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Sampling the Submesoscale Ocean in the Santa Barbara Channel (August, 2013)

Saturday, August 17, 2013

NASA’s Surface Water Ocean Topography (SWOT) satellite, expected to launch in 2020, will measure sea level height variations on spatial scales down to a few kilometers. This mission will ultimately allow the ocean’s submesoscale (spatial scales from kilometers to a few 10’s of kilometers) variations to be observed from space.  AIRSWOT is the airborne calibration and validation instrument to support the planned SWOT mission (, with a key objective to understand the spatial scales of ocean features that will potentially be resolved with SWOT. The AIRSWOT instrument utilizes across-track interferometry with Ka-band frequency (35 GHz) to obtain centimeter-level height maps of the ocean as well as land water surfaces. Recent AIRSWOT engineering flights were conducted over the Santa Barbara Channel, California, to obtain the first set of ocean data needed to test the instrument and evaluate the derived ocean surface topography observations. In support of these initial flights, a small team was enlisted to provide in situ sampling and aerial sea surface temperature (SST) maps of submesoscale ocean features in the Santa Barbara Channel. The team was composed of Jeroen Molemaker (UCLA), Carter Ohlmann (UCSB), and Ben Holt (JPL), who have developed a unique coordinated observational approach to measuring submesoscale ocean dynamics from field experiments in the Southern California Bight in support of a NASA-funded project targeting submesoscale flows. The sampling campaign in support of AIRSWOT flights in the Santa Barbara Channel is described here.
The Santa Barbara Channel is a unique coastal region, due to its location in the northwestern edge of the Southern California Bight (SCB), its direct connection with the equatorward flowing California Current, and its semi-enclosed nature due to the surrounding Channel Islands (Figure 1). A series of six seasonal circulation patterns or modes typically occur in the Santa Barbara Channel, including the common ‘cyclonic’ flow, characterized by closed loop counterclockwise flow in the western Channel (Harms and Winant, 1998). A strong northward jet between Santa Cruz and Santa Rosa Islands is also commonly present. Along the mainland coast, Santa Barbara Channel encompasses a string of the largest and most studied natural hydrocarbon seeps in the world (Figure 2).

Figure 1. Southern California Bight (SCB): showing prevailing near-surface circulation and buoy locations. Solid arrows indicated the general SCB pattern of poleward flow nearshore and equatorward flow offshore of the California Current, which migrates closer to shore in spring and summer as indicated by dashed arrows The Santa Barbara Channel (SBC), both the Santa Monica Bay (inshore) and Santa Monica Basin (offshore) (SMB/B), and San Pedro Channel and San Pedro Basin (SPC/B) are indicated. (After DiGiacomo and Holt, 2001).

Figure 2.  Photograph of oiled ocean surface coming from a natural  hydrocarbon seep near Santa Barbara.

The AIRSWOT field campaign consisted of in situ sampling and aerial imagery. In situ instruments included water-following drifters, surface salinity and temperature sensors, an acoustic Doppler current profiler (ADCP), and a conductivity-temperature-depth (CTD) sensor. The key aerial sensor was a near infrared (NIR) camera, which flew on a small plane chartered by UCLA. The ocean instruments were deployed using UCLA’s 28-foot inflatable boat (Figure 3), a comparatively small but high-speed craft specifically intended to access and sample submesoscale features including eddies and fronts that can evolve and move rapidly in time. The sampling approach involves first identifying submesoscale features with fine-resolution satellite and aircraft imagery, and then quickly deploying the UCLA boat and its instrumentation to the identified feature for rapid in-situ sampling. The aerial SST provides fine-resolution data over a region, 2-3 times per day, and utilizes onboard processing and air-to-boat data downlink to guide the UCLA boat to identified features.

Figure 3. a) UCLA research vessel. b) CTD on deck that is deployed by adjacent winch. c) Surface drifter showing buoy containing communication package and submerged drifter. d) Surface drifter being retrieved.

The first set of ocean observations in conjunction with AirSWOT measurements occurred on May 8, when clear skies, diminishing winds, and a choppy wave state existed. A total of 15 drifters were deployed in the afternoon before the flight in a nested grid pattern within the flight coverage area.  The initial idea was for the drifter observations to map out the prevailing flow field within the AIRSWOT coverage area through the night and following morning, leading up to the AIRSWOT overflight and aerial SST survey at noon. The buoys dispersed  widely throughout the Santa Barbara Channel overnight and the wave state worsened, so the buoy retrieval took up the entire flight day. One drifting buoy appeared to have been grounded onshore near the west end of Santa Cruz Island but suddenly moved away from an inaccessible near shore area and was quickly retrieved. The 15th and most westward buoy was not recovered and has since made its way southward within the main branch of the California Current. At the time of this writing, that drifter was beyond the southern tip of the Baja California peninsula, continuing to transmit its position every hour (check the location of buoy OHL-I-0067 at MODIS SST data showed a temperature front present extending northward from the west end of Santa Cruz Island (Figure 4). The preliminary AIRSWOT results from May 8 illustrated that the sensor was working, with surface waves well detected and reasonable wave heights estimated.

 Figure 4. 1-km MODIS-Terra SST data obtained on May 8, 2013, at 1921 UTC. An approximate 1°C temperature front is indicated by red arrow.

The final AIRSWOT and ocean sampling flight took place on June 5. Conditions were nearly ideal, with clear skies and calm winds and sea state over most of the Santa Barbara Channel, much calmer than previously found on May 8.  The buoys were deployed rapidly in the calmer morning conditions on the day of the flight in a nested grid pattern within the AIRSWOT coverage area, with the flight occurring around noon. A temperature front was found and sampled during the remainder of the afternoon, with the remainder of the in situ instruments (Figure 5). MODIS SST data the morning of June 5 was obtained at the edge of the swath, preventing a larger view of the channel, but coincidentally a Landsat 8 thermal IR image (with 90 m resolution) was obtained (Figure 6). Two significant temperature frontal gradients were clearly observed, again oriented in the north-south direction across the channel.  The only drawback was that aerial SST flights were delayed until the late afternoon due to fog in Santa Monica.

Figure 5. 1-km MODIS-Terra SST data obtained on June 5, 2013, at 1803 UTC. Approximate 1°C temperature fronts are indicated by red arrows.

Figure 6. Landsat 8 thermal infrared image obtained on June 5, 2013, at 1836 UTC. Approximate 1°C temperature fronts are indicated by black arrows. 

Thermal IR imagery from ASTER, a multispectral fine-resolution sensor on the NASA Aqua satellite, was obtained throughout the planned sampling period, but never coincided with the sampling dates. To illustrate the rapidly evolving nature of temperature fronts that may be found, a pair of ASTER thermal IR acquisitions (also with 90 m resolution) were obtained on May 27 and 28 (Figure 7), and, like Landsat (Figure 6), show extensive detail in the temperature gradients.  Also note the temporal variability in the patterns, taken 36 hours apart, related to the northward flowing jet between the two islands. Coincident MODIS SST data are shown in Figure 8, which capture the primary thermal gradients seen in ASTER but do not provide the detailed structure. The four different SST remote sensing datasets will be compared with each other and with the in situ SST measurements, to examine heating and the possible influence on upper ocean mixing, another valuable component of submesoscale oceanography.  
The flexibility of the boat and ocean measurement capabilities provided a means to rapidly sample submesoscale features identified in the field under varying conditions, with the features having short temporal and small spatial scales. Useful measurements were also obtained over a fairly large region (40 km by 40 km) during a single day of operation.  The team is working towards a comprehensive view of the submesoscale structure through the synthesis of all data collected. The in situ data, along with the aerial and satellite SS, T will be valuable for interpreting AIRSWOT observations of sea surface height. Future work entails resolving short-term and month-long variations in SST (from boat, aerial, and satellite) and salinity, along with submesoscale current flow and convergence using the drifters.

Figure 7. ASTER thermal infrared imagery obtained on top) May 27, 2013, at 0605 UTC; and bottom) May 28, 2013, at 1851 UTC.  Santa Rosa (left) and Santa Cruz (right) Islands are shown.

Figure 8. 1-km MODIS-Terra SST data on top) May 27, 2013, at 0605 UTC; bottom) May 28, 2013, at 1851 UTC.


PO.DAAC Science Team,
Jet Propulsion Laboratory, Pasadena, Calif.