Glacier Mass Balance Forecasting
NORTH CASCADE GLACIER CLIMATE PROJECT
Mauri S. Pelto, Director NCGCP
Nichols College, Dudley, MA 01571
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What is mass balance?
Crucial to the survival of a glacier is its mass balance, the difference between accumulation and ablation (melting and sublimation). Climate change may cause variations in both temperature and snowfall, causing changes in mass balance. Changes in mass balance control a glacier's long term behavior.A glacier with a sustained negative balance is out of equilibrium and will retreat. A glacier with a sustained positive balance is out of equilibrium and will advance.
Annual balance is the most sensitive annual glacier climate indicator. This sensitivity to climate indicates that climatic driving forces may allow forecasting of the mass balance. A mass balance forecast is valuable as it identifies the amount of glacier runoff that will be produced, which in turn will be key to water management decisions. North Cascade glaciers annual balance has averaged -0.54 m/a of water equivalent from 1984-2006, a cumulative loss of over 12.4 m in glacier thickness or 20-40 % of their total volume since 1984 due to negative mass balances. With more than twenty years of glaciers from ten different glaciers we have tested a forecasting tool for glacier mass balance. The forecasting relies on monthly data for October - April to forecast mass balance for the hydrologic year which ends in October. The forecast thus is given at the start of the melt season and provides an assessment of the amount of glacier runoff that will be generated. We are also working on predictions of mass balance based simply on monthly climate data. This years forecast issued May 1 is for negative glacier mass balance. Mass balance prediction.
Locations of glaciers, SNOTEL sites and weather stations used in this study.
Table 4. The columns are respectively: Mean value of winter PDOW (October-April). Winter ENSOW (October-April). Glacier net balance bn-SC from South Cascade Glacier 1960-2005. Glacier annual balance ba-NC for the 10 NCGCP glaciers (1984-2006). The relative phase: positive, >0.2 (p), negative, <-0.2 (n) and equilibrium -0.2 to 0.2 for equilibrium (e), for PDOW, ENSOW indices and glacier net balance and annual balance (bn-ba) respectively. The number of the rule utilized from section 5. Lastly if the rule correctly yields the annual balance in terms of negative, equilibrium or positive it is noted by a yes, if not a no for both bn-ba . The rule is correct in 41 of 46 years for the South Cascade Glacier for which the model was designed. The rule is correct in 20 of 23 years for the North Cascade Glacier Climate Project glaciers.
Mass Balance Forecasting from Climate Indices
A number of papers have examined the relationship of Pacific Northwest glacier mass balance to atmospheric circulation indices (McCabe and Fountain, 1995; Hodge et al., 1998; Bitz and Battisti, 1999; Pelto and Miller, 2001). It has been shown that the mass balance of these glaciers correlate with the PDO (Pacific Decadal Oscillation) and MEI (Mulitvariate ENSO index) particularly over a multi year period (McCabe and Fountain, 1995; Bitz and Batisti 1999; Hodge et al., 1998). Variations in the PDO and ENSO account for 35% and 20% of the variance in annual mass balance respectively of South Cascade Glacier (Bitz and Batisti 1999). The variances indicate the importance of each index, yet neither MEI or PDO are well enough correlated with annual balance to provide any reasonable predictive ability of the specific annual balance (Pelto and Miller, 2003).
After many attempts to calculate a specific mass balance in meters of water equivalent, from climate indices data, it was clear the results were just not going to be consistent enough to provide a reliable forecast. Instead if the goal was to provide a forecast of the mass balance as either negative (<-.20 m), equilibrium (-0.20m to 0.20 m) or positive (>0.20)could we make it work. To determine if these key climate indices could be useful for forecasting the glacier mass balance the mass balance records from North Cascade glaciers were correlated with two PDO and MEI (Bitz and Batisti 1999; Hodge et al., 1998; Pelto and Miller, 2003). Mass balance records from South Cascade Glacier (Krimmel, 2000) and NCGCP glaciers were used. Table 4 displays the annual values for each. Each index is a critical indicator for annual balance that when taken alone does not predict specific annual balance values reliably, but when considered together do provide an assessment of whether mass balance will be positive or negative. In much the same way that the seasonal forecasts for the number of hurricanes is determined from a suite of indicators, this seems to be the most reasonable approach for glacier mass balance forecasting. Five forecasting rules are developed that can be applied to 44 of the 46 possible years examined and provide a correct assessment in 39 of the 44 years. The forecast is not a specific quantity but a range either positive, negative or equilibrium mass balance.
Rule 1. If both PDO and ENSO are positive, than glacier mass balance will be negative on South Cascade Glacier. This rule works in all 14 of the 14 years. Same for NCGCP glaciers the rule is correct in 8 of nine years.
Rule 2. If PDO is negative and ENSO is equilibrium or negative, mass balance will be equilibrium or positive on South Cascade Glacier. This rule is successful in 13 of 16 years. Same for NCGCP glaciers, and is successful in 9 of 10 years.
Rule 3. If PDO is positive and ENSO is neutral the glaciers will have an equilibrium or negative balance on South Cascade Glacier. The rule is correct in 6 of 7 years. For NCGCP glaciers the result is positive glacier balance, and is successful in 4 of 6 years.
Rule 4. If PDO is negative and ENSO is positive the glacier balance will be negative on South Cascade Glacier. This is true in 5 of 6 years. For NCGCP glaciers the result is positive glacier balance, and is successful in 1 of 1 years.
Rule 5. If PDO is positive and ENSO is negative glacier mass balance will be negative on South Cascade Glacier. This rule provides an accurate result in 2 of 2 years. Same for NCGCP glaciers, and is successful in 2 of 2 years.
Rule 6. If PDO is neutral then glacier annual balance will be negative on South Cascade Glacier. This rule is accurate in 3 of 3 years. Same for NCGCP glaciers, and is successful in 2 of 2 years.
The rule is correct in 41 of 46 years for the South Cascade Glacier. The rule is correct in 20 of 23 years for the North Cascade Glacier Climate Project glaciers. These rules provide us with the capability to forecast glacier annual balance given winter PDO and ENSO values. Given the neutral nature of the PDO and a positive ENSO value in the winter of 2007, it is forecast as of May 1 that North Cascade glacier annual balance will be negative in 2007.
These rules provide us with the capability to better forecast glacier mass balance given PDO and MEI values.
Though the statistical and graphical relationship between annual balance and the PDO and ENSO climate indices is not quantitatively strong, both are a key influence of annual balance (Table 4). This is demonstrated being able to correctly forecast the sign of annual balance in 42 of 47 years simply by applying six forecasting rules using October-April PDO and ENSO index values. As predictors of glacier mass balance positive MEI values, El Nino, and warm phase PDO’s favor negative balances, and cool phase PDO’s and negative MEI values, La Nina, favor positive annual balances.