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El Nino-Southern Oscillation (ENSO) is by far the most energetic, and at present also the most predictable, short-term fluctuation in the Earth's climate system, though the limits of its predictability are still a subject of considerable debate. As a result of over two-decades of intensive observational, theoretical and modeling efforts, ENSO's basic dynamics is now well understood and its prediction has become a routine practice at application centers all over the world. The predictability of ENSO largely stems from the ocean-atmosphere interaction in the tropical Pacific and the low-dimensional nature of this coupled system. Present ENSO forecast models, in spite of their vast differences in complexity, exhibit comparable predictive skills, which seem to have hit a plateau at moderate level. However, mounting evidence suggests that there is still room for improvement. In particular, better model initialization and data assimilation, better simulation of surface heat and freshwater fluxes, and better representation of the relevant processes outside of the tropical Pacific, could all lead to improved ENSO forecasts. (c) 2007 Elsevier Inc. All rights reserved.

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Droughts over the central United States (US) are modulated by sea surface temperature (SST) variations in the eastern tropical Pacific. Many models, however, are unable to reproduce the severity and spatial pattern of the "Dust Bowl'' drought of the 1930s with SST forcing alone. We force an atmosphere general circulation model with 1930s SSTs and model-generated dust emission from the Great Plains region. The SSTs alone force a drought over the US similar to observations, but with a weaker precipitation anomaly that is centered too far south. Inclusion of dust radiative forcing, centered over the area of observed wind erosion, increases the intensity of the drought and shifts its center northward. While our conclusions are tempered by limited quantitative observations of the dust aerosol load and soil erosion during this period, our study suggests that unprecedented atmospheric dust loading over the continental US exacerbated the "Dust Bowl'' drought.

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Abstract:
We investigate relationships between Indonesian
drought, the state of the equatorial Indian Ocean, and
ENSO using three instrumental indices spanning 1884–
1997 A.D.: 1. EQWIN, a zonal wind index for the equatorial
Indian Ocean; 2. the Dipole Mode Index (DMI), an indicator
of the Indian Ocean SST gradient; and 3. tropical Pacific
Nin˜o-3.4 SSTs. A regression model of the Java Sep–Dec
Palmer Drought Severity Index (PDSI) using a combination
of these indices provides significant predictive skill (ar2 =
0.50). Both the DMI and EQWIN correlate strongly with
Java droughts (r = 0.71 and 0.66, respectively), but weakly
with wet events (r = 0.21 and 0.18, respectively), while
the Nin˜o SST index correlates moderately with both dry
and wet events (r = 0.31 and 0.36, respectively). Our
findings indicate that Java droughts are intensified during El
Nin˜os that coincide with negative EQWIN conditions,
which are also linked to a strengthened Indian monsoon.
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The controversial claim that El Ni (n) over tildeo events might be partially caused by radiative forcing due to volcanic aerosols is reassessed. Building on the work of Mann et al., estimates of volcanic forcing over the past millennium and a climate model of intermediate complexity are used to draw a diagram of El Ni (n) over tildeo likelihood as a function of the intensity of volcanic forcing. It is shown that in the context of this model, only eruptions larger than that of Mt. Pinatubo ( 1991, peak dimming of about 3.7Wm(-2)) can shift the likelihood and amplitude of an El Ni (n) over tildeo event above the level of the model's internal variability. Explosive volcanism cannot be said to trigger El Ni (n) over tildeo events per se, but it is found to raise their likelihood by 50% on average, also favoring higher amplitudes. This reconciles, on one hand, the demonstration by Adams et al. of a statistical relationship between explosive volcanism and El Ni (n) over tildeo and, on the other hand, the ability to predict El Ni (n) over tildeo events of the last 148 yr without knowledge of volcanic forcing.

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Abstract:
We investigate relationships between Indonesian
drought, the state of the equatorial Indian Ocean, and
ENSO using three instrumental indices spanning 1884–
1997 A.D.: 1. EQWIN, a zonal wind index for the equatorial
Indian Ocean; 2. the Dipole Mode Index (DMI), an indicator
of the Indian Ocean SST gradient; and 3. tropical Pacific
Nin˜o-3.4 SSTs. A regression model of the Java Sep–Dec
Palmer Drought Severity Index (PDSI) using a combination
of these indices provides significant predictive skill (ar2 =
0.50). Both the DMI and EQWIN correlate strongly with
Java droughts (r = 0.71 and 0.66, respectively), but weakly
with wet events (r = 0.21 and 0.18, respectively), while
the Nin˜o SST index correlates moderately with both dry
and wet events (r = 0.31 and 0.36, respectively). Our
findings indicate that Java droughts are intensified during El
Nin˜os that coincide with negative EQWIN conditions,
which are also linked to a strengthened Indian monsoon.
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The transfer of Pacific water into the Indian Ocean through the Indonesian seas affects the heat and freshwater budgets of both oceans. The observed transport in the Makassar Strait, the primary Indonesian throughflow pathway, from January 2004 through November 2006 is 11.6 +/- 3.3 Sv (Sv = 10(6) m(3)/s). This transport is 27% larger than observed during 1997 when a strong El Nino suppressed the flow. The 2004-06 Makassar transport displays clear seasonal behavior, with associated heat and freshwater variability, in contrast to the El Nino dominated 1997 transport. The 2004-06 transport reached maximum values towards the end of the northwest and southeast monsoons, with minimum transport are in October-December. A sustained high transport is observed in early 2006, perhaps in response to an La Nina condition. The maximum throughflow occurs within the thermocline, as in 1997, though the longer 2004-06 measurements also reveal a shallowing of transport as speeds increase. The transport-weighted temperature is 15.6 degrees C in 2004-06, nearly 1 degrees C warmer than that observed in 1997, presumably a consequence of El Nino. Citation: Gordon, A. L., R. D. Susanto, A. Ffield, B. A. Huber, W. Pranowo, and S. Wirasantosa (2008), Makassar Strait throughflow, 2004 to 2006, Geophys. Res. Lett., 35, L24605, doi: 10.1029/2008GL036372.

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The impact of Arctic sea ice concentrations, surface albedo, cloud fraction, and cloud ice and liquid water paths on the surface shortwave (SW) radiation budget is analyzed in the twentieth-century simulations of three coupled models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report. The models are the Goddard Institute for Space Studies Model E-R (GISS-ER), the Met Office Third Hadley Centre Coupled Ocean-Atmosphere GCM (UKMO HadCM3), and the National Center for Atmosphere Research Community Climate System Model, version 3 (NCAR CCSM3). In agreement with observations, the models all have high Arctic mean cloud fractions in summer; however, large differences are found in the cloud ice and liquid water contents. The simulated Arctic clouds of CCSM3 have the highest liquid water content, greatly exceeding the values observed during the Surface Heat Budget of the Arctic Ocean (SHEBA) campaign. Both GISS-ER and HadCM3 lack liquid water and have excessive ice amounts in Arctic clouds compared to SHEBA observations. In CCSM3, the high surface albedo and strong cloud SW radiative forcing both significantly decrease the amount of SW radiation absorbed by the Arctic Ocean surface during the summer. In the GISS-ER and HadCM3 models, the surface and cloud effects compensate one another: GISS-ER has both a higher summer surface albedo and a larger surface incoming SW flux when compared to HadCM3. Because of the differences in the models' cloud and surface properties, the Arctic Ocean surface gains about 20% and 40% more solar energy during the melt period in the GISS-ER and HadCM3 models, respectively, compared to CCSM3.

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The major North American droughts as per instrumental records are shown to be part of a larger, global pattern of low-frequency drought variability. Drought in western North America during the 1850s-1860s, 1870s, 1890s, 1930s and 1950s, is shown to coincide with the occurrence of prolonged dry spells in parts of Europe, southern South America and western Australia. Tropical land regions are mostly wet during these periods, with the exception of central east Africa, southern India and Sri Lanka, which are dry. The recent 1998-2003 period of drought in western North America reveals a similar global hydroclimatic lsquofootprintrsquo with the exception of a wet southern South America and continued dry conditions in the Sahel. Common to each of the six droughts is the persistence of anomalously cool east central tropical Pacific sea surface temperatures (SSTs). For the 1998-2003 case, the warming of SSTs everywhere outside of the east central tropical Pacific may be influencing precipitation and masking the influence of persistent precipitation anomalies driven from the tropical Pacific alone. In general, examination of these major historical extra-tropical droughts reveals a hemispherically and, in the extra-tropics, a zonally symmetric pattern consistent with forcing from the Tropics.Ensembles of model simulations forced by observed SSTs globally (Global Ocean Global Atmosphere, GOGA) and only within the tropical Pacific (Pacific Ocean Global Atmosphere-Mixed Layer, POGA-ML) are both able to capture the global pattern of the persistent extra-tropical drought regimes since the mid-nineteenth century. This implies that the recently demonstrated link between SST forcing and drought in North America is in fact only one part of a global hydroclimatic response to the persistence of cool SST anomalies in the tropical Pacific. Indian Ocean SST forcing is required to capture the droughts in central east Africa. Over Europe, the modelled, low-frequency precipitation signal is unrealistically ENSO dominated, as the model does not faithfully reproduce the observed history of low-frequency NAO variability. Overall, our results suggest that the global pattern of persistent drought appears to be a low-frequency version of interannual ENSO-forced variability. Copyright © 2008 Royal Meteorological Society

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The monthly summer monsoon rainfall over the Homogeneous Indian Monsoon region (HI) that represents most of the variance of all-India monsoon rainfall is investigated using observational data from 1880 to 2002. Severe droughts in July occur mostly during El Nino events of the boreal summer monsoon season. They occurred frequently in the late 19th to early 20th century, rarely in the middle of the 20th century, and again occasionally occurred after the 1960s.

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The state of the Indian Ocean dipole representing the SST anomaly difference between the western and southeastern regions of the ocean is investigated using historical SST reconstructions from 1880 to 2004. First, the western and eastern poles of the SST-based dipole mode index are analyzed separately. Both the western and eastern poles display warming trends over this period, particularly after the 1950s. The western pole tends to be anomalously colder than the eastern pole from 1880 to 1919, whereas in the interval 1950-2004 the SST anomalies over the western pole are comparable to those over the eastern pole though there are occasional outliers where the eastern pole is anomalously colder than the western pole.

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The relationship between all-India summer monsoon rainfall (ISMR) and the timing of (El Nino-Southern Oscillation) ENSO-related warming/cooling is investigated, using observational data during the period from 1881 to 1998. The analysis of the evolutions of Indo-Pacific sea surface temperature (SST) anomalies suggests that when ISMR is not below normal despite the co-occurrence of an El Nino event, warming over the eastern equatorial Pacific starts from boreal winter and evolves early so that the westerncentral Pacific and Indian Ocean are warmer than normal during the summer monsoon season. In contrast, when the more usual El Nino-dry ISMR relationship holds, the eastern equatorial Pacific starts warming rapidly only about a season before the reference summer so that the western-central Pacific and Indian Oceans remain cold during the monsoon season.

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Rifts in ice shelves accumulate a melange of snow and firn from above and marine ice from below, material that has been postulated to negatively influence iceberg calving. From measurements and modeling we show that a 100 m wide rift near the front of the Ross Ice Shelf captures all wind-transported snow traveling in saltation and a substantial fraction of the snow blowing in suspension across the rift. Moderate winds and snow transport appear sufficient to fill the rift in several years, whereas the melange surface profile remains relatively constant within the 30 m deep rift. Observed subsidence and ocean temperatures in the rift suggest that net basal melting dissolves the melange from below, limiting its effectiveness as an anti-calving agent near ice fronts.

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This study uses the most recent simulations from all available fully coupled atmosphere-ocean general circulation models (CGCMs) to investigate whether the North American monsoon (NAM) precipitation seasonal-interannual variations are simulated and, if so, whether the key underlying physical mechanisms are correctly represented. This is facilitated by first identifying key centers where observed large-scale circulation fields and sea surface temperatures (SSTs) are significantly correlated with the NAM precipitation averages over the core region (central-northwest Mexico) and then examining if the modeled and observed patterns agree.

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This study focuses on 12 years of physical oceanography data, collected during the Palmer, Antarctica, Long-Term Ecological Research program (PAL LTER) over the continental margin of the western Antarctic Peninsula (WAP). The dataset offers the most long-lived consistent CTD-gridded observations of Antarctic waters collected anywhere in the Southern Ocean. The physical characteristics, water column structure and spatio-temporal variability of the various properties are examined for physically consistent and ecologically important patterns and modes of variability. Unique findings of note include: (1) The average annual ocean heat flux (to the atmosphere) over the continental shelf shows a decreasing trend through time averaging 0.6 W m(-2) yr(-1), with an annual average ocean heat flux of similar to 19W m(-2). The ocean heat content over the shelf shows a linearly increasing trend of 2.6 x 10(7) J m(-2) yr(-1), due predominantly to increased upwelling of warm Upper Circumpolar Deep Water (UCDW) onto the shelf with a small contribution due to a slight warming of UCDW (but over longer time scales (50yr), the warming of UCDW dominates), (2) optimal multi-annual average vertical turbulent diffusivity coefficient (k(z)) is similar to 8.5 x 10(-5) m(2)s(-1), determined by inversion considering warming of trapped remnant winter mixed layer water, (3) the water masses in the grid are well separated according to bathymetrically controlled features, dividing the sample domain into 3 sub-regions: slope, shelf and coastal waters; (4) the Antarctic Circumpolar Current (ACC) was always present along the shelf-break (consistent with the Orsi et al. [1995. On the mericlional extent and fronts of the Antarctic Circumpolar Current. Deep-Sea Research 1 42 (5), 641-673.] climatology) where UCDW shows its farthest southern extent and forms the Southern ACC Front (SACCF). The spatio-temporal variability of the delivery and distribution of ocean heat is dictated by the dynamics that are consistent with changes in the state of ENSO (La Nina drives enhanced upwelling in this region) and in the strength of the Southern Annular Mode (SAM; +SAM drives a local response similar to that of La Nina). The large 1997-1998 El Nino, followed by the transition to the strong La Nina of 1998-1999 (amplified by a large +SAM) introduced a regime shift on the shelf, resulting in the elimination of similar to 0.5 m of sea ice melt (presumably from the loss of sea ice being grown). 2002 was an anomalous year coinciding with an extraordinary storm forcing driving a 4.5 sigma increase in the heat content on the shelf. These jumps coincide with considerable changes in sea ice distribution as well. Pure UCDW on the shelf is primarily restricted to the deep canyons, with occasional appearances on the shelf floor near the middle of the grid. Anomalies in summer sea surface temperatures reflect wind strength (stronger winds mixing more cold winter water to the surface, with cooler SST; light winds, the opposite). (C) 2008 Elsevier Ltd. All rights reserved.

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In September-October 2005, the juxtaposition of low-and high-pressure anomalies at 130 degrees W and 60 degrees W, respectively, created strong and persistent northerly airflow across the West Antarctic Peninsula (WAP). This had a major impact on regional sea ice conditions, with extreme ice compaction in the Bellingshausen and East Amundsen seas (60 degrees W-130 degrees W) but divergence in the West Amundsen and East Ross seas. This resulted in the former in a highly compact marginal ice zone and ice cover, mean modeled ice thicknesses of >5 m, and an earlier-than-average maximum extent (mid-August). While rapid ice retreat in late winter-spring created a major negative ice extent anomaly, compact ice persisted in the subsequent summer. Other effects were anomalies in air temperature (of +1 degrees C to +5 degrees C) and precipitation rates (to >2.5 mm/d). The patterns in late 2005 are consistent with the occurrence of a weak La Nina and a near-neutral Southern Annular Mode, with a quasi-stationary zonal wave three pattern dominating hemispheric atmospheric circulation. Once a compact ice edge was created, it took only one additional week of strong winds to "solidify'' the pack in place. Conditions in 2005 are analyzed in the context of 1979-2005 and compared with the springs of 1993, 1997, 1999, 2001, and 2004. A statistically significant increase of the northerly 10-m wind component between 110 degrees W and 125 degrees W occurred in the Septembers of 1979-2005. No clear trends occur in other spring months. This work underlines the key importance of ice dynamics in recent changes in the WAP sea ice regime.

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The Southern Ocean hosts the formation of the densest layers of the oceanic overturning circulation and provides a climatically sensitive element of deep ocean ventilation. An oceanographic section across the eastern Scotia Sea occupied in 1995, 1999, and 2005 reveals significant variability in the deep and bottom waters of Southern Ocean origin. Warming (similar to 0.1 degrees C) of the warm midlayer waters in the Scotia Sea between 1995 and 1999 reversed through to 2005, reflecting changes seen earlier upstream in the Weddell Sea. The volume of deep waters with potential temperature less than 0 degrees C decreased during 1995-2005, though such a reduction was only clear between 1995 and 1999 at the southern end of the section. The abyssal waters of the eastern Scotia Sea changed circulation between 1995 and 1999, with the dominant point of their entry to the basin shifting from the south to the northeast; by 2005, the former route had regained dominance. These changes are best explained by interannual variations in the deep waters exiting the Weddell Sea, superimposed on a longer-term (decadal) warming trend. The interannual variations are related to changes in the strength of the Weddell Gyre, reflecting large-scale atmospheric variability that may include the El Ni (n) over tildeo- Southern Oscillation phenomenon. The Scotia Sea is the most direct pathway for dense waters of the overturning circulation emanating from the Weddell Sea to fill much of the World Ocean abyss. The regional changes reported here have the potential to affect the climatically significant ventilation of the global ocean abyss.

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The temporal and spatial variability of phytoplankton size structure of waters west of the Antarctica Peninsula (WAP) was investigated between 1997 and 2006. Time series of satellite-derived (phytoplankton size structure index or [gamma]bbp, chlorophyll a concentration or chlT, and sea-ice extent) and shipboard (temperature, salinity, nutrients, and mixed-layer depth) variables were generated during spring-summer in slope, middle shelf and inshore waters and analyzed in relation to atmospheric anomalies (El NiÒo Southern Oscillation, ENSO, and Southern Annular Mode, SAM). The sampling design included stations north (northern, 62∞S) and within (central and southern, 64-68∞S) the Pal-LTER (Palmer Long Term Ecological Research) study site. It is hypothesized that contribution of [`]small' phytoplankton (<20†[mu]m) has increased in the last decade in WAP waters due the ongoing regional climate change. Relationships between [gamma]bbp, the spectral slope of particle backscattering, and environmental parameters were explored based on non-parametric trends (Mann-Kendall test) and cross-correlation coefficients (Spearman matrix). Three types of temporal patterns were detected in satellite-derived phytoplankton size distributions: (1) inter-annual variations of spring-summer [gamma]bbp related to monthly sea-ice extent, (2) abrupt transitions toward dominance of [`]small' (<20†[mu]m) phytoplankton cells (high [gamma]bbp) and low chlT values (<1†mg†m-3) during 1998 and 2003 summer seasons, and (3) positive or negative trends (decrease vs increase of mean cell size) in specific domains of central and northern stations. Temporal transitions in cell size coincided with a switch on ENSO and SAM anomalies as well as increase of heat content of shelf waters over the WAP region. The lack of offshore spring bloom and summer shelf bloom most likely explains the dominance of relative small phytoplankton cells during the 1998 and 2003 summer seasons. A greater frequency of southerly winds during spring and autumn is expected to favor the dominance of [`]small' (<20†[mu]m) phytoplankton cells over WAP waters. Conversely, the greater intensification of the Antarctic Circumpolar Current interaction with the WAP shelf-break during SAM+ years is expected to intensify topographically induced upwelling and favor the dominance of [`]large' (>20†[mu]m) phytoplankton cells on slope waters of central stations. The well-described 50-year warming trend in the Antarctic Peninsula has not resulted in a consistent trend in phytoplankton size structure, as originally hypothesized, but a mosaic of trends attributed to anomalous mesoscale changes of sea-ice extent and circulation patterns.

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A high-resolution numerical simulation of the Arctic Ocean is analyzed in order to study the fate of river runoff and freshwater fluxes in the Arctic Ocean. The model is driven by realistic winds and thermodynamic forcing from the European Centre for Medium-range Weather Forecasting (ECMWF) Reanalysis data set. Dye tracers have been added to visualize the pathways followed by low-salinity water from the major Arctic rivers and Bering Strait Inflow. The model is spun up using repeated forcing with the 1979 annual cycle for 20 years; then the 1979 through 1998 atmospheric forcing is applied. Under the influence of the 1979 through early 1980s winds, a large plume of river runoff exits the broad Eurasian shelf seas along the Lomonosov Ridge. Starting in about 1985, the locus of shelf-to-basin transport shifts eastward to the Alpha-Mendeleyev ridge complex. This shift in the model output is related to changes in the sea-surface height (SSH) fields, which we attribute primarily to shifts in surface wind stresses. Model resolution, runoff inputs, and relaxation terms in the Lena River delta region are analyzed in detail to expose issues with model performance at boundaries with freshwater inflow. Suggestions are made for improving future simulations of river runoff in basin-scale models.

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This paper addresses the retrospective detection of step changes at unknown time points in the correlation structure of two or more climate times series. Both the variance of individual series and the covariance between series are addressed. For a sequence of vector-valued observations with an approximate multivariate normal distribution, the proposed method is a parametric likelihood ratio test of the hypothesis of constant covariance against the hypothesis of at least one shift in covariance. The formulation of the test statistic and its asymptotic distribution are taken from Chen and Gupta (2000). This test is applied to the series comprised of the mean summer NINO3 index and the Indian monsoon rainfall index for the years 1871-2003. The most likely change point year was found to be 1980, with a resulting p-value of 0.12. The same test was applied to the series of NINO3 and Northeast Brazil rainfall observations from the years 1856-2001. A shift was detected in 1982 which is significant at the 1% level. Some or all of this shift in the covariance matrix can be attributed to a change in the variance of the Northeast Brazil rainfall. A variation of this methodology designed to increase power under certain multiple change point alternatives, specificallly when a shift is followed by a reversal, is also presented. Simulations to assess the power of the test under various alternatives are also included, in addition to a review of the literature on alternative methods.

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Variability in the temporal-spatial distribution and abundance of zooplankton was documented each summer on the Palmer Long-Term Ecological Research (LTER) grid west of the Antarctic Peninsula between Anvers and Adelaide Islands during a 12-yr time series. Oblique tows to 120 m with a 2 x 2 m fixed-frame net were made at about 50 stations each January/February between 1993 and 2004. The numerically dominant macro- and mesozooplanktonic species >2mm included three species of euphausiids (Euphausia superba, Antarctic krill; Thysano ssa macrura; Euphausia crystallorophias, ice krill), a shelled pteropod (Limacina helicina), and a salp (Salpa thompsoni). Life cycles, life spans, and habitat varied among these species. Abundance data from each year were allocated to 100 km by 20 km (alongshore by on/offshore) grid cells centered on cardinal transect lines and stations within the Palmer LTER grid. The long-term mean or climatology and means for each year were used to calculate annual anomalies across the grid. Principal components analysis (PCA) was used to analyze for patterns and trends in the temporal-spatial variability of the five species. Questions included whether there are groups of species with similar patterns, and whether population cycles, species interactions or seasonal sea-ice parameters were correlated with detected patterns. Patterns in the climatology were distinct, and matched those of physical parameters. Common features included higher abundance in the north than in the south, independent of the cross-shelf gradients, and cross-shelf gradients with higher abundance either inshore (E. crystallorophias) or offshore (S. thompsoni). Anomalies revealed either cycles in the population, as episodic recruitment in Antarctic krill, or changes in anomaly pattern between the first and second half of the sampling period. The 1998 year, which coincided with a rapid change from a negative to a positive phase in the SOI, emerged as a year with either significant anomalies or that marked a change in anomaly patterns for different species. PCA analysis showed that the pattern of cumulative variance with increasing number of modes was distinctly different for shorter-lived versus longer-lived species; the first mode accounted for nearly 50% of the variance in the shorter-lived species and less than 25% in the longer-lived species. This suggested that the mechanisms driving variability in the temporal-spatial distribution of the shorter-lived, more oceanic species were less complex and more direct than those for the longer-lived euphausiids. Evidence from both the anomaly plots and the trend analysis suggested that salps have been more consistently present across the shelf from 1999 to present, and that the range of L. helicina has been expanding. With shorter life spans, these two species can respond more quickly to the increasing heat content on the shelf in this region. The cross-correlation analysis illustrated the negative correlation between salps and ice retreat and the number of ice days, and the positive correlation between the presence of ice krill and the day of ice retreat. These results suggest that for these species, several environmental controls on distribution and abundance were linked to seasonal sea-ice dynamics. (C) 2008 Elsevier Ltd. All rights reserved.

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The possible role that tropical Pacific SSTs played in driving the megadroughts over North America during the medieval period is addressed. Fossil coral records from the Palmyra Atoll are used to derive tropical Pacific SSTs for the period from A. D. 1320 to A. D. 1462 and show overall colder conditions as well as extended multidecadal La Nina-like states. The reconstructed SSTs are used to force a 16-member ensemble of atmosphere GCM simulations, each with different initial conditions, with the atmosphere coupled to a mixed layer ocean outside of the tropical Pacific. Model results are verified against North American tree ring reconstructions of the Palmer Drought Severity Index. A singular value decomposition analysis is performed using the soil moisture anomaly simulated by another 16-member ensemble of simulations forced by global observed SSTs for 1856-2004 and tree ring reconstructions of the Palmer Drought Severity Index for the same period. This relationship is used to transfer the modeled medieval soil moisture anomaly (relative to the modern simulation) into a model-estimated Palmer Drought Severity Index. The model-estimated Palmer Drought Severity Index reproduces many aspects of both the interannual and decadal variations of the tree ring reconstructions, in addition to an overall drier climate that is drier than the tree ring records suggest. The model-estimated Palmer Drought Severity Index simulates two previously identified "megadroughts," A. D. 1360-1400 and A. D. 1430-60, with a realistic spatial pattern and amplitude. In contrast, the model fails to produce a period of more normal conditions in the early fifteenth century that separated these two megadroughts. The dynamical link between tropical SSTs and the North American megadroughts is akin to that operating in modern droughts. The model results are used to argue that the tropical Pacific played an active role in driving the megadroughts. However, the match between simulated and reconstructed hydroclimate is such that it is likely that both the coral-reconstructed SST anomalies contain significant errors and that SST anomalies in other basins also played a role in driving hydroclimate variations over North America during the late medieval period.

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Could the Dust Bowl drought of the 1930s have been predicted in advance if the SST anomalies of the 1930s had been foreknown? Ensembles of model simulations forced with historical observed SSTs in the global ocean, and also separately in the tropical Pacific and Atlantic Oceans, are compared with an ensemble begun in January 1929 with modeled atmosphere and land initial conditions and integrated through the 1930s with climatological SSTs. The ensemble with climatological SSTs produces values for the precipitation averaged over 1932-39 that are not statistically different from model climatology. In contrast, the ensembles with global SST forcing produce a drought centered in the central plains and southwestern North America that is clearly separated from the model climatology. Both the tropical Pacific and northern tropical Atlantic SST anomalies produce a statistically significant model drought in this region. The modeled drought has a spatial pattern that is different from the observed drought, which was instead centered in the central and northern plains and also impacted the northern Rocky Mountain states but not northeastern Mexico. The model error in extending the Dust Bowl drought too far south is attributed to an incorrect response of the model to warm subtropical North Atlantic SST anomalies. The model error in the northern states cannot be attributed to an incorrect response to tropical SST anomalies. The model also fails to reproduce the strong surface air warming across most of the continent during the 1930s. In contrast, the modeled patterns of precipitation reduction and surface air temperature warming during the 1950s drought are more realistic. Tree-ring records show that the Dust Bowl pattern of drought has occurred before, suggesting that while the extensive human-induced land surface degradation and dust aerosol loading of the 1930s drought may have played an important role in generating the observed drought pattern, natural processes, possibly including land interactions, are capable of generating droughts centered to the north of the main ENSO teleconnection region. Despite this caveat, advance knowledge of tropical SSTs alone would have allowed a high-confidence prediction of a multiyear and severe drought, but one centered too far south and without strong cross-continental warming.

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Abstract:
The regularized expectation maximization (RegEM) method has been used in recent studies to derive
climate field reconstructions of Northern Hemisphere temperatures during the last millennium. Original
pseudoproxy experiments that tested RegEM [with ridge regression regularization (RegEM-Ridge)] standardized
the input data in a way that improved the performance of the reconstruction method, but included
data from the reconstruction interval for estimates of the mean and standard deviation of the climate
field—information that is not available in real-world reconstruction problems. When standardizations are
confined to the calibration interval only, pseudoproxy reconstructions performed with RegEM-Ridge suffer
from warm biases and variance losses. Only cursory explanations of this so-called standardization sensitivity
of RegEM-Ridge have been published, but they have suggested that the selection of the regularization
(ridge) parameter by means of minimizing the generalized cross validation (GCV) function is the source of
the effect. The origin of the standardization sensitivity is more thoroughly investigated herein and is shown
not to be associated with the selection of the ridge parameter; sets of derived reconstructions reveal that
GCV-selected ridge parameters are minimally different for reconstructions standardized either over both
the reconstruction and calibration interval or over the calibration interval only. While GCV may select ridge
parameters that are different from those that precisely minimize the error in pseudoproxy reconstructions,
RegEM reconstructions performed with truly optimized ridge parameters are not significantly different
from those that use GCV-selected ridge parameters. The true source of the standardization sensitivity is
attributable to the inclusion or exclusion of additional information provided by the reconstruction interval,
namely, the mean and standard deviation fields computed for the complete modeled dataset. These fields
are significantly different from those for the calibration period alone because of the violation of a standard
EM assumption that missing values are missing at random in typical paleoreconstruction problems; climate
data are predominantly missing in the preinstrumental period when the mean climate was significantly
colder than the mean of the instrumental period. The origin of the standardization sensitivity therefore is
not associated specifically with RegEM-Ridge, and more recent attempts to regularize the EM algorithm
using truncated total least squares could theoretically also be susceptible to the problems affecting RegEMRidge.
Nevertheless, the principal failure of RegEM-Ridge arises because of a poor initial estimate of the
mean field, and therefore leaves open the possibility that alternative methods may perform better.
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The Palmer Long-Term Ecological Research (LTER) program seeks to obtain a comprehensive understanding of various components of the Antarctic marine ecosystem-the assemblage of plants, microbes, animals, ocean, and sea-ice south of the Antarctic Polar Front. A central hypothesis of the Palmer LTER is that the seasonal and interannual variability of sea ice affects all levels of the Antarctic marine ecosystem, from the timing and magnitude of seasonal primary production to the breeding success and survival of apex predators. In the context of this high-latitude ecosystem we use satellite imagery to examine physical forcing and possible mechanisms influencing the distribution of phytoplankton biomass in the region to the west of the Antarctic Peninsula. We evaluate the spatial and temporal variability of pigment biomass (estimated as chlorophyll-a concentrations using SeaWiFS data) in response to the spatial and temporal variability of sea-ice extent (estimated from passive microwave satellite data). While the ocean-color data record is relatively short (7 years) and contains high interannual variability, there are persistent spatial patterns of phytoplankton biomass that indicate important regional-scale physical mechanisms including: the marginal ice zone and its impact on the mixed-layer depth, the timing of spring sea-ice retreat, the importance of the southern Antarctic Circumpolar front, and teleconnections with sub-polar regions. The SeaWIFS imagery presented here provides the most complete synoptic space/time views of phytoplankton biomass within this region to date. These observations suggest that the southern Antarctic Circumpolar front may have a more profound influence on the western Antarctic Peninsula ecosystem than previously thought. (C) 2008 Elsevier Ltd. All rights reserved.

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The Antarctic Peninsula region is undergoing rapid change: a warming in winter of almost 6 degrees C since 1950, the loss of six ice shelves, the retreat of 87% of the marine glaciers, and decreases in winter sea-ice duration. Concurrently, there is evidence of ecosystem change along the western Antarctic Peninsula (wAP). Since the life histories of most polar marine species are synchronized with the seasonal cycle of sea ice, we assess how the seasonal sea-ice cycle is changing in the wAP region. Four new metrics of seasonal sea-ice variability were extracted from spatial maps of satellite derived daily sea-ice concentration: (a) day of advance, (b) day of retreat, (c) the total number of sea-ice days (between day of advance and retreat), and (d) the percent time sea-ice was present (or sea-ice persistence). The spatiotemporal variability describes distinct on-to-offshore and alongshore differences in ice-ocean marine habitats, characterized overall by a longer sea-ice season in coastal regions (6.8-7.9 months) versus a shorter sea-ice season over the shelf(4.1-5.3 months), with on-to-offshore differences increasing south-to-north. Large perturbations in the seasonality of the marine habitat occur in association with ENSO and Southern Annular Mode (SAM) variability. The local atmospheric response to these climate modes is largely a strengthening of the meridional winds during spring-to-autumn, which in turn affect the timing of the sea-ice retreat and subsequent advance. These perturbations are embedded in overall trends towards a later sea-ice advance, earlier retreat and consequently shorter sea-ice season, the impacts of which are expected to affect ecosystem functionality in the wAP region. A suite of ocean-atmosphere-ice interactions are described that are consistent with the amplified warming in late autumn, early winter. Published by Elsevier Ltd.

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Previous studies have shown strong contrasting trends in annual sea ice duration and in monthly sea ice concentration in two regions of the Southern Ocean: decreases in the western Antarctic Peninsula/southern Bellingshausen Sea ( wAP/sBS) region and increases in the western Ross Sea ( wRS) region. To better understand the evolution of these regional sea ice trends, we utilize the full temporal ( quasi-daily) resolution of satellite-derived sea ice data to track spatially the annual ice edge advance and retreat from 1979 to 2004. These newly analyzed data reveal that sea ice is retreating 31 +/- 10 days earlier and advancing 54 +/- 9 days later in the wAP/sBS region ( i.e., total change over 1979 - 2004), whereas in the wRS region, sea ice is retreating 29 +/- 6 days later and advancing 31 +/- 6 days earlier. Changes in the wAP/sBS and wRS regions, particularly as observed during sea ice advance, occurred in association with decadal changes in the mean state of the Southern Annular Mode ( SAM; negative in the 1980s and positive in the 1990s) and the high-latitude response to El Nino - Southern Oscillation ( ENSO). In general, the high-latitude ice-atmosphere response to ENSO was strongest when - SAM was coincident with El Nino and when +SAM was coincident with La Nina, particularly in the wAP/sBS region. In total, there were 7 of 11 -SAMs between 1980 and 1990 and the 7 of 10 +SAMs between 1991 and 2000 that were associated with consistent decadal sea ice changes in the wAP/sBS and wRS regions, respectively. Elsewhere, ENSO/SAM-related sea ice changes were not as consistent over time ( e. g., western Weddell, Amundsen, and eastern Ross Sea region), or variability in general was high ( e. g., central/ eastern Weddell and along East Antarctica).

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Results from diagnostic analyses to understand the seasonal evolution of the large-scale climatic state responsible for the development and melt of the winter snowpack, and spring-early summer precipitation in the Churchill Falls region on the Quebec-Labrador Peninsula, Canada, are presented in the context of the development of an empirical model for seasonal to annual streamflow forecasting, with a special emphasis on the May-July spring freshet. Teleconnection indices and gridded global measures of atmospheric circulation inferred from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis are used as climatic indicators. Composite and correlation analyses are applied to the climatic indicators conditioned on the spring streamflow for identification of potential predictors. Meridional and zonal atmospheric fluxes over the Atlantic and the Pacific Oceans emanating from regionally persistent sea surface temperature/sea level pressure modes are identified as potential carriers of information. We speculate on the ocean-atmosphere and regional hydrologic mechanisms that may be involved in lending multiseasonal predictability to streamflows in the region.

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The performance of different models and procedures for forecasting aggregated May-July streamflow for the Churchill Falls basin on the Quebec-Labrador peninsula is compared. The models compared have different lead times and include an autoregressive model using only past streamflow data, an autoregressive with exogenous input model utilizing both past streamflow and precipitation, and a linear regression model using the principal components of exogenous measures of atmospheric circulation inferred from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis project. The forecast skills of the different approaches are compared using a variety of measures of performance. The results indicate that relatively accurate forecasts using only measures of atmospheric circulation can be issued as early as in December of the prior year. A multimodel combination approach is found to be more effective than the use of a single forecast model. In addition, it is concluded that forecasting models utilizing atmospheric circulation data are useful, especially for basins where hydroclimatic observations are scarce and for basins where flows and other hydroclimatic variables are not strongly autocorrelated (do not depend on their past).

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Abstract:
The Lucky Strike segment between 37 and 38N on the
Mid-Atlantic Ridge is the focus of the international MoMAR program to
monitor seafloor-spreading processes. During the GRAVILUCK cruise in
August 2006, hydrographic, velocity and light-scattering data were
collected in the rift valley at Lucky Strike in order to investigate the
regional dynamics and to provide background information for the
monitoring effort. The survey observations reveal a remarkably simple
dynamical setting dominated by persistent northward flow transporting
~0.2Sv of water along the rift valley. Approximately half of
this transport must upwell within a deep basin that occupies the
northern half of the segment. In the comparatively shallow segment
center the along-valley transport takes place in two parallel,
hydraulically controlled overflows on both sides of an active volcano
that rises from the rift-valley floor. Within the better sampled of
these overflows instantaneous velocities recorded during the survey
were northward more than 95% of the time and occasionally exceeded
\cmps{20}. Similar to other laterally confined overflows in the deep
ocean, the cross-sill density gradients are characterized by a lower
layer with streamwise decreasing densities and an upper layer where the
densities increase along the path of the flow. This vertical
density-gradient dipole is the signature of the buoyancy flux
associated with high levels of diapycnal mixing at the sill.

Overall, the hydrography and dynamics in the rift valley of the Lucky Strike
segment are highly reminiscent of many ridge-flank canyons in the
western South Atlantic, where mean along-axial advection of density is
balanced by vigorous diapycnal mixing.

There is circumstantial evidence from historic hydrographic data
suggesting that northward flow below ~1800m in the rift
valley in the MoMAR region is persistent on time scales of years to
decades and that it extends more than 200km to the south. During
GRAVILUCK the northward flow at Lucky Strike extended well above 1600m, where
two previous one-year-long current meters had recorded southward mean
flows near the Lucky Strike hydrothermal vent field. While interannual
variability can potentially account for this difference, the data also
allow for the possibility of a cyclonic re-circulation around an
isolated topographic peak east of the vent field, resulting in the
southward mean flows observed there.
%
In addition to the light-scattering anomalies associated with plumes
rising from the Lucky Strike vent field near the segment center, the GRAVILUCK
data also show clear evidence for a separate hydrothermal particle
plume emanating from a not-yet-discovered vent field in the southern
half of the segment, probably near 2000m.
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In shelf waters of the western Antarctic Peninsula (wAP), with abundant macro- and micronutrients, water-column stability has been suggested as the main factor controlling primary production; freshwater input from sea-ice melting stabilizes the upper water column by forming a shallow summer mixed layer. Retreating sea ice in the spring and summer thus defines the area of influence, the sea-ice zone (SIZ) and the marginal ice zone (MIZ). A 12-year time series (1995–2006) was analyzed to address two main questions: (1) what are the spatial and temporal patterns in primary production; and (2) to what extent and in what ways is primary production related to sea-ice dynamics. Data were collected on cruises performed during January of each year, at the height of the growth season, within the region bounded by 64°S and 64°W to the north and 68°S and 66°W to the south. Average daily integrated primary production varied by an order of magnitude, from not, vert, similar250 to not, vert, similar1100 mg C m−2 d−1, with an average cruise primary production of 745 mg C m−2 d−1. A strong onshore–offshore gradient was evident along the shelf with higher production observed inshore. Inter-annual regional production varied by a factor of 7: maximum rates were measured in 2006 (1788 mg C m−2 d−1) and minimum in 1999 (248 mg C m−2 d−1). The results support the hypothesis that primary production in the wAP shelf is related to sea-ice dynamics. To first order, shallower summer mixed-layer depths in the shelf correlated with late sea retreat and primary production. Principal component analysis showed that high primary production in January was associated with enhanced shelf production toward the coast and in the south, explaining 63% of the variability in space and time. This first mode captured the inter-annual variability in regional production. Temporal variability in primary production (time series of anomalies defined for each location) showed spatial dependence: higher primary production correlated with shallow mixed-layer depths only at mid-shelf; in coastal and offshore waters, primary production correlated with deeper mixed layers. Thus, coastal primary production can show a non-linear relationship with summer mixed layers. Under conditions of large biomass (>20 mg chl a m−3) and shallow mixed-layer depth (e.g., 5 m) phytoplankton production becomes light limited. This limitation is reduced with a deepening of the summer mixed layer (e.g., 20 m). Dominance of diatoms and the ability to adapt and photosynthesize at higher light levels characterized the large phytoplankton blooms. No significant regional trend in primary production was detected within the 12-year series. We conclude that the regional average primary production on the wAP shelf is associated with shallow summer mixed layers in conjunction with late sea-ice retreat. An opposite relationship is observed for the highest production rates in coastal waters, associated with large biomass, where a deepening of the summer mixed layer relieves light limitation.

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This study investigates the influence of high-latitude climate variability on the Antarctic sea ice distribution. The climate variability examined here includes distinct climate modes, such as the Southern Annular Mode (SAM), quasi-stationary wave-3 pattern, Pacific South American pattern (PSA) and Semi-Annual Oscillation (SAO). The results reveal that the largest impact comes from PSA in the Antarctic Dipole (ADP) region of the western hemisphere at the interannual timescale, which is related to the teleconnection of El Ni (n) over tildeo-Southern Oscillation (ENSO). The wave-3 pattern also has a strong and similar influence on sea ice in the ADP regions as PSA does, suggesting a positive interaction between PSA and wave- 3 in the region. Measured by correlation coefficients and their significance, SAM has a relatively less significant influence on sea ice than other climate patterns in general, though this global assessment may not apply to particular regions. Sea ice usually responds to large-scale atmospheric anomalies with a 2-month delay. The singular value decomposition (SVD) analysis reveals that the coupled relationships between sea ice and atmospheric pressure, temperature, and wind fields are represented by these known climate modes. The leading coupled modes between sea ice and sea level pressure are accountable for 50% to 60% of total squared covariance for all seasons. The leading modes between sea ice and surface air temperature in winter and summer are also accountable for the same amount of total squared covariance. It indicates that these well-established climate modes/patterns are the dominant factors leading to a strong interaction between the atmosphere and sea ice field in the Antarctic.

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