Volume and Heat Budgets in the Coastal California Current System: Means, Annual Cycles, and Interannual Anomalies of 2014-16

The data-assimilating California State Estimate (CASE) enables the explicit evaluation of spatiotemporally varying volume and heat budgets in the coastal California Current System (CCS). An analysis of over 10 years of CASE model output (2007-17) diagnoses the physical drivers of the CCS mean state, annual cycles, and the 2014-16 temperature anomalies associated with a marine heat wave and an El Nino event. The largest terms in the mean mixed layer (from-50 to 0 m) volume budgets are upward vertical transport at the coast and offshore-flowing ageostrophic Ekman transport at the surface, the two branches of the coastal upwelling overturning cell. Contributions from onshore geostrophic flow in the Southern California Bight and alongshore geostrophic convergence in the central CCS balance the mean volume budgets. The depth-dependent annual cycle of vertical velocity exhibits the strongest upward velocity between -40- and -30-m depth in April. Interannual volume budgets show that over 50% of the 2013.5-16.5 time period experienced downwelling anomalies, which were balanced predominantly by alongshore transport convergence and, less often, by onshore transport anomalies. Mixed layer temperature anomalies persisted for the entirety of 2014-16, reaching a maximum of +3 degrees in October 2015. The mixed layer heat budget shows that intermittent high air-sea heat flux anomalies and alongshore and vertical heat advection anomalies all contributed to warming during 2014-16. A subsurface (from -210 to -100 m) heat budget reveals that in September 2015 anomalous poleward heat advection into the Southern California Bight by the California Undercurrent caused deeper warming during the 2015/16 El Nino.