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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 atmosphereāocean 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
Box modeling of the Eastern Mediterranean sea
In ā¼1990 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
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