So beeinflussen die pazifischen Ozeanzyklen das globale und nordamerikanische Klima

Die Ozeanzyklen sind ein wichtiger Klimatreiber, der von der Wissenschaft viel zu lange übersehen wurde. In unserem Buch “Die kalte Sonne” haben wir darauf hingewiesen, ernteten dafür aber zunächst nur Proteste. Nun sind die Kritiker verstummt und machen sich endlich ernsthafte Gedanken. Es gilt zu hoffen, dass bald auch die Modellierer die Ozeanzyklen vollständig und korrekt in ihre Simulationen aufnehmen. Denn nur dann sind die Simulationen auch realistisch.

Der ein oder andere Leser mag bereits stöhnen: Oh, schon wieder diese Ozeanzyklen. Das haben wir doch mittlerweile verstanden, dass die wichtig sind. Richtig. Umso wichtiger ist es, die Entwicklung in der aktuellen Literatur zu dokumentieren und diskutieren. Nachdem wir gestern den Atlantik beleuchtet haben, geht es heute in den Pazifik und zu den pazifischen Ozeanzyklen. Der Pazifisch Dekadischen Oszillation (PDO) kommt hier eine besonders herausstechende Bedeutung zu.

Wir beginnen mit einer Arbeit von Diane Thompson und Kollegen im Dezember 2014 in Nature Geoscience. Ko-Autor der Studie ist Gerald Meehl, der in den letzten Jahren viele wichtige Publikationen zum Thema Ozeanzyklen und Hiatus beigesteuert hat. In diesem Paper von 2014 beleuchten die Wissenschaftler die starke globale Erwärmungsphase 1910-1940, die nicht allein durch CO2 oder andere anthropogene Treibhausgase erklärt werden kann, da die ausgestoßenen Mengen zu jener Zeit noch relativ gering waren. Vielmehr sehen Thompson und Kollegen hier natürliche Zyklen am Werke, speziell eine Abschwächung der pazifischen Passatwinde, die zur Erwärmung beigetragen haben. Als sich die Winde ab 1940 dann wieder verstärkten, stoppte die globale Erwärmung. Eine wirklich gute Arbeit. Hier der Abstract:

Early twentieth-century warming linked to tropical Pacific wind strength
Of the rise in global atmospheric temperature over the past century, nearly 30% occurred between 1910 and 1940 when anthropogenic forcings were relatively weak1. This early warming has been attributed to internal factors, such as natural climate variability in the Atlantic region, and external factors, such as solar variability and greenhouse gas emissions. However, the warming is too large to be explained by external factors alone and it precedes Atlantic warming by over a decade. For the late twentieth century, observations and climate model simulations suggest that Pacific trade winds can modulate global temperatures2, 3, 4, 5, 6, 7, but instrumental data are scarce in the early twentieth century. Here we present a westerly wind reconstruction (1894–1982) from seasonally resolved measurements of Mn/Ca ratios in a western Pacific coral that tracks interannual to multidecadal Pacific climate variability. We then reconstruct central Pacific temperatures using Sr/Ca ratios in a coral from Jarvis Island, and find that weak trade winds and warm temperatures coincide with rapid global warming from 1910 to 1940. In contrast, winds are stronger and temperatures cooler between 1940 and 1970, when global temperature rise slowed down. We suggest that variations in Pacific wind strength at decadal timescales significantly influence the rate of surface air temperature change.

Die Pressemitteilung der NCAR gibt es hier. Auch der Deutschlandfunk berichtete dankenswerterweise über die Studie seinerzeit. Weiter gehts mit einem Paper von Kevin Trenberth und Kollegen im August 2014 in Nature Climate Change. Die Autoren beleuchten die Erwärmungspause 1998-2014, also den berühmten Hiatus, der erst vom El Nino beendet wurde. Ganz besonders ausgeprägt ist der Hiatus im zentralen und östlichen Pazifik. Wieder argumentieren die Wissenschaftler mit Änderungen in den Passatwinden sowie mit dem 60-jährigen PDO-Ozeanzyklus:

Seasonal aspects of the recent pause in surface warming
Factors involved in the recent pause in the rise of global mean temperatures are examined seasonally. For 1999 to 2012, the hiatus in surface warming is mainly evident in the central and eastern Pacific. It is manifested as strong anomalous easterly trade winds, distinctive sea-level pressure patterns, and large rainfall anomalies in the Pacific, which resemble the Pacific Decadal Oscillation (PDO). These features are accompanied by upper tropospheric teleconnection wave patterns that extend throughout the Pacific, to polar regions, and into the Atlantic. The extratropical features are particularly strong during winter. By using an idealized heating to force a comprehensive atmospheric model, the large negative anomalous latent heating associated with the observed deficit in central tropical Pacific rainfall is shown to be mainly responsible for the global quasi-stationary waves in the upper troposphere. The wave patterns in turn created persistent regional climate anomalies, increasing the odds of cold winters in Europe. Hence, tropical Pacific forcing of the atmosphere such as that associated with a negative phase of the PDO produces many of the pronounced atmospheric circulation anomalies observed globally during the hiatus.

Das nächste Paper stammt aus dem Mai 2017 und wurde von Bordbar et al. verfasst. Erneut geht es um den zentralen und östlichen Pazifik, der sich in den letzten zwei Jahrzehnten spürbar abgekühlt hat. Und wieder tauchen die Passatwinde als Erklärung auf. Bordbar und Kollegen fragen sich, ob die Pazifik-Entwicklung ungewöhnlich ist und möglicherweise durch den menschengemachten Klimawandel begründet werden kann. Das Resultat ist deutlich: Die Entwicklung ist nicht vollkommen ungewöhnlich und fällt noch immer in den klimasysteminternen Bereich der natürlichen Variabilität. Abstract:

Role of internal variability in recent decadal to multidecadal tropical Pacific climate changes
While the Earth’s surface has considerably warmed over the past two decades, the tropical Pacific has featured a cooling of sea surface temperatures in its eastern and central parts, which went along with an unprecedented strengthening of the equatorial trade winds, the surface component of the Pacific Walker Circulation (PWC). Previous studies show that this decadal trend in the trade winds is generally beyond the range of decadal trends simulated by climate models when forced by historical radiative forcing. There is still a debate on the origin of and the potential role that internal variability may have played in the recent decadal surface wind trend. Using a number of long control (unforced) integrations of global climate models and several observational data sets, we address the question as to whether the recent decadal to multidecadal trends are robustly classified as an unusual event or the persistent response to external forcing. The observed trends in the tropical Pacific surface climate are still within the range of the long-term internal variability spanned by the models but represent an extreme realization of this variability. Thus, the recent observed decadal trends in the tropical Pacific, though highly unusual, could be of natural origin. We note that the long-term trends in the selected PWC indices exhibit a large observational uncertainty, even hindering definitive statements about the sign of the trends.

Weiter mit einer Veröffentlichung von Aaron Levine und Kollegen am 28. April 2017 in den Geophysical Research Letters. Die Autoren zeigen eine Verknüpfung der pazifischen und atlantischen Ozeanzyklen. Die “Stadionwelle” von Judith Curry lässt grüßen. Abstract:

The impact of the AMO on multidecadal ENSO variability
Multidecadal shifts in El Niño–Southern Oscillation (ENSO) variability have been observed, but it is unclear if this variability is just a random variation in the ENSO cycle or whether it is forced by other modes of climate variability. Here we show a strong influence of the Atlantic on the multidecadal variability of ENSO. The Atlantic Multidecadal Oscillation (AMO) is the dominant mode of multidecadal sea surface temperature (SST) variability in the Atlantic Ocean. Changes in AMO-related tropical Atlantic SSTs are known to force changes in the Walker circulation in the tropical Pacific Ocean. Using conceptual and coupled model experiments, we show that these changes to the Walker circulation modify ENSO stability on both annual and multidecadal time scales leading to a distinctive pattern of multidecadal ENSO variability that we find in observations and ocean reanalyses.

Eine Arbeit von Johnstone & Mantua 2014 untersucht die Temperaturentwicklung der letzten 100 Jahre im Nordostpazifik und findet eine klare Handschrift des PDO-Ozeanzyklus. Die Wissenschaftler vergleichen die Ergebnisse mit Modellierungen, die jedoch allesamt danebenlieben. Die theoretischen Simulationen können die real gemessenen Temperaturzyklen und den PDO-Zusammenhang nicht nachvollziehen. Was für eine Pleite. Abstract:

Atmospheric controls on northeast Pacific temperature variability and change, 1900–2012
Over the last century, northeast Pacific coastal sea surface temperatures (SSTs) and land-based surface air temperatures (SATs) display multidecadal variations associated with the Pacific Decadal Oscillation, in addition to a warming trend of ∼0.5–1 °C. Using independent records of sea-level pressure (SLP), SST, and SAT, this study investigates northeast (NE) Pacific coupled atmosphere–ocean variability from 1900 to 2012, with emphasis on the coastal areas around North America. We use a linear stochastic time series model to show that the SST evolution around the NE Pacific coast can be explained by a combination of regional atmospheric forcing and ocean persistence, accounting for 63% of nonseasonal monthly SST variance (r = 0.79) and 73% of variance in annual means (r = 0.86). We show that SLP reductions and related atmospheric forcing led to century-long warming around the NE Pacific margins, with the strongest trends observed from 1910–1920 to 1940. NE Pacific circulation changes are estimated to account for more than 80% of the 1900–2012 linear warming in coastal NE Pacific SST and US Pacific northwest (Washington, Oregon, and northern California) SAT. An ensemble of climate model simulations run under the same historical radiative forcings fails to reproduce the observed regional circulation trends. These results suggest that natural internally generated changes in atmospheric circulation were the primary cause of coastal NE Pacific warming from 1900 to 2012 and demonstrate more generally that regional mechanisms of interannual and multidecadal temperature variability can also extend to century time scales.

Significance:
Northeast Pacific coastal warming since 1900 is often ascribed to anthropogenic greenhouse forcing, whereas multidecadal temperature changes are widely interpreted in the framework of the Pacific Decadal Oscillation (PDO), which responds to regional atmospheric dynamics. This study uses several independent data sources to demonstrate that century-long warming around the northeast Pacific margins, like multidecadal variability, can be primarily attributed to changes in atmospheric circulation. It presents a significant reinterpretation of the region’s recent climate change origins, showing that atmospheric conditions have changed substantially over the last century, that these changes are not likely related to historical anthropogenic and natural radiative forcing, and that dynamical mechanisms of interannual and multidecadal temperature variability can also apply to observed century-long trends.

Die Seattle Times berichtete seinerzeit über die Studie (via WUWT).

Lesenswert auch eine Studie von Bruce Kurtz in PLOS One aus dem Juni 2015. Kurtz analysiert die Schwankungen in der Temperaturentwicklung in den USA und identifiziert regionale Unterschiede. Einige US-Gebiete werden von der PDO angetrieben, andere von der AMO, und wiederum andere von beiden. Abstract:

The Effect of Natural Multidecadal Ocean Temperature Oscillations on Contiguous U.S. Regional Temperatures
Atmospheric temperature time series for the nine climate regions of the contiguous U.S. are accurately reproduced by the superposition of oscillatory modes, representing the Atlantic multidecadal oscillation (AMO) and the Pacific decadal oscillation (PDO),
on a monotonic mode representing, at least in part, the effect of radiant forcing due to increasing atmospheric CO2. The relative importance of the different modes varies among the nine climate regions, grouping them into three mega-regions: Southeastern comprising the South, Southeast and Ohio Valley; Central comprising the Southwest, Upper Midwest, and Northeast; and Northwestern comprising the West, Northwest, and Northern Rockies & Plains. The defining characteristics of the mega-regions are: Southeastern – dominated by the AMO, no PDO influence; Central – influenced by the AMO, no PDO influence, Northwestern – influenced by both the AMO and PDO. Temperature vs. time curves calculated by combining the separate monotonic and oscillatory modes agree well with the measured temperature time series, indicating that the 1938-1974 small decrease in contiguous U.S. temperature was caused by the superposition of the downward-trending oscillatory mode on the upward-trending monotonic mode while the 1980-2000 large increase in temperature was caused by the superposition of the upward-trending oscillatory mode on the upward-trending monotonic mode. The oscillatory mode, mostly representing the AMO, was responsible for about 72% of the entire contiguous U.S. temperature increase over that time span with the contribution varying from 86 to 42% for individual climate regions.

Siehe auch Besprechung von Judith Curry.

Und noch ein Paper von Gerald Meehl und Kollegen, diesmal aus dem Juli 2015 in den Geophysical Research Letters. In der zweiten Hälfte des 20. Jahrhunderts blieb die Erwärmung im Südosten der USA aus und setzte erst nach 2000 ein. Die Autoren sehen die Ursache in einem pazifischen Ozeanzyklus. Abstract:

Disappearance of the southeast U.S. “warming hole” with the late 1990s transition of the Interdecadal Pacific Oscillation
Observed surface air temperatures over the contiguous U.S. for the second half of the twentieth century showed a slight cooling over the southeastern part of the country, the so-called “warming hole,” while temperatures over the rest of the country warmed.
This pattern reversed after 2000. Climate model simulations show that the disappearance of the warming hole in the early 2000s is likely associated with the transition of the Interdecadal Pacific Oscillation (IPO) phase from positive to negative in the tropical Pacific in the late 1990s, coincident with the early 2000s slowdown of the warming trend in globally averaged surface air temperature. Analysis of a specified convective heating anomaly sensitivity experiment in an atmosphere-only model traces the disappearance of the warming hole to negative sea surface temperature anomalies and consequent negative precipitation and convective heating anomalies in the central equatorial Pacific Ocean associated with the negative phase of the IPO after 2000.

Siehe auch Besprechung auf The Hockeyschtick.