Teleconnections from the tropics energize variations of the North Pacific climate, but detailed diagnosis of this relationship has proven difficult. Simple univariate methods, such as regression on ...El Niño–Southern Oscillation (ENSO) indices, may be inadequate since the key dynamical processes involved—including ENSO diversity in the tropics, re-emergence of mixed layer thermal anomalies, and oceanic Rossby wave propagation in the North Pacific—have a variety of overlapping spatial and temporal scales. Here we use a multivariate linear inverse model to quantify tropical and extratropical multiscale dynamical contributions to North Pacific variability, in both observations and CMIP6 models. In observations, we find that the tropics are responsible for almost half of the seasonal variance, and almost three-quarters of the decadal variance, along the North American coast and within the Subtropical Front region northwest of Hawaii. SST anomalies that are generated by local dynamics within the northeast Pacific have much shorter time scales, consistent with transient weather forcing by Aleutian low anomalies. Variability within the Kuroshio–Oyashio Extension (KOE) region is considerably less impacted by the tropics, on all time scales. Consequently, without tropical forcing the dominant pattern of North Pacific variability would be a KOE pattern, rather than the Pacific decadal oscillation (PDO). In contrast to observations, most CMIP6 historical simulations produce North Pacific variability that maximizes in the KOE region, with amplitude significantly higher than observed. Correspondingly, the simulated North Pacific in all CMIP6 models is shown to be relatively insensitive to the tropics, with a dominant spatial pattern generally resembling the KOE pattern, not the PDO.
Off-equatorial wind anomalies on seasonal timescales from both the North and South Pacific, known as "precursors" of the El Niño Southern Oscillation (ENSO), have been shown to independently trigger ...the ENSO feedbacks in the tropics and its teleconnections to the extra-tropics. However, the impacts of ENSO precursors on Tropical Pacific Decadal-scale Variability (TPDV) is still not well understood and quantified. We show that the dynamic sequence from extra-tropical ENSO precursors to ENSO (tropics) to extra-tropical ENSO teleconnections is not only important for ENSO, but acts as a primary mechanism to filter (e.g. reddening) the low-frequency variability of the seasonal precursors into the decadal-scale variance of the Pacific basin, accounting for the largest fraction of the TPDV (~65%) and its phase. This process, which contrasts previous theories advocating for a TPDV generated internally in the tropics (e.g. ENSO residuals), is inherently unpredictable and not well reproduced in climate models and raises challenges for understanding and predicting the role of internal TPDV in future climate change scenarios.
Observational analyses suggest that a significant fraction of the tropical Pacific decadal variability (TPDV) (∼60%–70%) is energized by the combined action of extratropical precursors of El ...Niño–Southern Oscillation (ENSO) originating from the North and South Pacific. Specifically, the growth and decay of the basin-scale TPDV pattern (time scale = ∼1.5–2 years) is linked to the following sequence: ENSO precursors (extratropics, growth phase) → ENSO (tropics, peak phase) → ENSO successors (extratropics, decay phase) resulting from ENSO teleconnections. This sequence of teleconnections is an important physical basis for Pacific climate predictability. Here we examine the TPDV and its connection to extratropical dynamics in 20 models from phase 5 of the Coupled Model Intercomparison Project (CMIP). We find that most models (∼80%) can simulate the observed spatial pattern (R > 0.6) and frequency characteristics of the TPDV. In 12 models, more than 65% of the basinwide Pacific decadal variability (PDV) originates from TPDV, which is comparable with observations (∼70%). However, despite reproducing the basic spatial and temporal statistics, models underestimate the influence of the North and South Pacific ENSO precursors to the TPDV, and most of the models’ TPDV originates in the tropics. Only 35%–40% of the models reproduce the observed extratropical ENSO precursor patterns (R > 0.5). Models with a better representation of the ENSO precursors show 1) better basin-scale signatures of TPDV and 2) stronger ENSO teleconnections from/to the tropics that are consistent with observations. These results suggest that better representation of ENSO precursor dynamics in CMIP may lead to improved Pacific decadal variability dynamics and predictability.
The climate variability of the Kuroshio-Oyashio Extension (KOE) and North Pacific Transition Zone (NPTZ) exerts a strong control on marine populations that are sensitive to the strong productivity ...gradients between the subtropical and subpolar recirculation gyres. In observations, the relationship between KOE, NPTZ and productivity is evident in the first two dominant covariability modes between sea surface height and Chlorophyll-a anomalies, which are associated with a meridional shift in the location of the KOE (e.g. shift mode 1) and an intensification of the mean circulation of the KOE (e.g. intensification mode 2). To understand the projected impacts of anthropogenic forcing on the NPTZ, we examine these two dominant modes of variability in the Community Earth System Model Large Ensemble (CESM-LE) and an ensemble of climate models from the Coupled Model Intercomparing Project (CMIP5-E) under the Representative Concentration Pathways (RCP8.5). A significant poleward shift has been found in the KOE mean location associated with an equivalent shift of the Aleutian Low atmospheric pressure system. Superimposed to the changes of the mean, we find a significant increase (15–20%) in the variability of the shift mode in both the CESM-LE and CMPI5-E suggesting that variation in the meridional position of the NPTZ are also becoming stronger. These changes in variance of the shift mode are linked to an increase variability of the atmospheric forcing.
UNDERSTANDING ENSO DIVERSITY Capotondi, Antonietta; Wittenberg, Andrew T.; Newmaman, Matthew ...
Bulletin of the American Meteorological Society,
06/2015, Letnik:
96, Številka:
6
Journal Article
Recenzirano
Odprti dostop
El Niño–Southern Oscillation (ENSO) is a naturally occurring mode of tropical Pacific variability, with global impacts on society and natural ecosystems. While it has long been known that El Niño ...events display a diverse range of amplitudes, triggers, spatial patterns, and life cycles, the realization that ENSO’s impacts can be highly sensitive to this event-to-event diversity is driving a renewed interest in the subject. This paper surveys our current state of knowledge of ENSO diversity, identifies key gaps in understanding, and outlines some promising future research directions.
Pacific salmon are a dominant component of the northeast Pacific ecosystem. Their status is of concern because salmon abundance is highly variable—including protected stocks, a recently closed ...fishery, and actively managed fisheries that provide substantial ecosystem services. Variable ocean conditions, such as the Pacific Decadal Oscillation (PDO), have influenced these fisheries, while diminished diversity of freshwater habitats have increased variability via the portfolio effect. We address the question of how recent changes in ocean conditions will affect populations of two salmon species. Since the 1980s, El Niño Southern Oscillation (ENSO) events have been more frequently associated with central tropical Pacific warming (CPW) rather than the canonical eastern Pacific warming ENSO (EPW). CPW is linked to the North Pacific Gyre Oscillation (NPGO), whereas EPW is linked to the PDO, different indicators of northeast Pacific Ocean ecosystem productivity. Here we show that both coho and Chinook salmon survival rates along western North America indicate that the NPGO, rather than the PDO, explains salmon survival since the 1980s. The observed increase in NPGO variance in recent decades was accompanied by an increase in coherence of local survival rates of these two species, increasing salmon variability via the portfolio effect. Such increases in coherence among salmon stocks are usually attributed to controllable freshwater influences such as hatcheries and habitat degradation, but the unknown mechanism underlying the ocean climate effect identified here is not directly subject to management actions.
The marine heatwave of 2014/2015 in the Northeast Pacific caused significant impacts on marine ecosystems and fisheries. While several studies suggest that land and marine heatwaves may intensify ...under climate change, less is known about the prolonged multiyear nature (~2 years) of the Northeast Pacific events. Examination of reanalysis products and a 30‐member climate model ensemble confirms that prolonged multiyear marine heatwaves are linked to the dynamics of the two dominant modes of winter sea surface temperature variability in the North Pacific, the Pacific Decadal Oscillation (PDO), and the North Pacific Gyre Oscillation (NPGO). Specifically, we find a significant correlation between winter warm NPGO anomalies and the following winter PDO arising from extratropical/tropical teleconnections. In the model projections for 2100 under the RCP8.5 scenario, this NPGO/PDO 1 year lag correlation exhibits a significant positive trend (~35%) that favors more prolonged multiyear warm events (>1°C) with larger spatial coverage (~18%) and higher maximum amplitude (~0.5°C for events >2°C) over the Northeast Pacific.
Plain Language Summary
Between the winters of 2014 and 2015 the Northeast Pacific experienced the largest and longest marine heatwave ever recorded in the instrumental record. A distinguishing feature of this event is the multiyear persistence of the ocean warm anomalies from one winter to the other. By analyzing and comparing different reanalysis products and an ensemble of climate model projections for 2100, we find that the observational trend for stronger winter to winter persistence of anomalies in the Northeast Pacific is consistent with climate model projections under the RCP8.5 radiative forcing scenario. We link this trend to an increase coupling between the two dominant modes of North Pacific decadal variability.
Key Points
Multiyear SST warm events in the Northeast Pacific typically emerge as a winter NPGO‐like warm pattern and transition to a PDO‐like pattern in the following winter
The coupling between winter NPGO and the following winter PDO is a robust climate teleconnection in both observations and the CESM‐LENS over the period 1920‐2100
A stronger NPGO‐PDO coupling is predicted under anthropogenic forcing in the CESM‐LENS and leads to more prolonged and larger area multiyear marine heatwaves
Quasi-decadal climate of the Kuroshio Extension (KE) is pivotal to understanding the North Pacific coupled ocean-atmosphere dynamics and their predictability. Recent observational studies suggest ...that extratropical-tropical coupling between the KE and the central tropical Pacific El Niño Southern Oscillation (CP-ENSO) leads to the observed preferred decadal time-scale of Pacific climate variability. By combining reanalysis data with numerical simulations from a high-resolution climate model and a linear inverse model (LIM), we confirm that KE and CP-ENSO dynamics are linked through extratropical-tropical teleconnections. Specifically, the atmospheric response to the KE excites Meridional Modes that energize the CP-ENSO (extratropicstropics), and in turn, CP-ENSO teleconnections energize the extratropical atmospheric forcing of the KE (tropicsextratropics). However, both observations and the model show that the KE/CP-ENSO coupling is non-stationary and has intensified in recent decades after the mid-1980. Given the short length of the observational and climate model record, it is difficult to attribute this shift to anthropogenic forcing. However, using a large-ensemble of the LIM we show that the intensification in the KE/CP-ENSO coupling after the mid-1980 is significant and linked to changes in the KE atmospheric downstream response, which exhibit a stronger imprint on the subtropical winds that excite the Pacific Meridional modes and CP-ENSO.
Pacific decadal variability has strong impacts on the statistics of weather, atmosphere extremes, droughts, hurricanes, marine heatwaves, and marine ecosystems. Sea surface temperature (SST) ...observations show that the variance of the El Niño‐like decadal variability has increased by ~30% (1920–2015) with a stronger coupling between the major Pacific climate modes. Although we cannot attribute these trends to global climate change, the examination of 30 members of the Community Earth System Model Large Ensemble (LENS) forced with the RCP8.5 radiative forcing scenario (1920–2100) suggests that significant anthropogenic trends in Pacific decadal variance will emerge by 2020 in response to a more energetic North Pacific Meridional Mode (PMM)—a well‐known El Niño precursor. The PMM is a key mechanism for energizing and coupling tropical and extratropical decadal variability. In the LENS, the increase in PMM variance is consistent with an intensification of the winds‐evaporation‐SST thermodynamic feedback that results from a warmer mean climate.
Plain Language Summary
Decadal variability modulates weather, droughts, hurricanes, and marine heatwaves in the Pacific Ocean with dramatic societal and ecological impacts. Understanding how decadal variability may change in a warming climate remains difficult to assess because of the limited observational record and poor reproducibility of decadal dynamics in climate projection models. We combine theory with available reanalysis products and a large climate model ensemble, to show that the Pacific decadal variance increases under anthropogenic forcing as a result of stronger thermodynamic coupling between ocean and atmosphere. Given that thermodynamic coupling is also increasing in other ocean basins, this study provides a mechanistic framework to understand the amplification of climate variability on global scales under anthropogenic forcing.
Key Points
The low‐frequency variance of the main Pacific climate modes might increase under anthropogenic forcing
The coupling between ENSO and the Pacific Meridional mode increases under anthropogenic forcing
North and South Pacific Meridional Modes (NPMM and SPMM) are known precursors of El Niño–Southern Oscillation (ENSO) and Tropical Pacific decadal variability (TPDV). However, the relative importance ...of these precursors and the timescale on which they impact the tropics remain unclear. Using a 30‐member ensemble of the Community Earth System Model as the control climate, we generate two additional members where the NPMM and SPMM are selectively suppressed. We find that both meridional modes energize the tropical variance independently on different timescales. The absence of NPMM leads to a significant reduction of the tropical interannual variability (~35%), while the absence of the SPMM has no appreciable impact on ENSO but significantly reduces the TPDV (~30%). While the relative importance of the NPMM and SPMM may be model dependent, the stochastic atmospheric variability in the extratropics that energizes the meridional modes emerges as a key source of TPDV.
Plain Language Summary
El Niño–Southern Oscillation (ENSO) and the Tropical Pacific decadal variability (TPDV) exert a strong influence over the climate of large parts of the world. To understand and predict tropical Pacific variability, it is necessary to study all those oceanic and atmospheric processes that act as triggers and energizers of ENSO and TPDV, also referred to as the precursor dynamics. The North and South Pacific Meridional Modes (NPMM and SPMM) are well‐known precursors of ENSO and TPDV; however, their relative importance and the timescale on which they exert their influence remain unclear. We use a climate model to perform simulations where either NPMM or SPMM variability are selectively suppressed. We find that the absence of NPMM leads to a significant reduction of ENSO (~35%), while the absence of the SPMM has no appreciable impact on ENSO but significantly reduces the TPDV (~30%). The NPMM and SPMM modes impact the tropical variance independently on interannual and decadal timescale, respectively.
Key Points
The absence of NPMM variability in CESM leads to a significant reduction of the tropical interannual variability (~35%)
The absence of the SPMM does not impact the ENSO spectrum but leads to a significant reduction of the decadal variability (~30%)
The NPMM and SPMM modes energize the tropical variance independently on interannual and decadal timescale respectively
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