Variability In Air-sea Fluxes Research Articles

Carbon Air–Sea Flux in the Arctic Ocean from CALIPSO from 2007 to 2020

Quantified research on the Arctic Ocean carbon system is poorly understood, limited by the scarce available data. Measuring the associated phytoplankton responses to air–sea CO2 fluxes is challenging using traditional satellite passive ocean color measurements due to low solar elevation angles. We constructed a feedforward neural network light detection and ranging (LiDAR; FNN-LID) method to assess the Arctic diurnal partial pressure of carbon dioxide (pCO2) and formed a dataset of long-time-series variations in diurnal air–sea CO2 fluxes from 2001 to 2020; this study represents the first time spaceborne LiDAR data were employed in research on the Arctic air–sea carbon cycle, thus providing enlarged data coverage and diurnal pCO2 variations. Although some models replace Arctic winter Chl-a with the climatological average or interpolated Chl-a values, applying these statistical Chl-a values results in potential errors in the gap-filled wintertime pCO2 maps. The CALIPSO measurements obtained through active LiDAR sensing are not limited by solar radiation and can thus provide ‘fill-in’ data in the late autumn to early spring seasons, when ocean color sensors cannot record data; thus, we constructed the first complete record of polar pCO2. We obtained Arctic FFN-LID-fitted in situ measurements with an overall mean R2 of 0.75 and an average RMSE of 24.59 µatm and filled the wintertime observational gaps, thereby indicating that surface water pCO2 is higher in winter than in summer. The Arctic Ocean net CO2 sink has seasonal sources from some continental shelves. The growth rate of Arctic seawater pCO2 is becoming larger and more remarkable in sectors with significant sea ice retreat. The combination of sea surface partial pressure and wind speed impacts the diurnal carbon air–sea flux variability, which results in important differences between the Pacific and Atlantic Arctic Ocean. Our results show that the diurnal carbon sink is larger than the nocturnal carbon sink in the Atlantic Arctic Ocean, while the diurnal carbon sink is smaller than the nocturnal carbon sink in the Pacific Arctic Ocean.

Seasonal variability of air-sea fluxes in two contrasting basins of the North Indian Ocean

Latent Heat Flux (LHF) and Sensible Heat Flux (SHF) are the two important parameters in air-sea interactions and hence have significant implications for any coupled ocean-atmospheric model. These two fluxes are conventionally computed from met-ocean parameters using bulk aerodynamic formulations; or the Coupled Ocean Atmosphere Response Experiment (COARE) bulk flux algorithms. Here COARE 3.5 algorithm is used to estimate the heat flux from two Ocean Moored Buoy Network for northern Indian Ocean (OMNI) buoy met-ocean observations in Arabian Sea (AS) and the Bay of Bengal (BoB). The AS and BoB are two ocean basins which are situated in same latitudinal range, but experience drastically differing in their met-ocean conditions, especially during the monsoon seasons. In this study, we have computed and compared the LHF and SHF at two different buoy locations in the AS and BoB and analysed their variability during three different seasons from November 2012 to September 2013. Additionally, 20 years (1998–2017) of Objectively Analysed (OA) Flux data sets collocated with the OMNI buoy locations were also utilised to the analyse the long period seasonal variabilities. The flux terms show strong seasonal variability with several peaks during the monsoon seasons in both the ocean basins. LHF varies directly with wind speed (WS) and inversely with relative humidity (RH). The correlation of LHF with WS is greater than 0.7 and RH is nearly -0.6 with few exceptions during pre-monsoon season in the AS and southwest monsoon in the BoB. However, SHF is less correlated with WS (∼0.3 to 0.5). The difference of sea surface temperature and air temperature (denoted as SST-AT) plays a significant role in determining SHF with a correlation greater than 0.6 in both the basins.

Air-Sea Heat Flux Variability in the Southeast Indian Ocean and Its Relation With Ningaloo Niño

Previous studies suggest that both air-sea heat flux anomalies and heat advection caused by an anomalous Leeuwin Current play an important role in modulating the sea surface temperature (SST) variability associated with the Ningaloo Niño. However, the estimates of surface heat fluxes vary substantially with the datasets, and the uncertainties largely depend on the time scale and locations. This study investigates air-sea flux variability associated with the Ningaloo Niño using multiple datasets of surface fluxes. The climatological net surface heat flux off the west coast of Australia from six major air-sea flux products shows large uncertainties, which exceeds 80 W m-2, especially in the austral summer when the Ningaloo Niño develops. These uncertainties stem mainly from those in shortwave radiation and latent heat flux. The use of different bulk flux algorithms and uncertainties of bulk atmospheric variables (wind speed and air specific humidity) are mostly responsible for the difference in latent heat flux climatology between the datasets. The composite evolution of air-sea heat fluxes over the life cycle of Ningaloo Niño indicates that the anomalous latent heat flux is dominant for the net surface heat flux variations, and that the uncertainties in latent heat flux anomaly largely depend on the phase of the Ningaloo Niño. During the recovery period of Ningaloo Niño, large negative latent heat flux anomalies (cooling the ocean) are evident in all datasets and thus significantly contribute to the SST cooling. Because the recovery of winds occurs earlier than SST, high SST and strong winds favor large evaporative cooling during the recovery phase. In contrast, the role of latent heat flux during the developing phase is not clear, because the sign of the anomalies depends on the datasets in this period. The use of high-resolution SST data, which can adequately represent SST variations produced by the anomalous Leeuwin Current, could largely reduce the errors in latent heat flux anomalies during the onset and peak phases.

Interannual variability of carbon fluxes in the North Sea from 1970 to 2006 – Competing effects of abiotic and biotic drivers on the gas-exchange of CO2

The three-dimensional biogeochemical model ECOHAM was applied to the Northwest European Continental Shelf (NECS) (47° 41′ – 63° 53′ N, 15° 5′ W – 13° 55′ E) for the years 1970–2006. The development of annual carbon fluxes was analysed for the North Sea as the inner shelf region. We divided the North Sea into several regions, the northern North Sea, the southern North Sea, the German Bight and the Southern Bight for a more detailed analysis. To separate the effect of physical and biological processes a second simulation without biology was performed. The results of our method for calculating the biological pCO2 were in good agreement with the biological pCO2 calculated after the method of Takahashi et al. (2002). While in the standard run the North Sea acted as sink for atmospheric CO2, in the run without biology the North Sea was a continuous source for atmospheric CO2.The main drivers of the air-sea flux variability were identified as being temperature, net ecosystem production and pH. The eutrophication due to high riverine nutrient inputs during the 1980s had no significant effect on the air-sea flux of CO2 because in contrast to net primary production, net ecosystem production did not respond to the period of higher phosphate input. The increase of sea surface temperature of 0.027 °C yr−1 over the simulation period and the pH decline of 0.002 yr−1 led to a decline of the uptake of atmospheric CO2 by the North Sea of about 30% in the last decade of the simulation period. A special feature occurred in the year 1996, where a cold sea surface temperature anomaly led to an additional (physical) uptake of atmospheric CO2 and corresponded with a low primary and net ecosystem production, which on the other hand led to less biologically induced uptake of CO2.Our results indicate an ongoing decline of the uptake capacity for atmospheric carbon dioxide of the North Sea for future scenarios.

Role of wind stress and heat fluxes in interannual‐to‐decadal variability of air‐sea CO2 and O2 fluxes in the North Atlantic

A coupled ecosystem‐circulation model of the North Atlantic is used to examine the individual contributions by wind stress and surface heat fluxes to naturally driven interannual‐to‐decadal variability of air‐sea fluxes of CO2 and O2 during 1948–2002. The model results indicate that variations in O2 fluxes are mainly driven by variations in surface heat fluxes in the extratropics (15°N to 70°N), and by wind stress in the tropics (10°S to 15°N). Conversely, variations in simulated CO2 fluxes are predominantly wind‐stress driven over the entire model domain (18°S to 70°N); while variability in piston velocity and surface heat fluxes is less important. The simulated uptake of O2 by the North Atlantic amounts to 70 ± 11 Tmol yr−1 to which the subpolar region (45°N to 70°N) contributes by 62 ± 10 Tmol yr−1. Whereas the subpolar North Atlantic takes up more than 2/3 of the total carbon absorbed by the North Atlantic in our model (about 0.3 Pg C yr−1), interannual variability of air‐sea CO2 fluxes reaches similar values (about 0.01 Pg C yr−1 each) in the subpolar (45°N to 70°N), the subtropical (15°N to 45°N) and the equatorial (10°S to 15°N) Atlantic.