Surface and Subsurface Heatwaves in the Hypersaline Dead Sea Caused by Severe Dust Intrusion

Abstract

The relationship between global warming and heatwaves contributes to environmental risks. We investigate lake heatwaves (LHWs) in the Eastern Mediterranean, where dust intrusions are frequently observed. The dust intrusions are characterized by the arrival of warm air masses containing dust pollution from the desert. In saline lakes, LHWs caused by dust intrusions have not been investigated in previous studies. In our study we focus on this point. It was found for the first time that, in the hypersaline Dead Sea, a severe dust intrusion (aerosol optical depth of over 3) caused the formation of LHWs, as appeared in September 2015. At the water surface, the LHWs were represented by abnormally high daily maximal and minimal surface water temperature (SWT) in comparison with their seasonally varied 90th percentile thresholds for 10 consecutive days (7–17 September). The surface LHWs’ intensity was up to 3 °C. Satellite (MODIS-Terra and METEOSAT) SWT did not detect the LHWs. Surface LHWs were accompanied by subsurface LHWs down to a depth of 20 m. The subsurface LHWs lasted longer (16 days) than the surface LHWs (10 days). There was a 4-day delay between the first date of the surface LHWs (7 September) and the start date of the subsurface LHWs (11 September). The maximal intensity of the subsurface LHWs decreased with depth from 1 m (0.6 °C) down to 5 m (0.3 °C), followed by an increase (up to 0.6 °C) at the deeper layers (from 10 m to 20 m). Taking into account that, over the Eastern Mediterranean, desert dust has increased during the past several decades, one can expect frequent occurrence of dust-related intense persistent heatwaves in the Dead Sea in the coming years. This will contribute to additional water heating and further drying up of the Dead Sea.

1. Introduction

Saline lakes are located in mostly arid regions: they represent over 40% of the volume and over 20% of the area of all lakes on Earth, according to Messager et al. [1]. Saline lakes are sensitive to climate variability causing disturbances in their water balance [2]. This is due to anthropogenic activity, associated with water withdrawal and increasing regional warming [2]. The hypersaline Dead Sea is located at a distinctive location of ~400 m b.s.l. (below sea level). The Dead Sea is a rich source of minerals. These minerals are being harvested by the chemical industries of both Jordan and Israel [3]. In recent decades, the Dead Sea region has faced essential environmental problems, such as water resource scarcity, sinkhole formation, and seismic disturbance risks [4]. The drying up of the Dead Sea, which was observed during the past three decades, was caused mainly by the lack of water inflow from the Jordan River, as well as by decreasing precipitation and increasing evaporation [4,5,6]. The Dead Sea drying up was accompanied by the noticeable shrinking of its water area [5,7]. During the past two decades, the Moderate-Resolution Imaging Spectroradiometer (MODIS) recorded the increasing Dead Sea SWT trend of up to 0.6 °C decade⁻¹ [5,8]. Kishcha et al. [5] discussed a positive feedback loop between the shrinking of the Dead Sea water area and the observed SWT trend: this positive feedback loop contributed to acceleration in the lake water level drop.

Lake heatwaves (LHWs) are typically defined as prolonged periods of abnormally high SWT in comparison with its 90th PTH for at least five consecutive days [9,10,11]. Despite the growing body of literature on heatwaves, their understanding is still incomplete [12]. As an example, little is known about LHWs in Eastern Mediterranean lakes, including the Dead Sea. Regional warming over the Eastern Mediterranean since the end of the 20th century has been accompanied by steadily increasing atmospheric dust intrusions from active deserts located in this area [13,14,15,16,17]. Dust intrusions are characterized by the arrival of warm air masses containing dust pollution from the desert. These intrusions could produce both warming and cooling effects on the atmosphere and surface below. First, intrusions of warm air masses from deserts contribute to air heating in the near-ground atmospheric layer, which is in contact with the surface below. In addition, dust particles absorb both shortwave solar radiation and longwave terrestrial radiation [18,19,20], contributing to additional air heating. On the other hand, in the daytime, dust particles scatter shortwave solar radiation, reducing the incoming solar radiation and consequently causing cooling of the surface below [18]. A comprehensive understanding of the role of dust intrusions in the development of LHWs in Eastern Mediterranean lakes is essential to reduce the future disappearance of lakes due to increasing evaporation.

The impact of dust intrusion on lake water temperature (WT) was discussed in our paper published in 2023 (Kishcha et al. [21]). We found that a severe dust intrusion occurring in September 2015 caused water heating in fresh-water Lake Kinneret located ~100 km north of the hypersaline Dead Sea. Furthermore, in our most recent study published in 2024 (Kishcha et al. [22]), we discussed a particular phenomenon, namely LHWs in Lake Kinneret lasting for 10 consecutive days from 7 to 17 September 2015. Their intensity was up to 3 °C [22]. The fact that fresh-water Lake Kinneret is located near the hypersaline Dead Sea raises the following question: Could the same dust intrusion cause intense and persistent LHWs in the hypersaline Dead Sea as well? Taking into account that, in the Eastern Mediterranean, desert dust has increased over the past several decades, knowledge about the development of LHWs in saline lakes by dust intrusion has become crucial in understanding this phenomenon.

In this study, we analyzed surface and subsurface LHWs caused by severe dust intrusion in the hypersaline Dead Sea which appeared in September 2015. These LHWs were investigated using both buoy WT measurements and satellite observations.

2. Materials and Methods

2.1. Dead Sea Study Area

The length of the Dead Sea from north to south is ~45 km and its length from east to west is ~15 km (Figure 1). The Dead Sea is approximately 300 m deep. Because of the high salinity of Dead Sea water (~340 g/kg), up to 90% of SR is absorbed in the uppermost water layer down to a depth of 2 m, producing considerable water heating [23].

Desert dust is frequently observed over the Dead Sea (e.g., Singer et al. [24,25]). This is due to both long-range dust transport from remote dust sources and local emissions in the Dead Sea valley. In the Dead Sea valley, distinct wind systems can be observed, such as Dead Sea breezes, slope winds, foehn winds, and Mediterranean Sea breezes [26,27,28,29,30]. In the arid Dead Sea valley, such wind systems with strong wind speeds cause noticeable dust emissions. Frequent dust intrusions into the Dead Sea valley were responsible for noticeable dust dry deposition in this area [31]. An increasing trend in dust deposition was found over northern and central Israel in autumn due to increasing dust penetration from the Syrian deserts [31].

Haze layers occur above the Dead Sea, when the atmosphere is stable and the weather is fair [3]. The haze layers usually form in early morning and dissipate in late afternoon [3].

The thermal structure of subsurface Dead Sea water was investigated using WT vertical profile measurements [32,33]. A stable thermocline is formed during the last week of March [32]. Later, evaporation causes an increase in water salinity in the upper layer (above the thermocline). Maximum water salinity (340 g/kg) is reached at the same time as the maximum WT in the upper layer (~35 °C). The thickness of the upper layer is approximately 15 m in June and 25 m in September ([32], their Figure 6). The phenomenon of double-diffusive processes in the ocean, discovered by Stommel et al. [34] and Stern [35], is also observed in the hypersaline Dead Sea [36,37]. Arnon et al. [36] discussed the salinity and temperature differences between the upper water layer (above the thermocline) and the bottom water layer (below the thermocline) in the Dead Sea. During the warm season, water heating and evaporation led to higher temperature and salinity in the upper layer than in the bottom layer. This caused instability at the interface between the two layers and promoted mixing by means of double-diffusive fluxes [36]. According to Kishcha et al. [37], on 22 March 2013, a 1 h sharp drop in noon SR from 900 to 50 W/m² caused a noticeable increase in WT in the uppermost layer of the Dead Sea. The high SWT caused an increase in evaporation and consequently an increase in water salinity in the skin surface layer. The increased salinity and density caused the warm skin water to submerge, thereby contributing to water heating in the layers below [37].

It is worth noting that, aside from some microorganisms and algae, the Dead Sea is completely devoid of life. There are no seaweeds, fishes or any other creatures found in this hypersaline lake (Bardell [38]).

2.2. Severe Dust Intrusion in September 2015

In September 2015, mineral dust originating from the desert areas of northern Syria created a severe dust intrusion over Israel, including the Dead Sea valley [39,40,41]. Satellite imagery from MODIS-Terra started showing dust over the Dead Sea on 7 September, and large amounts of dust appeared on 8 and 9 September ([21], their Figure 2). During the severe intrusion, dust plumes were transported to the southwest [39,40,41]. During the dust intrusion, the CALIPSO satellite recorded a dust layer up to 5 km above sea level over the Dead Sea valley [40].

2.3. Method

The dust intrusion caused significant changes in air temperature (Tair), solar radiation (SR), and Dead Sea WT at various depths. To investigate the dust impact on each of the aforementioned parameters, we used a similar approach. Our approach is explained below using Tair-MAX as an example. In accordance with previous studies on marine and lake heatwaves [9,10,11,42], we conducted a comparison between time variations of Tair-MAX in September 2015 and its seasonally varying 90th PTH. The exceedance of Tair-MAX over its 90th PTH for at least five consecutive days was defined as a heatwave at the Dead Sea. The seasonally varying 90th PTH for Tair-MAX was obtained for each day in September 2015 by using the running 31-day window (centered on each day from 1 to 30 September) for all years during the baseline period (2007–2016). We used regular measurements of Tair, SR, and Dead Sea WT at various depths from August to October from 2007 to 2016 to represent the baseline period (2015 was not included as it was under investigation in this study).

To characterize quantitatively LHWs at each depth in the Dead Sea, we used anomalies of WT-MAX and WT-MIN from their seasonally varying 90th PTHs. In accordance with Hobday et al. [42], the following metrics were used: (1) start and end dates on which the LHW began and ended; (2) duration, which was the number $\left(N\right)$ of consecutive days that temperature exceeded the threshold; (3) maximal intensity $\left(MI\right)$ (°C), which was the highest temperature anomaly value during the LHW; and (4) cumulative intensity ($CI$) (°C∙day), which was determined as the sum of daily intensities during the LHW, according to the following expression: $CI=\sum _{i=1}^{N}Daily Anomaly$, where $Daily Anomaly$ is the difference between the observed WT and its 90th PTH for each day of the LHW, and $N$ is the LHW durations in days.

2.4. Data

We used available meteorological measurements from the Dead Sea hydrometeorological buoy (31.42° N, 35.44° E), anchored in the Dead Sea, ~5 km offshore from Ein Gedi. (Figure 2). The meteorological measurements (such as Tair, wind speed (WS), and SR) were conducted at the height of 2–3 m above the sea surface [37]. Water temperature vertical profiles were measured by the buoy down to a depth of 40 m: specifically at a depth of 1, 2, 5, 10, 15, 20, 25, 30, and 40 m. All above-mentioned in situ buoy measurements were obtained at 20 min intervals.

For measuring the incoming solar radiation (SR) (0.3–2.5 µm) we used the Solar Radiation Sensor 2770. The sensor specifications are available online at https://www.aanderaa.com/media/pdfs/Solar-Radiation-Sensor-2770.pdf (accessed on 21 April 2025). The sensor accuracy is better than ±20 W m⁻², and its resolution is 4 W m⁻². For measuring water temperature at a depth from 1 to 40 m, before May 2012, we used the AANDERAA Temperature string (https://www.aanderaa.com/, accessed on 21 April 2025). The sensor accuracy was 0.1 degree. Now there is no reference to this product on the AANDERAA site. Since May 2012 we have used SBE 39 string temperature sensors. The sensor specifications are available online at (https://www.seabird.com/sbe-39plus-with-plastic-housing-no-pressure-sensor-mcbh-connector-external-thermistor/product?id=54627895107&callback=pf) (accessed on 21 April 2025). The temperature accuracy is ±0.002 (−5 to +35 °C); ± 0.01 (+35 to +45 °C).

To study surface LHWs, the METEOSAT land surface temperature (LST) product (physical model) was used. The data were given on a 0.05° × 0.05° grid, according to https://wui.cmsaf.eu/safira/action/viewProduktHome (accessed on 21 April 2025). The hourly METEOSAT LST data contain LST retrievals and their estimated uncertainty [43,44]. We used METEOSAT data over the pixels within the Dead Sea water area (Figure 2a). Note that the Dead Sea buoy is located near the boundary between the two METEOSAT pixels. We compared buoy measurements of Tair (as SWT proxy [22]) with METEOSAT SWT averaged over the specified Dead Sea water area (31.40–31.45° N; 35.40–35.50° E) (Figure 2).

We also used Collection-6 (C6) of the MODIS MOD11A1 product from the NASA Terra satellite [45]. This product provides daily per-pixel land surface temperature data of 1 × 1 km² spatial resolution, at ~10:30 LT and ~22:30 LT. For consistency with METEOSAT SWT, we used MODIS SWT averaged over the specified Dead Sea water area (Figure 2a). Note that radiometers on board the satellites measure Dead Sea SWT in the surface skin layer of 10–20 µm [46].

To characterize dust pollution, we used daily Deep Blue Aerosol Optical Depth (AOD) data from the MOD08_D3 product during the period from 1 to 30 September 2015 [47]. The timeseries of AOD data was taken at the grid node (31.5° N, 35.5° E) located near the Dead Sea. The aforementioned satellite MODIS-Terra AOD data were complemented by the ground-based hourly measurements of total suspended particles (TSP) taken at the Lot monitoring site. This site was located in the south of the Dead Sea valley (31.069° N, 35.397° E) (Figure 1). TSP represents particulate matter with a characteristic size of less than 100 μm. The TSP measurements were conducted using the Thermo Scientific TM TEOM 1400a analyzer. The specifications of TM TEOM 1400a are available online at https://www.instrumentcompaniet.no/files/THERMO_Datablad/Thermo_Teom1400.pdf (accessed on 21 April 2025).

Similarly to our previous studies [21,22], all 20 min buoy data were processed as hourly means. For this purpose, buoy data were smoothed using a one-hour running mean. This was carried out for consistency with hourly METEOSAT SWT data.

3. Results

3.1. Dust Impact on SR

We analyzed the time series of daily Deep Blue Aerosol Optical Depth (AOD) data, taken at the grid node (31.5° N, 35.5° E) located within the Dead Sea valley. We found that from 6 to 16 September 2015, on each consecutive day, AOD exceeded 0.5, indicating the presence of dust pollution (Figure 3). On 8 September, dust pollution was maximal: it was characterized by AOD of over 3 (Figure 3). Then the AOD decreased until 16 September 2015.

As for the surface dust concentration, we used available measurements of total suspended particle (TSP) taken at the Lot monitoring site during the dust intrusion. This site was located in the south of the Dead Sea (Figure 1). The high TSP concentrations were measured from 7 to 10 September (Figure 4a). The maximal dust intrusion was characterized by TSP of up to 4200 µg m⁻³ on 8 September compared to TSP less than 100 µg m⁻³ on 6 September (Figure 4b). The TSP measurements were suspended on 10 September 2015 for technical reasons.

Buoy measurements of SR showed that the severe dust intrusion under study caused a sharp drop in SR due to the shortwave dust radiative effect. To illustrate this drop in SR, we compared day-to-day variations in SR-MAX with the baseline SR-MAX in September months during the baseline period (2007–2016) (Figure 5a).

A decrease in SR-MAX started after 6 September. On 8 September, in the presence of maximal dust pollution, SR-MAX reached its minimum of ~190 W m⁻². This was essentially lower than the baseline SR-MAX of ~860 W m⁻² on the corresponding day. From 8 to 11 September, the SR-MAX increased along with a decrease in AOD. Unexpectedly, after the dusty period, from 18 to 30 September 2015, the SR-MAX remained noticeably higher than the baseline SR-MAX (Figure 5a). To further examine this phenomenon, we compared day-to-day variations in SR-MAX with the seasonally varying 90th PTH for SR-MAX (Figure 5b). We found that, after the dusty period, from 18 to 30 September 2015, the SR-MAX continued increasing, and it exceeded the 90% PTH for the 10-day period (18–28 September) separated by only one day on 25 September. This exceedance of SR-MAX over its 90% PTH (after the disappearance of dust pollution) indicated that the atmosphere over the Dead Sea became clearer than usual. This phenomenon can be explained by the disappearance of haze particles due to the formation of atmospheric instability encouraging vertical motion (Section 4: Discussion).

Note that a similar analysis of SR-MAX at Lake Kinneret during the same dust intrusion in September 2015 showed a different picture. Specifically, after the dusty period (18–30 September), the SR-MAX occurred within the uncertainty of the baseline SR-MAX in September (Figure 6a). The latter was defined as the average SR-MAX in September months during the baseline period (2011–2023) [21]. Moreover, in Lake Kinneret, our comparison between the time variations of SR-MAX and its seasonally varying 90th PTH showed that after the disappearance of dust pollution on 18 September 2015, the SR-MAX was noticeably lower than its 90th PTH (Figure 6b).

3.2. Dust Impact on Tair

Our analysis of daily variations of Tair showed air heating in the near-ground atmospheric layer, which was caused by the dust intrusion in September 2015. To illustrate this phenomenon, we compared diurnal variations of buoy-measured Tair between clear-sky days and dusty days in September 2015 (Figure 7). To describe diurnal variations on the first six clear-sky days (1–6 September 2015), we used average Tair over these days (Figure 7a; the bold black line). In the nighttime, the average Tair decreased until sunrise. This was due to water cooling via the emission of longwave radiation. After sunrise, the average Tair increased, reaching its maximum (36.2 °C) at 17 LT. After 20 LT, Tair noticeably decreased due to water cooling via the emission of longwave radiation (Figure 7a).

On the first four dusty days from 7–10 September 2015, in the nighttime from 0–6 LT, the average Tair exceeded by 2 °C the average Tair on the previous clear-sky days. This happened because of the arrival of warm air masses originating from the desert areas of northern Syria [39,40,41]. In the nighttime on 8 September after sunset, a pronounced intrusion of dust pollution was observed, which was characterized by a significant increase in TSP concentration from 1000 to 4000 µg/m³ (Figure 4a). This increasing dust intrusion was accompanied by a rise in the nighttime Tair from 34.6 °C at 18 LT to 36.1 °C at 24 LT, on that date (8 September) (Figure 7b).

In the daytime on 8 September, in the presence of maximal dust pollution, Tair steadily increased in the daytime (Figure 7b), whereas winds on that day were minimal (Figure 8). There was no daytime Tair maximum on 8 September, and the daytime Tair was mainly lower (by up to 1 °C) than the daytime Tair on clear-sky 6 September (Figure 7a,b). The absence of the daytime maximum in Tair on 8 September can be explained by the shortwave dust radiative effect causing cooling of the surface below [18]. On the following two dusty days (9 and 10 September), the daytime Tair exceeded (by up to 3 °C) the daytime Tair on clear-sky 6 September, indicating the arrival of warm desert air masses.

3.3. AHWs over the Dead Sea

Now, when we know that the dust intrusion caused an increase in Tair over the Dead Sea, a question arises whether the dust intrusion could cause the formation of atmospheric heatwaves (AHWs). AHW is defined as a prolonged period of unusually high Tair compared to its seasonally varied 90th PTH. To answer that question, we analyzed time variations in the two parameters Tair-MAX and Tair-MIN in September 2015 (Figure 9). The time variations in Tair-MAX showed unusually high values in comparison with its seasonally varied 90th PTH for 10 consecutive days (9–18 September) (Figure 9a), whereas time variations in Tair-MIN showed that an AHW appeared over the Dead Sea also for 10 consecutive days (7–16 September) (Figure 9b). The slightly differing dates for the start of the above-mentioned AHWs were due to the following factors. On 8 September, Tair-MAX was lower than its 90th PTH due to the dramatic drop in SR on that day (Figure 5b and Figure 9a), whereas on that date (8 September), Tair-MIN exceeded its 90th PTH due to heating in the near-ground atmospheric layer. This was the reason the AHW based on Tair-MAX started on 9 September while the AHW based on Tair-MIN started on 7 September 2015. The maximal intensity of the AHW based on Tair-MAX was 3 °C, while the maximal intensity of the AHW based on Tair-MIN was 4 °C.

3.4. LHWs in the Dead Sea

The appearance of the above-mentioned AHWs over the Dead Sea suggests the development of LHWs at the lake water surface. This is because the near-ground atmospheric layer is in direct contact with the lake water surface. Previous studies on LHWs used satellite retrievals of lake SWT [9,10,11,22]. To investigate LHWs in the Dead Sea, we used both buoy measurements and satellite (METEOSAT and MODIS-Terra) SWT data.

3.4.1. Buoy SWT

As discussed in our previous study on LHWs in Lake Kinneret (Kishcha et al. [22]), Tair-MAX and Tair-MIN can be used as proxies for in situ-measured SWT-MAX and SWT-MIN, respectively. This was because of a good correspondence between the day-to-day variations of Tair (obtained from in situ meteorological measurements) and those of actual Kinneret SWT (obtained from in situ radiometer measurements) ([22], their Figure 10). It is reasonable to suggest that if it is true for Lake Kinneret, it is also true for the Dead Sea. Therefore, we used Tair-MAX and Tair-MIN as proxies for SWT-MAX and SWT-MIN, respectively. The obtained AHWs over the Dead Sea (characterized by the abnormally high Tair-MAX and Tair-MIN for 10 consecutive days) (Figure 9a,b) are evidence of the surface LHWs in the Dead Sea. These LHWs were represented by the abnormally high SWT-MAX and SWT-MIN in comparison with their 90th PTHs. The maximal intensity of the surface LHWs was the same as for the AHWs. From now on we will call in situ Tair (SWT proxy) actual SWT.

3.4.2. Satellite-Based SWT

To further investigate surface LHWs in the Dead Sea, we analyzed satellite SWT retrievals from METEOSAT and MODIS-Terra. MODIS-Terra provides us with SWT retrievals at approximately 10:30 LT and 22:30 LT. METEOSAT provides us with hourly SWT data. The above information allowed us to conduct two quantitative comparative analyses: (1) day-to-day variations in METEOSAT SWT and actual SWT averaged during the period from 10 to 11 LT were compared with daytime MODIS-Terra SWT at 10:30 LT; and (2) day-to-day variations in METEOSAT SWT and actual SWT averaged during the period from 22 to 23 LT were compared with nighttime MODIS-Terra SWT at 22:30 LT. For comparison purposes, satellite SWT data were averaged over the specified limited Dead Sea water area (31.40–31.45° N; 35.40–35.50° E) (Figure 2).

Note that, during the dust intrusion, over the specified limited water area, satellite SWT retrievals were not available on a daily basis, as would be expected on clear-sky days. Specifically, daytime METEOSAT SWT retrievals were unavailable only on 8 September, while nighttime METEOSAT SWT retrievals were unavailable during six days in a row, from 8 to 13 September 2015. Similarly, daytime MODIS-Terra SWT retrievals were unavailable from 8 to 10 September, while nighttime MODIS-Terra SWT retrievals were unavailable on 8, 10, 12, and 13 September.

We are going to show that satellite SWT data were not capable of reproducing the observed surface LHWs. First, in the presence of the severe dust intrusion, we analyzed spatial distribution of METEOSAT and MODIS-Terra SWT over the entire Dead Sea. Unexpectedly, both satellites showed that, over any part of the Dead Sea, SWT on dusty days was lower than SWT on clear-sky 6 September (Figure 10). This contradicted the increase in actual SWT in the presence of the dust intrusion. Next, we conducted quantitative comparison between satellite SWT and actual SWT. Our analysis showed that, when satellite SWT retrievals were available over the specified limited water area, their average SWT mainly coincided with the satellite SWT averaged over all available pixels. Therefore, for comparison purposes, on the dusty days, when satellite SWT retrievals were unavailable over the limited water area, their average SWT was replaced by the satellite SWT averaged over all available pixels within the Dead Sea. Using this approach, we compared quantitatively satellite-based SWT retrievals with actual SWT over the specified water area during the dust intrusion (Figure 11).

In the daytime, our quantitative comparison showed a strong disagreement between actual SWT and METEOSAT SWT retrievals (

Table 1. Comparison between METEOSAT and MODIS-Terra SWT retrievals (averaged over the specified limited water area) and actual SWT, during the period from 6 to 11 September 2015. On those days, measured AOD and SR-MAX are also shown.

Date	AOD	SR-MAX, W/m2	Actual SWT, °C	METEOSAT SWT, °C	MODIS-Terra SWT, °C
Daytime 10 LT–11 LT
6 September	0.4	940	32.1	39.2	32.7
7 September	0.6	830	32.3	34.8	28.0
8 September	3.2	190	34.0	-	-
9 September	1.8	470	34.8	27.9	-
10 September	1.8	580	34.3	30.0	-
11 September	1.0	680	33.8	36.8	29.8
Nighttime 22 LT–23 LT
6 September	-	-	33.4	37.8	30.0
7 September	-	-	35.3	36.2	29.6
8 September	-	-	36.1	-	-
9 September	-	-	35.9	-	26.9
10 September	-	-	37.1	-	-
11 September	-	-	35.7	-	29.1

). To illustrate, we compared actual SWT and METEOSAT SWT retrievals on the following two days: clear-sky 6 September and dusty 9 September. On 9 September, dust aerosol optical depth (AOD) (1.8) exceeded AOD on 6 September (0.4), indicating the presence of severe dust intrusion on 9 September. Due to the significant dust radiative effect, SR-MAX on 9 September (470 W/m²) was only half of SR-MAX (940 W/m²) on clear-sky 6 September. As a result of the dust impact, the daytime actual SWT (34.8 °C) on 9 September exceeded by 2.7 °C the daytime actual SWT on clear-sky 6 September (32.1 °C) (Table 1). In contrast, daytime METEOSAT SWT (27.9 °C) on 9 September was lower by 11.3 °C than that on 6 September (39.2 °C) (Table 1). Similarly, a strong disagreement between the daytime actual SWT and METEOSAT SWT retrievals was observed on two dusty days, 9 and 11 September 2015. Specifically, the daytime METEOSAT SWT on 9 September (27.9 °C) was lower by 8.9 °C than that on 11 September (36.8 °C). In contrast, the daytime actual SWT on 9 September (34.8 °C) was higher by 1 °C than actual SWT on 11 September (33.8 °C) (Table 1).

Furthermore, in the nighttime, a comparison between actual SWT and MODIS-Terra SWT on clear-sky 6 September and dusty 9 September also showed a strong disagreement (Table 1). In particular, on 9 September, nighttime actual SWT (35.9 °C) was higher by 2.5 °C than that on clear-sky 6 September (33.4 °C). In contrast, nighttime MODIS-Terra SWT (26.9 °C) on 9 September was lower by 3.1 °C than that on 6 September (30.0 °C) (Table 1). Finally, in the nighttime, actual SWT (35.9 °C) on 9 September was slightly higher than actual SWT (35.7 °C) on 11 September. In contrast, nighttime MODIS-Terra SWT (26.9 °C) on 9 September was lower by 2.2 °C than that on 11 September (29.1 °C) (Table 1).

As illustrated in Figure 11, instead of an increase in SWT, both satellites showed a decrease in SWT, underestimating actual SWT by up to 7 °C in the daytime and by up to 10 °C in the nighttime. This indicates that satellite SWT data were not capable of reproducing the surface LHWs in the presence of severe dust pollution. It is worth noting, however, that, from 14 to 30 September 2015 in the presence of low dust pollution, there was no dramatic difference between the day-to-day variations in actual SWT and in satellite-based SWT observations: they were compatible with each other both in the daytime and nighttime (Figure 11).

3.4.3. Subsurface Lake Heatwaves

A question arises as to whether the dust intrusion could cause the formation of subsurface LHWs. To answer this question, we compared day-to-day variations in both WT-MAX and WT-MIN at various depths with their seasonally varied 90th PTHs (Figure 12 and Figure 13). One can see that, at every depth, the blue lines, representing the 90th PTHs for WT-MAX and WT-MIN, showed a decrease from 1 to 30 September 2015, indicating the decreasing seasonal variations in WT in the Dead Sea.

We found that the dust intrusion caused the formation of subsurface LHWs down to a depth of 20 m, based on both WT-MAX and WT-MIN. The exceedance of WT-MAX over its 90th PTH for up to 16 days was observed at every depth, with the exception of a depth of 1 m (Figure 12 and Figure 13, the left panels). As for WT-MAX at a depth of 1 m (WT-1m-MAX), a number of peaks were observed of the abnormally high WT-1m-MAX on 12–13, 15, 17–18, and 20–22 September 2015 (Figure 12a). These peaks exceeded the 90th PTH by up to 0.7 °C. Between the peaks, however, WT-1m-MAX was slightly lower than the 90th PTH. Therefore, according to the LHW’s definition, there was no LHW at this depth (1 m) based on WT-1m-MAX. This was because there was no prolonged period of abnormally high WT-1m-MAX, despite the observed abnormally high peaks (Figure 12a). As for WT-MIN, it exceeded its 90th PTH for up to 16 consecutive days at every depth, with the exception of a depth of 5 m (Figure 12 and Figure 13, the right panels). At a depth of 5 m, WT-5m-MIN exceeded its 90th PTH only for two separated days (12–13 September) and three days (16–18 September) (Figure 12c). Therefore, at a depth of 5 m, there was no prolonged period of abnormally high WT-5m-MIN. This indicated the absence of LHW at the depth of 5 m based on WT-5m-MIN.

We calculated metrics of the observed LHWs at every depth down to 20 m to characterize them quantitatively. This was carried out using anomalies of WT-MAX and WT-MIN from their seasonally varying 90th PTHs (Figure 14). Our analysis highlighted the fact that the observed subsurface LHWs at different depths started on the same date, 11 September. Indeed, as illustrated in

Table 2. Metrics of the observed LHWs at various depths down to 20 m, in the Dead Sea in September 2015, including start and end dates, duration, maximal intensity $\left(MI\right)$, and cumulative intensity $\left(CI\right)$. The characteristics were obtained based separately on daily maximum water temperature anomalies (WT-MAX anomalies) and daily minimum water temperature anomalies (WT-MIN anomalies).

Depth, m	Start Date	End Date	Duration, Days	MI, °C	CI, °C Days
WT-MAX anomalies
1	-	-	-	-	-
2	12 September	22 September	11	0.68	2.52
5	12 September	17 September	6	0.32	1.00
10	11 September	26 September	16	0.56	3.88
15	11 September	26 September	16	0.60	4.11
20	11 September	27 September	17	0.60	4.00
WT-MIN anomalies
1	11 September	26 September	16	0.57	3.58
2	11 September	21 September	11	0.52	2.82
5	-	-	-	-	-
10	11 September	26 September	16	0.61	3.53
15	11 September	27 September	17	0.45	4.00
20	11 September	27 September	17	0.46	3.62

, all LHWs, based on WT-MIN, started on 11 September. As for the LHWs based on WT-MAX, they also mainly started on 11 September. Consequently, there was a 4-day delay between the first date of the surface LHWs (7 September) and the start date of the subsurface LHWs (11 September). With respect to the LHWs duration, the subsurface LHWs lasted longer (16 days) than the surface LHWs (10 days) (Table 2).

The intensity of the subsurface LHWs decreased with depth from 1m down to 5 m, followed by an increase at the deeper layers (from 10 m to 20 m) (Table 2). In particular, based on WT-MAX anomalies, at a depth of 5 m, the maximal intensity (0.32 °C) of the LHWs was only half of that (0.68 °C) observed at a depth of 2 m. However, at a depth of 10 m, the maximal intensity of LHWs increased up to 0.61 °C. The cumulative intensity of the subsurface LHWs varied slightly between 3.5 to 4 °C a day at the deep layers from 10 m to 20 m (Table 2).

4. Discussion

Buoy measurements in the hypersaline Dead Sea showed that the severe dust intrusion appearing in September 2015 caused the formation of both surface and subsurface LHWs. The surface LHWs were represented by unusually high SWT-MAX and SWT-MIN in comparison with their seasonally varied 90th PTHs for 10 consecutive days (7–17 September). The LHWs were caused by the arrival of warm air masses containing dust pollution from the desert. It was despite the dramatic drop in solar radiation due to the shortwave dust radiative effect. In situ-measured Tair-MAX and Tair-MIN were used as proxies of SWT-MAX and SWT-MIN, in accordance with [22]. The intensity of the surface LHWs was as high as 3 °C.

To further investigate surface LHWs in the Dead Sea, we compared satellite (METEOSAT and MODIS-Terra) SWT data with actual SWT based on buoy measurements. First, we analyzed spatial distribution of METEOSAT and MODIS-Terra SWT over the entire Dead Sea, during the severe dust intrusion. Both satellites showed that, over any part of the Dead Sea, SWT on dusty days was lower than SWT on clear-sky 6 September (Figure 10). This contradicted the increase in actual SWT in the presence of the dust intrusion (Figure 9). Next, we conducted quantitative comparison between satellite-based SWT and actual SWT. Our quantitative comparison highlighted the following important point. Satellite (MODIS-Terra and METEOSAT) SWT data did not detect the LHWs observed at the Dead Sea surface. Instead of an increase in SWT, both satellites showed a decrease in SWT, underestimating actual SWT by up to 10 °C. This finding is in line with [21,22] on dust-related LHWs in fresh-water Lake Kinneret. As discussed in [22], dust-caused infrared (IR) perturbations prevented satellites from conducting accurate IR measurements. Note that, in some previous studies [9,10,11], satellite observations were used to investigate heatwave properties for hundreds of lakes worldwide. However, based on our outcomes (that, in the presence of dust intrusions, both orbital and geostationary satellites did not detect LHWs observed in the Dead Sea), we suggest that dust-related lake heatwaves could not have been included in the analysis in the aforementioned studies.

The formation of surface LHWs was accompanied by the development of subsurface LHWs to a depth of 20 m. We found that, with respect to the LHW duration, the subsurface LHWs lasted longer (16 days) than the surface LHWs (10 days) (Table 2). There was a 4-day delay between the first date of the surface LHWs (7 September) and the start date of the subsurface LHWs (11 September). The maximal intensity of the subsurface LHWs decreased with depth from 1m (0.6 °C) to 5 m (0.3 °C), followed by an increase (up to 0.6 °C) at the deeper layers (from 10 m to 20 m) (Table 2).

With respect to solar radiation during the dust intrusion, day-to-day variations of SR-MAX showed a dramatic decrease: from ~900 W m⁻² on clear sky 6 September to ~200 W m⁻² on dusty 8 September (Figure 5). It was found that after the dusty period, from 18 to 30 September, SR-MAX noticeably increased and even exceeded its seasonally varied 90th PTH (Figure 5). This exceedance of SR-MAX over its 90th PTH indicated that, after the dust intrusion, the atmosphere over the Dead Sea became clearer than usual. This phenomenon can be explained by the dissipation of haze particles due to the formation of suitable conditions. In accordance with [3], over the Dead Sea, haze particles tend to disperse upward in unstable air. Consequently, our result (that the atmosphere became clearer than usual after the dust intrusion) indicated the formation of atmospheric instability encouraging upwelling vertical motion. A similar analysis of SR-MAX over Lake Kinneret during the same dust intrusion failed to show the same phenomenon (Figure 6). Therefore, this phenomenon is a specific feature of the Dead Sea. It is probable that this phenomenon can be explained by a distinctive topography of the Dead Sea valley where this hypersaline lake is located at approximately 400 m b.s.l. (below sea level). In addition, the chemical composition of hypersaline Dead Sea water is essentially different from that of Lake Kinneret water. This requires further research.

5. Conclusions

In saline lakes, heatwaves caused by dust intrusions have not been investigated in previous studies. In our study we focus on this point. It was found for the first time that a severe dust intrusion (aerosol optical depth of over 3, and TSP of over 4000 µg/m³) caused surface and subsurface LHWs in the hypersaline Dead Sea, which appeared in September 2015. The LHWs were observed despite the dramatic drop in solar radiation due to the dust shortwave radiative effect. The main points of our study are listed below:

• At the water surface, the LHWs were represented by abnormally high daily maximal and minimal SWT in comparison with their seasonally varied 90th PTHs for 10 consecutive days (7–17 September). In situ-measured Tair and its daily maxima (Tair-MAX) and minima (Tair-MIN) were used as proxies of SWT-MAX and SWT-MIN. The intensity of the surface LHWs was as high as 3 °C.
• Satellite SWT data from both orbital (MODIS-Terra) and geostationary (METEOSAT) satellites did not detect the LHWs observed at the Dead Sea surface, in the presence of severe dust intrusion. Instead of an increase in SWT, satellites showed a decrease in SWT, underestimating actual SWT by up to 10 °C. This is because dust-caused infrared (IR) perturbations prevented satellites from conducting accurate IR measurements of SWT.
• The formation of surface LHWs was accompanied by the development of subsurface LHWs to a depth of 20 m. The subsurface LHWs lasted longer (16 days) than the surface LHWs (10 days). There was a 4-day delay between the first date of the surface LHWs (7 September) and the start date of the subsurface LHWs (11 September). The maximal intensity of the subsurface LHWs decreased with depth from 1m (0.6 °C) to 5 m (0.3 °C), followed by an increase (up to 0.6 °C) at the deeper layers (from 10 m to 20 m).
• In the presence of dust intrusion, measurements of daily maximal solar radiation (SR-MAX) showed a dramatic decrease from ~900 W m⁻² on clear sky 6 September to ~200 W m⁻² on dusty 8 September. After the dusty period (18–30 September), SR-MAX noticeably increased and became higher than its seasonally varied 90th PTH (Figure 5b). This exceedance of SR-MAX over its 90th PTH indicated that, after the dusty period, the atmosphere over the Dead Sea became clearer than usual. This phenomenon can be explained by the disappearance of haze particles due to the formation of atmospheric instability encouraging upwelling vertical motion.
• Taking into account that, over the Eastern Mediterranean, desert dust has increased during the past several decades, one can expect frequent occurrence of dust-related intense persistent heatwaves in the Dead Sea. This will contribute to additional water heating and further drying up of the Dead Sea.