North Pacific Gyre Oscillation links ocean climate and ecosystem change

Abstract

International audienceDecadal 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

On the mechanisms of decadal variability of the North Pacific Gyre Oscillation over the 20th century

“…The EOF spatial pattern exhibits a dipole pattern corresponding to an intensification/a weakening of the central and eastern parts of the subtropical and subpolar gyre over the North Pacific Ocean (Figure 1a), which shares great similarities with the NPGO spatial pattern [Di Lorenzo et al, 2008]. A further examination shows that the associated time series is also highly correlated with the NPGO index defined by Di Lorenzo et al [2008] based on a model simulation, with a correlation of 0.64 (exceeding 95% confidence level). If the period is restricted from 1950 to 2004 used in their model (Regional Ocean modeling System-ROMS), the correlation can be up to 0.75 (not shown).…”

“…Di Lorenzo et al [2008] have shown that previously unexplained fluctuations of salinity, nutrients and chlorophyll observed in the northeast Pacific track the NPGO index. Di and Cloern et al [2010] further pointed out that the NPGO has more far-reaching impacts on the northeast Pacific ecosystem than the PDO, which is closely related to the primary factors of production such as the plankton and phytoplankton concentrations and thus can be used to track low-frequency climate changes in the central and eastern North Pacific oceanic ecosystem.…”

“…Previous studies have shown that the NPGO can fluctuate on decadal time scales in observation, oceanalone model and the fully coupled model [e.g., Di Lorenzo et al, 2008Chhak et al, 2009;Ceballos et al, 2009;Furtado et al, 2011Furtado et al, , 2012. However, it is very difficult to tell whether the decadal variations of the observed NPGO are significant or not due to the lack of long-term observational records.…”

“…One is the Pacific Decadal Oscillation (PDO) [Mantua et al, 1997], which features a horseshoe-like pattern, with basin-wide cold (warm) anomalies in the central North Pacific surrounded by warm (cold) anomalies along the west coast of the North America. The other is represented by a dipole, with cool (warm) anomalies north of 408N and warm (cold) anomalies south of 408N, which is commonly referred to as the North Pacific Gyre Oscillation (NPGO) [Di Lorenzo et al, 2008]. The NPGO index, defined as the time series of second empirical orthogonal function (EOF) of the SSH anomaly (SSHA) over the region from 258N to 628N and 1808W to 1108W, is statistically independent of the PDO and is more closely related to the Victoria mode proposed by Bond et al [2003].…”

“…Di and Cloern et al [2010] further pointed out that the NPGO has more far-reaching impacts on the northeast Pacific ecosystem than the PDO, which is closely related to the primary factors of production such as the plankton and phytoplankton concentrations and thus can be used to track low-frequency climate changes in the central and eastern North Pacific oceanic ecosystem. With regard to the strengthening of NPGO in recent years and its associated increasing ecological influence [Sydeman et al, 2013], the NPGO has been heavily investigated [e.g., Di Lorenzo et al, 2008Furtado et al, 2011Furtado et al, , 2012.…”