remote sensing

Article Sea Surface Salinity Response to Tropical Cyclones Based on Satellite Observations

Jingru Sun 1,* , Gabriel Vecchi 1,2,3 and Brian Soden 4

1 Department of Geosciences, Princeton University, Princeton, NJ 08544, USA; [email protected] 2 High Meadows Environmental Institute, Princeton University, Princeton, NJ 08544, USA 3 Atmospheric and Oceanic Sciences Program, Princeton University, Princeton, NJ 08540, USA 4 Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA; [email protected] * Correspondence: [email protected]

Abstract: Multi-year records of satellite remote sensing of sea surface salinity (SSS) provide an opportunity to investigate the climatological characteristics of the SSS response to tropical cyclones (TCs). In this study, the influence of TC winds, rainfall and preexisting ocean stratification on SSS evolution is examined with multiple satellite-based and in-situ data. Global storm-centered composites indicate that TCs act to initially freshen the ocean surface (due to precipitation), and subsequently salinify the surface, largely through vertical ocean processes (mixing and upwelling), although regional hydrography can lead to local departure from this behavior. On average, on the day a TC passes, a strong SSS decrease is observed. The fresh anomaly is subsequently replaced by a net surface salinification, which persists for weeks. This salinification is larger on the right (left)-hand side of the storm motion in the Northern (Southern) Hemisphere, consistent with the location of stronger turbulent mixing. The influence of TC intensity and translation speed on the ocean response



is also examined. Despite having greater precipitation, stronger TCs tend to produce longer-lasting,
 stronger and deeper salinification especially on the right-hand side of the storm motion. Faster

Citation: Sun, J.; Vecchi, G.; Soden, B. moving TCs are found to have slightly weaker freshening with larger area coverage during the Sea Surface Salinity Response to passage, but comparable salinification after the passage. The ocean haline response in four basins Tropical Cyclones Based on Satellite with different climatological salinity stratification reveals a significant impact of vertical stratification Observations. Remote Sens. 2021, 13, on the salinity response during and after the passage of TCs. 420. https://doi.org/10.3390/ rs13030420 Keywords: sea surface salinity; upper ocean response; tropical cyclone; SMAP; SMOS; Aquarius; Argo Academic Editor: Viviane V. Menezes Received: 18 December 2020 Accepted: 22 January 2021 Published: 26 January 2021 1. Introduction Tropical cyclones (TCs) are one of the most destructive natural hazards, in terms of Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in human and financial costs. So, understanding the mechanisms behind and improving published maps and institutional affil- predictions of TC evolution are of key scientific and societal concern. Due to strong winds iations. and intense heat, momentum and freshwater fluxes, TCs can have a profound impact on the thermal and salinity structure of the upper ocean. These upper ocean changes can feed back onto the evolution of the TC [1–6] and may impact the large-scale climate [7]. Therefore, characterizing and understanding the upper ocean response to TCs are important steps for understanding the air-sea interaction during the passage of TCs and consequently the Copyright: © 2021 by the authors. improvement of TC intensity forecasts. Licensee MDPI, Basel, Switzerland. This article is an open access article Both observational and modeling studies have shown that TCs drive strong vertical mixing distributed under the terms and and upwelling, which leads to surface cooling by entrainment of colder subsurface waters [8–13]. conditions of the Creative Commons The TC-induced cooling decreases the heat fluxes from the ocean to the storm, providing a Attribution (CC BY) license (https:// negative feedback with the potential to influence TC intensification [3–5,14–16]. It has been creativecommons.org/licenses/by/ suggested that subsurface warming driven by TC-induced mixing may impact the mean 4.0/). climate [17–19].

Remote Sens. 2021, 13, 420. https://doi.org/10.3390/rs13030420 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, 420 2 of 25

Although there is now substantial literature to provide a global perspective on the sea surface temperature (SST) response to TCs and its impact on TC evolution, the interaction between sea surface salinity (SSS) and TCs is less explored. However, studying the surface salinity response provides an additional perspective on the upper-ocean response to TCs, and may be important for understanding the ocean hydrologic cycle and biogeochemical dynamics [20–22]. A combined exploration of SSS and SST evolution will provide a more complete understanding of upper ocean dynamics and the ocean density response to TCs. Because of the sparsity of ocean salinity observations, previous observational analyses have tended to focus on individual case studies. Maneesha et al. [23] found SSS increased up to 0.74 psu on the right of the storm track due to Bay of Bengal (BoB) TC Nargis (2008) with in-situ data. The BoB has the freshest surface and great vertical salinity gradients in the Indian Ocean due to oceanic rainfall and river discharges. A more synoptic view of SSS response to another BoB TC (Vardah 2016) with satellite observations suggests significant increase of SSS (up to 1.5 psu) on the right of the storm and slight decrease on the left [24]. Chaudhuri et al. [25] also reveals the SSS increase of 1.5–3 psu over an area of nearly one million square kilometers after the passage of TC Phailin (2013) in the BoB. Like the BoB, the spreading of the Amazon–Orinoco River plume (AOP) formed a low salinity region in the northwestern tropical Atlantic. Various case studies indicate that the passages of hurricanes in the AOP region can leave a high haline wake with more than 1 psu salinity increase over large areas. However, the SSS change is much weaker outside the plume region [26,27]. Domingues et al. [28] found that the vertical mixing during Hurricane Gonzalo (2014) caused a ~0.6 psu salinity increase in above 20 m and a salinity decrease of 0.4 psu in the layer between 20 and 130 m using underwater gliders situated north of Puerto Rico. The large positive SSS anomaly induced by TCs reflects a strong haline stratification before the passage of TCs. Although these TCs induced large surface salinification, the existence of the strong haline stratification acts as a barrier layer, restricting vertical mixing and amplitude of surface cooling [29–31]. A consequence of a strong pre-TC near-surface fresh layer is a reduction of the negative feedback of SST cooling. For example, Androulidakis et al. [32] investigated the interaction between the AOP and three successive hurricanes in 2011. It is found that the existence of a barrier layer during the first storm increased the resistance of the upper ocean to cooling, while the effect was missing in the subsequent two storms as the barrier layer was eroded after the passage of the first hurricane. Observations and model studies have revealed that TC intensification rates can be significantly higher over regions with barrier layers [33–35]. In addition to strong vertical mixing and upwelling, intense rainfall and evaporation from the strong TC winds drive surface freshwater fluxes that would act to impact surface and upper-ocean salinity. Although many case studies have revealed a net positive SSS anomaly following a TC, as vertical mixing tends to entrain the subsurface saltier water to the surface, it does not mean that the influence of rainfall on freshening the ocean surface can be neglected. Robertson and Ginis [36] investigated the impact of TC rainfall on ocean salinity with idealized TC forcing and found that the inclusion of rain induced surface negative anomalies in front of the storm center. With underwater gliders, Hsu and Ho [37] found that TCs with heavy rainfall tended to dilute the SSS during TCs, freshening both surface and subsurface. Jourdain et al. [38] found that the positive buoyancy flux due to TC rainfall strengthens the vertical stratification, reduces the vertical mixing and decreases the salty wake left by the passage of a TC. Therefore, TC rainfall acts to reduce the mixed layer depth after the TC passage, hence reducing cold water entrainment [38–40]. A slightly fresher mixed layer was observed due to intense rainfall by a moored station south of Kuroshio Extension as typhoon Choi-Wan (2009) was approaching [41]. Kil et al. [42] also found that using satellite data the higher precipitation during Hurricane Isaac (2012) was associated with the lower SSS. A case study designed by Steffen and Bourassa [43] indicates that precipitation by hurricane Gonzalo (2014) produced near-surface freshening of about 0.3 psu and developed a barrier layer during its approach and passage. Lin and Oey [20] reported stronger negative salinity anomalies on the left-hand side of the storms Remote Sens. 2021, 13, 420 3 of 25

in the western North Pacific because of the leftward asymmetric TC rainfall. Subsurface freshening due to rainfall and vertical mixing is also clearly identified through case and composite studies with in-situ observations [44,45]. As will be shown below, looking across many TCs, the surface salinity response is initially dominated by the TC rainfall and subsequently by vertical ocean processes. To complement these case studies, we seek to describe the climatological characteris- tics of the ocean haline response to TCs and compare the salinity changes with the ocean temperature response, through a combined analysis of satellite and in-situ observations. The advent of satellite remote sensing of SSS starting in 2010 opens the possibility to system- atically explore the climatology of the SSS response to TCs, with an aim to provide insights into the climatological ocean haline response to TCs that can complement previous case studies (e.g., [24]). Through a series of observational composite analyses, we will describe the response of ocean salinity to TCs, which help elucidate the processes controlling this ocean salinity response. In particular, the present study tries to answer the following ques- tions: 1. What is the typical impact of TC rainfall, evaporation and winds on ocean salinity in terms of climate characteristics? 2. How do TC intensity and translation speed affect the ocean response? 3. What is the difference in ocean response between different basins, and how does it depend on the initial vertical salinity gradients? A recently published paper by Reul et al. [46] characterized the SSS response after TC passage with different methods using 10-year satellite SSS data. The influence of TC intensity, translation speed and vertical salinity gradient on the SSS response during relaxation stage was investigated. Although the SSS response during the forced stage of the TCs and the impact of the balance between precipitation and evaporation are not considered in their study, the climatological SSS response to TCs was studied for the first time at global scale. The consistency and differences between Reul et al. [46] and our study will also be discussed below. The remainder of this paper is organized as follows. Section2 introduces data and methods used in the study. Section3 presents the global averaged ocean salinity response to TCs and evaluates the impact of TC intensity, translation speed, and initial ocean salinification on ocean haline response to TCs. A conclusion and discussion are given in Section4.

2. Data and Methods 2.1. Data The center locations and maximum sustained wind speed data of TCs from 2002 to 2019 are obtained from International Best Track Archive for Climate Stewardship (IB- TrACS; [47]). TC intensity is defined by 1-minute sustained 10-m wind speed, grouped by TC category on the Saffir-Simpson scale. Supplementary Figure S1 shows the global TC tracks and the intensity category of track positions during the period 2010–2019. The counts of TC track positions for each TC intensity category are present in Table1. TCs with maximum wind speed of 17 m s−1 or less are excluded. The large-scale wind field from National Center Environmental Prediction (NCEP) daily reanalysis data [48] is used to calculate the vertical wind shear during TCs, which is defined as the difference in wind vectors between 200 and 850 hPa averaged over an annulus region of 200–800 km from the TC center.

Table 1. TC track position counts for each TC intensity category. The IBTrACS data from August 2011 to June 2015 (Aquarius), March 2015 to December 2019 (SMAP) and June 2010 to December 2019 (SMOS) are used.

Satellite TS Cat 1 Cat 2 Cat 3 Cat 4 Cat 5 Aquarius 9157 2103 899 894 581 97 SMAP 11,725 2710 1501 1533 1045 215 SMOS 23,060 5266 2561 2628 1725 320 Remote Sens. 2021, 13, 420 4 of 25

Aquarius/SAC-D and Soil Moisture Active-Passive (SMAP) satellite SSS provided by National Aeronautics and Space Administration (NASA) and the European Space Agency’s (ESA) Soil Moisture Ocean Salinity (SMOS) satellite SSS are used to calculate the SSS response to TCs [49–51]. The satellite observations have been widely used to investigate the interaction between ocean and TCs (e.g., [25–27]). The Aquarius data (version 5.0) covers 2011-08-25 to 2015-06-07 with 150 km spatial resolution. Both level 3 (L3) weekly running mean daily data with horizontal resolution 1◦ × 1◦ and level 2 (L2) swath daily data are applied in this study. SMAP satellite data provided by Remote Sensing Systems (version 4.0) are available since 2015-03-27 with a higher resolution (70 km). The SMAP L3 product provides eight-day running meaning daily SSS with horizontal resolution 0.25◦ × 0.25◦ while the L2C product contains the validated SSS swath daily data. The L3 eight-day running mean and L2C swath daily data during the period 2015-03-27 to 2019-12-31 are applied in later calculation. We have also included the daily swath data (L2) and ten-day running mean daily data (L3) from SMOS with resolution about 50 km as a complement for our study. The SMOS L2 data obtained from ESA are generated by version 662 of the Level 2 ocean salinity operational processor. The SMOS L3 ten-day running mean data are obtained from Centre Aval de Traitement des Données SMOS (CATDS). The salinity is based on L2Q products, corrected from land-sea contamination and latitudinal bias. SMOS L2 and L3 satellite data from 2010-06-01 to 2019-12-31 are used in this study. For the seven-, eight- or ten-day running daily mean satellite data, the daily SSS is the average between 3, 3.5 or 4.5 days before and after, respectively. SMAP and SMOS satellite data are analyzed to present both the time series and distribution of the SSS response to TCs. However, the Aquarius satellite data are only applied to calculate the time series of the TC related SSS response due to the relatively coarse horizontal resolution. As we will focus on the ocean surface salinity response during and after the passage of TCs mainly based on satellite observations, the accuracy of satellite SSS retrievals under high surface wind speeds is important to our results. Surface wind is used to estimate the sea surface roughness and its contribution to the brightness temperature during SSS retrievals [52]. However, the surface wind speed used in SSS retrievals is often underestimated for intense TCs, which would therefore make the retrieved SSS less accurate. For example, Meissner et al. [53] indicate that the retrieved SMAP L2C SSS may have moderate degradation when the surface wind speed is larger than 15 m s−1. SSS retrievals are also flagged as moderate degradation when surface wind speed is larger than 15 m s−1 and severe degradation when surface wind speed exceeds 20 m s−1 for Aquarius L2 data. Therefore, to decrease the uncertainty of satellite SSS L3 data, the retrieved L2 SMAP and Aquarius SSS under wind speed higher than 20 m s−1 are discarded during the gridding for the satellite L3 running mean data [53]. The SMOS L2 SSS associated with larger surface wind speed (>12 m s−1) are also removed before averaging to create L3 data due to inaccurate surface roughness for high surface wind speeds. In this study, we will do composite analysis using L2 SSS with and without removing SSS associated with surface wind speed larger than 15 m s−1. The microwave and infrared (MR_IR) optimally interpolated (OI) SST daily products, provided by the Remote Sensing Systems, from 2010 to 2019 are applied to study the SST response to TCs. Microwave measurements permit retrieval of SST even under overcast conditions and has been widely used in previous studies [4,54,55]. Daily SST observations with 9 km × 9 km are used in this study to calculate the TC related SST change before and after the storm passage. The SST cooling after the TC passage provides a proxy for the TC-induced mixing. Daily precipitation data are from NASA Global Precipitation Measurement (GPM; [56]) satellite observations with 0.1◦ horizontal resolution and 0.25◦ daily evaporation data are obtained by ERA5 reanalysis data [57]. The daily accumulated precipitation and evaporation from 2015 to 2019 are applied to estimate the surface net freshwater flux induced by TCs at daily intervals. In-situ Argo [58] profiles from 2002 to 2019 are also collected during the reference period (pre-storm) and post-storm within the 2◦ × 2◦ gridded Remote Sens. 2021, 13, x FOR PEER REVIEW 5 of 27

after the storm passage. The SST cooling after the TC passage provides a proxy for the TC- induced mixing. Daily precipitation data are from NASA Global Precipitation Measurement (GPM; [56]) satellite observations with 0.1° horizontal resolution and 0.25° daily evaporation data are obtained by ERA5 reanalysis data [57]. The daily accumulated precipitation and Remote Sens. 2021, 13, 420 evaporation from 2015 to 2019 are applied to estimate the surface net freshwater flux5 of 25 induced by TCs at daily intervals. In-situ Argo [58] profiles from 2002 to 2019 are also collected during the reference period (pre-storm) and post-storm within the 2° × 2° gridded box centered at a TC position to present the ocean stratification before TCs and box centered at a TC position to present the ocean stratification before TCs and the ocean thehaline ocean response haline response after the after TC passage.the TC passage.

2.2.2.2. Methods Methods TheThe SSS, SSS, SST, SST, precipitation precipitation and and evaporation evaporation in in a a10° 10 ×◦ ×10°10 gridded◦ gridded box box centered centered at atthe the TCTC position position from from 40 40 days days (day (day −40)−40) before before the the TC TC passed passed over over until until 40 40days days (day (day 40) 40) after after thethe TC TC passed passed are are collected. collected. Day Day 0 indicates 0 indicates the the time time the the TC TC reaches reaches the the position. position. Every Every 10°10 ×◦ 10°× 10 gridded◦ gridded box box is rotated is rotated so that so that the the TC TCtranslation translation direction direction is along is along the the x-axis x-axis and and pointingpointing to tothe the left. left. Two Two examples examples are are shown shown in inFigure Figure 1,1 where, where the the tracks tracks of of hurricane hurricane FlorenceFlorence (2018) (2018) and and Helene Helene (2018) (2018) are are overlaid overlaid on on the the distribution distribution of ofSMAP SMAP L2C L2C swath swath dailydaily SSS SSS on on 12 12September September 2018. 2018.

FigureFigure 1. Distribution 1. Distribution of sea of seasurface surface salinity salinity (psu) (psu) on 2018 on 2018 September September 12, from 12, from SMAP SMAP swath swath daily daily − datadata (L2C) (L2C) with with (lower (lower panel) panel) and and without without (upper (upper panel) panel) the the high high su surfacerface wind wind (>15 (>15 m ms−1 s; dashed1; dashed blueblue line) line) associated associated SSS SSS removed. removed. Two Two TC TC trac tracksks are are Florence Florence (left) (left) and and Helene Helene (right), (right), respectively. respectively.The larger The (smaller) largersquares (smaller) show squares the 10show◦ × th10e◦ 10°(2◦ ×× 10°2◦ )(2° TC-centered × 2°) TC-centered boxes. boxes. Larger Larger squares are squaresrotated are so rotated that the so translation that the translation direction isdirectio alongn the is along x-axis the and x-axis pointing and topointing the left. to the left.

TheThe Lagrangian Lagrangian composite composite method performedperformed toto compute compute the the evaluation evaluation and and distri- distributionbution of theof the SSS SSS and and SSS SSS response response before, before, during during and afterand after the passage the passage of a TC of followsa TC followsthat of that Lloyd of Lloyd and Vecchi and Vecchi [4]. The [4]. SSS The and SSS other and variablesother variables were sampled were sampled for a 2 ◦for× a2 ◦2°and × 2° 10and◦ × 10°10 ◦× gridded10° gridded box box centered centered on theon TCthe positionTC position (Figure (Figure1). The 1). The seasonal seasonal cycles cycles were wereremoved removed to obtain to obtain the anomaly the anomaly for each for variable. each variable. In order In to order examine to examine the ocean the response ocean to responseTCs, days to TCs,−12 days to −2 − are12 to defined −2 are asdefined reference as reference periods for periods all the for satellite all the swath satellite data, swath while days −20 to −10 are the reference periods for seven-, eight- or ten-day running mean data. The reference SSS and SST were calculated by averaging the SSS and SST anomaly during the reference period, respectively. The SSS (SST) anomaly relative to the reference SSS (SST) in the same day relative to day 0 were then averaged over all the TC positions to compute the composite SSS and SST anomalies. The composite anomalies were also computed separately by intensity category and translation speed. We define slow (fast)-moving TCs as those with translation speeds half standard deviation smaller (larger) than the group mean TC translation speed. To estimate the uncertainty in the composite mean, standard Remote Sens. 2021, 13, 420 6 of 25

√ error is also calculated with s/ n, where s is the sample standard deviation, n is the number of TC track points.

3. Results 3.1. Impact of TC Rainfall and Winds on Ocean Salinity 3.1.1. Time Evolution of Ocean Surface Response Using SMAP swath daily raw data (without removing the SSS associated with high surface wind speeds), the Lagrangian composite SSS anomaly (relative to the reference SSS) shows that on average, as the TC approaches day 0, ocean surface freshens rapidly, with a maximum decrease of mean SSS of more than 0.65 psu (Figure2b). Freshening varies with TC intensity with a maximum freshening (~0.9 psu) on day 0 for Category 3 TCs (Figure2a) . Remote Sens. 2021, 13, x FOR PEER REVIEWDue tothe degradation of the retrieved SSS under strong surface wind speeds, the SSS 7 of 27

response on day 0 has larger uncertainty than in other days, especially for intense TCs; this uncertainty on day 0 will also impact multi-day averages that include it. The impact of the possible contamination of SSS at high wind speeds will be discussed in Section 3.1.2.

Figure 2. Lagrangian composites relative to TC passage for: (a) daily SSS, (b) eight-day running mean SSS, (c) daily precipitation and (d) daily SST. Panel (a) shows SSS anomaly relative to the reference SSS forFigure the composite 2. Lagrangian average of different composites intensity relative categories duringto TC the passage period 2015–2019, for: (a) from daily SMAP SSS, (b) eight-day running swathmean daily SSS, raw (c data) daily (L2C). precipitation Panel (b) is as in ( aand) but using(d) daily SMAP eight-daySST. Panel running (a) mean shows data (L3).SSS anomaly relative to the Thereference dashed blue SSS (purple) for the line composite is calculated usingaverage SMAP of swath differe dailynt data intensity with (without) categories the high during the period 2015– surface wind (>15 m s−1) associated SSS removed. The blue and purple dots indicate the maximum negative2019, from SSS anomaly SMAP for theswath composite daily average raw of data all storms (L2C). with andPanel without (b) the is highas in surface (a) windbut using SMAP eight-day associatedrunning SSS mean removed, data respectively. (L3). The Panel dashed (c) shows blue composite (purple) daily line accumulated is calculated precipitation using SMAP swath daily data duringwith (without) the same period the with high (a). Thesurface corresponding wind dots(>15 are m the s accumulated−1) associated precipitation SSS removed. on day 0 The blue and purple fordots different indicate categories. the maximum Panel (d) shows negative composite SSTSSS anomaly anomaly response for relativethe composite to the reference average of all storms with SST during the same period with (a). The composited variables are averaged over a 2◦ × 2◦ region centeredand without over TC trackthe positionshigh surface from days wind−20 to associ 20. Colorated shading SSS indicates removed, the standard respectively. error of Panel (c) shows eachcomposite variable. daily accumulated precipitation during the same period with (a). The corresponding dots are the accumulated precipitation on day 0 for different categories. Panel (d) shows The local freshening of the surface lasts approximately one day and is rapidly replaced bycomposite a smaller but SST longer-lasting anomaly surface response salinification. relative The to largest the reference mean positive SST SSS during response the same period with (a). (~0.05The composited psu) to all storms variables is observed are on averaged day 2. The over weaker a surface2° × 2° salinification region centered after TCs over TC track positions from days −20 to 20. Color shading indicates the standard error of each variable.

Similar behavior is revealed when the analysis above is also applied to Aquarius and SMOS satellite data (Figure 3). On average, the stronger negative SSS response during TCs is followed by weaker but longer-lasting surface salinification after the passage of TCs. The magnitude of the composite mean surface freshening computed with Aquarius swath daily data is 0.25 psu weaker than that of SMAP swath daily data, probably due to the coarse resolution. With similar horizontal resolution, the composite mean surface freshening calculated with SMOS swath daily data is about 0.1 psu larger than that of SMAP data, which may be attributed to different sensors, sampling or data processing of SSS retrievals between these two data products. The reduced salinity response obtained with the seven-/ten-day running mean data on day 0 is also found (Figure 3b,d). Stronger TCs tend to induce larger surface salinification after the passage of TCs, which is consistent with composite analysis results using SMAP data and the results from Reul et al. [46]. The peak in surface freshening on day 0 reflects that the maximum rainfall under a TC is observed on the day the TC passes (day 0; Figure 2c). Figure 2c shows that stronger TCs tend to produce greater precipitation. The daily mean evaporation rate shows similar characters, yet its magnitude only about 10% of the precipitation rate (Supplementary Figure S2). Table 2 compares the values of the daily accumulated precipitation and evaporation on day 0 for different intensity categories, revealing a monotonically increasing relationship between TC intensity and net freshwater flux. Unlike rainfall, the TC induced cooling, which is primarily driven by turbulent mixing and upwelling, reaches its maximum one or two days after a TC has passed (Figure 2d; [4]). The mean SST recovery time after the passage of TCs is more than 20 days for the storms considered here.

Remote Sens. 2021, 13, 420 7 of 25

have passed is observed across the different intensity categories, although the average magnitude of salinification differs. The passage of Category 5 TCs results in the strongest positive SSS response (~0.2 psu) on day 2 and 3 (Figure2a). The SSS response to different intensity during and after TCs will be further compared in Section 3.2. The composite SSS response to TCs is also analyzed using SMAP L3 eight-day run- ning mean daily data (Figure2b). The TC-induced initial freshening and the subsequent salinification are also observed with the smoothed data. Stronger surface salinification after the passage of TCs is clearly evident for intense TCs, although the initial freshening becomes very weak in the smoothed data. As expected, the maximum surface salinification Remote Sens. 2021, 13, x FOR PEER REVIEW 8 of 27 using the L3 eight-day running mean data is observed on day 5, three days later than that calculated with L2C daily data. Despite the delayed response, the magnitude of the positive SSS response is comparable, indicating that the SMAP L3 eight-day running mean SSS data can still capture aspects of the TC-induced SSS response after the storm passage. TableSimilar 2. Composite behavior is mean revealed and when standard the analysis error above of daily is also accumulated applied to Aquarius precipitation and and evaporation onSMOS day satellite 0 for different data (Figure TC3 ).intensity On average, categories the stronger during negative March SSS 2015 response to December during 2019. The composite variablesTCs is followed are averaged by weaker over but longer-lastinga 2° × 2° TC-centered surface salinification box. after the passage of TCs. The magnitude of the composite mean surface freshening computed with Aquarius swath daily data is 0.25 psu TS weaker than Cat that 1 of SMAP Cat swath 2 daily Cat data, 3 probably Cat due 4 Cat 5 to the coarse resolution. With similar horizontal resolution, the composite mean surface fresheningPrecipitation calculated with66.87 SMOS swath102.39 daily data125.46 is about 0.1 psu148.49 larger than169.84 that of 202.06 SMAP(mm) data, which may be± 0.61 attributed to± different 1.43 sensors,± 1.94 sampling or±2.05 data processing±2.95 of ±6.73 SSS retrievals between these two data products. The reduced salinity response obtained withEvaporation the seven-/ten-day running−5.37 mean data−7.38 on day 0 is−8.96 also found (Figure−9.633 b,d). Stronger−10.22 −12.38 TCs tend(mm) to induce larger±0.02 surface salinification±0.07 after the±0.11 passage of TCs,±0.11 which is consistent±0.12 ±0.21 with composite analysis results using SMAP data and the results from Reul et al. [46].

Figure 3. (a) The same with Figure2a, but with Aquarius L2 swath daily data from 2011 to 2015. (b) As in Figure2b but using Aquarius seven-day running mean data (L3) and L2 swath daily data Figureduring the 3. same(a) The period same with with (a). (c) Figure The same 2a, with but Figure with2a, Aqua but withrius SMOS L2 L2swath swath daily daily datadata from 2011 to 2015. (fromb) As 2010 in to Figure 2019. (d )2b As but in Figure using2b but Aquarius using SMOS seven-day ten-day running running mean datamean (L3) data and L2 (L3) swath and L2 swath daily data duringdaily data the during same the sameperiod period with with (a ().c). (c) The same with Figure 2a, but with SMOS L2 swath daily data from 2010 to 2019. (d) As in Figure 2b but using SMOS ten-day running mean data (L3) and L2 swath daily data during the same period with (c).

3.1.2. Impact of the Possible Contamination of SSS at High Wind Speeds The composite mean SSS response with SMAP swath daily data after removing the SSS associated with surface wind speed larger than 15 m s−1 is also presented in Figure 2b (dashed blue line). Although surface freshening on day 0 is slightly weaker (~0.05 psu) compared to that with the raw data, the TC related SSS evolution is still characterized with the strong surface freshening and subsequent surface salinification. Similar to SMAP data, after removing the potentially contaminated SSS retrievals due to the high surface winds (>15 m s−1), the SSS negative response on day 0 calculated with Aquarius and SMOS is still evident, with a slightly reduced magnitude (dashed blue lines in Figure 3b,d). In order to assess the impact of high wind speeds (and their contamination of SSS retrievals) to uncertainty in the SSS response to TCs, the spatial distribution of the percentage of pixels retained from days −1 to 3 for different intensity categories is presented in Supplementary Figure S3 and S4. As expected, more SSS pixels are removed on day 0 compared to days 1 to 3. Also, more SSS pixels are removed during the passage of stronger TCs than with weaker TCs. Therefore, the largest uncertainties of the SSS response to TCs is likely to occur on day 0 for the most intense TCs. The impact of the uncertainty due to SSS retrievals due to high surface wind on tropical storms is likely smaller, with more than 70% of SSS pixels retained in the SSS response to tropical storms on day 0 (Supplementary Figure S3). Therefore, the strong mean negative response on day 0 computed with the high surface wind associated SSS removed satellite L2 data (dashed

Remote Sens. 2021, 13, 420 8 of 25

The peak in surface freshening on day 0 reflects that the maximum rainfall under a TC is observed on the day the TC passes (day 0; Figure2c). Figure2c shows that stronger TCs tend to produce greater precipitation. The daily mean evaporation rate shows similar characters, yet its magnitude only about 10% of the precipitation rate (Supplementary Figure S2). Table2 compares the values of the daily accumulated precipitation and evaporation on day 0 for different intensity categories, revealing a monotonically increasing relationship between TC intensity and net freshwater flux. Unlike rainfall, the TC induced cooling, which is primarily driven by turbulent mixing and upwelling, reaches its maximum one or two days after a TC has passed (Figure2d; [4]). The mean SST recovery time after the passage of TCs is more than 20 days for the storms considered here.

Table 2. Composite mean and standard error of daily accumulated precipitation and evaporation on day 0 for different TC intensity categories during March 2015 to December 2019. The composite variables are averaged over a 2◦ × 2◦ TC-centered box.

TS Cat 1 Cat 2 Cat 3 Cat 4 Cat 5 Precipitation (mm) 66.87 ± 0.61 102.39 ± 1.43 125.46 ± 1.94 148.49 ± 2.05 169.84 ± 2.95 202.06 ± 6.73 Evaporation (mm) −5.37 ± 0.02 −7.38 ± 0.07 −8.96 ± 0.11 −9.63 ± 0.11 −10.22 ± 0.12 −12.38 ± 0.21

3.1.2. Impact of the Possible Contamination of SSS at High Wind Speeds The composite mean SSS response with SMAP swath daily data after removing the SSS associated with surface wind speed larger than 15 m s−1 is also presented in Figure2b (dashed blue line). Although surface freshening on day 0 is slightly weaker (~0.05 psu) compared to that with the raw data, the TC related SSS evolution is still characterized with the strong surface freshening and subsequent surface salinification. Similar to SMAP data, after removing the potentially contaminated SSS retrievals due to the high surface winds (>15 m s−1), the SSS negative response on day 0 calculated with Aquarius and SMOS is still evident, with a slightly reduced magnitude (dashed blue lines in Figure3b,d). In order to assess the impact of high wind speeds (and their contamination of SSS retrievals) to uncertainty in the SSS response to TCs, the spatial distribution of the percent- age of pixels retained from days −1 to 3 for different intensity categories is presented in Supplementary Figures S3 and S4. As expected, more SSS pixels are removed on day 0 compared to days 1 to 3. Also, more SSS pixels are removed during the passage of stronger TCs than with weaker TCs. Therefore, the largest uncertainties of the SSS response to TCs is likely to occur on day 0 for the most intense TCs. The impact of the uncertainty due to SSS retrievals due to high surface wind on tropical storms is likely smaller, with more than 70% of SSS pixels retained in the SSS response to tropical storms on day 0 (Supplementary Figure S3). Therefore, the strong mean negative response on day 0 com- puted with the high surface wind associated SSS removed satellite L2 data (dashed blue line in Figures2 and3) is mainly the average of SSS response to tropical storms. Although significant surface freshening is also observed in relatively intense TCs (Category 1–5) with satellite L2 raw data, the uncertainties arising from stronger wind speeds are more pronounced.

3.1.3. Spatial Distribution of Ocean Surface Response The spatial distribution of the accumulated precipitation–evaporation (P-E) and SSS (and SST) anomalies relative to reference SSS (and SST) in the Northern Hemisphere from days −1 to 3 are presented in Figure4. The accumulated P-E is computed starting from day −2. The spatial pattern of accumulated P-E is similar to the pattern of the precipitation since the magnitude of evaporation in the TC center is much smaller than that of precipitation. This is consistent with Gao et al. [59] and indicates a substantial large scale atmospheric convergence of moisture is needed to supply the TC rainfall. The accumulated P-E is distributed symmetrically across the track of the storms. Chen et al. [60] point out that the maximum TC rainfall is on the downshear left (right) side of the storms in the Northern Remote Sens. 2021, 13, x FOR PEER REVIEW 9 of 27

blue line in Figures 2 and 3) is mainly the average of SSS response to tropical storms. Although significant surface freshening is also observed in relatively intense TCs (Category 1–5) with satellite L2 raw data, the uncertainties arising from stronger wind speeds are more pronounced.

3.1.3. Spatial Distribution of Ocean Surface Response The spatial distribution of the accumulated precipitation–evaporation (P-E) and SSS (and SST) anomalies relative to reference SSS (and SST) in the Northern Hemisphere from days −1 to 3 are presented in Figure 4. The accumulated P-E is computed starting from day −2. The spatial pattern of accumulated P-E is similar to the pattern of the precipitation since the magnitude of evaporation in the TC center is much smaller than that of precipitation. This is consistent with Gao et al. [59] and indicates a substantial large scale Remote Sens. 2021, 13, 420 9 of 25 atmospheric convergence of moisture is needed to supply the TC rainfall. The accumulated P-E is distributed symmetrically across the track of the storms. Chen et al. [60] point out that the maximum TC rainfall is on the downshear left (right) side of the storms in the(Southern) Northern (Southern) Hemisphere Hemisphere when the vertical when the wind vertical shear wind is more shear than is 5 more m s− than1. The composite 5 m s−1. Thevertical composite wind vertical shearon wind day shear 0 is only on 1.45day m0 sis− 1only, resulting 1.45 m in s a−1 relatively, resulting symmetric in a rainfall relatively symmetricpattern; ongoingrainfall pattern; work explores ongoing the work dependence explores on the the dependence shear direction on the ofthe shear ocean response direction of theto TCs. ocean response to TCs.

Figure 4. (upperFigure panels) 4. (upper Distribution panels) ofDistribution the accumulated of the accumulated P-E (mm; precipitation-evaporation) P-E (mm; precipitation-evaporation) from day − 2. The vector on day 0 showsfrom the day composite −2. The verticalvector on wind day shear,0 shows which the composit is 1.54 me s −vertical1. (middle wind panels) shear, Distributionwhich is 1.54 of m the s−1. SSS anomaly (psu), using SMAP(middle swath panels) daily Distribution data (L2C) with of the surface SSS anomaly wind higher (psu), than using 15 SMAP m s−1 associatedswath daily SSS data removed. (L2C) with The composite translation speedsurface is 5.18 wind m shigher−1 shown than on 15 daym s− 0.1 associated Contours showSSS removed. the positive The SSScomposite anomaly translation with contour speed interval is 5.18 0.03 psu. −1 (lower panels)m Distribution s shown on of day the SST0. Contours anomaly show (◦C). the All positive TCs in theSSS Northern anomaly Hemispherewith contour from interval March 0.03 2015 psu. to December (lower panels) Distribution of the SST anomaly (°C). All TCs in the Northern Hemisphere from 2019 are used for composite analysis. March 2015 to December 2019 are used for composite analysis. The spatial distribution of composited SSS response in Figure4 is calculated using SMAP L2C swath daily data after removing the SSS associated with the high surface wind. The pattern of SSS response one day before the TCs pass resembles the pattern of P-E,

indicating that the net freshwater flux drives the freshening of the ocean surface on day −1. Although more freshwater is accumulated in the rear quadrants of the storms by day 0, the sea surface freshening is concentrated in the center of the TCs. Meanwhile, stronger SST cooling is observed on the rear right quadrant. We hypothesize that the difference between the pattern of P-E and SSS response is caused by the TC-induced vertical mixing. By days 1 to 3, the spatial pattern of SSS response no longer resembles the pattern of accumulated P-E. After the TC passes, there is an overall tendency towards salinification, resulting in a weaker surface freshening (salinification) is found in the front (rear) quadrants on day 1. The mean surface salinification on days 2–3 shows a pronounced right-hand side bias, which coincides with the stronger right-hand side cooling resulting from TC induced vertical mixing. The TC-induced mixing results in stronger right-hand side cooling and salinification after the passage of TCs in the Northern Hemisphere because on the Remote Sens. 2021, 13, 420 10 of 25

right-hand side the storm wind vector rotates clockwise, which is in the same sense as inertial current vector, and consequently tends to result in resonance [11,61,62]. The distribution of the composited accumulated P-E, and SST response in the Southern Hemisphere resembles that of the Northern Hemisphere, except that the TC-induced mixing in the Southern Hemisphere is biased to the left of the storm’s tracks (Figure5). Consistent with our interpretations of the role of mixing on salinification, surface cooling and salinification after the passage of TCs are stronger on the left-hand side, a feature expected due to the resonance between inertial oscillations and wind on the left-hand side. In addition, the mean SSS salinification for the Southern Hemisphere storms on days 2–3 is slightly stronger than that of the Northern Hemisphere storms. The rightward (leftward) peaking surface salinification in the Northern (Southern) Hemisphere is also found by Reul et al. [46], although their post-storm SSS response is averaged from days 0 to 5. They have also pointed that the maximum salinification of the globally-averaged SSS response is at around twice the maximum wind radius. The spatial distribution of SSS response during and after TCs in the Northern and Southern Hemisphere is consistent with the results of composite SSS response evolution shown in Figure2. The SSS response is initially dominated by the rainfall, leading to a freshening surface. The impact of vertical Remote Sens. 2021, 13, x FOR PEER REVIEW mixing and upwelling overwhelms rainfall afterwards, offsetting the fresher11 of 27 surface by a longer-lasting salinification through bringing saltier and colder subsurface water to the surface.

Figure 5.FigureThe same 5. The as Figuresame 4as, but Figure in the 4, Southern but in the Hemisphere. Southern TheHemisphere. composite verticalThe composite wind shear vertical in upper wind panel day 0 is −1 −1 3.32 m sshear. The in compositeupper panel translation day 0 is speed 3.32 inm middles−1. The panel composite day 0 is translation 4.23 m s . speed in middle panel day 0 is 4.23 m s−1.

The spatial pattern of SSS responses to TCs is also examined with SMOS swath daily data (Figure 6). The SSS associated with the surface wind more than 15 m s−1 is removed. Initial surface freshening on days −1 to 0 and the subsequent salinification in both Northern and Southern Hemisphere are consistent with those seen in the SMAP data. Compared to SMAP, the maximum SSS decrease from SMOS on day 0 is slightly weaker in both Hemispheres. Slightly stronger salinification is also observed on days 1 to 3 with the significant right (left)-hand side biases in the Northern (Southern) Hemisphere. The qualitative consistency between the results with SMAP and SMOS satellite data suggests that the features of the composites represent the actual evolution of SSS following the passage of a TC.

Remote Sens. 2021, 13, 420 11 of 25

The spatial pattern of SSS responses to TCs is also examined with SMOS swath daily data (Figure6). The SSS associated with the surface wind more than 15 m s −1 is removed. Initial surface freshening on days −1 to 0 and the subsequent salinification in both Northern and Southern Hemisphere are consistent with those seen in the SMAP data. Compared to SMAP, the maximum SSS decrease from SMOS on day 0 is slightly weaker in both Hemispheres. Slightly stronger salinification is also observed on days 1 to 3 with the significant right (left)-hand side biases in the Northern (Southern) Hemisphere. The Remote Sens. 2021, 13, x FOR PEER REVIEW qualitative consistency between the results with SMAP and SMOS satellite data12 of suggests 27 that the features of the composites represent the actual evolution of SSS following the passage of a TC.

Figure 6. Distribution of the SSS anomaly using SMOS swath daily data (L2) from 2010 to 2019 with surface wind higher thanFigure 15 m s −6.1 Distributionassociated SSS removedof the SSS in thean Northernomaly using (upper SMOS panels) swath and Southern daily data (lower (L2) panels) from Hemisphere. 2010 to 2019 with surface wind higher than 15 m s−1 associated SSS removed in the Northern (upper panels) and Southern (lower panels)The along-trackHemisphere. composite of SSS (SST) response from −4◦ to +4◦ to the center between day −5 and day 20 in the Northern and Southern Hemisphere is presented in Figure7. The The along-trackSST composite response is of characterized SSS (SST) by response uniform coolingfrom − with4° to the +4° maximum to the center decrease between on days 1 to 2. Right (left)-hand side bias is clearly observed in the Northern (Southern) hemisphere, day −5 and day 20 inwhich the isNorthern consistent and with Southern previous studies Hemisphere [11,12,63 is]. Unlikepresented the SST in response,Figure 7. SSS The drops SST response is characterizedprimarily during by uniform the passage cooling of TCs duewith to the the maximum peak in rainfall. decrease It is worth on days noting 1 that to 2. Right (left)-handas the side satellite bias is swath clearly SSS ob pixelsserved under in highthe Northern surface wind (Southern) are removed, hemisphere, SSS response to which is consistentrelatively with previous intense TCs studies on day [11,12,63]. 0 may not beUnlike fully considered the SST response, in the composite SSS drops calculation. Although TCs produce heavy rainfall, the surface freshening is eventually offset by surface primarily during the passage of TCs due to the peak in rainfall. It is worth noting that as salinification caused by vertical mixing and upwelling, which explains the right (left)-hand the satellite swathside SSS bias pixels distribution under of high the surface surface salinification wind are in theremoved, Northern SSS (Southern) response Hemisphere. to relatively intense TCsUltimately, on day the 0 passagemay not of be TCs fully over considered the ocean results in the in a composite cooler and saltier calculation. surface, with Although TCs producerecovery heavy time more rainfall, than 15the days. surf Bothace SMAPfreshening and SMOS is eventually satellite data offset reveal by similar surface salinificationSSS caused response by features vertical during mixing and after and the upwelling, passage of TCs,which which explains makes the the results right more reliable, although the magnitude of the salinification after the passage of TCs is higher (left)-hand side biaswhen distributi SMOS satelliteon of datathe surface are used. salinification in the Northern (Southern) Hemisphere. Ultimately, the passage of TCs over the ocean results in a cooler and saltier surface, with recovery time more than 15 days. Both SMAP and SMOS satellite data reveal similar SSS response features during and after the passage of TCs, which makes the results more reliable, although the magnitude of the salinification after the passage of TCs is higher when SMOS satellite data are used.

Remote Sens. 2021Remote, 13, x Sens.FOR 2021PEER, 13 REVIEW, 420 13 of 27 12 of 25

Figure 7. Along-trackFigure 7. averaged Along-track composites averaged of composites SSS anomaly of relativeSSS anomal to they relative reference to SSS the calculatedreference SSS with calculated SMAP swath daily data (psu; upperwith panels) SMAP and swath SMOS daily swath data daily (psu; data upper (psu; panels) middle and panels) SMOS and swath SST anomalydaily data relative (psu; middle to the reference panels) SST (◦C; lower panels)and for NorthernSST anomaly (left panels)relative andto the Southern reference (right SST panels) (°C; lower Hemisphere. panels) for Both Northern the SMAP (left andpanels) SMOS and SSS associated Southern (right panels) Hemisphere. Both the SMAP and SMOS SSS associated with high surface with high surface wind are removed. The gray shading indicates the region where more than 30% of the SSS pixels are wind are removed. The gray shading indicates the region where more than 30% of the SSS pixels removed. The y-axis is the cross-track direction ±4◦ to the right and left of the track. Composite SSS (SST) anomaly relative are removed. The y-axis is the◦ cross-track direction ±4° to the right and left of the track. Composite to the referenceSSS SSS (SST) (SST) anomaly along-track relative±4 toto the the reference center is SSS averaged (SST) along-track from days − ±4°5 to to 20. the center is averaged from days −5 to 20. 3.1.4. Subsurface Response 3.1.4. Subsurface RTheesponse composite vertical stratification with in-situ Argo data during the reference time The compositeperiod (pre-storm)vertical stratification and days 1with to 5 (post-storm)in-situ Argo ondata the during right- andthe reference left-hand sidetime referenced to the TC heading for both Hemispheres are presented in Figure8. The salinity (temperature) period (pre-storm) and days 1 to 5 (post-storm) on the right- and left-hand side referenced stratification is defined by the vertical derivation of salinity (temperature), ∂S/∂z (∂T/∂z). to the TC heading for both Hemispheres are presented in Figure 8. The salinity Before the passage of TCs, the subsurface water is colder and saltier relative to the surface (temperature) stratification is defined by the vertical derivation of salinity (temperature), in both Hemispheres [38]. Therefore, it is expected that the mean vertical stratification of S/ (/). Before the passage of TCs, the subsurface water is colder and saltier relative temperature and salinity would result in cooling and salinification, respectively, of the to the surface in both Hemispheres [38]. Therefore, it is expected that the mean vertical surface from mixing and upwelling (Figure7). The Argo-derived composite profiles also stratification of temperature and salinity would result in cooling and salinification, reveal the subsurface freshening on both sides due to the surface freshwater flux during respectively, of the surface from mixing and upwelling (Figure 7). The Argo-derived the TCs. In the Northern Hemisphere, the subsurface freshening on the right (left)-hand composite profiles also reveal the subsurface freshening on both sides due to the surface side is observed between 30 to 150 m (5 to 55 m) with maximum salinity decrease about freshwater flux during the TCs. In the Northern Hemisphere, the subsurface freshening 0.03 psu while in the Southern Hemisphere, the subsurface decrease is found in the layer on the right (left)-hand side is observed between 30 to 150 m (5 to 55 m) with maximum between 5 and 78 m (5 and 76 m) with maximum negative salinity response about 0.06 (0.04) salinity decreasepsu onabout theleft 0.03 (right)-hand psu while side. in the Stronger Southern and deeperHemisphere, subsurface the subsurface freshening on the right decrease is found(left)-hand in the side layer is probablybetween 5 attributed and 78 m to (5 the and stronger 76 m) verticalwith maximum mixing on negative the right (left)-hand salinity responseside inabout the Northern0.06 (0.04) (Southern) psu on the Hemisphere, left (right)-h whichand side. could Stronger drive more and freshwaterdeeper from the subsurface fresheningsurface to on the the subsurface. right (left)-hand side is probably attributed to the stronger vertical mixing onOn the average, right (left)-hand the passage si ofde TCsin the is followed Northern by (Southern) a subsurface Hemisphere, salinity increase in the which could layerdrive betweenmore freshwater 5 and 30 mfrom on thethe right-hand surface to sidethe subsurface. in the Northern Hemisphere. However, the subsurface salinity increase is not found on the left-hand side of the Northern Hemisphere in above 50 m, despite the positive SSS response, indicating that the salinification is confined in the near-surface layer. In the layer between 80 to 150 m in the Southern Hemisphere, we hypothesize that the salinity increase is the result of entrainment and

Remote Sens. 2021, 13, 420 13 of 25

upwelling, which is also observed in the layer between 60 to 130 m on the left-hand side in the Northern Hemisphere. This response, combined with the salinity decrease in the layer between 5 to 80 m, acts to increases the upper ocean density stratification. The ocean haline response to TCs is therefore significantly different from the ocean temperature response: stronger subsurface cooling in above 40 m is followed by weaker cooling in the layer between 40 to 150 m due to turbulent mixing and upwelling, which decreases the ocean stratification (Figure8a,d). Therefore, the influence of ocean haline response to TCs on the vertical density change after the passage of TCs can be significantly distinct to influence the ocean temperature response due to the existence of surface freshwater flux [64]. Although 11 (5) days of the in-situ Argo profiles are averaged to estimate the pre-storm (post-storm) Remote Sens. 2021, 13, x FOR PEER REVIEWocean conditions, the statements above have to be put in light of the poor spatial14 of and 27

temporal sampling of the Argo profiles. The statements can be potentially strengthened by more in-situ observations in the future.

FigureFigure 8. CompositeComposite temperature temperature and and salinity salinity profiles profiles du duringring the the reference reference time time period period (solid (solid line) line) andand averaged averaged 1–5 1–5 days days after after the the passage passage of ofthe the TCs TCs (dashed (dashed line) line) for all for storms all storms during during 2002 2002to 2019 to in2019 the in Northern the Northern (a,b) and (a,b )Southern and Southern (d,e) Hemisphere, (d,e) Hemisphere, with in-situ with in-situ data from data Argo from floats. Argo floats.The The salinity difference between the composite average during reference time period and days 1 to 5 in salinity difference between the composite average during reference time period and days 1 to 5 the Northern and Southern Hemisphere is presented in (c) and (f), respectively. LHS (RHS) shows in the Northern and Southern Hemisphere is presented in (c,f), respectively. LHS (RHS) shows the composite profiles within the left (right) 2°◦ × 1° box◦ of the center. Color shading indicates the standardthe composite error profilesof each variable. within the left (right) 2 × 1 box of the center. Color shading indicates the standard error of each variable.

3.2. InfluenceOn average, of TC the Intensity passage and of Translation TCs is followed Speed on by Ocean a subsurface Response salinity increase in the layer between 5 and 30 m on the right-hand side in the Northern Hemisphere. However, the subsurfaceTo investigate salinity the influenceincrease ofis TCnot intensityfound on and the translation left-hand speedside of on the SSS Northern response, the Northern Hemisphere TCs are divided into several groups with different TC intensity Hemisphere in above 50 m, despite the positive SSS response, indicating that the categories and translation speeds. salinification is confined in the near-surface layer. In the layer between 80 to 150 m in the Southern3.2.1. Influence Hemisphere, of TC Intensity we hypothesize that the salinity increase is the result of entrainment and upwelling, which is also observed in the layer between 60 to 130 m on The along-track composite of the SSS response from day −5 to day 20 for different intensity the left-hand side in the Northern Hemisphere. This response, combined with the salinity categories exhibits similar features with the mean composite of all storms (Figure9) . As the decrease in the layer between 5 to 80 m, acts to increases the upper ocean density majority of the SSS pixels near the center of relatively intense TCs on day 0 are removed stratification. The ocean haline response to TCs is therefore significantly different from (Supplementary Figure S4), it is difficult to compare the ocean haline response on day the ocean temperature response: stronger subsurface cooling in above 40 m is followed by 0 to TC-induced precipitation. After the passage of TCs, all TC intensity groups exhibit weaker cooling in the layer between 40 to 150 m due to turbulent mixing and upwelling, which decreases the ocean stratification (Figure 8a,d). Therefore, the influence of ocean haline response to TCs on the vertical density change after the passage of TCs can be significantly distinct to influence the ocean temperature response due to the existence of surface freshwater flux [64]. Although 11 (5) days of the in-situ Argo profiles are averaged to estimate the pre-storm (post-storm) ocean conditions, the statements above have to be put in light of the poor spatial and temporal sampling of the Argo profiles. The statements can be potentially strengthened by more in-situ observations in the future.

3.2. Influence of TC Intensity and Translation Speed on Ocean Response To investigate the influence of TC intensity and translation speed on SSS response, the Northern Hemisphere TCs are divided into several groups with different TC intensity categories and translation speeds.

Remote Sens. 2021, 13, x FOR PEER REVIEW 15 of 27

3.2.1. Influence of TC Intensity The along-track composite of the SSS response from day −5 to day 20 for different intensity categories exhibits similar features with the mean composite of all storms (Figure Remote Sens. 2021, 13, 420 9). As the majority of the SSS pixels near the center of relatively intense TCs on day 140 are of 25 removed (Supplementary Figure S4), it is difficult to compare the ocean haline response on day 0 to TC-induced precipitation. After the passage of TCs, all TC intensity groups exhibitstrong strong asymmetric asymmetric surface surface salinification salinification structure, structure, with the with largest the positivelargest positive SSS response SSS responseon the right-hand on the right-hand side of side the stormsof the storms in the in Northern the Northern Hemisphere. Hemisphere. It is It also is also found found that thatintense intense TCs TCs tend tend to to drive drive stronger stronger and and longer-lasting longer-lasting surfacesurface salinification.salinification. The The along along tracktrack ±4°± 4composite◦ composite with with SMAP SMAP swath swath data data shows shows that that TS TSand and Category Category 1 TCs 1 TCs result result in meanin mean SSS SSSincrease increase of more of more than than0.04 psu 0.04 on psu th one right-hand the right-hand side of side the ofstorms the storms during during days 2 daysto 4. 2The to 4. positive The positive SSS response SSS response then then slightly slightly decreases. decreases. However, However, the the ocean ocean surface surface responseresponse remained remained positive positive until until day day 14. 14. The The maximum maximum positive positive SSS SSS response response to to Category Category 2–32–3 (Category (Category 4–5) 4–5) TCs TCs reaches reaches more more than than 0.06 0.06 (0.12) (0.12) psu psu from from days days 2 2to to 8. 8. The The positive positive salinitysalinity response response of of larger larger than than 0.04 0.04 psu psu is is maintained maintained until until 10 10 (17) (17) days days after after the the passage passage ofof TCs TCs for for Category Category 2–3 2–3 (Category (Category 4–5) 4–5) TCs. TCs. When When SMOS SMOS swath swath data data are are used, used, a asimilar similar phenomenonphenomenon is is observed, observed, although although the the SSS SSS posi positivetive response response is is systematically systematically stronger stronger thanthan that that calculated calculated with with SMAP SMAP data. data. Both Both data data indicate indicate that that although although stronger stronger TCs TCs tend tend toto produce produce more more precipitation precipitation and and consequently consequently input input more more freshwater freshwater into into the the ocean, ocean, the the surfacesurface freshening freshening is is eventually eventually overwhelmed overwhelmed by by the the more more intense intense vertical vertical mixing. mixing. Thus, Thus, strongerstronger TCs TCs ultimately ultimately induce induce a asaltier saltier surface surface with with a alonger longer recovery recovery time. time. Consistent Consistent withwith our our findings, findings, ReulReul et et al. al. [46 [46]] have have also also found found that morethat more intense intense storms storms tend to tend produce to producesaltier globally-averaged saltier globally-averaged SSS response SSS response on days on 0 to days 5. 0 to 5.

Figure 9. Along-track averaged composite of SSS anomaly (psu) relative to the reference SSS for Figuredifferent 9. Along-track intensity categories averaged (TS composite to Category of SSS 1, anom Categoryaly (psu) 2–3 and relative Category to the 4–5) reference using SSS SMAP for (left differentpanels) andintensity SMOS categories (right panels) (TS to swath Category daily 1, data. Category Both the 2–3 SMAP and Category and SMOS 4–5) SSS using under SMAP high surface(left panels) and SMOS (right panels) swath daily data. Both the SMAP and SMOS SSS under high wind speed are removed. The gray shading indicates the region where more than 30% of the SSS surface wind speed are removed. The gray shading indicates the region where more than 30% of ± ◦ thepixels SSS arepixels removed. are removed. The y-axis The isy-axis the cross-track is the cross-track4 to the±4° rightto the and right left and of theleft track.of the Composite track. SSS ◦ Compositeresponse alongSSS response track ±4 alongto the track center ±4° isto averaged the center from is averaged days −5 from to 20. days −5 to 20. The vertical structure under the TC center for different intensity groups is compared The vertical structure under the TC center for different intensity groups is compared using Argo in-situ data (Figure 10). Consistent with the composite of the Argo-derived using Argo in-situ data (Figure 10). Consistent with the composite of the Argo-derived vertical profiles before the passage of all storms (Figure8), colder and saltier subsurface vertical profiles before the passage of all storms (Figure 8), colder and saltier subsurface water is found for all TC intensity groups on both sides of the storms. The passage of TCs generally results in stronger subsurface cooling above 30 to 60 m and weaker subsurface cooling deeper than that. All the intensity groups exhibit stronger upper layer cooling on the right-hand side. Remote Sens. 2021, 13, x FOR PEER REVIEW 16 of 27

water is found for all TC intensity groups on both sides of the storms. The passage of TCs generally results in stronger subsurface cooling above 30 to 60 m and weaker subsurface Remote Sens. 2021, 13, 420 cooling deeper than that. All the intensity groups exhibit stronger upper layer cooling15 of on 25 the right-hand side.

Figure 10. The same with the upper panels of Figure 88,, exceptexcept thatthat thethe ArgoArgo profilesprofiles duringduring TSTS andand Category 1 TCs (upper panels),panels), CategoryCategory 2–32–3 (middle(middle panels) panels) and and Category Category 4–5 4–5 (lower (lower panels) panels) in in the the Northern Hemisphere are composited. Northern Hemisphere are composited.

The oceanocean haline haline response response shows shows different different features. features. For group For group TS and TS Category and Category 1, although 1, althoughthe surface the salinification surface salinification is presented is on pres bothented sides on after both the sides TCs after overpass the (FigureTCs overpass9) , the (Figuresubsurface 9), the upper subsurface layer (the upper above layer 30 m) (the salinification above 30 m) is salinification only found on is the only right-hand found on side the right-handdue to stronger side due vertical to stronger mixing. vertical On the mixing left-hand. On side,the left-hand the subsurface side, the layer subsurface from 5 layer m to fromabout 5 50 m mtodisplays about 50 a m weaker displays salinity a weaker decrease, salinity indicating decrease, that indicating the surface that salinification the surface salinificationexists in a very exists shallow in a very near-surface shallow near-surface layer on the layer left-hand on the side.left-hand For Categoryside. For Category 2–3 and 2–3Category and Category 4–5, both 4–5, sides both manifest sides subsurface manifest uppersubsurface layer upper salinification layer salinification with a larger salinitywith a largerincrease salinity and a increase deeper upper and a layer deeper on theupper right-hand layer on side. the right-hand For all the intensityside. For groups,all the intensitybelow the groups, uppersalinification below the upper layer, salinification the freshwater layer, flux the during freshwater the TCs flux results during in a the layer TCs of resultssalinity in decrease, a layer withof salinity greater decrease, freshening with on thegreater right-hand freshening side. on Therefore, the right-hand compared side. to Therefore,the right-hand compared side, both to the the right-hand temperature side, and both salinity the temperature response to TCsand leadsalinity to a response stronger tostratification TCs lead to on a stronger the left-hand stratification side in the on Northern the left-hand Hemisphere. side in the Northern Hemisphere.

3.2.2. Influence of TC Translation Speed We further explored the influence of TC translation speed on surface salinity response. Slow-moving TCs could increase the amount of TC-related daily accumulated rainfall and

consequently result in stronger freshwater flux (Supplementary Figure S5). In addition, slow-moving TCs also tend to input more momentum flux into the ocean, leading to Remote Sens. 2021, 13, x FOR PEER REVIEW 17 of 27

3.2.2. Influence of TC Translation Speed We further explored the influence of TC translation speed on surface salinity response. Slow-moving TCs could increase the amount of TC-related daily accumulated Remote Sens. 2021, 13, 420 rainfall and consequently result in stronger freshwater flux (Supplementary Figure S5).16 In of 25 addition, slow-moving TCs also tend to input more momentum flux into the ocean, leading to enhanced vertical mixing and SST cooling [65,66]. The composites of SSS responseenhanced to tropical vertical storms mixing on and day SST0 in coolingtwo groups [65, 66of]. translation The composites speed (see of SSS Method) response are to showntropical in Figure storms 11. on The day mean 0 in translation two groups speed of translation of slower speedand faster (see cases Method) is 2.48 are m shown s−1 andin 7.80 Figure m s −111, respectively.. The mean translation speed of slower and faster cases is 2.48 m s−1 and 7.80 m s−1, respectively.

Figure 11. Distribution of SSS response (psu) to tropical storms on day 0 in the Northern Hemisphere Figure 11. Distribution of SSS response (psu) to tropical storms on day 0 in the Northern Hemispherebased on based translation on translation speed, with speed, SMAP with (upper SMAP panels) (upper and panels) SMOS and (lower SMOS panels) (lower swath panels) daily data. swathSame daily time data. period Same from time March period 2015 from to DecemberMarch 2015 2019 to isDecember used for both2019 SMAPis used and for SMOSboth SMAP satellite data. andThe SMOS mean satellite translation data. The speed mean is 2.48 translation and 7.80 speed m s−1 ,is respectively. 2.48 and 7.80 m s−1, respectively.

To Tocompare compare the the ocean ocean haline haline response response to different to different translation translation speed speed on onday day 0, SSS 0, SSS responseresponse to totropical tropical storms storms is is used used becausebecause moremore SSS SSS pixels pixels are are retained retained for tropicalfor tropical storms storms(Supplementary (Supplementary Figure Figure S3). S3). The The degradation degradation of SSS of retrievalsSSS retrievals under under high high wind wind speeds speedsmakes makes it hard it hard to compare to compare the SSSthe responseSSS respon onse day on day 0 to relatively0 to relatively intense intense TCs. TCs. The The SMAP SMAPsatellite satellite data data show show that that the surfacethe surface freshening freshening induced induced by slow-moving by slow-moving tropical tropical storms stormscovers covers a smaller a smaller area area and concentratesand concentrates near near the compositethe composite center center of the of tropical the tropical storms, storms,while while the surface the surface freshening freshening for fast-moving for fast-moving tropical tropical storms storms covers covers a much a largermuch arealarger with areathe with maximum the maximum decrease decrease (~0.6 psu) (~0.6 weaker psu) weaker than that than (~1 that psu) (~1 ofpsu) the of slow-moving the slow-moving groups. Similar results are noted with SMOS satellite data, indicating that fast (slow)-moving groups. Similar results are noted with SMOS satellite data, indicating that fast (slow)- tropical storms tend to result in a weaker (stronger) surface freshening with larger (smaller) moving tropical storms tend to result in a weaker (stronger) surface freshening with larger area coverage. (smaller) area coverage. After the passage of TCs, the positive SSS response on days 1–2 for slower storms is concentrated near the center and rear quadrants, while greater negative SSS response is observed in the front quadrants (Supplementary Figure S6 and Figure 12). As more fresh- water is accumulated for slower cases (Supplementary Figure S5), the salinity stratification is enhanced, which inhibits the vertical mixing and surface salinification. Compared to slower cases, faster-moving TCs lead to weaker negative SSS anomalies with smaller area coverage in the front quadrants because of the less freshwater accumulation. Although slower-moving TCs tend to have larger momentum and evaporative fluxes and induce Remote Sens. 2021, 13, x FOR PEER REVIEW 18 of 27

After the passage of TCs, the positive SSS response on days 1–2 for slower storms is concentrated near the center and rear quadrants, while greater negative SSS response is observed in the front quadrants (Supplementary Figure S6 and Figure 12). As more freshwater is accumulated for slower cases (Supplementary Figure S5), the salinity stratification is enhanced, which inhibits the vertical mixing and surface salinification. Compared to slower cases, faster-moving TCs lead to weaker negative SSS anomalies with Remote Sens. 2021, 13, 420 smaller area coverage in the front quadrants because of the less freshwater accumulation.17 of 25 Although slower-moving TCs tend to have larger momentum and evaporative fluxes and induce cooler SST, the value of the maximum positive SSS response is comparable to the faster-movingcooler SST, theTCs, value as there of the is maximum more fres positivehwater SSSaccumulated response is and comparable stronger to surface the faster- fresheningmoving during TCs, as TCs, there offsetting is more the freshwater effect of accumulatedenhanced evaporation and stronger and mixing. surface freshening during TCs, offsetting the effect of enhanced evaporation and mixing.

Figure 12. Distribution of SSS response (psu) to Category 1–5 TCs on days 1–2 in the Northern Hemisphere based on translation speed, with SMAP (upper panels) and SMOS (lower panels) swath Figuredaily 12. data. Distribution Same time of SSS period response from March (psu) to 2015 Category to December 1–5 TCs 2019 on isdays used 1–2 for in both the Northern SMAP and SMOS Hemispheresatellite data.based The on meantranslation translation speed, speed with isSMAP 2.67 and(upper 7.44 panels) m s−1, and respectively. SMOS (lower panels) swath daily data. Same time period from March 2015 to December 2019 is used for both SMAP and 3.3.SMOS Ocean satellite Response data. The in Different mean translation Basins speed is 2.67 and 7.44 m s−1, respectively. The mean ocean tropical salinity and temperature stratification is characterized with 3.3. fresherOcean Response and warmer in Different surface Basins water and saltier and cooler subsurface water in both Northern andThe Southern mean ocean Hemisphere tropical salinity during and the temperature reference period stratification (Figure8 ).is However,characterized the surfacewith freshersalinity and and warmer salinity surface stratification water differand markedlysaltier and between cooler differentsubsurface basins, water and in we both choose Northernfour basins and Southern to investigate Hemisphere and illustrate during the the influence reference of period background (Figure salinity 8). However, and tempera- the surfaceture salinity stratification and salinity on ocean stratification response to differ TCs (Figure markedly 13). Thebetween Arabian different Sea (AS) basins, and theand Bay we ofchoose Bengal four (BoB) basins are to two investigate basins in and the northernillustrate Indianthe influence Ocean, of each background with distinct salinity surface andsalinity temperature and salinity stratification stratification. on ocean The response BoB is one to ofTCs the (Figure freshest 13). regions The Arabian in tropics Sea as (AS) a result of oceanic rainfall and river runoff. Compared to the BoB, the AS SSS is higher mainly due to enhanced evaporation and vertical mixing of surface water with subsurface water [67]. The Atlantic presents two other contrasting subbasins. The Subtropical North Atlantic

(SNA) exhibits the highest SSS values worldwide with mean SSS exceeding 37 psu [68,69]. Lower surface salinity in the Amazon–Orinoco Plume (AOP) region is attributed to the spreading of freshwater plumes from the Amazon and Orinoco River. Remote Sens. 2021, 13, x FOR PEER REVIEW 19 of 27

and the Bay of Bengal (BoB) are two basins in the northern Indian Ocean, each with distinct surface salinity and salinity stratification. The BoB is one of the freshest regions in tropics as a result of oceanic rainfall and river runoff. Compared to the BoB, the AS SSS is higher mainly due to enhanced evaporation and vertical mixing of surface water with subsurface water [67]. The Atlantic presents two other contrasting subbasins. The Subtropical North Atlantic (SNA) exhibits the highest SSS values worldwide with mean SSS exceeding 37 psu [68,69]. Lower surface salinity in the Amazon–Orinoco Plume (AOP) Remote Sens. 2021, 13, 420 region is attributed to the spreading of freshwater plumes from the Amazon and Orinoco18 of 25 River.

Figure 13. Distribution of the mean SSS (psu) during the Northern Hemisphere TC seasons (June to Figure 13. Distribution of the mean SSS (psu) during the Northern Hemisphere TC seasons (June November), with SMAP swath daily data. Four basins are highlighted with blue boxes: Arabian Sea to November),◦ ◦ with◦ SMAP◦ swath daily data. Four◦ basins◦ are◦ highlighted◦ with blue boxes: Arabian Sea(AS; (AS; 5 –255°–25°N,N, 40 40°–80°E);–80 E); Bay Bay of of Bengal Bengal (BoB; (BoB; 5 –255°–25°N,N, 80 80°–100°E);–100 E); Subtropical Subtropical North North Atlantic Atlantic (SNA; ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ (SNA;20 –40 20°–40°N,N, 100 –20100°–20°W)W) and Amazon–Orinocoand Amazon–Orinoco plume plume region region (AOP; (AOP; 0 –20 0°–20°N,N, 70 –40 70°–40°W).W). Figure 14 compares the Argo-derived temperature, salinity, and density stratification Figure 14 compares the Argo-derived temperature, salinity, and density stratification before the passage of TCs during the reference time period in four regions. BoB and AOP before the passage of TCs during the reference time period in four regions. BoB and AOP display greater salinity and density stratification, resulting in an approximately 20–30 m display greater salinity and density stratification, resulting in an approximately 20–30 m mean barrier layer, while the ocean preconditions in the AS and SNA are characterized by mean barrier layer, while the ocean preconditions in the AS and SNA are characterized weaker stratification and higher surface salinity. On average, the subsurface water in the by weaker stratification and higher surface salinity. On average, the subsurface water in SNA (AS) is slightly saltier (fresher) than the surface. the SNA (AS) is slightly saltier (fresher) than the surface. The composite evolution of SSS anomalies for each of the four basins are presented in Figure 15. To obtain more samples, both Aquarius and SMAP satellite data are used (Figure 15). The SSS response time series for these four basins calculated with SMOS satellite data (Supplementary Figure S7) shows similar features. The SSS anomaly calculated with satellite swath daily data exhibits great surface freshening on day 0 in both AS and SNA but much smaller freshening in the AOP. The initial surface freshening due to precipitation is not obvious during the passage of TCs in the BoB. As the SSS under high surface wind is removed from the swath daily data, the surface response on day 0 to intense TCs may not be well displayed. Because of the small sample size in each basin, the Aquarius and SMAP seven-/eight-day running mean data (Figure 15) and SMOS ten-day running mean data (Supplementary Figure S7) are applied to investigate the SSS response to different intensity categories after TCs overpass. It is found that the TC-induced mixing produces a weaker positive response even for intense TCs due to weaker salinity stratification in the SNA. The composite SSS response after TC passage is negative in the AS because of the fresher subsurface salinity (Figure 14), thus TCs in the AS eventually result in a fresher surface. In contrast, TCs in the BoB lead to strong surface salinification. Note that the y-axis of Figure 15b is modified to −1.5 to 2 psu (−1.5 to 3 psu in Supplementary Figure S7b). The mean salinification reaches the maximum on day 2 with the positive SSS response exceeding 0.5 psu, 10 times of the global mean SSS response on day 2. Relatively intense TCs (Category 3–5) in the BoB caused extremely large surface salinification. The amplitude of the positive SSS response after the passage of TCs is as high as 1.5 psu for Category 5 TCs, consistent with the dependence of SSS response amplitude to TCs on vertical salinity gradients and storm intensity shown in Reul et al. (their Figure3c, [ 46]). The mean salinification after the passage of TCs in AOP is approximately 0.1 psu, two times of the global mean. However, Category 4–5 TCs exhibit much higher surface salinification Remote Sens. 2021, 13, 420 19 of 25

(~0.6 psu) than the mean salinification in AOP, which is consistent with the case studies mentioned in the introduction. A possible interpretation is that the most intense TCs Remote Sens. 2021, 13, x FOR PEER REVIEW(Category 4–5) tend to develop after a strong river plume, and we hypothesize that fresher20 of 27

surface results in stronger vertical stratification, reduces the SST cooling, and facilitates the TC intensification.

Figure 14. Composite temperature, salinity and density profiles during the reference time period (solidFigure line) 14. Composite and averaged temperature, 1–5 days after salinity the passageand density of the profiles TCs (dashed during line) the reference for all storms time averagedperiod within(solid line) the centered and averaged 2◦ × 2 1–5◦ box days during after 2002 the topa 2019ssage in of the the (a TCs) AS, (dashed (b) BoB, line) (c) SNA for all and storms (d) AOP, with averaged within the centered 2° × 2° box during 2002 to 2019 in the (a) AS, (b) BoB, (c) SNA and in-situ data from Argo floats. (d) AOP, with in-situ data from Argo floats. The along-track composite of SSS response during and after the passage of TCs are comparedThe composite in Figure evolution16. The distribution of SSS anomalies of the fo along-trackr each of the SSS four response basins inare the presented SNA is similarin Figure to 15. the To mean obtain SSS more response samples, to all both storms Aquarius in the and Northern SMAP Hemisphere satellite data (Figure are used7). However,(Figure 15). the The surface SSS response intime the series AS shows for these a notably four differentbasins calculated pattern: thewith stronger SMOS surfacesatellite fresheningdata (Supplementary on day 0 is followed Figure withS7) shows weaker similar surface features. freshening The after SSS the anomaly passage ofcalculated TCs. As with a result satellite of the swath higher daily surface data exhibi salinity,ts great the vertical surface mixing fresheni inducesng on day subsurface 0 in both salinificationAS and SNA but in the much AS aftersmaller the freshening passage of in TCs the (Figure AOP. The14a). initial The subsurfacesurface freshening response due in theto precipitation AS is therefore is not distinct obvious from during the subsurface the passage response of TCs in in the the SNA BoB. (Figure As the 14 SSSc) and under the compositehigh surface mean wind subsurface is removed response from the in theswath Northern daily data, Hemisphere the surface (Figure response8), where on day strong 0 to mixingintense bringsTCs may surface not be freshwater well displayed. to the Because subsurface, of the and small reduces sample the size subsurface in each salinity.basin, the AquariusIt is also and confirmed SMAP seven-/eight-day in Figure 16 that running the surface mean salinitydata (Figure response 15) and to TCs SMOS in the ten-day SNA andrunning AS ismean much data weaker (Supplementa than thatry of Figure the BoB S7) andare applied AOP, as to would investigate be expected the SSS fromresponse the weakerto different salinity intensity stratification categories in theafter SNA TCsand over ASpass. basins. It is found In the that BoB the and TC-induced AOP, entrainment mixing actingproduces on thea weaker strong salinity positive stratification response makeseven for the initialintense surface TCs fresheningdue to weaker weaker salinity but is conducivestratification to in substantial the SNA. salinificationThe composite of SSS the response sea surface after after TC TCs passage pass. is The negative asymmetric in the patternAS because for the of averagethe fresher response subsurface is also salinity remarkable. (Figure The 14), SSS thus positive TCs responsein the AS spans eventually over a largeresult area in a andfresher mainly surface. on the In right-handcontrast, TCs side in of the storm BoB motionlead to instrong BoB andsurface AOP. salinification. Besides the surfaceNote that salinification, the y-axis subsurfaceof Figure salinification15b is modified is also to significant −1.5 to 2 in psu the upper(−1.5 to 80 3 m psu owing in toSupplementary the strong salinity Figure stratification S7b). The mean in these salini twofication basins reaches (Figure the 14b,d). maximum on day 2 with the positive SSS response exceeding 0.5 psu, 10 times of the global mean SSS response on day 2. Relatively intense TCs (Category 3–5) in the BoB caused extremely large surface salinification. The amplitude of the positive SSS response after the passage of TCs is as high as 1.5 psu for Category 5 TCs, consistent with the dependence of SSS response amplitude to TCs on vertical salinity gradients and storm intensity shown in Reul et al. (their Figure 3c, [46]). The mean salinification after the passage of TCs in AOP is approximately 0.1 psu, two times of the global mean. However, Category 4–5 TCs exhibit much higher surface salinification (~0.6 psu) than the mean salinification in AOP, which is consistent with the case studies mentioned in the introduction. A possible interpretation

Remote Sens. 2021, 13, x FOR PEER REVIEW 21 of 27

is that the most intense TCs (Category 4–5) tend to develop after a strong river plume, and Remote Sens. 2021, 13, 420 20 of 25 we hypothesize that fresher surface results in stronger vertical stratification, reduces the SST cooling, and facilitates the TC intensification.

Figure 15. Lagrangian composite SSS anomaly relative to the reference SSS during the period 2011–2019,Figure 15. Lagrangian from Aquarius composite and SSS SMAP anom seven/eight-dayaly relative to the runningreference meanSSS during daily the data period (L3) for the (a) AS, 2011–2019, from Aquarius and SMAP seven/eight-day running mean daily data (L3) for the (a) AS, (b) BoB, (c) SNA, and (d) AOP. The dashed blue lines are Lagrangian composite SSS response for all (b) BoB, (c) SNA, and (d) AOP. The dashed blue lines are Lagrangian composite SSS response for stormsall storms calculated calculated with with Aquarius Aquarius andand SMAPSMAP swath swath daily daily data data (L2) (L2) with with high high surface surface wind wind raised SSS Remote Sens. 2021, 13, x FOR PEER REVIEWremoved.raised SSS Theremoved. range The of y-axisrange of is y-axis 1.3 psu is for1.3 psu (a), (forc) and(a), (c (d) and) while (d) while 3.5 psu 3.5 for psu (b for). For(b). reference,For 22 the of 0 27 to 1.3reference, psu range the 0 is to marked 1.3 psu range in red is line marked in panel in red (b line). in panel (b).

The along-track composite of SSS response during and after the passage of TCs are compared in Figure 16. The distribution of the along-track SSS response in the SNA is similar to the mean SSS response to all storms in the Northern Hemisphere (Figure 7). However, the surface response in the AS shows a notably different pattern: the stronger surface freshening on day 0 is followed with weaker surface freshening after the passage of TCs. As a result of the higher surface salinity, the vertical mixing induces subsurface salinification in the AS after the passage of TCs (Figure 14a). The subsurface response in the AS is therefore distinct from the subsurface response in the SNA (Figure 14c) and the composite mean subsurface response in the Northern Hemisphere (Figure 8), where strong mixing brings surface freshwater to the subsurface, and reduces the subsurface salinity. It is also confirmed in Figure 16 that the surface salinity response to TCs in the SNA and AS is much weaker than that of the BoB and AOP, as would be expected from the weaker salinity stratification in the SNA and AS basins. In the BoB and AOP, entrainment acting on the strong salinity stratification makes the initial surface freshening weaker but is conducive to substantial salinification of the sea surface after TCs pass. The asymmetric pattern for the average response is also remarkable. The SSS positive response spans over a large area and mainly on the right-hand side of storm motion in BoB and AOP. Besides the surface salinification, subsurface salinification is also significant in the upper 80 m owing to the strong salinity stratification in these two basins (Figure 14b,d).

FigureFigure 16. 16. Along-trackAlong-track averaged averaged composite composite of of SSS SSS anomal anomalyy relative relative to to the the reference reference SSS SSS calculated calculated withwith SMAP SMAP swath swath daily daily data data (psu) (psu) and and SMOS SMOS swath swath daily daily data data (psu) (psu) for for basin basin AS, AS, BoB, BoB, SNA SNA and and AOP.AOP. Both Both the the SMAP SMAP and and SMOS SMOS SSS SSS associated associated wi withth high high surface surface wind wind are areremoved. removed. The gray The gray shading indicates the region where more than 30% of the SSS pixels are removed. The y-axis is the shading indicates the region where more than 30% of the SSS pixels are removed. The y-axis is the cross-track ±4° to the right and left of the track. Composite SSS anomaly along-track ±4° to the cross-track ±4◦ to the right and left of the track. Composite SSS anomaly along-track ±4◦ to the center is averaged from days −5 to 20. center is averaged from days −5 to 20. 4. Discussion and Conclusions In this study, the spatial and temporal signatures of ocean salinity response to TCs are investigated with Aquarius, SMAP, and SMOS satellite SSS data and in-situ Argo profiles, which show distinct characteristics from the ocean temperature response. The storm-following composites of ocean haline response are also analyzed for different TC intensity, translation speed, and basins. On average, as a TC passes over a position, rapid surface cooling is induced with maximum SST response reaching one or two days after the TC passes ([4,5]; Figures 2 and 7). The passage of TCs produces a cold wake that is enhanced to the right (left) of 2storm motion in the Northern (Southern) Hemisphere, and the recovery time is more than 20 days. Typically, strong winds cause a negative air-sea enthalpy flux and drive the cold water from below entrain into the mixed layer, cooling the surface. In addition, TC rainfall may also lead to surface cooling as rainfall is often colder than the surface ocean. Previous studies estimated a no more than 10% contribution of surface heat loss by TC rainfall [70,71]. The response of the SSS to TCs exhibits significantly different features from the SST response. Our results show that the SSS response to TCs is initially dominated by the TC induced rainfall and subsequently controlled by wind driven vertical mixing and upwelling. The dominance of precipitation over evaporation in the net salinity budget near the storm indicates that much of the atmospheric moisture necessary to sustain the storm rainfall is evaporated at a distance of at least several hundred kilometers away from the storm center. On average, SSS decreases significantly on day 0 due to the net surface freshwater flux (positive P-E), with negative SSS anomaly averaged over the TC-centered 2° box more than 0.60 psu when SMAP swath data are used. Strong surface freshening is followed by weaker and longer-lasting surface salinification after the passage of TCs.

Remote Sens. 2021, 13, 420 21 of 25

4. Discussion and Conclusions In this study, the spatial and temporal signatures of ocean salinity response to TCs are investigated with Aquarius, SMAP, and SMOS satellite SSS data and in-situ Argo profiles, which show distinct characteristics from the ocean temperature response. The storm-following composites of ocean haline response are also analyzed for different TC intensity, translation speed, and basins. On average, as a TC passes over a position, rapid surface cooling is induced with maximum SST response reaching one or two days after the TC passes ([4,5]; Figures2 and7) . The passage of TCs produces a cold wake that is enhanced to the right (left) of 2storm motion in the Northern (Southern) Hemisphere, and the recovery time is more than 20 days. Typically, strong winds cause a negative air-sea enthalpy flux and drive the cold water from below entrain into the mixed layer, cooling the surface. In addition, TC rainfall may also lead to surface cooling as rainfall is often colder than the surface ocean. Previous studies estimated a no more than 10% contribution of surface heat loss by TC rainfall [70,71]. The response of the SSS to TCs exhibits significantly different features from the SST response. Our results show that the SSS response to TCs is initially dominated by the TC induced rainfall and subsequently controlled by wind driven vertical mixing and upwelling. The dominance of precipitation over evaporation in the net salinity budget near the storm indicates that much of the atmospheric moisture necessary to sustain the storm rainfall is evaporated at a distance of at least several hundred kilometers away from the storm center. On average, SSS decreases significantly on day 0 due to the net surface freshwater flux (positive P-E), with negative SSS anomaly averaged over the TC-centered 2◦ box more than 0.60 psu when SMAP swath data are used. Strong surface freshening is followed by weaker and longer-lasting surface salinification after the passage of TCs. Vertical mixing and upwelling act to entrain the saltier subsurface water, leaving behind a right (left) biased wake of higher SSS in the Northern (Southern) Hemisphere with recovery time more than 15 days after the passage of TCs, as the storm wind vector on the right (left)-hand side rotates clockwise (anticlockwise) in the same sense as the inertial oscillation. With combined SMAP and SMOS satellite data, Reul et al. [46] also found that the passing hurricanes induced surface salinification with maximum positive anomaly on the right (left)-hand side in the Northern (Southern) Hemisphere at around twice the maximum wind radius. The comparable recovery time of SSS and SST is somewhat perplexing since surface heat fluxes would tend to damp SST anomalies, but net surface salinity fluxes are not fundamentally dependent on background salinity. The mechanisms for the relaxation of the storm induced SSS signal merit further exploration. The Argo-derived composite profiles reveal that the ocean temperature response to TCs is characterized by stronger subsurface cooling in above 40 m and weaker cooling in the layer below, which would act to decrease ocean temperature and density stratification. However, subsurface freshening is observed on both sides of storms with stronger and deeper freshening on the right (left)-hand side in the Northern (Southern) Hemisphere. The ocean haline response on TCs may therefore act to increase the vertical density stratification. The influence of TC intensity and translation speed on the surface response is also examined by dividing the Northern Hemisphere TCs into different groups. Although more intense TCs tend to input more freshwater flux into the ocean, the surface negative response of different intensity categories is eventually replaced by the surface salinification. It is found that longer-lasting and greater surface salinification is induced by stronger TCs due to stronger vertical mixing and upwelling. The saltier surface response for more intense TCs is also presented by Reul et al. [46]. It is distinct from the nonmonotonic SST response to different intensity categories: maximum cooling is observed in Category 3 TCs as extreme TCs tend to occur in the ocean with weaker upper-ocean stratification and larger ocean heat content (Figure2d; [ 4,5]). Besides stronger surface salinification, stronger TCs account for the larger and deeper subsurface upper layer salinity increase, especially on the right-hand side of storm motion in the Northern Hemisphere. Remote Sens. 2021, 13, 420 22 of 25

As to the ocean response to TCs with different translation speed, satellite observations indicate stronger (weaker) surface freshening during TCs but comparable surface salinifi- cation on days 1–2 after the passage of TCs for slow (fast)-moving TCs due to the negative effect of the TC-induced rainfall on vertical mixing. However, surface cooling after the passage of TCs tends to be larger for slow-moving than fast-moving TCs (e.g., Figure2,[ 4]). Reul et al. [46] investigated the averaged SSS response on days 0–5 to TCs with different translation speed. The nondimensional storm translation speed τ is used in their study, which is defined by the ratio of local inertial period to the TC residence time. The surface salinification for TCs with smaller τ (τ < 1) is found two times larger than that of larger τ (τ > 1) because of the longer storm residence and stronger vertical mixing [4]. The ocean response in four illustrative basins with different ocean climatological salinity stratification are compared. Strong salinity stratification enhances the positive SSS response following the passage of a TC in the BoB and AOP region. In contrast, the weaker salinity stratification in the AS and SNA results in a more muted response of salinity after the TC passage and a greater tendency for freshening during the TC. Previous case studies have found similar results to these composite analyses. For example, Grodsky et al. [26] found that the passage of Hurricane Katia (2011) resulted in a 1.5 psu positive salinity wake with weekly averaged Aquarius and SMOS satellite data in the AOP region. Based on ten-day averaged SMOS satellite data, a similarly strong surface salinification was also observed after Hurricane Igor (2011) in the AOP region [27]. Chacko [24] reported that BoB TC Vardah (2016) induced a significant increase of SSS (up to 1.5 psu) on the right of the storm with eight-day running SMAP data. These results are consistent with the composite salinity response to TCs in the AOP and the BoB region, in which the ocean response is characterized by strong positive salinity anomaly while surface freshening is hardly observed. Reul et al. [46] have also found that the vertical salinity stratification before TCs significantly contributes to the regional variability of SSS response to TCs. Consistent with our study, striking surface salinification is found in the BoB and AOP region due to the fresh surface river plume and stronger vertical salinity stratification. Besides BoB and AOP, river plume regions of Mississippi, Pearl, and Yangtze are also characterized with strong salinification after the passage of TCs. On the contrary, in the AS and SNA the mean SSS response is dominated by surface freshening on days 0–5. The negative surface response after the TC passage in the AS is consistent with our results. In the SNA, our results show striking surface freshening on day 0 and very weak surface salinification during days 2–8. Both the results in our study and that of Reul et al. [46] emphasize the significant dependency of ocean salinity response on the pre-storm ocean salinity stratification.

Supplementary Materials: The following are available online at https://www.mdpi.com/2072-429 2/13/3/420/s1, Figure S1: Global TC tracks during the period 2010–2019, from IBTrACS data. Colors indicate the TC category based on the Saffir-Simpson scale from Tropical Depression to Category 5. Figure S2: Lagrangian composite of daily accumulated evaporation during the same period with Figure2, from ERA5. Figure S3: The percentage of the SSS pixels retained for TS, Category 1 and Category 2 TCs after removing the SSS associated with the surface wind speed higher than 15 m s−1 from the SMAP L2 swath daily data. Figure S4: The percentage of the SSS pixels retained for Categories 3 to 5 TCs after removing the SSS associated with the surface wind speed higher than 15 m s−1 from the SMAP L2 swath daily data. Figure S5: Distribution of daily accumulated P-E (mm) on day 0 in the Northern Hemisphere based on translation speed during the same time period with Figure 11. The mean translation speed in upper (lower) panels is 2.48 (2.67) and 7.80 (7.44) m s−1, respectively. Figure S6: Distribution of SSS response (psu) to tropical storms on days 1–2 based on translation speed in the Northern Hemisphere, with SMAP (upper panels) and SMOS (lower panels) swath daily data. Same time period with Figure 12 is used for both SMAP and SMOS satellite data. Figure S7: The same with Figure 15, but with SMOS satellite data. The range of y-axis is 1.3 psu for (a), (c) and (d) while 4.5 psu for (b). Author Contributions: Conceptualization, J.S. and G.V.; methodology, J.S., G.V. and B.S.; validation, J.S., G.V. and B.S.; formal analysis, J.S.; writing—original draft preparation, J.S.; writing—review and Remote Sens. 2021, 13, 420 23 of 25

editing, J.S., G.V. and B.S.; visualization, J.S.; project administration, G.V.; funding acquisition, G.V. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded in part by Award 80NSSC19K0482 from the National Aeronautics and Space Administration and by the Carbon Mitigation Initiative at Princeton University. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: All Aquarius and SMAP satellite data created or used during this study are openly available from Physical Oceanography Distributed Active Archive Center at https: //podaac.jpl.nasa.gov/datasetlist. SMOS satellite salinity data used during this study are openly available from ESA SMOS Online Dissemination Service at https://smos-diss.eo.esa.int/smos/faq. html. OISST used in this study is openly available from http://www.remss.com/measurements/ sea-surface-temperature/. NCEP reanalysis data used in this study are openly available from NOAA Physical Sciences Laboratory at https://psl.noaa.gov/data/gridded/data.ncep.reanalysis. html. GPM precipitation data used in this study are openly available from https://disc.gsfc.nasa. gov/datasets/GPM_3IMERGDF_06/summary. ERA5 evaporation data used during this study are openly available from Copernicus Climate Change Service at https://www.ecmwf.int/en/forecasts/ datasets/reanalysis-datasets/era5. Argo in-situ data used in this study are openly available from North Carolina Government Data Analytics Center at ftp://ftp.ifremer.fr/ifremer/argo. Acknowledgments: The authors want to thank all the satellite groups participating in Aquarius, SMAP, SMOS, etc. for producing and sharing the satellite remote sensing data. Conflicts of Interest: The authors declare no conflict of interest.

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