Interannual Variability in the

[1] A control simulation of the GFDL CM2.1 global coupled GCM, run for 2000 years with its atmospheric composition, solar irradiance, and land cover held fixed at 1860 values, exhibits strong interdecadal and intercentennial modulation of its ENSO behavior. To the extent that such modulation is realistic, it could attach large uncertainties to ENSO metrics diagnosed from centennial and shorter records ― with important implications for historical and paleo records, climate projections, and model assessment and intercomparison. Analysis of the wait times between ENSO warm events suggests that such slow modulation need not require multidecadal memory; it can arise simply from Poisson statistics applied to ENSO's interannual time scale and seasonal phase-locking.

/pdf/are-historical-records-sufficient-to-constrain-enso-tqa7wt1ta5.pdf

Are historical records sufficient to constrain ENSO simulations

The El Nino-Southern Oscillation (ENSO), which originates in the Pacific, is the strongest and most well-known mode of tropical climate variability. Its reach is global, and it can force climate variations of the tropical Atlantic and Indian Oceans by perturbing the global atmospheric circulation. Less appreciated is how the tropical Atlantic and Indian Oceans affect the Pacific. Especially noteworthy is the multidecadal Atlantic warming that began in the late 1990s, because recent research suggests that it has influenced Indo-Pacific climate, the character of the ENSO cycle, and the hiatus in global surface warming. Discovery of these pantropical interactions provides a pathway forward for improving predictions of climate variability in the current climate and for refining projections of future climate under different anthropogenic forcing scenarios.

/pdf/pantropical-climate-interactions-408jgbbmm0.pdf

Pantropical Climate Interactions

The El Nino/Southern Oscillation in the Pacific Ocean influences temperature in other tropical ocean basins. Reanalysis data and model simulations suggest that temperature anomalies in the north tropical Atlantic may also influence the development of La Nina events.

Sea surface temperature in the north tropical Atlantic as a trigger for El Niño/Southern Oscillation events

Although persistent drought in West Africa is well documented from the instrumental record and has been primarily attributed to changing Atlantic sea surface temperatures, little is known about the length, severity, and origin of drought before the 20th century. We combined geomorphic, isotopic, and geochemical evidence from the sediments of Lake Bosumtwi, Ghana, to reconstruct natural variability in the African monsoon over the past three millennia. We find that intervals of severe drought lasting for periods ranging from decades to centuries are characteristic of the monsoon and are linked to natural variations in Atlantic temperatures. Thus the severe drought of recent decades is not anomalous in the context of the past three millennia, indicating that the monsoon is capable of longer and more severe future droughts.

https://science.sciencemag.org/content/sci/324/5925/377.full.pdf

Atlantic Forcing of Persistent Drought in West Africa

The 'Atlantic Nino' is a climate pattern similar to El Nino temperature fluctuations in surface waters of the tropical Eastern Pacific Ocean. The Atlantic Nino is recognized as an important influence on Atlantic climate, probably driven by its better known Pacific counterpart, but our current ability to predict it is limited. This is particularly perplexing since the Pacific El Nino can be predicted one or two seasons in advance by the current coupled climate models. A new study suggests that the difficulty of predicting the Atlantic Nino lies in the fact that the local ocean–atmosphere interaction in the equatorial Atlantic interferes destructively with the remote influence from Pacific El Nino. This imposes a more stringent requirement on the coupled climate models; they need not only to simulate the two competing processes, but also to predict their relative strengths and their destructive interference. El Nino, the most prominent climate fluctuation at seasonal-to-interannual timescales, has long been known to have a remote impact on climate variability in the tropical Atlantic Ocean, but a robust influence is found only in the northern tropical Atlantic region1. Fluctuations in the equatorial Atlantic are dominated by the Atlantic Nino2,3, a phenomenon analogous to El Nino, characterized by irregular episodes of anomalous warming during the boreal summer. The Atlantic Nino strongly affects seasonal climate prediction in African countries bordering the Gulf of Guinea4,5. The relationship between El Nino and the Atlantic Nino is ambiguous and inconsistent. Here we combine observational and modelling analysis to show that the fragile relationship is a result of destructive interference between atmospheric and oceanic processes in response to El Nino. The net effect of El Nino on the Atlantic Nino depends not only on the atmospheric response that propagates the El Nino signal to the tropical Atlantic, but also on a dynamic ocean–atmosphere interaction in the equatorial Atlantic that works against the atmospheric response. These results emphasize the importance of having an improved ocean-observing system in the tropical Atlantic, because our ability to predict the Atlantic Nino will depend not only on our knowledge of conditions in the tropical Pacific, but also on an accurate estimate of the state of the upper ocean in the equatorial Atlantic.

The cause of the fragile relationship between the Pacific El Niño and the Atlantic Niño

The authors argue that a reduction to the stochastic forcing of the El Nino–Southern Oscillation (ENSO) wrought by Pacific-wide climate changes in response to mid-Holocene (6000 BP) orbital forcing is a viable hypothesis for the observed reduction of ENSO activity during that time This conclusion is based on comprehensive analysis of an intermediate coupled model that achieves significant reduction to ENSO variance in response to mid-Holocene orbital forcing The model’s excellent simulation of the tropical Pacific interannual variability lends credibility to the results Idealized simulations demonstrate that the mid-Holocene influence is communicated to the tropical Pacific largely via climate changes outside of the tropical Pacific, rather than from insolation changes directly on the tropical Pacific This is particularly true for changes to the ENSO, but also with changes to the cold tongue annual cycle Previously proposed mechanisms for teleconnected mid-Holocene ENSO changes, including fo

Pacific Climate Change and ENSO Activity in the Mid-Holocene

The authors argue that a reduction to the stochastic forcing of the El Nino–Southern Oscillation (ENSO) wrought by Pacific-wide climate changes in response to mid-Holocene (6000 BP) orbital forcing is a viable hypothesis for the observed reduction of ENSO activity during that time. This conclusion is based on comprehensive analysis of an intermediate coupled model that achieves significant reduction to ENSO variance in response to mid-Holocene orbital forcing. The model’s excellent simulation of the tropical Pacific interannual variability lends credibility to the results. Idealized simulations demonstrate that the mid-Holocene influence is communicated to the tropical Pacific largely via climate changes outside of the tropical Pacific, rather than from insolation changes directly on the tropical Pacific. This is particularly true for changes to the ENSO, but also with changes to the cold tongue annual cycle. Previously proposed mechanisms for teleconnected mid-Holocene ENSO changes, including fo...

Pacific Climate Change and ENSO activity in the mid-Holocene

[1] We explore the impact of interhemispheric thermal gradients forcing on the tropical Pacific ocean-atmosphere climate in an intermediate coupled model. The equatorial zonal sea surface temperature (SST) gradient strengthens with an increased northward interhemispheric thermal gradient, the increase arising from earlier onset and later retreat of the seasonal cold tongue, and intensification during the peak cold season. When the mean interhemispheric thermal gradient is reversed, the central equatorial Pacific SST annual cycle abruptly reverses in phase, with its cold season in Mar–May rather than Sep–Nov. While startling, this response is consistent with a prevailing hypothesis that ties the cold tongue SST annual cycle phase to the hemispheric mean asymmetry of the Intertropical Convergence Zone. El Nino-Southern Oscillation activity is also sensitive to the interhemispheric thermal gradient, with peak activity occurring when the mean gradient is small, reducing rapidly as the mean gradient increases in either direction.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2008GL034166

Interhemispheric thermal gradient and tropical Pacific climate

[1] Evidence is presented that the dominant non-trend mode of interdecadal global SST variations is linked to significant modulation of ENSO variance during the past 150 years. The mode resembles the interhemispheric SST pattern linked to Sahel rainfall changes, with colder northern hemisphere conditions co-incident with higher ENSO variance. Simulations of an intermediate coupled global climate model demonstrate that this global SST pattern can drive the interdecadal changes to ENSO variance. The influence on the ENSO dynamics comes primarily from the portion of the SST change local to the tropical Pacific, which is warm over the entire equatorial Pacific particularly in the southeast during times of stronger ENSO activity. We speculate that the warming equatorial SST causes ENSO events occur earlier and thus have longer time to develop, as well as enhancing the vertical temperature gradient and thus increasing the effectiveness of oceanic entrainment.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2008GL033761

Yue Fang

Papers

The cause of the fragile relationship between the Pacific El Niño and the Atlantic Niño

Pacific Climate Change and ENSO Activity in the Mid-Holocene

Pacific Climate Change and ENSO activity in the mid-Holocene

Interhemispheric thermal gradient and tropical Pacific climate

Variation of mean sea surface temperature and modulation of El Niño–Southern Oscillation variance during the past 150 years