Ahmed, M., K. J. Anchukaitis, A. Asrat, H. P. Borgaonkar, M. Braida, B. M. Buckley, U. Buntgen, B. M. Chase, D. A. Christie, E. R. Cook, M. A. J. Curran, H. F. Diaz, J. Esper, Z. X. Fan, N. P. Gaire, Q. S. Ge, J. Gergis, J. F. Gonzalez-Rouco, H. Goosse, S. W. Grab, N. Graham, R. Graham, M. Grosjean, S. T. Hanhijarvi, D. S. Kaufman, T. Kiefer, K. Kimura, A. A. Korhola, P. J. Krusic, A. Lara, A. M. Lezine, F. C. Ljungqvist, A. M. Lorrey, J. Luterbacher, V. Masson-Delmotte, D. McCarroll, J. R. McConnell, N. P. McKay, M. S. Morales, A. D. Moy, R. Mulvaney, I. A. Mundo, T. Nakatsuka, D. J. Nash, R. Neukom, S. E. Nicholson, H. Oerter, J. G. Palmer, S. J. Phipps, M. R. Prieto, A. Rivera, M. Sano, M. Severi, T. M. Shanahan, X. M. Shao, F. Shi, M. Sigl, J. E. Smerdon, O. N. Solomina, E. J. Steig, B. Stenni, M. Thamban, V. Trouet, C. S. M. Turney, M. Umer, T. van Ommen, D. Verschuren, A. E. Viau, R. Villalba, B. M. Vinther, L. von Gunten, S. Wagner, E. R. Wahl, H. Wanner, J. P. Werner, J. W. C. White, K. Yasue, E. Zorita and Pages 2k Consortium, 2013: Continental-scale temperature variability during the past two millennia. Nature Geoscience, 6(5): 339-346.
Past global climate changes had strong regional expression. To elucidate their spatio-temporal pattern, we reconstructed past temperatures for seven continental-scale regions during the past one to two millennia. The most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the nineteenth century. At multi-decadal to centennial scales, temperature variability shows distinctly different regional patterns, with more similarity within each hemisphere than between them. There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between AD 1580 and 1880, punctuated in some regions by warm decades during the eighteenth century. The transition to these colder conditions occurred earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere regions. Recent warming reversed the long-term cooling; during the period AD 1971-2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years.
Assmann, K. M., A. Jenkins, D. R. Shoosmith, D. P. Walker, S. S. Jacobs and K. W. Nicholls, 2013: Variability of Circumpolar Deep Water transport onto the Amundsen Sea continental shelf through a shelf break trough. Journal of Geophysical Research-Oceans, 118(12): 6603-6620.
The presence of warm Circumpolar Deep Water (CDW) intrusions on the Amundsen continental shelf has been linked to recent thinning of the outlet glaciers draining the West Antarctic ice sheet into the Amundsen Sea. Inflow of the CDW onto the shelf is thought to occur within a series of troughs that intersect the continental shelf break. We use observations between 1994 and 2011 and a numerical model to investigate the variability of CDW transport in a trough intersecting the shelf break at 113 degrees W. The location of the main CDW inflow into the trough varies between its eastern flank and center, while the western part of the trough is filled by a recirculation that commonly entrains cooler water originating further south on the shelf. Thermocline depth decreases between the early and late 2000s with an indication that the depth of the 1994 thermocline was similar to the later years. Mooring results show that the CDW layer cools and thins in summer and thickens and warms in winter. In addition to a deeper thermocline in summer, we observe a stronger presence of Lower CDW in the bottom of the trough. Heat flux onto the shelf is controlled by current velocities rather than CDW temperature and the majority of the heat is carried onto the shelf by background flow rather than episodic events.
Camargo, S. J., M. F. Ting and Y. Kushnir, 2013: Influence of local and remote SST on North Atlantic tropical cyclone potential intensity. Climate Dynamics, 40(5-6): 1515-1529, DOI: 10.1007/s00382-012-1536-4.
We examine the role of local and remote sea surface temperature (SST) on the tropical cyclone potential intensity in the North Atlantic using a suite of model simulations, while separating the impact of anthropogenic (external) forcing and the internal influence of Atlantic Multidecadal Variability. To enable the separation by SST region of influence we use an ensemble of global atmospheric climate model simulations forced with historical, 1856-2006 full global SSTs, and compare the results to two other simulations with historical SSTs confined to the tropical Atlantic and to the tropical Indian Ocean and Pacific. The effects of anthropogenic plus other external forcing and that of internal variability are separated by using a linear, "signal-to-noise" maximizing EOF analysis and by projecting the three model ensemble outputs onto the respective external forcing and internal variability time series. Consistent with previous results indicating a tampering influence of global tropical warming on the Atlantic hurricane potential intensity, our results show that non-local SST tends to reduce potential intensity associated with locally forced warming through changing the upper level atmospheric temperatures. Our results further indicate that the late twentieth Century increase in North Atlantic potential intensity, may not have been dominated by anthropogenic influence but rather by internal variability.
Ducklow, H. W., W. R. Fraser, M. P. Meredith, S. E. Stammerjohn, S. C. Doney, D. G. Martinson, S. F. Sailley, O. M. Schofield, D. K. Steinberg, H. J. Venables and C. D. Amsler, 2013: West Antarctic Peninsula: An Ice-Dependent Coastal Marine Ecosystem in Transition. Oceanography, 26(3): 190-203.
The extent, duration, and seasonality of sea ice and glacial discharge strongly influence Antarctic marine ecosystems. Most organisms' life cycles in this region are attuned to ice seasonality The annual retreat and melting of sea ice in the austral spring stratifies the upper ocean, triggering large phytoplankton blooms. The magnitude of the blooms is proportional to the winter extent of ice cover, which can act as a barrier to wind mixing. Antarctic krill, one of the most abundant metazoan populations on Earth, consume phytoplankton blooms dominated by large diatoms. Krill, in turn, support a large biomass of predators, including penguins, seals, and whales. Human activity has altered even these remote ecosystems. The western Antarctic Peninsula region has warmed by 7 degrees C over the past 50 years, and sea ice duration has declined by almost 100 days since 1978, causing a decrease in phytoplankton productivity in the northern peninsula region. Besides climate change, Antarctic marine systems have been greatly altered by harvesting of the great whales and now krill. It is unclear to what extent the ecosystems we observe today differ from the pristine state.
Frierson, D. M. W., Y. T. Hwang, N. S. Fuckar, R. Seager, S. M. Kang, A. Donohoe, E. A. Maroon, X. J. Liu and D. S. Battisti, 2013: Contribution of ocean overturning circulation to tropical rainfall peak in the Northern Hemisphere. Nature Geoscience, 6(11): 940-944.
Rainfall in the tropics is largely focused in a narrow zonal band near the Equator, known as the intertropical convergence zone. On average, substantially more rain falls just north of the Equator(1). This hemispheric asymmetry in tropical rainfall has been attributed to hemispheric asymmetries in ocean temperature induced by tropical landmasses. However, the ocean meridional overturning circulation also redistributes energy, by carrying heat northwards across the Equator. Here, we use satellite observations of the Earth's energy budget(2), atmospheric reanalyses(3) and global climate model simulations to study tropical rainfall using a global energetic framework. We show that the meridional overturning circulation contributes significantly to the hemispheric asymmetry in tropical rainfall by transporting heat from the Southern Hemisphere to the Northern Hemisphere, and thereby pushing the tropical rain band north. This northward shift in tropical precipitation is seen in global climate model simulations when ocean heat transport is included, regardless of whether continents are present or not. If the strength of the meridional overturning circulation is reduced in the future as a result of global warming, as has been suggested(4), precipitation patterns in the tropics could change, with potential societal consequences.
Goddard, L., A. Kumar, A. Solomon, D. Smith, G. Boer, P. Gonzalez, V. Kharin, W. Merryfield, C. Deser, S. J. Mason, B. P. Kirtman, R. Msadek, R. Sutton, E. Hawkins, T. Fricker, G. Hegerl, C. A. T. Ferro, D. B. Stephenson, G. A. Meehl, T. Stockdale, R. Burgman, A. M. Greene, Y. Kushnir, M. Newman, J. Carton, I. Fukumori and T. Delworth, 2013: A verification framework for interannual-to-decadal predictions experiments. Climate Dynamics, 40(1-2): 245-272.
Decadal predictions have a high profile in the climate science community and beyond, yet very little is known about their skill. Nor is there any agreed protocol for estimating their skill. This paper proposes a sound and coordinated framework for verification of decadal hindcast experiments. The framework is illustrated for decadal hindcasts tailored to meet the requirements and specifications of CMIP5 (Coupled Model Intercomparison Project phase 5). The chosen metrics address key questions about the information content in initialized decadal hindcasts. These questions are: (1) Do the initial conditions in the hindcasts lead to more accurate predictions of the climate, compared to un-initialized climate change projections? and (2) Is the prediction model's ensemble spread an appropriate representation of forecast uncertainty on average? The first question is addressed through deterministic metrics that compare the initialized and uninitialized hindcasts. The second question is addressed through a probabilistic metric applied to the initialized hindcasts and comparing different ways to ascribe forecast uncertainty. Verification is advocated at smoothed regional scales that can illuminate broad areas of predictability, as well as at the grid scale, since many users of the decadal prediction experiments who feed the climate data into applications or decision models will use the data at grid scale, or downscale it to even higher resolution. An overall statement on skill of CMIP5 decadal hindcasts is not the aim of this paper. The results presented are only illustrative of the framework, which would enable such studies. However, broad conclusions that are beginning to emerge from the CMIP5 results include (1) Most predictability at the interannual-to-decadal scale, relative to climatological averages, comes from external forcing, particularly for temperature; (2) though moderate, additional skill is added by the initial conditions over what is imparted by external forcing alone; however, the impact of initialization may result in overall worse predictions in some regions than provided by uninitialized climate change projections; (3) limited hindcast records and the dearth of climate-quality observational data impede our ability to quantify expected skill as well as model biases; and (4) as is common to seasonal-to-interannual model predictions, the spread of the ensemble members is not necessarily a good representation of forecast uncertainty. The authors recommend that this framework be adopted to serve as a starting point to compare prediction quality across prediction systems. The framework can provide a baseline against which future improvements can be quantified. The framework also provides guidance on the use of these model predictions, which differ in fundamental ways from the climate change projections that much of the community has become familiar with, including adjustment of mean and conditional biases, and consideration of how to best approach forecast uncertainty.
Hays, J. D., D. G. Martinson and J. J. Morley, 2013: Biological and climatic consequences of a cold, stratified, high latitude ocean. Quaternary Science Reviews, 82: 78-92.
The flux from deep- and shallow-living radiolarian assemblages provides evidence of a glacial, high latitude, cold ocean stratification that increased biological pump efficiency and promoted ocean carbon sequestration. Greater deep (>200 m) than shallow-living (<200 m) radiolarian assemblage flux characterizes glacial North Pacific (>45 degrees N) sediments with the deep-living Cycladophora davisiana dominant (>24%). By contrast modern radiolarian flux consists primarily of shallow-living species (C davisiana <10%). Clues to the cause of this unusual glacial radiolarian flux come from the presently, strongly stratified Sea of Okhotsk. Here beneath a thin nutrient depleted mixed layer radiolarian and zooplankton faunas conform to the sea's physical stratification with lower concentrations of both in a Cold (-1.5 to 1 degrees C) Intermediate Layer (CIL) (20-125 m) and higher concentrations in waters between 200 and 500 m (Nimmergut and Abelmann, 2002). This biological stratification generates a radiolarian flux echoing that of the glacial northwest Pacific with C davisiana 26% of total flux. Widespread C davisiana percentages (>20%) in high latitude (>45 degrees) glacial sediments of both hemispheres is evidence that these oceans were capped with an Okhotsk-Like Stratification (O-LS). O-LS provides mechanisms to (1) strip nutrients from surface waters depriving the deep-ocean of preformed nutrients, increasing biological pump efficiency and (2) deepen carbon re-mineralization increasing deep-ocean alkalinity. Both may have contributed to lower glacial atmospheric CO2 concentrations. O-LS would also have amplified glacial climatic cycles by promoting the spread of high latitude sea ice in winter as occurs in the Sea of Okhotsk today, and reducing gas exchange between ocean and atmosphere in summer. (C) 2013 Elsevier Ltd. All rights reserved.
Kang, S. M., L. M. Polvani, J. C. Fyfe, S. W. Son, M. Sigmond and G. J. P. Correa, 2013: Modeling evidence that ozone depletion has impacted extreme precipitation in the austral summer. Geophysical Research Letters, 40(15): 4054-4059.
The impacts of stratospheric ozone depletion on the extremes of daily precipitation in the austral summer are explored using two global climate models. Both models indicate that stratospheric ozone losses since the late 1970s may have increased the frequency and intensity of very heavy precipitation in austral summer over southern high and subtropical latitudes, and may have decreased the frequency and intensity over southern midlatitudes. This hemispheric wide pattern of extreme precipitation response projects strongly onto a previously identified pattern of seasonal mean precipitation response, both of which are shown to be likely of dynamic rather than thermodynamic origin.
Karamperidou, C., M. A. Cane, U. Lall and A. T. Wittenberg, 2013: Intrinsic modulation of ENSO predictability viewed through a local Lyapunov lens. Climate Dynamics, 42(1-2): 253-270, Doi 10.1007/S00382-013-1759-Z.
The presence of rich ENSO variability in the long unforced simulation of GFDL's CM2.1 motivates the use of tools from dynamical systems theory to study variability in ENSO predictability, and its connections to ENSO magnitude, frequency, and physical evolution. Local Lyapunov exponents (LLEs) estimated from the monthly NINO3 SSTa model output are used to characterize periods of increased or decreased predictability. The LLEs describe the growth of infinitesimal perturbations due to internal variability, and are a measure of the immediate predictive uncertainty at any given point in the system phase-space. The LLE-derived predictability estimates are compared with those obtained from the error growth in a set of re-forecast experiments with CM2.1. It is shown that the LLEs underestimate the error growth for short forecast lead times (less than 8 months), while they overestimate it for longer lead times. The departure of LLE-derived error growth rates from the re-forecast rates is a linear function of forecast lead time, and is also sensitive to the length of the time series used for the LLE calculation. The LLE-derived error growth rate is closer to that estimated from the re-forecasts for a lead time of 4 months. In the 2,000-year long simulation, the LLE-derived predictability at the 4-month lead time varies (multi)decadally only by 9-18 %. Active ENSO periods are more predictable than inactive ones, while epochs with regular periodicity and moderate magnitude are classified as the most predictable by the LLEs. Events with a deeper thermocline in the west Pacific up to five years prior to their peak, along with an earlier deepening of the thermocline in the east Pacific in the months preceding the peak, are classified as more predictable. Also, the GCM is found to be less predictable than nature under this measure of predictability.
Sheffield, J., S. J. Camargo, R. Fu, Q. Hu, X. A. Jiang, N. Johnson, K. B. Karnauskas, S. T. Kim, J. Kinter, S. Kumar, B. Langenbrunner, E. Maloney, A. Mariotti, J. E. Meyerson, J. D. Neelin, S. Nigam, Z. T. Pan, A. Ruiz-Barradas, R. Seager, Y. L. Serra, D. Z. Sun, C. Z. Wang, S. P. Xie, J. Y. Yu, T. Zhang and M. Zhao, 2013: North American Climate in CMIP5 Experiments. Part II: Evaluation of Historical Simulations of Intraseasonal to Decadal Variability. Journal of Climate, 26(23): 9247-9290, doi: 10.1175/JCLI-D-12-00593.1.
This is the second part of a three-part paper on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that evaluates the twentieth-century simulations of intraseasonal to multidecadal variability and teleconnections with North American climate. Overall, the multimodel ensemble does reasonably well at reproducing observed variability in several aspects, but it does less well at capturing observed teleconnections, with implications for future projections examined in part three of this paper. In terms of intraseasonal variability, almost half of the models examined can reproduce observed variability in the eastern Pacific and most models capture the midsummer drought over Central America. The multimodel mean replicates the density of traveling tropical synoptic-scale disturbances but with large spread among the models. On the other hand, the coarse resolution of the models means that tropical cyclone frequencies are underpredicted in the Atlantic and eastern North Pacific. The frequency and mean amplitude of ENSO are generally well reproduced, although teleconnections with North American climate are widely varying among models and only a few models can reproduce the east and central Pacific types of ENSO and connections with U.S. winter temperatures. The models capture the spatial pattern of Pacific decadal oscillation (PDO) variability and its influence on continental temperature and West Coast precipitation but less well for the wintertime precipitation. The spatial representation of the Atlantic multidecadal oscillation (AMO) is reasonable, but the magnitude of SST anomalies and teleconnections are poorly reproduced. Multidecadal trends such as the warming hole over the central-southeastern United States and precipitation increases are not replicated by the models, suggesting that observed changes are linked to natural variability.
Tierney, J. E., J. E. Smerdon, K. J. Anchukaitis and R. Seager, 2013: Multidecadal variability in East African hydroclimate controlled by the Indian Ocean. Nature, 493(7432): 389-392, doi:10.1038/nature11785.
The recent decades-long decline in East African rainfall(1) suggests that multidecadal variability is an important component of the climate of this vulnerable region. Prior work based on analysing the instrumental record implicates both Indian(2) and Pacific(1) ocean sea surface temperatures (SSTs) as possible drivers of East African multidecadal climate variability, but the short length of the instrumental record precludes a full elucidation of the underlying physical mechanisms. Here we show that on timescales beyond the decadal, the Indian Ocean drives East African rainfall variability by altering the local Walker circulation, whereas the influence of the Pacific Ocean is minimal. Our results, based on proxy indicators of relative moisture balance for the past millennium paired with long control simulations from coupled climate models, reveal that moist conditions in coastal East Africa are associated with cool SSTs (and related descending circulation) in the eastern Indian Ocean and ascending circulation over East Africa. The most prominent event identified in the proxy record-a coastal pluvial from 1680 to 1765-occurred when Indo-Pacific warm pool SSTs reached their minimum values of the past millennium. Taken together, the proxy and model evidence suggests that Indian Ocean SSTs are the primary influence on East African rainfall over multidecadal and perhaps longer timescales.
Werner, J. P., J. Luterbacher and J. E. Smerdon, 2013: A Pseudoproxy Evaluation of Bayesian Hierarchical Modeling and Canonical Correlation Analysis for Climate Field Reconstructions over Europe. Journal of Climate, 26(3): 851-867.
A pseudoproxy comparison is presented for two statistical methods used to derive annual climate field reconstructions (CFRs) for Europe. The employed methods use the canonical correlation analysis (CCA) procedure presented by Smerdon et al. and the Bayesian hierarchical model (BHM) method adopted from Tingley and Huybers. Pseudoproxy experiments (PPEs) are constructed from modeled temperature data sampled from the 1250-yr paleo-run of the NCAR Community Climate System Model (CCSM) version 1.4 model by Ammann et al. Pseudoproxies approximate the distribution of the multiproxy network used by Mann et al. over the European region of interest. Gaussian white noise is added to the temperature data to mimic the combined signal and noise properties of real-world proxies. Results indicate that, while both methods perform well in areas with good proxy coverage, the BHM method outperforms the CCA method across the entire field and additionally returns objective error estimates.
Williams, A. P., C. D. Allen, A. K. Macalady, D. Griffin, C. A. Woodhouse, D. M. Meko, T. W. Swetnam, S. A. Rauscher, R. Seager, H. D. Grissino-Mayer, J. S. Dean, E. R. Cook, C. Gangodagamage, M. Cai and N. G. McDowell, 2013: Temperature as a potent driver of regional forest drought stress and tree mortality. Nature Climate Change, 3(3): 292-297.
As the climate changes, drought may reduce tree productivity and survival across many forest ecosystems; however, the relative influence of specific climate parameters on forest decline is poorly understood. We derive a forest drought-stress index (FDSI) for the southwestern United States using a comprehensive tree-ring data set representing AD 1000-2007. The FDSI is approximately equally influenced by the warm-season vapour-pressure deficit (largely controlled by temperature) and cold-season precipitation, together explaining 82% of the FDSI variability. Correspondence between the FDSI and measures of forest productivity, mortality, bark-beetle outbreak and wildfire validate the FDSI as a holistic forest-vigour indicator. If the vapour-pressure deficit continues increasing as projected by climate models, the mean forest drought-stress by the 2050s will exceed that of the most severe droughts in the past 1,000 years. Collectively, the results foreshadow twenty-first-century changes in forest structures and compositions, with transition of forests in the southwestern United States, and perhaps water-limited forests globally, towards distributions unfamiliar to modern civilization.
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