The influence of decadal oscillations on the oxygen and nutrient trends in the Pacific Ocean

Abstract

<p><strong>Abstract.</strong> A strong oxygen deficient layer is located in the upper layer of the tropical Pacific Ocean and at deeper depths in the North Pacific. Processes related to climate change (upper ocean warming, reduced ventilation) are expected to change ocean oxygen and nutrient inventories. In most ocean basins, a decrease in oxygen (‘deoxygenation’) and an increase of nutrients has been observed in subsurface layers. Deoxygenation trends are not linear and there could be other influences on oxygen and nutrient trends and variability. Here oxygen and nutrient time series since 1950 in the Pacific Ocean were investigated at 50 to 300 m depth, as this layer provides critical pelagic habitat for biological communities. In addition to trends related to ocean warming the oxygen and nutrient trends show a strong influence of the Pacific Decadal Oscillation (PDO) in the tropical and the eastern Pacific, and the North Pacific Gyre Oscillation (NPGO) especially in the North Pacific. In the Oyashio Region the PDO, the NPGO, the North Pacific Index (NPI) and a 18.6 year nodal tidal cycle overlay the long-term trend. In most regions oxygen increases and nutrients decrease in the 50 to 300 m layer during the negative PDO phase, with opposite trends during the positive PDO phase. The PDO index encapsulates the major mode of surface temperature variability in the Pacific and oxygen and nutrients trends throughout the basin can be described in the context of the PDO phases. An influence of the subtropical-tropical cell in the tropical Pacific cannot be proven with the available data. El Niño and La Niña years often influence the oxygen and nutrient distribution during the event in the eastern tropical Pacific, but do not have a multi-year influence on the trends.</p>

Moving on up: Vertical distribution shifts in rocky reef fish species during climate‐driven decline in dissolved oxygen from 1995 to 2009

“…While signals of ocean deoxygenation driven by anthropogenic climate change have emerged globally (Stramma et al, 2012 ; Keeling et al, 2010 ; Helm et al, 2011 ), natural climate oscillations are superimposed on these trends and can confound mechanistic explanations. The visual surveys in this study coincided with a period of anomalously low DO concentrations in the California Current region (Bograd et al, 2008 ; McClatchie et al, 2010 ), likely linked to a shallow thermocline characterized by a negative (cool) phase of the Pacific Decadal Oscillation (PDO) (Deutsch et al, 2011 ; Stramma et al, 2019 ). The rapid rates of change in depth distribution estimated for the rocky reef fish species in this study occurred during a period of rapid oxygen decline; future decreases in DO driven by anthropogenic climate change may be more gradual.…”

“…Additionally, while historical declines in DO provide some foresight into ecosystem response to impending anthropogenic climate change, other environmental factors such as nutrient availability will respond differently to natural and anthropogenic climate variability. For example, negative phases of the PDO are linked to colder and more productive environments (Deutsch et al, 2011 ; Stramma et al, 2019 ), although not necessarily in the California Current System (Di Lorenzo et al, 2008 ), whereas anthropogenic warming is projected to correspond to a decrease in nutrient availability (Behrenfeld et al, 2006 ; Polovina et al, 2008 ). Further study is required to tease apart the relative impacts of both natural and anthropogenic climate processes and predict future changes in fish distribution.…”

“…Marked declines in dissolved oxygen (DO) concentrations and shoaling of the oxygen minimum zone (OMZ) have been observed globally since 1960 (Stramma et al, 2008 ). These anthropogenic processes are superimposed on natural multidecadal oscillations (Deutsch et al, 2011 ; Stramma et al, 2019 ), seasonal patterns (Boyer et al, 1999 ; Connolly et al, 2010 ) and storm impacts (Van Dolah & Anderson, 1991 ; Xu et al, 2019 ) on ocean oxygen content.…”

Referee comment for bg-2019-91

“…For trends calculations, Thomson and Emery (2014) propose dividing the length of the time series N by an integral time scale T in order to obtain an effective number of degrees of freedom N* = N/T. Using their method for estimating an integral time scale by including lags of up to plus or minus 10 years for example, I obtained an integral time scale for the PDO time series of 2.8 years.…”

Biogeochemical feedbacks may amplify ongoing and future ocean deoxygenation: a case study from the Peruvian oxygen minimum zone

“…In addition to ocean warming, natural variability induced by e.g. the Pacific Decadal Oscillation has contributed to the observed oxygen depletion (Deutsch et al 2011;Stramma et al 2020). Moreover, biogeochemical feedbacks may have amplified the OMZ expansion since marine sediments release dissolved iron and phosphorus into the water column where low oxygen waters interact with the seafloor (Berelson et al 2003;Dale et al 2015;Ingall et al 1993;Ingall and Jahnke 1994;Klar et al 2018;McManus et al 1997;Plass et al 2020;Rapp et al 2019;Wallmann 2003Wallmann , 2010.…”