1. The Role of Melt Onset Timing in the Recent Extreme Arctic Summer Ice Extent Minima
W. N. Meier, J. C. Stroeve, and J. Smith (Correspondence:
Introduction
Melt Onset
2002
2003
2004
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+30
The recent reduction in summer ice cover in recent years has been one of the starkest indications of change in the Arctic. The September 2002 minimum extent was the lowest since the beginning of the satellite record in September 2003 was the second lowest on record and nearly as low as marks an unprecedented third year in a row of anomalously low ice extents (Figure 1, Table 1). This marks a change from past extreme low years, which have typically been followed by a rebound to higher ice extents the next year. Previous studies have indicated that a positive phase in the previous winter’s Arctic Oscillation (AO) can lead to a larger export of thicker multiyear ice out of the Arctic through the Fram Strait (Rigor et al., 2002). This results in a thinner average ice cover that is preconditioned for more extensive melt the following summer. However, in the winter AO has largely been negative. Another possible factor leading to the September minimum extent is the length of the melt season. Warmer spring and summer temperatures that can result in earlier melt (and hence a longer melt season) have been noted in many regions of anomalously low summer extents. Here, we investigate links between the timing of the melt onset, based on passive microwave data, with the following September’s extent as a possible factor in the extreme minima of the past three years.
Melt onset anomalies generally correspond to the September ice anomalies of the last three years (Figure 3).
Most widespread and extreme melt in corresponds to lowest extent of satellite record.
Earlier melt most pronounced in western Arctic, where extent anomalies have been most extreme (see Figure 1).
Earlier melt is consistent with higher air temperatures observed in the Arctic in the months preceding the past three Septembers.
Discrepancies between melt and extent anomalies in eastern Arctic, where despite earlier melt, there are positive extent anomalies; dynamics are likely playing an important role, advecting thicker ice into the region.
Figure 3. Melt onset anomaly (in days) for (left to right) 2002, 2003, 2004 compared to the mean. Red (negative) values indicate earlier than normal melt onset; blue (positive) values indicate later than normal melt onset.
Melt Onset
Figure 5. Decadal average melt onset anomaly (in days) for the 1980s (left) and 1990s (right) (note the scale difference from Figure 3). Red (negative) values indicate earlier than normal melt onset; blue (positive) values indicate later than normal melt onset.
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+15
Sep 2002
Sep 2003
Sep 2004
September mean sea ice cover during extreme minima years (x 106 km2)
Year
Total Arctic Ice Extent
Total Arctic Ice-Covered Area
2002
5.96
3.98
2003
6.18
4.04
2004
6.06
4.36
79-00
mean
(std dev)
7.04
(0.55)
4.67
(0.45)
Correlation between melt onset and the following September’s extent are fairly high (0.6 – 0.7), though with much variability (Figure 4).
Correlations are notably lower for anomalously low ice conditions (Figure 4 and Table 2), which suggests that dynamics play a key role in the extreme minimums, which is consistent with other findings (Rigor et al., 2002).
‘Low’ and ‘High’ years defined as being > 1 st. dev. from the average; ‘Normal’ years are within 0.5 st. dev. of the average.
There is a marked contrast between the 1980s and 1990s melt onset, with later melt in the 1980s and earlier melt in the 1990s (Figure 5), which corresponds to observed temperature trends.
There is also a contrast in the melt onset patterns for low ice years compared to normal and high years as listed in Table 2 (Figure 6). The patterns are quite different than those for the low years of (Figure 3).
In low ice years during , earlier melt is concentrated in eastern Arctic; for , earlier melt is generally stronger in the western Arctic.
There is actually later melt for much of the region east of Greenland for low years; in there are substantial areas of earlier melt east of Greenland.
The contrast between the time period and the past three years suggests that different and/or additional mechanisms are playing an important role in the unprecedented low summer minimum Arctic ice extents.
Figure 1. September average sea ice extent/area anomalies for (left to right) 2002, 2003, The median ice extent contour is overlaid in purple. Note the pronounced retreat of the ice edge north of Alaska and eastern Siberia, and east of Greenland. The table on the right summarizes the extents and area for the last three years compared to the mean.
Table 1. Summary of total extents and area for the last three Septembers compared to the mean.
Table 2. Years selected for ‘low’, ‘normal’, and ‘high’ conditions with corresponding September extents and correlation between melt and extent.
September mean sea ice cover (x 106 km2) and extent vs. melt correlation during low, normal and high years
Low
Normal
High
Year
Ext.
Corr.
1990
6.24
0.64
1984
7.17
0.67
1980
7.85
0.71
1993
6.50
0.62
1985
6.93
0.70
1983
7.52
0.65
1995
6.13
1989
7.04
1986
7.54
0.74
1999
0.55
1997
6.74
0.68
1992
7.55
2000
6.32
0.63
2001
6.75
0.66
1996
7.88
Mean
6.29
0.61
6.92
7.67
0.69
Melt Onset Product
Parameter: Julian day of onset of surface melt over sea ice at each 25 km-resolution pixel.
Source: Scanning Multichannel Microwave Radiometer (SMMR), ; Special Sensor Microwave/Imager (SSM/I),
Algorithm: Uses horizontally polarized 19 and 37 GHz frequencies (Anderson, 1997; Drobot and Anderson, 2001a).
More information on algorithm and data access:
Standard product spans and is available via ftp from the above web site. A preliminary update for was produced specially for this study.
An update of the standard product, including algorithm refinements, is under development.
The September ice concentration contours follow the melt onset contours fairly closely (Figure 2) because the length of the melt season is a key component in how much sea ice melts during the summer. However, dynamics also plays an important role as changes in circulation can advect ice out of or into regions. Dynamics can also change the thickness distribution of ice pack (through convergence and divergence) and alter the impact of melt.
Figure 4. September ice extent and the correlation between melt onset and extent. The extent has been scaled by 10 compared the values elsewhere to plot both parameters on the same scale (i.e., ice extent is in 107 km2 instead of 106 km2).
Figure 6. Melt onset anomaly (in days) for (left to right) low, normal, and high September ice extents. Red (negative) values indicate earlier than normal melt onset; blue (positive) values indicate later than normal melt onset.
Normal
High
Low
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Summary
References
Average September Sea Ice Concentration
Average Julian Day of Melt Onset
The melt onset patterns presented here generally correspond with the minimum ice cover patterns of the following summer. However, the correlation between the two is notably weaker during anomalously low extents compared to normal or high extents. This suggests that dynamics play a key role during extreme minimum events. There is a distinct difference between the low ice years of and the extreme summer minimums of the past three years. While a positive winter AO index has been linked to negative extent anomalies during (Rigor et al., 2002), the winter AO has been largely negative the past three winters, which indicates other mechanisms are important factors in the minimums of the past three years. Serreze et al. (2003) suggest that advection of warm air from the south during the spring followed by low SLP and high temperatures are key factors. The earlier melt for shown here corroborates this hypothesis.
Anderson, M Determination of a melt onset date for Arctic sea ice regions using passive microwave data. Annals of Glaciology, 25,
Drobot, S.D., and M.R. Anderson, 2001a. An improved method for determining snowmelt onset dates over Arctic sea ice using scanning multichannel microwave radiometer and Special Sensor Microwave/Imager data, J. Geophys. Res., 106, 24,033-24,050.
Drobot, S. and M. Anderson. 2001b. Snow melt onset over arctic sea ice from SMMR and SSM/I brightness temperatures. Boulder, CO, USA: National Snow and Ice Data Center. Digital media.
Rigor, I.G., J.M. Wallace, and R.L. Colony, Response of sea ice to the Arctic Oscillation, J. Climate, 15,
Serreze, M.C., J. Maslanik, T.A. Scambos, F. Fetterer, J. Stroeve, K. Knowles, C. Fowler, S. Drobot, R.G. Barry, and T.M. Haran, A record minimum Arctic sea ice extent and area in 2002, Geophys. Res. Lett., 30(3), 1110, doi: /2002GL
Thanks to Jeff Smith, NSIDC, for processing of melt onset data. 20 40 60 80
100 %
220
Figure 2. Comparison of September sea ice concentration (left) with melt onset (right). Concentration is in percent of ice covered area (0-100). Melt onset is defined by the Julian day of year that melt is first detected (60 = 1 March; 220 = 8 August). 60