The most significant decadal oscillation of the climate system is the sea surface temperature change in the North Atlantic, namely the Atlantic Decadal Oscillation (AMO)(Kerr, 2000). AMO cycle is very long, about 50~70 years. Whether this phenomenon is an internal change of the earth's climate system or just a response to external forces has always been an open question.
According to the meteorological data observed by the instrument, it is considered that AMO comes from the internal variability of the earth's climate system. The study of climate reconstruction based on tree rings and ice cores also shows that there is interdecadal variability in the average climate of the North Atlantic and the whole world in recent thousand years, but it is difficult to find the real attribution only through observation and reconstruction data. The attribution research based on model simulation is inconclusive. Some simulation studies show that AMO may be the response of sea surface temperature to atmospheric random disturbance, the external expression of Atlantic meridional circulation, or the response of external radiation forcing.
Recently, Michael. Mann of Pennsylvania State University and his collaborators published a paper in Science (Mannet al., 202 1), suggesting that the interdecadal temperature oscillation in the past thousand years was caused by volcanic activity, and there was no interdecadal oscillation in the climate system itself.
The author thinks that the biggest difference between internal variability and external forcing is that internal variability mainly affects the spatial redistribution of global energy, while external forcing affects the total income and expenditure of global energy, so the analysis of internal variability should start from the space of global climate anomalies.
In this paper, the multi-cone singular value decomposition (MTM-SVD) is used to analyze the variable field in frequency domain, which avoids the influence of the differences between modes on signal extraction, and thus obtains the fractional local variance (LFV) of the variable field. The advantage of this method is that the spatial characteristics (modes) of narrow signals with specific frequency bandwidth can be obtained. Using this method, the author analyzes the global temperature field of all model reference tests and simulation tests in the last thousand years that participated in the fifth stage of the fifth international coupled model comparison project (CMIP5 for short).
The results show that, unlike the obvious interannual signal of El Ni? o-Southern Oscillation, there is no obvious interdecadal to interdecadal component in the simulated global temperature without external forcing reference test (Figure 1A), but there are several obvious interdecadal to interdecadal oscillation signals in the result of external forcing, including the component with a period of 50-70 years (Figure 1B). Compared with the multi-modal ensemble average global average temperature power spectrum (figure 1C), it is found that the interdecadal oscillation signal of the simulation experiment in recent thousand years is consistent with the interdecadal variability of the global average temperature, that is, the interdecadal climate variability mode existing in the simulation experiment in recent thousand years is most likely a response to external forcing.
Figure 1 CMIP5 mode temperature data spectrum analysis results; (A)CMIP5 is the LFV spectrum of the test temperature field, and the dashed line indicates the confidence calculated by Monte Carlo method, which are respectively p=0.5, 0. 1, 0.05, 0.01from bottom to top; Each colored line represents a single member (N=44), and the thick black line is the average value; (b) Similar to A, analyze the simulation test of CMIP5 in recent thousand years (n =16); (c) The power spectrum of global average temperature of C)CMIP 5 in recent thousand years (N= 16), and the blue vertical dashed line indicates the spectral peaks of b and C*** (Mann et al., 202 1).
The author further analyzes the influence of the two most important external forcing factors (solar radiation and volcanic activity) on the global temperature in the past 1000 years by using the energy balance model (a simple climate model that calculates the temperature and its distribution according to the energy balance without considering the internal variability). Taking multiple volcanic activities and solar radiation reconstruction sequences as external forced combination or single driving model, it is found that only volcanic activities can lead to the interdecadal variation of global average temperature. It shows that volcanic activity is the main reason for the interdecadal oscillation of global average temperature in recent thousand years.
In addition, the author used the same method to analyze the observed data and the CMIP5 historical climate simulation experiment (Mannet et al., 2020). The results show that the global temperature results of the fully forced historical climate simulation are consistent with the observed data, and both of them have the characteristics of interdecadal variation with a period of about 50 years (Figure 2). However, in the historical climate simulation results with only artificial forcing, although there are decades of variability modes, their periods are different, about 60 years (Figure 3). Comparing the observed data with the results of several models, it is found that the phase of this interdecadal oscillation is also consistent in observation and different models, which obviously does not conform to the random characteristics of intrinsic variability. Considering that there is no interdecadal oscillation in the reference test, it shows that this interdecadal oscillation is a response to external forcing (artificial forcing and natural forcing).
Fig. 2 LFV spectrum analysis of CMIP 5 historical climate simulation test (natural forcing+artificial forcing) and observation data temperature field; Gray shading indicates the distribution range of set members (the dark gray area accounts for 68% of the distribution range of set members and the gray area accounts for 95%), the thick black line is the average value, the blue line and the purple line respectively represent the results of two different models, and the blue line is the observation result (Mann et al., 2020).
Fig. 3 LFV spectrum analysis of CMIP 5 historical climate simulation test (artificially forced) and observed data temperature field; The historical climate simulation test here refers to a single artificial forcing test without natural forcing, and the rest is the same as Figure 2(Mann et al., 2020).
In a word, the author thinks that there is probably no interdecadal intrinsic variability in the climate system, which is different from the interannual scale of El Ni? o-Southern Oscillation. Since the industrial revolution, the interdecadal variation of global temperature has been caused by both human and natural reasons. In the past thousand years, when human influence was weak, volcanic activity was the driving force of interdecadal oscillation of global temperature.
Because the author's research is based on climate model, the results obtained mainly show that there is no interdecadal intrinsic variability in CMIP5 model, and the reliability of the results is limited by the ability of current model to simulate intrinsic variability. Nevertheless, this result once again shows the important influence of volcanic activity on interdecadal climate change in the past Millennium. This is consistent with the view that volcanic activity is the main driving force of interdecadal climate change in the past Millennium global temperature reconstruction work (PAGES2k Consortium, 20 19). These studies inspire us to deeply explore the physical mechanism of the interdecadal climate impact process of volcanic activity, which is conducive to a deeper scientific explanation of climate change.
Acknowledgement: I am grateful to the new generation of associate researcher Xu for his comments and valuable suggestions for revision.
Main references
Pacemaker of North Atlantic climate for centuries [J]. Science, 2000,288 (5473):1984-1985.
[1] Maname, steinman, Miller. Lack of internal decadal and interdecadal oscillation in climate model simulation [J]. Natural Communication, 2020, 1 1: 49.
[2] Manman, steinman, Brulette, et al. Decades of climate oscillation driven by volcanic force in the past thousand years [J]. Science, 202 1, 371(6533):1kloc-0/4-/kloc.
PAGES2k consortium Consistent Decadal Variability in Global Temperature Reconstruction and Simulation in Common Era [J]. Natural Geoscience, 20 19, 12(8): 643-649.