All of the time series of annual tabulations of numbers of tropical cyclones (TCs) (from the annual global total, to the annual totals within individual ocean basins) appear highly erratic (i.e., there is no persistence, and the values seem to jump substantially from one year to the next). Time-lag autocorrelations of these time series confirm this aspect: all have small negative values at a one-year time lag. Also, most of the time series have prominent spikes of both exceptionally low and high values. The high years are referred to as prolific years, and the low years as meager years. The prolific years and meager years are identified for the global and basin distributions. The range in the annual numbers of TCs in every basin is large. The global annual average is 85 with a range of 66 to 105; the western North Pacific annual average is 27 with a range of 19 to 36; the North Atlantic annual average is 10 with a range of 4 to 19; the eastern North Pacific annual average is 16 with a range of 8 to 24; the North Indian Ocean annual average is 5 with a range of 2 to 13; and the Southern Hemisphere annual average is 27 with a range of 19 to 38. The difference of 39 between the maximum and minimum annual global number of TCs is more than twice that of any individual basin.
Given the relative rarity of TC formation (as a global atmospheric phenomenon) and couple it with the aforementioned phenomenon of basin prolific and meager years, a natural question arises as to whether there are compensations among the annual number of TCs within the several TC basins that act so as to stabilize the annual global number of TCs (i.e., negative correlations among some or all of the TC basins); or whether the annual global number of TCs is destabilized by positive correlations among some or all of the TC basins. The latter appears to be true. Cross-correlations among the basins reveal three statistically significant values (and each of these is a weak positive correlation). During the 27-year period 1969 to 1997 there were 16 years with at least one basin experiencing a prolific or a meager year. Five of these years had two, or more, prolific and-or meager years. Only two of the years (1969 and 1995) had both prolific and meager years occurring simultaneously in different basins; the prolific years during these two years were those of the North Atlantic.
There are strong relationships of the annual number of tropical cyclones in the North Atlantic with ENSO and with the QBO. In other basins these relationships are weaker, and, at least with ENSO, the effects are primarily upon the locations of the TCs and not the annual numbers. This paper explores the spatial and temporal properties of the global tropical cyclone distribution and examines the effects of ENSO, QBO, and other climate features on the global and basin tropical cyclone time series. The effects of the extreme ENSO event of 1997-98 on the global TC distribution are discussed.