Observational study of the remote forcing of Pacific subtropical highs

Background

We present preliminary investigations of remote forcing mechanisms of the Pacific subtropical highs. To streamline the discussion, the Northern and Southern Hemisphere surface subtropical highs will be called the “Californian high” and the “Chilean high” respectively.

Factors that cause the subtropical highs to exist where they do can be organized into: 1) local factors that enhance sinking near the high 2) remote rising motion that is linked to that sinking 3) local and remote factors that distort or displace the highs

There is general agreement regarding the local factors. There is less agreement as to what remotely forces these highs. Three remote forcing views are considered:

Current view: rising motion and diabatic heating (precipitation) equatorward and eastward of the subtropical high. A theoretical basis for this view extrapolates the mechanism identified in the Gill tropical circulation model.

Classical view: convection that is mainly equatorward (“Hadley cell”) and/or westward (“Walker cell”) of the subtropical high maintains a divergent circulation having sinking over the subtropical high. A theoretical basis for the Walker circulation link can be constructed from westward tilted (with height) planetary waves forced by diabatic heating to the west and cooling at the subtropical high. A divergent circulation may connect Amazonian rainfall with the Chilean high overlapping with the current view. Such overlap is much weaker for the Californian high.

Extratropical cyclone activity view: cyclones interact with the subtropical highs in possibly several different ways. At present, the nature of the primary interaction(s) is unclear. Some possible links include: (a) merging of transient anticyclones to reinforce the subtropical high, (b) storm track interaction with the subtropical high, (c) remote forcing of the time mean subtropical high by the transient eddies.

Our presentation:

The data include reanalysis sea level pressure (SLP), CMAP precipitation, and interpolated OLR from CDC-NOAA. The data are seasonally stratified with adjustment for the very different climatology of the Chilean and Californian highs. Parallel research lines include forming composites from the extreme months within a season and calculating one-point correlations.

Monthly average data show companion activity, but cannot sort cause from effect here. Our preliminary results show significant correlation between the subtropical highs and forcing by “classical” and “extratropical” mechanisms. The “current” view seems to play little or no role in the case of the Californian high and is ambiguous (but not prominent) for the Chilean high. Shifts of (ICZ, SPCZ, and extratropical cyclone) precipitation away from the high are correlated with strengthening of the high.

Initial work on cause and effect will examine “daily” data of OLR and SLP. (Daily OLR data have ascending and descending passes averaged and additional time filtering.) Preliminary results using daily anomaly fields shall be shown.