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.
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.
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.
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).
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.
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