Synoptic‐dynamic character of ‘bursts’ and ‘breaks’ in the South‐West U.S. summer precipitation singularity

Abstract

AbstractMarked intraseasonal variations in the intensity of the south‐west U.S. summer precipitation singularity (the ‘Monsoon’) are identified for the years 1980–1982 from a previously developed satellite cloud climatology. These ‘bursts’ and ‘breaks’ are examined synoptically by compositing sea level pressure and 500 mb height data. Both types of events are controlled by the upper‐level flow. This typically involves, for bursts, the intrusion into subtropical latitudes of a trough in the westerlies with attendant cold air advection. In conjunction with intense surface heating, this produces widespread atmospheric destabilization over the South‐west. Monsoon breaks are characterized by enhanced ridging of the Bermuda and North Pacific subtropical anticyclones with associated subsidence over southern Arizona/California. Between‐year differences in these composite patterns are identified, and it is suggested that wetter summers may be characterized by larger within‐season variations in 500 mb heights and isobaric curvature compared with drier summers. Several 500 mb synoptic types associated with burst events are determined subjectively, and composite patterns are derived. These indicate that bursts occasionally occur with anticyclonic conditions aloft, but the latitude of the ridge seems critical in determining the extent of moist air penetration from the south.Twice‐daily aerological soundings at five stations in the South‐west and northern Mexico are also used to determine the thermodynamic characteristics of bursts and breaks and the dominant source of monsoon moisture. Monsoon bursts in Arizona are evidently a combination of moisture from the Gulf of Mexico, transported on a light south‐easterly flow, and more rapid lower‐level ‘surges’ originating in the Gulf of California. Moisture from the latter source is transported rapidly into the Great Basin on a low‐level wind‐speed maximum of marked directional constancy. This study has implications for longer‐term summer rainfall‐synoptic circulation variations in the American South‐west.