Seasonal and Interannual Variability of the California Current System: Glider Observations and a Numerical State Estimate
- Author(s): Zaba, Katherine Dorothy
- Advisor(s): Rudnick, Daniel
- et al.
The California Current System (CCS) is an ecologically and economically important coastal upwelling zone. Large-scale climate variability, like El Niño Southern Oscillation (ENSO), modulates the regional fluctuations of physical and biological properties in the CCS. This dissertation work makes use of an observational dataset from autonomous underwater gliders and model output from an ocean state estimate to characterize seasonal cycles and local interannual responses to climate variability. The California Underwater Glider Network (CUGN) was established in 2006 to obtain sustained observations of the southern CCS. For over ten years, Spray gliders have continuously measured subsurface physical and biological variables along three cross-shore sections. Data from the CUGN were processed into a gridded climatology. They were also assimilated by a general circulation model to produce the California State Estimate (CASE) for 2007-2017, which is constrained by governing physics and observations.
Together the CUGN climatology and CASE are applied to study physical variability within the CCS. The observations are used to characterize spatiotemporally-varying anomalies and the model to provide a hypothesis of the physical forcings that caused them. The focus is on seasonal to interannual timescales, paying special attention to the prolonged period of anomalously warm upper ocean temperatures during 2014-16, which included the 2014-15 marine heat wave (MHW) and the 2015-16 El Niño. As observed by the CUGN, the MHW manifested locally as a shallow, highly-stratified, surface-intensified warm pool co-occurring with downwelling anomalies. During the El Niño year, CUGN observations show the persistence of warm and downwelling anomalies throughout the CCS and even a strengthening and deepening of the anomalies in some locations, like the Southern California Bight. A positive isopycnal salinity anomaly is observed at the turn of the year 2015-2016 and interpreted to represent anomalous advection from the south. The CASE model output is rigorously assessed relative to the CUGN climatology and shown to realistically reproduce the observed mean state, annual cycles, and interannual variability, including the temperature, downwelling, and isopycnal salinity anomalies. Volume and heat budgets are calculated from CASE, quantifying the contributions of anomalous air-sea heat flux, horizontal advection, vertical advection, and mixing towards circulation and heat content changes during 2014-2016. The budgets show that multiple mechanisms played a role in forcing the anomalies, though at different times and locations within the CCS region.