By Amanda Delaney, Mark Neiswender and Brian Whitley of Weather Routing, Inc.
Planning a trip to any of the exotic islands in the Indian Ocean would entice the imagination of any mariner. However, if one does not take into account the seasonal monsoons across the Northern Indian Ocean or the tropical cyclone frequency and tracks for this area of the world, a dream vacation can turn into a horrendous trip. That is why for a mariner it is very important to be knowledgeable about the weather, or to receive assistance from government sources or professional marine meteorologists while underway in order to transit on the safest and fastest route possible.
The Red Sea:
The weather conditions across the Red Sea, as well as the Northern Indian Ocean as we will later discuss, alter significantly between the winter and summer. From late November through March, strong cold fronts track into and across the far Northern Red Sea approximately every 3 days. Behind these cold fronts, cold air funnels through the Gulf of Suez southward to approximately 18N as a ridge of high pressure builds across Northeastern Egypt and the Arabian Peninsula. As a result, enhanced north-northwest winds surge across the Gulf of Suez and the Northern Red Sea of approximately Beaufort Force 5-7 and swells will build up to 8-12ft (see Figure 1A).
Behind exceptionally strong cold fronts, winds will increase up to sustained gale force (Beaufort Force 8-9) particularly near the Gulf of Suez and swells will build up to 10-15ft.
During April and May, and again in September through early November, the cold fronts become weaker and only track as far south as the extreme Northern Red Sea about every 4-5 days. The ridge of high pressure that builds over the Eastern Mediterranean Sea is also weaker during the spring and autumn, therefore the arctic air that usually funnels through the Northern Red Sea in the winter is moderated. Hence, the north-northwest wind surges occur less often and are weaker.
From May through early October a thermal trough of low pressure generally develops from Northern Sudan northeastward to the central Arabian Peninsula. This weather feature changes little in strength but does fluctuate its orientation during the summer. For instance, if the thermal trough is across the central Arabian Peninsula and a ridge of high pressure builds over the Eastern Mediterranean Sea, then northwest winds will become enhanced from the Gulf of Suez southward to approximately 20N. These winds are generally in the range of Beaufort Force 4-6 (highest near the Gulf of Suez and lowest near 20N) and swells build to 6-10ft across this region. When the thermal trough shifts farther west over the Red Sea, then the northwest winds ease and become variable of Beaufort Force 4 or less north of 15N (see Figure 1B).
However, south to southeast winds begin to funnel through Bab el Mandeb and surge northward to approximately 15N. Winds can reach up to Beaufort Force 7-8 through Bab el Mandeb, with the highest winds occurring during the late morning and afternoon hours, then become Beaufort Force 5-7 in the Southern Red Sea with swells building up to 10-15ft (highest near the entrance of Bab el Mandeb).
Aside from the wind surges across the Red Sea, winds are usually highest during the late morning and afternoon hours and then lower overnight during the late spring through autumn months. Also outside of surge events, winds are generally onshore by day and offshore by night 2-3 nautical miles from the Arabian and African coasts.
The Northern Indian Ocean:
The monsoonal winds are the weather event that defines the seasons across the Northern Indian Ocean. The northeast monsoon develops during late October and continues through early May before transitioning to the southwest monsoon in late May through early October. Several weather features are responsible for the development and transition of the monsoons.
The equatorial trough climatologically resides from 05N to 10S and from 50E eastward across the Southern Maldives to Sumatra and Western Indonesia. This trough is usually defined by numerous showers and squalls moving westward from Sumatra and Western Indonesia, and changes little in strength, while the trough itself remains quasi-stationary throughout the year. However, the trough does have a tendency to be a few degrees farther north in the summer, as well as a few degrees farther south in the winter.
From late October through March, a strong arctic high becomes established across much of Northern Asia. An associated high pressure ridge builds across the Northern Arabian Peninsula to the Bay of Bengal, and allows cold air to rush southward. This, in turn, produces enhanced north-northeast winds across the Northern Indian Ocean when this ridge interacts with the equatorial trough (see Figure 2A).
These north-northeast winds are usually strongest across the Bay of Bengal and Arabian Sea (east of the Gulf of Aden) with winds of Beaufort Force 5-7 and swells of 9-13ft. However, during December through February sustained gale force winds can develop in these regions when the arctic high is usually strongest. Beaufort Force 7-8 winds and swells reaching 11-16ft are not uncommon during these cases. Also, the Somali coast and Gulf of Mannar are prone to funneling winds, likely sustained Beaufort Force 6-7. Winds and swells gradually ease farther south and closer to the equatorial trough. The tropics allow for winds to shift out of the east and range from Beaufort Force 3-5. Swells in this vicinity are usually out of the east at approximately 5-8ft.
By April through early May the arctic high over northern Asia weakens and the equatorial trough begins to migrate farther north. During this time, the northeast monsoon slowly weakens and becomes more intermittent, with occasional wind shifts to the south to southwest. By the end of May, the winds strengthen and south to southwest winds become dominant, commencing the southwest monsoon (see Figure 2B).
Southwest winds are strongest from the coast of Somalia eastward several hundred nautical miles to approximately 65E, as well as from Southeastern India and Sri Lanka eastward across the Bay of Bengal. Winds are generally from the south-southwest of approximately Beaufort Force 5-7 and swells 8-12ft across the Bay of Bengal. However, southwest winds will funnel across the Gulf of Mannar and become more westerly along the southern coast of Sri Lanka of force 6-7.
The coast of Somalia consistently has the highest winds (Beaufort Force 8-9) and building swells (15-20ft) during June through August. Winds are usually lower south of 5N, generally coming from the west at approximately Beaufort Force 4 or less and swells become westerly 4-7ft. By late September through October the southwest monsoon gradually weakens and transitions to the northeast monsoon by the end of November.
The monsoon does influence other regions outside of the Northern Indian Ocean as well. Across the Gulf of Aden winds are generally from the northeast and east of Beaufort Force 3-5 and swells 4-7ft during the winter. During the summer, the winds shift to more east and southeast of Beaufort Force 4-6 (with the highest winds occurring over the extreme western gulf) and swells 5-8ft. However, within 2-3 nautical miles from the Yemen coast, winds are generally onshore by day and offshore by night with the highest winds occurring during the late morning and afternoon hours year round.
Winds funneling through the Strait of Malacca switch directions during the transition of the monsoons, like everything else in the North Indian Ocean. During the northeast monsoon, winds are generally out of the north and northwest of Beaufort Force 3-5. Swells generally subside through the strait to 3-6ft. Winds shift to more south and southeast of Beaufort Force 3-5 during the southwest monsoon and swells are generally 3-5ft. Winds are typically more variable and lighter year round, especially near the southern entrance of the strait, due to its proximity to the equator.
As the southwest monsoon becomes established in April and May, the threat for tropical cyclones increases across the Northern Indian Ocean. The tracks and frequency of tropical cyclones differ across the Arabian Sea and the Bay of Bengal. In the Arabian Sea, the tropical cyclone season usually begins in May and lasts through October. Tropical cyclones generally develop off the southwest coast of India. Once development occurs, one of two tracks will normally occur: either to the northwest to north-northwest, toward the Northern Arabian Sea; or west to west-northwest towards the Arabian Peninsula. There are times when a tropical cyclone tracks from the Bay of Bengal across central or Southern India and re-intensifies once emerging offshore the Western Indian coast. The peak in the tropical cyclone season occurs at the end of May through June and again in October. However, tropical cyclones develop less frequently across the Arabian Sea averaging only 1 to 3 systems a year.
The tropical cyclone season across the Bay of Bengal begins in April and continues through December. The peaks in the season occur in May and November during the transition between the southwest and northeast monsoons. Tropical cyclones usually develop in the Southeastern Bay of Bengal (north of approximately 6N) and track either north to northwestward towards Bangladesh or west to northwestward to the eastern coast of India. Overall, the Bay of Bengal generally spawns 3 to 5 tropical cyclones a year.
Now that all of the major weather features have been summarized across the Red Sea and Northern Indian Ocean, some questions may come to mind such as when is the best time of year to transit across these areas?
Overall the best times of year to make this transit are during April through early May and from late October through November. There are several reasons that these times of year are more favorable to plan a voyage. Spring and autumn are the transitional times between the northeast and southwest monsoons. Winds and swells are generally lowest across the Northern Indian Ocean as the wind direction becomes more variable. Across the Red Sea, cold fronts are weaker and higher northwest winds surging across the Northern Red Sea occur less often. Also, the thermal trough across the Red Sea and Arabian Peninsula is weakening in autumn, allowing for lighter southeasterly winds across the Southern Red Sea and through Bab el Mandeb.
April and early May also avoid the worst of the tropical cyclone season across the Northern Indian Ocean. The tropical cyclone season is just beginning across the Northern Indian Ocean during this time. Planning a trip during later October through November does increase the threat of tropical cyclone formation, especially in the Northern Indian Ocean. However, a tropical cyclone can be easily avoided as long as a mariner has access to the latest tropical advisories or consults a marine meteorologist prior to and during a voyage.
Planning a transit during the winter months across the Red Sea and Northern Indian Ocean is not ideal with higher north-northwest winds and swells across the northern Red Sea and higher north-northeast winds and swells across the Northern Indian Ocean. If one must depart during this time, one should be prepared for long delays and numerous stoppages to wait for a brief break in the northerly wind surges.
It is not recommended to plan a trip across the Northern Indian Ocean during the summer months. Sustained gale force winds, large southwest swells, a strong Somalia current in the Arabian Sea and a threat of tropical cyclones will make the transit extremely uncomfortable and, at times, can be dangerous especially for smaller vessels. Long delays are likely and window to transit across this region are few and far between.
Having this basic weather information and when to plan for generally the best conditions at hand will give a mariner a better idea of what to expect while navigating across the Northern Indian Ocean. However, the weather does not always go according to plan and it is best to have access to the latest weather information on board the vessel or to consult marine meteorologists who can monitor the weather and advise of any route alterations in order to avoid heavy weather. This way a mariner will have peace of mind knowing what lies ahead and to steer for clear skies.
Did you find this article useful? Read A Season of Transition: The Pacific Coast of Mexico and Central America in Late Winter and Spring by David Cannon and Amanda Delaney of WRI.
Mark Neiswender is Director of Yacht Operations and a Senior Meteorologist, and Amanda Delaney and Brian Whitley are both Senior Meteorologists. All are employed at Weather Routing Inc. (WRI Ltd.), which has provided professional routing and meteorological consultation to mariners worldwide since 1961.
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