Trends in oceanic precipitation characteristics inferred from shipboard
present-weather reports, 1950-2019
Abstract
Manual shipboard present weather reports from 1950 to 2019 are
aggregated and composited yearly and seasonally on a
$1^\circ \times
1^\circ$ grid to characterize the global climatology
and long-term trends in the relative frequency of four categories of
oceanic precipitation: drizzle, moderate and heavy non-drizzle,
precipitation associated with thunderstorms and deep convection, and
frozen-phase precipitation. Although ship reports are susceptible to
subjective interpretation, the inferred distributions of these phenomena
are consistent with datasets derived from other platforms. These
distributions highlight widespread 70-year trends that are often
consistent across both annual and seasonal frequencies, with statistical
significance at 95\% confidence. The relative frequency
of ship-reported drizzle has largely increased in the tropics annually
and seasonally, with linear best-fit relative increases by as much as
15\% per decade. Decreased relative frequencies have
been observed in parts of the subtropics and at higher latitudes.
Heavier precipitation has encompassed a growing fraction of non-drizzle
precipitation reports over the subtropical North Pacific and
Mediterranean. The relative frequency of thunderstorm reports has
declined over the open Atlantic but show positive trends over the
Mediterranean and the western Atlantic. The trends in relative frozen
precipitation occurrence suggest a poleward retreat of areas receiving
frozen precipitation in the Northern Hemisphere. Possible mechanisms for
these ship-observed trends are discussed and placed in the context of
the modeled effects of climate change on global precipitation.