Some changes on tropical surface charts

Recently a couple of changes have been made to surface analysis charts and surface forecast charts generated by NOAA that cover the tropical latitudes in the Atlantic and Pacific. While these changes are subtle, they do include the use of some new symbols on the charts, and also provide mariners with some additional information.

The Intertropical Convergence Zone (ITCZ) is defined as the boundary between the northeast trade winds that typically prevail in the tropical latitudes of the northern hemisphere, and the southeast trade winds that typically prevail in the tropical latitudes of the southern hemisphere. In the theoretical general circulation model of the atmosphere, the ITCZ lies right along the equator and it is a trough of low pressure. The pressure gradient between this trough and the subtropical highs of both hemispheres leads to a force pushing the air from the highs toward the ITCZ, or toward the equator. The Coriolis effect then leads to the deflection of this air to the right of its motion in the northern hemisphere, and to the left of its motion in the southern hemisphere. This results in the wind flow north of the equator coming from the northeast, and south of the equator from the southeast.

In reality, though, the ITCZ does not always lie right along the equator, but instead shifts north and south away from the equator due to many factors, including the change of the seasons and the proportion of land area versus ocean area. Because there is more land in the northern hemisphere, the ITCZ spends more time north of the equator than south of it, and tends to reach higher latitudes in the northern hemisphere summer than in the southern hemisphere summer.

When the ITCZ moves well north of the equator during the northern hemisphere summer, the wind direction to the south of the ITCZ ends up being different. This is because the air flowing from the southern hemisphere subtropical high toward the ITCZ crosses the equator, and the direction of the Coriolis deflection changes. This results in southwest winds in the northern hemisphere to the south of the ITCZ instead of the southeast trades of the southern hemisphere. In this situation, the National Oceanic and Atmospheric Administration (NOAA) is now designating this trough of low pressure as the Monsoon trough rather than the ITCZ. The symbol on the chart is slightly different as well. The naming of this feature has to do with its relationship to the seasonal monsoon wind flows that are most notable over the Indian subcontinent, where the ITCZ/Monsoon trough pushes as far north as 30°N over interior northern India during the summer.

Generally the biggest difference between the ITCZ and the Monsoon trough for ocean voyagers is the difference in wind direction to the south of the feature. When the feature is designated as the ITCZ, southeast winds can be expected to its south, and when it is designated as the Monsoon trough, southwest winds are likely to its south. Both features are troughs of low pressure, and thus will produce showers and thunderstorms of varying intensity in their vicinity. The Monsoon Trough is a favored place for tropical cyclones to develop, while this generally does not occur with the ITCZ due to its proximity to the equator where the Coriolis effect is zero, and thus rotary motion around a low-pressure center cannot be supported. Thus voyagers should be aware that a tropical depression or tropical storm could form in the vicinity of the Monsoon trough.

When cold fronts push south into tropical latitudes they typically slow down and often stall, and also the contrast between the two air masses on either side of the front is reduced and eventually disappears altogether. Previously, this was shown on the charts as a trough, and would be depicted as a dashed line. Now, these features are being designated “shear lines” and the symbol used to show them is different. This is being done to acknowledge that the feature is really no longer a front since there is no significant temperature contrast across the feature, and there is also no significant shift in wind direction. The feature can also be devoid of significant precipitation in contrast to active cold fronts which usually produce showers and thunderstorms. There is, however, a notable increase in wind speed on the poleward side of shear lines, and ocean voyagers should anticipate this when operating in the vicinity of a shear line.

The following link provides information from the National Hurricane Center on these changes, including the new symbols being used:

The two charts to the right show the ITCZ and Monsoon trough. In the wider view chart, notice the ITCZ in the Atlantic just a short distance north of the equator, transition to the Monsoon trough in the eastern Atlantic and extending over Africa as the feature extends farther away from the equator. Also, notice a Monsoon trough in central America. The Caribbean chart, which is more likely to be used by a mariner since it shows a bit more detail, is valid for the same time and shows the same features.

In summary, surface tropical charts now depict the difference between the ITCZ and the Monsoon trough, and they also will show shear lines rather than troughs on the tail end of weakening cold fronts in tropical latitudes.

About the author
Ken McKinley earned a bachelor’s degree in atmospheric science from Cornell University in 1980, and attended graduate school in meteorology at the Massachusetts Institute of Technology. After working as a meteorologist for nearly 10 years he founded his own meteorological consulting firm, Locus Weather, in Camden, Maine in 1991.‹    A large portion of his business at Locus Weather involves providing custom weather forecast services for oceangoing yachts, both racers and cruisers. Ken serves as an instructor for the Ocean Navigator School of Seamanship, and also as an adjunct instructor at the STAR Centers for Professional Maritime Officers in Dania, Fla., and Toledo, Ohio, and for MITAGS in Baltimore, Md. He has also taught meteorology at Maine Maritime Academy. He resides in Rockport, Maine, with his wife and two sons.‹ â€šKen’s website is:

Click on the charts to view them bigger.

By Ocean Navigator