Utilización de barnices de clorhexidina en el control de la caries de corona

The average cluster patterns for all extreme wind types are shown in Figure 4-21. Generally, sets of 2 to 4 clusters returned stable weather patterns. Four clusters were found for extreme SW winds with each showing varying positions of southern ocean troughs and varying strengths of a ridge of high pressure over or north of the North Island. The strong SW winds are always associated with the western sector of the trough, and are seen in clusters 1 through to 4 southeast-ward or over the Chatham Islands or south of NZ. In addition, the consistent location near or southeast of the Chatham Islands correlates with the frequency maxima of Figure 4-17. Furthermore, cluster 3 shows the extreme winds are closest to NZ when the southern low reaches 60°S (i.e. southern ocean is in most northern position).

Three stable patterns were found associated with extreme southerly winds and are very similar to that of SW winds. They are generated by southern ocean troughs over NZ or the Chatham Islands that have south to south-southwest airstreams on their western flanks (see cluster one and two, Figure 4.20) and large Tasman Sea anticyclones with no ridges. These patterns indicate more southerly flows affect NZ when the Tasman Sea anticyclone contains no ridges. Consequently, the area of strong southerly winds is over southern NZ and the area immediately eastward being consistent with the maximum in Figure 4-17. Cluster three shows the weakest airflow west of the Chatham Islands (Figure 4-21). Two stable patterns were returned for extreme southeast winds and are shown to be generated from a ridge of high pressure over the South Island extending from the Tasman Sea or from a subtropical trough of low pressure east of NZ combining with an anticyclone south of the Chatham Islands (Figure 4-21). Thus, cluster 1 displays SE winds over NZ while cluster two shows SE winds over and east of the Chatham Islands.

Extreme easterly winds were related to three dominant clusters. Two patterns (cluster 1 and 3) display easterly flows east and north of the North Island from the northern sectors of anticyclones that occupy the NZ and Chatham Islands region. The regions showing easterly flows are consistent with the maximum in extreme

easterly winds in Figure 4-18. The other cluster shows easterly winds being generated by a cyclone east of the North Island and an associated ridge in the southern ocean. Stable and identical cluster patterns for extreme winds were not found across all five runs from a set of eight clusters down to two clusters. The spatial distribution of extreme northeast winds (Figure 4-18) suggests two separate generation mechanisms are involved given that the two prominent maximum (one north of NZ and the other near 165°W) are spatially distinct and appear unrelated. This situation is not ideal for cluster analysis as the blurring or smoothing of rather different patterns results and stable clusters are hard to achieve.

To test this hypothesis, individual weather maps were generated for all northeast wind events and separated into winds directly affecting NZ (and therefore associated with the maximum near NZ) versus those winds eastward of the Chatham Islands by eyeballing the maps. Cluster analysis was then applied to the ‘NZ maximum of NE winds’ and ‘165°W maximum’. This separation enabled four stable clusters to be found for both the NZ maximum and 165°W maximum (Figure 4-21), and they are indeed generated by different mechanisms. The local maximum in extreme northeast winds over and north of NZ involve a cyclone-anticyclone pair or subtropical trough-anticyclone pair. The anticyclone is either south, east or over the Chatham Islands while the cyclone or trough is north of NZ or in the north Tasman Sea west of Northland. On the other hand, extreme northeast winds near 165°W are associated with subtropical troughs northeast of NZ combining with a ridge of high pressure east of the Chatham Islands (cluster 1 and 3) or southern ocean cyclonic activity colliding with a ridge of high pressure near 165°W (cluster 2 and 4). Furthermore, when a subtropical trough is involved the northeast winds are confined to the area north of 40°S while southern ocean cyclones or troughs generate strong northeast winds south of 40°S.

4.3.4 Discussion

The most frequent extreme winds over the oceans east of NZ are from the southwest and cluster in the high latitudes with maximum activity east of 175°E where they are generated by southern ocean troughs. While maximum activity is well away from eastern NZ, the distribution still shows large numbers of southwest winds over and

east of the lower South Island. Extreme southerly winds also show a high latitude maximum centred near the dateline, however they spread further northward and cover the entire eastern North Island. Southeast, east and northeast extreme winds, in contrast, have maximum activity in the midlatitudes between 30 and 45°S and cluster off the North Island. Both east and northeast winds cluster in two distinct areas, with one immediately north of the North Island, and the other east of 170°W. The centre of activity near the North Island is located such that the eastern North Island will be largely unaffected – the easterly wind maximum is too far northward, and northeast wind maximum is too far westward. Nonetheless, the widespread distribution of the winds shows that extreme east and northeast winds still affect the eastern North Island and will generate coastal storms. And, furthermore, the other centre of activity near 170°W acts as a distant wave generator.

The synoptic situations associated with extreme southwest winds show southern ocean troughs and ridges of high pressure over and north of the North Island (extending from Tasman Sea anticyclones). The strongest pressure gradients off eastern NZ occur when the trough extends from a southern low that is situated near 60°S (i.e. the parent cyclone is in a more northern position). In contrast, extreme southerly winds involve southern ocean troughs that extend further northward due to the absence of ridges of high pressure over the North Island. In other words, the anticyclone in the Tasman Sea appears to determine whether the trough has a south or southwesterly flow – if the Tasman Sea anticyclone has an eastward branching ridge, southwest flows will develop; if there is no ridge, southerly flows develop. These synoptic patterns are identical to the Trough/Ridge coastal storm type. Extreme southeast winds affecting the eastern North Island are created by a Tasman Sea anticyclone that is aligned on a NW-SE axis and a trough over or north of the North Island.

The synoptic patterns associated with extreme winds from the easterly quadrant involve large anticyclones to the east of NZ with troughs directly northward (easterly winds) or troughs and low pressure systems to the west or northwest of the North Island. These patterns strongly resemble the coastal storm type’s cyclone- anticyclone pair, subtropical low and Tasman Sea Low.

Wind Cluster 1 Cluster 2 Cluster 3 Cluster 4 SW

S

Figure 4-21

The leading cluster patterns for extreme wind events from the SW, S, SE, E and NE off eastern NZ.

SE 31% 36% 18% 15% 42% 38% 21% 37% 63%

E NE – NZ Maximum NE- 165°W maximum

Figure 4-21 (cont.) The leading cluster patterns for extreme wind events from the SW, S, SE, E and NE off eastern NZ.

43% _31% 26%

22% 32%

34% _12%

Rough estimates of the annual average frequencies of these extreme winds have been made. The area off the eastern South Island experiences around 15 extreme southwest wind events per year, greatly exceeding that of the eastern North Island (5 per year, on average). This greater frequency off the South Island is attributed to its close proximity to the source region. Extreme southerly wind events off the eastern North and South Islands, however, have the same annual average of 3 events, while those winds from the easterly quarter are more likely to affect the eastern North Island and occur only once a year.