By Andy May
The Aleutian Low – Beaufort Sea Anticyclone climate index or ALBSA is designed to compare the Aleutian Low Pressure and the Beaufort Sea High Pressure Centers. The intent is to relate air circulation patterns in the North Pacific and Arctic to climate and the timing of spring sea ice and snow melt.
Calculation method:
The ALBSA index is calculated using 4 points from the NCEP/NCAR Reanalysis Dataset: The following 850mb geopotential height points are used in the calculation:
N: 75° N, 170° W
S: 50° N, 170° W
E: 55° N, 150° W
W: 55° N, 200° W (160° E)
ALBSA = [E – W] – [N – S]
Use of the ALBSA index
Christopher Cox and his colleagues at NOAA developed ALBSA as an indicator of snowmelt timing in the Pacific Arctic on the North Slope of Alaska (Cox, et al., 2019). The timing is influenced by the marine air drawn (advected) to the Beaufort Sea Arctic region from the Aleutian low pressure region. When air is drawn from the Aleutians to the Beaufort Sea, it warms the area, and an early snow melt is observed on the North Slope of Alaska. The pattern illustrated in figure 1 is for 2002 when an early snowmelt was observed in May.
Figure 1 illustrates the typical circulation pattern for years with early melting snow and ice. The air from the Aleutian low pressure region moves eastward and then trends northward through the Bering Strait to the Chukchi and Beaufort Seas. The average ALBSA 850 mb geopotential height (GPH) anomaly in May 2002 was about 69 meters and for the entire spring (March-June) it was 91.1 meters.
For comparison the same map is presented for 1988 as figure 2, when the snowmelt was late. In that year it did not start until June.
The major characteristic of late years is the presence of the Beaufort Sea Anticyclone (BSA), this pushes cold Arctic air to the North Slope which delays melting. For the month of June, the ALBSA 850 mb geopotential height (GPH) anomaly was 7.9 meters and for the 1988 spring it was -90.3 meters. That is the North-South difference was much larger than the east-west difference in 850 mb geopotential height.
Like many other climate oscillations, the ALBSA index has been trending positive in recent decades. That means the Beaufort Sea Anticyclone has been weakening, causing a warmer North Slope. This is illustrated in figure 3.
As illustrated in figure 1, the 2002 spring had an early melt and no Beaufort Sea Anticyclone. In that year the May ALBSA anomaly was +68.9 m and the average spring ALBSA anomaly was +91.1 m, the melt occurred May 23. In 1988 the melt was very late, June 18, and the spring average ALBSA anomaly was -90.3 m. That spring had a strong Beaufort Sea anticyclone, which kept the North Slope of Alaska cold for a longer period.
The correlation between ALBSA and HadCRUT5 is poor, and the trends do not match. However, it does correlate decently with the NPI, which was discussed in post #8. NPI and ALBSA are compared in figure 4. They are not perfectly correlated but they both trend positively since the 1980s.
ALBSA correlates with snowmelt in Northern Alaska and the onset of sea ice melting in the adjacent seas. It also captures some of the variability in the NPI.
Discussion
The timing of snow and sea ice melting is important because the albedo of ice and snow is very high, whereas the albedo of meltwater is very low. This contrast makes a significant difference in the absorption of solar radiation and the resulting warming rate of the surface and lower troposphere as the sun re-enters the polar sky in the spring. Measurements of absorbed energy on the North Slope of Alaska have shown that early melts, for example May 13, 2016, can absorb 30% or more solar energy than late melts, for example June 18, 2017 (Cox C., et al., 2018). Further, as sea ice melts, it allows heat trapped under the ice to escape into the atmosphere.
AR6 does not mention ALBSA or the NPI or discuss if they are reproduced in the CMIP6 climate models. However, given that the models do not reproduce the NAO or AO (see post 9) or the Aleutian Low very well (AR6, page 1381) we assume that ALBSA is not reproduced well by the models. The PDO is discussed in AR6, and it is related to both the NPI and ALBSA. The PDO is very poorly reproduced in the CMIP6 climate models (AR6, page 427 & 503). AR6 often refers to the PDO as “PDV” and claims that since the CMIP6 models cannot duplicate it, it must be random internal variablity, even though the PDO oscillations are statistically significant (Mantua, et al., 1997) & (Ebbesmeyer, et al., 1990).
It is logical that ALBSA affects the pattern of Northern Hemisphere warming and cooling, but it does not correlate well with HadCRUT5. The next post will discuss the Oceanic Niño Index or ONI, which is used to define the El Niño and La Niña ENSO states.
Download the bibliography here.
Previous posts in this series:
Musings on the AMO
The Bray Cycle and AMO
Climate Oscillations 1: The Regression
Climate Oscillations 2: The Western Hemisphere Warm Pool (WHWP)
Climate Oscillations 3: Northern Hemisphere Sea Ice Area
Climate Oscillations 4: The Length of Day (LOD)
Climate Oscillations 5: SAM
Climate Oscillations 6: Atlantic Meridional Model
Climate Oscillations 7: The Pacific mean SST
Climate Oscillations 8: The NPI and PDO
Climate Oscillations 9: Arctic & North Atlantic Oscillations
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