They found that the strength of the global-average summer monsoon precipitation in the forced run exhibits a minimum during the Little Ice Age LIA and a maximum during Medieval Warm Period MWP , which follows the natural variations in the total amount of effective solar radiative forcing. The notable strengthening of the global summer monsoon rainfall in the last 30 years of the forced simulation — is unprecedented and owed in part to the increase of atmospheric CO 2 concentration.
However, the spatial patterns, the hemisphere differences in local summer monsoons and the associated mechanisms were not studied. Here we focus on addressing the following questions: What are the spatial—temporal structures of the global scale local summer monsoon precipitation GSMP change on the timescales longer than a decade over the past millennium?
Can we distinguish the forced response and internal variability of the GSMP? The description of the model physics and performance was given in details by Zorita et al. Here for convenience of the readers, we provide a brief summary. The model configuration has 19 vertical levels in the atmosphere and 20 levels in the ocean, and horizontal resolutions are approximately 3.
The ocean model HOPE-G has a grid refinement in the tropical regions, where the meridional grid point separation reaches 0. To enable the coupled model to sustain a simulated climate near to the real present day climate with minimal drift, both heat and fresh-water fluxes between atmosphere and ocean are modified by adding a constant in time field of adjustment with net-zero spatial average Roeckner et al.
Two millennial integrations with the ECHO-G model were conducted: a year control simulation CTL which was generated using fixed external annually cycling forcing set to the present-day values Zorita et al. The volcanic forcing is parameterized in this simulation as a simple reduction of the annual mean solar constant, starting in the year with a volcanic eruption and usually lasting a couple of years, according to the reconstructions of volcanic aerosol forcing Crowley The external forcing used in ECHO-G forced run: the effective solar radiation forcing insolation plus volcanic aerosol effect; solid line , the concentration of CH 4 long dashed line and CO 2 dotted line.
The initial conditions of the ERIK simulation were taken from year of the control run. Note that the uncertainty in the initial conditions, in turn, could potentially influence the relationship between applied forcing and simulated response in the first — years. This issue has been addressed by Zorita et al. How the model responds to annual variation of the insolation is critical for model evaluation because we examine how the model responds to long-term variability in insolation forcing.
The performance of ECHO-G in modeling the patterns and errors of the climatological annual mean and annual cycle of precipitation has been assessed by Liu et al. The overall agreement adds confidence to our further analysis of the decadal-centennial precipitation variability using the outputs generated by the ERIK run. Despite these successes, the simulated annual mean precipitation has notable biases in East Asia monsoon region, subtropical South Pacific and South Atlantic convergence zones, and the Mexican-North American monsoon regions Liu et al.
These biases are also common in most of coupled models in simulating annual mean precipitation Lee et al. Evidently, the simulated monsoon precipitation domains are reasonably realistic.
Major deficiency is the missing of the East Asian subtropical monsoon. Other minor differences are mainly due to different model 3.
Generally speaking, the model simulations on global scale are adequate for our study of the long-term modulations of the global monsoon under imposed natural and anthropogenic forcing.
For convenience, in the following analysis we will use CMAP monsoon domain. The monsoon domain is the regions where the annual range local summer mean minus winter mean of precipitation rate exceeds 2. We used linear regression to estimate trends for given periods. Therefore test of the significance between two trends becomes a significance test of the differences between two regression line slopes, or regression coefficients.
Denote the two slopes by b 1 and b 2. Where S x 1 and S x 2 denote the sums of squares for x in the first and second group, respectively, and. Can we distinguish forced and internal variability of the GSMP? The separation is possible if the forced response and internal variability have distinct spatial patterns and temporal behaviors.
For this reason, we examine the principal modes of the GSMP variability by empirical orthogonal function EOF analysis of the decadal mean precipitation in monsoon domains over the past millennium.
The first and second modes accounts for 9. The spatial structure is characterized by a nearly uniform increase or decrease of monsoon precipitation across all regional monsoon regions, which is in association with a global SST warming or cooling. The global SST pattern features obviously warming or cooling in the mid-latitude oceans.
But the warming or cooling signal is relative weak in the region from the southeastern to equatorial western-central Pacific. The sub-millennial variation and the bicentennial peak are primarily a response to the effective solar forcing because the SV forcing shows a similar sub-millennial variation and the spectrum of the SV forcing has a similar bicentennial peak Liu et al.
Of note is that the increase of GSMP during the twentieth century is unprecedented, resulting in a maximum that is significantly stronger than that during the MWP. Since the effective solar forcing in the MWP is about the same as in the twentieth century, the excessive warming in the twentieth century reflects the impact of the GHG forcing. The fraction on the top of panel a denotes the fractional variance.
The correlation matrix was used for EOF analysis. The second mode does not show a cohesive pattern of variability Fig. The NH monsoon rainfall tends to be in phase while the SH monsoon rainfall tends to be out of phase between the eastern and western hemisphere.
The corresponding principal component PC2 has triple spectral peaks on 2—7 decades Fig. This concentration of SST variability in the equatorial central Pacific is quite different from the SST pattern associated with the EOF1 where midlatitude warms more than the equatorial regions. What is the origin of the EOF 2 mode? The same as in Fig. To address this question, we examine the control CTL run in which the external forcing is fixed. Figure 5 shows the leading EOF mode derived from the control experiment.
Because the forcing is fixed in the CTL run, the leading mode must arise from internal feedback processes within the coupled climate system. We note that the spatial structure of the EOF 2 mode in the forced run Fig. The regressed SST patterns are also very similar figure not shown. In general, the power spectra are also similar. Revista: Climate Dynamics.
Here, we assess the stationarity of SAM spatial correlations with instrumental and paleoclimate proxy data for the past millennium.
The instrumental period shows that temporal non-stationarities in SAM teleconnections are not consistent across the SH land areas. This suggests that the influence of the SAM index is modulated by regional effects.
However, within key-regions with good proxy data coverage South America, Tasmania, New Zealand , teleconnections are mostly stationary over the instrumental period. Using different stationarity criteria for proxy record selection, we provide new austral summer and annual mean SAM index reconstructions over the last millennium. Our summer SAM reconstructions are very robust to changes in proxy record selection and the selection of the calibration period, particularly on the multi-decadal timescale.
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Citation Type. Has PDF. Publication Type. More Filters. The evolution and volcanic forcing of the southern annular mode during the past years. A positive change in the southern annular mode SAM , which is the primary pattern of climate variability in the Southern Hemisphere, has been induced predominantly by polar stratospheric ozone … Expand.
Reconstructions of the southern annular mode SAM during the last millennium. The leading mode of atmospheric variability in the Southern Hemisphere is the Southern Annular Mode SAM , which affects the atmosphere and ocean from the mid-latitudes to the Antarctic.
However, the … Expand. View 2 excerpts, cites background. SAM variability shows strong seasonal asymmetries in the … Expand.
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