Jerome Fiechter

Spatially resolved upwelling in the California Current System and its connections to climate variability

A historical analysis of California Current System (CCS) circulation, performed using the Regional Ocean Modeling System with four-dimensional variational data assimilation, was used to study upwelling variability during the 1988-2010 period. We examined upwelling directly from the vertical velocity field, which elucidates important temporal and spatial variability not captured by traditional coastal upwelling indices. Through much of the CCS, upwelling within 50 km of the coast has increased, as reported elsewhere. However, from 50 to 200 km offshore, upwelling trends are negative and interannual variability is 180 ◦ out of phase with the nearshore signal. This cross-shore pattern shows up as the primary mode of variability in central and northern CCS vertical velocity anomalies, accounting for ∼40% of the total variance. Corresponding time series of the dominant modes in the central and northern CCS are strongly correlated with large-scale climate indices, suggesting that climate fluctuations may alternately favor different biological communities.

ENSO and the California Current coastal upwelling response

A 31 year (1980–2010) sequence of historical analyses of the California Current System (CCS) is used to describe the central CCS (35–43˚N) coastal upwelling response to El Nino-Southern Oscillation (ENSO) variability. The analysis period captures 10 El Nino and 10 La Nina events, including the extreme El Ninos of 1982–1983 and 1997–1998. Data-assimilative model runs and backward trajectory calculations of passive tracers are used to elucidate physical conditions and source water characteristics during the upwelling season of each year. In general, El Nino events produce anomalously weak upwelling and source waters that are unusually shallow, warm, and fresh, while La Nina conditions produce the opposite. Maximum vertical transport anomalies in the CCS occur ∼1 month after El Nino peaks in midwinter, and before the onset of the upwelling season. Source density anomalies peak later than transport anomalies and persist more strongly through the spring and summer, causing the former to impact the upwelling season more directly. As nitrate concentration covaries with density in the central CCS, El Nino may exert more influence over the nitrate concentration of upwelled waters than it does over vertical transport, although both factors are expected to reduce nitrate supply during El Nino events. Interannual comparison of individual diagnostics highlights their relative impacts during different ENSO events, as well as years deviating from the canonical response to ENSO variability. The net impact of ENSO on upwelling is explored through an “Upwelling Efficacy Index”, which may be a useful indicator of bottom-up control on primary productivity.

Application of a data‐assimilative regional ocean modeling system for assessing California Current System ocean conditions, krill, and juvenile rockfish interannual variability

To be robust and informative, marine ecosystem models and assessments require parameterized biophysical relationships that rely on realistic water column characteristics at appropriate spatial and temporal scales. We examine how hydrographic properties off California from 1990 through 2010 during late winter and spring correspond to krill and juvenile rockfish (Sebastes spp.) abundances. We evaluated coherence among temperature, salinity, depth of 26.0 potential density isopycnal, and stratification strength at regionally and monthly time scales derived from shipboard and mooring observations, and a data-assimilative Regional Ocean Model System reanalysis. The reanalysis captures spatiotemporal physical variability of coastal ocean conditions in winter and spring months and elucidates mechanisms connecting the spatial and temporal upwelling and transport dynamics on observed krill and rockfish abundances in spring. This provides evidence for a mechanistic connection between the phenology of upwelling in the California Current System and seasonal development of the shelf ecosystem.

Air-sea CO2 fluxes in the California Current: Impacts of model resolution and coastal topography

The present study uses a suite of coupled physical-biogeochemical model simulations at 1/3°, 1/10°, and 1/30° to assess the impact of horizontal resolution on air-sea CO2 fluxes in the California Current System (CCS), a relevant issue for downscaling between coarser resolution global climate models and higher resolution regional models. The results demonstrate that horizontal resolution is important to reproduce the sharp transition between near-shore outgassing and offshore absorption, as well as to resolve regions of enhanced near-shore outgassing in the lee of capes. The width of the outgassing region is overestimated when horizontal resolution is not eddy resolving (i.e., 1/3°) but becomes more dependent on shelf topography for eddy-resolving simulations (i.e., 1/10° and 1/30°). Enhanced near-shore outgassing is associated with a local increase in wind-driven upwelling in the lee of capes (i.e., expansion fans), meaning that sufficient horizontal resolution is needed both in the ocean circulation model and in the wind field forcing the model. From a global carbon budget perspective, the model indicates that biological production generates sufficient absorption within a few hundred kilometers of the coast to offset near-shore outgassing, which is consistent with the notion that midlatitude eastern boundary current upwelling systems act both as a sink and source for atmospheric CO2. Based on the 1/30° solution, the CCS between 35 and 45 N and out to 600 km offshore is a net carbon sink of approximately 6 TgC yr−1, with the 1/10° solution underestimating this value by less than 10% and the 1/3° solution by a factor of 3.

A 4D-Var Analysis System for the California Current: A Prototype for an Operational Regional Ocean Data Assimilation System

In this chapter we will describe a comprehensive 4-dimensional variational ocean data assimilation system that is currently being used in the Regional Ocean Model System for the production of both near real-time and historical ocean analyses of the California Current circulation. The main focus of this article is on the practical aspects of the data assimilation system as applied to an energetic coastal mesoscale circulation environment.

Environmental conditions impacting juvenile Chinook salmon growth off central California: An ecosystem model analysis

A fully coupled ecosystem model is used to identify the effects of environmental conditions and upwelling variability on growth of juvenile Chinook salmon in central California coastal waters. The ecosystem model framework consists of an ocean circulation submodel, a biogeochemical submodel, and an individual-based submodel for salmon. Simulation results indicate that years favorable for juvenile salmon growth off central California are characterized by particularly intense early season upwelling (i.e., March through May), leading to enhanced krill concentrations during summer near the location of ocean entry (i.e., Gulf of the Farallones). Seasonally averaged growth rates in the model are generally consistent with observed values and suggest that juvenile salmon emigrating later in the season (i.e., late May and June) achieve higher weight gains during their first 90 days of ocean residency.

Projecting marine mammal distribution in a changing climate

Climate-related shifts in marine mammal range and distribution have been observed in some populations; however, the nature and magnitude of future responses are uncertain in novel environments projected under climate change. This poses a challenge for agencies charged with management and conservation of these species. Specialized diets, restricted ranges, or reliance on specific substrates or sites (e.g., for pupping) make many marine mammal populations particularly vulnerable to climate change. High-latitude, predominantly ice-obligate, species have experienced some of the largest changes in habitat and distribution and these are expected to continue. Efforts to predict and project marine mammal distributions to date have emphasized data-driven statistical habitat models. These have proven successful for short time-scale (e.g., seasonal) management activities, but confidence that such relationships will hold for multi-decade projections and novel environments is limited. Recent advances in mechanistic modeling of marine mammals (i.e., models that rely on robust physiological and ecological principles expected to hold under climate change) may address this limitation. The success of such approaches rests on continued advances in marine mammal ecology, behavior, and physiology together with improved regional climate projections. The broad scope of this challenge suggests initial priorities be placed on vulnerable species or populations (those already experiencing declines or projected to undergo ecological shifts resulting from climate changes that are consistent across climate projections) and species or populations for which ample data already exist (with the hope that these may inform climate change sensitivities in less well observed species or populations elsewhere). The sustained monitoring networks, novel observations, and modeling advances required to more confidently project marine mammal distributions in a changing climate will ultimately benefit management decisions across time-scales, further promoting the resilience of marine mammal populations.

Observing System Impacts on Estimates of California Current Transport

Abstract The impact of different observing platforms and control vector elements on 4D-Var analyses of California Current transport are explored using the adjoint Kalman gain matrix to map a transport metric into observation space. The contribution of control vector elements on the metric provide a useful measure for tracking performance characteristics of the 4D-Var algorithm, and they can be used to detect potential inconsistencies within the analysis system. Observation impact calculations provide detailed information about the contribution of each observation or observing platform on the transport. A novel aspect of this work is that it provides a direct quantitative measure of the observing system impact on ocean state estimates spanning three decades, and it reveals the complex interplay between different observing platforms within the 4D-Var analyses as different observing systems become available. The method employed here requires considerably less computational effort than more traditional ones.