Camargo, S. J., A. W. Robertson, A. G. Barnston and M. Ghil, 2008: Clustering of eastern North Pacific tropical cyclone tracks: ENSO and MJO effects. Geochemistry Geophysics Geosystems, 9.
A probabilistic clustering technique is used to describe tropical cyclone tracks in the eastern North Pacific, on the basis of their shape and location. The best track data set is decomposed in terms of three clusters; these clusters are analyzed in terms of genesis location, trajectory, landfall, intensity, seasonality, and their relationships with the El Niño-Southern Oscillation (ENSO) and Madden-Julian Oscillation (MJO). Longitudinal track location plays a strong discriminating role in the regression mixture model's solution, with the average track orientation becoming more zonal toward the west. This progression encapsulates well the relationship between tropical cyclones over the eastern tropical Pacific and the MJO or ENSO. Two of the clusters describe tropical cyclones (TCs) with tracks that lie near the coast of Mexico and Central America. The most frequent cluster contains tracks that trend west-northwestward, while the second most frequent one has genesis locations that lie slightly to the southeast of those in the most frequent cluster and tracks that run typically parallel to the Central American coast. This second cluster is shown to be significantly associated with the westerly phase of the MJO. The third, least frequent cluster contains TCs with westward trajectories over the central and eastern equatorial Pacific; some of these TCs have an impact on Hawaii and other islands, as far as the central and western North Pacific regions. The least frequent cluster is strongly related to ENSO, while the others are not; it occurs significantly more frequently during El Niño conditions. Examination of the large-scale patterns of atmospheric circulation and sea surface temperature associated with each of our three clusters are consistent with previous studies. Anomalous low-level westerly zonal winds from the monsoon trough and MJO meet anomalous easterlies near the region of genesis in each cluster.
Cook, B. I., R. L. Miller and R. Seager, 2008: Dust and sea surface temperature forcing of the 1930s "Dust Bowl" drought. Geophysical Research Letters, 35, L08710(8): doi:10.1029/2008GL033486.
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.
Emile-Geay, J., R. Seager, M. A. Cane, E. R. Cook and G. H. Haug, 2008: Volcanoes and ENSO over the past millennium. Journal of Climate, 21(13): 3134-3148. doi: 10.1175/2007JCLI1884.1.
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.
Fennel, K., J. Wilkin, M. Previdi and R. Najjar, 2008: Denitrification effects on air-sea CO2 flux in the coastal ocean: Simulations for the northwest North Atlantic. Geophysical Research Letters, 35(24): -.
The contribution of coastal oceans to the global air-sea CO2 flux is poorly quantified due to insufficient availability of observations and inherent variability of physical, biological and chemical processes. We present simulated air-sea CO2 fluxes from a high-resolution biogeochemical model for the North American east coast continental shelves, a region characterized by significant sediment denitrification. Decreased availability of fixed nitrogen due to denitrification reduces primary production and incorporation of inorganic carbon into organic matter, which leads to an increase in seawater pCO(2), but also increases alkalinity, which leads to an opposing decrease in seawater pCO(2). Comparison of simulations with different numerical treatments of denitrification and alkalinity allow us to separate and quantify the contributions of sediment denitrification to air-sea CO2 flux. The effective alkalinity flux resulting from denitrification is large compared to estimates of anthropogenically driven coastal acidification. Citation: Fennel, K., J. Wilkin, M. Previdi, and R. Najjar (2008), Denitrification effects on air-sea CO2 flux in the coastal ocean: Simulations for the northwest North Atlantic, Geophys. Res. Lett., 35, L24608, doi: 10.1029/2008GL036147.
Giannini, A., M. Biasutti and M. M. Verstraete, 2008: A climate model-based review of drought in the Sahel: Desertification, the re-greening and climate change. Global and Planetary Change, 64(3-4): 119-128.
We review the evidence that connects drought and desertification in the Sahel with climate change past, present and future. Advances in climate modeling point to the oceans, not land, as the cause of the recent persistence of drought in the Sahel. The current generation of global climate models reproduces the spatial extent, continental in scale, and the timing and duration of the shift to dry conditions that occurred in the late 1960's given knowledge of observed surface oceanic conditions only. The pattern statistically and dynamically associated with drought is one of warming of the tropical oceans, especially the Pacific and Indian Oceans, superimposed on an enhanced warming of the southern compared to the northern hemisphere most evident in the Atlantic. These models, which include a prognostic description of land surface and/or vegetation, albeit crude, indicate that positive feedbacks between precipitation and land surface/cover may act to amplify the ocean-forced component of continental climate. Despite the advances made in understanding the recent past, uncertainty dominates as we move forward in time, to the present, partial greening of the Sahel, and to the future of climate change projections. (C) 2008 Elsevier B.V. All rights reserved.
Gordon, A. L., R. D. Susanto, A. Ffield, B. A. Huber, W. Pranowo and S. Wirasantosa, 2008: Makassar Strait throughflow, 2004 to 2006. Geophysical Research Letters, 35(24): -.
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 Niño suppressed the flow. The 2004-06 Makassar transport displays clear seasonal behavior, with associated heat and freshwater variability, in contrast to the El Niño 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 Niña 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 Niño. 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.
Gorodetskaya, I. V., L. B. Tremblay, B. Liepert, M. A. Cane and R. I. Cullather, 2008: The influence of cloud and surface properties on the Arctic Ocean shortwave radiation budget in coupled models. Journal of Climate, 21(5): 866-882.
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.
Herweijer, C. and R. Seager, 2008: The global footprint of persistent extra-tropical drought in the instrumental era. International Journal of Climatology, 28(13): 1761-1774.
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
Ihara, C., Y. Kushnir and M. A. Cane, 2008: July droughts over Homogeneous Indian Monsoon region and Indian Ocean dipole during El Niño events. International Journal of Climatology, 28(13): 1799-1805.
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 Niño 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.
Ihara, C., Y. Kushnir and M. A. Cane, 2008: Warming trend of the Indian Ocean SST and Indian Ocean dipole from 1880 to 2004. Journal of Climate, 21(10): 2035-2046.
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.
Ihara, C., Y. Kushnir, M. A. Cane and A. Kaplan, 2008: Timing of El Niño-related warming and Indian summer monsoon rainfall. Journal of Climate, 21(11): 2711-2719.
The relationship between all-India summer monsoon rainfall (ISMR) and the timing of (El Niño-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 Niño 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 Niño-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.
Kim, D., J. S. Kug, I. S. Kang, F. F. Jin and A. T. Wittenberg, 2008: Tropical Pacific impacts of convective momentum transport in the SNU coupled GCM. Climate Dynamics, 31(2-3): 213-226.
Impacts of convective momentum transport (CMT) on tropical Pacific climate are examined, using an atmospheric (AGCM) and coupled GCM (CGCM) from Seoul National University. The CMT scheme affects the surface mainly via a convection-compensating atmospheric subsidence which conveys momentum downward through most of the troposphere. AGCM simulations-with SSTs prescribed from climatological and El Nino Southern Oscillation (ENSO) conditions-show substantial changes in circulation when CMT is added, such as an eastward shift of the climatological trade winds and west Pacific convection. The CMT also alters the ENSO wind anomalies by shifting them eastward and widening them meridionally, despite only subtle changes in the precipitation anomaly patterns. During ENSO, CMT affects the low-level winds mainly via the anomalous convection acting on the climatological westerly wind shear over the central Pacific-so that an eastward shift of convection transfers more westerly momentum toward the surface than would occur without CMT. By altering the low-level circulation, the CMT further alters the precipitation, which in turn feeds back on the CMT. In the CGCM, CMT affects the simulated climatology by shifting the mean convection and trade winds eastward and warming the equatorial SST; the ENSO period and amplitude also increase. In contrast to the AGCM simulations, CMT substantially alters the El Nino precipitation anomaly patterns in the CGCM. Also discussed are possible impacts of the CMT-induced changes in climatology on the simulated ENSO.
Lee, M. I., M. J. Suarez, I. S. Kang, I. M. Held and D. Kim, 2008: A moist benchmark calculation for atmospheric general circulation models. Journal of Climate, 21(19): 4934-4954.
A benchmark calculation is designed to compare the climate and climate sensitivity of atmospheric general circulation models (AGCMs). The experimental Setup basically follows that of the aquaplanet experiment (APE) proposed by Neale and Hoskins, but a simple mixed layer ocean is embedded to enable air-sea coupling and the Prediction of surface temperature. In calculations with several AGCMs, this idealization produces very strong zonal-mean flow and exaggerated ITCZ strength. but the model simulations remain sufficiently realistic to justify the use of this frame work in isolating key differences between models. Because Surface temperatures are free to respond to model differences, the simulation of the cloud distribution. especially in the subtropics. affects many other aspects of the simulations. The analysis of the simulated tropical transients highlights the importance of convection inhibition and air-sea coupling as affected by the depth of the mixed layer. These preliminary comparisons demonstrate that this idealized benchmark provides a discriminating framework for understanding the implications of differing physics parameterization in AGCMs.
Leonard, K. C., L. B. Tremblay, D. R. MacAyeal and S. S. Jacobs, 2008: Interactions of wind-transported snow with a rift in the Ross Ice Shelf, Antarctica. Geophysical Research Letters, 35(5): -.
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.
Liang, X. Z., J. H. Zhu, K. E. Kunkel, M. F. Ting and J. X. L. Wang, 2008: Do CGCMs simulate the North American monsoon precipitation seasonal-interannual variability? Journal of Climate, 21(17): 4424-4448.
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.
Lin, J. L., M. I. Lee, D. Kim, I. S. Kang and D. M. W. Frierson, 2008: The impacts of convective parameterization and moisture triggering on AGCM-simulated convectively coupled equatorial waves. Journal of Climate, 21(5): 883-909.
This study examines the impacts of convective parameterization and moisture convective trigger on convectively coupled equatorial waves simulated by the Seoul National University (SNU) atmospheric general circulation model (AGCM). Three different convection schemes are used, including the simplified Arakawa-Schubert (SAS) scheme, the Kuo (1974) scheme, and the moist convective adjustment (MCA) scheme, and a moisture convective trigger with variable strength is added to each scheme. The authors also conduct a "no convection" experiment with deep convection schemes turned off. Space-time spectral analysis is used to obtain the variance and phase speed of dominant convectively coupled equatorial waves, including the Madden-Julian oscillation (MJO), Kelvin, equatorial Rossby (ER), mixed Rossby-gravity (MRG), and eastward inertio-gravity (EIG) and westward inertio-gravity (WIG) waves.
Martinson, D. G., S. E. Stammerjohn, R. A. Iannuzzi, R. C. Smith and M. Vernet, 2008: Western Antarctic Peninsula physical oceanography and spatio-temporal variability. Deep-Sea Research Part Ii-Topical Studies in Oceanography, 55(18-19): 1964-1987.
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 Niña 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 Niña). The large 1997-1998 El Niño, followed by the transition to the strong La Niña 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.
Massom, R. A., S. E. Stammerjohn, W. Lefebvre, S. A. Harangozo, N. Adams, T. A. Scambos, M. J. Pook and C. Fowler, 2008: West Antarctic Peninsula sea ice in 2005: Extreme ice compaction and ice edge retreat due to strong anomaly with respect to climate. Journal of Geophysical Research-Oceans, 113(C2): -.
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 Niña 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.
Meredith, M. P., A. C. N. Garabato, A. L. Gordon and G. C. Johnson, 2008: Evolution of the deep and bottom waters of the Scotia Sea, Southern Ocean, during 1995-2005. Journal of Climate, 21(13): 3327-3343.
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.
Montes-Hugo, M. A., M. Vernet, D. Martinson, R. Smith and R. Iannuzzi, 2008: Variability on phytoplankton size structure in the western Antarctic Peninsula (1997-2006). Deep Sea Research Part II: Topical Studies in Oceanography, 55(18-19): 2106-2117.
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.
Newton, R., P. Schlosser, D. G. Martinson and W. Maslowski, 2008: Freshwater distribution in the Arctic Ocean: Simulation with a high-resolution model and model-data comparison. Journal of Geophysical Research-Oceans, 113(C5): -.
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.
Robinson, L. F., V. H. de la Pena and Y. Kushnir, 2008: Detecting shifts in correlation and variability with application to ENSO-monsoon rainfall relationships. Theoretical and Applied Climatology, 94(3-4): 215-224. DOI: 10.1007/s00704-007-0351-z.
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.
Ross, R. M., L. B. Quetin, D. G. Martinson, R. A. Iannuzzi, S. E. Stammerjohn and R. C. Smith, 2008: Palmer LTER: Patterns of distribution of five dominant zooplankton species in the epipelagic zone west of the Antarctic Peninsula, 1993-2004. Deep-Sea Research Part Ii-Topical Studies in Oceanography, 55(18-19): 2086-2105, doi: 10.1016/j.dsr2.2008.04.037.
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.
Seager, R., R. Burgman, Y. Kushnir, A. Clement, E. Cook, N. Naik(Henderson) and J. Miller, 2008: Tropical Pacific Forcing of North American Medieval Megadroughts: Testing the Concept with an Atmosphere Model Forced by Coral-Reconstructed SSTs. Journal of Climate, 21(23): 6175-6190.
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 Niña-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.
Seager, R., Y. Kushnir, M. Ting, M. Cane, N. Naik(Henderson) and J. Miller, 2008: Would advance knowledge of 1930s SSTs have allowed prediction of the dust bowl drought? Journal of Climate, 21(13): 3261-3281.
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.
Smerdon, J. E., A. Kaplan and D. Chang, 2008: On the Origin of the Standardization Sensitivity in RegEM Climate Field Reconstructions. Journal of Climate, 21(24): 6710-6723, DOI: 10.1175/2008JCLI2182.1.
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 RegEM-Ridge. 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.
Smith, R. C., D. G. Martinson, S. E. Stammerjohn, R. A. Iannuzzi and K. Ireson, 2008: Bellingshausen and western Antarctic Peninsula region: Pigment biomass and sea-ice spatial/temporal distributions and interannual variabilty. Deep-Sea Research Part Ii-Topical Studies in Oceanography, 55(18-19): 1949-1963, doi: 10.1016/j.dsr2.2008.04.027.
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.
Stammerjohn, S. E., D. G. Martinson, R. C. Smith, X. Yuan and D. Rind, 2008: Trends in Antarctic annual sea ice retreat and advance and their relation to El Niño-Southern Oscillation and Southern Annular Mode variability. Journal of Geophysical Research-Oceans, 113(C3): -.
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 Niño - Southern Oscillation ( ENSO). In general, the high-latitude ice-atmosphere response to ENSO was strongest when - SAM was coincident with El Niño and when +SAM was coincident with La Niña, 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).
Sveinsson, O. G. B., U. Lall, J. Gaudet, Y. Kushnir, S. Zebiak and V. Fortin, 2008: Analysis of climatic states and atmospheric circulation patterns that influence Quebec spring streamflows. Journal of Hydrologic Engineering, 13(6): 411-425.
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.
Sveinsson, O. G. B., U. Lall, V. Fortin, L. Perrault, J. Gaudet, S. Zebiak and Y. Kushnir, 2008: Forecasting spring reservoir inflows in Churchill Falls basin in Quebec, Canada. Journal of Hydrologic Engineering, 13(6): 426-437.
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).
Vernet, M., D.G. Martinson, R. Iannuzzi, S. Stammerjohn, W. Kozlowski, K. Sines, R. Smith and I. Garibotti, 2008: Primary production within the sea-ice zone west of the Antarctic Peninsula: I—Sea ice, summer mixed layer, and irradiance. Deep Sea Research Part II: Topical Studies in Oceanography, 55(18-19, doi: 10.1016/j.dsr2.2008.04.021): 2068-2085.
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.
Yuan, X. J. and C. H. Li, 2008: Climate modes in southern high latitudes and their impacts on Antarctic sea ice. Journal of Geophysical Research-Oceans, 113(C6): -.
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.
Zappa, C. J. , M. L. Banner, H. J. Schultz, A. Corrada-Emmanuel, L. B. Wolff and J. Yalcin, 2008: Retrieval of short ocean wave slope using polarimetric imaging. Measurement Science and Technology, 19(5).
We present a passive optical remote sensing technique for recovering shape information about a water surface, in the form of a two-dimensional slope map. The method, known as polarimetric slope sensing (PSS), uses the relationship between surface orientation and the change in polarization of reflected light to infer the instantaneous two-dimensional slope across the field-of-view of an imaging polarimeter. For unpolarized skylight, the polarization orientation and degree of linear polarization of the reflected skylight provide sufficient information to determine the local surface slope vectors. A controlled laboratory experiment was carried out in a wave tank with mechanically generated gravity waves. A second study was performed from a pier on the Hudson River, near Lamont-Doherty Earth Observatory. We demonstrated that the two-dimensional slope field of short gravity waves could be recovered accurately without interfering with the fluid dynamics of the air or water, and water surface features appear remarkably realistic. The combined field and laboratory results demonstrate that the polarimetric camera gives a robust characterization of the fine-scale surface wave features that are intrinsic to wind-driven air–sea interaction processes.
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