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 observations 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 frequent during the present century than before 1900. America. The peculiar "Atlas" position of the ICZ in Feb- c. The ICZ (1942 and 1943) moves less ruary is possibly due to the same cause—a long period anomaly in the general circulation during the first rapidly southward (about 4 latitude degrees decades of the present century, perhaps of cyclical character. in 3-4 months) from September-October to

Unauthenticated | Downloaded 10/01/21 11:45 PM UTC TABLE I. MEAN MONTHLY LATITUDINAL days, its wind direction was more easterly POSITION OF THE INTERTROPICAL CON- than in 1942, and it did not extend very far VERGENCE ZONE BETWEEN 90°W north of the Equator. AND 95°W LONGITUDE Comparing wind patterns on the daily charts in September 1942 and September Extreme Position 1943, it is noted that in the latter year cy- 1942 Mean North South clonic circulations were much more frequent, J and the west winds (of the southern quad- F (3.3N)1 (5N)1 (0-lN): rants of these circulations) extended to M 5.5N 9-10N 1-2N lower latitudes, to 5°N in 1943, but only to A 6.2N 9-10N 5N 7°N in 1942. As the main convergence M 8.1N 12-13N 5-6N usually occurred between the monsoon cur- J 9.5N < 14N 5N3 rent and the west-wind streams of these in- J 8.5N 13-14N 5-6N termittent cyclonic circulations, the mean A 9.2N 13-14N 5N position of the convergence zone was farther S 10.6N 14N 6N south in 1943. 0 9.IN 14N 5-6N It is interesting to note that these diffuse N 7.8N 13-14N 5-6N cyclonic circulations develop at approxi- 2 D 7.6N UN 5-6N mately 10°N (see map for September 27, 1943 1943 in FIGURE 1). Their west-wind quad- J 5.7N 9-1 ON 2-3N rants often cover a belt from 5°N to 10°N; F 1.9N 5-6N 0-1S4 however, sometimes a belt of such west winds M 1.0N 6-7N 5S continues eastward to the coast of Central A 4.2N 6-7N 0 America. Thus what has been considered to M 7.4N 11-13N 3-4N be a well-developed and deep west-wind sea- J 8.1N 13-14N 5N breeze on the coast of Central America, may J 7. IN 10-12N 5N often be a more widespread air stream aug- A 7. IN 10-1 IN 5N mented locally by sea breezes and valley S 7.0N ION 5N winds. Such a circulation persists for sev- 0 7.8N 12-13N 4-5N eral days, while increasing and decreasing N 7.4N 9-13N 5-6N in intensity, with its center moving in an D 5.7N 7-8N 2-4N east or west direction. It is possible that some of the hurricanes of the Eastern North 1944 Pacific Ocean develop from these circula- J 4.0N 6N 2N tions.4 The cyclonic circulations may be in- 1 Data for only five days. duced in some cases at least, by the pressure 2 On two days the zone may have been at 13°-14° N. 3 On one day the zone may have been at 3° N. falls associated with the upper portions of 4 On two days the zone may have been at 4°-5° S. easterly waves and hurricanes that cross the mountains of Central America from the the southernmost position in February-March Caribbean. (the dry season in Central America). Since a change of such major magnitude Although the seasonal movements of the in the mean circulation pattern as noted be- intertropical convergence zone showed a simi- tween 1942 and 1943 might be reflected in lar regime in 1942 and 1943, the zone was the "Easterly Index,"5 an attempt is being farther north in all months in 1942. No sat- made to correlate the latitude of the inter- isfactory explanation of this fact is available. tropical convergence zone with the value of Comparing wind patterns on the daily the "Easterly Index.99 It is noteworthy charts in March 1942 and 1943, it is noted that in the first third of 1942, the extension that in March 1942 the monsoon was present every day as a south wind which extended 4 Hurd, W. W., "Tropical Cyclones of the Eastern North Pacific Ocean," Pilot Chart of the North Pacific far north of the Equator; whereas in March Ocean, U. S. Hydrographic Office, August 1941. 5Namias, J., Methods of Extended Forecasting, 1943, the monsoon was present on fewer U. S. W. B., September 1943.

Unauthenticated | Downloaded 10/01/21 11:45 PM UTC TABLE II-A. MONTHLY PERCENTAGE FREQUENCY OF OCCURRENCE OF SOUTHERN HEMISPHERE AIR IN EACH UNIT AREA 1° LATITUDE AND 90°W-95°W LONGITUDE, SEA LEVEL TO 2,000 FEET ABOVE SEA LEVEL

1942 1943

Feb.1 Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Jan.

13-14N (0.0) 0.0 0.0 0.0 7.3 6.9 4.8 12.1 8.1 1.6 0.0 0.0 12-13N (0.0) 0.0 0.0 2.2 12.7 6.9 14.5 29.2 14.5 4.8 0.0 0.0 11-12N (0.0) 0.0 0.0 4.4 27.2 12.0 17.7 37.5 21.0 9.7 0.0 0.0 10-1 IN (0.0) 0.0 0.0 8.7 36.2 19.0 33.8 56.2 27.5 9.7 7.5 0.0

9-10N (0.0) 1.9 6.8 21.8 49.0 25.9 40.2 68.7 37.1 12.9 17.0 1.7 8-9N (0.0) 3.8 6.8 28.3 69.1 39.7 53.2 79.1 56.5 21.0 26.5 6.8 7-8N (0.0) 7.6 9.1 54.3 76.4 53.5 69.4 83.3 67.7 43.6 41.5 15.2 6-7N (0.0) 19.0 29.3 91.3 82.3 95.0 90.3 91.6 90.3 87.1 75.5 28.8 5-6N (0.0) 49.2 75.0 97.7 92.8 95.0 93.5 100 96.6 98.3 98.0 54.2

4-5N (30.0) 86.7 100 100 98.3 100 100 100 100 100 100 83.0 3-4N (50.0) 90.6 100 100 98.3 100 100 100 100 100 100 86.5 2-3N (80.0) 96.3 100 100 100 100 100 100 100 100 100 98.3 1-2N (80.0) 98.2 100 100 100 100 100 100 100 100 100 100 0-1N (90.0) 100 100 100 100 100 100 100 100 100 100 100

0-1S (100) 100 100 100 100 100 100 100 100 100 100 100 1-2S (100) 100 100 100 100 100 100 100 100 100 100 100 2-3S (100) 100 100 100 100 100 100 100 100 100 100 100 3-4S (100) 100 100 100 100 100 100 100 100 100 100 100 4-5S (100) 100 100 100 100 100 100 100 100 100 100 100

5-6S (100) 100 100 100 100 100 100 100 100 100 100 100 6-7S (100) 100 100 100 100 100 100 100 100 100 100 100 7-8S (100) 100 100 100 100 100 100 100 100 100 100 100

1 Data for only 5 days. of the Bermuda High into the Caribbean greater detail in TABLE II, it is apparent Sea and Gulf of Mexico was very weak. The that the Southern Hemisphere trade-wind results of this study will be published at a sometimes blows across the Equator far later date. A preliminary survey of this into the Northern Hemisphere during the problem indicates some interesting relation- northern-summer season. On certain days, ships that may be useful in long-range fore- this monsoon extends to the coast of Central casting of the mean position of the conver- America (and across the Isthmus of Panama gence zone. The analysis is hampered by into the Caribbean Sea). This condition was the fact that the "Easterly Index" is com- most frequent in 1942 (see map for June 15, puted for the area 35 °N, 130°W-10°W, and 1942, FIGURE 1). 20°N, 110°W-30°W, which is somewhat re- For periods of one to several days, as moved from the areas of major control for observed especially during March 1943, the the eastern Pacific. intertropical convergence zone may be found Examining the extreme northerly and ex- south of the Equator, and a rudimentary treme southerly daily positions of the con- northern monsoon will appear, best developed vergence zone shown in TABLE I and in just off the coast of South America (see

Unauthenticated | Downloaded 10/01/21 11:45 PM UTC TABLE II-B. MONTHLY PERCENTAGE FREQUENCY OF OCCURRENCE OF SOUTHERN HEMISPHERE AIR IN EACH UNIT AREA 1° LATITUDE AND 90°W-95°W LONGITUDE, SEA LEVEL TO 2,000 FEET ABOVE SEA LEVEL

1943 1944 Latitude Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Jan.

13-14N 0.0 0.0 0.0 0.0 1.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 12-13N 0.0 0.0 0.0 3.4 8.9 0.0 0.0 0.0 10.3 5.0 0.0 0.0 11-12N 0.0 0.0 0.0 3.4 12.5 1.9 0.0 0.0 10.3 5.0 0.0 0.0 10-1 IN 0.0 0.0 0.0 10.3 14.3 3.8 1.7 0.0 19.0 5.0 0.0 0.0

9-10N 0.0 0.0 0.0 14.0 25.0 11.5 15.0 6.0 20.7 5.0 0.0 0.0 8-9N 0.0 0.0 0.0 25.8 37.5 15.4 23.3 18.0 34.5 20.4 0.0 0.0 7-8N 0.0 0.0 0.0 44.8 48.3 30.8 33.3 42.0 44.8 36.4 3.7 0.0 6-7N 0.0 3.8 5.2 65.5 71.3 63.4 51.6 54.0 63.7 63.6 25.9 0.0 5-6N 4.1 7.5 25.1 81.0 91.1 84.6 80.0 78.0 84.5 97.7 59.3 13.7

4-5N 21.0 15.1 51.7 94.8 100 100 100 100 96.5 100 88.9 35.3 3-4N 36.7 20.7 65.6 96.5 100 100 100 100 100 100 96.2 62.7 2-3N 57.1 28.3 82.7 100 100 100 100 100 100 100 96.2 92.2 1-2N 77.5 51.0 93.2 100 100 100 100 100 100 100 100 100 0-1N 89.7 56.6 93.2 100 100 100 100 100 100 100 100 100

0-1S 91.7 69.8 100 100 100 100 100 100 100 100 100 100 1-2S 91.7 79.2 100 100 100 100 100 100 100 100 100 100 2-3S 91.7 83.0 100 100 100 100 100 100 100 100 100 100 3-4S 96.0 92.5 100 100 100 100 100 100 100 100 100 100 4-5S 100 94.3 100 100 100 100 100 100 100 100 100 100

5-6S 100 100 100 100 100 100 100 100 100 100 100 100 6-7S 100 100 100 100 100 100 100 100 100 100 100 100 7-8S 100 100 100 100 100 100 100 100 100 100 100 100

map for March 7, 1943, FIGURE l).6 In cer- is well developed, the northern monsoon tain years when the "El Nino"7 phenomena pushes several degrees south of the Equator in the vicinity of the Galapagos Archipelago, 6 Note that the convergence in the intertropical convergence zone is confined to a very shallow layer below and along the coast of South America, as it 4,000 feet, above which divergence is present. The weather associated with this part of the zone is rela- did in February and March 1943. tively "mild" compared to the weather where active In certain months a distance of 300-540 convergence takes place in a deep layer. The greater weather activity between 2°N-4°N (850W-95°W) is miles (5-9 latitude degrees) separates the associated with active convergence aloft above the northerly winds, that is, above 4,000 feet. A situation extreme northern and southern daily position of this sort may lead to the erroneous conclusion that of the convergence zone. It is not yet cer- the intertropical convergence zone has a "double structure" (Grimes). tain whether the convergence zone actually 7 Schott, G., Der Peru-Strom und Seine Nordlichen Nachbargebiete in Normaler und Anormaler Ausbildung, moves as an entity within these limits, or Ann. der Hydrogr. und Marit. Meteorol., Vol. 59, 1931, forms, moves, dissipates and reforms again pp. 161-169, 200-213, and 240-253: Der Peru-oder Humboldstrom, Forschungen und Fortschritte, Vol. 7,1931. in a new location, existing as an entity only Murphy, R. C., Oceanic and Climatic Phenomena Along the West Coast of South America During 1925, for isolated periods of one to several consecu- Geographical Review, Vol. 16, 1936. tive days. Sheppard, G., The Rainy Season of 1932 in South- western Ecuador, Geographical Review, Vol. 13, 1933, pp. 210-216.

Unauthenticated | Downloaded 10/01/21 11:45 PM UTC