Global ocean sea surface temperatures
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Sea‐surface temperature (SST) is a key driver for various interactions and feedbacks between components of the Earth System and can control local weather and climate. The formation of marine fog, for example, can be sensitive to small changes in SST at a scale of a few kilometres. As a contribution to understanding processes at the interface between air and sea, this article discusses results from a state‐of‐the‐art fully coupled regional atmosphere–land–ocean–wave prediction system for the UK at km scale. This study focuses on the impact of the changes in surface forcing resulting from coupling SST in the marine boundary layer and formation of summertime coastal fog over the North Sea. A study from July 2013 provided a good case to evaluate the role of SST in fog evolution. The benefit of an evolving SST in the coupled simulation is shown in capturing a warming trend in observed SST over the five‐day case study period, with a root‐mean‐square error (RMSE) against in situ observations of 1.1 K. In contrast, in uncoupled atmosphere‐only simulations, the initial‐condition SST is persisted for the duration of the case, as is more typical in current operational numerical weather prediction (NWP). In the uncoupled simulations, a cold bias develops over the modelling period and the RMSE against observed SST is 2.4 K. The impact of coupling is shown to propagate into the overlying marine boundary layer and therefore affect the formation of coastal fog. Increased heat flux from a relatively warmer sea surface in the coupled simulations led to near‐surface atmospheric instability, hampering stratus lowering and destroying the fog‐promoting inversion layer. This significantly reduced fog fractions in selected regions. The value of model coupling was assessed by comparing coupled and uncoupled simulations initialized at different times ahead of fog development.
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Thermal expansion and contraction affect the ocean surface level. With the installation of the ARGO temperature measurements in 2004, we now have high-precision records of the changes in temperature both in horizontal and vertical dimension. In the Equatorial region (30 °N-30 °S) there has been a warming of about 0.25 °C in the uppermost 50-70m. This would rise sea level by about 1.2mm. In the Arctic region (55 °N-65 °N) there has been a cooling of about 0.20 °C in the upper 1400m. This would lower sea level by about 4.9cm. This invalidates previous global thermal expansion estimates.
Argo
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This paper reviews the current state of observation, parameterization and evaluation of surface air-sea energy and gas fluxes, and sea ice, for the purposes of monitoring and predicting the state of the global ocean.The last 10 years have been marked by the development of more accurate parameterizations of turbulent fluxes, in particular COARE-3 (Coupled Ocean-Atmosphere Response Experiment).A seamless approach to surface flux observing systems is also being developed ranging from highly accurate observations on buoys and research ship campaigns to the longstanding Voluntary Observing Ship (VOS) scheme.In addition to flux products based on in situ data, satellite measurements and numerical weather prediction, several hybrid products have been developed which combine data from these different sources.Satellite monitoring of sea ice has been extended to more accurate and higher resolution estimation of ice extent and quantification of ice thickness.Global air-sea CO 2 flux products are now based on significantly better-sampled datasets reducing the uncertainty in the ocean carbon budget.Despite these advances, considerable gaps remain in our understanding of air-sea fluxes, for example, at both high and low wind speeds, for gas and aerosol exchange and in marginal ice zones.Furthermore, there are serious concerns about the recent decline in the number of VOS observations.Closure of global and regional energy balances still cannot be achieved without adjustments to the flux fields and/or the underlying surface meteorological variables.The impact of sampling on interannual variability of fluxes makes estimates of climate tendencies in air-sea exchanges highly uncertain.In order to meet these challenges we formulate a future vision of a surface flux observing system, which provides a synergy of in situ measurements (buoys, research vessels and merchant ships), remote sensing and models.
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Analysis of the oscillations of the yearly average ocean levels, measured around the coasts of the world ocean, establishes a general rise in all observed ocean levels. This recent rise in ocean level is closely related to the general warming of the earth.
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We have investigated the effects of assimilating sea ice concentration (SIC) data on a simulation of Arctic Ocean climate using an atmosphere-ocean-sea ice coupled model. Our results show that the normal overestimation of summertime SIC in the East Siberian Sea and the Beaufort Sea in simulations without sea-ice data input can be greatly reduced by assimilating sea-ice data and that this improvement is also evident in a following hindcast experiment for 3-4 years after the initialization of the assimilation. In the hindcast experiment, enhanced heat storage in both sea ice and in the ocean surface layer plays a central role in improving the accuracy of the sea ice distribution, particularly in summer. Our detailed investigation suggests that the ice-albedo feedback and the feedback associated with the atmospheric pressure pattern generated by the improved estimation of SIC work more effectively to retain the heat signal after initialization for a coupled atmosphere-ocean-sea ice system prediction. In addition, comparison with field observations confirms that the model fails to produce a realistic feedback loop, which is (presumably) due to inadequacies in both the ice-cloud feedback model and the feedback via the Beaufort Gyre circulation. Further development of coupled models is thus required to better define Arctic Ocean climate processes and to improve the accuracy of their predictions.
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