Trends in the local Hadley and local Walker circulations

Abstract The linear trend in the local Hadley and Walker circulations from 1979 to 2009 is calculated. These local circulations are defined through a decomposition of the vertical mass flux into its zonal and meridional components. Defining the local circulation this way ensures that the two orthogonal circulations (the local Hadley and Walker circulations) sum to the original circulation even after averaging the circulations regionally. Large regional differences in changes in the local Hadley and Walker circulations over a 31 year period are found. For example, the local Hadley circulation has shifted southward over Africa, the Maritime Continent, and the western and central Pacific by about 1°. Over the Americas and the Atlantic the local Hadley circulation has strengthened by about 1–5%. The zonal component of the vertical mass flux has increased by about 10–20% in the tropics over all continents and decreased over the adjacent oceans by about 10–20%. Although the local Walker circulations in the Indian Ocean and the Atlantic have weakened, the circulation in the Pacific has changed little (about 1–2%). The local Walker circulations in all ocean basins have shifted westward by about 1–2°on average.