The Response of Atmospheric Boundary Layer and Its clouds to Oceanic Eddies in the Kuroshio Extension: Observations and Simulations
The Response of Atmospheric Boundary Layer and Its clouds to Oceanic Eddies in the Kuroshio Extension: Observations and Simulations
Abstract:
A research vessel (R/V), Dongfanghong 2 of Ocean University of China, sailed across a cold eddy in April 2014 and a warm eddy in April 2016 in the Kuroshio Extension. During both of the cruises, the R/V observations captured clear modulations of the marine atmospheric boundary layer (MABL) and developments of marine low clouds, mainly stratocumulus, over the two eddies under different synoptic pressures. Based on the analysis of the observations, possible mechanisms involved in the atmospheric responses to oceanic eddies were suggested. Regional atmospheric model experiments were conducted to aid the interpretations of the in situ observations. Comparisons between the atmospheric responses to the two eddies indicated that air pressure adjustment plays major part in the warm eddy case and that both air pressure adjustment and vertical mixing in the MABL are important in the cold eddy case.
In cold eddy case, enhanced turbulent heat fluxes promoted a well-mixed MABL up to 1900 m deep and a stratocumulus layer over the warm water west of the eddy. Over the cold eddy, the MABL was free of cloud and shrank to 1200 m deep. In the transition zone from the warm water to the cold eddy, the MABL deepened and convective cumulus developed under a temperature inversion at 2800 m. Model results indicate that cold air over the eddy induces high pressure anomaly, which drives anomalous easterly winds in the transition zone, causing surface convergence to deepen the MABL and clouds. The atmospheric asymmetric responses in surface divergence are due to the combined effects of the vertical mixing and pressure adjustment.
In warm eddy case, the increased surface heat flux over the warm ocean eddy lowered surface pressure (low pressure anomaly) and thereby accelerated southeasterly winds. The southeasterly winds enhanced the air-sea interface heat flux, which in turn deepened the low pressure anomaly and promoted low-level convergence and rising motion over the warm eddy. The lifting condensation level lowered and the top of the marine atmospheric boundary layer rose, thereby aiding the development of the stratocumulus. Sea surface temperature anomalies associated with the warm eddy accounted for 80% of the total ascending motion and 95% of total cloud water mixing ratio in the MABL during the development of stratocumulus.
In cold eddy case, enhanced turbulent heat fluxes promoted a well-mixed MABL up to 1900 m deep and a stratocumulus layer over the warm water west of the eddy. Over the cold eddy, the MABL was free of cloud and shrank to 1200 m deep. In the transition zone from the warm water to the cold eddy, the MABL deepened and convective cumulus developed under a temperature inversion at 2800 m. Model results indicate that cold air over the eddy induces high pressure anomaly, which drives anomalous easterly winds in the transition zone, causing surface convergence to deepen the MABL and clouds. The atmospheric asymmetric responses in surface divergence are due to the combined effects of the vertical mixing and pressure adjustment.
In warm eddy case, the increased surface heat flux over the warm ocean eddy lowered surface pressure (low pressure anomaly) and thereby accelerated southeasterly winds. The southeasterly winds enhanced the air-sea interface heat flux, which in turn deepened the low pressure anomaly and promoted low-level convergence and rising motion over the warm eddy. The lifting condensation level lowered and the top of the marine atmospheric boundary layer rose, thereby aiding the development of the stratocumulus. Sea surface temperature anomalies associated with the warm eddy accounted for 80% of the total ascending motion and 95% of total cloud water mixing ratio in the MABL during the development of stratocumulus.