Stephan Howden

Effects of river inputs into the Bay of Bengal

The effect of river runoff in the Bay of Bengal is examined using a reduced gravity primitive equation ocean model coupled to an atmospheric boundary layer model. Model simulations are carried out by including river discharges as surface freshwater forcing at the mouths of the rivers. To assess the effect of river inputs on the dynamics and thermodynamics of the tropical Indian Ocean, parallel simulations are carried out by neglecting the river inputs. Additionally, another set of parallel runs without penetrative radiation loss through the mixed layer is carried out. The freshwater flux due to rivers results in lower salinities and shallower mixed layers, as expected. However, the influence of this additional freshwater flux into the bay is rather counterintuitive. With the inclusion of river discharges more heat is absorbed by the ocean, but sea surface temperatures are slightly cooler in the bay because of enhanced entrainment cooling of the shallower mixed layer, enhanced penetrative radiation, and an enhanced effect of latent heat loss on the temperature tendency. This is despite the greater latent heat loss when river input is neglected. Conversely, neglect of penetrative radiation results in a shallower but slightly warmer mixed layer with river input. River input and penetrative radiation each affect the mixed layer depths, the salinity and temperature structure, and currents in the Bay of Bengal, but they have a small effect on SST. Annual SST, averaged over the Bay of Bengal, is only 0.1°C colder with river input. Neglecting penetrative radiation in the river run results in an increase of only 0.2°C for the annual SST. The lack of persistence of a barrier layer in the bay helps regulate SST even in the presence of enhanced buoyancy forcing due to river input. Averaged over the bay, a barrier layer forms as mixed layer detrainment occurs, and the thermocline deepens just after the southwest monsoon and the northeast monsoon. The barrier layer is short-lived in each case it is eroded by mixing. The effect of riverine input in the bay is not confined to the surface waters. A pool of cold anomaly (−1°C) and fresher waters is centered near 100 m depth in the bay with riverine input. This cold pool beneath the mixed layer allows entrainment cooling of the mixed layer to be more effective, even though mass entrainment is lower relative to the case neglecting river input. The more diffuse thermocline in the bay is consistent with enhanced vertical mixing despite the large positive buoyancy forcing.

The Integrated Ocean Observing System High-Frequency Radar Network: Status and Local, Regional, and National Applications

A national high-frequency radar network has been created over the past 20 years that provides hourly 2-D ocean surface current velocity fields in near real time from a few km offshore out to approximately 200 km. This preoperational network is made up of more than 100 radars from 30 different institutions. The Integrated Ocean Observing System efforts have supported the standards-based ingest and delivery of these velocity fields to a number of applications such as coastal search and rescue, oil spill response, water quality monitoring, and safe and efficient marine navigation. Thus, regardless of the operating institution or location of the radar systems, emergency response managers, and other users, can rely on a common source and means of obtaining and using the data. Details of the history, the physics, and the application of high-frequency radar are discussed with successes of the integrated network highlighted.

Were Multiple Stressors a ‘Perfect Storm’ for Northern Gulf of Mexico Bottlenose Dolphins (Tursiops truncatus) in 2011?

An unusual number of near term and neonatal bottlenose dolphin (Tursiops truncatus) mortalities occurred in the northern Gulf of Mexico (nGOM) in 2011, during the first calving season after two well documented environmental perturbations; sustained cold weather in 2010 and the Deepwater Horizon oil spill (DWHOS). Preceding the stranding event, large volumes of cold freshwater entered the nGOM due to unusually large snowmelt on the adjacent watershed, providing a third potential stressor. We consider the possibility that this extreme cold and freshwater event contributed to the pattern of perinatal dolphin strandings along the nGOM coast. During the 4-month period starting January 2011, 186 bottlenose dolphins, including 46% perinatal calves (nearly double the percentage for the same time period from 2003–2010) washed ashore from Louisiana to western Florida. Comparison of the frequency distribution of strandings to flow rates and water temperature at a monitoring buoy outside Mobile Bay, Alabama (the 4th largest freshwater drainage in the U.S.) and along the nGOM coast showed that dolphin strandings peaked in Julian weeks 5, 8, and 12 (February and March), following water temperature minima by 2–3 weeks. If dolphin condition was already poor due to depleted food resources, bacterial infection, or other factors, it is plausible that the spring freshet contributed to the timing and location of the unique stranding event in early 2011. These data provide strong observational evidence to assess links between the timing of the DWHOS, other local environmental stressors, and mortality of a top local predator. Targeted analyses of tissues from stranded dolphins will be essential to define a cause of death, and our findings highlight the importance of considering environmental data along with biological samples to interpret stranding patterns during and after an unusual mortality event.

Annual Rossby Wave in the Southern Indian Ocean: Why Does It “Appear” to Break Down in the Middle Ocean?

Abstract Using both altimetry data (TOPEX/Poseidon and Geosat) and Levitus climatology and a linear reduced-gravity model, the authors studied the annual Rossby waves in the southern Indian Ocean from 19° to 9°S. The most striking feature from the data analysis is that the westward phase propagation of the annual variability appears to break up in the midocean, which results in two local maxima for annual variability in both the sea level and the depth of the 18°C isotherm, with one in the eastern basin and the other in the western basin. Separating the two maxima is a midocean minimum. Decomposition of the annual variability into Rossby waves and localized response indicates that the two local maxima of the annual variability simply result from the constructive interference between the localized response and the Rossby waves in the eastern and western basin. On the other hand, the midocean local minimum results from the destructive interference between the Rossby waves and the localized response. Modelin...

A Comparison of Methods for Determining Significant Wave Heights—Applied to a 3-m Discus Buoy during Hurricane Katrina

Abstract In August 2005, the eye of Hurricane Katrina passed 90 km to the west of a 3-m discus buoy deployed in the Mississippi Sound and operated by the Central Gulf of Mexico Ocean Observing System (CenGOOS). The buoy motions were measured with a strapped-down, 6 degrees of freedom accelerometer, a three-axis magnetometer, and from the displacement of a GPS antenna measured by postprocessed-kinematic GPS. Recognizing that an accelerometer experiences a large offset due to gravity, the authors investigated four different means of computing wave heights. In the most widely used method for a buoy with a strapped-down, 1D accelerometer, wave heights are overestimated by 26% on average and up to 56% during the peak of the hurricane. In the second method, the component of gravity is removed from the deck relative z-axis accelerations, requiring pitch and roll information. This is most similar to the motion of the GPS antenna and reduces the overestimation to only 5% on average. In the third method, the orient...

Hurricane Katrina Winds Measured by a Buoy-Mounted Sonic Anemometer

Abstract The eye of Hurricane Katrina passed within 49 n mi of an oceanographic observing system buoy in the Mississippi Bight that is part of the Central Gulf of Mexico Ocean Observing System. Although a mechanical anemometer failed on the buoy during the hurricane, a two-axis sonic anemometer survived and provided a complete record of the hurricane’s passage. This is the first reported case of a sonic anemometer surviving a hurricane and reporting validated data, and it demonstrates that this type of anemometer is a viable alternative to the mechanical anemometers traditionally used in marine applications. The buoy pitch and roll record during the storm show the importance of compensating the anemometer records for winds oblique to the horizontal plane of the anemometers. This is made apparent in the comparison between the two wind records from the anemometers during the hurricane.

Interannual variability in the South China Sea from expendable bathythermograph data

Using 29 years of expendable bathythermograph data, we studied the interannual temperature variability in the upper 300 m of the South China Sea (SCS). It is found that the temperature field has significant quasi-penta-annual variability throughout the 29 years. The interannual variability has a large decadal modulation. Although outside the SCS, in the Kuroshio, significant interannual variability extends to a much deeper layer beyond the 300 m depth, inside the SCS, especially in the central SCS, strong interannual variability is located near the 100 m depth and is capped by a surface layer of much weaker interannual variability. Similarities and differences in the temporal characteristics between the interannual variability inside and outside the SCS suggest that the interannual variability inside the SCS is primarily generated locally inside the SCS. In addition to the strong interannual variability, there exist strong decadal variability and a linear trend, which are also stronger in the subsurface than near the surface. It is found that below 100 m, the SCS has been cooling (linear trend) at a rate of ∼0.4°C per decade, decreasing from the eastern to the western basin.

IES Calibration and Mapping Procedures

Abstract The procedures used in the Synoptic Ocean Prediction experiment for calibration of inverted echo sounder (IES) travel time τ data have led to substantially improved accuracy. In previous work, τ was converted to main thermocline depth ZT using point measurements of the 12°C depth from coincident expendable bathythermograph (XBT) casts. The new method presented in this study vertically integrates the XBT temperature to produce the quantity QT, which is very tightly related to τ. The advantage of this method is that the calibration constants have roughly half the uncertainty obtained with the traditional method. Seasonal changes in τ determined from hydrographic data in the Gulf Stream region were found to vary with geographic region; near 68°W τ changed by 1.8 ms from March to September, whereas near Cape Hatteras the annual change was only 1.0 ms. The fully calibrated IES Z*T has an estimated error of 19 m (one standard deviation). An iterative optimal interpolation scheme is described, by which ...

The Three-Dimensional Secondary Circulation in Developing Gulf Stream Meanders

From June 1988 until August 1990 an array of 12 inverted echo sounders (IES), 12 additional IES with pressure gauges to measure the abyssal pressure signal, and 12 tall current meter moorings instrumented at depths 400 m, 700 m, 1000 m, and 3500 m was deployed in the Gulf Stream near 688W. The current meter array spanned an area of approximately 260 km by 200 km with nominal horizontal instrument spacing of 56 km in the cross-stream direction and 65 km in the downstream direction. Six events were observed during the two year deployment during which a steep Gulf Stream meander trough and an abyssal cyclone developed concurrently. Two of the meander troughs eventually pinched off to form cold core rings. As the abyssal flows spun up they became nearly barotropic with strong [ O(30 cm s21)] swirl speeds that advected the main thermocline and helped steepen the meanders. Although the geostrophic barotropic flow played a large role in the development of the system, ageostrophic flow was also required. In this study the three-dimensional mesoscale ageostrophic flow within the main thermocline is diagnosed during two steep meander trough events and one steep meander crest event. It is shown that in addition to the alongstream horizontal ageostrophic flow associated with the curvature of the main jet (cyclostrophic speeds greater than 25 cm s21 are not unusual on trough axes) there is significant horizontal cross-stream ageostrophic flow. Cross-stream ageostrophic flow as high as 18 cm s 21 is diagnosed. Vertical momentum advection is also shown to be an important contributor to cross-stream ageostrophic flow, with values as high as 7 cm s 21 diagnosed. The patterns of the ageostrophic flow are shown to be tied to the phasing between the upper-level meander and the abyssal vortex. Both rotational and divergent patterns in the ageostrophic flow are evident. For parcels on main thermocline isopycnals, the ageostrophic flow provides an asymmetric cross-frontal flow relative to the symmetric cross-frontal flow associated with the barotropic vortices.

Passive Active L/S-band microwave aircraft sensor for ocean salinity measurements

A Passive/Active L/S-band (PALS) microwave aircraft instrument to measure ocean salinity has been built and tested. Because the L-band brightness temperatures associated with salinity changes are expected to be small, it was necessary to build a very sensitive and stable system. This new instrument has dual-frequency, dual polarization radiometer and radar sensors. The antenna is a high beam efficiency conical horn. The PALS instrument was installed on the NCAR C-130 aircraft and salinity measurement missions were flown on July 17-19, 1999, southeast of Norfolk, Virginia over the Gulf Stream. The measurements indicated a clear and repeatable salinity signal during these three days, which was in good agreement with the R/V Cape Hatteras salinity data. Data was also taken in the open ocean and a small decrease of 0.2 K was measured in the brightness temperature, which corresponded to the salinity increase of 0.4 psu measured by the M/V Oleander vessel.