3) The spectral signature of the series analyzed reveals that while there are well separated, narrow band interannual, and interdecadal oscillatory components shared between the Devils Lake and the climate indices, their expression is rather time dependent and the recent record of the Devils Lake is manifested as a singularity in the system where the dominant frequencies of interest, both for the lake and for the climate indices (NINO3, PDO, and NAO) are concurrently at anomalous levels, their interaction (i.e., cross-ocean factors) is important in determining the local precipitation and lake response. This combination of factors and the lake?s state does not have an analog in the 1905-1999 record.
4) The connection of the Devils Lake fluctuations to the climate indices raises hopes of climate based forecasts of the lake and associated regional hydroclimatology. However, given that no successful means of forecasting these indices for the long run yet exists, despite their clear-cut low-frequency character, makes the task difficult. An additional complication is that the Devils Lake basin hydrology and lake dynamics appear to be acting as a nonlinear, notch filter on the climate system, in that until some thresholds of wetness (unknown but definable) are crossed the lake volume squelches the climate signal. The changes in seasonality of the precipitation forcing may play a critical role in determining the nature and magnitude of this threshold behavior. Conversely, after the threshold is crossed, the climate signal is amplified. This is similar to but more dramatic than what happens with the Great Salt Lake.
5) A question that has been brought up in the climatic context of Devils Lake has been the possibility that a changed climate due to increased carbon dioxide (CO2) in the atmosphere may be responsible for the changes in the precipitation and in the lake volume. Such questions are invariably difficult to answer given the limitations of numerical models of the Earth?s climate and the limited time history over which such assessments can be done. We did not directly pursue investigations to investigate such an attribution. However, given the longer, paleoclimatic context for the region and for other lakes such as the Great Salt Lake, it is evident that the type of conditions being currently experienced have occurred in the past (see for instance the marker (X) in Figure 1) prior to our notion of anthropogenic climate change. Consequently, such questions can be answered in a useful way only through investigation of climatic mechanisms, i.e. modes of the ocean-atmosphere system, that would lead to anomalous moisture transport to the region, and to investigate whether the frequency of such modes is likely to undergo changes over time, in particular due to anthropogenic forcing. If changes in the frequency of such events are indicated, then the relative risk of such occurrences is likely to increase. We noted that the regime residence time and regularity/duration of switching of the low-frequency climate conditions indicated by the climate indices used have varied quite a bit over the historical period. Whether such variations occur in the natural climate or whether they are forced by greenhouse effects is difficult to diagnose, given that current coupled ocean-atmosphere models do not adequately reproduce these low frequency modes. However there are indications from several such models of the increased incidence of El Ni?o like conditions under a warming scenario, which may in turn translate into positive summer/fall precipitation in the region as indicated by the correlations identified here. However, the models are unable to define the nature of the PDO/NAO variations that have longer time scales and may be just as important for the region. Indeed, the persistent nature of the current event would likely be linked to the more slowly varying ocean states (PDO, NAO) than the tropical Pacific (El Ni?o).
6) Various methods of near-term and long-term forecasting were tested. In particular, the nonlinear time series methods based on state space reconstruction using Multivariate Adaptive Regression Splines (MARS) that have been very successful for Great Salt Lake forecasts, were used for the 1-4 year ahead forecasts. While for certain combinations of parameters, and at certain times, these models produced forecasts that looked quite promising, there was significant variation in the actual predictors selected and the complexity of the fitted models over the combinations explored. Further, many of the fitted models were unstable, in that small perturbations led to dramatically varying and unrealistic forecasts. The indicated model confidence limits, accounting for the degrees of freedom were often very wide. Our confidence in these forecasts is consequently limited. In this respect the performance was quite different from our applications to the Great Salt Lake volume data. Various differences in the character of the Devils Lake and Great Salt Lake volume fluctuations were highlighted earlier. The longer Great Salt Lake record actually included analogs for the extreme 1980?s rise and fall for the lake, and consequently, the forecasts for that period were more robust. The Great Salt Lake forecasts for the subsequent period (to date) have been remarkably accurate even as far as 4 years ahead.
7) The longer run forecasts based on ARFIMA or ARCOMP type of models suffered from similar problems. Confidence limits were very wide, particularly as the 1990?s data was included in these models, and the models indicated solutions on the non-stationary boundary. This is hardly surprising in light of the earlier results, and the anomalous nature of the recent period relative to the rest of the record. Spectral forecasting methods that may have been able to account for such nonstationarities were considered, but their direct application to the problem was not pursued. We were discouraged by the pronounced amplitude variation of the signals for Devils Lake over the period of record, and were concerned that robust forecasts from these methods would not be likely.
8) Given the historical perspective of Devils Lake and the Great Salt Lake, one can say that while there is considerable uncertainty associated with numerical forecasts of Devils Lake?s future levels, such conditions or excursions for closed basin lakes are not uncommon. They reflect the complex interaction between basin scale and hemispheric scale threshold processes. Key characteristics of such excursions are that, once the threshold is crossed in either direction, the lake will rise or fall rapidly. The rate and amount of such changes are generally far larger than what strategies for human adaptation, such as pumping or diversions, can handle. Thus, flood plain zoning or other measures that moves critical facilities out of such areas are desirable. A more intensive regional/continental analysis of climatic factors and their land surface projection using model, historical and paleo information may be useful for improving the perception and estimates of risk.
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