Wind-Driven Sea Level Variability on the California Coast: An Adjoint Sensitivity Analysis

Verdy, Ariane; Mazloff, Matthew R.; Cornuelle, Bruce D.; Kim, Sung Yong

doi:10.1175/jpo-d-13-018.1

Cited by 33 publications

(48 citation statements)

References 39 publications

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“…Our results and previous 2014-16 CCS anomaly attribution studies (Chao et al 2017;Frischknecht et al 2017;Myers et al 2018) are sensitive to model configurations and the geometry of the ocean volume being analyzed. As discussed in Zaba et al (2018), the 3-month assimilation window used here is shorter than that of a former long-term (2007-10) version of CASE Todd et al 2011Todd et al , 2012Verdy et al 2014) to increase model controllability of eddy variability. This 3-month window is longer than that of other CCS modeling studies (Chao et al 2018;Kurapov et al 2017;Neveu et al 2016) to capture the continuous evolution of ocean dynamics over monthly time scales.…”

Section: Discussionmentioning

confidence: 99%

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

Zaba

Rudnick

Cornuelle

et al. 2020

Journal of Physical Oceanography

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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 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 Niño event. The largest terms in the mean mixed layer (from250 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 depthdependent annual cycle of vertical velocity exhibits the strongest upward velocity between 240and 230-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 138 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 2210 to 2100 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 Niño.

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Section: Discussionmentioning

confidence: 99%

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

Zaba

Rudnick

Cornuelle

et al. 2020

Journal of Physical Oceanography

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“…Notably, while Ekman transport overestimates vertical transport, the regression line between the two still crosses through (0,0), indicating that cross‐shore geostrophic transport tends to oppose Ekman transport whether it is upwelling or downwelling favorable (Figure ) and supporting the notion that at least on short (weekly) timescales, the alongshore surface pressure gradient sets up in response to the local wind. On longer timescales, the surface pressure gradient is influenced more strongly by remote (greater than O[100 km] away) wind variability and subsequent coastal trapped wave propagation (e.g., Verdy et al, ).…”

Section: Improved Upwelling Indices For the Us West Coastmentioning

confidence: 99%

Coastal Upwelling Revisited: Ekman, Bakun, and Improved Upwelling Indices for the U.S. West Coast

et al. 2018

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Coastal upwelling is responsible for thriving marine ecosystems and fisheries that are disproportionately productive relative to their surface area, particularly in the world's major eastern boundary upwelling systems. Along oceanic eastern boundaries, equatorward wind stress and the Earth's rotation combine to drive a near-surface layer of water offshore, a process called Ekman transport. Similarly, positive wind stress curl drives divergence in the surface Ekman layer and consequently upwelling from below, a process known as Ekman suction. In both cases, displaced water is replaced by upwelling of relatively nutrient-rich water from below, which stimulates the growth of microscopic phytoplankton that form the base of the marine food web. Ekman theory is foundational and underlies the calculation of upwelling indices such as the "Bakun Index" that are ubiquitous in eastern boundary upwelling system studies. While generally valuable first-order descriptions, these indices and their underlying theory provide an incomplete picture of coastal upwelling. Here we review the relevant dynamics and limitations of classical upwelling indices, particularly related to representation of the surface wind stress, the influence of geostrophic currents, and the properties of upwelled water. To address these shortcomings, we present two new upwelling indices for the U.S. West Coast (31-47°N), which are available from 1988 to present. The Coastal Upwelling Transport Index and the Biologically Effective Upwelling Transport Index provide improved estimates of vertical transport and vertical nitrate flux, respectively, by leveraging technological and scientific advances realized since the introduction of the Bakun Index nearly a half century ago. Plain Language Summary The California Current System, running along the North American WestCoast, hosts a rich and diverse marine ecosystem that provides considerable socioeconomic benefit. The process underlying this exceptional biological productivity is wind-driven coastal upwelling, which delivers deep, nutrient-rich water to the sunlit surface layer and stimulates growth of phytoplankton that form the base of the marine food web. Given the ecological importance of upwelling, indices designed to monitor its intensity (e.g., the "Bakun Index") were introduced nearly 50 years ago. While these indices have proved extremely useful, they have a number of limitations as they are derived from relatively coarse resolution atmospheric pressure fields. In particular, uncertainties arise in the estimation of wind stress and from the omission of the influence of ocean circulation. Furthermore, historical indices estimate only the amount of water upwelled, not the nutrient content of that water. Here we present new indices that leverage ocean models, satellite data, and in situ observations to more accurately estimate upwelling strength as well as the amount of nitrate being upwelled. The new indices are publicly available, extend from 1988 to present, and will be valuable for monitoring upwelling in...

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“…We separate the linear and nonlinear responses of a given quantity by imposing positive and negative perturbations of the same magnitude in two different model runs following Verdy et al (2014) and Jones et al (2018). Given a perturbation ΔQ = Q − Q 0 , in a quantity Q, then the response of a variable H(Q) can be approximated by Taylor series expansions as:…”

Section: Appendix A: Separating Linear and Nonlinear Responsesmentioning

confidence: 99%

Heat Distribution in the Southeast Pacific Is Only Weakly Sensitive to High‐Latitude Heat Flux and Wind Stress

et al. 2019

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The Southern Ocean features regionally varying ventilation pathways that transport heat and carbon from the surface ocean to the interior thermocline on timescales of decades to centuries, but the factors that control the distribution of heat along these pathways are not well understood. In this study, we use a global ocean state estimate (ECCOv4) to (1) define the recently ventilated interior Pacific (RVP) using numerical passive tracer experiments over a 10-year period and (2) use an adjoint approach to calculate the sensitivities of the RVP heat content (RVPh) to changes in net heat flux and wind stress. We find that RVPh is most sensitive to local heat flux and wind stress anomalies north of the sea surface height contours that delineate the Antarctic Circumpolar Current, with especially high sensitivities over the South Pacific Gyre. Surprisingly, RVPh is not especially sensitive to changes at higher latitudes. We perform a set of step response experiments over the South Pacific Gyre, the subduction region, and the high-latitude Southern Ocean. In consistency with the adjoint sensitivity fields, RVPh is most sensitive to wind stress curl over the subtropical gyre, which alter isopycnal heave, and it is only weakly sensitive to changes at higher latitudes. Our results suggest that despite the localized nature of mode water subduction hot spots, changes in basin-scale pressure gradients are an important controlling factor on RVPh. Because basin-scale wind stress is expected to change in the coming decades to centuries, our results may have implications for climate, via the atmosphere/ocean partitioning of heat.Despite recent efforts to better understand the regionally specific nature of subduction and ventilation (e.g., Cerovecki et al., 2013;Jones et al., 2016), we still have relatively little knowledge on how regional variations in surface forcing and surface ocean properties can ultimately impact subduction and the properties of the ventilated region. We need a more sophisticated understanding of how changes in the location, magnitude, and variability of surface forcing can impact this critical aspect of the overturning circulation. Improvements

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Wind-Driven Sea Level Variability on the California Coast: An Adjoint Sensitivity Analysis

Cited by 33 publications

References 39 publications

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

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

Coastal Upwelling Revisited: Ekman, Bakun, and Improved Upwelling Indices for the U.S. West Coast

Heat Distribution in the Southeast Pacific Is Only Weakly Sensitive to High‐Latitude Heat Flux and Wind Stress

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