Archive for the ‘Climate’ Category

Ascension Island and the ITCZ



“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 (8^oS). A visitor expecting to find lush tropical vegetation is suprised, just as Darwin was surprised by the climate of the Galapagos.  Saint Helena, 1300km to the South-West (16^oS) is also dry. So too is the island of Santiago, Capo Verde 2700km to the North (15^oN). On a recent trip to these islands I  was left wondering where the tropical rainfall was.

Rendered by

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 uplift of moist air produces 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^o resolution based on the years 1979-2015. The ITCZ corresponds to the band of strongly negative divergence (convergence) near the equator (in mathematical notation \bf{\nabla} \cdot \bf{U} < 0).


ITCZ moves with seasons as expected. However ERA data show that over much of the Atlantic and Pacific oceans, it is (a) narrow, and (b) shifted significantly towards the NH (by about 5^o). This is why the convergence manages to miss Capo Verde, Ascension and Saint Helena almost entirely which accounts for the surprisingly dry climate of these islands. On the other hand, ITCZ dips far enough South to bring heavy rain to Fernando de Noronha (3^oS off the coast of Brazil) during April.

The large NH bias of ITCZ[1] is a fundamental fact of the climate system. For example, it is a factor in the rarity of hurricanes in the SH. NH bias is believed to be related to the observation that SH is \approx1.25^oC cooler than NH, which is in turn related to imbalance of ocean heat transport between hemispheres.

[1] Why the ITCZ Is Mostly North of the Equator?

Coffee prices, long range forecasts and drought

The tropical highlands of Minas Gerais, Brazil are responsible for 25% of the world’s Arabica coffee production. In 2014, the region experienced drought during the critical austral summer months of January and February. World coffee prices moved sharply higher at the end of January. By early March, prices had nearly doubled.

In an efficient market the price of a commodity reflects all available information. Did the coffee price assimilate long-range weather forecast information available in 2014 ?


The above chart show monthly rainfall (crosses) and average rainfall (black line). Rainfall totals were extracted from ERA-interim reanalysis. Periods of deficit relative to average rainfall are indicated in red. Rainfall was less than 50% of average in both January and February 2014.


Despite dryness, coffee futures actually trended slightly lower during January 2014 (above). However this situation reversed dramatically after January 29 (indicated by the red arrow). It is as though the market abruptly woke up to the fact that drought would continue well into February and that this would impact Arabica coffee fruit development.

In fact, well in advance of January 29, long range weather forecasts were indicating a high probability of continued drought in February. The graph below shows a large ensemble of CFSv2 rainfall forecasts for Minas Gerais for December, January and February. Such forecasts[*] are made every 6 hours up to 9 months prior to the forecast month. A high probability of anomalous rainfall for the months of January and February is evident some 2-3 weeks in advance.



This analysis points to a surprising conclusion. For perhaps two weeks, world coffee market prices did not properly reflect probabilistic information available from long range weather forecasts.

[*] Raw forecasts, not bias corrected.