“Considering that these islands are placed directly under the equator, the climate is far from being excessively hot; this seems chiefly caused by the singularly low temperature of the surrounding water, brought here by the great southern Polar current.. very little rain falls, and even then it is irregular.” – Darwin (Voyage of the Beagle) describing the climate of the Galapagos.
Ascension Island is located in the tropical South Atlantic (8S). A visitor expecting to find lush tropical vegetation might be disappointed, just as Darwin was surprised by the climate of the Galapagos. Saint Helena, 1300km to the South-West (16S) is also dry. So too is the island of Santiago, Capo Verde 2700km to the North (15N). On a recent trip to these islands I was left wondering where the tropical rainfall was.
Tropical rainfall is tied to the intertropical convergence zone (ITCZ) where trade winds from Northern and Southern hemispheres (NH & SH) converge. Convergence implies uplift and convective rainfall. ITCZ tends to follow maximum sunshine seasonally about the equator, which explains the timing of the wet season at a given location. This begs the question, why are some tropical oceanic islands almost entirely dry while others at similar latitude are very wet?
I used historical atmospheric circulation data from ERA-interim reanalysis to try to shed light on this puzzle. Mean monthly divergence of surface (10m) wind field was computed at 0.75 resolution based on the years 1979-2015. The ITCZ corresponds to the band of strongly negative divergence (convergence) near the equator (mathematically, ).
While ITCZ moves with seasons as expected, ERA data show that over much of the Atlantic and Pacific oceans, the ITCZ is (a) narrow, and (b) shifted significantly towards the NH ( 5). This is why the convergence manages to miss Capo Verde, Ascension and Saint Helena almost entirely and this explains the surprisingly dry climate of these islands. On the other hand, ITCZ dips far enough South to bring heavy rain to Fernando de Noronha (3S off the coast of Brazil) during April.
The large NH bias of ITCZ is a fundamental fact of the climate system. For example, it may be a factor in the relative absence of hurricanes in the SH. NH bias is poorly understood but is believed to be related to the fact that NH is 1.25C warmer than SH. This may in turn be explained by the imbalance of ocean heat transport between hemispheres.