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The Northwest Atlantic, which has exhibited evidence of accelerated warming compared to the global ocean, also experienced several notable marine heatwaves (MHWs) over the last decade. We analyze spatiotemporal patterns of surface and subsurface temperature structure across the Northwest Atlantic continental shelf and slope to assess the influences of atmospheric and oceanic processes on ocean temperatures. Here we focus on MHWs from 2015/16 and examine their physical drivers using observational and reanalysis products. We find that a combination of jet stream latitudinal position and ocean advection, mainly due to warm core rings shed by the Gulf Stream, plays a role in MHW development. While both atmospheric and oceanic drivers can lead to MHWs they have different temperature signatures with each affecting the vertical structure differently and horizontal spatial patterns of a MHW. Northwest Atlantic MHWs have significant socio-economic impacts and affect commercially important species such as squid and lobster.

The instrumental record reveals that tropical cyclone activity is sensitive to oceanic and atmospheric variability on inter-annual and decadal scales. However, our understanding of the influence of climate on tropical cyclone behaviour is restricted by the short historical record and the sparseness of prehistorical reconstructions, particularly in the western North Pacific, where coastal communities suffer loss of life and livelihood from typhoons annually. Here, to explore past regional typhoon dynamics, we reconstruct three millennia of deep tropical North Pacific cyclogenesis. Combined with existing records, our reconstruction demonstrates that low-baseline typhoon activity prior to 1350 ce was followed by an interval of frequent storms during the Little Ice Age. This pattern, concurrent with hydroclimate proxy variability, suggests a centennial-scale link between Pacific hydroclimate and tropical cyclone climatology. An ensemble of global climate models demonstrates a migration of the Pacific Walker circulation and variability in two Pacific climate modes during the Little Ice Age, which probably contributed to enhanced tropical cyclone activity in the tropical western North Pacific. In the next century, projected changes to the Pacific Walker circulation and expansion of the tropics will invert these Little Ice Age hydroclimate trends, potentially reducing typhoon activity in the deep tropical Pacific. Atmospheric circulation shifts during the Little Ice Age led to greater typhoon generation in the tropical North Pacific according to a comparison of sediment proxy records of past storm activity and outputs of general circulation models.

More frequent high pressure conditions associated with atmospheric blocking episodes over Greenland in recent decades have been suggested to enhance melt through large-scale subsidence and cloud dissipation, which allows more solar radiation to reach the ice sheet surface. Here we investigate mechanisms linking high pressure circulation anomalies to Greenland cloud changes and resulting cloud radiative effects, with a focus on the previously neglected role of topography. Using reanalysis and satellite data in addition to a regional climate model, we show that anticyclonic circulation anomalies over Greenland during recent extreme blocking summers produce cloud changes dependent on orographic lift and descent. The resulting increased cloud cover over northern Greenland promotes surface longwave warming, while reduced cloud cover in southern and marginal Greenland favors surface shortwave warming. Comparison with an idealized model simulation with flattened topography reveals that orographic effects were necessary to produce area-averaged decreasing cloud cover since the mid-1990s and the extreme melt observed in the summer of 2012. This demonstrates a key role for Greenland topography in mediating the cloud and melt response to large-scale circulation variability. These results suggest that future melt will depend on the pattern of circulation anomalies as well as the shape of the Greenland Ice Sheet. Supplemental information related to this paper is available at the Journals Online website: https://doi.org/10.1175/JCLI-D-19-0527.s1. © 2020 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

The Indian Ocean Dipole (IOD) affects climate and rainfall across the world, and most severely in nations surrounding the Indian Ocean1–4. The frequency and intensity of positive IOD events increased during the twentieth century5 and may continue to intensify in a warming world6. However, confidence in predictions of future IOD change is limited by known biases in IOD models7 and the lack of information on natural IOD variability before anthropogenic climate change. Here we use precisely dated and highly resolved coral records from the eastern equatorial Indian Ocean, where the signature of IOD variability is strong and unambiguous, to produce a semi-continuous reconstruction of IOD variability that covers five centuries of the last millennium. Our reconstruction demonstrates that extreme positive IOD events were rare before 1960. However, the most extreme event on record (1997) is not unprecedented, because at least one event that was approximately 27 to 42 per cent larger occurred naturally during the seventeenth century. We further show that a persistent, tight coupling existed between the variability of the IOD and the El Nino/Southern Oscillation during the last millennium. Indo-Pacific coupling was characterized by weak interannual variability before approximately 1590, which probably altered teleconnection patterns, and by anomalously strong variability during the seventeenth century, which was associated with societal upheaval in tropical Asia. A tendency towards clustering of positive IOD events is evident in our reconstruction, which—together with the identification of extreme IOD variability and persistent tropical Indo-Pacific climate coupling—may have implications for improving seasonal and decadal predictions and managing the climate risks of future IOD variability. Coral records indicate that the variability of the Indian Ocean Dipole over the last millennium is strongly coupled to variability in the El Nino/Southern Oscillation and that recent extremes are unusual but not unprecedented.

Iberia is predicted under future warming scenarios to be increasingly impacted by drought. While it is known that this region has experienced multiple intervals of enhanced aridity over the Holocene, additional hydroclimate-sensitive records from Iberia are necessary to place current and future drying into a broader perspective. Toward that end, we present a multi-proxy composite record from six well-dated and overlapping speleothems from Buraca Gloriosa (BG) cave, located in western Portugal. The coherence between the six stalagmites in this composite stalagmite record illustrates that climate (not in-cave processes) impacts speleothem isotopic values. This record provides the first high-resolution, precisely dated, terrestrial record of Holocene hydroclimate from west-central Iberia. The BG record reveals that aridity in western Portugal increased secularly from 9.0 ka BP to present, as evidenced by rising values of both carbon (δ13C) and oxygen (δ18O) stable isotope values. This trend tracks the decrease in Northern Hemisphere summer insolation and parallels Iberian margin sea surface temperatures (SST). The increased aridity over the Holocene is consistent with changes in Hadley Circulation and a southward migration of the Intertropical Convergence Zone (ITCZ). Centennial-scale shifts in hydroclimate are coincident with changes in total solar irradiance (TSI) after 4 ka BP. Several major drying events are evident, the most prominent of which was centered around 4.2 ka BP, a feature also noted in other Iberian climate records and coinciding with well-documented regional cultural shifts. Substantially, wetter conditions occurred from 0.8 ka BP to 0.15 ka BP, including much of the ‘Little Ice Age’. This was followed by increasing aridity toward present day. This composite stalagmite proxy record complements oceanic records from coastal Iberia, lacustrine records from inland Iberia, and speleothem records from both northern and southern Spain and depicts the spatial and temporal variability in hydroclimate in Iberia.

Initialized Earth System predictions are made by starting a numerical prediction model in a state as consistent as possible to observations and running it forward in time for up to 10 years. Skilful predictions at time slices from subseasonal to seasonal (S2S), seasonal to interannual (S2I) and seasonal to decadal (S2D) offer information useful for various stakeholders, ranging from agriculture to water resource management to human and infrastructure safety. In this Review, we examine the processes influencing predictability, and discuss estimates of skill across S2S, S2I and S2D timescales. There are encouraging signs that skilful predictions can be made: on S2S timescales, there has been some skill in predicting the Madden–Julian Oscillation and North Atlantic Oscillation; on S2I, in predicting the El Niño–Southern Oscillation; and on S2D, in predicting ocean and atmosphere variability in the North Atlantic region. However, challenges remain, and future work must prioritize reducing model error, more effectively communicating forecasts to users, and increasing process and mechanistic understanding that could enhance predictive skill and, in turn, confidence. As numerical models progress towards Earth System models, initialized predictions are expanding to include prediction of sea ice, air pollution, and terrestrial and ocean biochemistry that can bring clear benefit to society and various stakeholders. The foundations of this Review emerged from a workshop held by the National Academies of Sciences, Engineering and Medicine in 2015 at Woods Hole, MA, USA, and the authors gratefully acknowledge support from A. Purcell and N. Huddleston. G.A.M., J.H.R. and N.R. were supported in part by the Regional and Global Model Analysis (RGMA) component of the Earth and Environmental System Modeling Program of the US Department of Energy’s Office of Biological & Environmental Research (BER) via National Science Foundation IA 1844590, and by the National Center for Atmospheric Research (NCAR), which is a major facility sponsored by the National Science Foundation (NSF) under Cooperative Agreement No. 1852977. M.H.E. acknowledges support from the Australian Research Council (Grant CE170100023). H.T. was partly supported by DOE/BER RGMA HiLAT-RASM. M.E.M. was supported by a grant from the NSF Paleoclimate Program #1748097. F.D.-R. and M.G.D. were supported by the H2020 EUCP project under Grant agreement no. 776613, and M.G.D also by the Ramón y Cajal 2017 grant reference RYC-2017-22964. A.C. acknowledges support from the National Oceanic and Atmospheric Administration (NOAA) Climate Program Office’s Modeling Analysis, Prediction and Projections (MAPP) Program and from the NOAA Climate Program Office’s Climate Variability and Predictability (CVP) Program. A.C.S. acknowledges support from the NOAA Climate Variability and Predictability Program (Award NA18OAR4310405) and NOAA-MAPP (NA17OAR4310106). N.S.L. is grateful for support from the NSF (OCE-1752724). D.T. acknowledges support from the NCAR Advanced Study Program and NSF (OCE-1931242). S.C.S was supported by the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) Postdoctoral Fellowship. A.A.S. and D.S were supported by the Met Office Hadley Centre Climate Programme funded by the Department for Business, Energy & Industrial Strategy (BEIS) and Department for Environment, Food and Rural Affairs (Defra) and by the European Commission Horizon 2020 EUCP project (GA 776613). Peer Reviewed "Article signat per 26 autors/es: Gerald A. Meehl, Jadwiga H. Richter, Haiyan Teng, Antonietta Capotondi, Kim Cobb, Francisco Doblas-Reyes, Markus G. Donat, Matthew H. England, John C. Fyfe, Weiqing Han, Hyemi Kim, Ben P. Kirtman, Yochanan Kushnir, Nicole S. Lovenduski, Michael E. Mann, William J. Merryfield, Veronica Nieves, Kathy Pegion, Nan Rosenbloom, Sara C. Sanchez, Adam A. Scaife, Doug Smith, Aneesh C. Subramanian, Lantao Sun, Diane Thompson, Caroline C. Ummenhofer & Shang-Ping Xie"

South-Eastern Brazil experienced a devastating drought associated with significant agricultural losses in austral summer 2014. The drought was linked to the development of a quasi-stationary anticyclone in the South Atlantic in early 2014 that affected local precipitation patterns over South-East Brazil. Previous studies have suggested that the unusual blocking was triggered by tropical Pacific sea surface temperature (SST) anomalies and, more recently, by convection over the Indian Ocean related to the Madden–Julian Oscillation. Further investigation of the proposed teleconnections appears crucial for anticipating future economic impacts. In this study, we use numerical experiments with an idealized atmospheric general circulation model forced with the observed 2013/2014 SST anomalies in different ocean basins to understand the dominant mechanism that initiated the 2014 South Atlantic anticyclonic anomaly. We show that a forcing with global 2013/2014 SST anomalies enhances the chance for the occurrence of positive geopotential height anomalies in the South Atlantic. However, further sensitivity experiments with SST forcings in separate ocean basins suggest that neither the Indian Ocean nor tropical Pacific SST anomalies alone have contributed significantly to the anomalous atmospheric circulation that led to the 2014 South-East Brazil drought. The model study rather points to an important role of remote forcing from the South Pacific, local South Atlantic SSTs, and internal atmospheric variability in driving the persistent blocking over the South Atlantic. Electronic supplementary material The online version of this article (10.1007/s00382-020-05442-9) contains supplementary material, which is available to authorized users.

Cave deposits can be valuable sources of paleoclimate data, especially when atmospheric circulation patterns, precipitation variability, vegetation changes, and the chemical evolution of waters moving through the karst environment can be mechanistically linked to speleothem proxies. In particular, an evaluation of the factors that control the isotopic composition of precipitation and the evolution of rainwater during migration from the land surface to the cave are needed to robustly develop speleothems as hydroclimate-sensitive proxies. One area in which precipitation and atmospheric variability are closely linked is western Iberia, where rainfall is strongly influenced by the Azores High, part of the North Atlantic Oscillation (NAO) dipole. Therefore, in order to better characterize the factors that influence the isotopic composition of precipitation in Portugal and to evaluate the potential of using stalagmites from this region as hydroclimate (and NAO-sensitive) proxies, we investigated Global Network of Isotopes in Precipitation (GNIP) data from ten mainland Portugal sites spanning multiple decades. In addition, we use more than one hydrologic year of precipitation amount and isotope data from Buraca Gloriosa (BG), a cave in western Portugal, the site of on-going speleothem analyses, as well as six years of environmental monitoring from BG. We present an integrated analysis of rainfall and vegetation through the normalized difference vegetation index (NDVI) following extremely wet and dry winters. Summer vegetation density, related to the amount of precipitation in the preceding winter wet season, as well as prior calcite precipitation (PCP), would largely control the stable carbon isotopic signature (delta C-13) in stalagmites at BG. Cool season recharge is likely the dominant factor for the oxygen isotopic composition (delta O-18) of water percolating through the cave system, while amount effects exert the primary control on precipitation delta O-18 values. Based on HYSPLIT modeling, moisture sources overwhelmingly originate from the Atlantic Ocean as opposed to the Mediterranean or elsewhere thus, variability in delta O-18 values in the precipitation (and, thus, by inference, those of the dripwater and stalagmites) are primarily reflecting changes in precipitation amount and not changing temperatures or source regions. Together these data constitute an important analysis of the controls of isotope proxies in Portuguese cave systems. US National Science FoundationNational Science Foundation (NSF) [1804528, 1804635, 1804132, 1805163] info:eu-repo/semantics/publishedVersion

The subtropical dry zones, including the broader Mediterranean region, are likely to experience considerable changes in hydroclimate in a warming climate. An expansion of the atmosphere’s meridional overturning circulation, the Hadley circulation, over recent decades has been reported, with implications for regional hydroclimate. Yet, there exists considerable disagreement in magnitude and even sign of these trends among different metrics that measure various aspects of the Hadley circulation, as well as discrepancies in trends between different analysis periods and reanalysis products during the 20th century. In light of these uncertainties, it is therefore of interest to explore variability and trends in subtropical hydroclimate and its dominant driver, the Hadley Circulation. We focus on the North Atlantic sector and explore variability in the Azores High, the manifestation of the Hadley Circulation’s downward branch, and hydroclimate across the Iberian Peninsula using a combination of observational/reanalysis products, state-of-the-art climate model simulations, and hydroclimatically-sensitive stalagmite records over the past 1200 yr. The Last Millennium Ensemble (LME) with the Community Earth System Model provides thirteen transient simulations covering the period 850 to 2005 A.D. with prescribed external forcing (e.g. greenhouse gas, solar, volcanic, land use, orbital, and aerosol) and smaller subsets with individual forcing only. The LME is shown to accurately simulate the variability and trends in the Azores High when compared to observational records from the 20th century. We evaluate variability in the Azores High (e.g., size, intensity, position) in relation to other key metrics that measure different aspects of the Hadley circulation throughout the course of the last millennium, as well as during key periods, such as the Little Ice Age or Medieval Climate Anomaly. The smaller subsets of LME simulations with individual forcing factors (e.g., solar, volcanic) allow for an attribution of past changes in regional hydroclimate to external drivers. Results from the climate model simulations are compared with hydroclimate reconstructed from stalagmites from Portuguese caves.

The dominant mode of sea surface temperature (SST) variability in the southeast Indian Ocean off the coast of Western Australia is called Ningaloo Niño/Niña. An unprecedented Ningaloo Niño, or marine heatwave, occurred during the austral summer of 2010/2011 with mean SSTs at 3°C above the long-term mean and had drastic impacts on the ecosystem. This event was attributed to a combination of an anomalous strong Leeuwin Current and high local air-sea heat fluxes. A number of local and remote forcing mechanisms have been investigated in recent years, however, little is known about the depth-structure of these ocean extremes and their general connections to large-scale ocean interannual to decadal variability. Using a suite of simulations with a high-resolution global Ocean General Circulation Model from 1958-2016, we investigate eastern Indian Ocean variability with focus on Ningaloo Niño and corresponding cold Ningaloo Niña events. In particular, we are interested in the impacts of large-scale ocean and climate variability, such as the Indonesian Throughflow, El Niño - Southern Oscillation and the Indian Ocean Dipole (IOD), on the study region. Spatial composites reveal large-scale surface and subsurface anomalies that extend from the western Pacific across the Indonesian Archipelago into the tropical eastern Indian Ocean. In particular, strong anomalies in temperature, salinity and mixed layer depth are found to the west of Sumatra and Java, a region that is generally strongly impacted by the IOD. We therefore investigate the connection with Ningaloo Niño/Niña events, at surface and subsurface, with a focus on 2010/2011 where a strong negative IOD event occurred prior to the unprecedented Ningaloo Niño. Furthermore, we find that major heatwaves in 2000 and 2011 are associated with pronounced fresh anomalies. Sensitivity experiments allow us to assess the relative role of buoyancy and wind-forcing as drivers of the observed patterns. Our work can provide valuable contributions for advancing the understanding of Ningaloo Niño/Niña drivers from surface to depth and regional to large scales.

The Caucasus region experiences recurrent droughts that affect natural vegetation and the agriculture-based economies of several countries. Because meteorological records are in general scarce and of short timespan, little is known about the magnitude and frequency of past climate variability. Despite the recent increase of climate reconstructions for parts of Eurasia, no study has focused on past hydroclimate variability in the Caucasus. Here, we use a multispecies network of tree-ring width chronologies from the Lesser Caucasus to develop the first precipitation reconstruction for the region back to 1752 CE. Despite the high annual precipitation in the region, our reconstruction accounted for 51.2 % of the variability in May–June precipitation from 1930 to 2001. In comparison with reconstructions in the eastern Mediterranean, our new reconstruction revealed important and distinct drought periods and pluvials. Previous winter North Atlantic Oscillation (NAO), and spring East Atlantic/Western Russia (EA/WR) and North Sea Caspian patterns are likely key drivers of May–June precipitation in the Caucasus and Anatolia. NAO appeared to negatively affect rainfall low-frequency variability while effects of EA/WR were more apparent at the interannual timescales. We also show a potential positive effect of Black Sea surface temperatures on May–June precipitation. In the Caucasus, May–June represents the period of major water supply in semi-arid areas and the period with the highest potential of water scarcity in mesic areas. It is also a period of potential catastrophic flood events. Thus, changes to the precipitation regime during this season will be critical to both human and natural systems of the Caucasus region.

Author Posting.© The Author(s), 2015. This is the authors version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Remote Sensing of Environment 174 (2016): 44-55, doi:10.1016/j.rse.2015.12.003. Pastoralist households across East Africa face major livestock losses during drought periods that can cause persistent poverty. For Kenya and southern Ethiopia, an existing index insurance scheme aims to reduce the adverse effects of such losses. The scheme insures individual households through an area-aggregated seasonal forage scarcity index derived from remotely-sensed normalized difference vegetation index (NDVI) time series. Until recently, insurance contracts covered animal losses and indemnity payouts were consequently made late in the season, based on a forage scarcity index incorporating both wet and dry season NDVI data. Season timing and duration were fixed for the whole area (March-September for long rains, October-February for short rains). Due to demand for asset protection insurance (pre-loss intervention) our aim was to identify earlier payout options by shortening the temporal integration period of the index. We used 250m-resolution 10-day NDVI composites for 2001-2014 from the Moderate Resolution Imaging Spectroradiometer (MODIS). To better describe the period during which forage develops, we first retrieved per-pixel average season start- and end-dates using a phenological model. These dates were averaged per insurance unit to obtain unit-specific growing period definitions. With these definitions a new forage scarcity index was calculated. We then examined if shortening the temporal period further could effectively predict most (>90%) of the interannual variability of the new index, and assessed the effects of shortening the period on indemnity payouts. Our analysis shows that insurance payouts could be made one to three months earlier as compared to the current index definition, depending on the insurance unit. This would allow pastoralists to use indemnity payments to protect their livestock through purchase of forage, water, or medicines. AV was funded under a contract from the International Livestock Research Institute. CCU was supported by the U.S. National Science Foundation under grant OCE-1203892. 2016-12-17

Changes in subtropical precipitation and the Hadley circulation (HC) are inextricably linked. The original Halley–Hadley model cannot explain certain aspects of the Earth’s meridional circulation in the tropics and is of limited use in understanding regional changes in precipitation. Here, we expand on previous work on the regional and seasonal aspects of the HC, in particular how land–sea temperature contrasts contribute to the strength and width of the HC. We show that the Earth’s HC is well described by three regionally distinct cells along the eastern edges of the major ocean basins with opposite circulations elsewhere. Moreover, comparable summertime hemisphere cells emerge in each region. While it has been recognized that continents modify the meridional pressure gradient, we demonstrate that a substantial part of the Earth’s HC is driven by zonal pressure gradients (ZPGs) that only exist due to continental heating and air–sea interaction. Projected changes in land–sea temperature contrasts in a warming climate due to the relatively low thermal capacity of land will also affect ZPGs and thus HC strength and width, with implications for extremes in hydroclimate and freshwater resources across the increasingly vulnerable subtropics.

The seasonal north-south migration of the intertropical convergence zone (ITCZ) defines the tropical rain belt (TRB), a region of enormous terrestrial and marine biodiversity and home to 40% of people on Earth. The TRB is dynamic and has been shown to shift south as a coherent system during periods of Northern Hemisphere cooling. However, recent studies of Indo-Pacific hydroclimate suggest that during the Little Ice Age (LIA; AD 1400–1850), the TRB in this region contracted rather than being displaced uniformly southward. This behaviour is not well understood, particularly during climatic fluctuations less pronounced than those of the LIA, the largest centennial-scale cool period of the last millennium. Here we show that the Indo-Pacific TRB expanded and contracted numerous times over multi-decadal to centennial scales during the last 3,000 yr. By integrating precisely-dated stalagmite records of tropical hydroclimate from southern China with a newly enhanced stalagmite time series from northern Australia, our study reveals a previously unidentified coherence between the austral and boreal summer monsoon. State-of-the-art climate model simulations of the last millennium suggest these are linked to changes in the structure of the regional manifestation of the atmosphere’s meridional circulation.

Drought patterns across monsoon and temperate Asia over the period 1877���2005 are linked to Indo-Pacific climate variability associated with the El Ni��o-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Using the Monsoon Asia Drought Atlas (MADA) composed of a high-resolution network of hydroclimatically sensitive tree-ring records with a focus on the June���August months, spatial drought patterns during El Ni��o and IOD events are assessed as to their agreement with an instrumental drought index and consistency in the drought response amongst ENSO/IOD events. Spatial characteristics in drought patterns are related to regional climate anomalies over the Indo-Pacific basin, using reanalysis products, including changes in the Asian monsoon systems, zonal Walker circulation, moisture fluxes, and precipitation. A weakening of the monsoon circulation over the Indian subcontinent and Southeast Asia during El Ni��o events, along with anomalous subsidence over monsoon Asia and reduced moisture flux, is reflected in anomalous drought conditions over India, Southeast Asia and Indonesia. When an IOD event co-occurs with an El Ni��o, severe drought conditions identified in the MADA for Southeast Asia, Indonesia, eastern China and central Asia are associated with a weakened South Asian monsoon, reduced moisture flux over China, and anomalous divergent flow and subsidence over Indonesia. Insights into the relative influences of Pacific and Indian Ocean variability for Asian monsoon climate on interannual to decadal and longer timescales, as recorded in the MADA, provide a useful tool for assessing long-term changes in the characteristics of Asian monsoon droughts in the context of Indo-Pacific climate variability.

Fidelity and projected changes in the climate models, used for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), are assessed with regard to the Southern Hemisphere extratropical ocean and sea ice systems. While individual models span different physical parameterizations and resolutions, a major component of intermodel variability results from surface wind differences. Projected changes to the surface wind field are also central in modifying future extratropical circulation and internal properties. A robust southward shift of the circumpolar current and subtropical gyres is projected, with a strong spinup of the Atlantic gyre. An associated increase in the core strength of the circumpolar circulation is evident; however, this does not translate into robust increases in Drake Passage transport. While an overarching oceanic warming is projected, the circulation-driven poleward shift of the temperature field explains much of the midlatitude warming pattern. The effect of this shift is less clear for salinity, where, instead, surface freshwater forcing dominates. Surface warming and high-latitude freshwater increases drive intensified stratification, and a shoaling and southward shift of the deep mixed layers. Despite large intermodel differences, there is also a robust weakening in bottom water formation and its northward outflow. At the same time the wind intensification invigorates the upwelling of deep water, transporting warm, salty water southward and upward, with major implications for sequestration and outgassing of CO2. A robust decrease is projected for both the sea ice concentration and the seasonal cycling of ice volume, potentially altering the salt and heat budget at high latitudes.

Late twentieth-century trends in New Zealand precipitation are examined using observations and reanalysis data for the period 1979–2006. One of the aims of this study is to investigate the link between these trends and recent changes in the large-scale atmospheric circulation in the Southern Hemisphere. The contributions from changes in Southern Hemisphere climate modes, particularly the El Niño–Southern Oscillation (ENSO) and the southern annular mode (SAM), are quantified for the austral summer season, December–February (DJF). Increasingly drier conditions over much of New Zealand can be partially explained by the SAM and ENSO. Especially over wide parts of the North Island and western regions of the South Island, the SAM potentially contributes up to 80% and 20%–50% to the overall decline in DJF precipitation, respectively. Over the North Island, the contribution of the SAM and ENSO to precipitation trends is of the same sign. In contrast, over the southwest of the South Island the two climate modes act in the opposite sense, though the effect of the SAM seems to dominate there during austral summer. The leading modes of variability in summertime precipitation over New Zealand are linked to the large-scale atmospheric circulation. The two dominant modes, explaining 64% and 9% of the overall DJF precipitation variability respectively, can be understood as local manifestations of the large-scale climate variability associated with the SAM and ENSO.

The autumn break is the first significant rainfall event of the winter growing season. Two definitions of the autumn break have been developed for northwestern Victoria; a so-called ideal break and a minimum rainfall condition for sowing a wheat crop termed a minimal break. Application of the ideal break definition for an eight-station average reveals that 41 autumn breaks occurred in the first half of the record (1889-1947) and 34 in the second half (1948-2006) with a trend towards breaks occurring later in the season. In the decade to 2006, there have been only 3 ideal breaks (1999, 2000 and 2005) and none of the selected rainfall stations has recorded an 'extreme' wet autumn over the last 11 years, the longest period recorded for this criterion. A synoptic analysis for the period 1956-2006 has established that breaks are predominantly associated with systems known as cutoff lows. The influence of these systems has varied markedly throughout the analysis period and only one autumn break has been caused by a cutoff low in the final decade of the analysis. Additionally, the total rainfall associated with cutoff lows in the April to June period has declined significantly over the past 30 years. A farming system model has been employed to simulate a wheat crop in northwestern Victoria under historical conditions for a range of management options. Average yield across all years of the simulation declines with delay in the sowing date after late April, but there is a marked interannual variability in yield response to the sowing date which is related to rainfall distribution in the growing season in each year. The simulated in-crop rainfall indicates that the most recent drought in southeastern Australia is comparable in severity with the two major droughts in the 20th Century.

Paleoclimate data suggest that the Horn of Africa will become drier under global warming, in contrast to model projections., The recent decline in Horn of Africa rainfall during the March–May “long rains” season has fomented drought and famine, threatening food security in an already vulnerable region. Some attribute this decline to anthropogenic forcing, whereas others maintain that it is a feature of internal climate variability. We show that the rate of drying in the Horn of Africa during the 20th century is unusual in the context of the last 2000 years, is synchronous with recent global and regional warming, and therefore may have an anthropogenic component. In contrast to 20th century drying, climate models predict that the Horn of Africa will become wetter as global temperatures rise. The projected increase in rainfall mainly occurs during the September–November “short rains” season, in response to large-scale weakening of the Walker circulation. Most of the models overestimate short rains precipitation while underestimating long rains precipitation, causing the Walker circulation response to unrealistically dominate the annual mean. Our results highlight the need for accurate simulation of the seasonal cycle and an improved understanding of the dynamics of the long rains season to predict future rainfall in the Horn of Africa.

In our reconstruction of extreme rainfall events from the Australian tropics (1), we relied on a method still in its infancy: linking accumulations of sediment trapped within stalagmites to discrete cave flooding events. This sedimentological approach (2, 3) represents an important complement to speleothem-based isotopic methods (4, 5) that expand our understanding of prehistoric tropical cyclone activity.

Assessing temporal variability in extreme rainfall events before the historical era is complicated by the sparsity of long-term “direct” storm proxies. Here we present a 2,200-y-long, accurate, and precisely dated record of cave flooding events from the northwest Australian tropics that we interpret, based on an integrated analysis of meteorological data and sediment layers within stalagmites, as representing a proxy for extreme rainfall events derived primarily from tropical cyclones (TCs) and secondarily from the regional summer monsoon. This time series reveals substantial multicentennial variability in extreme rainfall, with elevated occurrence rates characterizing the twentieth century, 850–1450 CE (Common Era), and 50–400 CE; reduced activity marks 1450–1650 CE and 500–850 CE. These trends are similar to reconstructed numbers of TCs in the North Atlantic and Caribbean basins, and they form temporal and spatial patterns best explained by secular changes in the dominant mode of the El Nino/Southern Oscillation (ENSO), the primary driver of modern TC variability. We thus attribute long-term shifts in cyclogenesis in both the central Australian and North Atlantic sectors over the past two millennia to entrenched El Nino or La Nina states of the tropical Pacific. The influence of ENSO on monsoon precipitation in this region of northwest Australia is muted, but ENSO-driven changes to the monsoon may have complemented changes to TC activity.

Asian monsoon extremes critically impact much of the globe's population. Key gaps in our understanding of monsoon climate remain due to sparse coverage of paleoclimatic information, despite intensi ...

Since 1995, a large region of Australia has been gripped by the most severe drought in living memory, the so-called ‘‘Big Dry’’. The ramifications for affected regions are dire, with acute water shortages for rural and metropolitan areas, record agricultural losses, the dryingout of two of Australia’s major river systems and farreaching ecosystem damage. Yet the drought’s origins have remained elusive. For Southeast Australia, we show here that the ‘‘Big Dry’’ and other iconic 20th Century droughts, including the Federation Drought (1895–1902) and World War II drought (1937–1945), are driven by Indian Ocean variability, not Pacific Ocean conditions as traditionally assumed. Specifically, a conspicuous absence of Indian Ocean temperature conditions conducive to enhanced tropical moisture transport has deprived southeastern Australia of its normal rainfall quota. In the case of the ‘‘Big Dry’’, its unprecedented intensity is also related to recent higher temperatures. Citation: Ummenhofer, C. C., M. H. England, P. C. McIntosh, G. A. Meyers, M. J. Pook, J. S. Risbey, A. S. Gupta, and A. S. Taschetto (2009), What causes southeast Australia’s worst droughts?

Spatially extensive and persistent drought episodes have repeatedly influenced human history, including the ?Strange Parallels? drought event in monsoon Asia during the mid-18th century. Here we explore the dynamics of sustained monsoon failure using observed and tree-ring reconstructed drought patterns and a 1300-year pre-industrial community earth system model control run. Both modern observational and climate model drought patterns during years with extremely weakened South Asian monsoon resemble those reconstructed for the Strange Parallels drought. Model analysis reveals that this pattern arises during boreal spring over Southeast Asia, with decreased precipitation and moisture flux, while related summertime climate anomalies are confined to the Indian subcontinent. Years with simulated South Asian drying exhibit canonical El Ni?o conditions over the Pacific and associated shifts in the Walker circulation. In contrast, multi-year drought periods, resembling those sustained during the Strange Parallels drought, feature anomalous Pacific warming around the dateline, typical of El Ni?o Modoki events.

In this study we investigate the teleconnection between the central-western Pacific sea surface temperature (SST) warming, characteristic of El Nino Modoki events, and Australian rainfall using observations and atmospheric general circulation model experiments. During Modoki events, wet conditions are generally observed over northwestern Australia at the peak of the monsoon season (i.e. January and February) while dry conditions occur in the shoulder-months (i.e. December and March). This results in a shorter but more intense monsoon season over northwestern Australia relative to the climatology. We show that, apart from the well-known displacement of the Walker circulation, the anomalous warming in the central-western equatorial Pacific also induces a westward-propagating disturbance associated with a Gill-type mechanism. This in turn generates an anomalous cyclonic circulation over northwestern Australia that reinforces the climatological mean conditions during the peak of the monsoon season. The anomalous circulation leads to convergence of moisture and increased precipitation over northern Australia. This response, however, only occurs persistently during austral summer when the South Pacific Convergence Zone is climatologically strengthened, phase-locking the Gill-type response to the seasonal cycle. The interaction between the interannual SST variability during El Nino Modoki events and the evolution of the seasonal cycle intensifies deep convection in the central-west Pacific, driving a Gill-type response to diabatic heating. The intensified monsoonal rainfall occurs strongly in February due to the climatological wind conditions that are normally cyclonic over northwestern Australia.