Surface Water Hydrology

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.

SURFACE WATER HYDROLOGY 3
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