Climate in the study region is classified as ‘Monsoon Subtype’ (Am) of the ‘Tropical Rain Climate’ by the Köppen system, with year-round average temperatures > 18° C and seasonally distributed precipitation. The Holdridge system classifies this region as tropical dry, with mean monthly temperatures ranging between 25°–27° C throughout the year.
We measured daily rainfall (data) at the Marajoara field site from October 1995–October 2001, and from October 1995–June 2000 at the sawmill community called ‘Marajoara Serraria’ located 27 km northeast of Marajoara on state highway PA-150. Mean annual precipitation totals over five years at Marajoara and Marajoara Serraria were 1855 mm and 1797 mm respectively, with > 90% falling during seven months from October–April. Annual wet season totals, measured from August–July, ranged from 1636–2170 mm at Marajoara and 1630–2043 mm at Marajoara Serraria. The timing and intensity of wet season onset varied widely from year to year, ranging from early September to as late as mid November. During most years a pronounced lull in precipitation lasting three to four weeks occurred between January and March (see discussion below of the ITCZ).
Though less than 30 km apart, monthly rainfall totals between the two sites differed by up to 200 mm, demonstrating the highly localized nature of rainy season precipitation on this landscape. That is, single rainfall events exceeding 50 mm commonly occurred at Marajoara with little or no accumulation at Marajoara Serraria, and vice versa. This was especially true during the first months of the wet season when most rain fell in quick bursts on narrow fronts one to several kilometers wide. Alternatively, region-wide stationary fronts during the rainy season distributed rainfall consistently over large areas for days at a time as steady slow rains. These periods often alternated with equally long dry spells. During the dry season, no rain typically fell for two to three months from June to August.
Weather patterns in the study region are controlled by semi-permanent high pressure systems centered over the north and south Atlantic, with an intervening low pressure cell positioned over the continental land mass. These high pressure zones migrate south in January as the sun’s vertical position shifts to the Tropic of Capricorn. Pressure in the intervening low is barometrically deepest and shifts furthest south during this period, allowing inland penetration of warm, moist northeast tradewinds that generate the eastern Amazon’s rainy season. By July, the two high pressure systems return north with the sun, the continental low retreats north and west to the interior Basin, and cooler, drier southeast trade winds off the south Atlantic high pressure cell supplant the northeast trades, initiating the dry season by reducing oceanic moisture inputs into the region.
The Intertropical Convergence Zone (ITCZ), the transitional zone between the two Atlantic high pressure systems where winds converge and where, when convergence intensifies, turbulent 'easterly waves' are created across the center of the zone, descends as far as 25° S in January over the continental interior, returning north of the equator in July and back again through the year. In southeastern Pará, as the sun migrates south from its equatorial position in September, clouds and precipitation build to a peak in December and January as the ITCZ reaches its southernmost limit; rainfall continues during its northerly retreat through April–May. With minor inversions Eidt’s 1968 description for Venezuela’s Llanos del Orinoco north of the equator admirably describes the transition to dry weather in south Pará: “In [May] precipitation drops off rapidly and remains low while the dynamically warmed winds from the [South] Atlantic high pressure cell stabilize and dry the atmosphere. Floods recede and large rivers decrease in size until nothing but acres of boulders and a small trickle of water in the old channel are visible” (p. 70).
Clapperton C (1993) Quaternary Geology and Geomorphology of South America. Elsevier Science Publishers BV, Amsterdam, the Netherlands.
Eidt RC (1968) The climatology of South America. In: Fittkau EJ, Illies J, Klinge H, Schwabe GH & Sioli H (eds.), Biogeography and Ecology in South America, pp. 54-81. Dr. W Junk NV Publishers, The Hague, Belgium.
Grogan, JE (2001) Bigleaf mahogany (Swietenia macrophylla King) in southeast Pará, Brazil: a life history study with management guidelines for sustained production from natural forests. PhD dissertation, Yale University School of Forestry & Environmental Studies, New Haven, CT.
Grogan J & Galvão J (2006) Physiographic and floristic gradients across topography in transitional seasonally dry evergreen forests of southeastern Amazonia, Brazil. Acta Amazonica 36: 483-496.
Grogan J, Schulze M (2012) The impact of annual and seasonal rainfall patterns on growth and phenology of emergent tree species in southeastern Amazonia, Brazil. Biotropica 44: 331-340.