Investigations of probability density distributions and draft statistics were already presented for the data of AWI 207-2, 208-3, 209-3, 210-2, 212-2 and 217-1 byStrass and Fahrbach [1998].
An annual cycle could be found in three statistical quantities - draft mean, draft mode and ice coverage - only in the central Weddell gyre at AWI-208. In the other regions the annual cycle was most evident in the ice coverage [Strass and Fahrbach, 1998]. It was also found that the summer minima in the three statistical quantities occur simultaneously in February, while the time of occurrence of the winter maxima is different: ice coverage in June/July, draft mode in September and draft mean in September/October. Spatially, the ice draft mean takes its maximum values at the boundaries of the Weddell gyre (AWI-207 and AWI-212), while the minimum occurs in the central gyre (AWI-209). For the ice draft mode the situation is reversed:
maximum values in the central gyre and minima in the boundary regions [Strass and Fahrbach, 1998]. The authors also estimated that almost 75% of the thin ice at AWI-212 is transformed to thicker ice at AWI-207 during circulation through the Weddell gyre. Furthermore, they demonstrated that the statistical ice draft distributions allow for a discrimination between level ice and deformed ice.
As the ice draft variation is described in detail in the previous section, only selected data with significant variations are discussed here. The detailed treatment of the ice draft statistics may give some additional information for the correlation analysis in the next chapter. Therefore, the discussion is mainly restricted to the ULS positions with the highest amount of data. Contrary to Strass and Fahrbach [1998] the statistical distributions are considered here on a monthly scale to allow for a more detailed investigation of thermodynamic growth and ice deformation.
Besides the distributions, the considered quantities are: monthly mean, mode, median and 95%
percentile.
The plot of AWI 207-2 (Fig. 6.5) shows that most of the ice drafts remained below 11 m in this record. The ice draft median mainly follows the pattern of the mean ice draft. In 1991 the most frequently measured ice class (mode) was 0-10 cm (except for May). A secondary mode formed in March at around 1.2 m and could be observed until August in the range 1.2-1.5 m, except for June, where it dropped to 1 m. In 1992 the main mode varied between 0-10 cm and 1.4-1.5 m. While the mode increased, the mean draft decreased, and vice versa. Compared to 1991 the second mode at 1.5 m became more dominant and increased to 1.7 m in August. The monthly draft distributions of 1992 were less skewed than in the year before.
In 1996 and 1997 most of the measured ice drafts remained below 8 m. The bimodal shape with modes at 0-10 cm and around 1-1.5 m was still present. Compared to 1996 the monthly distributions in 1997 were less skewed. Whereas in 1996 small ice drafts (0-10 cm) were most abundant throughout the year, in July and August 1997 the mode around 1.5 m dominated.
The period 2005-2007 shows a completely different picture, reflecting significant changes in the ice regime since 1997. Most of the ice drafts remained below 4 m. The bimodal shape was not as consistent as in the years 1991-1997, and the monthly ice draft distributions were more variable. The main mode was still largely at 0-10 cm and increased only between May and August.
The two data with the clearest annual cycles (Fig. 6.6) display thermodynamic ice growth in the central Weddell gyre, where the drift speeds are lower than in the boundary regions [Fahrbach, 1994]. At AWI-208 the ice draft distributions took an almost Gaussian shape in May 1993. As the mode increased until October, the monthly distributions widened and became mode skewed
Fig. 6.5: Monthly mean, mode, median and 95% percentile of ice draft and monthly draft distributions of AWI 207-2, 207-4 and 207-6. There were no valid data in August 2005. The mean value is theeffective mean, i.e. the mean ice draft calculated by including data cycles classified as open water. Also the mode (thick blue line) was calculated for data including ice and water. All histograms have been scaled by the maximum bar of the respective month to ensure equal distance between the time steps in the plot. The bin width of 10 cm was selected according to the measurement accuracy of 5-12 cm.
towards larger ice drafts. In November the median still continued on its level at around 1 m, whereas the mode dropped to 0-10 cm, indicating that open water and thin ice became dominant. Between November and January the second (thermodynamic) mode dropped rapidly together with the median. In the following year significant ice growth started one month earlier.
Fig. 6.6: The same as in figure 6.5, but for AWI 208-3, AWI 209-3, AWI 229-1 and AWI 229-2.
At AWI-209 the three quantities mean, median and mode almost coincide over the entire record.
The distributions were almost Gaussian between July and October. The maximum mode be-tween August and October remained bebe-tween 70 cm and 80 cm, displaying the maximum of thermodynamic ice growth 2.
At AWI 229-1 (Fig. 6.6) the median and the mode largely coincided, except for times when the class 0-10 cm was dominant. The larger mean draft indicates the degree of skewness of the distributions. Thus, the deviation of the mean from the median and the mode can be assumed to display the degree of ice deformation. Depending on the year, the mode reached maximum values of 60-70 cm and 70-80 cm, which displays changing thermodynamic conditions.
Fig. 6.7: The same as in figure 6.5, but for AWI 229-4, AWI 229-5 and AWI 229-6.
2Note that the mode is also determined by the snowload on top of the ice
At AWI 229-2 mean, mode and median took the same values, which indicates a low degree of ice deformation and a dominating influence of thermodynamics. The draft distributions of AWI 229-4 and AWI 229-5 (Fig. 6.7) are very similar to AWI 229-1. AWI 229-6 indicates a low degree of ice deformation, i.e. almost Gaussian shaped distributions in winter, which were only slightly skewed towards larger drafts.
The plot of AWI 231-1 (Fig. 6.8) shows slightly higher ice deformation in 1997 compared to 1996. In 1998 the monthly distributions were almost Gaussian between July and October, with the respective medians, modes and means almost coinciding. In November the ice started disintegrating, and thin ice classes and open water became dominant.
Fig. 6.8: The same as in figure 6.5, but for AWI 231-1 and AWI 231-2.
At AWI 231-4 (Fig. 6.9) the monthly distributions were almost symmetric and displayed an only weak influence of ice deformation in 2001. One year later, mean, mode and median did not differ significantly from each other until September. In contrast to 2001, the distributions in August and September 2002 became flatter from zero ice draft to the draft mode. Mean, mode and median started deviating from each other in August. The same applies to AWI 231-6. At AWI 231-7 the deviation of median and mode in June and November indicates dominating open water and thin ice. The positive deviation of the mean from the median between June and October indicates deformed ice. However, as discussed in the chapter about data quality, the record of AWI 231-7 is affected by biases resulting from high ice concentration. The distributions of winter ice drafts in 2006 corrected by the sound model are shown in the inset. Mainly the
distributions in July and October shift towards higher values, which corresponds to thicker ice and low ice concentration.
Fig. 6.9: The same as in figure 6.5, but for AWI 231-4, AWI 231-6 and AWI 231-7. The inset shows data corrected by the sound model.
At AWI 232-1 (Fig. 6.10) the draft mode reached a maximum value of 0.9 m. Strongly skewed monthly distributions and deviating statistical quantities indicate a high degree of ice deformation. This changes at AWI 232-4. The record shows a monotonic growth of mean, mode and median until September. The mode reached maximum values of only 0.6 m in August/September. In 2000 it increased to almost 1 m in September, which indicates colder temperatures in that year. In contrast to 1996, most of the ice drafts did not exceed 2 m.
AWI 232-5 shows monotonic increase of the three main statistical quantities until August in
2001 and until July in 2002. After that, the rising mean indicates the growing influence of ice dynamics. The August mode was comparable in both years. However, after August the ice deformation seems to have played a larger role in the first year of AWI 232-5.
Fig. 6.10: The same as in figure 6.5, but for AWI 232-1, AWI 232-4 and AWI 232-5.
AWI 232-6 (Fig. 6.11) displays a high degree of ice deformation. A monotonic increase of the draft mode is hardly detectable. Most of the ice drafts remained below 6-7 m. Mean, mode and median deviated by up to 2 m from each other. The modes of 2003 and 2004 were comparable.
Less deformation occurred in 2004. In 2006 and 2007 ice drafts of up to 10 m were measured.
The modes never exceeded 0.7 m, except for October 2007. However, the thermodynamic modes were still visible around 1 m between August and November 2007. It seems that the thicker
ice in 2007 was due to a combination of both thermodynamics and dynamics.
Fig. 6.11: The same as in figure 6.5, but for AWI 232-6 and AWI 232-8.
Averaged statistical values, such as monthly mean ice draft and monthly draft mode, are shown below (Table VII & Table VIII).
Table VII: Monthly mean ice drafts [m]
The monthly averages were calculated as effective means from all available draft measurements. The respective lower values (±) are
standard errors of the means.
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
207-2 1.32 0.87 1.85 2.32 2.79 2.44 3.4 3.39 3.2 3.4 2.07 1.57
212-2 1.86 1.33 0.94 0.74 1.58 2.84 3.78 1.3 3.29 1.29 2.93 3.07
±0.02 ±0.02 ±0.02 ±0.01 ±0.02 ±0.02 ±0.03 ±0.02 ±0.03 ±0.02 ±0.03 ±0.03
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
232-1 0.22 - - 0.71 1.13 1.47 1.47 2 2.43 2.34 1.47 1.32
±0.01 ±- ±- ±0.03 ±0.03 ±0.03 ±0.03 ±0.04 ±0.06 ±0.05 ±0.03 ±0.03
232-4 - - - 0.19 0.47 0.56 0.63 0.85 0.94 0.99 1.18 1.09
±- ±- ±- ±0.00 ±0.00 ±0.00 ±0.01 ±0.01 ±0.01 ±0.01 ±0.01 ±0.01
232-5 0.4 - 0.01 0.19 0.5 0.55 0.73 0.88 1.21 1.11 1.28 1.13
±0.01 ±- ±0.00 ±0.00 ±0.00 ±0.00 ±0.01 ±0.01 ±0.01 ±0.01 ±0.01 ±0.01
232-6 0.09 0.04 - 0.21 0.64 1.52 1.83 2.18 1.87 1.42 1.6 0.52
±0.00 ±0.00 ±- ±0.00 ±0.01 ±0.01 ±0.01 ±0.02 ±0.02 ±0.01 ±0.01 ±0.01
232-8 0.77 - - 0.54 1.41 1.97 2.22 2.6 2.48 3.1 2.06 1.56
±0.01 ±- ±- ±0.00 ±0.01 ±0.01 ±0.01 ±0.01 ±0.01 ±0.02 ±0.01 ±0.01
233-2 0.81 0.22 - 0.55 1.21 1.64 2.09 2.46 2.51 1.89 2.43 0.54
±0.01 ±0.01 ±- ±0.01 ±0.01 ±0.02 ±0.02 ±0.02 ±0.02 ±0.02 ±0.02 ±0.01
233-6 1.95 1.13 1.01 1.68 1.94 2.06 2.07 3.15 2.24 1.52 2.75 2.72
±0.01 ±0.01 ±0.01 ±0.01 ±0.01 ±0.01 ±0.02 ±0.02 ±0.02 ±0.01 ±0.02 ±0.02
Table VIII: Monthly ice draft modes [m]
All data cycles of open water were removed before the calculation to better capture the thermodynamic modes.
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
207-2 0.1 0.1 0.1 0.1 1.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
207-4 0.1 - - 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
207-6 0.1 0.1 0.1 0.1 0.1 0.4 0.1 0.1 0.1 0.1 0.1 0.1
208-3 0.1 - 0.1 0.1 0.5 0.6 0.6 0.6 0.7 0.9 0.1 0.1
209-3 0.1 - - 0.1 0.5 0.5 0.7 0.8 0.7 0.8 0.9 0.1
210-2 0.1 - 0.1 0.1 0.4 0.7 0.8 0.7 0.8 0.7 0.1 0.1
212-2 0.1 0.1 0.1 0.1 0.3 0.1 0.1 0.3 0.3 0.2 0.1 0.1
217-1 0.1 0.1 0.1 0.3 0.4 0.6 0.9 0.1 0.1 0.1 0.1 0.1
227-4 - - - - - 0.1 0.1 0.5 0.1 0.7 0.1 0.1
227-6 - - - - - 0.1 0.1 0.1 0.1 0.1 0.1
-229-1 - - - - - 0.1 0.5 0.5 0.4 0.5 0.1 0.1
229-2 - - - - - 0.1 0.3 0.5 0.6 0.7 0.8 0.1
229-3 - - - - 0.1 0.1 0.5 0.4 - - -
-229-4 0.1 - - - - 0.1 0.6 0.4 0.5 0.6 0.6 0.1
229-5 - - - - 0.1 0.3 0.4 0.4 0.4 0.1 0.1 0.1
229-6 - - - - - - 0.5 0.5 0.5 0.3 0.1 0.1
230-2 - - - - 0.1 0.4 - - - - -
-230-3 0.1 - - - 0.1 0.1 0.4 0.5 0.5 0.5 0.1 0.1
231-1 - - - - 0.1 0.6 0.5 0.4 0.5 0.4 0.1 0.1
231-2 - - - - - 0.1 0.3 0.4 0.4 0.5 0.1 0.1
231-3 - - - - 0.1 0.5 - - - - -
-231-4 0.1 - - - 0.1 0.1 0.4 0.4 0.5 0.5 0.1 0.1
231-6 - - - - 0.1 0.4 0.4 0.4 0.1 0.1 0.1 0.1
231-7 0.1 - - - 0.1 0.1 0.5 0.5 0.5 0.5 0.5 0.1
232-1 0.1 - - 0.1 0.1 0.6 0.5 0.4 0.9 0.7 0.1 0.1
232-4 - - - 0.1 0.4 0.5 0.4 0.5 0.7 0.1 0.9 0.9
232-5 0.1 0.1 0.1 0.1 0.5 0.4 0.6 0.5 0.5 0.1 0.1 0.1
232-6 0.1 0.1 - 0.1 0.1 0.4 0.4 0.4 0.1 0.4 0.3 0.1
232-8 0.1 - - 0.1 0.4 0.4 0.3 0.6 0.6 0.1 0.1 0.1
233-2 0.1 0.1 - 0.1 0.1 0.1 0.1 0.1 0.7 0.1 0.1 0.1
233-6 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1