Magdalena Andres

Direct interaction between the Gulf Stream and the shelfbreak south of New England

Sea surface temperature imagery, satellite altimetry, and a surface drifter track reveal an unusual tilt in the Gulf Stream path that brought the Gulf Stream to 39.9°N near the Middle Atlantic Bight shelfbreak—200 km north of its mean position—in October 2011, while a large meander brought Gulf Stream water within 12 km of the shelfbreak in December 2011. Near-bottom temperature measurements from lobster traps on the outer continental shelf south of New England show distinct warming events (temperature increases exceeding 6°C) in November and December 2011. Moored profiler measurements over the continental slope show high salinities and temperatures, suggesting that the warm water on the continental shelf originated in the Gulf Stream. The combination of unusual water properties over the shelf and slope in late fall and the subsequent mild winter may affect seasonal stratification and habitat selection for marine life over the continental shelf in 2012.

Recent accelerated warming of the continental shelf off New Jersey: Observations from the CMV Oleander expendable bathythermograph line†

Expendable bathythermographs (XBTs) have been launched along a repeat track from New Jersey to Bermuda from the CMV Oleander through the NOAA/NEFSC Ship of Opportunity Program about 14 times per year since 1977. The XBT temperatures on the Middle Atlantic Bight shelf are binned with 10 km horizontal and 5 m vertical resolution to produce monthly, seasonally, and annually averaged cross-shelf temperature sections. The depth-averaged shelf temperature, Ts, calculated from annually averaged sections that are spatially averaged across the shelf, increases at 0.026 ± 0.001°C yr−1 from 1977 to 2013, with the recent trend substantially larger than the overall 37 year trend (0.11 ± 0.02°C yr−1 since 2002). The Oleander temperature sections suggest that the recent acceleration in warming on the shelf is not confined to the surface, but occurs throughout the water column with some contribution from interactions between the shelf and the adjacent Slope Sea reflected in cross-shelf motions of the shelfbreak front. The local warming on the shelf cannot explain the regions amplified rate of sea level rise relative to the global mean. Additionally, Ts exhibits significant interannual variability with the warmest anomalies increasing in intensity over the 37 year record even as the cold anomalies remain relatively uniform throughout the record. Ts anomalies are not correlated with annually averaged coastal sea level anomalies at zero lag. However, positive correlation is found between 2 year lagged Ts anomalies and coastal sea level anomalies, suggesting that the regions sea level anomalies may serve as a predictor of shelf temperature.

On the Recent Destabilization of the Gulf Stream Path downstream of Cape Hatteras

Mapped satellite altimetry reveals interannual variability in the position of initiation of Gulf Stream meanders downstream of Cape Hatteras. The longitude where the Gulf Stream begins meandering varies by 1500 km. There has been a general trend for the destabilization point to shift west, and 5 of the last 6 years had a Gulf Stream destabilization point upstream of the New England Seamounts. Independent in situ data suggest that this shift has increased both upper-ocean/deep-ocean interaction events at Line W and open-ocean/shelf interactions across the Middle Atlantic Bight (MAB) shelf break. Mooring data and along-track altimetry indicate a recent increase in the number of deep cyclones that stir Deep Western Boundary Current waters from the MAB slope into the deep interior. Temperature profiles from the Oleander Program suggest that recent enhanced warming of the MAB shelf may be related to shifts in the Gulf Streams destabilization point.

Second-Mode Internal Tides in the East China Sea Deduced from Historical Hydrocasts and a Model

during the two-year deployment period. To obtain temperature (T) and specific volume anomaly (d) profiles from t and sea surface temperature (SST), observed respectively by PIES and satellite, we constructed multi-index gravest empirical mode (MI-GEM) fields [Park et al., 2005] using historical hydrocasts. However, comparison between the MI-GEM fields and the historical hydrocasts reveals that the MI-GEM fields fail to capture 44% and 52% of the T and d variance, respectively, in the p = 100–200-dbar layer. This may be caused by 2nd- or higher-mode variations of vertical structure, since t is insensitive to all but the lowest baroclinic mode [Watts and Rossby, 1977]. [4] In this study, we demonstrate that 2nd-mode ITs in the ECS are responsible for failure of the MI-GEM fields to captureadequatelyTand dvariationsinthep=100–200-dbar layer as seen in historical hydrographic measurements. We verify our observations with a numerical simulation of ITs.

Eddy‐Kuroshio interaction processes revealed by mooring observations off Taiwan and Luzon

The influence and fate of westward propagating eddies that impinge on the Kuroshio were observed with pressure sensor-equipped inverted echo sounders (PIESs) deployed east of Taiwan and northeast of Luzon. Zero lag correlations between PIES-measured acoustic travel times and satellite-measured sea surface height anomalies (SSHa), which are normally negative, have lower magnitude toward the west, suggesting the eddy-influence is weakened across the Kuroshio. The observational data reveal that impinging eddies lead to seesaw-like SSHa and pycnocline depth changes across the Kuroshio east of Taiwan, whereas analogous responses are not found in the Kuroshio northeast of Luzon. Anticyclones intensify sea surface and pycnocline slopes across the Kuroshio, while cyclones weaken these slopes, particularly east of Taiwan. During the 6 month period of overlap between the two PIES arrays, only one anticyclone affected the pycnocline depth first at the array northeast of Luzon and 21 days later in the downstream Kuroshio east of Taiwan.

Effect of a sheared flow on iceberg motion and melting

Icebergs account for approximately half the freshwater flux into the ocean from the Greenland and Antarctic ice sheets and play a major role in the distribution of meltwater into the ocean. Global climate models distribute this freshwater by parameterizing iceberg motion and melt, but these parameterizations are presently informed by limited observations. Here we present a record of speed and draft for 90 icebergs from Sermilik Fjord, southeastern Greenland, collected in conjunction with wind and ocean velocity data over an 8 month period. It is shown that icebergs subject to strongly sheared flows predominantly move with the vertical average of the ocean currents. If, as typical in iceberg parameterizations, only the surface ocean velocity is taken into account, iceberg speed and basal melt may have errors in excess of 60%. These results emphasize the need for parameterizations to consider ocean properties over the entire iceberg draft.

Icebergs and sea ice detected with inverted echo sounders

AbstractA 1-yr experiment using a pressure-sensor-equipped inverted echo sounder (PIES) was conducted in Sermilik Fjord in southeastern Greenland (66°N, 38°E) from August 2011 to September 2012. Based on these high-latitude data, the interpretation of PIESs’ acoustic travel-time records from regions that are periodically ice covered were refined. In addition, new methods using PIESs for detecting icebergs and sea ice and for estimating iceberg drafts and drift speeds were developed and tested. During winter months, the PIES in Sermilik Fjord logged about 300 iceberg detections and recorded a 2-week period in early March of land-fast ice cover over the instrument site, consistent with satellite synthetic aperture radar (SAR) imagery. The deepest icebergs in the fjord were found to have keel depths greater than approximately 350 m. Average and maximum iceberg speeds were approximately 0.2 and 0.5 m s−1, respectively. The maximum tidal range at the site was ±1.8 m and during neap tides the range was ±0.3 m, ...

Adjustment of a wind-driven two-layer system with mid-basin topography

A linear primitive equations model is used to simulate spin-up of a two-layer ocean bisected by a meridional ridge. The ocean is forced with steady zonal winds east of the ridge. When wind-driven barotropic planetary Rossby waves propagate across the ridge, barotropic and baroclinic anomalies are generated as the barotropic flow adjusts. These ridge-generated anomalies propagate westward from the ridge as planetary Rossby waves and their arrival along the basin’s western boundary modulates the western boundary current (WBC) transport and vertical structure. Model results suggest that at short (<1 year) and long (>10 years) delay relative to a change in wind stress curl, net WBC transport, TWBC, is that predicted by the Sverdrup balance for a flat ocean, TSv, but at intermediate delay this balance is disrupted by arrival of the additional barotropic ridge-generated anomalies. The magnitude of the anomalous transport, T ′ WBC, depends on the meridional deflection of the flow at the ridge relative to the length-scale over which wind stress curl varies. The timescale, tBT, associated with adjustment at the ridge is a function of latitude, density contrast between layers and ridge width.

Moored observations of the Deep Western Boundary Current in the NW Atlantic: 2004–2014

A moored array spanning the continental slope southeast of Cape Cod sampled the equatorward-flowing Deep Western Boundary Current (DWBC) for a 10-year period: May 2004 - May 2014. Daily profiles of subinertial velocity, temperature, salinity and neutral density are constructed for each mooring site and cross-line DWBC transport time series are derived for specified water mass layers. Time-averaged transports based on daily estimates of the flow and density fields in stream coordinates are contrasted with those derived from the Eulerian-mean flow field, modes of DWBC transport variability are investigated through compositing, and comparisons are made to transport estimates for other latitudes. Integrating the daily velocity estimates over the neutral density range of 27.8 - 28.125 kg/m3 (encompassing Labrador Sea and Overflow Water layers), a mean equatorward DWBC transport of 22.8 x 106 m3/s ± 1.9 x 106 m3/s is obtained. Notably, a statistically-significant trend of decreasing equatorward transport is observed in several of the DWBC components as well as the current as a whole. The largest linear change (a 4% decrease per year) is seen in the layer of Labrador Sea Water that was renewed by deep convection in the early 1990s whose transport fell from 9.0 x 106 m3/s at the beginning of the field program to 5.8 x 106 m3/s at its end. The corresponding linear fit to the combined Labrador Sea and Overflow Water DWBC transport decreases from 26.4 x 106 m3/s to 19.1 x 106 m3/s. In contrast, no long-term trend is observed in upper-ocean Slope Water transport. These trends are discussed in the context of decadal observations of the North Atlantic circulation, and subpolar air-sea interaction/water mass transformation.

Combining Observations from Multiple Platforms across the Kuroshio Northeast of Luzon: A Highlight on PIES Data

AbstractThis study presents amended procedures to process and map data collected by pressure-sensor-equipped inverted echo sounders (PIESs) in western boundary current regions. The modifications to the existing methodology, applied to observations of the Kuroshio from a PIES array deployed northeast of Luzon, Philippines, consist of substituting a hydrography-based mean travel time field for the PIES-based mean field and using two distinct gravest empirical mode (GEM) lookup tables across the front that separate water masses of South China Sea and North Pacific origin. In addition, this study presents a method to use time-mean velocities from acoustic Doppler current profilers (ADCPs) to reference (or “level”) the PIES-recorded pressures in order to obtain time series of absolute geostrophic velocity. Results derived from the PIES observations processed with the hydrography-based mean field and two GEMs are compared with hydrographic profiles sampled by Seagliders during the PIES observation period and wi...