S. Fournier

SMAP observes flooding from land to sea: the Texas event of 2015

Floods can have damaging impacts on both land and sea, yet studies of flooding events tend to focus on only one side of the land/sea continuum.xa0 Here we present the first two-sided analysis, focusing on the May 2015 severe flooding in Texas.xa0Our investigation benefits from simultaneous measurements of land-surface soil moisture and sea surface salinity from NASAs recent Soil Moisture Active Passive (SMAP) mission as well as ancillary data.xa0We report the comprehensive chronology of the flooding: above average rainfall preceding the flood caused soils to saturate; record rainfall then generated record river discharge; subsequently, an unusual freshwater plume associated with anomalous ocean currents formed in the north-central Gulf of Mexico. Together with the Mississippi River plume, a rare horseshoe pattern was created that may have significant biogeochemical implications. Such integrated land/sea analysis of flood evolution can improve impact assessments of future extreme flooding events.

Modulation of the Ganges‐Brahmaputra River Plume by the Indian Ocean Dipole and Eddies Inferred From Satellite Observations

The Bay of Bengal receives large amounts of freshwater from the Ganga-Brahmaputra (GB) river during the summer monsoon. The resulting upper-ocean freshening influences seasonal rainfall, cyclones, and biological productivity. Sparse in situ observations and previous modeling studies suggest that the East India Coastal Current (EICC) transports these freshwaters southward after the monsoon as an approximately 200 km wide, 2,000 km long “river in the sea” along the East Indian coast. Sea surface salinity (SSS) from the Soil Moisture Active Passive (SMAP) satellite provides unprecedented views of this peculiar feature from intraseasonal to interannual timescales. SMAP SSS has a 0.83 correlation and 0.49 rms-difference to 0–5 m in situ measurements. SMAP and in stu data both indicate a SSS standard deviation of ∼0.7 to 1 away from the coast, that rises to 2 pss within 100 km of the coast, providing a very favorable signal-to-noise ratio in coastal areas. SMAP also captures the strong northern BoB, postmonsoon cross-shore SSS contrasts (∼10 pss) measured along ship transects. SMAP data are also consistent with previous modeling results that suggested a modulation of the EICC/GB plume southward extent by the Indian Ocean Dipole (IOD). Remote forcing associated with the negative Indian Ocean Dipole in the fall of 2016 indeed caused a stronger EICC and “river in the sea” that extended by approximately 800 km further south than that in 2015 (positive IOD year). The combination of SMAP and altimeter data shows eddies stirring the freshwater plume away from the coast.

Interannual Variation in Offshore Advection of Amazon‐Orinoco Plume Waters: Observations, Forcing Mechanisms, and Impacts

This study investigates sea surface salinity (SSS) and sea surface temperature (SST) variations in the tropical Atlantic east of the Lesser Antilles, a region where freshwater advection from the Amazon and Orinoco rivers, may potentially impact air-sea interaction. Observations are used to document later-summer variability and evaluate offshore riverine transport from 2010-2014. During the period 2010-2014, the largest difference in plume-affected areas, defined as the extent covered by SSS lower than 35.5 pss, is found between 2011 and 2014. Plume waters covered 92% of the study region in 2011 and 60% in 2014, with the average SSS in the study region being 2-pss lower in 2011. Lagrangian particle tracking based on satellite-derived ocean currents is used to diagnose the impact of the river plumes on SSS and SST from 2010 through 2014. Northward freshwater flux in summer 2014 was significantly weaker than fluxes in 2010-2013. This difference is not due to interannual discharge variability, but to significant changes in eddy-driven transport and cross-shore winds. In particular, the stronger cross-shore wind in May 2014 restricted offshore freshwater flow and lead to a smaller plume-affected area. Persistent SST gradients are often found near the plume edge, which may have implications for ocean-atmosphere coupling associated with atmospheric convection. SST in the study region was 1°C higher in 2010 than in other years, and is related to basin-scale ocean-atmosphere processes. Interannual variation in Amazon advective pathways and the associated SSS changes are also influenced by changes in the ITCZ position between 2011 and 2014.

Intercomparison of In-Situ and Remote Sensing Salinity Products in the Gulf of Mexico, a River-Influenced System

The recent emergence of satellite-based sea surface salinity (SSS) measurements provides new opportunities for oceanographic research on freshwater influence in coastal environments. Several products currently exist from multiple observing platforms and processing centers, making product selection for different uses challenging. Here we evaluate four popular SSS datasets in the Gulf of Mexico (GoM) to characterize the error in each product versus in-situ observations: Two products from NASA’s Soil Moisture Active Passive (SMAP) mission, processed by Remote Sensing Systems (REMSS) (40 km and 70 km); one SMAP 60 km product from the Jet Propulsion Laboratory (JPL); and one 60 km product from ESA’s Soil Moisture Ocean Salinity (SMOS) mission. Overall, the four products are remarkably consistent on seasonal time scales, reproducing dominant salinity features. Towards the coast, 3 of the 4 products (JPL SMAP, REMSS 40 km SMAP, and SMOS) show increasing salty biases (reaching 0.7–1 pss) and Root Mean Square Error (RMSD) (reaching 1.5–2.5 pss), and a decreasing signal to noise ratio from 3 to 1. REMSS 40 km generally shows a lower RMSD than other products (~0.5 vs. ~1.1 pss) in the nearshore region. However, at some buoy locations, SMOS shows the lowest RMSD values, but has a higher bias overall (>0.2 vs. <0.1 pss). The REMSS 70km product is not consistent in terms of data availability in the nearshore region and performs poorly within 100 km of the coast, relative to other products. Additional analysis of the temporal structure of the errors over a range of scales (8/9-day to seasonal) shows significantly decreasing RMSD with increasing timescales across products.