Tropical cyclones (TCs) are cyclonic vortex systems with warm-core structures. TCs with wind speeds over 32.7 m/s in the Northwest Pacific are commonly called typhoons. TCs cause intense vertical mixing in the upper ocean, forming “cold wakes” via carrying cold water upward. The sea surface temperature (SST) cooling is usually 1–6 °C [1
] and, in some cases, can even reach up to 11 °C [4
]. In the Northern Hemisphere, the surface cooling is generally biased to the right of TCs’ moving direction [6
]. The warm ocean provides heat and water vapor for TC development, and the intensified TC absorbs more vapor and energy, further strengthening the intensity of TC. This process is often called TC-ocean positive feedback [11
]. However, a TC cannot infinitely strengthen partly because TC-induced SST cooling inhibits the heat flux and water vapor transport from the ocean to the TC and restricts TC development or even weakens the TC [13
], which is called TC-ocean negative feedback.
Temperature variations caused by a TC is closely related to salinity variations. Previous studies [16
] have shown that variations in salinity structure affect the mixed layer temperature by changing the static stability of the upper ocean. A study of the effects of salinity found that freshening of the mixed layer can inhibit downward entrainment of heat and weaken SST cooling by changing the seawater density as well as ocean stratification [19
]. In tropical oceans and some other areas, the isothermal layer is thicker than the isopycnic layer, with a high-salinity layer between the two layers, i.e., the salinity barrier layer (BL) [21
]. The BL inhibits surface cooling via preventing the kinetic energy input of TC penetrated the thermocline and reducing the vertical mixing of cold water, which favors TC development [23
]. In the Bay of Bengal, it was found that salinity stratification had a remarkable effect on TC intensity: the haline stratification caused a cooling decrease between post-monsoon and pre-monsoon seasons of up to 40%, while thermal stratification accounts for the remaining 60% of the cooling reduction [26
]. Therefore, the upper ocean salinity response and its feedback to TCs require further study.
Under the influence of Typhoon Choi-Wan (2009), a mooring to the left side of the TC track showed an increase in salinity in the mixed layer (MLS) [27
]. Before and after the passage of Hurricane Gonzalo (2014), an underwater glider observed a salinity increase of 0.6 practical salinity units (psu) in the water above 20 m and a salinity reduction of 0.4 psu between depths of 30 m and 130 m [28
]. The sea surface salinity (SSS) response to Hurricane Isaac (2012) studied using microwave remote sensing data presented a freshening signature near the center of the TC [18
]. Both mooring observations and remote sensing data showed that the cyclone Phailin (2013) caused a SSS increase on the right side of the track [29
]. Argo profiles during Typhoon Tingting (2004) also observed significant MLS freshening; statistics of Argo profiles from 2000 to 2005 found an almost symmetrical variations on two sides of the track [30
]. Several observations from moored buoys showed an SSS increase (decrease) to the right (left) of the track [31
]. Although there have been many studies on the salinity response during TCs, the response symmetry, changing tendency, and other characteristics of the TC-induced salinity response are still uncertain.
The main factors that affect the ocean responses caused by TCs generally include storm intensity [33
], translation speed [1
] and pre-storm oceanic conditions, such as the mixed layer thickness, which can modulate vertical mixing [36
], and thermohaline stratification, which modulates vertical advection [28
]. Using numerical model simulations, a complex relationship was found between the BL and TC-induced upper ocean responses. For a weak TC, when its kinetic energy is not enough to spread below the mixed layer, the air-sea heat flux controlled the SST cooling, while the TC is strong enough to trigger strong entrainment into the BL, the heat loss at the sea surface can be partly compensated for by the warmer water in the BL [25
To some extent, the salinity response is similar with the temperature response. The SSS is mainly affected by five processes: rainfall, evaporation, vertical mixing, entrainment and advection [24
]. Given the pre-storm salinity structure, the final salinity change depends on the competition among those five processes. Generally, seawater salinity increases monotonously with depth. The vertical mixing and entrainment can induce salinity increases in the mixed layer and decreases in deeper layer [1
]. A moored buoy captured a slight increase in SSS during cyclone Nargis (2008) caused by intense entrainment of high-salinity subsurface water [41
]. The effect of advection varies with the horizontal and vertical gradients of salinity. Generally, downwelling (upwelling) reduces (increases) the salinity at a certain depth. In the study of the upper ocean variation induced by Hurricane Katia (2011), a region where SSS is reduced by approximately 1 psu was found, which was considered to be caused by the horizontal advection of freshwater from a low-SSS plume [42
] with the additional contribution of direct rainfall [38
]. An analysis based on Argo salinity profiles of typhoons in the Northwest Pacific Ocean showed that upwelling controls the salinity anomalies whenever the vertical salinity stratification of seawater is not so weak [36
]. Evaporation transfers energy to the atmosphere as latent heat. This action not only cools the upper ocean but also salinizes the ocean mixed layer [43
]. In contrast, precipitation can cause a drop in surface salinity [18
]. During the approach of Typhoon Choi-Wan (2009), a freshening signature in the MLS due to intense rainfall was observed at a mooring located in the Kuroshio Extension area [27
]. Another study which analyzed the underwater glider data found that typhoons, accompanied by heavy rainfall, can cause subsurface layer freshening [44
]. A series of numerical simulations showed that the desalination rate due to precipitation can exceed the salination rate due to the entrainment of high-salinity water from below, resulting in a salinity decrease in the mixed layer rather than an increase [45
In situ observation data is an important basis of scientific research. A cross-shaped observational array was deployed in the South China Sea (hereafter SCS) to capture information about the TC-induced upper ocean response [4
]. Fortunately, Typhoon Kalmaegi (2014) passed over this array in September 2014, and the variation of atmospheric conditions and oceanic thermal conditions induced by Kalmaegi (2014) were both documented by this array. Based on this in situ observations and other datasets, the upper ocean thermodynamic and dynamic responses to Typhoon Kalmaegi had been investigated in [3
]. It was found that the maximum cooling of SST reached 6 °C, which was mainly induced by vertical mixing and upwelling [3
]. The dynamic response was studied in [4
], which indicated that the effect of upwelling in the subsurface layer was comparable to vertical mixing, and the cold suction effect need to be taken seriously. Furthermore, the net modulation mechanism of upper ocean temperature variations caused by Typhoon Kalmaegi was studied in [46
]. Different from the three studies, this study focus on the role of precipitation on the TC-induced upper ocean salinity response rather than the thermal response induced by Kalmaegi.
The main goal of this paper is to investigate the influence of TC precipitation on TC-induced upper ocean response. The simultaneous precipitation and salinity data provided by in situ observations can help us understand the relationship between these two elements. The infrared (IR) satellite cloud images and satellite-based retrieval of precipitation provide us with consistent and reliable data over an open ocean, which help us to investigate the evolution of TC’s structure and precipitation distribution. Moreover, the possible influence of typhoon precipitation on the upper ocean temperature and salinity responses is investigated via a three-dimensional oceanic numerical model. In summary, this study is based on satellite observations, in situ observations, and a numerical model to reveal the air-sea responses, precipitation characteristics, and the effect of precipitation on ocean responses during Typhoon Kalmaegi (2014).
The data and numerical model in use are introduced in Section 2
. The results are showed in Section 3
, which includes the observed atmospheric features, the corresponding ocean responses, and the influence of precipitation on ocean responses. Section 4
presents the discussions of this work.
4. Discussion and Conclusions
The atmospheric characteristics of Typhoon Kalmaegi (2014) and its induced upper ocean response have been studied in detail using multiple-satellite observations and in situ observations. More importantly, the role of TC precipitation on TC-induced upper ocean variations were also studied by using the 3DPWP model.
The asymmetric structure shown in the satellite IR images corresponds to the asymmetric distribution of the CMORPH precipitation. Different structures caused different responses when TC was passing over the observational stations. The wider and more convective cloud clusters on the left side of Kalmaegi’s track caused stronger precipitation at Station 5 than at the other stations. The evolution of meteorological elements at the three stations during the passage of Kalmaegi had also been documented. When the typhoon eye passed by, the air pressure and relative humidity decreased, while the air temperature increased rapidly. Additionally, the maximum wind speed appeared twice when the annular eyewall passed.
Comparing the temperature profiles before and after Kalmaegi, the water column at Stations 2 and 5 were cooled, while a cooling-warming-cooling vertical structure appeared at Station 4. Those vertical profiles indicated that the upwelling is stronger than the vertical mixing at Stations 2 and 5, while the vertical mixing at Station 4 is strong enough to reach the depth affected by upwelling. The MLS became saltier at Station 4, while the salinity at Stations 2 and 5 experienced several dilution processes that were related to the intense precipitation. The strongest SSS increase at Station 4 was approximately 0.176 psu, and the strongest SSS decreases (increases) at Stations 2 and 5 were 0.145 psu (0.155 psu) and 0.278 psu (0.103 psu), respectively. It is uncertain whether the salinity response had the same rightward bias feature similar to temperature. All these differences between salinity and temperature responses prompt us to think about the dynamic differences that dominate these variations.
The 3DPWP model was capable to simulate the upper ocean variations regardless of the precipitation forcing. However, the simulation results with precipitation input is fresher and warmer. When precipitation was added to the model, the overestimation of SSS in the simulation without rainfall was suppressed. A model-based analysis suggests that considering precipitation in the 3DPWP model may cause the surface salinity to be fresher and the temperature to be slightly warmer. The mean ratios of impact within seven days after the forced stage were approximately 3.8%, 15.1%, and 2.7% (63.6%, 52.6%, and 63.0%) for the SST (SSS) at Stations 2, 4, and 5, respectively. Precipitation had a larger effect on salinity than on temperature, that is, salinity was more sensitive to precipitation than temperature. The precipitation is likely to work by changing the salinity stratification and impact the static stability, thus influencing vertical mixing and other internal ocean currents. The temperature variation induced by TC precipitation was a reflection of the above process.
The results of our study highlight the importance of precipitation for the TC-induced upper ocean response, especially for the ocean salinity. The SSS reduction during Kalmaegi, indicates that the effect of precipitation can be stronger than that of vertical mixing. Precipitation can directly reduce ocean salinity by adding freshwater into the ocean. Once the salinity becomes lower, the stratification would enhance, resulting in a shallower mixed layer depth and a weaker vertical mixing. Then the SST cooling would be suppressed because of the weakening vertical mixing. The weakened sea surface cooling caused by the enhanced upper-ocean stratification can in turn promote the TC to intensify and increase the vertical turbulent mixing. It looks like this forms a complete negative feedback process. Actually, the air-sea interaction during a TC is a complicated system in which multiple factors work together. Once precipitation occurs, the intensity, TC translation speed and other factors are constantly changed, which will also have an impact on the effect of precipitation. What’s more, water vapor transfers energy from the ocean to the atmosphere, and then returns to the ocean in the form of precipitation, changes the ocean conditions, and further feed back into the atmosphere above it. In a word, precipitation is very important for air-sea interaction during TCs, and the effect of precipitation (or freshwater flux) during a TC period should be taken seriously.
The model simulation in this study is an idealized test. Factors such as lack of surface heat flux and evaporation, inaccurate atmospheric forcing fields, and lack of atmospheric feedback mechanisms for ocean variations all affected the simulations. Therefore, a comprehensive ocean-atmosphere coupled model will be used to investigate the role of TC precipitation in TC-ocean response. Moreover, the impact of precipitation is likely concentrated in the mixed layer, so it is essential to choose a higher vertical resolution in the ocean model.