共查询到20条相似文献,搜索用时 31 毫秒
1.
F. Kucharski A. A. Scaife J. H. Yoo C. K. Folland J. Kinter J. Knight D. Fereday A. M. Fischer E. K. Jin J. Kröger N.-C. Lau T. Nakaegawa M. J. Nath P. Pegion E. Rozanov S. Schubert P. V. Sporyshev J. Syktus A. Voldoire J. H. Yoon N. Zeng T. Zhou 《Climate Dynamics》2009,33(5):615-627
The ability of atmospheric general circulation models (AGCMs), that are forced with observed sea surface temperatures (SSTs),
to simulate the Indian monsoon rainfall (IMR) variability on interannual to decadal timescales is analyzed in a multimodel
intercomparison. The multimodel ensemble has been performed within the CLIVAR International “Climate of the 20th Century”
(C20C) Project. This paper is part of a C20C intercomparison of key climate time series. Whereas on the interannual timescale
there is modest skill in reproducing the observed IMR variability, on decadal timescale the skill is much larger. It is shown
that the decadal IMR variability is largely forced, most likely by tropical sea surface temperatures (SSTs), but as well by
extratropical and especially Atlantic Multidecadal Oscillation (AMO) related SSTs. In particular there has been a decrease
from the late 1950s to the 1990s that corresponds to a general warming of tropical SSTs. Using a selection of control integrations
from the World Climate Research Programme’s (WCRP’s) Coupled Model Intercomparison Project phase 3 (CMIP3), it is shown that
the increase of greenhouse gases (GHG) in the twentieth century has not significantly contributed to the observed decadal
IMR variability. 相似文献
2.
G. A. Meehl P. R. Gent J. M. Arblaster B. L. Otto-Bliesner E. C. Brady A. Craig 《Climate Dynamics》2001,17(7):515-526
Historically, El Nino-like events simulated in global coupled climate models have had reduced amplitude compared to observations.
Here, El Nino-like phenomena are compared in ten sensitivity experiments using two recent global coupled models. These models
have various combinations of horizontal and vertical ocean resolution, ocean physics, and atmospheric model resolution. It
is demonstrated that the lower the value of the ocean background vertical diffusivity, the greater the amplitude of El Nino
variability which is related primarily to a sharper equatorial thermocline. Among models with low background vertical diffusivity,
stronger equatorial zonal wind stress is associated with relatively higher amplitude El Nino variability along with more realistic
east–west sea surface temperature (SST) gradient along the equator. The SST seasonal cycle in the eastern tropical Pacific
has too much of a semiannual component with a double intertropical convergence zone (ITCZ) in all experiments, and thus does
not affect, nor is it affected by, the amplitude of El Nino variability. Systematic errors affecting the spatial variability
of El Nino in the experiments are characterized by the eastern equatorial Pacific cold tongue regime extending too far westward
into the warm pool. The time scales of interannual variability (as represented by time series of Nino3 SSTs) show significant
power in the 3–4 year ENSO band and 2–2.5 year tropospheric biennial oscillation (TBO) band in the model experiments. The
TBO periods in the models agree well with the observations, while the ENSO periods are near the short end of the range of
3–6 years observed during the period 1950–94. The close association between interannual variability of equatorial eastern
Pacific SSTs and large-scale SST patterns is represented by significant correlations between Nino3 time series and the PC
time series of the first EOFs of near-global SSTs in the models and observations.
Received: 17 April 2000 / Accepted: 17 August 2000 相似文献
3.
Some features associated with Eastern China Precipitation (ECP), in terms of mean climatology, sea-sonal cycle, interannual
variability are studied based on monthly rainfall data. The rainfall behavior over Eastern China has fine spatial structure
in the seasonal variation and interannual variability. The revealed characteristics of ECP motivate us dividing Eastern China
into four sub—regions to quantify significant lag—correlations of the rainfalls with global sea surface temperatures (SSTs)
and to study the ocean’s pre-dominant role in forcing the eastern China summer monsoon rainfalls. Lagged correlations between
the mid—eastern China summer monsoon rainfalls (MECSMRs) and the global SSTs, with SST leading to rain-fall, are investigated.
The most important key SST regions and leading times, in which SSTs are highly corre-lated with the MECSMRs, are selected.
Part of the results confirms previous studies that show links between the MECSMRs and SSTs in the eastern equatorial Pacific
associated with the El Nino — Southern Oscillation (ENSO) phenomenon. Other findings include the high lag correlations between
the MECSMRs and the SSTs in the high and middle latitude Pacific Ocean and the Indian Ocean, even the SSTs over the Atlantic
Ocean, with SST leading—time up to 4 years. Based on the selected SST regions, regression equa-tions are developed by using
the SSTs in these regions in respective leading time. The correlation coefficient between the observed rainfalls and regressed
rainfalls is over 0.85. The root mean square error (RMSE) for regressed rainfall is around 65% of the standard deviation and
about 15% of the mean rainfall. The regression equation has also been evaluated in a forecasting mode by using independent
data. Discussion on the consistence of the SST—rainfall correlation with circulation field is also presented.
This work was jointed supported by Chinese Academy of Sciences under Grant “Hundred Talents” for “Validation of Coupled Climate
models” and by U.S. Department of Energy under Grant DEFG0285ER 60314 to SUNY at Stony Brook. The authors are grateful to
Professor R. D. Cess at SUNY, Stony Brook for his supports. 相似文献
4.
Understanding natural atmospheric decadal variability is an important element of climate research, and here we investigate
the geographic and seasonal diversity in the balance between its competing sources. Data are provided by an ensemble of multi-decadal
atmospheric general circulation model experiments, forced by observed sea surface temperatures (SSTs), and verified against
observations. First, the nature of internal atmospheric variability is studied. By assessing its spectral character, we refute
the idea that internal modes may persist or oscillate on multi-annual time-scales, either through mechanisms purely internal
to the atmosphere, or via coupling to the land surface; instead, they behave as a white noise process. Second, and more importantly,
the role of oceanic forcing, relative to internal variability, is investigated by extending the ‘analysis of variance’ technique
to the frequency domain. Significance testing and confidence intervals are also discussed. In the tropics, atmospheric decadal
variability is usually dominated by oceanic forcing, although for some regions less so than at interannual time-scales. A
moderate oceanic impact is also found for some extratropical regions in some seasons. Verification against observed mean sea-level
pressure (MSLP) data suggests that many of these influences are realistic, although some model errors are also revealed. In
other mid- and high-latitude regions, local simulated decadal variability is dominated by random processes, i.e. the integrated
effects of chaotic weather systems. Third, we focus on the mechanisms of decadal variability in two specific regions (where
the model is well behaved). Over the tropical Pacific, the relative impact of SSTs on decadal MSLP is strongly seasonal such
that it peaks in September to November (SON). This is explained by noting that the model atmosphere is responsive to SSTs
a little farther west in SON than it is in other seasons, and here it picks up relatively more decadal power from the ocean
(the western Pacific being less dominated by ENSO time-scales), causing atmospheric ‘signal-to-noise ratios’ to be enhanced
at decadal timescales in SON. Over southern North America, a strong SST impact is found in summer and autumn, resulting in
an upward trend of MSLP over recent decades. We suggest this is caused by decadal SST variability in the Caribbean (and to
some extent the tropical northeast Pacific in summer), which induces anomalous convective heating over these regions and hence
the wider MSLP response.
Received: 30 November 1998 / Accepted: 22 April 1999 相似文献
5.
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
6.
Results are first presented from an analysis of a global coupled climate model regarding changes in future mean and variability of south Asian monsoon precipitation due to increased atmospheric CO2 for doubled (2 × CO2) and quadrupled (4 × CO2) present-day amounts. Results from the coupled model show that, in agreement with previous studies, mean area-averaged south Asian monsoon precipitation increases with greater CO2 concentrations, as does the interannual variability. Mechanisms producing these changes are then examined in a series of AMIP2-style sensitivity experiments using the atmospheric model (taken from the coupled model) run with specified SSTs. Three sets of ensemble experiments are run with SST anomalies superimposed on the AMIP2 SSTs from 1979–97: (1) anomalously warm Indian Ocean SSTs, (2) anomalously warm Pacific Ocean SSTs, and (3) anomalously warm Indian and Pacific Ocean SSTs. Results from these experiments show that the greater mean monsoon precipitation is due to increased moisture source from the warmer Indian Ocean. Increased south Asian monsoon interannual variability is primarily due to warmer Pacific Ocean SSTs with enhanced evaporation variability, with the warmer Indian Ocean SSTs a contributing but secondary factor. That is, for a given interannual tropical Pacific SST fluctuation with warmer mean SSTs in the future climate, there is enhanced evaporation and precipitation variability that is communicated via the Walker Circulation in the atmosphere to the south Asian monsoon to increase interannual precipitation variability there. This enhanced monsoon variability occurs even with no change in interannual SST variability in the tropical Pacific. 相似文献
7.
This paper uses recent gridded climatological data and a coupled general circulation model (GCM) simulation in order to assess the relationships between the interannual variability of the Indian summer monsoon (ISM) and the El Niño-Southern Oscillation (ENSO). The focus is on the dynamics of the ISM-ENSO relationships and the ability of the state-of-the-art coupled GCM to reproduce the complex lead-lag relationships between the ISM and the ENSO. The coupled GCM is successful in reproducing the ISM circulation and rainfall climatology in the Indian areas even though the entire ISM circulation is weaker relative to that observed. In both observations and in the simulation, the ISM rainfall anomalies are significantly associated with fluctuations of the Hadley circulation and the 200 hPa zonal wind anomalies over the Indian Ocean. A quasi-biennial time scale is found to structure the ISM dynamical and rainfall indices in both cases. Moreover, ISM indices have a similar interannual variability in the simulation and observations. The coupled model is less successful in simulating the annual cycle in the tropical Pacific. A major model bias is the eastward displacement of the western North Pacific inter-tropical convergence zone (ITCZ), near the dateline, during northern summer. This introduces a strong semiannual component in Pacific Walker circulation indices and central equatorial Pacific sea surface temperatures. Another weakness of the coupled model is a less-than-adequate simulation of the Southern Oscillation due to an erroneous eastward extension of the Southern Pacific convergence zone (SPCZ) year round. Despite these problems, the coupled model captures some aspects of the interannual variability in the tropical Pacific. ENSO events are phase-locked with the annual cycle as observed, but are of reduced amplitude relative to the observations. Wavelet analysis of the model Niño34 time series shows enhanced power in the 2–4 year band, as compared to the 2–8 year range for observations during the 1950–2000 period. The ISM circulation is weakened during ENSO years in both the simulation and the observations. However, the model fails to reproduce the lead-lag relationship between the ISM and Niño34 sea surface temperatures (SSTs). Furthermore, lag correlations show that the delayed response of the wind stress over the central Pacific to ISM variability is insignificant in the simulation. These features are mainly due to the unrealistic interannual variability simulated by the model in the western North Pacific. The amplitude and even the sign of the simulated surface and upper level wind anomalies in these areas are not consistent with observed patterns during weak/strong ISM years. The ISM and western North Pacific ITCZ fluctuate independently in the observations, while they are negatively and significantly correlated in the simulation. This isolates the Pacific Walker circulation from the ISM forcing. These systematic errors may also contribute to the reduced amplitude of ENSO variability in the coupled simulation. Most of the unrealistic features in simulating the Indo-Pacific interannual variability may be traced back to systematic errors in the base state of the coupled model. 相似文献
8.
M. Pontaud J.-P. Céron M. Kimoto F. Pluviaud L. Terray A. Vintzileos 《Climate Dynamics》2000,16(12):917-933
The interannual variability over the tropical Pacific and a possible link with the mean state or the seasonal cycle is examined
in four coupled ocean-atmosphere general circulation models (GCM). Each model is composed of a high-resolution ocean GCM of
either the tropical Pacific or near-global oceans coupled to a moderate-resolution atmospheric GCM, without using flux correction.
The oceanic subsurface is considered to describe the mean state or the seasonal cycle through the analytical formulations
of some potential coupled processes. These coupled processes characterise the zonal gradient of sea surface temperature (hereafter
SST), the oceanic vertical gradient of temperature and the equatorial upwelling. The simulated SST patterns of the mean state
and the interannual signals are generally too narrow. The grid of the oceanic model could control the structure of the SST
interannual signals while the behaviour of the atmospheric model could be important in the link between the oceanic surface
and the subsurface. The first SST EOFs are different between the coupled models, however, the second SST EOFs are quite similar
and could correspond to the return to the normal state while that of the observations (COADS) could favour the initial anomaly.
All the models seem to simulate a similar equatorial wave-like dynamics to return to the normal state. The more the basic
state is unstable from the coupled processes point of view, the more the interannual signal are high. It seems that the basic
state could control the intensity of the interannual variability. Two models, which have a significant seasonal variation
of the interannual variance, also have a significant seasonal variation of the instability with a few months lag. The potential
seasonal phase locking of the interannual fluctuations need to be examined in more models to confirm its existence in current
tropical GCMs.
Received: 30 July 1999 / Accepted: 25 April 2000 相似文献
9.
Analyses of proxy based reconstructions of surface temperatures during the past 330 years show the existence of a distinct
oscillatory mode of variability with an approximate time scale of 70 years. This variability is also seen in instrumental
records, although the oscillatory nature of the variability is difficult to assess due to the short length of the instrumental
record. The spatial pattern of this variability is hemispheric or perhaps even global in scale, but with particular emphasis
on the Atlantic region. Independent analyses of multicentury integrations of two versions of the GFDL coupled atmosphere-ocean
model also show the existence of distinct multidecadal variability in the North Atlantic region which resembles the observed
pattern. The model variability involves fluctuations in the intensity of the thermohaline circulation in the North Atlantic.
It is our intent here to provide a direct comparison of the observed variability to that simulated in a coupled ocean-atmosphere
model, making use of both existing instrumental analyses and newly available proxy based multi-century surface temperature
estimates. The analyses demonstrate a substantial agreement between the simulated and observed patterns of multidecadal variability
in sea surface temperature (SST) over the North Atlantic. There is much less agreement between the model and observations
for sea level pressure. Seasonal analyses of the variability demonstrate that for both the model and observations SST appears
to be the primary carrier of the multidecadal signal.
Received: 8 June 1999 / Accepted: 11 February 2000 相似文献
10.
—Upper ocean thermal data and surface marine observations are used to describe the three-dimensional, basinwide co-evolution
of interannual variability in the tropical Pacific climate system. The phase propagation behavior differs greatly from atmosphere
to ocean, and from equatorial to off-equatorial and from sea surface to subsurface depths in the ocean. Variations in surface
zonal winds and sea surface temperatures (SSTs) exhibit a standing pattern without obvious zonal phase propagation. A nonequilibrium
ocean response at subsurface depths is evident, characterized by coherent zonal and meridional propagating anomalies around
the tropical North Pacific: eastward on the equator but westward off the equator. Depending on geographic location, there
are clear phase relations among various anomaly fields. Surface zonal winds and SSTs in the equatorial region fluctuate approximately
in-phase in time, but have phase differences in space. Along the equator, zonal mean thermocline depth (or heat content) anomalies
are in nonequilibrium with the zonal wind stress forcing. Variations in SSTs are not in equilibrium either with subsurface
thermocline changes in the central and western equatorial Pacific, with the former lagging the latter and displaced to the
east. Due to its phase relations to SST and winds, the basinwide temperature anomaly evolution at thermocline depths on an
interannual time scale may determine the slow physics of ENSO, and play a central role in initiating and terminating coupled
air-sea interaction. This observed basinwide phase propagation of subsurface anomaly patterns can be understood partially
as water discharge processes from the western Pacific to the east and further to high latitudes, and partially by the modified
delayed oscillator physics.
Received: 17 January 1997 / Accepted: 10 March 1998 相似文献
11.
A Coupling Experiment of an Atmosphere and an Ocean Model with a Monthly Anomaly Exchange Scheme 总被引:5,自引:0,他引:5
A nine-layer spectral atmospheric general circulation model is coupled to a twenty-layer global oceanic general circulation model with the “prediction-correction” monthly anomaly exchange scheme which has been proposed at the Institute of Atmospheric Physics (IAP). A forty-year integration of the coupled model shows that the CGCM is fairly successful in keeping a reasonable pattern of the modelled SST although most of the Pacific become warmer than those given by the uncoupled ocean model. The model tends to reach a more realistic state than the uncoupled one in terms of downward surface heat flux into ocean particularly in the equatorial Pacific region. Also, the model is capable to simulate interannual variability of sea surface temperature in tropical region. 相似文献
12.
Atmospheric general circulation models (AGCMs) are often “coupled” with time varying observations of boundary conditions
or some other aspect of the climate system. A typical example is the Atmospheric Model Intercomparison Project (AMIP) experimental
protocol, which required the specification of sea surface temperature and sea-ice extent from observed monthly means. AGCMs
ordinarily incorporate the prescribed conditions by evaluating an interpolating function at each time step. Typical schemes,
such as that used in the second generation GCM (GCM2) of the Canadian Centre for Climate Modelling and Analysis (CCC), do
not preserve monthly means and have a smoothing effect on the interpolated time series which tends to reduce the amplitude
of annual cycle and interannual variability of sea surface temperature (SST). By solving a large set of linear equations,
a simple linear time-interpolation scheme that preserves the observed monthly mean SST and hence its variability can be obtained.
The new scheme improves upon that used previously in CCC GCM2 by eliminating the substantial loss of interannual variability
(up to 20%) and the small attenuation of the annual cycle (less than 4% on average) incurred with the old scheme. The improved
linear interpolation scheme is easily adapted to other quantities.
Received: 4 August 1997 / Accepted: 26 November 1997 相似文献
13.
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
14.
A nine-member ensemble of simulations with a state-of-the-art atmospheric model forced only by the observed record of sea
surface temperature (SST) over 1930–2000 is shown to capture the dominant patterns of variability of boreal summer African
rainfall. One pattern represents variability along the Gulf of Guinea, between the equator and 10°N. It connects rainfall
over Africa to the Atlantic marine Intertropical Convergence Zone, is controlled by local, i.e., eastern equatorial Atlantic,
SSTs, and is interannual in time scale. The other represents variability in the semi-arid Sahel, between 10°N and 20°N. It
is a continental pattern, capturing the essence of the African summer monsoon, while at the same time displaying high sensitivity
to SSTs in the global tropics. A land–atmosphere feedback associated with this pattern translates precipitation anomalies
into coherent surface temperature and evaporation anomalies, as highlighted by a simulation where soil moisture is held fixed
to climatology. As a consequence of such feedback, it is shown that the recent positive trend in surface temperature is consistent
with the ocean-forced negative trend in precipitation, without the need to invoke the direct effect of the observed increase
in anthropogenic greenhouse gases. We advance plausible mechanisms by which the balance between land–ocean temperature contrast
and moisture availability that defines the monsoon could have been altered in recent decades, resulting in persistent drought.
This discussion also serves to illustrate ways in which the monsoon may be perturbed, or may already have been perturbed,
by anthropogenic climate change. 相似文献
15.
The intraseasonal oscillation in ECHAM4 Part I: coupled to a comprehensive ocean model 总被引:1,自引:0,他引:1
This study discusses the representation of the intraseasonal oscillation (ISO) in three simulations with the ECHAM4 atmosphere general circulation model (GCM). First, the model is forced by AMIP sea surface temperatures (SST), then coupled to the OPYC3 global ocean GCM and third forced by OPYC3 SSTs to clarify possible air-sea interactions and connections of the ISO and the ENSO cycle. The simulations are compared to ECMWF reanalysis data and NOAA outgoing longwave radiation (OLR) observations. Although previous studies have shown that the ECHAM4 GCM simulates an ISO-like oscillation, the main deficits are an overly fast eastward propagation and an eastward displacement of the main ISO activity, which is shown with a composite analysis of daily data between 1984 to 1988 for the reanalysis and the AMIP simulation, 25 years of the coupled integration, and a five year subset of the coupled SST output used for the OPYC3 forced atmosphere GCM experiment. These deficits are common to many atmospheric GCMs. The composites are obtained by principal oscillation pattern (POP). The POPs are also used to investigate the propagation speed and the interannual variability of the main ISO activity. The present coupled model version reveals no clear improvements in the ISO simulation compared to the uncoupled version forced with OPYC3 SSTs, although it is shown that the modeled ISO influences the simulated high-frequency SST variability in the coupled GCM. Within the current analysis, ECHAM4 forced by AMIP SSTs provides the most reasonable ISO simulation. However, it is shown that the maximum amplitudes of the annual cycle of the ISO variability in all analyzed model versions are reached too late in the year (spring and summer) compared to the observations (winter and spring). Additionally, the ENSO cycle influences the interannual variability of the ISO, which is revealed by 20 years of daily reanalysis data and 100 years of the coupled integration. The ENSO cycle is simulated by the coupled model, although there is a roughly 1 K cold bias in the East Pacific in the coupled model. This leads to a diminished influence of the ENSO cycle on the spatial variability of the modeled ISO activity compared to observations. This points out the strong sensitivity of the SST on the ISO activity. Small biases in the SST appear to cause large deterioration in the modeled ISO. 相似文献
16.
A new winter Aleutian Low (AL) intensity index was defined in this paper. A centurial-long time series of this index was constructed
using the sea level pressure (SLP) data of nearly 100 years. The features of interannual and decadal variability of the winter
AL intensity since 1900 were analyzed by applying the wavelet analysis. The relationship between the winter AL intensity and
atmospheric circulation was examined. The cross-wavelet analysis technique was used to further reveal the relationship between
the AL intensity and sea surface temperature (SST) in the equatorial eastern Pacific (EEP) and tropical Indian Ocean (TIO)
in winter. The results indicate that: 1) On the interannual timescale, the winter AL intensity displays 3–7-yr oscillations,
while on the decadal timescale, 8–10-yr and 16–22-yr oscillations are more obvious. 2) Of the linkage to atmospheric circulation,
both AO (Arctic Oscillation) and PNA (Pacific North America pattern) are closely associated with winter AL intensity on the
interannual timescale, but only PNA contributes to the variation of winter AL intensity on the decadal timescale. 3) As to
the ocean impact, winter EEP SST is a major factor affecting the winter AL intensity on the interannual timescale, especially
on the 3–7-yr periods. However, on the decadal timescale, though both the TIO and EEP SSTs are associated with the AL intensity
in winter, the TIO SST impact is more significant. 相似文献
17.
Boreal winter North Atlantic climate change since 1950 is well described by a trend in the leading spatial structure of variability, known as the North Atlantic Oscillation (NAO). Through diagnoses of ensembles of atmospheric general circulation model (AGCM) experiments, we demonstrate that this climate change is a response to the temporal history of sea surface temperatures (SSTs). Specifically, 58 of 67 multi-model ensemble members (87%), forced with observed global SSTs since 1950, simulate a positive trend in a winter index of the NAO, and the spatial pattern of the multi-model ensemble mean trend agrees with that observed. An ensemble of AGCM simulations with only tropical SST forcing further suggests that variations in these SSTs are of primary importance. The probability distribution function (PDF) of 50-year NAO index trends from the forced simulations are, moreover, appreciably different from the PDF of a control simulation with no interannual SST variability, although chaotic atmospheric variations are shown to yield substantial 50-year trends. Our results thus advance the view that the observed linear trend in the winter NAO index is a combination of a strong tropically forced signal and an appreciable noise component of the same phase. The changes in tropical rainfall of greatest relevance include increased rainfall over the equatorial Indian Ocean, a change that has likely occurred in nature and is physically consistent with the observed, significant warming trend of the underlying sea surface. 相似文献
18.
A series of recent papers showed that sea surface temperature (SST) anomalies in the south equatorial tropical Atlantic modulate the interannual variability of the African and Indian monsoon rainfall. Physically this teleconnection can be explained by a simple Gill-Matsuno mechanism. In this work, the output from five different models chosen within the CMIP3 (Coupled Model Intercomparison Project version 3) ensemble of coupled general circulation models (CGCMs) are analyzed to investigate how state-of-the-art CGCMs represent the impact of the South Tropical Atlantic (STA) SSTs on the Indian and African region. Using a correlation-regression technique, it is found that four out of the five models display a teleconnection between STA and Indian region which is generally weaker than in the observations but in agreement in the rainfall field pattern. This teleconnection is also noticeable in the ensemble mean of the five models. Over Africa, however, the significant changes in rainfall displayed in the observation are properly caught by only one of the CGCMs. Additionally, none of the models reproduces the symmetric upper-level wind response around the Equator seen over the Indian Ocean in the observations and all have significant biases also in the surface pressure field response to the tropical Atlantic SSTs. Nonetheless the STA response, particularly over the southern hemisphere, is indicative of the Gill-Matsuno-type mechanism identified in previous studies using idealized experiments with atmospheric GCMs and observational data. With a suite of atmospheric-only GCM integrations it is shown that the differences in amplitude and pattern are not only due to the strong biases and reduced variabilities of the CGCMs over the tropical Atlantic but they are also caused by the different physical parameterizations used in models. 相似文献
19.
Arthur Prigent Joke F. Lbbecke Tobias Bayr Mojib Latif Christian Wengel 《Climate Dynamics》2020,54(5):2731-2744
A prominent weakening in equatorial Atlantic sea surface temperature (SST) variability, occurring around the year 2000, is investigated by means of observations, reanalysis products and the linear recharge oscillator (ReOsc) model. Compared to the time period 1982–1999, during 2000–2017 the May–June–July SST variability in the eastern equatorial Atlantic has decreased by more than 30%. Coupled air–sea feedbacks, namely the positive Bjerknes feedback and the negative net heat flux damping are important drivers for the equatorial Atlantic interannual SST variability. We find that the Bjerknes feedback weakened after 2000 while the net heat flux damping increased. The weakening of the Bjerknes feedback does not appear to be fully explainable by changes in the mean state of the tropical Atlantic. The increased net heat flux damping is related to an enhanced response of the latent heat flux to the SST anomalies (SSTa). Strengthened trade winds as well as warmer SSTs are suggested to increase the air–sea specific humidity difference and hence, enhancing the latent heat flux response to SSTa. A combined effect of those two processes is proposed to be responsible for the weakened SST variability in the eastern equatorial Atlantic. The ReOsc model supports the link between reduced SST variability, weaker Bjerknes feedback and stronger net heat flux damping. 相似文献
20.
F. Chen 《Climate Dynamics》2005,24(7-8):667-684
The International Satellite Land-Surface Climatology Project (ISLSCP) Initiative-I 1-degree 1987–1988 data were used to drive
a land surface model (LSM) to simulate global surface energy budgets. Simulated surface heat fluxes show remarkable spatial
variability and seem to capture well their annual and interannual variability. A shift of maximum evaporation across the equator
is more closely related to the seasonal shifting of precipitation pattern than to surface radiation changes. The NCEP/NCAR
reanalysis did not reflect this shift, presumably due to its dominant rainfall maximum in the Southern Hemisphere. To assess
the “reliability” of these fields, both Global Soil Wetness Project (GSWP) and reanalysis were verified against observations,
at two sites. Monthly mean ISLSCP forcing conditions agree fairly well with observations, but its precipitation is usually
lower during spring and summer. Low summer GSWP evaporation may be due to low precipitation and incorrect specification of
vegetation and soil conditions. The reanalysis had larger seasonal variability than GSWP and observations, and overestimated
summer heat fluxes because of its large rainfall and surface radiation. Despite uncertainty in ISLSCP data, an LSM with a
modest treatment of vegetation was able to capture reasonably well the seasonal variations in surface heat fluxes at global
scales. With some caution, these types of simulations can be used as “pseudo-observations” to evaluate climate-model simulations
and to investigate global energy budgets. For the next phase of ISLSCP data development, higher resolution data, which can
reflect local heterogeneity of vegetation and soil characteristics, include more rain gauge data are highly desirable to improve
model simulations. 相似文献