Mechanisms of Pacific Ocean Climate & Ecosystem Variability (original) (raw)
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We examine processes that influence North Pacific sea surface temperature (SST) anomalies including surface heat fluxes, upper ocean mixing, thermocline variability, ocean currents, and tropical-extratropical interactions via the atmosphere and ocean. The ocean integrates rapidly varying atmospheric heat flux and wind forcing, and thus a stochastic model of the climate system, where white noise forcing produces a red spectrum, appears to provide a baseline for SST variability even on decadal time scales. However, additional processes influence Pacific climate variability including the "reemergence mechanism," where seasonal variability in mixed layer depth allows surface temperature anomalies to be stored at depth during summer and return to the surface in the following winter. Wind stress curl anomalies in the central/east Pacific drive thermocline variability that propagates to the west Pacific via baroclinic Rossby waves and influences SST by vertical mixing and the change in strength and position of the ocean gyres. Atmospheric changes associated with the El Niño-Southern Oscillation (ENSO) also influence North Pacific SST anomalies via the "atmospheric bridge." The dominant pattern of North Pacific SST anomalies, the Pacific Decadal Oscillation (PDO), exhibits variability on interannual as well as decadal time scales. Unlike ENSO, the PDO does not appear to be a mode of the climate system, but rather it results from several different mechanisms including (1) stochastic heat flux forcing associated with random fluctuations in the Aleutian Low, (2) the atmospheric bridge augmented by the reemergence mechanism, and (3) wind-driven changes in the North Pacific gyres.
Progress in Oceanography, 2000
Basin-scale variations in oceanic physical variables are thought to organize patterns of biological response across the Pacific Ocean over decadal time scales. Different physical mechanisms can be responsible for the diverse basin-scale patterns of sea-surface temperature (SST), mixed-layer depth, thermocline depth, and horizontal currents, although they are linked in various ways. In light of various theories and observations, we interpret observed basinwide patterns of decadal-scale variations in upper-ocean temperatures. Evidence so far indicates that large-scale perturbations of the Aleutian Low generate temperature anomalies in the central and eastern North Pacific through the combined action of net surface heat flux, turbulent mixing and Ekman advection. The surface-forced temperature anomalies in the central North Pacific subduct and propagate southwestwards in the ocean thermocline to the subtropics but apparently do not reach the equator. The large-scale Ekman pumping resulting from changes of the Aleutian Low forces western-intensified thermocline depth anomalies that are approximately consistent with Sverdrup theory. These thermocline changes are associated with SST anomalies in the Kuroshio/Oyashio Extension that are of the same sign as those in the central North Pacific, but lagged by several years. The physics of the possible feedback from the SST anomalies to the Aleutian Low, which might close a coupled ocean-atmosphere mode of decadal variability, is poorly understood and is an area of active research. The possible responses of North Pacific Ocean ecosystems to these complicated physical patterns is summarized.
ENSO and the North Pacific Gyre Oscillation: an integrated view of Pacific decadal dynamics
2009
We show that decadal dynamics of the Pacific Decadal Oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO) are linked through their relationships to ENSO. The PDO and NPGO are first order autoregressive responses of the ocean to the two dominant modes of North Pacific atmospheric variability --the Aleutian Low (AL) and the North Pacific Oscillation (NPO), respectively. The links between the PDO/AL and NPGO/NPO are found by computing the ocean/atmosphere covariability modes in the Pacific sector [40S-62N]. The first co-variability mode captures the mature phase of ENSO and its atmospheric teleconnections to the AL,
Decadal atmosphere-ocean variations in the Pacific
Climate Dynamics, 1994
Considerable evidence has emerged of a substantial decade-long change in the north Pacific atmosphere and ocean lasting from about 1976 to 1988. Observed significant changes in the atmospheric circulation throughout the troposphere revealed a deeper and eastward shifted Aleutian low pressure system in the winter half year which advected warmer and moister air along the west coast of North America and into Alaska and colder air over the north Pacific. Consequently, there were increases in temperatures and sea surface temperatures (SSTs) along the west coast of North America and Alaska but decreases in SSTs over the central north Pacific, as well as changes in coastal rainfall and streamflow, and decreases in sea ice in the Bering Sea. Associated changes occurred in the surface wind stress, and, by inference, in the Sverdrup transport in the north Pacific Ocean. Changes in the monthly mean flow were accompanied by a southward shift in the storm tracks and associated synoptic eddy activity and in the surface ocean sensible and latent heat fluxes. In addition to the changes in the physical environment, the deeper Aleutian low increased the nutrient supply as seen through increases in total chlorophyll in the water column, phytoplankton and zooplankton. These changes, along with the altered ocean currents and temperatures, changed the migration patterns and increased the stock of many fish species. A north Pacific (NP) index is defined to measure the decadal variations, and the temporal variability of the index is explored on daily, annual, interannual and decadal time scales. The dominant atmosphere-ocean relation in the north Pacific is one where atmospheric changes lead SSTs by one to two months. However, strong ties are revealed with events in the tropical Pacific, with changes in tropical Pacific SSTs leading SSTs in the north Pacific by three months. Changes in the storm tracks in the north Pacific help to reinforce and maintain the anomalous circulation in the upper troposphere. A hypothesis is put forward outlining the tropical and extratropical realtionships which stresses the role of tropical forcing but with important feed-backs in the extratropics that serve to emphasize the decadal relative to interannual time scales. The Pacific decadal timescale variations are linked to recent changes in the frequency and intensity of El Niño versus La Nina events but whether climate change associated with “global warming” is a factor is an open question.
The role of ocean dynamics in producing decadal climate variability in the North Pacific
Climate Dynamics, 2001
Decadal time scale climate variability in the North Paci®c has implications for climate both locally and over North America. A crucial question is the degree to which this variability arises from coupled ocean/ atmosphere interactions over the North Paci®c that involve ocean dynamics, as opposed to either purely thermodynamic eects of the oceanic mixed layer integrating in situ the stochastic atmospheric forcing, or the teleconnected response to tropical variability. The part of the variability that is coming from local coupled ocean/atmosphere interactions involving ocean dynamics is potentially predictable by an ocean/atmosphere general circulation model (O/A GCM), and such predictions could (depending on the achievable lead time) have distinct societal bene®ts. This question is examined using the results of fully coupled O/A GCMs, as well as targeted numerical experiments with stand-alone ocean and atmosphere models individually. It is found that coupled ocean/atmosphere interactions that involve ocean dynamics are important to determining the strength and frequency of a decadal-time scale peak in the spectra of several oceanic variables in the Kuroshio extension region o Japan. Local stochastic atmospheric heat¯ux forcing, integrated by the oceanic mixed layer into a red spectrum, provides a noise background from which the signal must be extracted. Although teleconnected ENSO responses in¯uence the North Paci®c in the 2±7 years/cycle frequency band, it is shown that some decadal-time scale processes in the North Paci®c proceed without ENSO. Likewise, although the eects of stochastic atmospheric forcing on ocean dynamics are discernible, a feedback path from the ocean to the atmosphere is suggested by the results.
ENSO drives near-surface oxygen and vertical habitat variability in the tropical Pacific
Environmental Research Letters, 2019
El Niño-Southern oscillation (ENSO) is the leading cause of sea surface temperature variability in the tropical Pacific with known impacts on tuna geographic range, but its effects on oxygen and available oxygenated habitat space are less clear. Variations in oxygenated vertical habitat space in the upper-ocean can alter interactions between predator and prey, as well as drive changes in the vulnerability of economically important tuna and other pelagic fish to surface fishing gear. Usingin situmeasurements, we show that ENSO is the primary driver of upper-ocean oxygen partial pressure (pO2) variability on year-to-year time scales in the tropical Pacific. Mechanistically, these pO2variations are primarily caused by vertical shifts in thermocline depth, which alternately elevate and depress cold, hypoxic waters from the ocean interior depending on the ENSO phase and location. Transport-driven, isopycnal pO2variations within the thermocline also play an important but secondary role. I...
Theoretical and Applied Climatology, 2013
The Pacific decadal oscillation (PDO) is defined as the first empirical orthogonal function (EOF) mode of the North Pacific sea surface temperature anomalies. In this study, we reconstructed the PDO using the first-order autoregressive model from various climate indices representing the El Niño-Southern oscillation (ENSO), Aleutian Low (AL), sea surface height (SSH), and thermocline depth over the Kuroshio-Oyashio extension (KOE) region. The climate indices were obtained from observation and twentiethcentury simulations of the eight coupled general circulation models (CGCMs) participating in the Climate Model Intercomparison Project Phase III (CMIP3). In this manner, we quantitatively assessed the major climate components generating the PDO using observation and models. Based on observations, the PDO pattern in the central to eastern North Pacific was accurately reconstructed by the AL and ENSO indices, and that in the western North Pacific was best reconstructed by the SSH and thermocline indices. In the CMIP3 CGCMs, the relative contribution of each component to the generation of the PDO varied greatly from model to model, and observations, although the PDO patterns from most of the models were similar to the pattern observed. In the models, the PDO pattern in the eastern and western North Pacific were well reconstructed using the AL and SSH indices, respectively. However, the PDO pattern reconstructed by the ENSO index was quite different from the observed pattern, which was possibly due to the model's common deficiency in simulating the amplitude and location of the ENSO. Furthermore, the differences in the contribution of the KOE thermocline index between the observed pattern and most of the models indicated that the PDO pattern associated with ocean wave dynamics is not properly simulated by most models. Therefore, the virtually well simulated PDO pattern by models is a result of physically inconsistent processes.