Surface Water Hydrology

This article discusses surface water hydrology, including ENSO, El Nino, La Nina, frontal systems, and evaporation. It explores the factors influencing evaporation and the effects of cold fronts and the Southern Annular Mode on weather patterns.

Running head: SURFACE WATER HYDROLOGY
Engin5201 – surface water hydrology
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SURFACE WATER HYDROLOGY 2
Problem 1
ENSO
This is an acronym which stands for El Nino Southern Oscillation. It involves changes in the
temperature of waters in the central and eastern tropical Pacific Ocean. It is a recurring
phenomenon whose effects are felt from about 3 to 7 years, where the surface waters involved
are 1 to 3°C warmer or cooler than is normal (US National Weather Service, 2019). This
oscillating warming and cooling cycle is what is called the ENSO cycle. It affects rainfall
distribution in the tropics and significantly influences the weather in parts of the world. The
cycle involves two intense phases, the El Nino and the La Nina, and a third intermediate phase
known as ENSO-neutral.
El Nino
In this the ocean surface warms to above average Surface Sea Temperatures (SST) in Central and
Eastern Tropical Pacific Ocean (Guilyardi, et al., 2009). This causes less rain to fall over
Indonesia and more over the ocean. The low level ‘easterly’ wind which under normal
circumstances blow from east to west start blowing in the opposite direction earning them the
name ‘westerly’ winds. In summary, the strength of the El Nino is directly proportional to the
ocean’s temperature anomaly (Cobb, et al., 2013). Below are some of the ways in which El Nino
has affected Victoria’s weather;
Reduced rainfall-In Victoria which is in South Eastern Australia, there was significantly less
rainfall observed during the very strong 1997-1998 El Nino (Guilyardi, et al., 2009). It is
important to note that another weaker El Nino caused widespread drought in the country, which
shows that rainfall is not necessarily impacted by the strength of the El Nino.

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Higher temperatures- most of Southern Australia has abnormally warm weather during El Nino.
The warmest daytimes through the years in all four seasons have been experienced during El
Nino. The temperatures have been lately furthered by warming trends meaning El Nino years
have been gradually getting warmer. The heats have been more intense but prolonged warm
spells have been less frequent. This is because weather systems are more mobile during this time
and there are no high pressure systems to act as buffers.
More frosting- due to reduced cloud cover, eastern areas experience abnormally cool nights.
Northern Victoria can experience up to 30% more days with frost than average, which can
negatively affect agriculture if it occurs in spring (Cobb, et al., 2013).
When an El Nino happens concurrently with a positive Indian Ocean Dipole (IOD) event it
creates a risk of fire like Ash Wednesday in February 1983 (Guilyardi, et al., 2009).
La Nina
This is the exact opposite of El Nino. It refers to a cooling of the ocean surface to below average
SST. This causes more rain in Indonesia and less in the ocean (Kug & Ham, 2011). Easterly
winds in this case become stronger. Similarly, a bigger the ocean temperature anomaly results in
a stronger La Nina.
ENSO-neutral
This is neither of the two extremes. The Tropical Pacific SSTs are close to average and on
instances where the temperatures a noticeably higher or lower, there is no change in atmospheric
conditions.
Frontal systems

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These are commonly called cold fronts. A cold front occurs when a mass of relatively cool air
moves into a region of warm. In Australia the cold, dry southerly winds move north and meet the
warm and moist northerly winds moving south. The difference temperature causes a figurative
line to form, one spanning up to several hundred thousand kilometres, to separate the two winds
(Kala, Lyons, & Nair, 2011). The line is called a front. Cold air moving over the area causes
temperature drops of up to 20°C within less than an hour. The higher density of the cold air
makes it move below the warm, which moves above to form clouds, rainfall and thunderstorms
(Kala, Lyons, & Nair, 2011). As cold air surges, it compresses the atmosphere ahead of it thus
strengthening the winds in front. In summer, strong north-western winds ahead of a cold front
bring hot dry winds to south-eastern areas like Victoria leading to a risk of fire.
Common effects of cold fronts are abruptly lowered temperatures, rainfall, increased wind speed
and change in direction of winds. Cold fronts are known to have devastating effects. In 2014 a
sequence of them brought great gusts at velocities greater than 90km/hr. accompanying low
pressure systems in Melbourne’s bay side suburbs caused tidal flooding and large waves which
disrupted transport and severely eroded the coast (Kala, Lyons, & Nair, 2011). Substantial
snowfall was reported in Victoria’s alpine regions. Heavy rainfall and damaging winds occur
with passage of strong fronts, with snow being observed when very cold air passes. Cold fronts
form throughout the year.
Southern Annular Mode (SAM)
This is also known as the Antarctic Oscillation (AAO). It refers to the northward and southward
change in position of the westerly wind belt that circles Antarctica (Abram, Mulvaney, Vimeux,
Turner, & England, 2014). It influences the strength of cold fronts and their position, and storm

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systems in the middle latitudes. It also has a hand in determining rainfall availability in southern
Australia. They are described as positive or negative depending on the direction the winds move
towards.
Positive SAM – in this, the winds move towards Antarctica. This typically happens in summer,
resulting in weaker westerly winds and therefore high pressures in the south which restricts cold
fronts. This cumulatively results in calm but dry conditions. Storms too are restricted further
south thus limiting rain in autumn and winter (Abram at al., 2014). In spring and summer a
positive SAM can mean more easterly winds bearing moist air from the Tasmanian Sea. This can
translate to rainfall when the winds hit the coast.
Negative SAM- the winds move towards the equator. Westerly winds shift resulting in low
pressure and storms over southern Australia (Abram et al., 2014). This is typically expected to
happen in winter which explaining why southern Australia including Victoria experiences wet
winter while the rest of the country typically has wet summers.
Problem 2
Evaporation
This is the process by which liquids change into vapour. Moisture in the atmosphere is
contributed by evaporation of water from the earth’s surface. Surface water accounts for up to
90% of atmospheric moisture. The rest is derived from transpiration in plants.

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Factors influencing evaporation
 Temperature- Evaporation happens faster in the presence of high temperature. This is
because water molecules have more freedom in high and can be detached from the mass.
In other words temperature determines whether the molecules will have enough kinetic
energy to convert into gas or not. Air with high temperature also has a greater volume
than that of low temperature which means that it can hold more water vapour (Gunckel,
Covitt, Salinas, & Anderson, 2012). This means that warmer air is a larger container for
water vapour which obviously encourages evaporation. It can be concluded, then, that
there is more evaporation in summer than in winter.
 Humidity- Evaporation will only occur when there is dry air to be occupied. For this
reason places with low humidity will experience more evaporation. This explains why
seasonal rivers running through arid areas will quickly become dry once supply of water
from the source halts (Breitenbach, et al., 2010). Very humid areas like the peaks of
mountains experience little to no evaporation. In winter when it is cold and the volume
of air is limited, evaporation too will be limited.
 Wind speeds- when wind blows over a water body, it carries with evaporated moisture
contained in the air and leaving it free to contain moisture. Fast winds encourage
evaporation both on water bodies and in plants because it blows away the saturated air
on the surface. Windy seasons such as those in which offshore winds prevail will have
more evaporation that calm seasons.
 Surface area for evaporation- Evaporation by definition only takes place on the surface.
Consequently more evaporation will be registered from a water body with a large surface
are. Similarly more water evaporates from a wide leaf than a slender leaf.

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 Latitude- Seas and water bodies located near the equator experience more evaporation.
The earth bulges out at the equator because of centripetal forces drawing it the sun, and
the equator to be closer to the sun than any other latitude. The surfaces of water a hit
almost perpendicularly by rays of light. It goes without saying that the temperatures here
would greatly encourage evaporation. Since all areas within the tropics receive sunshine
throughout the year, it can be said that rates of evaporation are fairly constant regardless
of season.
 Altitude- Within the troposphere, temperatures get lower with increase with increase in
altitude. Since temperature is an important factor for evaporation, it can be concluded
that very little evaporation occurs at high altitudes. In these areas, in fact, moisture is
released into the atmosphere by sublimation when strong winds blow over the ice. Lower
altitudes are considerably warmer and more supportive of evaporation.

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References
Abram, N. J., Mulvaney, R., Vimeux, F., Phipps, S. J., Turner, J., & England, M. H. (2014).
Evolution of the Southern Annular Mode during the past millennium. Nature Climate
Change, 4(7), 564.
Breitenbach, S. F., Adkins, J. F., Meyer, H., Marwan, N., Kumar, K. K., & Haug, G. H. (2010).
Strong influence of water vapor source dynamics on stable isotopes in precipitation
observed in Southern Meghalaya, NE India. Earth and Planetary Science Letters, 292(1-
2), 212-220.
Cobb, K. M., Westphal, N., Sayani, H. R., Watson, J. T., Di Lorenzo, E., Cheng, H., .. &
Charles, C. D. (2013). Highly variable El Niño–Southern Oscillation throughout the
Holocene. Science, 339(6115), 67-70.
Gunckel, K. L., Covitt, B. A., Salinas, I., & Anderson, C. W. (2012). A learning progression for
water in socio‐ecological systems. Journal of Research in Science Teaching, 49(7), 843-
868.
Guilyardi, E., Wittenberg, A., Fedorov, A., Collins, M., Wang, C., Capotondi, A.,... & Stockdale,
T. (2009). Understanding El Niño in ocean–atmosphere general circulation models:
Progress and challenges. Bulletin of the American Meteorological Society, 90(3), 325-
340.
Kala, J., Lyons, T. J., & Nair, U. S. (2011). Numerical simulations of the impacts of land-cover
change on cold fronts in south-west Western Australia. Boundary-layer
meteorology, 138(1), 121-138.

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Kug, J. S., & Ham, Y. G. (2011). (Kug & Ham, 2011) Are there two types of La
Nina?. Geophysical Research Letters, 38(16).
Pfahl, S., & Sodemann, H. (2014). What controls deuterium excess in global
precipitation?. Climate of the Past, 10(2), 771-781.
US National Weather Service. (2019, Jan 20). What is ENSO. Retrieved March 23, 2019, from
https://www.weather.gov/mhx/ensowhat