Half-precessional dynamics of monsoon rainfall near the East African Equator

External climate forcings-such as long-term changes in solar insolation-generate different climate responses in tropical and high latitude regions(1). Documenting the spatial and temporal variability of past climates is therefore critical for understanding how such forcings are translated into regional climate variability. In contrast to the data-richmiddle and high latitudes, high-quality climate-proxy records from equatorial regions are relatively few(2-4), especially from regions experiencing the bimodal seasonal rainfall distribution associated with twice-annual passage of the Intertropical Convergence Zone. Here we present a continuous and well-resolved climate-proxy record of hydrological variability during the past 25,000 years from equatorial East Africa. Our results, based on complementary evidence from seismic-reflection stratigraphy and organic biomarker molecules in the sediment record of Lake Challa near Mount Kilimanjaro, reveal that monsoon rainfall in this region varied at half-precessional (similar to 11,500-year) intervals in phase with orbitally controlled insolation forcing. The southeasterly and northeasterly monsoons that advect moisture from the western Indian Ocean were strengthened in alternation when the inter-hemispheric insolation gradient was at a maximum; dry conditions prevailed when neither monsoon was intensified and modest local March or September insolation weakened the rain season that followed. On sub-millennial timescales, the temporal pattern of hydrological change on the East African Equator bears clear high-northern-latitude signatures, but on the orbital timescale it mainly responded to low-latitude insolation forcing. Predominance of low-latitude climate processes in this monsoon region can be attributed to the low-latitude position of its continental regions of surface air flow convergence, and its relative isolation from the Atlantic Ocean, where prominent meridional overturning circulation more tightly couples low-latitude climate regimes to high-latitude boundary conditions.

Satellite tracking of the World's largest bony fish, the ocean sunfish (Mola mola L.) in the North East Atlantic

Satellite-linked archival transmitters were used to record the movements of three ocean sunfish (Mola mola) in the North East Atlantic. Patterns of depth use and temperature experienced by individual fish were integrated into 4-hour intervals throughout the tracking period and relayed via the Argos system. Data were recorded for 42, 90 and 54 days respectively from the three fish. The first two were tagged off southern Portugal at the end of February 2007 and travelled principally northward, while the third fish was tagged off west Ireland in August 2007 and travelled southward. These patterns are consistent with seasonal migration of ocean sunfish to high latitudes and their Subsequent return south. Maximum depths recorded by the three fish were 432 m, 472 m and 320 m respectively. All three individuals showed a diel pattern in depth use, occurring deeper during the day and shallower at night, a pattern consistent with sunfish tracking normally vertically migrating prey. Sunfish sometimes remained continuously at deeper (>200 m) depths during the day, but at other times they showed extensive movement through the water column typically travelling between their maximum depth and the surface within each 4-h period. The overall pattern to emerge was that ocean sunfish travel extensively in both horizontal and vertical dimensions, presumably in search of their patchily-distributed jellyfish prey.