Frictionally modified flow in a deep ocean channel: Application to the Vema Channel

The modification of the exchange flow in a deep southern hemisphere passage, resembling the Vema Channel, by frictionally induced secondary circulation is investigated numerically. The hydrostatic primitive equation model is a two-dimensional version of the sigma-coordinate Princeton Ocean Model. The time dependent response of a stratified along-channel flow, forced by barotropic or baroclinic pressure gradients, is examined. Near the bottom, where the along-channel now is retarded, there is cross-channel Ekman nux that is associated with downwelling on the eastern side and upwelling on the western side of the channel. In the presence of stratification the cross-channel flow rearranges the density structure, which in turn acts on the along-channel velocity via the thermal wind relation. Eventually the cross-isobath Ekman flux is shut down. In the case of baroclinically driven flow of Antarctic Bottom Water through the Vema Channel the model reproduces the observed shape of the deep temperature profiles and their cross-channel asymmetry. The model offers an explanation that is alternative or supplementary to inviscid multilayer hydraulic theory that;was proposed in earlier studies. It explains the extremely thick bottom boundary layers in the center and on the western slope of the channel. The deep thermocline is spread out in the west and sharpened in the east, and the coldest water is found on the eastern side of the deep trough; The modified density field reduces the along-channel flow near the bottom and focuses it into a narrow jet on the eastern side of the channel

A decadally delayed response of the tropical Pacific to Atlantic multidecadal variability

North Atlantic sea surface temperature anomalies are known to affect tropical Pacific climate variability and El Niño-Southern Oscillation (ENSO) through thermocline adjustment in the equatorial Pacific Ocean. Here coupled climate simulations featuring repeated idealized cycles of the Atlantic Multidecadal Oscillation (AMO) generated by nudging its tropical branch demonstrate that the tropical Pacific response to the AMO also entails a substantial decadally delayed component. The simulations robustly show multidecadal fluctuations in central equatorial Pacific sea surface temperatures lagging the AMO by about three decades and a subdecadal cold-to-warm transition of the tropical Pacific mean state during the AMO's cooling phase. The interplay between out-of-phase responses of seawater temperature and salinity in the western Pacific and associated density anomalies in local thermocline waters emerge as crucial factors of remotely driven multidecadal variations of the equatorial Pacific climate. The delayed AMO influences on tropical Pacific dynamics could help understanding past and future ENSO variability.North Atlantic sea surface temperature anomalies are known to affect tropical Pacific climate variability and El Niño-Southern Oscillation (ENSO) through thermocline adjustment in the equatorial Pacific Ocean. Here coupled climate simulations featuring repeated idealized cycles of the Atlantic Multidecadal Oscillation (AMO) generated by nudging its tropical branch demonstrate that the tropical Pacific response to the AMO also entails a substantial decadally delayed component. The simulations robustly show multidecadal fluctuations in central equatorial Pacific sea surface temperatures lagging the AMO by about three decades and a subdecadal cold-to-warm transition of the tropical Pacific mean state during the AMO's cooling phase. The interplay between out-of-phase responses of seawater temperature and salinity in the western Pacific and associated density anomalies in local thermocline waters emerge as crucial factors of remotely driven multidecadal variations of the equatorial Pacific climate. The delayed AMO influences on tropical Pacific dynamics could help understanding past and future ENSO variability. © 2016. American Geophysical Union. All Rights Reserved

Clarifying the Relative Role of Forcing Uncertainties and Initial‐Condition Unknowns in Spreading the Climate Response to Volcanic Eruptions

Radiative forcing from volcanic aerosol impacts surface temperatures; however, the background climate state also affects the response. A key question thus concerns whether constraining forcing estimates is more important than constraining initial conditions for accurate simulation and attribution of posteruption climate anomalies. Here we test whether different realistic volcanic forcing magnitudes for the 1815 Tambora eruption yield distinguishable ensemble surface temperature responses. We perform a cluster analysis on a superensemble of climate simulations including three 30‐member ensembles using the same set of initial conditions but different volcanic forcings based on uncertainty estimates. Results clarify how forcing uncertainties can overwhelm initial‐condition spread in boreal summer due to strong direct radiative impact, while the effect of initial conditions predominate in winter, when dynamics contribute to large ensemble spread. In our setup, current uncertainties affecting reconstruction‐simulation comparisons prevent conclusions about the magnitude of the Tambora eruption and its relation to the “year without summer.

Coupled stratosphere-troposphere-Atlantic multidecadal oscillation and its importance for near-future climate projection

Northern Hemisphere (NH) climate has experienced various coherent wintertime multidecadal climate trends in stratosphere, troposphere, ocean, and cryosphere. However, the overall mechanistic framework linking these trends is not well established. Here we show, using long-term transient forced coupled climate simulation, that large parts of the coherent NH-multidecadal changes can be understood within a damped coupled stratosphere/troposphere/ocean-oscillation framework. Wave-induced downward propagating positive stratosphere/troposphere-coupled Northern Annular Mode (NAM) and associated stratospheric cooling initiate delayed thermohaline strengthening of Atlantic overturning circulation and extratropical Atlantic-gyres. These increase the poleward oceanic heat transport leading to Arctic sea-ice melting, Arctic warming amplification, and large-scale Atlantic warming, which in turn initiates wave-induced downward propagating negative NAM and stratospheric warming and therefore reverse the oscillation phase. This coupled variability improves the performance of statistical models, which project further weakening of North Atlantic Oscillation, North Atlantic cooling and hiatus in wintertime North Atlantic-Arctic sea-ice and global surface temperature just like the 1950s-1970s

Is the observed NAO variability during the instrumental record unusual?

Observed multidecadal variability (30 yr running means, trends, and moving standard deviations) of the North Atlantic Oscillation (NAO) during the instrumental record is compared to that simulated by two different coupled general circulation models in extended-range control experiments. Simulated NAO exhibits strong low frequency fluctuations, even on multi-centennial time scale. Observed multi-decadal NAO variations agree well with the model variability. Trend probability distribution functions, observed and simulated, were not found to be different with statistical significance. Thus, multi-decadal NAO changes similar to those observed during the instrumental record, including the recent increase in 1965–1995, may be internally generated within the coupled atmosphere-ocean system without considering external forcing

Land–Atmosphere coupling sensitivity to GCMs resolution: a multimodel assessment of local and remote processes in the Sahel hot spot

Land–atmosphere interactions are often interpreted as local effects, whereby the soil state drives local atmospheric conditions and feedbacks originate. However, nonlocal mechanisms can significantly modulate land–atmosphere exchanges and coupling. We make use of GCMs at different resolutions (low ~1° and high ~0.25°) to separate the two contributions to coupling: better represented local processes versus the influence of improved large-scale circulation. We use a two-legged metric, complemented by a process-based assessment of four CMIP6 GCMs. Our results show that weakening, strengthening, and relocation of coupling hot spots occur at high resolution globally. The northward expansion of the Sahel hot spot, driven by nonlocal mechanisms, is the most notable change. The African easterly jet’s horizontal wind shear is enhanced in JJA due to better resolved orography at high resolution. This effect, combined with enhanced easterly moisture flux, favors the development of African easterly waves over the Sahel. More precipitation and soil moisture recharge produce strengthening of the coupling, where evapotranspiration remains controlled by soil moisture, and weakening where evapotranspiration depends on atmospheric demand. In SON, the atmospheric influence is weaker, but soil memory helps to maintain the coupling between soil moisture and evapotranspiration and the relocation of the hot spot at high resolution. The multimodel agreement provides robust evidence that atmospheric dynamics determines the onset of land–atmosphere interactions, while the soil state modulates their duration. Comparison of precipitation, soil moisture, and evapotranspiration against satellite data reveals that the enhanced moistening at high resolution significantly reduces model biases, supporting the realism of the hot-spot relocation

Aerosol size confines climate response to volcanic super-eruptions

Extremely large volcanic eruptions have been linked to global climate change, biotic turnover, and, for the Younger Toba Tuff (YTT) eruption 74,000 years ago, near-extinction of modern humans. One of the largest uncertainties of the climate effects involves evolution and growth of aerosol particles. A huge atmospheric concentration of sulfate causes higher collision rates, larger particle sizes, and rapid fall out, which in turn greatly affects radiative feedbacks. We address this key process by incorporating the effects of aerosol microphysical processes into an Earth System Model. The temperature response is shorter (9–10 years) and three times weaker (−3.5 K at maximum globally) than estimated before, although cooling could still have reached −12 K in some midlatitude continental regions after one year. The smaller response, plus its geographic patchiness, suggests that most biota may have escaped threshold extinction pressures from the eruption

Toward predicting volcanically-forced decadal climate variability

International audienc

Artic-North Atlantic interactions and multidecadal variability of the thermohaline circulation

Analyses of a 500-yr control integration with the non-flux-adjusted coupled atmosphere–sea ice–ocean model ECHAM5/Max-Planck-Institute Ocean Model (MPI-OM) show pronounced multidecadal fluctuations of the Atlantic overturning circulation and the associated meridional heat transport. The period of the oscillations is about 70–80 yr. The low-frequency variability of the meridional overturning circulation (MOC) contributes substantially to sea surface temperature and sea ice fluctuations in the North Atlantic. The strength of the overturning circulation is related to the convective activity in the deep-water formation regions, most notably the Labrador Sea, and the time-varying control on the freshwater export from the Arctic to the convection sites modulates the overturning circulation. The variability is sustained by an interplay between the storage and release of freshwater from the central Arctic and circulation changes in the Nordic Seas that are caused by variations in the Atlantic heat and salt transport. The relatively high resolution in the deep-water formation region and the Arctic Ocean suggests that a better representation of convective and frontal processes not only leads to an improvement in the mean state but also introduces new mechanisms determining multidecadal variability in large-scale ocean circulation