Natural Climate Variability on Decade-to-Century Time Scales (1995) / Chapter Skim
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COUPLED SYSTEMS
COUPLED SYSTEMS
Pages 413-488
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From page 414... ...
If we are to understand and simulate climate variability on decade-to-century time scales, model runs of thousands of years are required. Up to this time, fully coupled models of satisfactory (but never sufficient)
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From page 415... ...
The coupled model could be run for only 100 years of ocean model time, due to computational limitations, but at the end of this time it had reached a quasi-equilibrium in which only the deeper parts of the ocean were still changing. The resulting distribution of surface temperature, while not directly comparable to observations, looked quite reasonable, with the ocean heat transport warming higher latitudes and cooling the tropics.
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From page 416... ...
The lessons from the Manabe-Stouffer model are that there can be two climate states (one with and one without NATURAL CLIMATE VARIABILITY ON DECADE-TO-CENTURY TIME SCALES a THC) in the presence of identical external forcings, and that it is difficult to achieve a good simulation of the surface salinity field unless both the atmospheric and oceanic processes that control salinity are correctly modeled.
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From page 417... ...
Even if longer-term variability is not predictable, the ability of models to successfully simulate the spectrum of climate variability is a necessary prerequisite to our full understanding of the natural climate system. CONCLUSION As we have seen, the basic problem in coupled modeling is the correct simulation of the climatology, particularly the annual cycle.
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From page 418... ...
· General increases in available resources and computational infrastructure, allowing a wider community to gain access to coupled modeling. · Improvement and extension of time series of physical quantities in the atmosphere, ocean, land, and cryosphere.
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From page 419... ...
time-scale variability in climate is that the instrumental records for all climate variables either do not exist or are too short to detect these phenomena with any measure of statistical confidence. Hence, the methodologies and strategies of the research activities related to the intermediate time scales of climate variability will be distinctly different from those related to interannual variability.
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From page 420... ...
Together, these studies and related analytical, modeling, and observational studies have elevated the deep ocean circulation into the arena of potential mechanisms for-or indicators of climate variability on the intermediate time scales. In 1964, Bjerknes examined the contemporaneous fields of sea surface temperature (SST)
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From page 421... ...
Nonetheless, Bjerknes's hypothesis that ocean dynamics play an important role in interdecadal variability is supported by recent studies of the decadal variability of the North Atlantic atmosphere-ocean system (e.g., Kushnir, 1992; Pan and Oort, 1983~. Nearly all the numerical and theoretical research activities on intermediate-scale climate variability have taken place during the last half-decade, for a variety of reasons.
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From page 422... ...
Thus, prior to the l950s the instrumental data records exist for only a few key variables in isolated regions, and provide only a blurred glimpse at climate variability. The data are insufficient for deducing the attending atmospheric and oceanic circulations and heat transport, and the energy exchange between the media.
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From page 423... ...
The goals of this program are as follows: · To determine the seasonal-to-decadal and multidecadal variability in the climate system due to interactions between the Atlantic Ocean, sea ice, and the global atmosphere using observed data, proxy data, and numerical models. · To develop and utilize coupled ocean-atmosphere models to examine seasonal-to-decadal climate variability in and around the Atlantic Basin, and to determine the predictability of the Atlantic climate system on seasonal-to-decadal time scales.
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From page 424... ...
, using the historical observational data of sea-surface temperature, sea-level pressure, and the zonal wind over the Atlantic Basin, have demonstrated that there is a complex wintertime atmosphere-ocean interaction in the North Atlantic with a preferred time scale of about 10 years. The anatomy of this decadal variability appears to be somewhat different from that of the interannual climate anomaly in the North Atlantic documented in Wallace et al.
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From page 425... ...
IDENTIFYING POTENTIAL CLIMATE VARIABILITY ON DECADE-TO-CENTURY TIME SCALES FROM NUMERICAL MODELS: STRATEGIES AND LIMITATIONS Numerical models will be the primary tools by which the mechanisms responsible for decade-to-century-scale climate variability are identified. More important, these same models will frequently be the instruments that scientists use to identify target intermediate-time-scale climate phenomena, because instrumental data exist only for the last century and, as mentioned above, for only a limited domain of the climate system.
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From page 426... ...
examined the response of the GFDL AGCM to the observed 1950-1979 SST and an annual cycle in insolation.7 Upon isolating the circulation anomalies associated with the North Pacific SST anomalies, LN found that the model anomalies were indeed consistent with those observed (i.e., a quasi-linear relationship between anomalies in SST and geopotential)
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From page 427... ...
It is reasonable to assume, however, that the climate models least prone to yielding non-physical solutions on the intermediate time scales are the models that can accurately simulate many of the directly observed and well-documented higher-frequency phenomena in the climate system. Thus, a prerequisite for a model for intermediate-time-scale climate studies is accurate reproduction of many of the already well-observed target phenomena and features.
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From page 428... ...
Subtle discrepancies between the observed and simulated interracial fields and fluxes formed with the standalone atmosphere and ocean models are often amplified in the coupling process. This amplification leads to a coupled climate state that is qualitatively different from either of the states achieved by the two uncoupled models in isolation.
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From page 429... ...
The primary limitation on the study of climate variation on the intermediate time scale is that the instrumental records for many climate variables either do not exist or are too short to detect a phenomenon with any statistical measure of confidence. Proxy data for climate, therefore, will play an important role in documenting the past climate variability on the intermediate time scales and, most likely, in evaluating the simulated climate variability.
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From page 430... ...
I tend to approach the question of long-time-scale change in an engineering fashion: I wonder whether we can tell from all the different records that we have whether there really is any anthropogenic forcing, and whether we are seeing any response to it. I recognize that there are many other reasons for studying these time scales, but this is certainly an important one.
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From page 431... ...
LEVITUS: It seems to me essential that we understand decadalscale variability better before we can go on to longer time scales or develop better models. First we have to be able to parameterize the processes better and to understand the system on shorter time scales.
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From page 432... ...
The fluctuations appear to be driven by density anomalies in the sinking region of the thermohaline circulation combined with much smaller density anomalies of opposite sign in the broad, rising region. Anomalies of sea surface temperature associated with this oscillation induce surface air temperature anomalies over the northern North Atlantic, the Arctic, and northwestern Europe.
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From page 433... ...
The region in which most of the sinking occurs in the model North Atlantic is principally confined to the latitudinal belt from 52°N to 72°N; hereafter, the term "sinking region" will refer to the 3In subsequent figures and the text, it is demonstrated that the intensity of the meridional overturning in the model North Atlantic has enhanced variance at a time scale of 40 to 50 years, but the spectrum of model SST in Figure 1 is not characterized by enhanced variance in this frequency band. As shown in Figure 4a, there is a spatial pattern of SST anomalies associated with these overturning variations.
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From page 434... ...
Sea Surface Temperature, Salinity, and Surface Air Temperature Variations The spatial pattern of the changes of model sea surface temperature associated with fluctuations in the intensity of the thermohaline circulation is shown in Figure 4a. These differences are computed by subtracting the mean of four decades having anomalously small values of the THC index from the mean of four decades with anomalously large values of the THC.
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From page 435... ...
.1J 60 45 30 15W 0 15E FIGURE 4 (a) Differences in annual mean model sea surface temperature between four decades with anomalously large thermohaline circulation index values and four decades with anomalously small thermohaline circulation index values.
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From page 436... ...
However, the enhanced surface absorption of summer insolation due to the reduced coverage of sea ice also contributes to the reduction of sea ice thickness and the increase of surface air temperature in winter. The above results highlight the potential importance to climate of such fluctuations in the thermohaline circulation over the North Atlantic, Europe, and the Arctic.
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From page 437... ...
The regression coefficients were computed using various lags in order to obtain an estimate of the temporal evolution of the three-dimensional fields of density, temperature, and salinity with respect to variations in the thermohaline circulation. These regression coefficients were then averaged vertically and horizontally over the sinking region of the North Atlantic (defined in text)
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From page 438... ...
As previously shown in Figure 6, the temporal variations of the vertically averaged density field in the sinking region precede the variations in the thermohaline circulation's intensity, implying that the former drives the latter. A map of the regression coefficients between the time series of the near-surface density anomalies and the TlIC is shown in Figure 8.
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From page 439... ...
Sea surface temperature fluctuations associated with the 439 variations in the thermohaline circulation have a spatial pattern that bears an encouraging resemblance to a pattern of observed interdecadal variability (Kushnir, 1994~. The SST anomalies induce surface air temperature anomalies of substantial magnitude over the North Atlantic, northern Europe, and the Arctic.
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From page 440... ...
DIAZ: Have you seen anything indicating an out-of-phase sea surface temperature relationship between the North and South Atlantic? DELWORTH: I haven't looked for that.
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From page 441... ...
In fact, the sea ice is virtually in phase with the fluctuations. In this model, and on this time scale, the ice appears to be reacting to changes in circulation rather than shaping them.
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From page 442... ...
There is variation in the amplitude of the extreme phases of the cycle warm and cold events and in the interval between events. There is variation in the evolution of events: no two cycles are exactly alike, and occasionally the Southern Oscillation signals in the west Pacific assert their independence from the E1 Nino sea surface temperature (SST)
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From page 443... ...
. In this paper we speculate on these three possible causes of the variability of the ENSO cycle: variations in the base state, in the noise, and in the internal dynamics.
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From page 444... ...
Two regime types are evident: one in which strong and regular ENSO cycles of approximately 4-year periodicity persists, and another with weaker, more erratic behavior. segments of 24 years' length (overlapping each other by 4 years)
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From page 445... ...
, standard deviation, dominant frequency (years- If, and fraction-ofvar~ance indices from the 1024-year simulation. The values of each index computed from observed sea-surface temperature between 1970 and 1987 are indicated by vertical line segments.
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From page 446... ...
5% Fourth EOF - CAC SSTA (1970-1987) 3% ':_~N ~2g°~: 125 E 150-E 175.E 160-W 135-W 110-W 85-W 125-E 150-E 175-E 160-W 135W 110-W 85.W Lor~itude Longitude FIGURE 4 First four EOFs of observed SST anomalies, calculated from monthly values between the years 1970 and 1987, inclusive (Climate Analysis Center analysis)
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From page 447... ...
Across each row, the total set of results in each plot has been stratified according to the three distinct values of the indicated parameter. The heavy solid line represents the distribution for the subset with a 5 percent reduction of the parameter, the dashed line represents the subset with the standard value, and the dotted line represents the subset with a 5 percent increase.
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From page 448... ...
RWRJ. Any variations in such features as mean atmospheric temperature or to to 2 en ID too zip z COUPLED SYSTEMS decadally averaged tropical Pacific SST are more likely to be consequences than causes of ENSO variations.
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From page 449... ...
It is more likely a consequence of nonlinearities internal to the ENSO cycle itself.
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From page 450... ...
a a 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Frequency in cycles/year 1.6 1.8 2.0 FIGURE 9 Power spectrum of the NINO3 SST anomaly from a run of the ZC nonlinear model. As described in the text, the 1024year (monthly)
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From page 451... ...
INTERNAL DYNAMICS The analysis above builds a case for attributing the aperiodicity in the ZC model to nonlinear dynamics intrinsic to the ENSO cycle. Under the assumption of linear dynamics, it was shown to be unlikely that the regime-like behavior could be driven by noise.
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From page 452... ...
One is to add an annual cycle to K, taking crude account of effects of the mean annual cycle in such factors as SST and wind speed. The other is to include in A the asymmetry between the warm (upper)
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From page 453... ...
According to Occam's Razor, these results recommend the following as a working hypothesis: The principal cause of all the variability associated with the ENSO cycle is low order chaotic dynamics of ocean-atmosphere interactions within the tropical Pacific. "All the variability" includes the basic quasi-quadrennial cycle, quasi-biennial variability (Rasmusson et al., 1990; Barnett, 1991; Ropelewski et al., 1992)
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From page 454... ...
There is now great interest in studying natural variability on time scales of decades and longer. Since the climate system contains both nonlinear chaotic subsystems and sources of what may be viewed as noise, even an impeccable model cannot reproduce a long natural time series.
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From page 455... ...
~1~ ~l ~ c I .~ ~r''' ~l~ ~ 1 loo :zoo Boa ~PQ tall-(Ye ~ FIGURE 16 NINO3 SST anomaly from ZC model run 02011; see Figure 6.
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From page 456... ...
If they were to increase the depth of the thermocline and change the phase speeds and so on, they would be in a different regime altogether, possibly one without E1 Nino. It seems to me that such changes probably involve the global thermohaline circulation, which determines the depth of the tropical thermocline.
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From page 457... ...
Mark, did you find that modifying your nonlinear model parameters to fit the ice-core records made any difference to the model's predictability characteristics on the 2-year time scale? CANE: These runs started from the same initial conditions.
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From page 458... ...
We have examined the secular variability of the ENSO cycle as revealed by commonly used indices, i.e., sea level pressure and sea surface temperature from the lowlatitude core region of the oscillation. We view the low-frequency variations (periods longer than approximately 30 years)
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From page 459... ...
ENSO CYCLE After years of analysis of global correlation patterns in surface temperature, pressure, and precipitation, Sir Gilbert Walker (Walker, 1924) identified three large atmospheric oscillations: two "Northern Oscillations", centered in the North Atlantic and the North Pacific, and a larger, more pervasive "Southern Oscillation" (SO)
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From page 460... ...
The phase and amplitude of the ENS O cycle is usually indexed by sea level pressure variations at stations representative of the SO pressure dipole or by SST anomalies in the central and eastern equatorial Pacific. In characterizing and indexing the ENSO cycle it is important to give due consideration to the seasonality of the anomaly fields, as emphasized by Walker and Bliss (19321.
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From page 461... ...
They found that seasonal correlations between eastern equatorial Pacific SST and SLP at a number of index stations have "been greatest since World War II, and were relatively high prior to World War I." They suggest that there have been changes in the character of the cycle: i.e., prior to World War I the pressure difference between the eastern Pacific and Darwin was more representative of the SO, but since World War II the pressure difference between the central Pacific and Darwin has better reflected the SO. Further Analysis The view of what constitutes "climate noise" and what distinguishes climate change from climate variability is largely dependent on the time scales of interest and the spectrum of climate variability.
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From page 462... ...
. SSA modes 2 through 7 reflect ENSO cycle variability and account for a total of 49 percent of the Bombay SLP variance and 65 percent of the SST*
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From page 463... ...
Base States Figure 8 shows an example of the base-state curve for Bombay SLP, superimposed on the original time series. Figure 9 shows the base-state time series and 31-year mean
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From page 464... ...
Thus, changes in tropical Pacific SST were generally of the same sense and larger than the centuryscale changes in globally averaged SST. Bombay exhibits a substantial century-time-scale trend toward lower pressure until 1955-1960, when values turned upward.
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From page 465... ...
exhibit a dominant century-scale variation with high variability late in the nineteenth century, the lowest variability during the third through fifth decades of the twentieth century, and increasing variability thereafter. The changes in the intensity of the ENSO cycle during the past century are substantial, with the 31year mean maxima and minima differing by a factor of 2 or more.
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From page 466... ...
Units are °C. The broadly similar character of the century-scale changes in SLP variability at the three stations shown suggests that the changes reflect primarily changes in the intensity of the ENSO cycle rather than shifts in one or both poles of the ENSO-cycle pressure seesaw.
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From page 467... ...
Upper panel, basic anomalies; lower panel, detrended series. A light median filter (Rabiner et al., 1975)
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From page 468... ...
Similar problems are likely to arise with any proxy record if ENSO fluctuations are classified by categories of intensity. These analysis results provide no clear evidence of a relationship between base-state changes and changes in the statistics of the ENSO cycle.
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From page 469... ...
The EOF3 coefficients imply an increase in tropical Pacific SST after the mid-1940s, and while this represents only a small part of the total SST variability, it is of the same sign as the trend in the SST base state. Finally, the time series of the percentage of the U.S.
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From page 470... ...
This may mean that the basic state is just one of the low-frequency, maybe non-linear manifestations of the ENSO cycle itself, and thus it may not be the true basic state for the phenomenon we are dealing with. The reference to a 30-year interval as the basic state is a helpful approach, however, at least for following the change in the main spectral peaks of ENSO, the four- to five-year cycle and the biennial cycle.
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From page 471... ...
There might be some way we could use the spatial patterns and multivariate changes to discriminate between what might be caused by some shift in the tropical Pacific and what might be caused by something else.
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From page 472... ...
Significant changes observed in the atmospheric circulation throughout the troposphere during that period show that a deeper, eastward-shifted Aleutian Low pressure system in the winter half-year advected warmer and moister air along the west coast of North America and into Alaska and colder air over the North Pacific. This advection caused substantial changes in sea surface temperatures over the North Pacific, as well as in coastal rainfall and streamflow, and in sea ice in the Bering Sea.
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From page 473... ...
NORTH PACIFIC LARGE-SCALE OBSERVED TRENDS In the North Pacific, a close association between SST anomalies and the atmospheric circulation has been well recognized. Changes in surface temperatures arise from
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From page 474... ...
{=~ , _~ ~ ~ ~ Jul Aug S.p Oct Nov Dec Jon Feb ~or ~p, llo, Jun Jul . ~ ~, , ·~ ~ ( ~1~ -~_1,____ S5 - _ ~ 1~: Jul Aug Sep Oct Nov Dec Jon hb 61~r Ap, Idoy Jun Jul Month FIGURE 2 Time series of mean North Pacific sea level pressures averaged over 30°N to 65°N, 160°E to 140°W as a function of month and time.
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From page 475... ...
_ ~ 1.5 ~ 2~0E -2.5r. ., .,,,,,,,,,,,,,,,,,,,,,,, 1,,,,,,,,,,,,,,, 1930 1940 1950 1960 1970 198_ Year 1990 FIGURE 4 Time series of mean North Pacific sea level pressures for November through March, as in Figure 3, but beginning in 1925 and smoothed with the low-pass filter.
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From page 476... ...
Across the North Pacific and North American regions, these patterns show that the anomaly featured in Figure 5 is consistent with the whole record: Below-normal NP values are associated with below-normal temperatures over the North Pacific and southeast United States and above-normal surface temperatures along the West Coast, extending throughout Alaska and across most of Canada. We have also investigated these relationships as a function of various lags.
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From page 477... ...
Once again, the PNA pattern emerges strongly, with all the PNA centers showing up, although the associated anomaly departure pattern clearly emphasizes the North Pacific. CAUSES OF CHANGE IN THE NORTH PACIFIC Examination of the possible causes of the various types of changes focuses attention on the association between the large-scale coherent climate variations and changes in atmospheric waves.
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From page 478... ...
When possible causes of changes are considered for the North Pacific, one prospect is in situ forcing through the influence of extratropical SST anomalies in the North Pacific on the circulation (Namias, 1959, 1963~. It has been difficult to substantiate such influences either statistically (Davis, LINKS WITH THE TROPICAL PACIFIC The period of the deeper Aleutian Low regime extends from 1977 to 1988; during it there were three E1 Nino (warm)
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From page 479... ...
Correlations of the SOI with the Northern Hemisphere sea level pressures for the November-to-March winter months combined show the link with the North Pacific and the extension across North America (Figure 10~. Note the values of opposite sign over North America, which are very important as part of the overall pattern.
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From page 480... ...
Nevertheless, these results emphasize the involvement of the tropical SST variations in the atmospheric and surface temperature variations over the North Pacific and North America. DISCUSSION AND CONCLUSIONS The picture emerging from these empirical and modeling studies is not yet fully clear, but the evidence suggests the following hypothesis.
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From page 481... ...
But the mean patterns in the North Pacific and
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From page 482... ...
TRENBERTH: There's a nice figure in my 1991 paper showing the annual change in the wind stress. It's directly related to the change in the Aleutian system, and contributes substantially to a change in both the subtropical and the polar gyres in the North Pacific.
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From page 483... ...
1983. A general circulation model study of January climate anomaly patterns associated with interannual variation of equatorial Pacific sea surface temperatures.
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From page 484... ...
1976. Predictability of sea surface temperature and sea level pressure anomalies over the North Pacific Ocean.
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From page 485... ...
1994. Interdecadal variations in North Atlantic sea surface temperature and associated atmospheric conditions.
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From page 486... ...
1988. The effects of North Pacific sea surface temperature anomalies on the January climate of a general circulation model.
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From page 487... ...
1984. The association between the North Atlantic oscillation and the southern oscillation in the northern hemisphere.
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From page 488... ...
1992. Singular value decomposition of wintertime sea surface temperature and 500 COUPLED SYSTEMS Weaver, A.J., E.S.
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