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Climate and weather

Ocean Navigator
In a gross sense, the Southern Hemisphere cyclone season dictated our ocean crossing. In Australia, we would be sailing the Coral Sea and the Gulf of Carpentaria during cyclone season to be in Darwin and ready to leave in June. We felt that the thorough forecasting coverage of the coastal areas by the Australia Bureau of Meteorology mitigated any risk from a late-season cyclone: With adequate warning there were ample marinas along this route in which to take refuge. On the other hand, once out of Darwin, we would be in open ocean and would take no risks with cyclones there. We would leave Darwin no earlier than June 1.

A rainbow in the Chagos Islands.
   Image Credit: Jeff Williams

The weather of the southern Indian Ocean is dominated by two major synoptic features that migrate north and south with the seasons: the Indian Ocean high and the Intertropical Convergence Zone (ITCZ). In the southern winter — the noncyclone season — the high generally moves north to about 30° S, and the ITCZ lies along the equator. The resultant pressure field sets up southeast trade winds across the whole of the tropical southern Indian Ocean. These winds get quite boisterous, especially in July and August, or any other time that the high is extremely strong. In 2003, there were several weeklong periods where the high was more than 1,040 millibars and the trades steady at 25 knots or more.

The high is regularly punctuated by low-pressure systems and cold fronts that circulate in the Southern Ocean. The effects of the cold fronts can be felt up to about 20° S, certainly impacting voyagers on the southern route through the Mascarene Islands. Farther north in the tropics, the cold fronts are generally not felt other than as a disturbance to the Indian Ocean high, altering wind strength and varying the wind direction from ESE to ENE.
 

During the same period, the ITCZ lies generally within 5° of the equator, its intensity varying in an apparently whimsical manner, sending tongues of moisture and unsettled weather to probe south as far as 7° or 8° S. Thick overcast, squalls, torrential rains, lightning and westerly winds can all accompany these vast disturbed areas.

In the southern summer, the ITCZ drifts south and broadens as the Indian Ocean high also moves farther south and generally weakens. As the pressure gradient eases, the trades slacken, and a northwest monsoon sets up in the lower latitudes. Of course, this is also cyclone breeding season and typically not the time people cruise this area. (It is a good time to make easting in the northern Indian Ocean however.)

The southern summer, however, is the time to sail around South Africa. Cold fronts are weaker, though still frequent, and lighter trade winds prevail. Intense thunderstorms and coastal lows also play into the summer climate here, but given the luxury of time, one can find ample, short weather windows for port-to-port hops.

The high is regularly punctuated by low-pressure systems and cold fronts that circulate in the Southern Ocean. The effects of the cold fronts can be felt up to about 20� S, certainly impacting voyagers on the southern route through the Mascarene Islands. Farther north in the tropics, the cold fronts are generally not felt other than as a disturbance to the Indian Ocean high, altering wind strength and varying the wind direction from ESE to ENE.

During the same period, the ITCZ lies generally within 5� of the equator, its intensity varying in an apparently whimsical manner, sending tongues of moisture and unsettled weather to probe south as far as 7� or 8� S. Thick overcast, squalls, torrential rains, lightning and westerly winds can all accompany these vast disturbed areas.

In the southern summer, the ITCZ drifts south and broadens as the Indian Ocean high also moves farther south and generally weakens. As the pressure gradient eases, the trades slacken, and a northwest monsoon sets up in the lower latitudes. Of course, this is also cyclone breeding season and typically not the time people cruise this area. (It is a good time to make easting in the northern Indian Ocean however.)

The southern summer, however, is the time to sail around South Africa. Cold fronts are weaker, though still frequent, and lighter trade winds prevail. Intense thunderstorms and coastal lows also play into the summer climate here, but given the luxury of time, one can find ample, short weather windows for port-to-port hops.

By Ocean Navigator