The Southern Hemisphere winter of 2007

Transcription

1 The Southern Hemisphere winter of 2007 The Southern Hemisphere winter of 2007 was marked by a number of significant weather events: 11m swells were reported to hit Reunion Island in late May; huge waves were reported to hit the Maldives in late May; the MV Pasha Bulker ran aground at Newcastle, Australia, after 70 knot winds developed across the Tasman Sea in early June; as many as ten vessels were sheltered along the South African coast as very heavy conditions battered the coast in late July. These significant events, as well as a few others, indicate that the winter of 2007 was particularly bad for ships at sea in the Southern Hemisphere. Figure 1 shows mean sea level pressure anomaly for June, July and August the heart of the winter season in the Southern Hemisphere. This graph indicates that sea level pressures were lower than average across much of the South Atlantic and South Indian oceans, south of South Africa, and in the Tasman Sea. While some ships do sail through the South Pacific and around Cape Horn, the majority of shipping traffic in the Southern Hemisphere is through the South Atlantic, around the Cape of Good Hope, and through the South Indian Ocean. Therefore, the low pressure anomalies across the South Atlantic, south of South Africa and through the South Indian Ocean are of particular importance. Figures 2 and 3, show the sea level pressure composite mean for the months June, July and August for 2007 (figure 2) and for the years , which we can consider to be the 38 year climatological mean (figure 3). Comparing the two figures, several significant differences are shown between the winter of 2007 and the climatological mean. In 2007, Subtropical high pressure ridge was shifted northward through South Atlantic, with a troughing feature present along the Argentina-Brazil coast. Subtropical high pressure ridge over the South Indian Ocean was weaker and shifted northward Strong high pressure was situated over the Bight of Australia Troughing feature was present over the Tasman Sea and the western/central South Pacific Subtropical high pressure ridge over the eastern South Pacific was significantly stronger than average, and shifted southward The high pressure shifted further north over the South Atlantic, and a weaker subtropical ridge over the South Indian Ocean allowed the main low track across the Southern Ocean to shift northward from the climatological mean. This allowed low pressure systems to dip further north into the normal shipping lanes through these basins. Since low pressure systems were able to move further north, swells associated with these lows were also moved further north. This resulted in more frequent, higher swell conditions north of 40S, in the main shipping lanes of these two basins. Figure 4 shows the number of days 6m or greater wave conditions were present in the various basins of the Southern Hemisphere over the past five years. These data corroborate what is seen in the synoptic analysis: the South Atlantic Ocean, Cape of Good Hope and the South Indian Ocean saw above average numbers of days with 6m or greater seas. Figure 5 shows the number of days with low pressure below 1000mb in each of the basins of the Southern Hemisphere. This figure is consistent with figure 4 and the synoptic analysis: the South Atlantic, South Indian and particularly South Africa

2 experienced higher than average amount of significant low activity through the season. The weather during July, the peak of the winter season, was particularly severe in these regions. Figure 6 shows the sea level pressure composite anomaly from July of This figure shows much lower than average anomalies across the South Atlantic, South Africa and the South Indian Ocean. During the month of July, Weathernews was forced (due to safety considerations) to shelter or slow vessels along the South African coast, as well as along the southwestern corner of Australia, due to very heavy conditions present in these regions during this period. These figures also indicate low pressure anomalies across the Tasman Sea/New Zealand regions during the winter of 2007, with above average number of days with significant low pressure present. Combined with a high pressure anomaly over the Bight of Australia, the pressure gradient between these two features caused several incidences of gale-to-storm force conditions along the eastern Australia coast this past winter, including the event that grounded the MV Pasha Bulker at Newcastle in early June. Other features to note in these figures are (as mentioned above) anomalously high pressure situated over the Bight of Australia and over the eastern South Pacific. These features caused lower than average number of days with 6m or greater waves and lower than average number of days with significant low pressure across the Bight and near Cape Horn. In particular, the Bight of Australia was under a high pressure ridge for much of the season, which resulted in calmer conditions for ships sailing across the Bight. The strong high pressure anomaly over the eastern South Pacific caused calmerthan-average conditions over Cape Horn, but shifted the cyclogenesis area typically situated over southern Argentina east into the western south Atlantic. This caused an increase in the number of low pressure systems developing across the western and central South Atlantic (note the troughing feature in Figure 1 in this region), further impacting shipping traffic to and from Brazil and Argentina. The synoptic pattern across the Southern Hemisphere this past winter set up such that the major shipping lanes across the South Atlantic, South Indian and around the Cape of Good Hope experienced heavier than average weather conditions. The main low track was shifted northward, allowing more and stronger lows to track through the main shipping lanes of these basins. This also caused significant swell conditions to track further north through these basins. It was a challenging season for routeing ships, particularly around the Cape of Good Hope.

3 Figure 1. Sea Level Pressure (mb) composite anomaly for June August 2007, as compared to climatogical mean From NCAR/NCEP global Reanalysis Data provided by Earth System Research Labratory.

4 Figure 2. Composite Mean Sea Level Pressure in millibars for June August From NCAR/NCEP global Reanalysis Data provided by Earth System Research Labratory

5 Figure 3. Climatological Composite Mean Sea Level Pressure in millibars for June August From NCAR/NCEP global Reanalysis Data provided by Earth System Research Labratory

6 Number of Days with 6m+ Waves Number of days South Atlantic South Africa South Indian Bight of Aus. Tasman Sea/NZ South Pacific Cape Horn Year Figure 4. Number of days with 6m or greater wave conditions. For South Atlantic, South Africa, South Indian and Bight of Australia, days were counted for wave conditions 6m or greater present north of 40S. For Tasman Sea, South Pacific, and Cape Horn, days were counted for 6m or greater wave conditions present north of 50S. Wave conditions were determined using WAVAD analyses.

7 Number of days with Low Pressure < 1000mb Number of Days South Atlantic South Africa South Indian Bight of Aus. Tasman Sea/N.Z. South Pacific Cape Horn Year Figure 5. Number of days with low pressure less than 1000mb. For South Atlantic, South Africa, South Indian and Bight of Australia, days were counted for significant low pressure present north of 40S. For Tasman Sea, South Pacific, and Cape Horn, days were counted for significant low pressure present north of 50S.

8 Figure 6. Sea Level Pressure (mb) composite anomaly for July 2007, as compared to climatogical mean From NCAR/NCEP global Reanalysis Data provided by Earth System Research Labratory.