The Effect of Milankovitch Variations in Insolation on Equatorial Seasonality

Although the sun crosses the equator 2 times per year at the equinoxes, at times in the past the equatorial insolation has had only one maximum and one minimum throughout the seasonal cycle because of Milankovitch orbital variations. Here a state-of-the-art coupled atmosphere–ocean general circulation model is used to study the effect of such insolation forcing on equatorial surface properties, including air and sea temperature, salinity, winds, and currents. It is shown that the equatorial seasonality is altered according to the insolation with, for example, either maximum sea surface temperature (SST) close to the vernal equinox and minimum SST close to the autumnal equinox or vice versa. The results may have important implications for understanding tropical climate as well as for the interpretation of proxy data collected from equatorial regions

The diurnal cycle and temporal trends of surface winds

Winds play an essential role in the climate system. In this study, we analyze the global pattern of the diurnal cycle of surface (10 m) winds from the ERA5 reanalysis data. We find that over the land and especially over sand dune regions, the maximal wind speed and wind drift potential (DP) occur during the hours around midday. However, over the ocean, the wind also peaks at night. Using the sensible heat flux, we show that the weaker winds over land at night are due to a nocturnal cooling that decouples upper atmospheric levels and their associated stronger winds from the surface -- nocturnal cooling is much smaller over the ocean. We also analyze wind data from more than 400 meteorological stations in the USA and find a similar diurnal trend as in the reanalysis data. The timing (during the day) of the maximum wind speed has not varied much over the past 70 years. Yet, the wind speed, wind power, and wind drift potential exhibit significant increases with time over the ocean and, to a much lesser degree, over the land and sand dune regions. We compare the USA and Europe DP and wind speed of the ERA5 to that of meteorological stations and find that the ERA5 significantly underestimates real winds; however, the temporal patterns of the two are similar

Europa's dynamic ocean: Taylor columns, eddies, convection, ice melting and salinity

The deep ocean (~100 km) of Europa, Jupiter's moon, is covered by a thick (tens of km) icy shell, and is one of the most probable places in the solar system to find extraterrestrial life. Yet, its ocean dynamics and its interaction with the ice cover have so far received little attention. Previous studies suggested that Europa's ocean is turbulent, yet neglected to take into account the effects of ocean salinity and appropriate boundary conditions for the ocean's temperature. Here, the ocean dynamics of Europa is studied using global ocean models that include non-hydrostatic effects, a full Coriolis force, consistent top and bottom heating boundary conditions, and including the effects of melting and freezing of ice on salinity. The density is found to be dominated by salinity effects and the ocean is very weakly stratified. The ocean exhibits strong transient vertical convection, eddies, low latitude zonal jets and Taylor columns parallel to Europa's axis of rotation. In the equatorial region, the Taylor columns do not intersect the ocean bottom and propagate equatorward, while off the equator, the Taylor columns are static. The meridional oceanic heat transport is intense enough to result in a nearly uniform ice thickness, that is expected to be observable in future missions

Asymmetry of Daily Temperature Records

The authors study the NCEP–NCAR reanalysis temperature records and find that surface daily mean temperature cools rapidly and warms gradually at the midlatitudes (around 40°N and 40°S). This “asymmetry” is partially related to the midlatitude cyclone activity, in which cold fronts are significantly faster and steeper than warm fronts, and to intrusions of cold air. The gradual warming may be attributed also to the radiative relaxation to average atmospheric conditions after the passage of cold fronts or other intrusions of cold air. At the high latitudes there is an opposite asymmetry with rapid warming and gradual cooling; this asymmetry may be attributed to the radiative relaxation to average cold atmospheric conditions after the passage of warm fronts or intrusions of warm air.Earth and Planetary Science

Box modeling of the Eastern Mediterranean sea

In ∼1990 a new source of deep water formation in the Eastern Mediterranean was found in the southern part of the Aegean sea. Till then, the only source of deep water formation in the Eastern Mediterranean was in the Adriatic sea; the rate of the deep water formation of the new Aegean source is 1 Sv, three times larger than the Adriatic source. We develop a simple three-box model to study the stability of the thermohaline circulation of the Eastern Mediterranean sea. The three boxes represent the Adriatic sea, Aegean sea, and the Ionian seas. The boxes exchange heat and salinity and may be described by a set of nonlinear differential equations. We analyze these equations and find that the system may have one, two, or four stable flux states. We conjecture that the change in the deep water formation in the Eastern Mediterranean sea is attributed to a switch between the different states on the thermohaline circulation; this switch may result from decreased temperature and/or increased salinity over the Aegean sea