Haline hurricane wake in the Amazon/Orinoco plume: aquarius/sacd and smos observations

Semyon A. Grodsky¹, Nicolas Reul², Gilles Reverdin³, James A. Carton¹, Bertrand Chapron², Vladimir N. Kudryavtsev⁴, Gary Lagerloef⁵

Rev. July 19, 2012

To be submitted to: Geophysical Research Letters

³Laboratoire d’Océanographie et de Climatologie par Expérimentation et Analyse Numérique, Institut Pierre Simon Laplace, CNRS/UPMC/IRD/MNHN, Paris, France.

Hurricane strength increases dramatically with increasing sea surface temperature (SST) and decreases in response to entrainment of cooler sub-mixed layer water into the ocean mixed layer. At its seasonal peak the near-surface Amazon/Orinoco plume covers a region of one million square kilometers in the western tropical Atlantic with more than 1m of extra freshwater, creating a near-surface barrier layer that inhibits mixing and warms to temperatures exceeding 29^oC. Two previous observational studies of the ocean response to hurricanes at fixed moorings in other regions of barrier layer have shown a substantial impact on salinity, although the lack of spatial information makes discussion of mechanisms difficult (Price, 1981; McPhaden et al. 2009). Here new sea surface salinity (SSS) observations from the Aquarius/SACD and SMOS satellites are used to explore the ocean response to hurricane Katia, which crossed the plume in early fall, 2011. Its passages left a +1.5psu high salinity wake (in its impact on density, the equivalent of cooling to 25.5^oC) due to mixing of the shallow barrier layer. Destruction of this barrier layer preserved higher SST and evaporation rates that otherwise would have declined in response to upwelling and mixing. Since hurricanes that form in the eastern tropical Atlantic in fall frequently pass 60^oW longitude at latitudes south of 22^oN we anticipate that many fall season hurricanes are affected by their passage across the Amazon/Orinoco plume.

Hurricane strength is known to be impacted by changes in the underlying ocean thermal stratification (e.g. Shay et al., 2000). In a provocative article Ffield (2007) used a historical statistical analysis to suggest that passage over the Amazon/Orinoco freshwater plume could also impact the growth of Atlantic hurricanes due to the effects of reduced salinity on enhancing stratification of the mixed layer. Such a result would be important because many hurricanes forming in the eastern basin in fall pass over the plume. Previous studies have shown that mixing and upwelling act to entrain cool thermocline water into the mixed layer, leaving a cool wake behind the storm of SST depressed by a few degrees (e.g. Price, 1981). This SST depression is associated with enhanced vertical heat transport (Sriver and Huber, 2007) and a reduction in hurricane strength (Shay et al., 1992; Bender and Ginis, 2000). Previously, Price (1981) and McPhaden et al. (2009) have reported on the impact of hurricanes on other regions with reduced SSS. However both studies lacked information about the horizontal structure of the salinity field. Here we combine in situ and observations from two recent satellite salinity sensors to examine the spatial and temporal structure of the interaction between Katia and the Amazon/Orinoco freshwater plume.

The western tropical Atlantic is characterized by high SST, in excess of 29^oC, and a plume of low SSS caused by Amazon and to a lesser extent Orinoco river discharge as well as local rainfall (Yoo and Carton, 1990; Dessier and Donguy, 1994; Lentz, 1995). This river discharge induces a shallow low salinity plume with a strong halocline in the upper 3-30m, below which salinity exceeds 36psu. The plume layer deepens seaward and acts as a barrier layer in density whose presence is associated with elevated SST (Pailler et al., 1999; Mignot et al., 2012). The plume extends eastward from the mouth of the Amazon at 0-2^oN, 50^oW with widths of 200-300km in June through December when Amazon discharge is at a seasonal minimum (0.08x10⁶ m³/s in November), expanding to 400-500km in March to May when Amazon discharge reaches a seasonal maximum of 0.24x10⁶ m³/s and winds are weak. At this rate of discharge an area of 1x10¹² m² (Hu et al., 2004) is diluted by 2psu down to the 20m depth in around two months. Further north the seasonality of the plume is the reverse, in that it expands northward and eastward away from the coastal current, reaching its maximum extent in August-September when the zone of weak winds shifts northward. In late summer-fall discharge from the Orinoco also contributes to the areas to the west of 55^oW, extending into the Caribbean. The plume contracts in November, which is coincident with the reappearance of the northeast trade winds and shifts in the surface currents (Dessier and Donguy, 1994).

The hurricane season in the Atlantic extends from early June through the end of November, with a peak in late August and early September. The most intense hurricanes, according to the National Hurricane Center (NHC), tend to form off the Cape Verde Islands in the eastern basin in fall, growing in intensity as they progress westward across the warm waters of the tropical North Atlantic. In 2011 the first of two Cape Verde hurricanes and the second hurricane of the season began as tropical storm Katia in late August when it began moving westward at 10m/s across the tropical Atlantic. Katia reached hurricane wind speeds mid-morning on September 1 with minimum central pressure of 988mb and hurricane force winds extending to 55km from the eye center. By the afternoon of September 4 NHC upgraded Katia to a Category 2 hurricane (maximum sustained winds > 44m/s) while the minimum central pressure had dropped to 961mb and the radius of sustained hurricane force winds had expanded to 75km. By the evening of the following day, after passing over the Amazon plume, Katia had strengthened to a Category 4 hurricane (winds >60m/s), a minimum central pressure of 942mb, and the radius of hurricane-force winds had expanded to 95km. Twenty-four hours later as Katia continued to shift northwestward the winds weakened substantially and the hurricane was downgraded to a Category 1 hurricane.

Here we examine the near surface response to the passage of Katia over the Amazon plume, its impact on SST and SSS, and its temporal relationship to the growth of Katia using a suite of remote sensing data including a newly available SSS data from the Aquarius/SACD and SMOS missions. In line with Wang et al. (2011) simulations we find that the presence of the plume did appear to reduce the cooling in SST that would have otherwise occurred due to the passage of the hurricane, and thus the plume did appear to have contributed to the intensification of Katia.

2. Data

SSS is provided by US/Argentina Aquarius/SACD (Lagerloef et al., 2008) and the European Space Agency Soil Moisture and Ocean Salinity (SMOS, Reul et al., 2012a) missions. Aquarius/SACDversion 1.3, level 3 data are available since 25 August, 2011 via the NASA/JPL/PODAAC ( ftp://saltmarsh.jpl.nasa.gov/L3/mapped/V1.3 ). These data provide global coverage after weekly averaging and are available daily on a 1^o x 1^o grid with approximately 150km spatial resolution. SMOS was launched in November 2009. Due to the polarimetric interferometric technology employed by SMOS it has higher spatial resolution of 25 km and provides global coverage after 10-day?? averaging. Here we use L3 SMOS SSS from ’Centre Aval de Traitement des Données SMOS’ (CATDS, www.catds.fr, see product documentation at www.salinityremotesensing.ifremer.fr/documentation-cec-products).

Both Aquarius/SACD and SMOS operate in the 21 cm microwave L-band and thus sample salinity within a few centimeters of the surface. Since surface roughness strongly affects L-band brightness temperature (Lagerloef et al., 2008; Reul et al., 2012a), and SMOS does not measure roughness, SSS is estimated with a lag of 1 to 2dy after hurricane passage to allow the sea state to calm. Still, despite this roughness-dependence L-band frequencies have a compensating advantage in that they are less affected by clouds and rain than at higher frequencies, and thus can provide a hurricane strength winds (Reul et al., 2012b).

The accuracy of the 10-day SMOS SSS at 25 km resolution is ~ 0.3 psu in the tropical oceans (Reul et al., 2012a). Information about the accuracy of Aquarius/SACD SSS is provided in Auxiliary Materials by comparison to all available in situ ~1m depth measurements from the Indian, Atlantic, and Pacific tropical mooring arrays, during August, 2011 - May, 2012. This comparison shows Aquarius/SACD SSS and the in situ observations to have an root mean squared (RMS) weekly difference < 0.25psu, with a slight 0.1psu negative bias relative to in situ salinity at 1m depth. The negative bias is larger in regions of low salinity, as expected, where intense rainfall and moderate winds lead to the formation of surface freshwater lenses (Henocq et al., 2010; Reverdin et al., 2012). Comparison of the much more limited set of 121 spatially and temporally collocated Argo near surface salinity and Aquarius/SACD SSS observations in our region of interest during September to November, 2011 suggests a somewhat larger range of differences. Prior to its use in this study the temporal variations of Aquarius/SACD SSS averaged zonally and with latitude in the band 50^oS-50^oN are removed to lessen the impact of a known 0.2psu time-dependent bias thought to be due to an unaccounted for component of radiometer drift (D. Levine, Personal Communication, 2012). To increase stability and accuracy we only use weekly averages of Aquarius/SACD SSS.

A comparison of the two SSS products with in situ 5m depth thermosalinograph (TSG) observations along a single ship track between France and French Guiana October 19-24, 2011, processed by the French Coriolis Argo Data center (Reul et al., 2012a), is presented in Fig. 1. This track section shows the ship passing through 500km of the Amazon plume (latitudes 8^o-13^oN) during which the intake salinity drops by 3-4psu to well below 34psu. The coincident Aquarius/SACD and SMOS SSS both show a similar signature of the Amazon plume, although displaced slightly southward (however this apparent shift may result from the spatial and temporal averaging).

In addition to SSS we examine daily SST based on a combined infrared satellite-in situ analysis, available daily at 0.25^ox0.25^o resolution (Reynolds et al., 2007). Intercomparisons of collocated in situ and satellite SST reviewed in Reynolds et al. suggest that the SST error is < 0.25^oC. To explore the varying strength of surface winds in the region of the plume we use the Bentamy and Croize-Fillon (2012) 0.25^ox0.25^ox1dy gridded analysis of Advanced scatterometer (ASCAT) 10m neutral winds. Uncertainty estimates for satellite wind speeds typically lie in the range of 1m/s in rain free and moderate wind conditions. In-situ vertical profiles of temperature and salinity are provided by the Argo program (Roemmich et al., 2009).

The region of low SSS< 34psu has a maximum area in September, 2011 of greater than a million square kilometers (see also Hu et al., 2004), extending northward to 20^oN and westward into the Caribbean (partly due to the contribution of the Orinoco). The plume extends eastward along the eastward flowing North Equatorial Countercurrent in the latitude band 5^o-10^oN to 40^oW (Fig. 2a). It is separated from the coast by a few hundred kilometer wide patch of higher salinity water around 55^oW, transported by the North Brazil Current and causing a near-shore increase in SSS evident along the transect (Fig. 1), and is mainly confined to regions where SST>29^oC (Fig. 2b). This plume differs from the September climatological description of Dessier and Donguy (1994) by the generally lower values of SSS (which are confirmed by in-situ data in Fig.1) and the further northward extension of the plume in the 1000 km wide longitude band between 60^o-50^oW. Wind speed over the plume (averaged over fixed area of SSS<34 psu in September, 2011, see Fig. 2b) has a monthly minimum of 3m/s in September (Fig. 2c). Climatological boreal summer mixed layer depth in this region <30m is determined by the depth of the salinity barrier layer (e.g. Mignot et al., 2012). After September the winds over the plume strengthen as the northeasterly trade winds reappear and, consistent with the Dessier and Donguy (1994) climatology, the area of the plume decreases perceptively (in part due to stronger wind mixing), reaching a minimum in January-March, 2012.

As tropical storm, then hurricane, Katia approached the plume from the east (August 30 - September 2, Figs. 3a,e) the cool wake it left behind was relatively weak (ΔSST < 0.5^oC comparing the week prior to the week following, see thin contours in Figs. 3c,f). The oceanic response changed dramatically on September 2 as the storm entered the region of the plume and Katia rapidly strengthened to Category 2. SST under the hurricane increased to 28.5^oC, while the cooling due to the passage of the hurricane intensified to 1^oC (Fig. 3c,f). Hurricane-induced mixing also caused a 1-2psu rise in SSS in the region of the plume (Figs. 3c,f, 4) and Katia continued to strengthen to Category 4 by September 4.

The strongest SSS increase induced by hurricane occurs within the plume (Figs. 3c,f, 4) and is explained by an erosion of the barrier layer. This is evident by Argo profiles collected within the plume (Fig. 5, #2, #3, see also Supplementary Material Fig.S3 for location #1) that indicate the presence of shallow, about 15m deep mixed layer overlying the halocline. Mixed layer salinity (diluted by transformed Amazon/Orinoco water) is lower by 2 to 3 psu than the water beneath. This haline stratification is partially destroyed by hurricane-forced entrainment (mixed layer deepening and upwelling), which is stronger on the right side of hurricane eye. But further along the hurricane trajectory the SSS change is weak because of the weak vertical salinity stratification outside the plume (Fig. 5, #8 and supplemental Fig.S3, #7).

On the left side of the trajectory there is an area of SSS decrease (Fig. 4) sampled by Argo #4 and #5. In contrast to the increase in SSS within the plume where the barrier layer is eroded, the surface (down to 30m, Fig. 5 #5) adjacent to the northwestern corner of the plume is 1psu fresher after the passage of hurricane. The decrease in salinity implies an addition of 1m of freshwater, much larger than could have come from direct rainfall. Indeed, the most likely explanation is freshwater advection from the plume with some addition due to direct rainfall.

SSS changes observed by Aquarius/SACD and SMOS also qualitatively agree (Fig. 4) and also agree with those from Argo data (presented in Supplementary Material, Fig. S4). The strongest change is found within and adjacent to the plume, while SSS change is weak at locations away from the plume. The change in SMOS is closer to the changes observed by the Argo data than Aquarius/SACD, likely reflecting better spatial resolution.

Time series of SST and SSS along the hurricane path shows that the thermal wake is detectable along the entire path and gradually intensifies with the intensifying hurricane (Figs. 3c,f and 6b). If the hurricane impact were only due to mixing (if it were enthalpy conserving), assuming a uniform property temperature and salinity layers with vertical stratification of temperature and salinity () across their bases, then the change in SST and SSS in its wake would be simply and where are the fractional deepening of the mixed layer defined by temperature or salinity. For =5^oC and = -1^oC then , while for = -4psu, = 2psu, , consistent with the presence of a shallow barrier layer of low salinity that is embedded within the uniform temperature mixed layer prior to the arrival of Katia. In the region of the plume the stratification of salinity increases up to = -4psu, leaving a haline wake increase of 1psu or more as evident in Fig. 6a.

In the week following hurricane passage SST in the plume remains warmer by 0.5^oC than SST outside the plume (Fig. 6b), which is in line with the Wang et al. (2011) simulations. Outside the plume is much smaller. Hence, for the same amount of mechanical stirring and the same we may expect a weaker SST cooling in the plume because of work to be done to mixed the barrier layer. In turn, this weaker cooling in the plume suggests weaker negative feedback on the growth of hurricane.

The availability of complementary SSS data from SMOS and Aquarius/SACD represents a revolution in our ability to track the oceanic component of the hydrologic cycle. Here we present a combined analysis of the impact of a hurricane passage on SSS providing the reader with information about the consistency of the results from these instruments, as well as using them to examine the role of salinity stratification in affecting the strength of hurricane passage across the massive Amazon/Orinoco plume.

Previous observational (Price, 1981; McPhaden et al. 2009) and modeling (Robertson and Ginis, 2002; Wang et al. 2011) studies have found the haline wake and suggested that the presence of a haline barrier layer leads to a reduction in hurricane-induced surface cooling, and thus favors hurricane development. In the one dimensional mixed layer experiments of Wang et al. SST cooling in the presence of a barrier layer with a thickness of 5-15 m is 0.4-0.8^oC less than it would have been following a hurricane if such a barrier layer were not present. The elevated SST and high evaporation enhances the hurricane’s maximum potential intensity (Saunders and Lea, 2008). The results shown here suggest that the passage of major hurricane Katia across the Amazon/Orinoco plume had an even larger effect on SST than the idealized model and thus an even bigger effect on evaporation.

The geographic location and seasonality of the Amazon/Orinoco plume makes hurricane overpasses a not-infrequent occurrence. Indeed, the expansion of the plume in August-September coincides with the peak of the production of Cape Verde hurricanes, a group which includes many of the most intense (Category 4-5) hurricanes. Thus the results presented here strongly suggest that the salinity stratification role in mixed layer dynamics should be taken into account when forecasting hurricane growth over the plume. The availability of satellite observations of SSS from Aquarius/SACD and SMOS are critical to making such model improvements practical.

This research was supported by the NASA Ocean Surface Salinity Team (OSST) through grant NNX12AF68G. VK and BC acknowledge support provided by Russian Government Mega-grant No.11.G34.31.0078. TSG data are provided by ESA's Soil Moisture and Ocean Salinity (SMOS) Cal/Val effort supported by the TOSCA program of CNES and are collected from the merchant vessels within the observatory SSS (ORE SSS) supported by French public agencies IRD and INSU.

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Figure 2. September 2011 mean (a) AQUARIUS SSS (shaded, psu) with September climatological surface drifter currents of Lumpkin and Garraffo (2005) overlain; (b) SST (shaded, degC), 34 psu (dashed, black) and 35psu (solid, black) contours are overlain, hot spots SST>29.5C are emphasized. (c) Fresh water area (number of 1^ox1^o deg² boxes within 34psu contour) and winds averaged over the September 2011 SSS<34psu area (shown in panel b).

Figure 5. Argo profiles of temperature and salinity before (bold) and after (open circles) the passage of Katia (see Figure 4 for profile locations and Supplements for the rest of profiles).

Figure 6. Temporal diagram of (a) weekly running mean AQUARIUS SSS and (b) SST along the hurricane trajectory (see Fig. 5). Hurricane path from NOAA/NHC analysis is overlain. Fresh pool is encompassed by dashed lines.

Comparisons to the mooring salinity use same-day Aquarius salinity data (SSS_AQ) averaged over the four nearest grid points (corresponding to a roughly 200km spatial average) from the shallowest mooring level, which is 1 m depth in most cases. A preliminary comparison has allowed us to flag several mooring time series as unreliable, which have been removed from this comparison. More than 98% of the daily differences are less than 1psu, with a STD of daily data of 0.36 psu (Fig. S1). If we assume the errors are random the uncertainty would reduce to 0.25 psu for weekly means (approximately 10 Aquarius observations a month are available at each grid point). The mean bias is <0.1psu, but the histogram of differences shows significant fresh outliers. In fact systematic differences between in situ nearsurface salinity and satellite SSS may be expected in low salinity tropical regions where SSS is fresher than bulk salinity (Henocq et al., 2010). Reverdin et al. (2012) have found that individual rainfall-induced freshening events reduce salinities by an average 0.56 psu at 50 cm depth and amplify towards the surface, being larger by more than 20% at 15 cm below the surface. Our comparison (Fig. S2) of the much more limited set of spatially and temporally collocated Argo near surface salinity and Aquarius SSS in our region of interest during September to November, 2011 (when the plume is present) suggests a somewhat larger range of differences.

Both MW sensors indicate similar spatial patterns of haline changes in the wake of Katia (Fig. 4). Some differences between the two sensors are related to the differences in temporal coverage and spatial averaging, with the spatial smoothing being notorious for the Aquarius. Fig. S4 shows that due to better spatial resolution SMOS compares better with in-situ salinity changes (after minus before the passage of Katia) deducted from Argo profiles from Figs. 5 and S3 (see Fig. 4 for Argo locations).

Figure S1 Comparison of daily mean 1m depth tropical mooring salinity () from the TAO/Triton, RAMA, and Pirata moorings ( http://www.pmel.noaa.gov/tao/index.shtml ) and contemporaneous, nearby AQUARIUS version 1.3 gridded SSS (SSS_AQ). (a) Scatter diagram of contemporaneous and SSS_AQ. Solid line is mean and cross-hatching shows ±σ as evaluated from in 1 psu bins. (b) Histogram of differences divided into 0.1psu bins. Solid line is a least squares Gaussian fit. Bias and STD are shown in upper left.

Figure S2. The same day AQUARIUS SSS and Argo float near surface salinity (the shallowest data from 5-10m depth range is taken, normally z=5m) in the western tropical Atlantic during the 2011 boreal fall.