5 Baroclinic processes
where uC is the circulation velocity, SI the salinity of the water entering the bay and SO is the salinity of the outflowing water. Using (5.3) and (5.4) gives:
uC = 2 (E−P)A b h
SI
SO−SI
(5.5) This simple model describes how, at a given rate of evaporation, water leaves the bay with higher salinities than the salinities of the inflowing waters. Further if the salinity difference increases, the circulation velocityuC has to decrease.
In Fig. 5.9 a transect through the opening of the bay at 24.8°S is shown. Fig. 5.9a shows the average salinity distribution for the whole simulation time (1990-2008). This is used to estimateSI with 35.5 psu and SO with 36 psu. bis taken as 60 km and h as 20 m. (E−P) is estimated with 0.8 m/yr (Tab. 2.1). This yields a circulation velocityuC of approx. 2 cm/s. To compare the performance of this simple analytical model, Fig. 5.9b shows the average velocity of the north/south component of the flow. All barotropic residuals have been removed here, therefore, only the evaporation induced velocity fields are visible. The peak inflow/outflow velocity is in the range of 3 cm/s and therefore the estimation of uC with 2 cm/s agrees well with the model output. Also visible is that the residual flow shows a tilted east/west separation. Therefore, Hervey Bay does not show the typical two-layered structure with a clear separation of the inflow of low saline water in the surface layer and the outflow of dense high saline water at the bottom. The bay shows a superposition of a horizontal circulation and a weak two-layered structure in the vertical. This is the result of the strong tidal mixing in and at the northern part of the bay (Fig. 4.1c). Because a classical vertical two-layer structure cannot be established, the water exchange is realised by an inflow of ocean water in the eastern part of the bay and an outflow at the western shore. The east/west component of the velocity (Fig. 5.9c) shows the fingerprint of the inverse circulations. At the western shore, there is a weak eastward flow close to the bottom. This agrees well with the salinity distribution (Fig. 5.9). This tilting of the isolines indicates an outflow of saline water down the slope. Therefore, Hervey Bay shows an inverse circulation pattern (in the zonal direction) with inflow of fresh water at the surface and an outflow of dense/saline water at the bottom.
To quantify the overall residual mass flow, the salinity flux of the bay has been calculated explicitly by computing the transport by advection and diffusion across the open boundaries (Ω) of Hervey Bay. The northern boundary is defined in Fig. 3.1 and the southern boundary is located in the Great Sandy Strait at 25.5°S.
FSalt(t) = Z
Ω
v(x, z, t)S(x, z, t) + KH(x, z, t) ∂
∂yS(x, z, t)
dΩ (5.6)
The first term represents the flux by advection (meridional velocity times salinity) whereas the second term represents the diffusive fluxes. KH is the turbulent scalar horizontal diffusivity.
A first estimate, to quantify the importance of both contributions to the integral, can be given
5.4 Evaporation induced circulations
Depth in m
b)
−20
−10 0
−2 0 2 a)
−20
−10 0
35.5 36
Longitude c)
152.5 152.6 152.7 152.8 152.9 153 153.1
−20
−10 0
−4
−2 0
Figure 5.9: (a) Average vertical salinity distribution at the northern opening of Hervey Bay in psu, (b) average north/south velocity distribution in cm/s. Positive values indicate a northward directed flow (out of the bay) and (c) average east/west velocity distribution in cm/s. Positive values indicate a eastward directed flow (directed to Fraser Island). The thick black line indicates the change in sign of the velocity components. The transect is placed along 24.8°S latitude. The data are averaged for the whole simulation period (1990-2008).
by estimating the average advective transport with 4 kgm/s, assuming a residual current of 0.1 m/s. The model predicts a bay average turbulent diffusivity of 30 m2/s. which is used to estimate the diffusive transport. The salinity gradient is estimated from the climatology (10−5 psu/m). This results in an average diffusive transport of approx. 3·10−4kgm/s. Therefore, the advective transport is at least three orders of magnitude larger than the diffusive transport.
Integrating both fluxes explicitly along sigma-coordinates over the domain, the export of salin-ity is estimated to be in the order of about 4.0 tons/s (Fig. 5.10a). Using the climatological values (Tab. 2.1), the net loss of 800 mm would result in an outflow of 3.7 tons/s, which is in good agreement with the numerical results.
Finally, the magnitude of these fluxes can be compared with estimates for Spencer Gulf, Aus-tralia [Nunes Vaz et al. , 1990]. Both coastal embayments do not differ significantly in size and atmospheric forcing. The estimated volumetric flux for Spencer Gulf is of the order of 0.05 Sv [Ivanov et al. , 2004]. Converting the peak flux (Fig. 5.8b) into a volume flux, this is estimated to be 0.006 Sv and therefore one order of magnitude smaller. This is not surprising, because Hervey Bay only covers 1/5 of the area of Spencer Gulf. Secondly, the aspect ratio (length to width ratio) of Hervey Bay is nearly one whereas for Spencer Gulf this is in the range of three. Hence Hervey Bay is more an open environment than that of a classical gulf shape and can therefore not support high salinity gradients and it is also much more affected by water
5 Baroclinic processes
1990 1992 1995 1997 2000 2002 2005 2007
0 10 20
Salinity flux
Year
Figure 5.10:Time series of salinity flux (daily averages) - [ton/s]. To indicate the trend, linear fits are added. The red dashed lines indicate the standard deviation. The grey bars show El Ni˜no/La Ni˜na events.
exchange with the open ocean. Taking these factors into account (assuming linear scaling, by multiplying the flow of Hervey Bay by an area correction of 5 and an aspect ratio correction of 2-3), the relative volume transport is comparable with Spencer Gulf even if Hervey Bay is smaller in size and constrained by the geometry.
6 Impact of climate variability
The climate along the subtropical east coast of Australia is changing significantly. Rainfall has decreased by about 50 mm per decade during the last fifty years. These changes are likely to impact upon episodes of hypersalinity and the persistence of inverse circulations which are controlled by the balance between evaporation, precipitation, and freshwater discharge . In this chapter it is investigated how current climate trends have affected upon the physical characteristics of the Hervey Bay. During the last two decades, mean precipitation in Hervey Bay deviates by 13 % from the climatology (1941-2000). Contrary to the drying trend, the occurrence of severe rainfalls, associated with floods, lead to short-term fluctuations in the salinity content of the bay.
6.1 The drying trend
6.1.1 Trends in freshwater supply
Tab. 6.1 shows the climatology of freshwater supply (river discharge and precpitation) for the three observation stations surrounding Hervey Bay. The deviations from the climatology (1941-2000) between 1941-1970 and 1971-2000 are less than 5%. The reduction in freshwater supply during the last two decades varies between 10-20% and is therefore higher than the long term variability. This is caused by severe droughts and the ongoing drying trend on the east coast of Australia. Despite the general trend, the precipitation gradient between Bundaberg and Sandy Cape remains nearly the same. To show the reduction in freshwater supply in detail, Table 6.1: Detailed climatological data of precipitation and river discharge (precipitation equivalent)
in mm/yr.
Bundaberg Sandy Cape Maryborough Mary River
1941-1970 1119.8 1172.7 1187.7 294.1
1971-2000 1029.7 1306.9 1221.8 315.9
1941-2000 1074.8 1239.8 1204.8 305.0
variability 45.1 (4%) 67.1 (5%) 7 (1%) 10.9 (4%)
1990-2008 988.7 1052.4 1008.1 235.2
reduction 8% 15% 16% 23%
Fig. 6.1 depicts the deviation of freshwater input into Hervey Bay from the climatology. Shown are the cumulative sum plots of monthly bay averaged precipitation and Mary river discharge.
6 Impact of climate variability
Two major events are visible. During 1992 strong rainfalls and river floods occurred, caused by an El Ni˜no event. The classification into El Ni˜no/La Ni˜na are based on the Oceanic Ni˜no Index (ONI, [NOAA, 2009]). The floods and rainfalls in 1999-2000 occurred during a La Ni˜na event. In 1992, the freshwater supply recovered to the climatology. Although the rainfalls in 1999-2000 were significant, they could not replenish the water deficit. Due to long/persistent droughts, the soil moisture in the catchments were low, thus a certain amount of rainfall was needed first to recharge soil moisture and ground water, until significant runoff could be released. The La Ni˜na events 1996 and 2008 show a signature in the river discharge but are in general of minor importance.
In the following, the numerical model is used to quantify, how this reduction affects Hervey Bay.
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008
−3
−2
−1 0 1
Year
Cummulative precipitation [mm/yr]
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008−0.3
−0.2
−0.1 0 0.1
Cummulative river discharge [mm/yr]
Precipitation Mary River
Figure 6.1: Deviation of freshwater flow into Hervey Bay from the climatology (1941-2000). Shown are the cumulative sum plots of monthly bay averaged precipitation and Mary river discharge. The grey bars indicate El Ni˜no/La Ni˜na events. Note that the river discharge has a different scaling to emphasise details.
6.1.2 Hypersalinity and inverse state
The density (Fig. 5.8a) and salinity (Fig. 5.8b) gradient times series clearly show the impact of the 1999 and 2008 La Ni˜na and also the 1992 El Ni˜no event. Further during the last decade less frequent reversals of the salinity gradient occurred. To understand the impact of the drying trend, the days in the year are computed, where the salinity gradient and the density gradient exceed the critical thresholds. A year is defined from July to June and therefore the complete southern hemisphere summer is included in one year. The results are shown in
6.1 The drying trend
Fig. 5.8c. A linear fit has been added to both time series. Hervey Bay is, on average, during 240 days of the year in a hypersaline state and for 108 days in the inverse state, respectively.
Interesting to note is that due to the reduction in freshwater supply, both time series show a rising trend. The model simulations indicate an increase of 2.7 days per year, where Hervey Bay is hypersaline and an increase of 1.8 days per year for inverse conditions. These trends are clearly biased by the El Ni˜no/La Ni˜na events. The 1999 floods and rains lowered the mean but only slightly biased the trend. The 2008 La Ni˜na decreased the trend. Therefore, these trends should be judged with care. If ignoring the 1999 La Ni˜na, the observed trend would be in the range of the inter-annual variations and indicates significance.
The time series indicate that Hervey Bay shows different behaviour before/past the 1999 La Ni˜na event. Before 1999, the bay was on average on 250 days in a hypersaline state. After the 1999 La Ni˜na event this increased to 300 days on average. This switch is mainly caused by the ongoing drought on the east coast of Australia.
6.1.3 Residual circulations
To understand the impact of the reduced freshwater supply, a time series of the evaporation induced residual flow is given in Fig. 6.2. To remove any barotropic influence, a model run was started, where temperature and salinity were switched off. The induced barotropic residual flow was then subtracted from the baroclinic case. The mean flow is about 2 cm/s.
1990 1995 2000 2005
1 1.5 2 2.5 3 3.5
Year
Residual flow
Figure 6.2: Time series of residual circulation (fortnightly averages) - [cm/s]. To indicate the trend, linear fits are added. The red dashed lines indicate the standard deviation. The grey bars show El Ni˜no/La Ni˜na events.
6 Impact of climate variability
A closer inspection of the time series shows that the flow is weaker during summer than during winter. This seems puzzling but the weakening of the evaporation induced residuals during summer is caused by the EAC.
In Tab. 2.1 the transport of the EAC is given (see also Fig. 2.1). The current is strongest during summer (18 Sv) and weaker during winter (12 Sv). The EAC induces an anti clockwise circulation within Hervey Bay. This residual flow is estimated to be in the range of 1-2 cm/s and therefore of the same order as the evaporation induced clockwise flow. Thus during summer the EAC can slow down the evaporation induced flow. Furthermore, this flow shows a rising trend. The increase during the simulation period is about 18%. Fig. 6.2 also shows the standard deviation, which is in the same range as the estimated trend. Thus, during the two decades of simulations the reduction of freshwater leads to an acceleration of the residual circulation.
6.1.4 Salinity flux
The model indicates that since 1990, the salinity flux has increased by about 22 % (linear fit in Fig. 5.10). This corresponds to a rise of approx. 0.9 ton/s during the simulation period. The mean flux is estimated to be 3.95 ton/s. Again the standard deviation is indicated, which is now with 4.1 ton/s the fourfold on the trend. Thus, the model indicates a trend, but to show that this increase is significant, the simulation period has to be at least doubled.
The analysis of the simulations further showed that the annual mean heat content of the bay, solar heat flux and air temperature remain nearly constant over the whole simulation period.
They are only responsible for the intra-annual variability. The most important factor influ-encing the rising trend in the salinity gradient/salinity flux is therefore the positive difference between evaporation and precipitation/river discharge.
6.1.5 Impact of the East Australian Current (EAC)
To quantify the importance of the EAC on the hydrodynamics in Hervey Bay, two additional experiments were conducted. The aim was to reduce the southward transport of the EAC.
The average transport is approx. 7.1 Sv (Tab. 2.1). In the first experiment, the transport was reduced to 3.5 Sv and in the second experiment; the EAC was completely switched off. These modifications were implemented by reducing the background sea surface gradient, causing the EAC. To preserve the dynamics, the sea surface height anomalies were left unchanged.
The comparison of the numerical results with the measurements from the field trips shows that the impact on the temperature field was of minor importance. The variations in the salinity field were noticeable. The impact of the EAC was further visible in the salinity and the density gradient time series, shown in Fig. 6.3. The EAC acts as low pass filter and smoothes the salinity and the density differences. By completely switching off the EAC, the peak values of the salinity and density gradient increased by approx. 10%. Especially from