Atmospheric Response to Mesoscale Ocean Eddies over the South China Sea

The South China Sea（SCS） is an eddy-active area. Composite analyses based on 438 mesoscale ocean eddies during 2000–2012 revealed the status of the atmospheric boundary layer is influenced remarkably by such eddies. The results showed cold-core cyclonic（warm-core anticyclonic） eddies tend to cool（warm） the overlying atmosphere and cause surface winds to decelerate（accelerate）. More than 5% of the total variance of turbulent heat fluxes, surface wind speed and evaporation rate are induced by mesoscale eddies. Furthermore, mesoscale eddies locally affect the columnar water vapor, cloud liquid water, and rain rate. Dynamical analyses indicated that both variations of atmospheric boundary layer stability and sea level pressure are responsible for atmospheric anomalies over mesoscale eddies. To reveal further details about the mechanisms of atmospheric responses to mesoscale eddies, atmospheric manifestations over a pair of cold and warm eddies in the southwestern SCS were simulated. Eddy-induced heat flux anomalies lead to changes in atmospheric stability. Thus, anomalous turbulence kinetic energy and friction velocity arise over the eddy dipole, which reduce（enhance） the vertical momentum transport over the cold（warm） eddy, resulting in the decrease（increase） of sea surface wind. Diagnoses of the model＇s momentum balance suggested that wind speed anomalies directly over the eddy dipole are dominated by vertical mixing terms within the atmospheric boundary layer, while wind anomalies on the edges of eddies are produced by atmospheric pressure gradient forces and atmospheric horizontal advection terms.

Delayed Atmospheric Temperature Response to ENSO SST:Role of High SST and the Western Pacific

Tropical zonally symmetric atmospheric warming occurs during ENSO＇s warm phase, and lags the equatorial east Pacific sea surface temperatures （SSTs） by 3-4 months. The role of the Indian and Atlantic oceans on the atmospheric delayed response has been pointed out by earlier studies. For 1951-2004, a regression analysis based on the observed SST data shows the western Pacific has a similarly important role as the Indian and Atlantic. Nevertheless, there is time mismatch of around 1-2 months between the zonally averaged tropical SST anomalies and the atmospheric temperature anomalies. It is expected that the tropospheric temperature should be controlled by diabatic heating forcing, which is sensitive primarily to SST anomalies over regions of high climatological SST, rather than to the tropical mean SST anomalies. To describe this mechanism, we propose a parameterization scheme of diabatic heating anomalies dependant on SST anomalies and climatological SST. The 1-2 month mismatch between tropical mean SST anomalies and air temperature anomalies is reconciled by the fact that the tropical mean heating anomalies are dominated by the SST anomalies over regions of high climatological SST, and lag the tropical mean SST anomalies by 1 month. The mechanism described by this parameterization scheme joins several physical processes of ENSO with reasonable time intervals. And the parameterized heating anomalies work better than the tropical mean SST anomalies for capturing the atmospheric temperature signal relative to ENSO.

Impacts of Oceanic Fronts and Eddies in the Kuroshio-Oyashio Extension Region on the Atmospheric General Circulation and Storm Track

This paper reviews the progress in our understanding of the atmospheric response to midlatitude oceanic fronts and eddies,emphasizing the Kuroshio-Oyashio Extension(KOE)region.Oceanic perturbations of interest consist of sharp oceanic fronts,temperature anomalies associated with mesoscale eddies,and to some extent even higher-frequency submesoscale variability.The focus is on the free atmosphere above the boundary layer.As the midlatitude atmosphere is dominated by vigorous transient eddy activity in the storm track,the response of both the time-mean flow and the storm track is assessed.The storm track response arguably overwhelms the mean-flow response and makes the latter hard to detect from observations.Oceanic frontal impacts on the mesoscale structures of individual synoptic storms are discussed,followed by the role of oceanic fronts in maintaining the storm track as a whole.KOE fronts exhibit significant decadal variability and can therefore presumably modulate the storm track.Relevant studies are summarized and intercompared.Current understanding has advanced greatly but is still subject to large uncertainties arising from inadequate data resolution and other factors.Recent modeling studies highlighted the importance of mesoscale eddies and probably even submesoscale processes in maintaining the storm track but confirmation and validation are still needed.Moreover,the atmospheric response can potentially provide a feedback mechanism for the North Pacific climate.By reviewing the above aspects,we envision that future research shall focus more upon the interaction between smaller-scale oceanic processes(fronts,eddies,submesoscale features)and atmospheric processes(fronts,extratropical cyclones etc.),in an integrated way,within the context of different climate background states.

A Theoretical Explanation of Anomalous Atmospheric Circulation Associated with ENSO Modoki during Boreal Winter

Based on a linear model, the present study provides analytical solutions for ideal triple forcing sources similar to sea surface temperature anomaly （SSTA） pat- terns associated with El Nino-Southern Oscillation （ENSO） Modoki in winter. The ideal triple pattern is composed of an equatorially symmetric heat source in the middle and equatoriaUy asymmetric cold forcing in the southeast and northwest. The equatorially symmetric heat source excites low-level cyclonic circulation anomalies associated with Rossby waves in both hemispheres, while the northwest- ern and southeastern equatorially asymmetric cold sources induce low-level anomalous anticyclones associated with Rossby waves in the hemisphere where the forcing source is located. Low-level zonal winds converge toward the heat sources associated with Kelvin and Rossby waves. Due to unequal forcing intensity in the northwest and southeast, atmospheric responses around the equatorially symmetric forcing become asymmetric, and low-level cyclonic circulation anomalies in the Southern Hemisphere become greater than those in the Northern Hemisphere. Ascending （descending） flows coincide with heat （cold） sources, resulting in a double-cell structure over the regions of forcing sources. Ideal triple patterns similar to SSTA patterns associated with La Nina Modoki produce opposite atmospheric responses. The theoretical atmospheric responses are consistent with observed circulation anomalies associated with ENSO Modoki. Therefore, the theoretical solutions can explain the dynamics responsible for atmospheric circulation anomalies associated with ENSO Modoki events.

Seasonal variation of atmospheric coupling with oceanic mesoscale eddies in the North Pacific Subtropical Countercurrent

This study investigated the seasonal variation in the atmospheric response to oceanic mesoscale eddies in the North Pacific Subtropical Countercurrent(STCC)and its mechanism,based on satellite altimetric and reanalysis datasets.Although mesoscale eddy in the study area is more active in summer,the sea surface temperature(SST)anomaly associated with mesoscale eddies is more intense and dipolar in winter,which is largely due to the larger background SST gradient.Similarly,the impact of the oceanic eddy on sea surface wind speed and heat flux is strongest in winter,whereas its effect on precipitation rate is more significant in summer.The study revealed that the SST gradient in STCC could impact the atmosphere layer by up to 800 h Pa(900 h Pa)in boreal winter(summer)through the dominant vertical mixing mechanism.Moreover,the intensity of the SST gradient causes such seasonal variation in mesoscale air-sea coupling in the study region.In brief,a stronger(weaker)background SST gradient field in wintertime(summertime)leads to a larger(smaller)eddy-induced SST anomaly,thus differently impacting atmosphere instability and transitional kinetic energy flux over oceanic eddies,leading to seasonal variation in mesoscale air-sea coupling intensity.

RESPONSE OF TROPICAL ATMOSPHERIC CIRCULATION TO THE OCEAN-LAND SURFACE HEATING——AN ANALYTICAL SIMULATION

A linear steady model is constructed to investigate the response of the tropical atmosphere to diabatic heating.The basic equations are similar to those used by Gill(1980),but the long-wave approximation is removed and periodic boundary conditions are taken in longitude.According to the features of the underlying surface temperature(including oceans and land),the heat sources(sinks)are given.Using this analytical model,we have simulated the climatological fields of wind and air pressure in the lower layers of the tropical and subtropical atmosphere in summer(June—August)and winter(December—February). The main features of observations are depicted in simulated fields.

A NUMERICAL STUDY ON THE MEDIUM-RANGE RESPONSES OF THE ATMOSPHERE TO THE ABNORMAL SOIL MOISTURE

By employing the improved T42L9 spectral model introduced by NMC (Beijing) from ECMWF and utilizing the FGGE-IIIb data covering the period of 14—19 June 1979, the atmospheric responses to the abnormal soil moisture during the medium-range period have been studied numerically. According to the initial field at 12 GMT 14 June, a five-day numerical experiment under different conditions of the soil moisture has been carried out respectively. The monthly mean climatological soil moisture for June has been used in the control experiment in the initial time and it changes with time according to the moisture budget equation at the land surface. Comparing with the experiments with dry or wet soil. one can conclude that: 1) Source of precipitation over continents in summer consists of the land-surface evaporation and the moisture transfer from oceans. Their intensities are comparable during the medium-range time scale when the soil evaporates its moisture sufficiently. Therefore, the soil moisture can influence the global precipitation and the general circulation significantly; 2) By influencing the thermodynamic difference between land and sea,the soil moisture can change the intensity of monsoon and precipitation distribution; 3) The response of the atmosphere to the abnormal soil moisture has the characteristics of geographical distribution and nonlinear interactions; 4) Human activi- ties on the world can influence the environment greatly.