KhanRaphael_AGUPoster_2015
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This study examines the influence of the zonal wave three (ZW3) atmospheric pattern on Antarctic sea ice concentrations from 1979-2009. The authors find: 1) Sea ice concentrations were positively correlated with the ZW3 index during advance season (March-August) in the Ross and Weddell Seas and off Amery ice shelf, where the ZW3's cold southerly flow is found. 2) Sea ice concentrations were negatively correlated with the ZW3 index in the Amundsen-Bellingshausen Seas, where the ZW3's warm northerly flow is found. 3) Regression analysis showed sea ice concentrations significantly depended on the ZW3 in parts
KhanRaphael_AGUPoster_2015
- 1. Abstract Summary 2 Previous works have looked at the in鍖uence of key atmospheric circulation patterns on sea ice in the Antarctic in terms of the atmospheres seasonal cycle. This study examines the in鍖uence of one of these atmospheric patterns, the zonal wave three (ZW3), in terms of the sea ices seasons from 1979足2009 in order to better understand the response of the sea ice. An index to represent the amplitude of the ZW3 was calculated using zonal anomalies of 850 hPa geopotential heights taken from the ERA足Interim data set. Sea ice concentrations (SIC), taken from the Hadley Center sea ice and sea surface temperature data set, were found to be signi鍖cantly positively correlated with the ZW3 index during the ice advance season (March to August) in the Ross and Weddell Seas and off the Amery ice shelf. These regions align with where cold, southerly 鍖ow associated with the ZW3 are found. In the Amundsen足Bellingshausen Seas region, SIC was found to be negatively correlated with the ZW3 index, which coincides with where the warm, northerly 鍖ow of the wave is found in this region. Regression analysis showed SIC to be signi鍖cantly dependent upon the ZW3 in parts of the Ross Sea, the ice edge in the Amundsen足 Bellingshausen Seas and off the Amery ice shelf during ice advance season. The results suggest that the ZW3 plays a role in the occurrence of the observed sea ice trends in the Ross Sea, Amundsen足Bellingshausen Seas (ABS), Weddell Sea and off the Amery ice shelf regions during the ice advance season, the critical period for sea ice growth. The results also demonstrate that re足examining the in鍖uence of relevant atmospheric patterns on sea ice in terms of the ices seasonal cycles could allow 鍖rmer connections to be established between sea ice trends and atmospheric patterns. Figure 1. a) Composite differences in the 850 hPa geopotential height of positive minus negative ZW3 phases over the period 1979-2014. 10m ERA- Interim winds for b) advance and c) retreat seasons averaged over the period 1980-2013. Figure 2. a) Monthly ZW3 index spanning from 1979-2013 representing the amplitude of the ZW3 and b) the ZW3 index averaged over the advance season (Mar-Aug) from 1979-2009 and c) retreat season (Oct-Feb) from 1979-2009. The index was calculated using zonal anomalies at the 850 hPa geopotential height from the ERA-Interim data set. Positive index values indicate a strongly meridional geopotential height 鍖eld and stronger north/south air鍖ow. Negative index values signify strongly zonal geopotential height 鍖eld resulting in reduced north/south air鍖ow. Figure 3. Correlations between zonal anomalies at the 850 hPa geopotential height and sea ice extent (SIE) during advance season for the a) Ross/Amundsen b) ABS and c) Weddell Sea sectors. SIE in the Ross/Amundsen sea sector is negatively correlated with zonal anomalies over the ABS and Ross/Amundsen sea regions, suggesting clockwise 鍖ow around a low in this region contributing to an increase in SIE in the Ross/Amundsen sector. This also corresponds to a trough of the ZW3 found in this region. Positive correlation with anomalies were found over the Weddell Sea, corresponding to a ridge of the ZW3, and parts of East Antarctic. The positive correlation between SIE in the ABS sector with zonal anomalies over the Ross/Amundsen region imply a clockwise 鍖ow around a low pressure over the ABS-Ross/ Amundsen area that is bringing warmer air polewards towards the ABS region through its eastern 鍖ank, contributing to a decrease in SIE in the ABS sector. Negative correlation between SIE in the Weddell Sea with zonal anomalies over the southern Atlantic region suggests clockwise 鍖ow over the region bringing equatorward 鍖ow and contributing to an increase in SIE in the Weddell Sea sector. Correlations are signi鍖cant at values of 賊 0.4 and higher. Figure 4. Correlations between the ZW3 and SIC from 1979-2009 with de-trended data. Coef鍖cients of 賊 0.35 or higher are considered signi鍖cant. During the advance season, signi鍖cant positive correlations were found in parts of the Ross Sea, NW regions of the Weddell Sea and off the Amery ice shelf. These regions align with where equatorward 鍖ows are found, which contribute to increasing SIC in these regions. Signi鍖cant negative correlations found in the ABS region near the northern edge and along the west Antarctic Peninsula are where poleward 鍖ows are found that bring warmer air and reduce SIC. Correlations during the retreat seasons also show a wave three pattern. Some differences between the two seasons include a larger area of signi鍖cant correlation off the Amery ice shelf region and an area of signi鍖cant negative correlation appearing in the western Ross Sea region during the retreat season. Overall, the correlations show presence of the ZW3s in鍖uence on SIC. The results of this study suggest that ZW3 plays a role in the occurrence of the observed sea ice trends in the Ross Sea, ABS and off the Amery ice shelf regions as well as parts of the Weddell Sea during the ice advance season, the critical period for sea ice growth. Statistical analyses showed a signi鍖cant dependence of SIC on ZW3 in the key regions associated with the wave. Sea ice extent reduced (increased) in the ABS region when ZW3 strengthened (weakened) since a poleward arm of the wave is found here during the advance period. Sea ice increased (reduced) in the Ross Sea, off the Amery ice shelf and parts of the Weddell Sea when the ZW3 strengthened (weakened). These regions fall under the equatorward arms of ZW3. The results demonstrate that the ZW3 plays a role in in鍖uencing sea ice during the advance season in the Ross Sea, ABS as well as parts of the Weddell Sea and may help explain the recent sea ice trends, particularly in the Ross and ABS regions where the largest SIC trends have been observed in the past three decades. This study, however, did not quantify the relative contribution of the ZW3 on in鍖uencing SIC/SIE nor did it take into account the spatial shift of ZW3 from its winter to spring location. Creation of an index for the ZW3 with a phase shift component also deserves important consideration in future works. Finally, the in鍖uence of the ZW3, a predominantly winter mode, on sea ice during the retreat season also deserves further exploration. The In'luence of the Zonal Wave Three on Antarctic Sea Ice during Ice Advance Season Hana Khan and Marilyn Raphael UCLA, Department of Geography, Los Angeles, CA. Raphael, M.N., & Hobbs, W. (2014). The in鍖uence of the largescale atmospheric circulation on Antarctic sea ice during ice advance and retreat seasons. Geophysical Research Letters, 41, 50375045. doi:10.1002/2014GL060365. a) c) The authors would like to thank Matt Zebrowski of the UCLA Geography Department for his assistance with this poster. a) b) c) References Acknowledgements a) b) c) b)