Rethinking the Spanish Plume: Airstream Analysis Challenges the Canonical Conceptual Model

The canonical Spanish plume is a summertime southerly airstream that is heated while traveling across the Meseta Central on the Iberian Peninsula, developing into an elevated mixed layer farther poleward. A recent review of the literature paints a sometimes unevidenced, inconsistent, unclear, and inaccurate picture of the Spanish Plume, raising questions about the evidence supporting this canonical conceptual model. A case study of a Spanish plume synoptic pattern from 1–2 July 2015 is studied, using air-parcel trajectories calculated from a mesoscale model simulation to identify four distinct airstreams responsible for the unstable thermodynamic profile over the United Kingdom: (1) a near-surface continental airstream transporting hot, moist boundary-layer air from France to the United Kingdom; (2) a lid (850-hPa) airstream descended from the mid-troposphere over the eastern North Atlantic and western Iberian Peninsula, initially warm and dry, but gradually cooling, moistening, and rising while traveling northward, forming a layer of convective inhibition; (3) a subtropical airstream of hot, dry air with steep lapse rates traveling poleward from North Africa and the Mediterranean then ascending near the United Kingdom; and (4) a middle-latitude upper-level trough airstream traveling eastward from the North Atlantic Ocean then ascending near the United Kingdom. These results challenge the canonical Spanish plume synoptic pattern in three ways. First, most air reaching the United Kingdom did not travel over the Iberian Peninsula, particularly in the near-surface and lid airstreams. Second, steep lapse rates were pre-existing from the subtropics rather than created when passing over the Iberian Peninsula. Third, the lid resulted from subsidence rather than surface heating. Thus, the synoptic-scale pattern appeared to exert a larger control over the thermodynamics of the Spanish plume airstream than heating from the Iberian Peninsula. A new conceptual model is proposed that does not require surface heating over the Iberian Peninsula.