Ocean Surface Changes Research Articles

Competition between ocean thermal structure and tropical cyclone characteristics modulates ocean environmental responses in the Yellow and Bohai Seas

To study the environmental responses of tropical cyclones (TCs) in continental shelf regions, TCs passing over the Yellow Sea and Bohai Sea (YBS) during 2002–2020 were investigated, with a special focus on how competition between ocean thermal structure and TC characteristics modulates ocean surface changes. The spatial distributions of the climatic mixed layer depth (MLD), accumulated wind forcing power index (WPi), accumulated sea surface temperature (SST) changes and accumulated chlorophyll (Chl-a) changes in the YBS were calculated. The linear regressions indicate that both the TC-induced SST cooling and TC-induced Chl-a increase are correlated with the TC wind speed rather than the translation speed, especially when the TC forcing depth (Zmixing) is greater than the MLD. Otherwise, both the changes in SST and Chl-a are correlated with the TC translation speed when Zmixing is shallower than the MLD. Further study has shown that whether TCs can break the MLD is also a key condition for oceanic responses. In the southern YBS, which has a deep-sea basin and MLD, the TC wind speed is the major factor affecting SST cooling and Chl-a increase, as TCs need more strength to reach the MLD. However, in the northern YBS, which has the shallowest sea basin and MLD, even weak TCs can easily break the MLD and reach the seabed; thus, ocean surface changes are associated mainly with the TC translation speed. The composite results reveal that both the maximum SST cooling center (1.64 °C) and the maximum Chl-a increasing center (0.14 log10(mg/m3)) are located on the right and behind the TC center, respectively. In addition, TC-induced SST cooling and Chl-a increase were initiated two days prior to TC passage and then reached their maximum values after 1 day. It takes approximately 7–8 days for the Chl-a concentration to recover, but it takes a much longer time (>15 days) for the SST to recover.

Identifying Oceanic Responses with Validated Satellite Observations after the Passage of Typhoons in the Northern South China Sea

Tropical cyclone-induced upwelling has an important influence on ocean temperature and chlorophyll-a (Chl-a) concentrations, which are modified by the existence of mesoscale eddies. This paper investigates the regional dynamics and associated variability in temperature and Chl-a during the passage of three typhoons (Kammuri, Nuri, and Hagupit) with similar tracks in the northern South China Sea (SCS) during 2008 using remote sensing and in situ observations. The measurements of wind and sea surface temperature obtained by a buoy and satellite were found to be similar, and both showed that typhoons have prominent impacts on the ocean’s upper layer. Sea surface cooling is first identified during the passage of each typhoon, particularly on the right side of typhoon tracks. Increased Chl-a concentrations were observed in the surrounding areas after the passage of typhoons Nuri and Hagupit, with large offshore blooms (Chl-a increases of 0.27–0.33 mg m−3) identified along the tracks of Nuri and Hagupit 3 to 4 days after their passage. Such blooms are highly dependent on the cooling associated with typhoon-induced mixing and upwelling. The pre-existing eddies modified the surface variations, and a perfect match was identified between the polarities of the eddies and the ocean surface changes. An anomalously high offshore Chl-a enhancement entrained by a cyclonic eddy occurred along the track of Nuri 4 days after its passage; in this instance, typhoon-induced upwelling was reinforced by the cyclonic eddy. In comparison, typhoon-induced upwelling is inhibited by anticyclonic eddies, resulting in much less prominent changes. The combination of typhoon winds and eddies can modify the productivity of marine phytoplankton, and a comprehensive understanding of typhoon-induced dynamics will aid in understanding ecosystem responses to typhoons.

Typhoon footprints on ocean surface temperature and chlorophyll-a in the South China Sea

Typhoons pose great threats to coastal regions, leaving distinctive ocean surface changes along the typhoon track before landing. However, a quantitative description of the spatial and temporal patterns of typhoon-induced oceanic responses remains unclear. Sixteen years of composited data of sea surface temperature (SST) and sea surface chlorophyll-a (Chl-a) in the South China Sea demonstrate that typhoon-induced changes initiate immediately before the arrival of the typhoon and continue to increase during its passage. The largest decrease in temperature occurred one day after the typhoon, leading the maximum Chl-a increase by four days. The spatial distribution of changes in SST and Chl-a clearly shows the prominent enhancement to the right of the typhoon's path. Both typhoon features and ocean status determine the corresponding impact on the oceans, as large temperature responses are associated with strong wind, slow translation speed and shallow mixed layer depth (MLD). The Chl-a responses generally followed the same rule, but the maximum Chl-a increase was found with a moderate value of the MLD that was most favorable to induce phytoplankton blooms. Quantifying the typhoon footprints will help to predict how the ocean responds to extreme events in the future climate.

Errors of Airborne Bathymetry LiDAR Detection Caused by Ocean Waves and Dimension-Based Laser Incidence Correction

Ocean waves are a vital environmental factor that affects the accuracy of airborne laser bathymetry (ALB) systems. As the regional water surface undulates with randomness, the laser propagation direction through the air–water surface will change and impact the underwater topographic result from the ALB system, especially for the small laser divergence system. However, the natural ocean surface changes rapidly over time, and uneven ocean surface point clouds from ALB scanning will cause an uncertain estimation of the laser propagation direction; therefore, a self-adaptive correction method based on the characteristics of the partial wave surface is key to improving the accuracy and applicability of the ALB system. In this paper, we focused on the issues of spatial position deviation caused by surface waves and position correction of the underwater laser footprint, and the dimension-based adaptive method is applied to attempt to correct the laser incidence angle. Simulation experiments and analysis of the actual measurement data from different ALB systems verified that the method can effectively suppress the influence of ocean waves. Furthermore, the inversion result of sea surface inclination changes is consistent with the surface wind wave reanalysis products. Based on the laser underwater propagation model in the strategy, we also quantitatively analyzed the influence of surface waves on laser bathymetry, which can guide the operation selection and data processing of the ALB system at specific water depths and under dynamic ocean conditions.

Modulation Effect of Mesoscale Eddies on Sequential Typhoon-Induced Oceanic Responses in the South China Sea

The impacts of mesoscale eddies on the modulation of typhoon-induced oceanic responses are important for understanding ocean dynamics. Satellite observations identified prominent ocean surface temperature and chlorophyll changes over the regions with mesoscale eddies after two sequential typhoons, e.g., Linfa and Nangka, in the South China Sea. The impacts of typhoons on the ocean surface were more prominent within cyclonic eddies than within anticyclonic eddies. The wind speed (translation speed) of Linfa was much larger (slower) than that of Nangka; thus, the changes induced by Linfa were stronger. However, the second typhoon easily generated mixing through the weak stratification induced by the first typhoon and impacted the upper ocean. The strong chlorophyll enhancement induced by Nangka was identified at a cyclonic eddy. Using a combination of reanalysis data, the depth of water origin (DWO) was applied to quantify the depth to which a typhoon’s impact could be exerted. Prominent changes were identified when the DWO reached the depth at which the temperature and nutrients differed from those within the mixed layer. This method can overcome the impacts of cloud coverage when examining a typhoon’s influence with remotely sensed data and offers a quantitative approach to determine the mechanisms responsible for typhoon-induced ocean surface changes.

Ocean surface warming in Krossfjorden, Svalbard, during the last 60 years

A high-resolution marine sediment core NP16-Kro1-MCB from Krossfjorden, Western Svalbard is studied to investigate changes in sea surface conditions in the fjord during the last 60 years (1953–2014). The diatom-based reconstruction of August sea surface temperature (aSST) demonstrates a clear warming trend of 0.6 °C through the record. As inferred from Marginal Ice Zone (MIZ) diatoms, surface warming occurs in parallel with a decline in sea ice extent (SIE) during recent decades. Factor analysis identified variations in diatom assemblages representing different water masses, showing a dominance of Arctic water diatoms throughout the period and decadal variations in the sea ice assemblage during periods of peak sea ice extent. The strong dominance of Arctic water diatoms along with increasing aSST suggest prolonged open-water conditions and increased sea ice melting in the region throughout the observed period. The reconstructed ocean surface changes are in line with the background warming occurring over the Arctic region. A comparison with instrumental records from neighboring regions supports the quality of the reconstructions, including the average reconstructed aSST and the magnitude of the warming trend. We suggest that increased CO2 forcing together with ocean–atmospheric interaction have caused the increasing SST trend and decreasing sea ice presence in Krossfjorden rather than an increasing influence from Atlantic Water, which has amplified changes in many regions of Svalbard.

An observation‐based method for reconstructing ocean surface changes using a 340,000‐year deuterium excess record from the Dome Fuji ice core, Antarctica

An extended deuterium excess (d or d‐excess) record covering the last three glacial‐interglacial cycles (ca. 340,000 years) was obtained from the Dome Fuji ice core, Antarctica. In order to interpret the Antarctic d record as a climatic history of vapor source regions, the record should be corrected for the Antarctic site temperature effect. We propose an observation‐based method for correcting the d record. That method uses a regression curve of the observed spatial relationship between δD and d in Antarctica on the assumption that the present slope has not changed. Unlike the previously proposed model‐based method, our correction method results in considerable change in the d profile of the Dome Fuji ice core. The corrected d record anticorrelates with a mean ocean δ18O record (R2 = 0.74) and correlates with a SST record (R2 = 0.58) throughout the observed period. These results emphasize the significance of our empirical correction method.

Solid Earth geophysics and satellites

Part of the frustrations in geophysics arise from the inability to adequately control the conditions for measurements and experiments. The earth is just too big to put into a conventional laboratory. Satellites provide a method of quickly‐making geophysical measurements globally.Early measurements of the earth and its gravity field were obtained from tracking satellite orbits. This led to experiments such as the geodetic satellites Geos‐1 and ‐2, which carried laser reflectors for improved tracking accuracy, and Geos‐3, which included a radar altimeter to measure directly ocean surface height. Radar altimeter technology has been improved and was tested on Skylab and used on the Seasat mission. These altimeter data make it possible to define the position of the geoid with respect to the spheroid more accurately over the oceans than over much of the continental masses. Uncertainties of the geoid's position over the oceans are 1 m to 5 m averaged over 5° by 5° areas. These uncertainties arise from our inability to account for all the variables, such as currents and tides, and from the lack of adequate data in some localities. The NASA Gravsat mission could provide a geoid accurate to 10 cm. This accuracy would make it possible to use future radar altimeter missions to measure ocean surface changes that deviate from the geoid. An additional product of radar altimetry would be a gravity map of the world that would include both oceans and continents. The map would be accurate to 1 to 5 mgal averaged over 1° by 1° areas.