Rainfall - sporadic vs. widespread
In the near-equatorial Pacific there is a limited occurrence of sporadic rainfall. This is illustrated here in the distribution of accumulated rainfall for a one month period in the area near Christmas Island, and by the remarkable contrast between this and the occurrence of rainfall at higher latitudes.
Accumulated Rainfall for August, 2007
The figure shows an estimate of the total rainfall for the one month period after the beginning of PASE observations near Christmas Island (from satellite data analyzed for the TRIMM project).
The island is visible as a white spot in the left/center area of the image. Small amounts of rain fell on the upwind side of the island, 30-60 mm total during this period. Over most of the remainder of the area from 5 S to 5 N there were even lower amounts of rainfall. The color scale was chosen to highlight these small amounts, and around 10 N the totals are far higher, > 100 mm, and far off of the scale. The underlying difference is between rain associated with structured disturbances and sporadic rainfall.
Structured Disturbances vs. Sporadic Rainfall
The intense rainfall that occurs in the Inter Tropical Convergence Zone (ITCZ) is largely associated with transient disturbances. Their effect can be seen most directly in a Hovmöller diagram. This is a time vs. longitude section, compositing data from a limited range of latitude.
This shows the rainfall from the TRIMM archive in the region from 9-10 degrees North, in the region from 165 degrees east to 130 degrees east, for the first 16 days of the PASE field campaign. The estimates are at intervals of 3 hours. The bands that extend from SE to NW as time increases show the impact of transient, wave-like disturbances that propagate along the ITCZ. (The ITCZ is at its highest northern latitude at this time of year.) There is a range of apparent disturbance speeds (i.e., the slope of the nearly linear segments is variable) and a variation in the intensity and frequency of occurrence of these disturbances. What is most notable for our purposes, however, is the extent to which this copious rainfall is confined to the ITCZ region.
The corresponding diagram for the 5-6 degree North band indicates a much smaller amount of rainfall, and makes it more apparent that the occurrence of disturbed conditions is far more common west of 145 degrees East than it is to the east of there.
The Hovmöller diagram for the 1-2 degree North band indicates that for most of the period there was little or no rainfall in the area of interest to the PASE investigators. The band of rainfall in the period August 8 - 10 resulted in a period of intense rain lasting about 3 hours on Christmas Island; it happened to occur between scheduled flight days. This area was selected for the PASE program precisely because of the absence of rainfall, together with the efflux of DMS from the ocean surface and the resulting oxidation of sulfur gases.