Evaporation Anomalies Research Articles

Ekman Transport Dominates Local Air–Sea Fluxes in Driving Variability of Subantarctic Mode Water

Subantarctic Mode Water (SAMW) is formed by deep convection in winter on the equatorward side of the Antarctic Circumpolar Current. Observations south of Australia show that the SAMW temperature (T) and salinity (S) vary significantly from year to year. The magnitude and density-compensating nature of the temperature and salinity changes cannot be explained by variations in air–sea exchange of heat and freshwater in the subantarctic zone where SAMW is formed. Rather, the T and S variability reflects variations in the equatorward Ekman transport of cool, low salinity water across the subantarctic front. Experiments with a coupled climate model suggest that the observations south of Australia are typical of the subantarctic zone. The model changes in SAMW properties are correlated significantly (at 99% level) with changes in wind stress and northward Ekman transport of cool low-salinity water. In contrast, air–sea heat flux anomalies are mostly a response to changes in SST, and anomalies in precipitation minus evaporation in the subantarctic zone are too small to account for the model SAMW salinity variations. Mode waters provide significant reservoirs of heat and freshwater that extend below the depth of the seasonal thermocline and, hence, can persist from year to year. The fact that wind stress variations can drive changes in mode water properties therefore has implications for climate variability.

Stochastic Aspects of Lake Ontario Evaporation

Time series analysis was carried out on long‐term monthly mean values of evaporation from Lake Ontario, which were generated in an earlier study, and on related meteorologic parameters. Correlation and spectral analyses showed that the annual cycle was dominant in all the time series. Evaporation is usually high in autumn and winter and low in spring and summer. A warming trend was observed in the air temperature and a drying trend in the relative humidity series. Except for the wind speed, no significant trend was found for the other time series. Cross correlation and cross‐spectral analyses showed a close relationship between evaporation anomalies and the anomalies of the other parameters. A first order Markov model adequately described the evaporation, air temperature, and relative humidity anomalies, whereas a second order model fitted the anomalies of wind speed and water surface temperature. Morton's [1967] water budget method yields higher evaporation estimates in the summer and lower estimates in the winter than the mass transfer estimates.

A parametric method for computing the mean water budget of the atmosphere

Using the equation of conservation of water vapor in the troposphere, the difference, evaporation at the surface minus precipitation, is expressed as a function of total cloudiness (or mean tropospheric relative humidity) and of temperature and horizontal wind at the surface and in mid-troposphere. Computations of the annual normal values of evaporation minus precipitation using the resulting formulas together with monthly averages of the required parameters for the Northern Hemisphere, agree well in the general patterns with estimates made by Starr, Peixoto and Crisi, as well as with climatological values. However, there are large small-scale discrepancies in the three estimates. The computations have a strong dependence on the divergence of the horizontal wind, and some of the discrepancies must be attributed to errors in evaluating the latter. To determine the importance of anomalies of the difference, evaporation minus precipitation, computations for February 1964 and February 1962 are carried out, showing that the values of the difference between these two months are of the same order of magnitude as the monthly values themselves. If positive values indicate wind from the south, the values of evaporation minus precipitation are inversely correlated to the meridional wind component on a broad scale. Furthermore the computations show that the anomalies in evaporation minus precipitation depend strongly not only on the anomalies of the meridional wind component but also on the anomalies of precipitable water, which in turn depend on the anomalies of surface temperature and cloudiness (or mean tropospheric relative humidity). DOI: 10.1111/j.2153-3490.1968.tb00404.x

A parametric method for computing the mean water budget of the atmosphere

Using the equation of conservation of water vapor in the troposphere, the difference, evaporation at the surface minus precipitation, is expressed as a function of total cloudiness (or mean tropospheric relative humidity) and of temperature and horizontal wind at the surface and in mid-troposphere. Computations of the annual normal values of evaporation minus precipitation using the resulting formulas together with monthly averages of the required parameters for the Northern Hemisphere, agree well in the general patterns with estimates made by Starr, Peixoto and Crisi, as well as with climatological values. However, there are large small-scale discrepancies in the three estimates. The computations have a strong dependence on the divergence of the horizontal wind, and some of the discrepancies must be attributed to errors in evaluating the latter. To determine the importance of anomalies of the difference, evaporation minus precipitation, computations for February 1964 and February 1962 are carried out, showing that the values of the difference between these two months are of the same order of magnitude as the monthly values themselves. If positive values indicate wind from the south, the values of evaporation minus precipitation are inversely correlated to the meridional wind component on a broad scale. Furthermore the computations show that the anomalies in evaporation minus precipitation depend strongly not only on the anomalies of the meridional wind component but also on the anomalies of precipitable water, which in turn depend on the anomalies of surface temperature and cloudiness (or mean tropospheric relative humidity). DOI: 10.1111/j.2153-3490.1968.tb00404.x