’s influence by including only the Indian Ocean while prescribing monthly-varying climatological SSTs for the
Pacific Ocean (the Indian-Ocean Run). In the Indo-Pacific Run, an oscillatory mode of the Indian Ocean SST variability is
identified by a multi-channel singular spectral analysis (MSSA). The oscillatory mode comprises two patterns that can be identified
with the Indian Ocean Zonal Mode (IOZM) and a basin-wide warming/cooling mode respectively. In the model, the IOZM peaks about
3–5 months after ENSO reaches its maximum intensity. The basin mode peaks 8 months after the IOZM. The timing and associated
SST patterns suggests that the IOZM is related to ENSO, and the basin-wide warming/cooling develops as a result of the decay
of the IOZM spreading SST anomalies from western Indian Ocean to the eastern Indian Ocean. In contrast, in the Indian-Ocean
Run, no oscillatory modes can be identified by the MSSA, even though the Indian Ocean SST variability is characterized by
east–west SST contrast patterns similar to the IOZM. In both control and anomaly runs, IOZM-like SST variability appears to
be associated with forcings from fluctuations of the Indian monsoon. Our modeling results suggest that the oscillatory feature
of the IOZM is primarily forced by ENSO. 相似文献
15.
Kenneth R. Sperber Silvio Gualdi Stephanie Legutke Veronika Gayler 《Climate Dynamics》2005,25(2-3):117-140
The Madden-Julian oscillation (MJO) dominates tropical variability on timescales of 30–70 days. During the boreal winter/spring,
it is manifested as an eastward propagating disturbance, with a strong convective signature over the eastern hemisphere. The
space–time structure of the MJO is analyzed using simulations with the ECHAM4 atmospheric general circulation model run with
observed monthly mean sea-surface temperatures (SSTs), and coupled to three different ocean models. The coherence of the eastward
propagation of MJO convection is sensitive to the ocean model to which ECHAM4 is coupled. For ECHAM4/OPYC and ECHO-G, models
for which ~100 years of daily data is available, Monte Carlo sampling indicates that their metrics of eastward propagation
are different at the 1% significance level. The flux-adjusted coupled simulations, ECHAM4/OPYC and ECHO-G, maintain a more
realistic mean-state, and have a more realistic MJO simulation than the nonadjusted scale interaction experiment (SINTEX)
coupled runs. The SINTEX model exhibits a cold bias in Indian Ocean and tropical West Pacific Ocean sea-surface temperature
of ~0.5°C. This cold bias affects the distribution of time-mean convection over the tropical eastern hemisphere. Furthermore,
the eastward propagation of MJO convection in this model is not as coherent as in the two models that used flux adjustment
or when compared to an integration of ECHAM4 with prescribed observed SST. This result suggests that simulating a realistic
basic state is at least as important as air–sea interaction for organizing the MJO. While all of the coupled models simulate
the warm (cold) SST anomalies that precede (succeed) the MJO convection, the interaction of the components of the net surface
heat flux that lead to these anomalies are different over the Indian Ocean. The ECHAM4/OPYC model in which the atmospheric
model is run at a horizontal resolution of T42, has eastward propagating zonal wind anomalies and latent heat flux anomalies.
However, the integrations with ECHO-G and SINTEX, which used T30 atmospheres, produce westward propagation of the latent heat
flux anomalies, contrary to reanalysis. It is suggested that the differing ability of the models to represent the near-surface
westerlies over the Indian Ocean is related to the different horizontal resolutions of the atmospheric model employed. 相似文献
16.
The predictability of atmospheric responses to global sea surface temperature (SST) anomalies is evaluated using ensemble
simulations of two general circulation models (GCMs): the GENESIS version 1.5 (GEN) and the ECMWF cycle 36 (ECM). The integrations
incorporate observed SST variations but start from different initial land and atmospheric states. Five GEN 1980–1992 and six
ECM 1980–1988 realizations are compared with observations to distinguish predictable SST forced climate signals from internal
variability. To facilitate the study, correlation analysis and significance evaluation techniques are developed on the basis
of time series permutations. It is found that the annual mean global area with realistic signals is variable dependent and
ranges from 3 to 20% in GEN and 6 to 28% in ECM. More than 95% of these signal areas occur between 35 °S–35 °N. Due to the
existence of model biases, robust responses, which are independent of initial condition, are identified over broader areas.
Both GCMs demonstrate that the sensitivity to initial conditions decreases and the predictability of SST forced responses
increases, in order, from 850 hPa zonal wind, outgoing longwave radiation, 200 hPa zonal wind, sea-level pressure to 500 hPa
height. The predictable signals are concentrated in the tropical and subtropical Pacific Ocean and are identified with typical
El Ni?o/ Southern Oscillation phenomena that occur in response to SST and diabatic heating anomalies over the equatorial central
Pacific. ECM is less sensitive to initial conditions and better predicts SST forced climate changes. This results from (1)
a more realistic basic climatology, especially of the upper-level wind circulation, that produces more realistic interactions
between the mean flow, stationary waves and tropical forcing; (2) a more vigorous hydrologic cycle that amplifies the tropical
forcing signals, which can exceed internal variability and be more efficiently transported from the forcing region. Differences
between the models and observations are identified. For GEN during El Ni?o, the convection does not carry energy to a sufficiently
high altitude, while the spread of the tropospheric warming along the equator is slower and the anomaly magnitude smaller
than observed. This impacts model ability to simulate realistic responses over Eurasia and the Indian Ocean. Similar biases
exist in the ECM responses. In addition, the relationships between upper and lower tropospheric wind responses to SST forcing
are not well reproduced by either model. The identification of these model biases leads to the conclusion that improvements
in convective heat and momentum transport parametrizations and basic climate simulations could substantially increase predictive
skill.
Received: 25 April 1996 / Accepted: 9 December 1996 相似文献
17.
Annual precipitation, July and January temperatures were reconstructed from a continuous Holocene pollen sequence from the
Middle Atlas, Morocco, using the best modern analogues method. The reconstructions show a clear difference between the early
and late Holocene: from ∼10 ka to ∼6.5 ka the climate was drier and warmer than during the period since 6.5 ka. The average
value of annual precipitation was ∼870 mm until 6.5 ka, then rose to ∼940 mm. Between 10 ka and 6.5 ka January and July temperatures
were about 4 °C higher than the present. Both temperatures show a marked decrease between 7 ka and 6 ka. After 6.5 ka July
and January temperatures fluctuated between 21 and 23 °C, and 2.5 and 5 °C respectively. January temperatures show a period
of intermediate values (∼3.5 °C) between 4 ka and 5.5 ka. The reconstructed climate values generally match palaeolimnological
data from the same core, which show five intervals of low lake level during the Holocene. They are also consistent with regional-scale
COHMAP simulated palaeoclimate that shows contrasting patterns of rainfall variation between the northwesternmost part of
Africa and the intertropical band.
Received: 7 July 1997 / Accepted: 28 May 1998 相似文献
18.
Results from nine coupled ocean-atmosphere simulations have been used to investigate changes in the relationship between the
variability of monsoon precipitation over western Africa and tropical sea surface temperatures (SSTs) between the mid-Holocene
and the present day. Although the influence of tropical SSTs on the African monsoon is generally overestimated in the control
simulations, the models reproduce aspects of the observed modes of variability. Thus, most models reproduce the observed negative
correlation between western Sahelian precipitation and SST anomalies in the eastern tropical Pacific, and many of them capture
the positive correlation between SST anomalies in the eastern tropical Atlantic and precipitation over the Guinea coastal
region. Although the response of individual model to the change in orbital forcing between 6 ka and present differs somewhat,
eight of the models show that the strength of the teleconnection between SSTs in the eastern tropical Pacific and Sahelian
precipitation is weaker in the mid-Holocene. Some of the models imply that this weakening was associated with a shift towards
longer time periods (from 3–5 years in the control simulations toward 4–10 years in the mid-Holocene simulations). The simulated
reduction in the teleconnection between eastern tropical Pacific SSTs and Sahelian precipitation appears to be primarily related
to a reduction in the atmospheric circulation bridge between the Pacific and West Africa but, depending on the model, other
mechanisms such as increased importance of other modes of tropical ocean variability or increased local recycling of monsoonal
precipitation can also play a role. 相似文献
19.
Climate forcing by carbonaceous and sulfate aerosols 总被引:3,自引:0,他引:3
An atmospheric general circulation model is coupled to an atmospheric chemistry model to calculate the radiative forcing
by anthropogenic sulfate and carbonaceous aerosols. The latter aerosols result from biomass burning as well as fossil fuel
burning. The black carbon associated with carbonaceous aerosols is absorbant and can decrease the amount of reflected radiation
at the top-of-the-atmosphere. In contrast, sulfate aerosols are reflectant and the amount of reflected radiation depends nonlinearly
on the relative humidity. We examine the importance of treating the range of optical properties associated with sulfate aerosol
at high relative humidities and find that the direct forcing by anthropogenic sulfate aerosols can decrease from −0.81 W m-2 to −0.55 Wm-2 if grid box average relative humidity is not allowed to increase above 90%. The climate forcing associated with fossil fuel
emissions of carbonaceous aerosols is calculated to range from +0.16 to +0.20 Wm-2, depending on how much organic carbon is associated with the black carbon from fossil fuel burning. The direct forcing of
carbonaceous aerosols associated with biomass burning is calculated to range from −0.23 to −0.16 Wm-2. The pattern of forcing by carbonaceous aerosols depends on both the surface albedo and the presence of clouds. Multiple
scattering associated with clouds and high surface albedos can change the forcing from negative to positive.
Received: 29 September 1997 / Accepted: 10 June 1998 相似文献
20.
ENSO dynamics and seasonal cycle in the tropical Pacific as simulated by the ECHAM4/OPYC3 coupled general circulation model 总被引:3,自引:0,他引:3
The new version of the atmospheric general circulation model (AGCM), ECHAM4, at the Max Planck Institute for Meteorology,
Hamburg, has been coupled to the OPYC3 isopycnic global ocean general circulation and sea ice model in a multi-century present-day
climate simulation. Non-seasonal constant flux adjustment for heat and freshwater was employed to ensure a long-term annual
mean state close to present-day climatology. This study examines the simulated upper ocean seasonal cycle and interannual
variability in the tropical Pacific for the first 100 years. The coupled model’s seasonal cycle of tropical Pacific SSTs is
satisfactory with respect to both the warm pool variation and the Central and Eastern Pacific, with significant errors only
in the cold tongue around April. The cold phase cold tongue extent and strength is as observed, and for this the heat flux
adjustment does not play a decisive role. A well-established South Pacific convergence zone is characteristic for the new
AGCM version. Apart from extending the southeast trades seasonal maximum to midbasin, wind stress pattern and strength are
captured. Overall the subsurface structure is consistent with the observed, with a pronounced thermocline at about 150 m depth
in the west and rising to the surface from 160 °W to 100 °W. The current system is better resolved than in some previous global
models and, on the whole, has the expected shape. The equatorial undercurrent is correctly positioned but the core is only
half as strong as observed. The north equatorial current and counter-current also have reduced maximum speeds but the April
minimum is captured. As with the companion publication from Roeckner et al. this study finds pronounced tropical Eastern and
Central Pacific interannual variability. Simulated and observed NINO3 sea surface temperature (SST) variability is represented
by a single, rather broadband, maximum of power spectral density, centered on about 28 months for the simulation and four
years for the observations. For simulation and observations, SST, windstress, and upper ocean heat content each exhibit a
single dominant large-scale amplitude and phase pattern, suggesting that the model captures the essential dynamics. The amplitude
of the essentially standing oscillation in SST in the NINO3 region attains the observed strength, but is weaker at the eastern
boundary. Anomalies of upper ocean heat content show off-equatorial westward and equatorial eastward propagation, the latter’s
arrival in the east of the basin coinciding with the SST anomalies. Equatorial wind stress anomalies near the date line provide
the appropriate forcing and clearly form a response to the anomalous SST.
Received: 14 June 1996 / Accepted: 11 November 1997 相似文献