Ocean-atmosphere Interactions Research Articles

Monthly extended ocean predictions based on a convolutional neural network via the transfer learning method

Sea surface temperature anomalies (SSTAs) and sea surface height anomalies (SSHAs) are indispensable parts of scientific research, such as mesoscale eddy, current, ocean-atmosphere interaction and so on. Nowadays, extended-range predictions of ocean dynamics, especially in SSTA and SSHA, can provide daily prediction services in the range of 30 days, which bridges the gap between synoptic-scale weather forecasts and monthly average scale climate predictions. However, the forecast efficiency of extended range remains problematic. With the development of ocean reanalysis and satellite remote sensing products, large amounts datasets provide an unprecedented opportunity to use big data for the extended range prediction of ocean dynamics. In this study, a hybrid model, combing convolutional neural network (CNN) model with transfer learning (TL), was established to predict SSTA and SSHA at monthly scales, which makes full use of these data resources that arise from delayed gridding reanalysis products and real-time satellite remote sensing observations. The proposed model, where both ocean and atmosphere reanalysis datasets serve as the pretraining dataset and the satellite remote sensing observations are employed for fine-tuning based on the transfer learning (TL) method, can effectively capture the evolving spatial characteristics of SSTAs and SSHAs with low prediction errors over the 30 days range. When the forecast lead time is 30 days, the root means square errors for the SSTAs and SSHAs model results are 0.32°C and 0.027 m in the South China Sea, respectively, indicating that this model has not only satisfactory prediction performance but also offers great potential for practical operational applications in improving the skill of extended-range predictions.

Mid-Holocene expansion of the Indian Ocean warm pool documented in coral Sr/Ca records from Kenya

Proxy reconstructions suggest that mid-Holocene East African temperatures were warmer than today between 8 and 5 ka BP, but climate models cannot replicate this warming. Precessional forcing caused a shift of maximum insolation from boreal spring to fall in the mid-Holocene, which may have favored intense warming at the start of the warm season. Here, we use three Porites corals from Kenya that represent time windows from 6.55 to 5.87 ka BP to reconstruct past sea surface temperature (SST) seasonality from coral Sr/Ca ratios in the western Indian Ocean during the mid-Holocene. Although the Indian monsoon was reportedly stronger in the mid-Holocene, which should have amplified the seasonal cycle of SST in the western Indian Ocean, the corals suggest reduced seasonality (mean 3.2 °C) compared to the modern record (mean 4.3 °C). Warming in austral spring is followed by a prolonged period of warm SSTs, suggesting that an upper limit of tropical SSTs under mid-Holocene conditions was reached at the start of the warm season, and SSTs then remained stable. Similar changes are seen at the Seychelles. Bootstrap estimates suggest a reduction in SST seasonality of 1.3 ± 0.22 °C at Kenya and 1.7 ± 0.32 °C at the Seychelles. SST seasonality at Kenya corresponds to present-day SST seasonality at 55° E–60° E, while SST seasonality at the Seychelles corresponds to present day SST seasonality at ~ 65° E. This implies a significant westward expansion of the Indian Ocean warm pool. Furthermore, the coral data suggests that SST seasonality deviates from seasonal changes in orbital insolation due to ocean–atmosphere interactions.

An 1800-year oxygen-isotope record of short- and long-term hydroclimate variability in the northern neotropics from a Jamaican marl lake

Hydroclimate variability on multi-decadal timescales has been a prominent feature of the circum-Caribbean region over the common era, with marked dry intervals noted in particular for the period 800–950 CE coinciding with the Terminal Classic Period (the so-called Terminal Classic Drought: TCD) in Mesoamerica, and with the Little Ice Age from about 1500 to 1800 CE, linked to complex ocean-atmosphere interactions. Previous compilations of palaeoclimate reconstructions have revealed a clear precipitation dipole between northern and southern Mesoamerica over the common era, which is consistent with meteorological data and modelling experiments. However, patterns of variability elsewhere within the region are less well understood, although palaeoclimate records do point to spatial complexity. Here, we present a ∼sub-decadal-scale lake-sediment hydroclimate reconstruction based on ostracod-shell stable isotopes from Wallywash Great Pond, Jamaica, covering the past ∼1800 years, which fills a spatial gap in records for the region. Variations in δ18O values at this site are a proxy for changes in effective moisture and they reveal a marked wet phase over the Terminal Classic Period (TCP), suggesting that the precipitation dipole over northern and southern Mesoamerica may have an east to west component. This is supported by some previous studies, although additional sites are required from strategic localities within the region to confirm this. The Little Ice Age interval at Wallywash is drier than the TCP, although the signal is less clear than at some sites within the wider region, suggesting that regional complexity in hydroclimate has characterised this interval as well.

A Case Study of Ocean-Atmosphere Interactions during the Passage of an Extra-Tropical Cyclone in the Vicinity of Cape Hatteras, North Carolina

A Case Study of Ocean-Atmosphere Interactions during the Passage of an Extra-Tropical Cyclone in the Vicinity of Cape Hatteras, North Carolina

IMPACT PROFILE OF ENSO AND DIPOLE MODE ON RAINFALL AS ANTICIPATION OF HYDROMETEOROLOGICAL DISASTERS IN THE PROVINCE OF SOUTH SUMATRA

El Niño Southern Oscillation (ENSO) is a weather phenomenon in the Pacific Ocean. At the same time, Dipole Mode (DM) is an ocean-atmosphere interaction phenomenon in the Indian Ocean. The area of South Sumatra Province, which is in the Monsunal area, makes the influence of ENSO and DM very instrumental in determining the length of the rainy season and throughout the year. The South Sumatra region is very vulnerable to forest and land fire disasters due to the condition of the area in the form of swamps and peatlands, which are very easy to burn if the dry season occurs in the South Sumatra area. In this study, an analysis of the effect of ENSO and DM on rainfall in the South Sumatra Region from 1981 to 2020 was carried out using a simple linear regression method and the correlation coefficient using Niño 3.4 index data and Dipole Mode with rainfall data and consecutive rainy days data. Consecutive Wet Day (CWD). The purpose of this study was to determine the effect of Enso and Dipole Mode on rainfall and CWD in the South Sumatra Region. The results show that the correlation between ENSO and rainfall is 0.0017-0.002573, DM and rainfall is 0.05972, and ENSO and CWD is -0.068. The correlation between DMI and CWD is 0.513. So it can be said that ENSO and DMI have no effect on rainfall in the South Sumatra Province. Still, the amount of CWD in South Sumatra Province is significantly determined by the Dipole Mode Index (DMI) at a moderate level. The number of consecutive rainy days in South Sumatra Province is influenced by the dynamics of the ocean and atmosphere of the Indian Ocean, which is characterized by the presence of positive IOD and negative IOD phenomena.

A deep learning approach to predict sea surface temperature based on multiple modes

Sea surface temperature (SST) is an essential physical parameter and plays a vital role in ocean-atmosphere interactions. Accurate SST prediction relies on numerical model methods, which require the understanding of complex dynamical and thermal processes. Data-driven SST prediction methods based on deep learning have been studied to obtain quick results. However, recent deep learning models can hardly accurately capture and simulate complex SST patterns, and thus give only lower resolution predictions. Here, SST map prediction is regarded as a spatiotemporal sequence prediction task. The memory in memory (MIM) model and variational mode decomposition (VMD) are combined (VMD-MIM) to accurately detect variations in SST patterns. The original SST data is firstly decomposed into different modes with different frequencies using VMD. For each mode extracted by VMD, MIM is then applied to obtain the corresponding prediction maps separately. SST data from the South China Sea are used to estimate the VMD-MIM method, in which we enter the ten-day SST maps and predict the next seven days. The results show that VMD-MIM can noticeably improve prediction skills by accurately presenting the fine structures (1/10°) of SST maps, which provides a valuable pathway for fast and lightweight short-term spatiotemporal SST predictions.

Variations in Asian summer monsoon and hydroclimate during Heinrich stadials 4 revealed by stalagmite stable isotopes and trace elements

The iceberg melting-induced Heinrich events (HEs), which were first reported in 1988, caused a series of centennial-millennial global climate changes during the last glacial period. The patterns and details of low-latitude monsoons, oceans, and atmospheric circulation responses to HEs still need to be understood. Here, we report 11a resolved δ13C, δ18O and 36a resolved Mg/Ca, Sr/Ca and Ba/Ca multi-proxy stalagmite records during 42.29–37.14 ka BP from Yangkou cave, southwestern China. The Greenland ice cores and the δ18O of stalagmites from northern China recorded a rapid onset/termination of Heinrich stadials (HS) 4 (∼0.2 ka). However, the transition interval for the weakening/strengthening of the Asian summer monsoon (ASM) during HS4, as recorded by the southern Chinese stalagmite δ18O, is about 0.5 ka. Low latitude monsoons and hydrological variations are influenced by tropical oceans and interhemispheric thermal contrasts, which prolong the onset/termination transition process of HS4. Our findings highlight the importance of atmosphere-ocean interactions at low latitudes and in the Southern Hemisphere for ASM dynamics, which triggered the termination of millennial-scales abrupt climate events. During HS4, variations of the δ13C and trace elements ratios are inconsistent with δ18O, indicating that while the ASM is weakened, regional hydroclimate and local ecology are not significantly degraded, resulting in a relatively moderate hydrological environment. Prolonged southerly subtropical westerlies, super El Niño Southern Oscillation, western China autumn rains, and lower regional evaporation, may increase regional precipitation in different seasons and regulate hydrological conditions during HS4. As a result, while the ASM has weakened, regional hydrological conditions in the Yangtze River basin in southwest China maintain wet environmental conditions. The gradual northward shift of the Intertropical Convergence Zone and subtropical westerlies during the latter phase of HS4 will cause a limited northward shift of the rainfall bands. Although regional hydrological conditions deteriorate slightly in comparison to the early phase, it remains humid overall. We present new evidence that local hydrological conditions and ASM intensity were not fully coupled during the abrupt climate events on millennial scales in the last glacial period.

Causal relationship between sea surface temperature and precipitation revealed by information flow

The atmosphere and the ocean are coupled with each other through various processes. Therefore, it is of great importance to understand the causality relationship between the atmosphere and the ocean for predicting their states. Here we apply the normalized information flow (NIF) to sea surface temperature (SST) and precipitation data to investigate the causality from the atmosphere to the ocean and from the ocean to the atmosphere. When the global spatial structure of the local NIFs is calculated for both directions, it is found that the ocean affects the atmosphere more in the tropics, while the atmosphere affects the ocean more in the extratropics. This causality relationship between the ocean and the atmosphere agrees with previous studies. To examine the teleconnections, the remote NIFs are then calculated and compared with the local NIFs. The local impact from SST to precipitation is dominant in almost all tropical regions, while the relative importance of remote impacts is higher for the precipitation-to-SST NIFs, except for a small area in the central-eastern equatorial Pacific and southeastern tropical Indian Ocean. Regional analyses for six selected areas within the tropics are also presented. This study suggests that NIFs may be a powerful tool to study ocean-atmosphere interactions.

Upper Ocean Responses to the Tropical Cyclones Ida and Felicia (2021) in the Gulf of Mexico and the Eastern North Pacific

Tropical cyclones (TCs) are a significant component of ocean–atmosphere interactions and the climate system. These interactions determine both the development and strength of TCs, as well as various biogeochemical processes in the upper oceans, including vertical mixing and primary production. We investigated the impact of the TCs Felicia and Ida that emerged in 2021 in the eastern North Pacific and the Gulf of Mexico, respectively, using satellite observations of sea-surface temperature (SST) and surface chlorophyll a (chl-a) concentrations, and vertical profiles of temperature and salinity derived from Argo floats. Observations differed between the two study areas. Cooling of SST associated with TC Ida was observed throughout the Gulf of Mexico (<0.5 °C), except for warming in a region off the Mexican coast east of Ida’s track (by about 0.5 °C). The passing of TC Felicia cooled SST in the eastern region (15°N, 115°W) and a central region (15°N, 125°W) by 0.5 °C and 0.36 °C, respectively. The passing of the TCs caused enhanced vertical mixing of the upper ocean layer in the Gulf of Mexico, with a deepening of the mixed layers from 38 m to 68 m (TC Ida). In contrast, the mixed layer in the eastern North Pacific decreased from 50 m to 20 m. For the eastern North Pacific, mixing could be related to an increase in surface chl-a and thus enhanced phytoplankton biomass was observed for 2 months after the passing of TC Felicia with a chl-a increase of 0.15 mg m−3. In the Gulf of Mexico, however, TC Ida caused the injection of a coastal phytoplankton bloom into the open Gulf, resting for more than a month after the cyclone had passed. Our findings contribute to the understanding of potential SST cooling, destratification, and enhanced primary production due to the passage of TCs in two distinct ocean regions, i.e., the open eastern North Pacific and the semi-enclosed Gulf of Mexico.

Impact of Indian Ocean Dipole Events on Phytoplankton Size Classes Distribution in the Arabian Sea

Changes in the environmental condition associated with climatic events could potentially influence the PSC dynamics of the regional marine ecosystem. The Indian Ocean dipole (IOD) is one of the critical ocean–atmosphere interactions that affects the climate of the Arabian Sea, and it could be a potential factor influencing the regional PSC distribution. However, the relationship between PSC and IOD remains unclear and less explored. In this study, using the in-situ database acquired from the Arabian Sea, we reparametrized the three−component abundance−based phytoplankton size class model and applied it to reconstructed satellite−derived chlorophyll−a concentration to extract the fractional contribution of phytoplankton size classes to chlorophyll−a concentration. Further, we investigated the influence of IOD on the changes in the biological–physical properties in the Arabian Sea. The results showed that the biological–physical processes in the Arabian Sea are interlinked and the changes in the IOD mode control the physical variables like sea surface temperature (SST), sea surface height (SSH), and mixed layer depth (MLD), which influence the specific PSC abundance. Unprecedented changes in the PSC distribution and physical properties were observed during the extreme positive and negative IOD events, which clearly indicated the potential role of IOD in altering the PSC distribution in the Arabian Sea. This study highlights the impact of extreme climate events on PSC distribution and the need for a better understanding of the associated physical–biological–climate interactions.

Orbital-scale thermocline temperature variability in the western equatorial Pacific during the last 370 kyr

Tropical subsurface circulation plays a key role in regulating heat and salt transport between the hemispheres, and in ocean–atmosphere interactions (e.g., El Niño-Southern Oscillation; ENSO). Therefore, understanding the past behavior of tropical thermoclines is important for predicting long-term global climate change. Here, we provide an orbital-scale thermocline temperature (TT) record based on Mg/Ca measurements of planktonic foraminifer Pulleniatina obliquiloculata in sediment cores obtained from the western equatorial Pacific (WEP) for the last 370 kyr. The vertical temperature gradient between sea surface temperature (SST) and TT in WEP showed weak positive correlations with the tropical Pacific zonal SST gradient in long-term trend and obliquity signal. The relationship of the two gradients does not coincide with modern ENSO variability, suggesting that the long-term TT variation depend primarily on another mechanism. Spectral analysis indicated that TT variability has strong signals in both the obliquity and precession bands. Since the obliquity signal is very weak in local insolation, the significant obliquity in TT variability implies an extratropical influence on tropical thermocline circulation. Therefore, our findings suggest that the subsurface circulation connecting the tropics and extratropics plays an essential role in orbital-scale tropical thermocline variability.

Weakened Impacts of the East Asia-Pacific Teleconnection on the Interannual Variability of Summertime Precipitation over South China since the Mid-2000s

The current study concentrates on the interdecadal shift in the interannual variability of summertime precipitation (IVSP) over South China (SC). Possible causes for the interdecadal shift are explored. The IVSP on a decadal time scale presents a significant weakening after the mid-2000s. The results show that the variances of the interannual precipitation variability over the SC region between 1993 and 2004 (hereafter S1) and 2005 and 2020 (hereafter S2) are 1.40 mm d−1 and 0.58 mm d−1, respectively. The variance of the IVSP has decreased by 58.6% since the mid-2000s. The current study reveals that the reduction in the IVSP over SC after the mid-2000s is prominently attributed to the weakened impact of the East Asia-Pacific (EAP) teleconnection. Before the mid-2000s, the interannual variation of the east-west movement of the western Pacific subtropical high was more significant. The warming over the tropical central-eastern Pacific (CEP) and cooling over the western Pacific (WP) suppress the Walker cell in the tropical Pacific and induce anomalous Hadley cell with its descending branch over the WP in the wet years. The anomalies of SST and atmospheric circulation show opposite phases in the dry years. This SSTA pattern enhances the northward propagation of the EAP teleconnection through a Rossby-wave-type response, which triggers an ascending/descending branch with active/suppressed convection over the northwestern Pacific in the wet/dry years. Therefore, the cooling WP and El Niño in its developing phase provide an ideal condition for more precipitation over SC. However, the above ocean–atmosphere interactions changed after the mid-2000s. The significant SST changes in the tropical CEP and the WP weaken the EAP teleconnection and atmospheric circulation anomalies over SC, leading to a significant interdecadal reduction in the IVSP over SC after the mid-2000s.

Different moisture regimes during the last 150 years inferred from a tree-ring δ18O network over the transitional zone of the Asian summer monsoon

The East Asian summer monsoon (EASM) and Indian summer monsoon (ISM) are two interactive climate systems dominating the moisture variability of Monsoon Asia. However, ISM-EASM interactions and their effects on regional moisture dynamics remain unclear. This study investigated the spatiotemporal characteristics of monsoon-related hydroclimate variability by establishing a new tree-ring oxygen isotope ratio (δ18Otree) network of eight δ18Otree records covering the last 150 years in southern China, which differs from previous studies that used data from individual sites. The δ18Otree chronologies were found to be sensitive to regional hydroclimatic changes during the monsoon season. The δ18Otree network indicated an east–west clustering pattern in the ISM-EASM transitional zone, which demonstrates an asynchrony in δ18Otree variability in the west and east of this region. Regional variability of the west and east δ18Otree modes reflected the different moisture signals, as indicated by their significant correlations with modern observations and paleo-δ18O records in disparate upstream regions of moisture transport (the Indochina Peninsula and Southeast China). This finding was confirmed by the east–west distinct pattern of spatial isotopic depletion of monsoon rainfall that originates from coastal areas of tropical oceans (Bay of Bengal and South China Sea) to the study area, as well as the consistent present-day pattern of atmospheric vapor transport during the monsoon season. In addition, the west and east modes exhibited stronger associations with ISM variability, whereas the east–west δ18Otree gradient captured the EASM-related hydroclimatic signal, suggesting that summer moisture variability in the ISM-EASM transitional zone was regulated more by ISM than by EASM during the last 150 years. The relationships between δ18Otree and large-scale ocean–atmosphere interaction modes revealed that the El Niño-Southern Oscillation (ENSO) dominantly modulates δ18Otree variability across southern China.

On the importance of wind generated waves in embayments with complex orographic features – A South African case study

The current study explores the importance of local ocean-atmosphere interactions in modelling wave records in embayments surrounded by significant orographic features and semi-sheltered against swell waves. The numerical code Simulating WAves in the Nearshore (SWAN) was employed for this purpose. False Bay and Table Bay, both located adjacent to the Cape Peninsula in South Africa, were used as case studies due to their geographical location and significance at the southern tip of Africa, their proximity to each other and their surrounding orography. Various combinations of atmospheric forcing were investigated (e.g. various atmospheric forcing horizontal resolutions) to distill the importance of remotely originating swell and locally generated wind waves on both peripheries of False Bay and within Table Bay. Generally, both embayments require high resolution, spatially varying wind forcing to capture the effects of the complex orography. The waves on the western periphery of False Bay and Table Bay are mainly generated by local spatially varying winds, while the waves on the eastern periphery of False Bay are mainly the incoming swell. Results are presented via statistical correlations with measured data, time-series comparisons, wind roses and wave field difference plots.

Continued weakening of the equatorial Pacific upwelling annual cycle in CMIP5 future projections

This study explores the dynamics of the equatorial Pacific upwelling annual cycle under global warming using the Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations. Through a linear-weighted theory developed recently, the theoretical upwelling annual cycles under global warming helped reasonably characterize the patterns of the original upwelling annual cycles simulated in CMIP5 models; however, an apparent weakening in magnitude as compared to that during the present stage was observed. To verify the above, we divided 90-year outputs in the CMIP5 future projections into three 30-year windows and set side by side. The long-term evolution of the upwelling annual cycle reconfirmed an overall weakening tendency in the entire equatorial Pacific. Moreover, the weakening of the Ekman upwelling could most likely be attributed to the meridional surface wind stress divergence, while the gradually smoothing inclination in the overall equatorial thermocline depth was responsible for the weakening of the wave upwelling. The weakening of the wave upwelling in the east and the Ekman upwelling in the west jointly contributed to the gradual weakening of the equatorial Pacific upwelling annual cycle. The above projected changes are robust among the 19 chosen CMIP5 models. Equatorial upwelling largely influences the sea surface temperature, associated atmosphere–ocean interactions, and convection and precipitation in tropical areas; hence, a continuous weakening of the upwelling annual cycle over the equatorial Pacific Ocean could likely affect the major climate phenomena variability with strong seasonal-locking characteristics by modifying the background strength at their peak phases in the future. The theoretical results can provide us the equatorial upwelling annual cycle patterns based on the Ekman and wave dynamics, which would be a strong tool for our investigations on the climate variability under global warming.

What causes a heavy precipitation period to become extreme? The exceptional October of 2018 in the Western Mediterranean

The Mediterranean region is particularly exposed to heavy precipitation and flash flooding. Every autumn the region is affected by these weather-related hazards, frequently with immense costly and deadly consequences. What makes an already potentially damaging period in terms of heavy precipitation, even more intense? This is the underlying question in this study, in which the atmosphere and the ocean conditions in October 2018 are examined to identify anomalies favoring this intensification. Furthermore, the model representativity of the over-averaged precipitation period and underlying anomalies is analyzed across scales using climatological, seasonal, and event-based COSMO high-resolution model simulations.Our investigation shows that October-2018, in the context of the climatological series from 1982 to 2018, could be marked as an unprecedented period because of the presence of intense and numerous low-pressure systems. Additionally, atmospheric moisture values placed this time above the climatological average, mainly for the high percentiles of the TCWV hourly anomalies. Specific humidity showed similar behaviour as TCWV except for pressure levels lower than 700 hPa, probably in relation to the evolution of the former Hurricane Leslie. The atmosphere-ocean interaction presented combined strong sea surface temperature (SST) and evaporation anomalies. April to October SST clearly exceeds climatological values while October-2018 presents both strong monthly anomaly and intense evaporation peaks preceding the most intense precipitation events. These large-scale features’ anomalies were in general well captured by the high-resolution regional climate model simulations at climatic and seasonal scales leading to an accurate representation of accumulated precipitation for the October period. However, the numerical weather prediction simulations on an event scale revealed low predictability, in agreement with former investigations, due to differences at the location and intensity of the cut-off lows and particularly at the atmospheric moisture field.The conclusions of this study show that it is not the most extreme period in terms of single anomalies which lead to extreme wet seasons, but the synergy of atmospheric and oceanic anomaly conditions with a constant interplay which made Autumn/October 2018 an extreme season/month.

Decadal variation in the frequency of tropical cyclones originating in the South China Sea and migrating from the western North Pacific

A decadal variation in the frequency of tropical cyclones (TCs) that reached their lifetime maximum intensity (LMI) in the South China Sea (SCS; 5°N-25°N, 107°E-121°E) from 1978 to 2020 was identified. TCs that generated and reached LMI in the SCS were named “local TCs,” while those that generated in the western North Pacific (WNP) and reached LMI in the SCS were named “migratory TCs.” A seesaw phenomenon in the frequencies of these two types of TCs was found before and after 1997. From 1978 to 1996, TC frequency was generally lower in local TCs but higher in migratory TCs. The opposite was true from 1997 to 2020. The main factors responsible for this “seesaw” phenomenon include changes in the genesis positions of TCs and the interdecadal variation of large-scale environmental flow patterns. From 1997 to 2020, during which the large-scale steering flow was favorable for local TCs, the monsoon trough over the WNP withdrew westward along with the warm pool and the subtropical high strengthened westward. Meanwhile, the sea surface temperature (SST) gradient between the equator and mid-latitudes decreased and the north wind weakened near 120°E. Easterly winds were strengthened in the equatorial region, which led to an abnormal anticyclone and the divergence of water vapor in the WNP. In contrast, the SST of the SCS, an internal sea, increased significantly. Under local atmosphere-ocean interaction, abnormal cyclonic circulation appeared in the SCS, which led to intensified convergence and intensified wet convection. Changes in the environmental fields in the WNP and SCS are the main reasons for the seesaw phenomenon observed in these two types of TCs.

Mesoscale impact of the sea surface on the performance of offshore wind farms

The impact of the ocean on the performance of offshore wind farms is a challenging study in the wind energy field. In the present study, by coupling a mesoscale numerical weather prediction model, Weather Research and Forecasting (WRF), with a sea surface model and a refined wind farm parameterization, a systematic atmosphere-sea surface-wind farm simulation method is established. The question of how the ocean-atmosphere interaction changes the wake effect and power output of wind farms based on the method is explored. The results show that the momentum deficit due to the operating wind turbines is remarkable, and the wind speed deficit has a strong horizontal diffusion on a sea surface. Wake from the offshore wind farm is prone to mix and extends with a shorter length than that on the flat land. The wake mixing enhances the interference from upwind to downwind turbines, resulting in the power outputs of downwind turbines being decreased by 10%. Besides, the increasing wind shear due to the ocean waves decreases the wind power by around 3.5%. Accordingly, it is significant for applying such a coupled air-sea surface-wind farm simulation method when assessing the offshore wind farm performance and the impact of the wind farms on the marine atmospheric boundary layer.

Validating Salinity from SMAP and HYCOM Data with Saildrone Data during EUREC4A-OA/ATOMIC

The 2020 ‘Elucidating the role of clouds-circulation coupling in climate-Ocean-Atmosphere’ (EUREC4A-OA) and the ‘Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign’ (ATOMIC) campaigns focused on improving our understanding of the interaction between clouds, convection and circulation and their function in our changing climate. The campaign utilized many data collection technologies, some of which are relatively new. In this study, we used saildrone uncrewed surface vehicles, one of the newer cutting edge technologies available for marine data collection, to validate Level 2 and Level 3 Soil Moisture Active Passive (SMAP) satellite and Hybrid Coordinate Ocean Model (HYCOM) sea surface salinity (SSS) products in the Western Tropical Atlantic. The saildrones observed fine-scale salinity variability not present in the lower-spatial resolution satellite and model products. In regions that lacked significant small-scale salinity variability, the satellite and model salinities performed well. However, SMAP Remote Sensing Systems (RSS) 70 km generally outperformed its counterparts outside of areas with submesoscale SSS variation, whereas RSS 40 km performed better within freshening events such as a fresh tongue. HYCOM failed to detect the fresh tongue. These results will allow researchers to make informed decisions regarding the most ideal product and its drawbacks for their applications in this region and aid in the improvement of mesoscale and submesoscale SSS products, which can lead to the refinement of numerical weather prediction (NWP) and climate models.

Intraseasonal Air–Sea Interaction Over the Southeastern Indian Ocean and its Impact on Indian Summer Monsoon

The weak monsoon rainfall simulation in the CMIP6 models calls for further process understanding about the Indian summer monsoon (ISM), especially the intraseasonal variabilities. Here, the remote forcing from the Southern Hemisphere on the Indian summer monsoon is examined. Over the southeastern Indian Ocean (SEIO), intraseasonal warm SST anomalies can induce low-level southeasterly wind anomalies and accelerate the background southeasterly wind. According to the mechanism of Wind-Evaporation-SST (WES) feedback, the wind acceleration gives rise to the positive anomalies of surface latent heat flux (LHF). The intraseasonal wind anomalies propagate equatorward along with the background southeasterlies; the positive LHF increases the moist static energy over the equator. As a result, deep convections are reinforced over tropics, which strengthen the northward-propagating monsoon intraseasonal oscillations. During boreal summer, the northward intraseasonal oscillation prompts enhanced rainfall events over the monsoon region. Current results indicate the inter-hemispheric impacts as an inevitable contributor to the heavy precipitation during ISM in the Northern Hemisphere. In CMIP6, the models with better SST simulations over SEIO can have stronger equatorial rainfall and more realistic northward propagation. The unsatisfactory simulations of CMIP6 are associated with the defective ocean–atmosphere interaction over SEIO, and one clue is the feeble variances of intraseasonal oceanic signals over SEIO, which is far from the observation. This research offers a new perspective on the chronic dry monsoon bias in the Northern Hemisphere; the cross-equatorial process and the bias of intraseasonal oceanic variation over SEIO deserve further attention in the coupled models.