摘要:

In situ measurements are used, together with sea surface height data, to study the development and variability of the Great Whirl (GW), a large quasi‐stationary anticyclone that appears off the coast of Somalia during the southwest monsoon season. We find that anticyclonic circulation indicative of the GW appears on average in April, almost two months before the onset of the southwest monsoon winds. This early initiation is coincident with the arrival of annual Rossby waves at the western boundary. With the onset of the monsoon winds in early June, the GW‐proper intensifies quickly, remaining at its peak throughout July, August, and September, and dissipating about one month after the winds have died. The GW is present for an average 166 ± 30 days per year and the position of its northern flank, close to 9°N, coincides with the latitude of zero wind stress curl. The intraseasonal variability of the GW is intense as a result of mutual advection with one to three flanking cyclones, which accompany the GW 70% of the time and tend to circulate clockwise around it. There is no consistent seasonal pattern for the development or dissipation of the GW, although movement to the southwest is common toward the end of the season. The GW of 1995 deepened from 200 m in June to over 1000 m in September, and strengthened from a swirl transport of 10 to 60 Sv. Cool waters in its core resulted from advection via the Somali Current and some local

ISSN:0148-0227    

DOI:10.1029/2012JC008198

年代:2013

数据来源: WILEY

摘要:

The late austral summer (February–April) phytoplankton bloom that occurs east of Madagascar exhibits significant interannual variability and at its largest extent covers ~1% of the world's ocean surface area. The bloom raises many intriguing questions about how it begins, is sustained, propagates to the east, exports carbon, and ends. It has been observed and studied using satellite ocean color observations, but the lack of in situ data makes it difficult to address these questions. Here we describe observations that were made serendipitously on a cruise in February 2005. These show clearly for the first time the simultaneous existence of a deep chlorophyll maximum at ~70–110 m depths (seen in SeaSoar fluorimeter data) and a surface chlorophyll signature [seen in Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) satellite ocean color data]. The observations also show the modulation of the biological signature at the surface by the eddy field but not of the deep chlorophyll maximum.Trichodesmiumdominates the bloom nearer to Madagascar, while the diatomRhizosolenia clevei(and its symbiontRichelia intracellularis) dominates further from the island. The surface bloom seen in the SeaWiFS data is confined to the shallow (~30 m) mixed layer. It is hypothesized that the interannual variability in bloom intensity may be due to variations in coastal upwelling and thus the supply of iron, which is a micronutrient that can limit diazotro

ISSN:0148-0227    

DOI:10.1029/2012JC008339

年代:2013

数据来源: WILEY

摘要:

Quantifying the spatial coverage of floating macroalgae from satellite imagery, using methods such as the normalized difference vegetation index (NDVI) and the floating algae index (FAI), requires the use of a scene‐wide threshold to isolate and then compute the number of floating macroalgae pixels. The problem faced is the sensitivity of the NDVI and, to a lesser extent, the FAI to radiance contributions from atmospheric aerosols and turbid water. Both these factors can vary significantly across a satellites' field‐of‐view generating irregular apparent reflectance of ocean and floating macroalgae pixels across an NDVI/FAI scene, leading to inaccuracies in spatial coverage estimates. We present a simple image processing algorithm, termed the scaled algae index (SAI), that removes any variability present in ocean and floating macroalgae pixels in NDVI or FAI imagery. The SAI does this by subtracting a given pixel's index by that of a local ocean pixel, effectively scaling ocean pixels to values near zero, and macroalgae pixels to positive values. The SAI algorithm has been tested on NDVI and FAI scenes of the 2008/2009 floating macroalgae blooms that occurred in the Yellow Sea, China. These SAI images show a major reduction in variability with scene‐wide histograms being unimodal. Histogram analysis also indicates that sufficient contrast exists between ocean and floating macroalgae pixels to enable segmentation by a scene‐wide threshold. A semiautomated threshold determination procedure is also presented, which together with the SAI algorithm can be used to compute accurate estimates of the spatial coverage of floating m

ISSN:0148-0227    

DOI:10.1029/2012JC008292

年代:2013

数据来源: WILEY

摘要:

In the Eastern Bering Sea, changes in sea ice have been implicated in recent major upper‐trophic level shifts. However, the underlying relationships between sea ice and primary producers have not been well tested. Here, we combine data from multiple satellite platforms, reanalysis model results and biophysical moorings to explore the dynamics of spring and summer primary production in relation to sea ice conditions. In the northern Bering Sea, sea ice consistently retreated in late spring, leading to ice‐edge phytoplankton blooms in cold (0–1 °C) waters. However, in the southeastern Bering Sea, sea ice retreat was far more irregular. Although this did not significantly alter bloom timing, late retreat led to blooms at the ice‐edge while early retreat led to blooms in open waters that were warmer (≤5.4 °C) and>70% more productive. Early sea ice retreat also led to higher productivity in summer, likely due to weaker thermal stratification. Overall, annual net primary production during warm years of early sea ice retreat was enhanced by 40–50% compared to years with late sea ice retreat in the southeastern Bering Sea. These findings suggest the potential for future sea ice loss to enhance overall carrying capacity of the southeastern Bering Sea ecosystem. Consistently warm blooms in the future may also channel more energy flow toward the pelagic, rather than benthic, environment. To date, however, neither sea ice extent nor the timing of its retreat have undergone long‐term changes in the

ISSN:0148-0227    

DOI:10.1029/2012JC008034

年代:2013

数据来源: WILEY

摘要:

AbstractThe pelagic province of the Southern Ocean generally has low levels of primary production attributable to a short growing season in the higher latitudes, a deep mixed layer, and iron limitation. Exceptions include phytoplankton blooms in the marginal ice zone (MIZ) during spring and summer sea‐ice retreat. The prevailing hypothesis as to the drivers of the blooms is that sea‐ice retreat increases the vertical stability of the water column through the production of melt water and provides shelter from wind mixing in areas of partial sea‐ice coverage. These conditions are favorable to phytoplankton growth by allowing them to maintain their position in the upper reaches of the water column. This work investigates the drivers of MIZ blooms using a biochemically coupled global circulation model. Results support the hypothesis in that physical conditions related to a shallow, vertically stable water column (e.g., mixed layer depth and available light) were the most significant predictors of bloom dynamics, while nutrient limitation was of lesser importance. We estimate that MIZ blooms account for 15% of yearly net primary production in the Southern Ocean and that the earlier phases of the MIZ bloom, occurring under partial ice coverage and invisible to remote sensing, account for about two thirds of this production. MIZ blooms were not found to enhance depth‐integrated net primary production when compared to similar ecological provinces outside of the MIZ, although the elevated phytoplankton concentrations in surface waters are hypothesized to provide important feeding habitats for grazing organisms such a

ISSN:0148-0227    

DOI:10.1029/2012JC008418

年代:2013

数据来源: WILEY

摘要:

AbstractRadiometric satellite measurements over the ocean are greatly affected by the contribution of the direct sunlight reflected on the ruffled ocean (so‐called sunglint). Sunglint produces radiance that can far exceed the radiance scattered by both the atmosphere and ocean layers. Knowledge of the sunglint radiance is required in many remote sensing applications using radiance and polarization information (e.g., retrieval of aerosol or hydrosol optical properties, sensor calibration). The Cox and Munk model is currently used for estimating sunglint signal, but its accuracy is mainly limited by the mandatory use of wind speed data sets. An algorithm (so‐called polarization‐based atmospheric correction glint) was developed based on the original multidirectional and polarization radiometric measurements of the Polarization and Anisotropy of Reflectances for Atmospheric Sciences Coupled with Observations from a Lidar satellite mission. The method enables to accurately estimate the radiance and the polarization terms of the sunglint signal. The strength of the algorithm is to quantify the sunglint radiation using the Polarization and Anisotropy of Reflectances for Atmospheric Sciences Coupled with Observations from a Lidar data without any a priori information on the actual sea state A relevant application of the algorithm is proposed to better detect the pixels influenced by clouds provided that ancillary data of wind speed are

ISSN:0148-0227    

DOI:10.1029/2012JC008221

年代:2013

数据来源: WILEY

摘要:

AbstractA new rheology that explicitly accounts for the subcontinuum anisotropy of the sea ice cover is implemented into the Los Alamos sea ice model. This is in contrast to all models of sea ice included in global circulation models that use an isotropic rheology. The model contains one new prognostic variable, the local structure tensor, which quantifies the degree of anisotropy of the sea ice, and two parameters that set the time scale of the evolution of this tensor. The anisotropic rheology provides a subcontinuum description of the mechanical behavior of sea ice and accounts for a continuum scale stress with large shear to compression ratio and tensile stress component. Results over the Arctic of a stand‐alone version of the model are presented and anisotropic model sensitivity runs are compared with a reference elasto‐visco‐plastic simulation. Under realistic forcing sea ice quickly becomes highly anisotropic over large length scales, as is observed from satellite imagery. The influence of the new rheology on the state and dynamics of the sea ice cover is discussed. Our reference anisotropic run reveals that the new rheology leads to a substantial change of the spatial distribution of ice thickness and ice drift relative to the reference standard visco‐plastic isotropic run, with ice thickness regionally increased by more than 1 m, and ice speed reduced by

ISSN:0148-0227    

DOI:10.1029/2012JC007990

年代:2013

数据来源: WILEY

摘要:

AbstractRecent geoids from the Gravity Recovery and Climate Experiment (GRACE) and the Gravity field and steady state Ocean Circulation Explorer satellite mission (GOCE) contain useful short‐scale information for the construction of a geodetic ocean mean dynamic topography (MDT). The geodetic MDT is obtained from subtracting the geoid from a mean sea surface (MSS) as measured by satellite altimetry. A gainful use of the MDT and an adequate assessment needs an optimal filtering. This is accomplished here by defining a cutoff length scaledmaxfor the geoid and applying a Gaussian filter with half‐width radiusron the MDT. A series of MDTs (GRACE, GOCE, and combined satellite‐only (GOCO) solutions) is tested, using different sets of filter parametersdmaxandr. Optimal global and regional dependent filter parameters are estimated. To find optimal parameters and to assess the resulting MDTs, the geostrophic surface currents induced by the filtered geodetic MDT are compared to corrected near‐surface currents obtained from the Global Drifter Program (GDP). The global optimal cutoff degree and order (d/o)dmax(half‐width radiusrof the spatial Gaussian filter) is 160 (1.1°) for GRACE; 180 (1.1–1.2°) for 1st releases of GOCE (time‐ and space‐wise methods) and GOCO models; and 210 (1.0 degree) for 2nd and 3rd releases of GOCE and GOCO models. The cutoff d/o is generally larger (smaller) and the filter length smaller (larger) for regions with strong, small‐scale (slow, broad scale) currents. The smallest deviations from the drifter data are obtained with the GOCO03s geoid model, although deviations of other models are o

ISSN:0148-0227    

DOI:10.1029/2012JC008149

年代:2013

数据来源: WILEY

摘要:

AbstractOur understanding of past sea‐ice variability is limited by the short length of satellite and instrumental records. Proxy records can extend these observations but require further development and validation. We compare methanesulfonic acid (MSA) and chloride (Cl–) concentrations from a new firn core from coastal West Antarctica with satellite‐derived observations of regional sea‐ice concentration (SIC) in the Amundsen Sea (AS) to evaluate spatial and temporal correlations from 2002–2010. The high accumulation rate (~39 g∙cm–2∙yr–1) provides monthly resolved records of MSA and Cl–, allowing detailed investigation of how regional SIC is recorded in the ice‐sheet stratigraphy. Over the period 2002–2010 we find that the ice‐sheet chemistry is significantly correlated with SIC variability within the AS and Pine Island Bay polynyas. Based on this result, we evaluate the use of ice‐core chemistry as a proxy for interannual polynya variability in this region, one of the largest and most persistent polynya areas in Antarctica. MSA concentrations correlate strongly with summer SIC within the polynya regions, consistent with MSA at this site being derived from marine biological productivity during the spring and summer. Cl–concentrations correlate strongly with winter SIC within the polynyas as well as some regions outside the polynyas, consistent with Cl–at this site originating primarily from winter sea‐ice formation. Spatial correlations were generally insignificant outside of the polynya areas, with some notable exceptions. Ice‐core glaciochemical records from this dynamic region thus may provide a proxy for reconstructing AS and Pine Island Bay polynya v

ISSN:0148-0227    

DOI:10.1029/2012JC008077

年代:2013

数据来源: WILEY

摘要:

AbstractWe investigate processes leading to uncertainty in forecasts of the Atlantic meridional overturning circulation (AMOC). A climate model is used to supply initial conditions for ensemble simulations in which members initially have identical ocean states but perturbed atmosphere states. Baroclinic transports diverge on interannual time scales even though the ocean is not eddy‐permitting. Interannual fluctuations of the model AMOC in the subtropical gyre are caused by westward propagating Rossby waves. Divergence of the predicted AMOC with time occurs because the waves develop different phases in different ensemble members predominantly due to differences in eastern boundary windstress curl. These windstress fluctuations communicate with interior ocean transports via modifications to the vertical velocity and the vortex stretching term dw/dz. Consequently, errors propagate westward resulting in longer predictability times in the interior ocean compared with the eastern boundary. Another source of divergence is transport anomalies propagating along the Gulf Stream (and other boundary currents). The propagation mechanism seems to be predominantly advection by mean currents, and we show that the arrival of westward propagating waves can trigger development of these anomalies. The mean state of the AMOC has a small effect on interannual predictability in the subtropical gyre, most likely because eastern boundary windstress curl predictability is not strongly dependent on the state of the AMOC in the subtropics. Eastern boundary windstress curl was more predictable at 45°N when the AMOC was in a strongly decreasing state, but, unlike at 30°N, no mechanism was found linking windstress curl fluctuations with deep transpo

ISSN:0148-0227    

DOI:10.1029/2012JC008334

年代:2013

数据来源: WILEY