Ham, Y.-G., J.-S. Kug, J.-Y. Park, and F.-F. Jin, 2013: “Sea surface temperature in the north tropical Atlantic as a trigger for El Niño/Southern Oscillation events.” Nature Geoscience, doi: 10.1038/ngeo1686.
El Niño events, the warm phase of the El Niño/Southern Oscillation (ENSO), are known to affect other tropical ocean basins through teleconnections. Conversely, mounting evidence suggests that temperature variability in the Atlantic Ocean may also influence ENSO variability. Here we use reanalysis data and general circulation models to show that sea surface temperature anomalies in the north tropical Atlantic during the boreal spring can serve as a trigger for ENSO events. We identify a subtropical teleconnection in which spring warming in the north tropical Atlantic can induce a low-level cyclonic atmospheric flow over the eastern Pacific Ocean that in turn produces a low-level anticyclonic flow over the western Pacific during the following months. This flow generates easterly winds over the western equatorial Pacific that cool the equatorial Pacific and may trigger a La Niña event the following winter. In addition, El Niño events led by cold anomalies in the north tropical Atlantic tend to be warm-pool El Niño events, with a centre of action located in the central Pacific, rather than canonical El Niño events. We suggest that the identification of temperature anomalies in the north tropical Atlantic could help to forecast the development of different types of El Niño event.
Subscription Required
Condrona, A., and P. Winsor, 2012: “Meltwater routing and the Younger Dryas.” Proceedings of the National Academy of Sciences, v. 109, 19,928-19,933, doi: 10.1073/pnas.1207381109.
The Younger Dryas—the last major cold episode on Earth—is generally considered to have been triggered by a meltwater flood into the North Atlantic. The prevailing hypothesis, proposed by Broecker et al. [1989 Nature 341:318-321] more than two decades ago, suggests that an abrupt rerouting of Lake Agassiz overflow through the Great Lakes and St. Lawrence Valley inhibited deep water formation in the subpolar North Atlantic and weakened the strength of the Atlantic Meridional Overturning Circulation (AMOC). More recently, Tarasov and Peltier [2005 Nature 435:662-665] showed that meltwater could have discharged into the Arctic Ocean via the Mackenzie Valley ~4,000 km northwest of the St. Lawrence outlet. Here we use a sophisticated, high-resolution, ocean sea-ice model to study the delivery of meltwater from the two drainage outlets to the deep water formation regions in the North Atlantic. Unlike the hypothesis of Broecker et al., freshwater from the St. Lawrence Valley advects into the subtropical gyre ~3,000 km south of the North Atlantic deep water formation regions and weakens the AMOC by <15%. In contrast, narrow coastal boundary currents efficiently deliver meltwater from the Mackenzie Valley to the deep water formation regions of the subpolar North Atlantic and weaken the AMOC by >30%. We conclude that meltwater discharge from the Arctic, rather than the St. Lawrence Valley, was more likely to have triggered the Younger Dryas cooling.
Subscription Required
Winschall, A., S. Pfahl, H. Sodemann, and H. Wernli, 2012: “Impact of North Atlantic evaporation hot spots on southern Alpine heavy precipitation events.” Quarterly Journal of the Royal Meteorological Society, v. 138, pp. 1245-1258, doi: 10.1002/qj.987.
This study investigates transient events of intense ocean evaporation with an amplitude exceeding 250 W m-2, a duration of a few days, and a spatial extent of about 106 km2 over the eastern North Atlantic (referred to as ‘evaporation hot spots’) and their impact on southern Alpine heavy precipitation. First, moisture sources for a heavy precipitation event in the Piedmont in November 2002 are studied using a water-tagging simulation with a regional model. The results reveal three main moisture sources: land evapotranspiration, and evaporation from the Mediterranean and the North Atlantic, with the last source contributing the most. This was partly due to an evaporation hot spot that appeared along the western edge of a prominent upper-level trough about two days prior to the onset of heavy precipitation. In the hot spot area strong surface winds induced by the upper-level trough led to intense evaporation of water that was transported around the trough to the Piedmont region during subsequent days, where it contributed to the heavy precipitation. Secondly, analyses by the European Centre for Medium-Range Weather Forecasts (ECMWF) are used to investigate climatologically the potential relationship between eastern North Atlantic evaporation hot spots and southern Alpine precipitation. During a 10-year time period, 42 hot spots have been identified in the eastern North Atlantic. It is shown that they typically occur along the western flank of prominent upper-level troughs, and that the evaporating moisture is transported to Europe within one to four days. A climatological analysis of southern Alpine heavy precipitation events shows that they are frequently preceded by intense North Atlantic evaporation. Hence the climatological analysis further supports the conclusion from the Piedmont 2002 tagging experiment that intense evaporation over the North Atlantic and the subsequent moisture transport, both induced by the upper-level trough, are potential key factors for the development of southern Alpine heavy precipitation events.
Subscription Required
Hannachi, A., T. Woollings, and K. Fraedrich, 2012: “The North Atlantic jet stream: a look at preferred positions, paths and transitions.” Quarterly Journal of the Royal Meteorological Society, v. 138, pp. 862–877, doi: 10.1002/qj.959.
Preferred jet stream positions and their link to regional circulation patterns over the winter North Atlantic/European sector are investigated to corroborate findings of multimodal behavior of the jet positions and to analyze patterns of preferred paths and transition probabilities between jet regimes using ERA-40 data. Besides the multivariate Gaussian mixture model, hierarchical clustering and data image techniques are used for this purpose. The different approaches all yield circulation patterns that correspond to the preferred jet regimes, namely the southern, central and the northern positions associated respectively with the Greenland anticyclone or blocking, and two opposite phases of an East Atlantic-like flow pattern. Growth and decay patterns as well as preferred paths of the system trajectory are studied using the mixture model within the delay space. The analysis shows that the most preferred paths are associated with central to north and north to south jet stream transitions with a typical time-scale of about 5 days, and with life cycles of 1–2 weeks. The transition paths are found to be consistent with transition probabilities. The analysis also shows that wave breaking seems to be the dominant mechanism behind Greenland blocking.
Subscription Required
Not, C., and C. Hillaire-Marcel, 2012: “Enhanced sea-ice export from the Arctic during the Younger Dryas.” Nature Communications, v. 3, article no. 647, doi: 10.1038/ncomms1658.
The Younger Dryas cold spell of the last deglaciation and related slowing of the Atlantic meridional overturning circulation have been linked to a large array of processes, notably an influx of fresh water into the North Atlantic related to partial drainage of glacial Lake Agassiz. Here we observe a major drainage event, in marine sediment cores raised from the Lomonosov Ridge, in the central Arctic Ocean marked by a pulse in detrital dolomitic-limestones. This points to an Arctic-Canadian sediment source area with about fivefold higher Younger Dryas ice-rafting deposition rate, in comparison with the Holocene. Our findings thus support the hypothesis of a glacial drainage event in the Canadian Arctic area, at the onset of the Younger Dryas, enhancing sea-ice production and drifting through the Arctic, then export through Fram Strait, towards Atlantic meridional overturning circulation sites of the northern North Atlantic.
Subscription Required
Hobbs, W.R., and J.K. Willis, 2012: “Midlatitude North Atlantic heat transport: A time series based on satellite and drifter data.” Journal of Geophysical Research, v. 117, paper no. C01008, doi: 10.1029/2011JC007039.
Using temperature, salinity, and displacement data from Argo floats combined with satellite sea surface height, a time series of the Atlantic meridional heat transport from January 2002 to August 2010 has been estimated for 41°N. The calculation method is validated against hydrographic climatologies and output from the ECCO2 ocean data assimilation model, and the assumptions are shown to be reasonable; the greatest source of error is from the sparse distribution of Argo floats. The mean heat transport is 0.50 ± 0.1 PW, which is consistent with previous estimates made using surface flux data but is low compared estimates from hydrographic cruise data. Consistent with results from the RAPID array, the heat transport has a significant annual cycle and high degree of subannual variability, indicating that statistical uncertainty in previous calculations may have been underestimated. There is little evidence of a trend over the short period of available data. Correlations with sea surface temperature suggest clear physical relationships between heat transport and SST, even on the short time scales of available data.
Subscription Required
Boer, G.J., 2012: “Long time-scale teleconnection patterns in the northern Atlantic and Pacific.” Journal of Climate, v. 25, no. 1, pp. 414-422, doi: 10.1175/2011JCLI4107.
Long time-scale teleconnection patterns, with common features in both the northern Atlantic and Pacific regions, are identified. The teleconnection patterns arise in an investigation of the internally generated variability in a multimodel ensemble of coupled climate model control simulations. The large amount of data involved offers statistical robustness and the benefits of combining results across models. Maxima of decadal potential predictability identify regions where long time-scale variability is an appreciable fraction of the total variability and serve as index regions for the teleconnection analysis. Annual, 5-yr, and decadal mean temperatures over these Atlantic and Pacific index regions are correlated with corresponding temperatures and precipitation rates over the globe. The resulting teleconnection patterns are reasonably similar despite the different long time-scale variability mechanisms thought to exist in the two ocean basins. Although lacking statistical robustness, some aspects of the temperature teleconnection patterns are obtained based on the Hadley Centre Sea Ice and Sea Surface Temperature (HadISST) dataset. The similarity of the teleconnection patterns in the two northern ocean regions suggests that common variability mechanisms may be involved.
Subscription Required
