3.1. Trend Analysis
[9] Table 1 presents the trends in wind over the period 1991 to 2011, in order to afford a direct comparison to the results presented by Young et al. [2011] The trend was calculated using a simple linear regression on the monthly mean values of u and v wind components, as well as the wind speed, after the seasonal cycle was removed. Also shown in Table 1is the correlation coefficient between the monthly mean values of wind and the monthly SAM index, and the correlation coefficient between the monthly mean surface wind and monthly mean upper level winds at 850 hPa, 700 hPa and 500 hPa. Again, the seasonal cycle in the wind and SAM index were removed prior to calculating the statistics. The twotailed pvalue describing the significance of the trends is also shown. For this study, a pvalue of less than 1 × 10^{−1}, which indicates significance to the 10% level, will be considered a significant trend, and a pvalue greater than this will be considered insignificant. All the correlation coefficients shown throughTables 1–4 are significant.
Table 1. Trends for the uWind,vWind and Wind Speed for Macquarie Island From 1991–2011, Along With Significance Level Given by the TwoTailed pValue^{a}  Trend (cm/s/year)  pValue  Correlation With SAM  Correlation With Surface 


uWind 
Surface  0.528  7.63 × 10^{−1}  0.435  1 
850 hPa  4.597  1.08 × 10^{−1}  0.521  0.862 
700 hPa  6.099  5.50 × 10^{−2}  0.510  0.841 
500 hPa  7.649  6.29 × 10^{−2}  0.460  0.796 

vWind 
Surface  −3.825  1.62 × 10^{−2}  −0.396  1 
850 hPa  −0.560  8.08 × 10^{−1}  −0.239  0.672 
700 hPa  −0.658  8.10 × 10^{−1}  −0.225  0.638 
500 hPa  −1.340  7.06 × 10^{−1}  −0.232  0.591 

Wind Speed 
Surface  2.999  1.13 × 10^{−2}  0.447  1 
850 hPa  2.879  1.61 × 10^{−1}  0.500  0.851 
700 hPa  5.148  3.87 × 10^{−2}  0.496  0.796 
500 hPa  7.290  2.95 × 10^{−2}  0.432  0.719 
Table 2. Trends for the uWind,vWind and Wind Speed for ERAInterim From 1991–2010, Along With Significance Level Given by the TwoTailed pValue^{a}  Trend (cm/s/year)  pValue  Correlation With SAM  Correlation With Surface 


uWind 
Surface  0.200  9.22 × 10^{−1}  0.422  1 
850 hPa  4.515  1.42 × 10^{−1}  0.503  0.957 
700 hPa  5.442  1.19 × 10^{−1}  0.474  0.937 
500 hPa  6.244  1.66 × 10^{−1}  0.432  0.897 

vWind 
Surface  −1.957  2.27 × 10^{−1}  −0.375  1 
850 hPa  −1.418  5.56 × 10^{−1}  −0.232  0.935 
700 hPa  −1.166  6.87 × 10^{−1}  −0.224  0.901 
500 hPa  −2.872  4.50 × 10^{−1}  −0.231  0.864 

Wind Speed 
Surface  −0.221  8.54 × 10^{−1}  0.371  1 
850 hPa  3.806  8.96 × 10^{−2}  0.472  0.942 
700 hPa  4.952  6.59 × 10^{−2}  0.452  0.893 
500 hPa  5.163  1.59 × 10^{−1}  0.402  0.838 
Table 3. Same as Table 1 but for Macquarie Island From 1973–2011  Trend (cm/s/year)  pValue  Correlation With SAM  Correlation With Surface 

uWind 
Surface  1.094  4.47 × 10^{−1}  0.469  1 
850 hPa  0.124  9.22 × 10^{−1}  0.505  0.861 
700 hPa  0.969  4.97 × 10^{−1}  0.493  0.838 
500 hPa  2.072  2.62 × 10^{−1}  0.454  0.787 

vWind 
Surface  −5.899  6.81 × 10^{−6}  −0.417  1 
850 hPa  0.399  6.85 × 10^{−1}  −0.163  0.673 
700 hPa  0.141  9.03 × 10^{−1}  −0.147  0.624 
500 hPa  1.067  4.69 × 10^{−1}  −0.153  0.571 

Wind Speed 
Surface  4.259  1.59 × 10^{−5}  0.454  1 
850 hPa  −1.611  7.17 × 10^{−2}  0.433  0.798 
700 hPa  −0.390  7.16 × 10^{−1}  0.448  0.746 
500 hPa  1.452  3.30 × 10^{−1}  0.408  0.672 
Table 4. Same as Table 2 but for ERAInterim From 1979–2010  Trend (cm/s/year)  pValue  Correlation With SAM  Correlation With Surface 

uWind 
Surface  1.528  1.46 × 10^{−1}  0.440  1 
850 hPa  2.951  6.48 × 10^{−2}  0.510  0.964 
700 hPa  3.151  8.38 × 10^{−2}  0.483  0.942 
500 hPa  3.802  1.07 × 10^{−1}  0.441  0.902 

vWind 
Surface  −0.139  8.58 × 10^{−1}  −0.380  1 
850 hPa  0.614  5.92 × 10^{−1}  −0.209  0.928 
700 hPa  0.997  4.68 × 10^{−1}  −0.197  0.889 
500 hPa  1.006  5.77 × 10^{−1}  −0.206  0.844 

Wind Speed 
Surface  0.638  2.54 × 10^{−1}  0.375  1 
850 hPa  2.314  3.99 × 10^{−2}  0.473  0.948 
700 hPa  3.010  2.83 × 10^{−2}  0.456  0.897 
500 hPa  3.722  4.77 × 10^{−2}  0.408  0.843 
[10] For the period 1991 to 2011, positive trends exist for MAC in the u wind, and increase with height. The only significant trends are those at 700 hPa and 500 hPa which are the greatest trends of 6.1 and 7.6 cm/s/year respectively. The trends in the v component are all negative, however the only significant trend is at the surface which is −3.8 cm/s/year. On average, v is negative, so a negative trend in the v component implies a strengthening of the northerlies. Put together, these result in overall positive trends for the wind speed, with the only insignificant trend being at 850 hPa. The surface wind speed shows an increase in magnitude of approximately 3 cm/s/year, with the trends at 700 hPa and 500 hPa increasing to 5.1 and 7.3 cm/s/year respectively.
[11] The correlation coefficient of the winds with SAM indicate positive correlations of between 0.43 and 0.52 for the u component, and weaker negative correlations of between −0.22 and −0.39 for the v component. This yields correlations with SAM of between 0.43 and 0.5 for the wind speed over the 4 levels. Correlation coefficients between the surface wind and upper level wind are all large. For the wind speed, the size of the correlations are between about 0.72 and 0.85, and decrease with height.
[12] These results agree very well with those presented by Young et al. [2011] for the trend in surface wind speed in the area around Macquarie Island, who find a trend of between 2–3 cm/s/year over the past two decades. Young et al. [2011] also note a correlation between SAM and significant wave height of up to 0.4 over large areas of the Southern Ocean.
[13] The same analysis was performed on the ERA data set, with the results shown in Table 2, for the period 1991 to 2010, in order to compare to the MAC data set over a similar time period. In general, ERA reproduces similar trends to MAC, with slightly lower magnitudes, however none of the trends in the u and vwind components are significant. The trend in the surface wind speed for ERA is insignificant with a pvalue of 0.85, hence one can not even infer the sign of the trend. The size of the trends for 850 hPa to 500 hPa wind speed are similar to MAC, however only the 850 hPa and 700 hPa trends are significant. ERA produces similar correlations between the winds and SAM, having similar values foru, v winds, and wind speed to those found in MAC. The correlations between the surface wind and upper level winds are higher in ERA than the ones in MAC, approximately equal to or greater than 0.9, and show the same pattern of decreasing with height.
[14] Both the MAC and ERA data sets extend back earlier than 1991, so the same statistics were calculated again using the full time periods in order to assess the longer term trends in each individual data set. Table 3 presents the trends and correlation coefficients for MAC for the period 1973 to 2011, a total of 39 years. Again, note that the continuous record of surface wind measurements is from 1987 to 2011, and hence represents a shorter time series. The only significant trend in the u and v components is for the surface v wind. Here we find a trend of −5.9 cm/s/year, which again implies the surface wind is becoming more northerly. The wind speed trends at the surface and 850 hPa are significant, and the trend at 850 hPa actually changes sign with the additional 18 years data. The surface trend shows an increase in magnitude and maintains the same sign with the addition of 4 extra years of data, resulting in a surface wind speed trend of 4.2 cm/s/year.
[15] All the correlation coefficients with the extended MAC wind data set and SAM remain between about 0.45 and 0.5 for the u component, and between −0.14 and −0.41 for the v component. This results in little change of the correlation coefficients of wind speed and SAM with the additional MAC data. Similarly, the correlations between surface wind and upper level winds remains high, typically between 0.67 and 0.8 for wind speed.
[16] Table 4 shows the same statistics for the ERA data set from 1979 to 2010. The only significant trends here are in the u wind at 850 hPa and 700 hPa, which show increases of 2.9 and 3.2 cm/s/year respectively. This results in the wind speed trends in ERA increasing from about 0.64 to 3.72 cm/s/year from the surface to 500 hPa, however the surface is insignificant. The correlation coefficients between the winds and SAM, and surface wind and upper level winds change very little with the additional reanalysis data.
3.2. Clustering Analysis
[17] A Kmeans clustering analysis [Anderberg, 1973; Pope et al., 2009] was performed on the MAC data set in order to identify naturally occurring regimes in the wind over Macquarie Island. The variables chosen for the clustering analysis were the u and v wind components at the surface, and 500 hPa, along with the surface pressure, totalling 5 variables. The aim is to determine if there is a shift in meteorological regimes rather than the mean state of the atmosphere.
[18] The initial number of clusters was chosen by employing a series of objective and subjective criteria, similar to those used by Rossow et al. [2005]. The most appropriate number of clusters was found to be K = 4. Only soundings from 1987 onwards were included in the cluster analysis. Figure 1 shows histograms of the variables for the 4 cluster centroids. In all panels, the yaxis represents wind speed in ms^{−1}, or pressure anomaly from the mean in hPa, where the mean pressure is 998.63 hPa.
[19] The first cluster represents 30.3% of the data, and is characterised by nearly average pressure and a strong surface westerly with a small northerly component. The strong westerlies persist aloft, and the vcomponent changes to a relatively large positive value. This represents an anticlockwise rotation in the wind field from the surface to 500 hPa. Cluster 2 represents 24.5% of the data, and is characterised by high pressure and moderately strong winds. There is a moderate north westerly at the surface turning to mostly westerly, with a small northerly component at 500 hPa. The third cluster represents 28.3% of the data, and indicates low surface pressure is associated with this regime. Here there exists persistent and strong north westerlies from the surface to 500 hPa. There appears to be little change in the wind direction with height for the soundings associated with this cluster. The final cluster is also the smallest, representing 16.9% of the MAC data. Again, this cluster is characterised by low pressure, however with weak winds. The wind is mostly southerly at the surface, changing to mostly westerly with a smaller southerly component at 500 hPa.
[20] To elucidate any trends in the occurrence of these clusters, the frequency with which they occur from 1987 was evaluated. The result is shown in Figure 2. For visual clarity, the time series for Cluster 3 has been offset by +0.1. There appears to be no trend in the occurrence of Cluster 1, and there are weak negative trends in Clusters 2 and 4, however these are insignificant with pvalues of 0.18 and 0.16 respectively. The trend in Cluster 3 shows a clear increase in occurrence of 0.19 % per year, which is statistically significant with a pvalue of 0.02. Thus, there appears to be a shift in the meteorological regimes over Macquarie Island, towards strong north westerly winds at the surface and 500 hPa.