The Intertropical Convergence Zone of the Eastern Pacific Region (I)* MAJOR LEO ALPERT, A.C. Hq. AAF Weather Service, Asheville, N. C. INTRODUCTION Recently a large number of weather ob- servations have become available from air- ITTLE INFORMATION concerning the craft patrols in the Eastern Pacific Ocean structure and position of the inter- L between 90°W to 95°W Longitude, and 7°S tropical convergence zone in the East- to 14°N Latitude. From December 1941 to ern Pacific Ocean was available prior to the late 1944, hourly weather observations were war. As daily weather reports from ships made daily by patrol aircraft, flying between have never been numerous from this area, the Republic of Guatemala in Central Amer- all attempts to show the position of the ica, the Galapagos Archipelago and the main- intertropical convergence zone based upon land of Ecuador in South America. Data an interpretation of summarized ship re- for the period February 1942 to January ports are open to question. The charts pub- 1944 have been plotted on daily charts and lished in the " Atlas of Climatic Charts of analyzed by the author. the Oceans'n are probably the best of these The grand total of observations available for this region. They are based upon fre- from the aircraft patrols in the area from quencies of true-wind directions determined 10°N to 5°S for the period February 1942 by observing the effect of wind on the sea to January 1944, inclusive, is two to four surface from a moving ship at Greenwich times as great as the grand total of observa- Noon during the period 1885-1933. tions for all months, 1885-1933, from the The limitations of the wind data used in surface ship-reports used in the '1 Atlas." the "Atlas'' may be summarized with spe- However, north of 10°N there are seven cial reference to the area in question:— times as many ships' reports as aircraft a. Observations are scarce, especially south reports. of 10°N Latitude. The wind observations were made mostly b. Wind observations from ships are in- by double-drift methods at altitudes ranging herently inaccurate. from close to sea level to 10,000 feet above c. Observations are confined to sea-level sea level. Every hour in flight the navi- wind directions so that there is no in- gators entered the wind direction (to 36 formation about the winds aloft. points of the compass) and speed in the d. Observations are confined to those Weather Bureau " Aircraft Weather Re- taken at or near Greenwich Noon so port' ' form. Uncorrected altimeter altitudes that any diurnal variations, though were used, the altimeter being set at 29.92 probably small, are neglected. inches of mercury before takeoff. As a rule, e. Compilations are based upon relatively wind observations were taken only when the large unit areas (5° in longitude by 5° plane was flying in the clevar, either with the in latitude), so that details of wind- ocean surface visible below, or the sky visible direction distribution along the meri- above the plane. Thus, wind observations dians are masked. are generally lacking from areas of obscured /. Data for a 48-year period are totalled, ocean surface or sky. smoothing out any large variations of The aircraft wind observations appear to a shorter period. be generally accurate; however, several sources of error in the wind reports are * Published with permission of the War Department. present:— 1 Atlas of Climatic Charts of the Oceans, U. S. Weather Bureau, Washington, 1938. Earlier charts of Maury, Buchan, Koppen and the various 1'Pilot a. The small inaccuracy of the double- Charts" were based on even fewer observations in this drift wind observation. area. Unauthenticated | Downloaded 10/01/21 11:45 PM UTC b. Errors in encoding and decoding the zones are often present which may be associ- reports. ated with heavier clouds and weather than c. Errors in enciphering and deciphering those in the intertropical convergence zone the reports. proper. On these days, the analysis requires d. Errors in radio transmission of the careful interpretation of clouds and weather, reports. and reference to the comments of the navi- e. Errors in plotting the reports. gators in the '1 remarks'' section of the "Aircraft Weather Report'' form and of The weather observations were made daily the pilots in the "weather section" of the and hourly in flight during the hours from "Aircraft Commander's Report." 1000 GCT to 2300 GCT. Most of the obser- Four typical daily wind-stream patterns vations, however, cover the period 1300 GCT forming the intertropical convergence zone to 2000 GCT. Since the diurnal variation of are shown in FIGURE 1. The streamlines wind direction is believed to be very small represent only the direction of the wind over the open ocean, the observations have streams; no attempt has been made to been interpreted as the " chance of occur- show the wind speed by the spacing of the rence at any hour." streamlines. The aircraft observations are of particular On March 7, 1943, the pattern was com- interest, as on each day a relatively complete plex but the intertropical convergence zone horizontal and vertical coverage of the area was evidently 3-4 degrees south of the Equa- was obtained—in contrast to the one or two tor at a 90°-180° wind-shift; the northerly surface ships reporting daily in the area current shows some recurvature below the before the War.2 Equator. On June 15, 1942, the zone was far The position of the intertropical con- to the north in Central America; easterly vergence zone was drawn on each daily winds were present in the Caribbean, but the chart; and from these charts the mean posi- convergence line is indistinct over the land, tion, the extreme northern daily position, due to local winds and topographic effects. and the extreme southern daily position for On June 11, 1943, the zone was formed by a each month were determined and entered in sharp 180° wind-shift between southerly and TABLE I. northerly wind streams; there was some cy- The position of the intertropical con- clonic effect in the northerly current causing vergence zone on the daily charts is usually an apparent divergent zone around 95°W, clearly defined by a 90 to 180-degree wind- 7°N. On September 27, 1943, the zone was shift between southeast, south or southwest marked by a 90° wind-shift between south- winds of the Southern Hemisphere air erly and westerly wind streams; a weak cy- stream and east, northeast, north, northwest clonic circulation is noticeable around 10 °N, or west winds of the Northern Hemisphere air stream; however, in the period from the causing a secondary convergence zone. second half of February to the first half of In the mean, the inaccuracies on certain April in 1943 the convergence zone was not daily charts when it was difficult to deter- so easily located, as easterly winds were mine the position of the zone, tend to balance often present on both sides of the conver- each other, and the positions shown in TABLE gence zone with a wind-shift of less than 90 I and FIGURE 2 are, therefore, believed to be degrees. In addition, secondary convergence representative of prevailing conditions. 1. MEAN POSITION OF THE INTERTROPICAL CONVERGENCE ZONE (ICZ) It is apparent from FIGURE 2 that the throughout the year. The mean positions mean monthly positions of the intertropical interpreted from the 1' Atlas of Climatic convergence zone lie north of the Equator Charts of the Ocean" (dotted line) agree 2 See the "Daily Synoptic Series, Historical Weather reasonably well with those of 1942 and 1943 Maps, Northern Hemisphere Sea Level," published by combined (solid line). The analysis of the the U. S. Weather Bureau, for the JMC, 1943-44. Unauthenticated | Downloaded 10/01/21 11:45 PM UTC FIGURE 1. Typical Daily Wind-Stream Patterns Forming the Intertropical Convergence Zone i(Atlas'9 data is therefore apparently reli- in common which also characterize the curve able, except perhaps in February (see below). from the "Atlas'' data: — The curves (FIGURE 2) of the position of a. The ICZ (1942, the dash line and 1943, the ICZ in 1942 and 1943 have three features the dash-dot line) is farthest south in Unauthenticated | Downloaded 10/01/21 11:45 PM UTC FIGURE 2. Mean Monthly Latitudinal Positions of the Intertropical Convergence Zone Between 90°W and 95°W Longitude February-March. However, the curve inter- ward rapidly (about 6 latitude degrees in preted from the ''Atlas'' has a secondary 3-4 months) to a maximum north position in maximum in February, which may be a pe- June-October, with a slight return southward culiarity of certain years.3 These are the during July-August. This is the rainy sea- rainy season months in the Galapagos Archi- son in Central America and the dry season pelago, and the dry season in Central in the Galapagos Archipelago. The slight America. southward movement of the zone in July- b. The ICZ (1942 and 1943) moves north- August results from the westward extension of a ridge from the Bermuda High into the 3 S. B. Jones ("The Weather Element in the Ha- western Caribbean, causing a midsummer waiian Climate," Ann. Assoc. Amer. Geographers, Vol. 29, 1939, pp. 29-57) reported that low February rain- secondary decrease in rainfall in Central fall totals at Eastern Pacific stations were more fre- quent during the present century than before 1900. America. The peculiar "Atlas" position of the ICZ in Feb- c.