Seasonality and the Baltic Dry Index

I'm doing some research on other economic indicators showing seasonality for a later post. An article on the BDI (Baltic Dry Index) and how shipping rates show a seasonal tendency caught my eye.

From the Financial Times:

It is with interest, therefore, that we note Icap’s latest monthly shipping report — which errs towards the notion that the move wasn’t necessarily so unusual.

According to the broker, for example, the BDI has fallen in June on 20 separate occasions since 1985 — making the recent declines relatively consistent.
As they noted:
..no other month can claim to have such a poor track record, although the month of July has shown a similar tendency towards weakness over the years.

As you can see from the graph from Stockcharts the BDI can be ... volatile.

It's important to know if a data series contains seasonal affects to it.  That's one reason I tend to show data on a year over year format which eliminates time of year influences.