Cycle Of Solar Heating Research Articles

Upper Ocean Shear and Density Variability at the Equator during TROPIC HEAT

Abstract A four and one-half day time series of upper-ocean shear and density observations was collected in the tropical Pacific Ocean in November 1984. The measurements were made on the equator at 139°50′W during a period when the equatorial undercurrent was well developed and 20–30 day period velocity fluctuations were prominent. Shear observations were collected with a ship-mounted acoustic-Doppler velocity profiler; density data were obtained from a loosely tethered microstructure instrument. The mean shear profile during the series strongly reflected the structure of the undercurrent; however, the meridional component contributed significantly to the magnitude of the total shear. The mean Richardson number was large near the undercurrent core, but fell to values less than 0.5 25 m above and below the core, and was below 0.25 in the upper 40 m for most of the sample period. Buoyancy frequency varied on a diurnal time scale in the upper 50 m owing to the solar heating cycle, but a compensating diurnal ...

Tropical climatic phase lags and Earth's precession cycle

The Earth's precession cycle causes changes in the seasonal cycle of incident solar radiation, affecting high-latitude and tropical climate in the period range ∼19–24 kyr (refs 1–3). Atmospheric general circulation models (GCMs) have shown that thermally forced monsoon circulations are enhanced by the strengthened seasonal cycle of solar heating that results when summer solstice occurs near perihelion4,5. However, observations indicate that the tropical climatic response reaches a maximum thousands of years after solstice–perihelion alignment2,6. Because the GCM experiments included slowly evolving boundary conditions (such as ice sheet extent and surface albedo), whose influences on tropical climate are not well understood, reasons for the observed lag are unclear. We have addressed this uncertainty by examining the response to the precession cycle. Our model suggests that some features of the monsoon circulation may reach maximum intensity ∼3,000 yr after solstice–perihelion alignment as the result of a direct thermal response to the astronomical forcing.

The response of the upper ocean to solar heating. I: The mixed layer

The results of two earlier papers on convection in the mixed layer and on the solar heating profile are here introduced into a one-dimensional model in order to investigate the following consequences of the daily cycle of solar heating in the upper ocean: 1. the daytime convection depth becomes less than the turbocline depth; 2. the convective power supply to turbulence in the mixed layer is reduced; 3. the mixed layer below the convection layer becomes stably stratified; 4. the depth of the turbocline is reduced, leaving a diurnal thermocline between it and the top of the seasonal thermocline; 5. the heat content and potential energy of the diurnal and seasonal thermoclines are increased, slowing down the subsequent nocturnal descent of the turbocline. These diurnal changes are illustrated by integrating a one-dimensional model forced by the astronomical cycle of solar heating and seasonal variation of surface meteorology derived from Bunker's climatology. The model is integrated for 18 months to show the seasonal modulation of the diurnal cycle. Nocturnal convection plays a dominant role. The convection depth closely follows the thermal compensation depth during the day when they are less than the turbocline depth. Integrating the model with a 24-hour time step leads to large errors in the seasonal variation of mixed layer temperature and depth, and in the source term of isopycnic potential vorticity. The errors are reduced by using two time steps per day, one for the daytime when convection is quenched, the other for the night when it is active. A novel parametrization based on tuning the daily equivalent solar elevation to surface temperature further reduces the error. This parametrization is used to investigate the sensitivity of the seasonal cycles of mixed layer depth and temperature to: (1) seasonality in the surface fluxes; (2) systematic changes in the net annual solar heating; (3) random changes in the seasonal cycles of solar heating induced (i) monthly and (ii) daily. The sensitivity to uncertainty in seawater turbidity is investigated in the same way. The profile of isopycnic potential vorticity subducted into the thermocline depends on the vernal correlation of mixed layer depth and density, so gyre circulation is sensitive to solar heating in spring.

The response of the upper ocean to solar heating. I: The mixed layer

AbstractThe results of two earlier papers on convection in the mixed layer and on the solar heating profile are here introduced into a one‐dimensional model in order to investigate the following consequences of the daily cycle of solar heating in the upper ocean:1. the daytime convection depth becomes less than the turbocline depth;2. the convective power supply to turbulence in the mixed layer is reduced;3. the mixed layer below the convection layer becomes stably stratified;4. the depth of the turbocline is reduced, leaving a diurnal thermocline between it and the top of the seasonal thermocline;5. the heat content and potential energy of the diurnal and seasonal thermoclines are increased, slowing down the subsequent nocturnal descent of the turbocline.These diurnal changes are illustrated by integrating a one‐dimensional model forced by the astronomical cycle of solar heating and seasonal variation of surface meteorology derived from Bunker's climatology. The model is integrated for 18 months to show the seasonal modulation of the diurnal cycle. Nocturnal convection plays a dominant role. The convection depth closely follows the thermal compensation depth during the day when they are less than the turbocline depth. Integrating the model with a 24‐hour time step leads to large errors in the seasonal variation of mixed layer temperature and depth, and in the source term of isopycnic potential vorticity. The errors are reduced by using two time steps per day, one for the daytime when convection is quenched, the other for the night when it is active. A novel parametrization based on tuning the daily equivalent solar elevation to surface temperature further reduces the error. This parametrization is used to investigate the sensitivity of the seasonal cycles of mixed layer depth and temperature to: (1) seasonality in the surface fluxes; (2) systematic changes in the net annual solar heating; (3) random changes in the seasonal cycles of solar heating induced (i) monthly and (ii) daily. The sensitivity to uncertainty in seawater turbidity is investigated in the same way. The profile of isopycnic potential vorticity subducted into the thermocline depends on the vernal correlation of mixed layer depth and density, so gyre circulation is sensitive to solar heating in spring.

Thermally driven wind variability in the planetary boundary layer above Lima, Peru

Land‐sea thermal forcing of the coastal wind near Lima, Peru, is examined as a mechanism to explain the observed wind variability there, in particular the similarity between monthly anomalies of sea surface temperature and wind speeds on the interannual time scales typical of El Niño occurrences. Aerological and surface meteorological observations from Peru coastal sites are analyzed for two time scales: over 15–20 year periods (1958–1977) for monthly averaged data and over a year (1976–1977) for daily data. Wind profile characteristics and annual cycles, and their relation to the cross‐coastal temperature gradient are qualitatively and quantitatively consistent with the thermally forced boundary‐layer model proposed by Lettau (1978) to explain the Peru coastal wind. The cross‐coastal air temperature difference between Lima and Callao explained 55% of the variance in the surface wind speeds at Lima during 1976–1977. The mean annual cycles of the surface wind at Lima and other Peru coastal locations are in phase with the annual solar heating cycle where the seasonality of low cloudiness is large but with the annual cycle of the southeast trade circulation where it is not. This suggests that insolation over the desert is the principal source of thermal forcing variability. During El Niño periods the alongshore wind and the crosscoastal circulation above Lima intensify, in conjunction with anomalously warm sea surface temperatures. The planetary boundary layer is destabilized, and the base of the winter inversion is eroded upward. The observations are consistent with a reduction of desert cloudiness during El Niño events owing to air‐sea exchange and a consequent increase in the thermal forcing of the wind.

Eigenvector Analysis for Prediction of Time Series

Abstract The theorem of singular value decomposition is used to represent a data matrix X as the product of a system with a response R to a forcing function F. Algebraically, R is the matrix of principal components and F the transpose of the matrix of eigenvectors of X′X. If the data are such that the eigenvectors are orthogonal functions of time and they have some recognizable non-random structure permitting predictability in time, then the observed response at time t can be used with the extrapolated forcing function to predict some physical quantity (e.g., temperature, pressure). This method is called the time extrapolated eigenvector prediction (TEEP). An example is given to illustrate the method with a known forcing function, the annual solar heating cycle. We have access to efficient computer routines which will facilitate an extension to much larger data sets.

Thermal stratification in lakes: Analytical and laboratory studies

Theories are developed for the time dependent vertical temperature distribution in a deep lake during the yearly cycle of solar heating and cooling. A portion of the incoming solar radiation is assumed to be absorbed at the water surface, whereas the remainder is absorbed exponentially beneath the surface. Heat is also conducted downward by molecular diffusion. A heat flux balance at the water surface, which accounts for back radiation and evaporative heat loss, is formulated as a boundary condition. The linear, second‐order heat transfer equation is solved by superposition of solutions for the surface absorbed radiation and the internally absorbed radiation. Analytical solutions are given for three different assumptions regarding the time dependence of the incoming radiation and the surface heat losses. At certain times, the resulting temperature and density distribution in the epilimnion are unstable. Under these conditions convective mixing and turbulent diffusion are accounted for by generating a surface mixed layer of uniform temperature. The depth of the surface mixed layer is determined by a thermal energy balance. The theory shows good agreement with field observations of temperature distributions in Lake Tahoe. Experiments are performed using artificial insolation (mercury vapor and infrared lamps) on a laboratory tank. We conclude that it is possible to simulate the development of thermal stratification under laboratory conditions.