GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 5423–5428, doi:10.1002/2013GL056756, 2013 Inner shelf intrusions of offshore water in an upwelling system affect coastal connectivity N. J. Nidzieko 1 and J. L. Largier 2 Received 1 August 2013; revised 19 September 2013; accepted 1 October 2013; published 17 October 2013. [1] Poleward warm water flows around Point Conception are an important transport mechanism linking biogeographic provinces along central California. These flows are initi- ated by relaxation of upwelling winds; the intensity and duration of upwelling is an important driver of how far north warm water penetrates against the prevailing sur- face currents. In this paper we present evidence of off- shore surface water intrusions to the inner shelf, 130 km north of Point Conception, near Cambria, California. Satellite observations show that the intrusions origi- nate as eddies generated offshore along the upwelling front. These eddies may form as submesoscale instabili- ties or by interaction of upwelling centers with offshore waters. The intrusions move southern waters to the central California inner shelf further northward than is typical for a coastal relaxation plume, and therefore, inner shelf con- nectivity and ecology may be governed over timescales and distances longer than those set by the intermittent relaxation of upwelling winds. Citation: Nidzieko, N. J., and J. L. Largier (2013), Inner shelf intrusions of offshore water in an upwelling system affect coastal connectivity, Geophys. Res. Lett., 40, 5423–5428, doi:10.1002/2013GL056756. 1. Introduction [2] The poleward propagation of warm water around Point Conception and along the central California coast during periods of relaxed upwelling-favorable winds is a well-documented feature [Winant et al., 1999, 2003; Cudaback et al., 2005; Melton et al., 2009; Washburn et al., 2011]. The westward flow from the Santa Barbara Channel and the southward surface flow along the central Califor- nia coast form a convergence zone that can extend up to 100 km southwest of Point Arguello [Winant et al., 2003]. When upwelling winds relax (down-coast (equatorward) winds generally < 5 ms –1 )[Melton et al., 2009], the equatorward pressure gradient to the north relaxes more quickly than the poleward pressure gradient in the Santa Barbara Channel generating a poleward coastal-trapped plume. These periodic flows are important because they provide a mechanism for transporting larval organisms northward against the southernward flowing California 1 Horn Point Laboratory, University of Maryland Center for Environ- mental Science, Cambridge, Maryland, USA. 2 Bodega Marine Laboratory, University of California Davis, Bodega Bay, California, USA. Corresponding author: N. J. Nidzieko, Horn Point Laboratory, Univer- sity of Maryland Center for Environmental Science, 2020 Horns Point Rd., Cambridge, MD 21613, USA. (nidzieko@umces.edu) ©2013. American Geophysical Union. All Rights Reserved. 0094-8276/13/10.1002/2013GL056756 current offshore [Winant et al., 1999]. The distance over which these plumes can move material is thought to be limited by the duration of the relaxation period between upwelling events [Melton et al. 2009; Washburn et al., 2011]. While the seasonal relaxation of wind stress (November to February) leads to a wintertime mean pole- ward flow capable of transporting material to northern California [Winant et al., 2003], during the upwelling sea- son (March to October), these relaxation events are on the order of 1–5 days [Cudaback et al., 2005] and so transport is limited to less than 100 km [Washburn et al., 2011]. [3] Point Conception is a major biogeographic bound- ary along the western North America coastline between Oregonian and Californian marine biota to the north and south, respectively. Studies of how transport affects species distributions frequently rely on temporally averaged oceano- graphic phenomena [Watson et al., 2010; White et al., 2010]. Few biogeographic breaks coincide with phylogenetic dis- continuities [Burton, 1998], however, and episodic transport phenomena that are not represented in temporally averaged model may partially explain this discrepancy. [4] In this paper we present evidence of intrusions of warm offshore surface water to the inner shelf during upwelling-favorable winds 130 km north of Point Con- ception, near White Rock, Cambria, California (Figure 1). Rather than propagating as a bottom advected buoyant coastal current [Melton et al., 2009; Washburn et al., 2011], satellite observations show that the warm water originates offshore along the upwelling front. Because these intrusions move warm Southern California Bight (SCB) waters along the central California inner shelf further northward than is typically observed within a coastal plume, they may play an important role in the ecology of upwelling areas. 2. Data Collection [5] A mooring array was deployed at White Rock, near Cambria, California from 22 April 2012 to 31 October 2012 (Figure 1, inset). The mooring array consisted of three moorings, each with an upward looking acoustic current profiler and thermistors at 2.5 m intervals throughout the water column to a subsurface float; a small spar buoy connected to the float contained a near-surface thermis- tor. The moorings were located in 10, 15, and 25 m of water, spanning the kelp forest. Thermistors sampled at 180 s intervals. The 1200 kHz profiler at 15 m depth sampled at 180 s intervals with 0.25 m vertical bin res- olution. Hourly offshore wave and meteorological data collected at National Data Buoy Center buoys 46028, 46011, and 46054 (Figure 1) were downloaded via the NDBC data portal (http://www.ndbc.noaa.gov, accessed 22 March 2013). 5423