Altimeter-Derived poleward Lagrangian pathways in the California Current System: Part 2 – 2-D patterns of parcel transports

• Altimeter-derived velocities produce realistic parcel pathways in the CCS. • Stronger poleward transports from 35°N past 43°N occur mostly during El Niños. • Local alongshore poleward winds are almost always a strong forcing factor. • Tropical forcing along the Oceanic Pathway is only a factor during strong EP events. • The ‘large-cape’ region is a critical ‘leaky obstacle’ for poleward transports. In this paper, we analyze two-dimensional distributions of water parcels as they travel poleward from 35°N in the California Current System (CCS). Daily, altimeter-derived velocities are used to carry the parcels, with and without the addition of Ekman transports. Poleward-traveling parcels use the Inshore Countercurrent (ICC) off southern and central California in summer and autumn, connecting to the Davidson Current (DC) off northern California, Oregon and Washington in autumn–winter. Water parcels from southern/central California are more successful in reaching north of 43°N (Cape Blanco) when they travel in the geostrophic flow beneath the Ekman layer. The years when the greatest number of parcels arrive north of 43°N are often (not always) El Niño years. During years with successful poleward transports, local wind forcing is stronger north of 38°, especially around the large capes, 39°-43°N. These winds increase poleward geostrophic currents north of 38°-40°N and increase transports past the large-cape region. Examination of individual years demonstrates that this region is a ‘leaky obstacle’ for the poleward parcel transports, resisting those transports when there are ‘broken’ (discontinuous) patterns of poleward geostrophic velocities in the large-cape region. Distant forcing of signals from the equator along the ‘Oceanic Pathway’ primarily increase the poleward transports during El Niños with dominant eastern Pacific signatures, especially 1997–98 and 2015–16. However, this oceanic distant forcing enhances poleward geostrophic currents south of 40°N and is less effective in moving parcels around the large capes. Local wind forcing farther north is more effective.