Wind direction as a tool for climate analysis

The characteristic ridge of high pressure, which dominates surface climate across of the sub tropical ocean, develops within the divergent flow in the descending limb of their Hadley Cells. The Trade Winds form within the surface air streaming towards the meteorological equator. Atmospheric circulation around the ridge is anticyclonic with the axis separating trade winds from mid latitude westerlies. Wind along the axis is variable and shows least direction bias over time. The close association between the Hadley Circulation and meridional temperature distribution makes a record of latitude for the anticyclone axis a valuable tool for monitoring climate variation. The Comprehensive Ocean Atmosphere Data Set (COADS) is a compilation of marine weather observations drawn largely from routine weather reports from merchant ships. Any analysis of COADS runs foul of irregularities in observing practice. Wind direction is one of the simplest observations to make; it is relatively idiot proof and remains untainted by any changes in observation technique. If any systematic trend in surface wind climate is detected it may reflect a real underlying change in atmospheric circulation. In this study wind direction data are compared with pressure to assess their relative merits for identifying axes of sub-tropical anticyclones, (STA) over the oceans. The nature of the axes is examined and time series are developed for these across the North Atlantic and North Pacific Oceans for the months of January and February 1950 to 1992. Streamline charts of surface wind flow are produced for the period 1950 to 1979. Results from these show that for this application wind direction requires a significantly smaller data sample than surface pressure. The implications are twofold: 1. Wind direction provides an effective tool for examining data sparse regions. 2. Corollary to this, shorter duration samples may be employed which enhances the temporal resolution of the analysis. The time series resolve small fluctuations in the axis of STA while the streamline charts illustrate regions of confluence and diffluence.