A new index, the western North Pacific directional index (WNPDI), based on historical wind direction observations taken aboard sailing ships, has been developed to characterize the western North Pacific summer monsoon (WNPSM) since 1898. The WNPDI measures the persistence of the surface westerly winds in the region 5°–15°N, 100°–130°E and easterly winds in the region 20°–30°N, 110°–140°E, exhibiting a consistent relationship with the summer precipitation in the areas affected by the WNPSM throughout the entire twentieth century. Its length doubles that of the previous WNPSM index (1948–2014) based on reanalysis products, which allows uncovering different relevant features of the WNPSM variability. The WNPSM had a significant interdecadal variability throughout the twentieth century. In particular, the period 1918–48 was characterized by less variable and stronger monsoons than in recent decades. Additionally, the relationship between the WNPSM and ENSO or El Niño Modoki has been evaluated during the entire twentieth century for the first time. It is confirmed that the WNPSM tends to be strong (weak) when El Niño (La Niña) develops during the whole record. Nevertheless, the relationship during the ENSO-decaying phase is not stable in time. Thus, the WNPSM tended to be strong (weak) when La Niña (El Niño) decayed only since the late 1950s, with an opposite relationship in the earliest part of the record. El Niño Modoki shows a rather stable and high correlation with the WNPDI during the whole study period throughout the twentieth century.
Abstract. In this paper, we develop an instrumental index based on
historical wind direction observations aimed to quantify the moisture
transport from the tropical Pacific to Central and northern South America at a monthly scale. This transport is mainly driven by the so-called “Chocó
jet”, a low-level westerly jet whose core is located at 5∘ N and 80∘ W. The Chocó jet is profoundly related to the
dynamics of the Intertropical Convergence Zone in the eastern equatorial
Pacific and it is responsible for up to 30 % of the total precipitation in
these areas. We have been able to produce an index for this transport
starting in the 19th century, adding almost a century of data to
previous comparable indices. Our results indicate that the seasonal
distribution of the precipitation in Central America has changed throughout the
20th century as a response to the changes in the Chocó jet, decreasing
(increasing) its strength in July (September). Additionally, we have found
that in general, the relationship between the Chocó jet and the El Niño–Southern Oscillation has been remarkably stable throughout the entire 20th century, a finding particularly significant because the stability of this relation is usually the basis of the hydrologic reconstructions in northern South America.
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