E. Di Lorenzo

North Pacific Gyre Oscillation links ocean climate and ecosystem change

Decadal fluctuations in salinity, nutrients, chlorophyll, a variety of zooplankton taxa, and fish stocks in the Northeast Pacific are often poorly correlated with the most widely-used index of large-scale climate variability in the region - the Pacific Decadal Oscillation (PDO). We define a new pattern of climate change, the North Pacific Gyre Oscillation (NPGO) and show that its variability is significantly correlated with previously unexplained fluctuations of salinity, nutrients and chlorophyll. Fluctuations in the NPGO are driven by regional and basin-scale variations in wind-driven upwelling and horizontal advection - the fundamental processes controlling salinity and nutrient concentrations. Nutrient fluctuations drive concomitant changes in phytoplankton concentrations, and may force similar variability in higher trophic levels. The NPGO thus provides a strong indicator of fluctuations in the mechanisms driving planktonic ecosystem dynamics. The NPGO pattern extends beyond the North Pacific and is part of a global-scale mode of climate variability that is evident in global sea level trends and sea surface temperature. Therefore the amplification of the NPGO variance found in observations and in global warming simulations implies that the NPGO may play an increasingly important role in forcing global-scale decadal changes in marine ecosystems.

Decadal variations in the California Current upwelling cells

uted to anomalous changes in the nutrient content of waters brought to the euphotic zone caused by a deeper thermocline [Roemmich and McGowan, 1995], this study focuses purely on changes in coastal upwelling (i.e., vertical velocities) as a potential mechanism for the aforementioned ecosystem changes. The goal of this study is to understand how climate variations in the North Pacific, specifically the PDO, influence the upwelling cell and the vertical fluxes of important tracers along the coast in the northeastern Pacific and to explore the potential impacts on coastal ecosystems. In this study, we use model adjoint passive tracers to elucidate how different phases of the PDO alter the threedimensional upwelling cells in the CCS.

ENSO and meridional modes: A null hypothesis for Pacific climate variability

Pacific low-frequency variability (timescale > 8 years) exhibits a well-known El Nino-like pattern of basin-scale sea surface temperature, which is found in all the major modes of Pacific decadal climate. Using a set of climate model experiments and observations, we decompose the mechanisms contributing to the growth, peak, and decay of the Pacific low-frequency spatial variance. We find that the El Nino-like interdecadal pattern is established through the combined actions of Pacific meridional modes (MM) and the El Nino–Southern Oscillation (ENSO). Specifically, in the growth phase of the pattern, subtropical stochastic excitation of the MM energizes the tropical low-frequency variance acting as a red noise process. Once in the tropics, this low-frequency variance is amplified by ocean-atmospheric feedbacks as the pattern reaches its peak phase. At the same time, atmospheric teleconnections distribute the variance from the tropics to the extratropics, where the pattern ultimately decays. In this stochastic red noise model of Pacific climate, the timescale of the extra-tropical/tropical interactions (1–2 years) permits the stochastic excitation of the ENSO-like pattern of decadal and interdecadal variance.

The Inverse Ocean Modeling System. Part II: Applications

The Inverse Ocean Modeling (IOM) System is a modular system for constructing and running weakconstraint four-dimensional variational data assimilation (W4DVAR) for any linear or nonlinear functionally smooth dynamical model and observing array. The IOM has been applied to four ocean models with widely varying characteristics. The Primitive Equations Z-coordinate-Harmonic Analysis of Tides (PEZHAT) and the Regional Ocean Modeling System (ROMS) are three-dimensional, primitive equations models while the Advanced Circulation model in 2D (ADCIRC-2D) and Spectral Element Ocean Model in 2D (SEOM-2D) are shallow-water models belonging to the general finite-element family. These models, in conjunction with the IOM, have been used to investigate a wide variety of scientific phenomena including tidal, mesoscale, and wind-driven circulation. In all cases, the assimilation of data using the IOM provides a better estimate of the ocean state than the model alone.

Correction to “North Pacific Gyre Oscillation modulates seasonal timing and ecosystem functioning in the California Current upwelling system”

[1] In the paper “North Pacific Gyre Oscillation modulates seasonal timing and ecosystem functioning in the California Current upwelling system, ” by F. Chenillat et al. (Geophysical Research Letters, 39, L01606, doi:10.1029/ 2011GL049966, 2012), Figure 4b is modified without change to the caption: black and white dots (averaged Chl-a CalCOFI data) were erroneous. The second sentence of paragraph 17 is changed in consequence to “General patterns of the twin simulations are in agreement with the main characteristics of the dynamics of the CCS (Figure 4b), albeit with a high Chl-a bias nearshore.”

Mechanisms for the emergence of ocean striations in the North Pacific

Recent observations suggest that the mean mesoscale oceanic zonal velocity field is dominated by alternating jet-like features often referred to as striations. Here the generating dynamics of Northeast Pacific striations are explored with a set of 120 year eddy-permitting model simulations. Simulations are conducted with decreasing complexity toward idealized configurations retaining the essential dynamics and forcing necessary for striation development. For each simulation, we diagnose the spin-up of the ocean model and the sensitivity of striation generation to topography, coastal geometry, and wind stress, which modulates the gyre circulation and the nonlinearity of the flow field. Results indicate that Northeast Pacific striations develop predominantly at the eastern boundary and migrate westward in congruence with beta-plumes in both the nonlinear and quasi-linear regimes. Mean striations are governed by coastline geometry, which provides quasi-steady vorticity sources energized by eastern boundary current instabilities.