Learning to Predict Climate Variations Associated with El Nino and the Southern Oscillation Accomplishments and Legacies of the TOGA Program (1996) / Chapter Skim
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4. WHAT WE'VE LEARNED
4. WHAT WE'VE LEARNED
Pages 81-109
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From page 81... ...
The predictability of sea surface temperature in the eastern Pacific, with lead times of about a year, has been demonstrated. Explorations of the effects of ENSO on the rest of the globe were pursued.
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From page 82... ...
Attempts to define a climatology by averaging only during "normal" periods-i.e., those without significant warm or cold phases of ENSO will give an incorrect climatology if ENSO produces rectified effects. Sea surface temperature is one of the key quantities that change during ENSO, and its climatology is therefore one of the most crucial.
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From page 83... ...
warmed uniformly over large parts of the central Pacific, followed by warming near the coast of South America. It is clear that in some sense the anomalies of sea surface temperature did propagate eastward, but in another sense the warm sea surface temperature developed over a large region of the tropical Pacific without propagation.
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From page 84... ...
The winds, subsurface thermal structure, sea surface temperatures and sea level are measured directly by the TOGA Observing System, and other quantities such as precipitation and surface fluxes are obtained by other methods. These
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From page 85... ...
extending from 20°S to 20°N in the east Pacific and crossing the equator at the dateline in the west. The predictions of sea surface temperature in the NINO3 region have been poorer during this time, with some notable misses by all dynamical and statistical models.
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From page 86... ...
Also unusual in the period 1990-1994 is that the magnitude of the anomalies of sea surface temperature in the equatorial Pacific peaks in the boreal spring, coincident with the normal annual warming, rather than the normal case for ENSO with the magnitude of the warm anomalies peaking in late boreal winter. -hi ~A inch 4~ fr~ -V~ REV ~-~ - ~ ~4r\ ~I: 1950 1960 1970 1980 1990 b V~1~ CON 4ON 7ON HP LP HP F .
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From page 87... ...
variations of sea surface temperature throughout the entire Pacific (Zhang et al. 1996; similar results were found earlier by Nitta and Yamada 1989~.
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From page 88... ...
Lau and Nath 1994) have indicated that variations of sea surface temperature in the tropics, characteristic of those found during the warm and cold phases of ENSO, affect the middle latitudes in recognizable patterns, and that midlatitude variations of sea surface temperature have only a small and indistinct effect.
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From page 89... ...
First, rather small changes in both sea surface temperature and sea-surface-temperature gradients can greatly influence the locations of strongest convection and low-level convergence. The f~rst-order response of the tropical atmosphere to anomalies of sea surface temperature is a shift in the location of organized convection.
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From page 90... ...
This helps explain why the response of the extratropical atmosphere to sea surface temperatures in the tropics can be highly nonlinear, as was found by Geisler et al.
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From page 91... ...
From 1915 to 1960, variability in NINO3 (the anomaly of sea surface temperature averaged over the region between 5°N and 5°S, 150°W and 90°W) was quite i_ Table 2.
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From page 92... ...
. A principal mandate of MONEG was studying, using integrations of atmospheric general-circulation models, the effects of prescribed anomalies of sea surface temperature on the monsoon.
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From page 93... ...
Observed sea-surface-temperature anomalies In the Indian and Atlantic Oceans did have some impact on the atmosphere over the Indian and Atlantic Oceans, respectively. Still the role of interannual variability In the Indian Ocean on the Asian monsoon seems to be weaker than the remote effect of Pacific Ocean sea surface temperatures, at least during
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From page 94... ...
have concluded that during cold phases of ENSO, warm anomalies of sea surface temperature in the tropical northwest Pacific may be of more direct importance in influencing the Asian summer monsoon than the cold equatorial anomalies of sea surface temperature in the central and eastern Pacific that determine the phase of ENSO. In summary, it is clear that there is a predictable element in the summer monsoon of south Asia.
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From page 95... ...
Therefore, the largest CO2 imbalances observed during climatically anomalous years of major ENSO phases represent only 1 percent fluctuations in the balanced exchange fluxes of atmospheric CO2 with the oceanic and terrestrial biospheric reservoirs of carbon. The 1982-83 ENSO warm event provided the first opportunity to quantify with direct measurements the major reduction of the sea-to-air CO2 flux from the equatorial Pacific Ocean.
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From page 96... ...
This can be understood most simply by imagining a warm anomaly of sea surface temperature that creates surface winds, which in turn enhance the warmth of the anomaly. The anoma
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From page 97... ...
When anomalies of sea surface temperature are assumed to be simply proportional to anomalies of thermocline depth, then the unstable coupled mode resembles a free, eastward-propagating Kelvin mode in the ocean, with atmospheric convection following. When anomalies of sea surface temperature are changed by surface advection, the unstable coupled mode resembles a free, westwardly propagating Rossby mode in the ocean, with atmospheric convection following.
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From page 98... ...
After a time ~ (the length of time it takes for the signal to return to the region of increasing sea surface temperature) the cooling signal, growing exponentially with an amplitude ~ as a function of its retarded time t-I, reaches the east Pacific and begins to erode the warming signal, eventually cooling the region.
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From page 99... ...
If the remote signal, multiplied by b, has a much greater ability to change sea surface temperature than the direct local effects, in the term multiplied by c, then growing oscillatory solutions can result. The cubic term is required for oscillatory finite-amplitude solutions.
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From page 100... ...
In these studies, Jin and Neelin explored the range of coupled atmosphere-ocean modes that exist in the parameter space covered by varying the coupling strength, the ratio of the ocean dynamical adjustment time to the time scale associated with the sea-surface-temperature changes, and the relative strength of the upwelling versus horizontal-advection terms in the ocean thermodynamic equation. By varying the parameters within realistic values, introducing increasingly complete ocean (sea surface temperature)
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From page 101... ...
Kessler (1990) has examined the observed wind-stress data, the anomalies of sea surface temperature, and the variability in the upper-ocean thermal structure obtained from the expendable bathythermographs launched by VOS.
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From page 102... ...
(1991) examined a streamlined, nonlinear coupled model and found that irregular interannual variability can result from the coupling of the atmosphere and ocean.
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From page 103... ...
This hypothesis for the irregularity in ENSO is based on the very different physical processes that seem to govern the interannual ENSO mode and the annual cycle (see, e.g., Koberle and Philander 19949. It is not clear, however, to what extent the spectrum of interannual variability in the tropical Pacific is stationary.
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From page 104... ...
. In both of these studies, the annual cycles in the tropical Pacific of sea surface temperature, surface fluxes, and wind are qualitatively consistent with those observed.
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From page 105... ...
built a similar autoregressive model us~ng the observed sea surface temperature. In contrast to Blumenthal, they
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From page 106... ...
predictability. In addition, because Penland and Sardeshmukh used sea-surface-temperature data but no thermocline data in their analysis, their hypothesis that the coupled system is globally stable required that the ocean dynamics equilibrate on time scales that are rapid compared with the changes in sea surface temperature.
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From page 107... ...
The TOGA Program promoted the free distribution of observational data and of numericalmodel analyses of wind, surface and subsurface temperature, and other variables at sites remote from land. Many of the observations were made available within a day, and all of them within 30 days, of acquisition.
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From page 108... ...
, where an interpretative analysis of the June 1987 El Nino oceanographic conditions (sea surface temperature, near-surface currents, thermocline depth, sea-surface height, surface wind, and other variables) in the tropical Pacific was presented.
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From page 109... ...
Achieving a balance among monitoring, modeling, empirical studies, and process studies required unprecedented cooperation among NOAA, NSF and other federal agencies when reviewing funding proposals and when filling crucial gaps during the implementation phase of the program. At the same time, meteorologists and oceanographers at universities and federal laboratories took a more active role in providing balanced scientific advice to the relevant federal agencies through the National Research Council's TOGA Panel and other structures.
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