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Modelling changes in Mediterranean thermohaline circulation 1987–1995
Journal of Marine Systems 33 – 34 (2002) 51 – 62 www.elsevier.com/locate/jmarsys
Modelling changes in Mediterranean thermohaline circulation 1987–1995 K. Stratford *, K. Haines 1 Department of Meteorology, The University of Edinburgh, Edinburgh, Scotland, UK Received 20 January 2001; accepted 24 July 2001
Abstract The observed changes in the eastern Mediterranean thermohaline circulation between 1987 and 1995 are investigated using an ocean general circulation model. A number of different surface forcing regimes are imposed to investigate the possible causes of the formation of new Aegean deep water. The formation and spreading of the Aegean deep water is assessed in terms of chlorofluorocarbon distributions and the changes in salt content in comparison with observation. It is found that three observed cold winters (of 1987, 1992 and 1993) are enough to generate new Aegean deep water in the model. The addition of anomalous winter wind stresses for the period 1988 – 1995 increases the southward transport of cool north Aegean water, but does not lead to a significant extra outflow of deep water. The modelled water properties agree well with the observation, but the overall volume of the new deep water is too low and there is no outflow at the straits to the west of Crete. The salt budget is examined to highlight the roles of both vertical redistribution and net inflow of salt at Sicily. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Modelling changes; Mediterranean thermohaline circulation; Surface forcing regimes
1. Introduction The Mediterranean thermohaline circulation has been much studied of late, with particular interest in the changed water masses which developed in the eastern Mediterranean around 1990 (Roether et al., 1996; Klein et al., 1999; Lascaratos et al., 1999; Malanotte-Rizzoli et al., 1999). Before these changes, the main eastern Mediterranean water masses consisted
*
Corresponding author. Tel.: +44-131-650-6759; fax: +44-131650-5780. E-mail address: [email protected] (K. Stratford). 1 Now at Environmental System Science Centre, University of Reading, Reading, UK.
of a relatively fresh inflow of modified Atlantic surface water at Sicily, highly saline Levantine Intermediate Water (LIW) at a depth of around 500 m and fresher eastern Mediterranean Deep Water (EMDW) of Adriatic origin at the bottom (see, e.g., Ovchinnikov et al., 1985; POEM Group, 1992). The main outflow to the west at Sicily, which is around 320 m deep, consisted (and still consists) of LIW. There were also some signs of a second intermediate water mass of Aegean origin in the depth range 500 –1500 m west of Crete, termed Cretan Intermediate Water (CIW), which was not dense enough to reach the bottom (Schlitzer et al., 1991). Tracer data from the September 1987 Meteor M5 cruise, in particular chlorofluoromethane (CFC12) data and the extreme uniformity of the deep water temperature and salinity properties at that time, strongly
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suggested that this water mass arrangement had been prevalent for perhaps the previous 100 years. In January 1995, the Meteor M31 cruise showed definitively that a large portion of the deep Adriatic waters of the eastern basin had been replaced within a period of only 7– 8 years by water of Aegean origin. The new deep water from the Aegean was found to be warmer and more salty than the previous Adriatic water and its high CFC12 concentrations indicated recent ventilation. The magnitude of the changes in water properties is best appreciated by considering the salt budget for the eastern Mediterranean. Roether et al. (1996) estimated that the increase in the salt observed below 1500 m would have required the inflow and evaporation of an additional 20 cm of water per year over the entire eastern Mediterranean for 7 years (140 cm in total). This is equivalent to a total of about 7 1013 kg of salt gain for the eastern basin. More recent estimates have suggested that some redistribution of salt from the upper water column occurred, accounting for about one-third of the total gain below 1500 m (Klein et al., 1999). However, this leaves a net salt gain in the eastern basin of 4 –5 1013 kg between 1987 and 1995. This large increase in salt has focused attention on changes in climate over the Mediterranean leading to enhanced net evaporation as the trigger of this transient event. The possible causes discussed in the literature include damming of large river inflows such as the Nile or changes in the salt exchange with the Black Sea and at the strait of Sicily (Rohling and Bryden, 1992; Zervakis et al., 2000). The impact of several cold winters over the northeastern Mediterranean has also been documented (Sur et al., 1992; Theocharis et al., 1999). However, the scarcity of appropriate observational data, particularly between 1987 and 1995, has made it difficult to pinpoint the exact cause of the transient event. Understanding such large changes and analysing the sequence of events leading up to the changes in water masses is therefore a difficult challenge. Using models to simulate the changed thermohaline circulation is one of the more promising ways of assessing the likely causes. Models may also be useful in determining the full consequences of the changed thermohaline circulation across the entire Mediterranean and its long-term effects, particularly on biogeochemistry, which are still unknown. Several studies have attempted to model the changed circulation patterns in different ways using ocean
general circulation models (OGCMs). Samuel et al. (1999) used wind fields compiled by Josey et al. (1999) to show that a shift in winter winds over the Aegean sea occurred around 1987, with a stronger northerly flow occurring over the following 6 years to 1993. Samuel et al. (1999) then used a 1/4j resolution OGCM to show that this wind shift considerably increased the water exchanges between the Aegean sea and the rest of the eastern basin, with greater LIW flow into the Aegean and considerably more outflow of Aegean water (CIW). However, without significant additional air –sea fluxes this water was insufficiently dense to reach the bottom of the eastern basin. Lascaratos et al. (1999) ran an OGCM simulation using the Princeton Ocean Model (POM), employing changes in winds and air – sea fluxes from European Centre for Medium-range Weather Forecasts (ECMWF) re-analyses. This model showed increased production of deep saline waters in the Aegean, as well as outflow into the rest of the eastern basin after 1990, although the exact causes of the changes and whether the new waters were able to reach the bottom outside the Cretan arc was not clear from the study. Wu et al. (2000) presented a more idealized OGCM study at 1/8j resolution to look at the impact of colder winters over the Aegean sea on dense water production. They showed that additional cooling in the North and Central Aegean generates stronger thermohaline flows. The mixing of the colder waters as they flow south, first into the Cretan sea, and then over the Kasos sills into the rest of the eastern basin, increases their salinity and leads to very realistic water properties and distributions at the bottom of the eastern Mediterranean basin. They also carried out a budget analysis which showed that 3/4 of the extra salt found in the deep eastern basin of the model (below 1000 m) was the result of redistribution from the upper 1000 m. The Wu et al. (2000) study produced the most detailed and realistic modelled water properties to date, but the cooling was idealized and has been criticised as unrealistically strong for too long a period (eight winters from 1987 to 1995). In addition, it did not examine the changed wind patterns identified as being beneficial to additional water formation in Samuel et al. (1999). This study will present results using a combination of the momentum and buoyancy forcing used by Samuel et al. (1999) and Wu et al. (2000). In particular, the study uses more moderate thermal forcing and looks at the sensitivity of the model to the
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combination of both cooling and changed wind patterns in producing the new water masses of the eastern basin. Section 2 briefly presents the model setup and forcing arrangements. Section 3 describes the new water properties produced by additional cooling over the Aegean and by changing winter winds. Section 3 also presents a salt budget analysis for the basin, and how the budget is changed by new water production. Section 4 discusses these results in comparison with observations and draws conclusions.
2. Model description The model used as a basis for this work is a freesurface version of the Geophysical Fluid Dynamics Laboratory Modular Ocean Model (Bryan, 1969; Pacanowski, 1995; Killworth et al., 1991). The Mediterranean implementation is based on work by a number of authors (Roussenov et al., 1995; Wu and Haines, 1996; Wu et al., 2000). The following sections highlight the new features in the present model which have not been described previously. 2.1. Model physics The time evolution of the prognostic variables, which are the horizontal component of the velocity field uh=(u, v) and the tracers Ti (temperature T, salinity S and CFC12), is described by: Duh =Dt þ f k uh ¼ ð1=q0 Þrp Ah r4 uh þ Av ðuh Þzz , DTi =Dt ¼ Kh r4 Ti þ Kv ðTi Þzz : Here, D/Dt is the total derivative, while vertical derivatives are denoted by subscript z. The Coriolis parameter is f and k is the unit vector in the vertical direction. The reference density is q0 and p is the hydrostatic pressure pz = gq. The vertical velocity w is computed via ju = 0. Mixing owing to processes unresolved by the model grid is parametrised via biharmonic terms in the horizontal and Laplacian terms in the vertical. The uniform viscosities are
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Ah = 3 1010 m4 s 1 and Av = 1.5 10 4 m2 s 1, while the uniform diffusivities are Kh = 2 1010 m4 s 1 and Kv = 3 10 5 m2 s 1. The density is related to the active tracers by the UNESCO equation of state q = q(T,S,p) (see Gill, 1982). 2.2. Topography The model domain includes the entire Mediterranean at a horizontal resolution of 1/8j with 41 vertical levels on a stretched grid (10 m in thickness at the surface to 200 m at the bottom). The topography is based on the 1/12j Digital Bathymetric Data Base averaged onto the 1/8j grid. In the Aegean region, the topography at temperature – salinity points is smoothed enough to ensure that at least one (u,v) model point is present at the bottom everywhere, but is unaltered elsewhere except at the Strait of Gibraltar. The model is adapted to include a partial step representation of the topography (e.g., Pacanowski and Gnanadesikan, 1998) which relaxes the constraint that the depth at each point must correspond to a whole number of model levels. Adcroft et al. (1997) show that this greatly improves the near-bottom flow compared with the conventional block-like topography used in the GFDL model. Overall, the present model has a more accurate representation of the topography than previous work at this resolution. For example, the islands of Kasos and Karpathos are present in the current model. The combination of high resolution, the partial step topography, and a relatively high linear bottom drag have been shown (Stratford and Haines, 2000) to improve the modelled dispersal of deep water from the Adriatic compared with previous versions of this model (Wu et al., 2000). 2.3. Surface forcing This section describes the surface boundary conditions employed in a number of different model integrations used to investigate the role of surface forcing in the initiation of the new deep water event. Air – sea fluxes of heat and freshwater are introduced by restoring the 10-m surface layer temperature and salinity to values linearly interpolated from the monthly Mediterranean Oceanographic Data Base (MODB-MED5, Brasseur et al., 1996).
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The forcing time scales are different: 2 h for temperature, but 5 days for salinity. The fast restoring time scale for temperature keeps the surface temperature very close to the specified values, whereas salinity is allowed to vary rather more. However, in the Levantine basin the salinity forcing is also increased to a 2-h time scale to insure that highly salty LIW is formed successfully. This forcing has been thoroughly discussed in several previous papers (e.g., Wu and Haines, 1996). The surface forcing of the CFC12 tracer in the model is determined by the history of atmospheric concentrations and the surface water temperature and salinity which control its solubility. Atmospheric CFC12 concentrations from 1970 to 1995 were provided by Roether (personal communication). The model initial conditions are taken from the end of a 100-year run with a 1/4j resolution model with the same surface temperature and salinity forcing, as described in Wu and Haines (1998). The 1/8j 41 level model is run on for another 20 years using monthly wind forcing from the US National Meteorological Center (NMC) based on a climatology for 1980 –1988 (see Roussenov et al., 1995). At the end of this spinup, the CFC12 tracer is initialised with zero everywhere and surface atmospheric concentrations appropriate to 1970 are introduced. The model is then run for a further 17 years with the same forcing to provide a baseline climatological state for 1986. At this point, three separate model integrations are started which investigate the role of the surface forcing in the transient. The aim is to determine whether the three particularly cold winters of 1987, 1992 and 1993, together with the observed changes in winter winds, could be sufficient to produce the new water masses. The first integration (model run 1) is a verification of the new model against the results of Wu et al. (2000) and introduces enhanced cooling in the region of the northern Aegean in February of every year 1987 –1995. This is done by reducing the temperature to which the model is relaxed in February by around 2 jC compared with the MODB. This gives a February mean temperature in the northern Aegean of typically 12 jC compared with the climatological 14 jC. Fig. 3 of Wu et al. (2000) shows the exact fields. Satellite sea surface temperature observations for the period 1983– 1992 (Marullo et al., 1999) show that the winter (January – March) mean temperature does vary by as much as 2j from year to
year and can be as low as 11 jC in the region. This suggests that the magnitude of the Wu et al. (2000) perturbation is reasonable, but would not apply to every year. Further observations suggest there were only three particularly cold winters (or winters with extremely cold events) over the Aegean Sea in the period of interest. These occurred in the early months of 1987, 1992 and 1993 (e.g., Theocharis et al., 1999; Zervakis et al., 2000). To address this, model run 2 applies the cooler conditions of Wu et al. (2000) only in these 3 years, while retaining the same winds in all years. Model run 3 again employs only 3 years of enhanced cooling, but also introduces an anomalous winter (January – March) wind stress from a climatology from the ECMWF for 1988– 1993. In this period, there is a stronger northerly component to the wind in the Aegean region compared with the 1980 – 1988 fields (see Myers et al., 1998). For model run 3, the anomalous winter wind stress is applied in each year in the period 1988– 1993, while the original climatological winds are retained for the final period 1994– 1995.
3. Model results 3.1. Water mass distributions The extent of new deep water formation in the model is most readily identified from CFC12 concentrations, which are higher in more recently ventilated water. The modelled concentrations at the deepest level in the eastern basin (where the depth is greater than 500 m) are shown in Fig. 1. The situation in the climatological state in 1986 (i.e., before any changes to boundary conditions are imposed) shows the single source of deep water is the Adriatic Sea (see also Stratford and Haines, 2000). The outflowing western boundary current is clearly revealed in the Ionian, while a large part of the deep eastern basin is unventilated by CFC12-rich water (Roether and Schlitzer, 1991). However, ventilation does take place in the Aegean and Cretan Seas to intermediate levels. The equivalent CFC12 distributions in 1995 are shown for the three different model integrations (Fig. 1b– d). High CFC12 concentrations associated with new Aegean deep water are widespread in each case. This water has originated in the Cretan Sea
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Fig. 1. Modelled CFC12 concentration (nmol m 3) in the deepest model layer (where deeper than 500 m) for (a) the ‘‘climatological state’’ of 1986, (b) model run 1 with enhanced winter cooling for all 8 years, (c) the enhanced cooling in 1987, 1992 and 1993 only, and (d) as for (c) except with the addition of the anomalous winter wind stress. The contour interval is 0.2 nmol m 3.
and overflowed the Kasos and Karpathos sills. The southward and eastward extent of the high CFC12 values in the Levant is greatest in model run 1 (Fig. 1b), and less in the two cases with only 3 years of enhanced winter cooling. Maximum concentrations in the range 0.7 – 0.8 nmol m 3 compare well with observation. However, there is no overflow of new dense water to the west of Crete through the Antikithera strait, and the westward extent of high CFC12 water is somewhat less than that revealed by observation (see, for example, Fig. 14e of Klein et al., 1999). Fig. 2 shows the vertical sections (south of Crete) of the same modelled CFC12 concentrations. In the pre-1987 state (Fig. 2a), the sole source of deep water is the Adriatic Sea and high CFC12 concentrations in the deep water are present only in the Ionian. How-
ever, there is some ventilation of CIW at around 1000 m directly to the south of Crete. Again, all the 1995 model runs show high CFC12 concentrations associated with new Aegean deep water in the eastern part of the section (Fig. 2b –d). The westward extent of new deep water in model run 1 is again notably greater than model runs 2 and 3. The intermediate waters (at around 1000 m) remain relatively unventilated in the eastern part of the section in each model run. Overall, the modelled CFC12 concentrations are in good agreement with observations (Roether et al., 1996), exceeding 0.6 nmol m 3 at the bottom in each of the integrations. The vertical redistribution of water masses is also well illustrated by the change in salinity observed between 1987 and 1995. Klein et al. (2000) have
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Fig. 2. Modelled CFC12 concentrations (nmol m 3) on a section at approximately 35jN (shown inset). (a) shows the climatological state in 1986 with recently ventilated deep water, revealed by the elevated CFC12 values, in the Ionian only. The model state in 1995 is shown in (b – d) for runs 1 – 3, respectively. The contour interval is 0.1 nmol m 3. Values near the surface are typically greater than 1 nmol m 3 at this time.
presented profiles of net salinity changes in different geographical regions of the eastern basin between the Meteor cruises M5 and M31 based on hydrographic data. The same analysis is performed in the model and the results are presented in Fig. 3. Note that the observational analysis ignores regions which are poor in data, and that the model analysis is matched as closely as possible to the same geographical regions. In the region to the south of Crete (defined as the Cretan passage 22 –28jE, 30 –35jN), the reduction in salinity at intermediate levels and associated increase in salinity in the deep levels are reproduced reasonably well by the model (Fig. 3a). The maximum decrease in salinity of 0.07 psu occurs at around 400 m, while the maximum increase of 0.18 psu occurs below 3000 m in model run 2. In the Levant (defined as 28 –35jE, 30 –37jN), the model again captures the increased salinity in the deep
waters (Fig. 3b). The increase is about 0.1 psu toward the bottom in each model run, which is again reasonably consistent with observations. However, the observed increase in salinities in the surface and intermediate waters of around 0.05 psu is not captured by the model, which exhibits a small net decrease everywhere above 1000 m. In the Ionian basin (defined as 15– 22jE, 30– 41jN), the lack of overflow of deep water to the west of Crete means that the model results do not show the salinity increases in the deep waters revealed by observation. Fig. 4 shows the time evolution of the water properties below 1500 m in the Cretan passage (defined as before: 22 –28jE, 30 –35jN) on temperature – salinity diagrams. The water in this region is binned at intervals of 0.0625 jC and 0.025 psu to give contours of a volume per T – S bin in m3 (jC psu) 1. In 1986, the climatological state reflects a uniform deep water
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Fig. 3. Changes in the salinity (psu) between 1986 and 1995 for different geographical regions as vertical profiles. In each panel, the crosses represent the observational estimate of the mean salinity change from the hydrographic data (Klein et al., 2000), the solid line is model run 1, the dashed line is model run 2, and the dotted line is model run 3. The geographical extent of each area is given in the text.
of Adriatic origin with temperature around 13.4 jC and salinity around 38.7 psu. These values compare well with observed bottom water with temperature 13.3– 13.4 jC and salinity 36.65 – 36.70 psu (Klein et al., 1999). In model run 1, the new deep water (Fig. 4b) is uniform with increased temperature around 13.8 jC and salinity 38.85 – 38.90 psu, values which again compare well with observation. The situation in 1995 for runs 2 and 3 is more mixed: some new water with the correct properties is present but there is not enough to replace completely the original Adriatic deep water. However, it can be seen that run 3 has produced slightly more new dense water of the correct properties than run 2. In all cases, the density of the new deep water has increased by 0.03– 0.05 kg m 3. Overall, the production of deep water, as revealed by the extent of high CFC12 concentrations, is greatest in model run 1. This is reflected in the increase in deep water salinities, which is again largest in model run 1, but generally smaller in model runs 2 and 3. New deep water production and redistribution is modelled reasonably well even with only 3 years of enhanced Aegean cooling, although salt increases in the basin are generally too small. This suggests that increased net evaporation over the basin is important. Although the change in wind forcing produces slightly better final water properties, it is found to make
only a small difference to new water formation in the model compared with the changes in buoyancy forcing. The total change in the budget of salt for the eastern basin is discussed in more detail in the following section. 3.2. Time evolution of the event Fig. 5 provides a time series of salt content changes (year by year) for the entire eastern basin above and below 1500 m. The results for the climatological state for 1980 –1987 are included (as part of model run 1) to illustrate the degree to which the model is in a steady state prior to the imposed change in forcing. Fig. 5a shows the salt changes above 1500 m which, prior to 1987, are on average slightly negative, indicating that the upper levels are not entirely in equilibrium. After 1987, the salt losses in the upper layer increase significantly. Below 1500 m (Fig. 5b), the model salt content is steady before 1987, after which significant gains in salt are produced. This is most pronounced in model run 1, but is also well marked in model runs 2 and 3. The gain in salt in the deep water is partly offset by the loss in the upper layer water, but the size of the loss indicates that this redistribution is not sufficient to account for the whole change, and there must be a net inflow of salt in the model. (Note that in the model, this
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Fig. 4. Contours of specific volume in m3 (jC psu) 1 on temperature salinity diagrams for the region of the Cretan passage below 1500 m show the quantity of water with given properties. The outermost contour in each panel is 2 1015 m3 (jC psu) 1 while the contour interval is 4 1015 m3 (jC psu) 1. Density contours (potential density 1000 kg m 3) are also shown.
inflow of salt is distributed between a physical inflow of salt owing to the circulation, and an unphysical surface salt flux arising from the boundary conditions. However, the net inflow of salt should be interpreted as being provided entirely by the circulation.) Fig. 6 shows a time series of the net heat and salt changes for the whole water column in the eastern basin. Years of enhanced cooling are easily identified by the strong heat losses (Fig. 6a). The overall salt change is again predominantly positive, but is only consistently so in model run 1 (Fig. 6b). The largest salt gain of 4 – 6 1012 kg year 1 is comparable to that suggested by observation, but this figure is not sustained over the entire period 1987 –1995. This is reflected in the total
salt changes over the period (shown in Table 1) which are smaller than those estimated by Klein et al. (1999). The observations show approximately one third of the additional salt below 1500 m has originated in the upper 1500 m. In the model, this fraction is also around one third in model runs 1 and 2, but increases to almost two thirds in model run 3. Few observational data are available in the 1987– 1995 period, but one useful record of the time-evolution of the event is provided by Theocharis et al. (1999), who record the deep water properties in the Cretan Sea. Unfortunately, no comparable data appear to be available for the period directly before 1987. The mean temperature and salinity below 1000 m in
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Fig. 5. Time series of the salt change in the water column for the eastern basin (a) above 1500 m and (b) below 1500 m. For each model integration, year-on-year values (kg salt year 1) are given.
June in the model Cretan Sea, which depend upon the previous winter’s water formation, are presented in Fig. 7. The model results for the period 1980 – 1987 show that the deepwater properties are relatively steady before the introduction of changed forcing. In model run 1, the onset of enhanced cooling in winter 1987 initiates a sharp drop in the temperature (Fig. 7a) of around 0.1 jC, and a sharp rise in the salinity (Fig. 7b) of 0.05 psu. In the following years, the temperature shows a steady downward trend until 1992, while the salinity remains roughly unchanged. This contrasts strongly with the results for model runs 2 and 3, where the initial decrease in temperature after 1987 is reversed in the period 1988 – 1992. There is then a further significant decrease in temperature of around 0.2 jC caused by the enhanced cooling of winters 1992 and 1993. The salinity trends for model runs 2 and 3 also differ from model run 1: the initial increase in salinity is again reversed in both model runs, while the enhanced cooling of 1992 induces a second increase in salinity. Model runs 2 and 3 differ in that the Cretan Sea water is cooler and fresher in the
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Fig. 6. Time series of modelled (a) net heat and (b) net salt changes for the whole water column. Year-on-year values are again presented.
latter, suggesting that the stronger northerly winter wind in model run 3 contributes to the southward transport of north Aegean water. However, there is no additional outflow of deep water into the eastern basin in model run 2 compared with run 3. Zervakis et al. (2000) report an overturning in the north Aegean associated with an extreme cold event in March 1987. However, the observations of Theocharis et al. (1999) show no decrease in temperature in the deep Cretan Sea after March 1987, but actually show an increase of around 0.15 jC between March 1987 and March 1992. This increase in temperature is reflected in model runs 2 and 3, but only after an Table 1 The change in salt content between 1987 and 1995 (1013 kg) estimated from hydrographic data (Klein et al., 1999) and computed in the entire eastern basin for model runs 1 – 3
Observation Model run 1 Model run 2 Model run 3
Above 1500 m
Below 1500 m
Net total
2 to 3 1.6 0.7 1.2
7 3.8 1.7 1.7
4 to 5 2.2 1 0.5
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added over the period to drive sufficient dense water production. However, the cooling events of 1987, 1992 and 1993 are clearly important, and the model runs resolving these events (2 and 3) induce realistic changes in temperature in the deep Cretan waters at this time.
4. Summary
Fig. 7. Time series of modelled deep-water properties in the Cretan Sea (below 1000 m): (a) potential temperature and (b) salinity. The open circles represent model run 1 with 8 years of enhanced winter cooling, the squares model run 2 with only 3 years of enhanced winter cooling, and the triangles model run 3 with 3 years of enhanced cooling together with the anomalous winter wind stress.
initial cooling driven by the cold 1987 winter. The observed decrease in temperature between March 1992 and March 1994 is 0.3 jC, which model runs 2 and 3 again reproduce well. The observed rise in deep Cretan Sea salinity of about 0.2 psu is relatively gradual over the whole period 1987 – 1994 and is much greater than the model increase. The model results are consistent with the use of the relaxation boundary conditions, where only the temperature is changed. The use of only 3 years of enhanced cooling in model runs 2 and 3 allows the surface temperature to recover in the intervening period. This means that surface temperatures revert to the climatological state and no cold convection occurs in the north Aegean between 1988 and 1991, which in turn leads to warming in the Cretan Sea owing to vertical mixing with the overlying water. Taken together, these results suggest that the model reproduces most qualitative aspects of the observed changes, but not enough salt is
This work extends the original investigation of Wu et al. (2000) by performing a number of different model integrations to assess the impact of only three extremely cold winters in the north Aegean, together with the influence of differing surface forcing (both buoyancy and momentum) on the development of the new deep water regime in the eastern Mediterranean. A new model with a number of improved features related to the model physics, including a better representation of the topography by partial cells, is employed. This model is capable of providing a good representation of the bottom water flows and CFC12 concentrations in the climatological state where the deep water is formed exclusively in the Adriatic Sea (Stratford and Haines, 2000). The topography in the region of interest for this work (the Aegean Sea and the surrounding area) is more faithfully represented than in previous work at the same horizontal resolution. The first clear result of the present work is that, in contrast to Wu et al. (2000), only three winters of enhanced cooling in the north Aegean are required to initiate the formation of new eastern Mediterranean deep water in the model. The mechanism is the same as that of Wu et al., with cool north Aegean water entering the Cretan Sea and mixing with saline LIW to give rise to the new dense water which overflows into the deep eastern basin. Furthermore, when the observed cool winters of 1992 and 1993 are resolved the trend in model temperatures in the Cretan Sea follows observation more consistently. New deep water overflowing from the Cretan Sea is identified by high CFC12 concentrations in the model which are consistent with observation except in their westward extent in the Ionian Sea. The vertical redistribution of salt, which accompanies the new dense water formation, is shown to be broadly consistent with observation in regions where new deep water is present. However, there is no
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outflow of new dense water west of Crete, suggesting insufficient water is formed in the Cretan Sea to force an overflow at Antikithera, or a problem with the resolution of the topography. The role of wind forcing, as investigated by Samuel et al. (1999), is found to be secondary to that of the buoyancy forcing in the current model. However, the model suggests that stronger northerly winter winds for the period 1988– 1993 do promote the southward transport of cool north Aegean water as far as the Cretan Sea, and lead to slightly improved water properties in the deep eastern basin. In reality, the wind and buoyancy fluxes are not decoupled as in the present model. Increased wind strength should account for not only increased momentum fluxes, but also increased buoyancy fluxes via increased heat loss and evaporation. Increasing wind strength also provides extra mechanical energy for deep mixing, but it is likely that the buoyancy fluxes will be dominant in producing the deep water in the area of interest in this study. While the qualitative performance of the model is good, a comparison of the total salt increase in the eastern basin with that estimated from observation reveals that the model changes are not large enough. For example, hydrographic data suggest a net increase of 4 –5 1013 kg of salt in the eastern basin between 1987 and 1995 (Klein et al., 1999). The model records at most half this increase, which indicates that there is insufficient net evaporation in the eastern basin in the model. A relaxation to more realistic salinities (e.g., Brankart and Pinardi, 2001) may produce some improvement in the salt budget, but the impact of changing freshwater budgets on the model would be more easily addressed using flux boundary conditions (e.g., Myers and Haines, 2000). In conclusion, the model results capture a number of important features of the observed eastern Mediterranean transient, including deep CFC12 distributions and vertical redistributions of salt. The results suggest an important role for both freshwater and heat fluxes in driving the event. There remain important outstanding questions involving the transient, such as the ultimate source of the extra salt observed in the basin. A second important issue to be resolved in the current model is the lack of new deepwater overflow to the west of Crete. Improved models of the Mediterranean bathymetry and improved forcing data offer ways to address such questions.
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Acknowledgements The work was inspired by discussions at a Commision Internationale pour l’Exploration Scientifique de la mer Me´diterrane´e (CIESM) workshop in Trieste in March 2000. We are grateful to Birgit Klein for kindly providing the observational data shown in Fig. 3 and to Wolfgang Roether for the CFC12 concentrations. The work was funded by the European Community MAST programme under grant MAS3-CT96-0051 (MATER) and MAS3-CT98-0189 (MEDNET—see http://www.met.ed.ac.uk/mednet/). References Adcroft, A., Hill, C., Marshall, J., 1997. Representation of topography by shaved cells in a height coordinate ocean model. Mon. Weather Rev. 125, 2293 – 2315. Brankart, J.-M., Pinardi, N., 2001. Abrupt cooling of the Mediterranean Levantine intermediate water at the beginning of the 1980s: observational evidence and model simulation. J. Phys. Oceanogr. 31, 2307 – 2320. Brasseur, P., Beckers, J.-M., Brankart, J.-M., Schoenauen, R., 1996. Seasonal temperature and salinity fields in the Mediterranean Sea: climatological analyses of an historical data set. DeepSea Res. 43, 159 – 192. Bryan, K., 1969. A numerical method for the study of the circulation of the world ocean. J. Comp. Phys. 4, 347 – 376. Gill, A.E., 1982. Atmosphere – Ocean Dynamics Academic Press, New York. Josey, S.A., Kent, E.C., Taylor, P.K., 1999. New insights into the ocean heat budget closure problem from analysis of the SOC air – sea flux climatology. J. Clim. 12, 2856 – 2880. Killworth, P.D., Staniforth, D., Webb, D.J., Patterson, S.M., 1991. The development of a free-surface Bryan – Cox – Semtner ocean model. J. Phys. Oceanogr. 21, 1333 – 1348. Klein, B., Roether, W., Manca, B., Bregant, D., Beitzel, V., Kovacevic, V., Luchetta, A., 1999. The large deep water transient in the eastern Mediterranean. Deep-Sea Res. 46, 371 – 414. Klein, B., Roether, W., Manca, B., Theocharis, A., 2000. The evolution of the eastern Mediterranean climatic transient during the last decade: the tracer viewpoint. CIESM Workshop Abstracts, Trieste. Lascaratos, A., Roether, W., Nittis, K., Klein, B., 1999. Recent changes in deep water formation and spreading in the eastern Mediterranean Sea. Prog. Oceanogr. 44, 5 – 36. Malanotte-Rizzoli, P., Manca, B., Theocharis, A., Brenner, S., Bu¨ zsoy, E., 1999. The eastern Mediterranean in the dillon, G., O 80s and in the 90s: the big transition in the intermediate and deep circulations. Dyn. Atmos. Oceans 29, 365 – 395. Marullo, S., Santoleri, R., Malanotte-Rizzoli, P., Bergamasco, A., 1999. The sea surface temperature field in the eastern Mediterranean from advanced very high resolution radiometer (AVHRR) data: Part II. Interannual variability. J. Mar. Syst. 20, 83 – 112.
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Myers, P.G., Haines, K., 2000. Seasonal and interannual variability in a model of the Mediterranean under derived flux forcing. J. Phys. Oceanogr. 30, 1069 – 1082. Myers, P.G., Haines, K., Josey, S., 1998. On the importance of the choice of wind stress forcing to the modeling of the Mediterranean Sea circulation. J. Geophys. Res. 103, 15729 – 15749. Ovchinnikov, I.M., Zats, V.I., Krivosheya, V.G., Udodov, A.I., 1985. Formation of deep eastern Mediterranean waters in the Adriatic Sea. Oceanology 25, 704 – 707. Pacanowski, R.C., 1995. MOM2 documentation, user’s guide and reference manual. GFDL Ocean Group Tech. Rep. 3. Geophysical Fluid Dynamics Laboratory/NOAA, Princeton University, Princeton, NJ. Pacanowski, R.C., Gnanadesikan, A., 1998. Transient response in a z-level ocean model that resolves topography with partial cells. Mon. Weather Rev. 126, 3248 – 3270. Physical Oceanography of the Eastern Mediterranean (POEM) Group, 1992. General circulation of the eastern Mediterranean. Earth Sci. Rev. 32, 285 – 309. Roether, W.R., Schlitzer, R., 1991. Eastern Mediterranean deep water renewal on the basis of chlorofluoromethane and tritium data. Dyn. Atmos. Oceans 15, 333 – 354. Roether, W.R., Manca, B., Klein, B., Bregant, D., Georgopoulos, D., 1996. Eastern Mediterranean deep waters found in an entirely new state. Science 271, 333 – 335. Rohling, E., Bryden, H., 1992. Man-induced salinity and temperature increases in the western Mediterranean deep water. J. Geophys. Res. 97, 11191 – 11198. Roussenov, V., Stanev, E., Artale, V., Pinardi, N., 1995. A seasonal model of the Mediterranean Sea general circulation. J. Geophys. Res. 100, 13513 – 13538.
Samuel, S., Haines, K., Josey, S., Myers, P.G., 1999. Response of the Mediterranean Sea thermohaline circulation to observed changes in the winter wind stress field in the period 1980 – 1993. J. Geophys. Res. 104, 7771 – 7784. Schlitzer, R., Roether, R., Oster, H., Junghans, H.-G., Hausmann, M., Johannsen, H., Michelato, A., 1991. Chlorofluoromethane and oxygen in the eastern Mediterranean. Deep-Sea Res. 38, 1531 – 1551. Stratford, K., Haines, K., 2000. Frictional sinking of the dense water overflow in a z-coordinate model of the Mediterranean Sea. Geophys. Res. Lett. 27, 3973 – 3976. ¨ zsoy, E., U ¨ nlu¨ata, U ¨ ., 1992. Simultaneous deep and Sur, H.I., O intermediate convection in the northern Levantine Sea, winter 1992. Oceanol. Acta 16, 33 – 43. Theocharis, A., Nittis, K., Kontoyiannis, H., Papageorgiou, E., Balopoulos, E., 1999. Climatic changes in the Aegean influence the eastern Mediterranean thermohaline circulation (1986 – 1997). Geophys. Res. Lett. 26, 1617 – 1620. Wu, P., Haines, K., 1996. A modelling study of deep water formation in the Mediterranean. J. Geophys. Res. 101, 6591 – 6607. Wu, P., Haines, K., 1998. The general circulation of the Mediterranean Sea from a 100-year simulation. J. Geophys. Res. 103, 1121 – 1135. Wu, P., Haines, K., Pinardi, N., 2000. Toward an understanding of deep-water renewal in the eastern Mediterranean. J. Phys. Oceanogr. 30, 443 – 458. Zervakis, V., Georgopoulos, D., Drakopoulos, P.G., 2000. The role of the North Aegean in triggering the recent eastern Mediterranean climatic changes. J. Geophys. Res. 105, 26103 – 26116.
Modelling changes in Mediterranean thermohaline circulation 1987–1995 K. Stratford *, K. Haines 1 Department of Meteorology, The University of Edinburgh, Edinburgh, Scotland, UK Received 20 January 2001; accepted 24 July 2001
Abstract The observed changes in the eastern Mediterranean thermohaline circulation between 1987 and 1995 are investigated using an ocean general circulation model. A number of different surface forcing regimes are imposed to investigate the possible causes of the formation of new Aegean deep water. The formation and spreading of the Aegean deep water is assessed in terms of chlorofluorocarbon distributions and the changes in salt content in comparison with observation. It is found that three observed cold winters (of 1987, 1992 and 1993) are enough to generate new Aegean deep water in the model. The addition of anomalous winter wind stresses for the period 1988 – 1995 increases the southward transport of cool north Aegean water, but does not lead to a significant extra outflow of deep water. The modelled water properties agree well with the observation, but the overall volume of the new deep water is too low and there is no outflow at the straits to the west of Crete. The salt budget is examined to highlight the roles of both vertical redistribution and net inflow of salt at Sicily. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Modelling changes; Mediterranean thermohaline circulation; Surface forcing regimes
1. Introduction The Mediterranean thermohaline circulation has been much studied of late, with particular interest in the changed water masses which developed in the eastern Mediterranean around 1990 (Roether et al., 1996; Klein et al., 1999; Lascaratos et al., 1999; Malanotte-Rizzoli et al., 1999). Before these changes, the main eastern Mediterranean water masses consisted
*
Corresponding author. Tel.: +44-131-650-6759; fax: +44-131650-5780. E-mail address: [email protected] (K. Stratford). 1 Now at Environmental System Science Centre, University of Reading, Reading, UK.
of a relatively fresh inflow of modified Atlantic surface water at Sicily, highly saline Levantine Intermediate Water (LIW) at a depth of around 500 m and fresher eastern Mediterranean Deep Water (EMDW) of Adriatic origin at the bottom (see, e.g., Ovchinnikov et al., 1985; POEM Group, 1992). The main outflow to the west at Sicily, which is around 320 m deep, consisted (and still consists) of LIW. There were also some signs of a second intermediate water mass of Aegean origin in the depth range 500 –1500 m west of Crete, termed Cretan Intermediate Water (CIW), which was not dense enough to reach the bottom (Schlitzer et al., 1991). Tracer data from the September 1987 Meteor M5 cruise, in particular chlorofluoromethane (CFC12) data and the extreme uniformity of the deep water temperature and salinity properties at that time, strongly
0924-7963/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 7 9 6 3 ( 0 2 ) 0 0 0 5 2 - 0
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suggested that this water mass arrangement had been prevalent for perhaps the previous 100 years. In January 1995, the Meteor M31 cruise showed definitively that a large portion of the deep Adriatic waters of the eastern basin had been replaced within a period of only 7– 8 years by water of Aegean origin. The new deep water from the Aegean was found to be warmer and more salty than the previous Adriatic water and its high CFC12 concentrations indicated recent ventilation. The magnitude of the changes in water properties is best appreciated by considering the salt budget for the eastern Mediterranean. Roether et al. (1996) estimated that the increase in the salt observed below 1500 m would have required the inflow and evaporation of an additional 20 cm of water per year over the entire eastern Mediterranean for 7 years (140 cm in total). This is equivalent to a total of about 7 1013 kg of salt gain for the eastern basin. More recent estimates have suggested that some redistribution of salt from the upper water column occurred, accounting for about one-third of the total gain below 1500 m (Klein et al., 1999). However, this leaves a net salt gain in the eastern basin of 4 –5 1013 kg between 1987 and 1995. This large increase in salt has focused attention on changes in climate over the Mediterranean leading to enhanced net evaporation as the trigger of this transient event. The possible causes discussed in the literature include damming of large river inflows such as the Nile or changes in the salt exchange with the Black Sea and at the strait of Sicily (Rohling and Bryden, 1992; Zervakis et al., 2000). The impact of several cold winters over the northeastern Mediterranean has also been documented (Sur et al., 1992; Theocharis et al., 1999). However, the scarcity of appropriate observational data, particularly between 1987 and 1995, has made it difficult to pinpoint the exact cause of the transient event. Understanding such large changes and analysing the sequence of events leading up to the changes in water masses is therefore a difficult challenge. Using models to simulate the changed thermohaline circulation is one of the more promising ways of assessing the likely causes. Models may also be useful in determining the full consequences of the changed thermohaline circulation across the entire Mediterranean and its long-term effects, particularly on biogeochemistry, which are still unknown. Several studies have attempted to model the changed circulation patterns in different ways using ocean
general circulation models (OGCMs). Samuel et al. (1999) used wind fields compiled by Josey et al. (1999) to show that a shift in winter winds over the Aegean sea occurred around 1987, with a stronger northerly flow occurring over the following 6 years to 1993. Samuel et al. (1999) then used a 1/4j resolution OGCM to show that this wind shift considerably increased the water exchanges between the Aegean sea and the rest of the eastern basin, with greater LIW flow into the Aegean and considerably more outflow of Aegean water (CIW). However, without significant additional air –sea fluxes this water was insufficiently dense to reach the bottom of the eastern basin. Lascaratos et al. (1999) ran an OGCM simulation using the Princeton Ocean Model (POM), employing changes in winds and air – sea fluxes from European Centre for Medium-range Weather Forecasts (ECMWF) re-analyses. This model showed increased production of deep saline waters in the Aegean, as well as outflow into the rest of the eastern basin after 1990, although the exact causes of the changes and whether the new waters were able to reach the bottom outside the Cretan arc was not clear from the study. Wu et al. (2000) presented a more idealized OGCM study at 1/8j resolution to look at the impact of colder winters over the Aegean sea on dense water production. They showed that additional cooling in the North and Central Aegean generates stronger thermohaline flows. The mixing of the colder waters as they flow south, first into the Cretan sea, and then over the Kasos sills into the rest of the eastern basin, increases their salinity and leads to very realistic water properties and distributions at the bottom of the eastern Mediterranean basin. They also carried out a budget analysis which showed that 3/4 of the extra salt found in the deep eastern basin of the model (below 1000 m) was the result of redistribution from the upper 1000 m. The Wu et al. (2000) study produced the most detailed and realistic modelled water properties to date, but the cooling was idealized and has been criticised as unrealistically strong for too long a period (eight winters from 1987 to 1995). In addition, it did not examine the changed wind patterns identified as being beneficial to additional water formation in Samuel et al. (1999). This study will present results using a combination of the momentum and buoyancy forcing used by Samuel et al. (1999) and Wu et al. (2000). In particular, the study uses more moderate thermal forcing and looks at the sensitivity of the model to the
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combination of both cooling and changed wind patterns in producing the new water masses of the eastern basin. Section 2 briefly presents the model setup and forcing arrangements. Section 3 describes the new water properties produced by additional cooling over the Aegean and by changing winter winds. Section 3 also presents a salt budget analysis for the basin, and how the budget is changed by new water production. Section 4 discusses these results in comparison with observations and draws conclusions.
2. Model description The model used as a basis for this work is a freesurface version of the Geophysical Fluid Dynamics Laboratory Modular Ocean Model (Bryan, 1969; Pacanowski, 1995; Killworth et al., 1991). The Mediterranean implementation is based on work by a number of authors (Roussenov et al., 1995; Wu and Haines, 1996; Wu et al., 2000). The following sections highlight the new features in the present model which have not been described previously. 2.1. Model physics The time evolution of the prognostic variables, which are the horizontal component of the velocity field uh=(u, v) and the tracers Ti (temperature T, salinity S and CFC12), is described by: Duh =Dt þ f k uh ¼ ð1=q0 Þrp Ah r4 uh þ Av ðuh Þzz , DTi =Dt ¼ Kh r4 Ti þ Kv ðTi Þzz : Here, D/Dt is the total derivative, while vertical derivatives are denoted by subscript z. The Coriolis parameter is f and k is the unit vector in the vertical direction. The reference density is q0 and p is the hydrostatic pressure pz = gq. The vertical velocity w is computed via ju = 0. Mixing owing to processes unresolved by the model grid is parametrised via biharmonic terms in the horizontal and Laplacian terms in the vertical. The uniform viscosities are
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Ah = 3 1010 m4 s 1 and Av = 1.5 10 4 m2 s 1, while the uniform diffusivities are Kh = 2 1010 m4 s 1 and Kv = 3 10 5 m2 s 1. The density is related to the active tracers by the UNESCO equation of state q = q(T,S,p) (see Gill, 1982). 2.2. Topography The model domain includes the entire Mediterranean at a horizontal resolution of 1/8j with 41 vertical levels on a stretched grid (10 m in thickness at the surface to 200 m at the bottom). The topography is based on the 1/12j Digital Bathymetric Data Base averaged onto the 1/8j grid. In the Aegean region, the topography at temperature – salinity points is smoothed enough to ensure that at least one (u,v) model point is present at the bottom everywhere, but is unaltered elsewhere except at the Strait of Gibraltar. The model is adapted to include a partial step representation of the topography (e.g., Pacanowski and Gnanadesikan, 1998) which relaxes the constraint that the depth at each point must correspond to a whole number of model levels. Adcroft et al. (1997) show that this greatly improves the near-bottom flow compared with the conventional block-like topography used in the GFDL model. Overall, the present model has a more accurate representation of the topography than previous work at this resolution. For example, the islands of Kasos and Karpathos are present in the current model. The combination of high resolution, the partial step topography, and a relatively high linear bottom drag have been shown (Stratford and Haines, 2000) to improve the modelled dispersal of deep water from the Adriatic compared with previous versions of this model (Wu et al., 2000). 2.3. Surface forcing This section describes the surface boundary conditions employed in a number of different model integrations used to investigate the role of surface forcing in the initiation of the new deep water event. Air – sea fluxes of heat and freshwater are introduced by restoring the 10-m surface layer temperature and salinity to values linearly interpolated from the monthly Mediterranean Oceanographic Data Base (MODB-MED5, Brasseur et al., 1996).
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The forcing time scales are different: 2 h for temperature, but 5 days for salinity. The fast restoring time scale for temperature keeps the surface temperature very close to the specified values, whereas salinity is allowed to vary rather more. However, in the Levantine basin the salinity forcing is also increased to a 2-h time scale to insure that highly salty LIW is formed successfully. This forcing has been thoroughly discussed in several previous papers (e.g., Wu and Haines, 1996). The surface forcing of the CFC12 tracer in the model is determined by the history of atmospheric concentrations and the surface water temperature and salinity which control its solubility. Atmospheric CFC12 concentrations from 1970 to 1995 were provided by Roether (personal communication). The model initial conditions are taken from the end of a 100-year run with a 1/4j resolution model with the same surface temperature and salinity forcing, as described in Wu and Haines (1998). The 1/8j 41 level model is run on for another 20 years using monthly wind forcing from the US National Meteorological Center (NMC) based on a climatology for 1980 –1988 (see Roussenov et al., 1995). At the end of this spinup, the CFC12 tracer is initialised with zero everywhere and surface atmospheric concentrations appropriate to 1970 are introduced. The model is then run for a further 17 years with the same forcing to provide a baseline climatological state for 1986. At this point, three separate model integrations are started which investigate the role of the surface forcing in the transient. The aim is to determine whether the three particularly cold winters of 1987, 1992 and 1993, together with the observed changes in winter winds, could be sufficient to produce the new water masses. The first integration (model run 1) is a verification of the new model against the results of Wu et al. (2000) and introduces enhanced cooling in the region of the northern Aegean in February of every year 1987 –1995. This is done by reducing the temperature to which the model is relaxed in February by around 2 jC compared with the MODB. This gives a February mean temperature in the northern Aegean of typically 12 jC compared with the climatological 14 jC. Fig. 3 of Wu et al. (2000) shows the exact fields. Satellite sea surface temperature observations for the period 1983– 1992 (Marullo et al., 1999) show that the winter (January – March) mean temperature does vary by as much as 2j from year to
year and can be as low as 11 jC in the region. This suggests that the magnitude of the Wu et al. (2000) perturbation is reasonable, but would not apply to every year. Further observations suggest there were only three particularly cold winters (or winters with extremely cold events) over the Aegean Sea in the period of interest. These occurred in the early months of 1987, 1992 and 1993 (e.g., Theocharis et al., 1999; Zervakis et al., 2000). To address this, model run 2 applies the cooler conditions of Wu et al. (2000) only in these 3 years, while retaining the same winds in all years. Model run 3 again employs only 3 years of enhanced cooling, but also introduces an anomalous winter (January – March) wind stress from a climatology from the ECMWF for 1988– 1993. In this period, there is a stronger northerly component to the wind in the Aegean region compared with the 1980 – 1988 fields (see Myers et al., 1998). For model run 3, the anomalous winter wind stress is applied in each year in the period 1988– 1993, while the original climatological winds are retained for the final period 1994– 1995.
3. Model results 3.1. Water mass distributions The extent of new deep water formation in the model is most readily identified from CFC12 concentrations, which are higher in more recently ventilated water. The modelled concentrations at the deepest level in the eastern basin (where the depth is greater than 500 m) are shown in Fig. 1. The situation in the climatological state in 1986 (i.e., before any changes to boundary conditions are imposed) shows the single source of deep water is the Adriatic Sea (see also Stratford and Haines, 2000). The outflowing western boundary current is clearly revealed in the Ionian, while a large part of the deep eastern basin is unventilated by CFC12-rich water (Roether and Schlitzer, 1991). However, ventilation does take place in the Aegean and Cretan Seas to intermediate levels. The equivalent CFC12 distributions in 1995 are shown for the three different model integrations (Fig. 1b– d). High CFC12 concentrations associated with new Aegean deep water are widespread in each case. This water has originated in the Cretan Sea
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Fig. 1. Modelled CFC12 concentration (nmol m 3) in the deepest model layer (where deeper than 500 m) for (a) the ‘‘climatological state’’ of 1986, (b) model run 1 with enhanced winter cooling for all 8 years, (c) the enhanced cooling in 1987, 1992 and 1993 only, and (d) as for (c) except with the addition of the anomalous winter wind stress. The contour interval is 0.2 nmol m 3.
and overflowed the Kasos and Karpathos sills. The southward and eastward extent of the high CFC12 values in the Levant is greatest in model run 1 (Fig. 1b), and less in the two cases with only 3 years of enhanced winter cooling. Maximum concentrations in the range 0.7 – 0.8 nmol m 3 compare well with observation. However, there is no overflow of new dense water to the west of Crete through the Antikithera strait, and the westward extent of high CFC12 water is somewhat less than that revealed by observation (see, for example, Fig. 14e of Klein et al., 1999). Fig. 2 shows the vertical sections (south of Crete) of the same modelled CFC12 concentrations. In the pre-1987 state (Fig. 2a), the sole source of deep water is the Adriatic Sea and high CFC12 concentrations in the deep water are present only in the Ionian. How-
ever, there is some ventilation of CIW at around 1000 m directly to the south of Crete. Again, all the 1995 model runs show high CFC12 concentrations associated with new Aegean deep water in the eastern part of the section (Fig. 2b –d). The westward extent of new deep water in model run 1 is again notably greater than model runs 2 and 3. The intermediate waters (at around 1000 m) remain relatively unventilated in the eastern part of the section in each model run. Overall, the modelled CFC12 concentrations are in good agreement with observations (Roether et al., 1996), exceeding 0.6 nmol m 3 at the bottom in each of the integrations. The vertical redistribution of water masses is also well illustrated by the change in salinity observed between 1987 and 1995. Klein et al. (2000) have
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Fig. 2. Modelled CFC12 concentrations (nmol m 3) on a section at approximately 35jN (shown inset). (a) shows the climatological state in 1986 with recently ventilated deep water, revealed by the elevated CFC12 values, in the Ionian only. The model state in 1995 is shown in (b – d) for runs 1 – 3, respectively. The contour interval is 0.1 nmol m 3. Values near the surface are typically greater than 1 nmol m 3 at this time.
presented profiles of net salinity changes in different geographical regions of the eastern basin between the Meteor cruises M5 and M31 based on hydrographic data. The same analysis is performed in the model and the results are presented in Fig. 3. Note that the observational analysis ignores regions which are poor in data, and that the model analysis is matched as closely as possible to the same geographical regions. In the region to the south of Crete (defined as the Cretan passage 22 –28jE, 30 –35jN), the reduction in salinity at intermediate levels and associated increase in salinity in the deep levels are reproduced reasonably well by the model (Fig. 3a). The maximum decrease in salinity of 0.07 psu occurs at around 400 m, while the maximum increase of 0.18 psu occurs below 3000 m in model run 2. In the Levant (defined as 28 –35jE, 30 –37jN), the model again captures the increased salinity in the deep
waters (Fig. 3b). The increase is about 0.1 psu toward the bottom in each model run, which is again reasonably consistent with observations. However, the observed increase in salinities in the surface and intermediate waters of around 0.05 psu is not captured by the model, which exhibits a small net decrease everywhere above 1000 m. In the Ionian basin (defined as 15– 22jE, 30– 41jN), the lack of overflow of deep water to the west of Crete means that the model results do not show the salinity increases in the deep waters revealed by observation. Fig. 4 shows the time evolution of the water properties below 1500 m in the Cretan passage (defined as before: 22 –28jE, 30 –35jN) on temperature – salinity diagrams. The water in this region is binned at intervals of 0.0625 jC and 0.025 psu to give contours of a volume per T – S bin in m3 (jC psu) 1. In 1986, the climatological state reflects a uniform deep water
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Fig. 3. Changes in the salinity (psu) between 1986 and 1995 for different geographical regions as vertical profiles. In each panel, the crosses represent the observational estimate of the mean salinity change from the hydrographic data (Klein et al., 2000), the solid line is model run 1, the dashed line is model run 2, and the dotted line is model run 3. The geographical extent of each area is given in the text.
of Adriatic origin with temperature around 13.4 jC and salinity around 38.7 psu. These values compare well with observed bottom water with temperature 13.3– 13.4 jC and salinity 36.65 – 36.70 psu (Klein et al., 1999). In model run 1, the new deep water (Fig. 4b) is uniform with increased temperature around 13.8 jC and salinity 38.85 – 38.90 psu, values which again compare well with observation. The situation in 1995 for runs 2 and 3 is more mixed: some new water with the correct properties is present but there is not enough to replace completely the original Adriatic deep water. However, it can be seen that run 3 has produced slightly more new dense water of the correct properties than run 2. In all cases, the density of the new deep water has increased by 0.03– 0.05 kg m 3. Overall, the production of deep water, as revealed by the extent of high CFC12 concentrations, is greatest in model run 1. This is reflected in the increase in deep water salinities, which is again largest in model run 1, but generally smaller in model runs 2 and 3. New deep water production and redistribution is modelled reasonably well even with only 3 years of enhanced Aegean cooling, although salt increases in the basin are generally too small. This suggests that increased net evaporation over the basin is important. Although the change in wind forcing produces slightly better final water properties, it is found to make
only a small difference to new water formation in the model compared with the changes in buoyancy forcing. The total change in the budget of salt for the eastern basin is discussed in more detail in the following section. 3.2. Time evolution of the event Fig. 5 provides a time series of salt content changes (year by year) for the entire eastern basin above and below 1500 m. The results for the climatological state for 1980 –1987 are included (as part of model run 1) to illustrate the degree to which the model is in a steady state prior to the imposed change in forcing. Fig. 5a shows the salt changes above 1500 m which, prior to 1987, are on average slightly negative, indicating that the upper levels are not entirely in equilibrium. After 1987, the salt losses in the upper layer increase significantly. Below 1500 m (Fig. 5b), the model salt content is steady before 1987, after which significant gains in salt are produced. This is most pronounced in model run 1, but is also well marked in model runs 2 and 3. The gain in salt in the deep water is partly offset by the loss in the upper layer water, but the size of the loss indicates that this redistribution is not sufficient to account for the whole change, and there must be a net inflow of salt in the model. (Note that in the model, this
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Fig. 4. Contours of specific volume in m3 (jC psu) 1 on temperature salinity diagrams for the region of the Cretan passage below 1500 m show the quantity of water with given properties. The outermost contour in each panel is 2 1015 m3 (jC psu) 1 while the contour interval is 4 1015 m3 (jC psu) 1. Density contours (potential density 1000 kg m 3) are also shown.
inflow of salt is distributed between a physical inflow of salt owing to the circulation, and an unphysical surface salt flux arising from the boundary conditions. However, the net inflow of salt should be interpreted as being provided entirely by the circulation.) Fig. 6 shows a time series of the net heat and salt changes for the whole water column in the eastern basin. Years of enhanced cooling are easily identified by the strong heat losses (Fig. 6a). The overall salt change is again predominantly positive, but is only consistently so in model run 1 (Fig. 6b). The largest salt gain of 4 – 6 1012 kg year 1 is comparable to that suggested by observation, but this figure is not sustained over the entire period 1987 –1995. This is reflected in the total
salt changes over the period (shown in Table 1) which are smaller than those estimated by Klein et al. (1999). The observations show approximately one third of the additional salt below 1500 m has originated in the upper 1500 m. In the model, this fraction is also around one third in model runs 1 and 2, but increases to almost two thirds in model run 3. Few observational data are available in the 1987– 1995 period, but one useful record of the time-evolution of the event is provided by Theocharis et al. (1999), who record the deep water properties in the Cretan Sea. Unfortunately, no comparable data appear to be available for the period directly before 1987. The mean temperature and salinity below 1000 m in
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Fig. 5. Time series of the salt change in the water column for the eastern basin (a) above 1500 m and (b) below 1500 m. For each model integration, year-on-year values (kg salt year 1) are given.
June in the model Cretan Sea, which depend upon the previous winter’s water formation, are presented in Fig. 7. The model results for the period 1980 – 1987 show that the deepwater properties are relatively steady before the introduction of changed forcing. In model run 1, the onset of enhanced cooling in winter 1987 initiates a sharp drop in the temperature (Fig. 7a) of around 0.1 jC, and a sharp rise in the salinity (Fig. 7b) of 0.05 psu. In the following years, the temperature shows a steady downward trend until 1992, while the salinity remains roughly unchanged. This contrasts strongly with the results for model runs 2 and 3, where the initial decrease in temperature after 1987 is reversed in the period 1988 – 1992. There is then a further significant decrease in temperature of around 0.2 jC caused by the enhanced cooling of winters 1992 and 1993. The salinity trends for model runs 2 and 3 also differ from model run 1: the initial increase in salinity is again reversed in both model runs, while the enhanced cooling of 1992 induces a second increase in salinity. Model runs 2 and 3 differ in that the Cretan Sea water is cooler and fresher in the
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Fig. 6. Time series of modelled (a) net heat and (b) net salt changes for the whole water column. Year-on-year values are again presented.
latter, suggesting that the stronger northerly winter wind in model run 3 contributes to the southward transport of north Aegean water. However, there is no additional outflow of deep water into the eastern basin in model run 2 compared with run 3. Zervakis et al. (2000) report an overturning in the north Aegean associated with an extreme cold event in March 1987. However, the observations of Theocharis et al. (1999) show no decrease in temperature in the deep Cretan Sea after March 1987, but actually show an increase of around 0.15 jC between March 1987 and March 1992. This increase in temperature is reflected in model runs 2 and 3, but only after an Table 1 The change in salt content between 1987 and 1995 (1013 kg) estimated from hydrographic data (Klein et al., 1999) and computed in the entire eastern basin for model runs 1 – 3
Observation Model run 1 Model run 2 Model run 3
Above 1500 m
Below 1500 m
Net total
2 to 3 1.6 0.7 1.2
7 3.8 1.7 1.7
4 to 5 2.2 1 0.5
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added over the period to drive sufficient dense water production. However, the cooling events of 1987, 1992 and 1993 are clearly important, and the model runs resolving these events (2 and 3) induce realistic changes in temperature in the deep Cretan waters at this time.
4. Summary
Fig. 7. Time series of modelled deep-water properties in the Cretan Sea (below 1000 m): (a) potential temperature and (b) salinity. The open circles represent model run 1 with 8 years of enhanced winter cooling, the squares model run 2 with only 3 years of enhanced winter cooling, and the triangles model run 3 with 3 years of enhanced cooling together with the anomalous winter wind stress.
initial cooling driven by the cold 1987 winter. The observed decrease in temperature between March 1992 and March 1994 is 0.3 jC, which model runs 2 and 3 again reproduce well. The observed rise in deep Cretan Sea salinity of about 0.2 psu is relatively gradual over the whole period 1987 – 1994 and is much greater than the model increase. The model results are consistent with the use of the relaxation boundary conditions, where only the temperature is changed. The use of only 3 years of enhanced cooling in model runs 2 and 3 allows the surface temperature to recover in the intervening period. This means that surface temperatures revert to the climatological state and no cold convection occurs in the north Aegean between 1988 and 1991, which in turn leads to warming in the Cretan Sea owing to vertical mixing with the overlying water. Taken together, these results suggest that the model reproduces most qualitative aspects of the observed changes, but not enough salt is
This work extends the original investigation of Wu et al. (2000) by performing a number of different model integrations to assess the impact of only three extremely cold winters in the north Aegean, together with the influence of differing surface forcing (both buoyancy and momentum) on the development of the new deep water regime in the eastern Mediterranean. A new model with a number of improved features related to the model physics, including a better representation of the topography by partial cells, is employed. This model is capable of providing a good representation of the bottom water flows and CFC12 concentrations in the climatological state where the deep water is formed exclusively in the Adriatic Sea (Stratford and Haines, 2000). The topography in the region of interest for this work (the Aegean Sea and the surrounding area) is more faithfully represented than in previous work at the same horizontal resolution. The first clear result of the present work is that, in contrast to Wu et al. (2000), only three winters of enhanced cooling in the north Aegean are required to initiate the formation of new eastern Mediterranean deep water in the model. The mechanism is the same as that of Wu et al., with cool north Aegean water entering the Cretan Sea and mixing with saline LIW to give rise to the new dense water which overflows into the deep eastern basin. Furthermore, when the observed cool winters of 1992 and 1993 are resolved the trend in model temperatures in the Cretan Sea follows observation more consistently. New deep water overflowing from the Cretan Sea is identified by high CFC12 concentrations in the model which are consistent with observation except in their westward extent in the Ionian Sea. The vertical redistribution of salt, which accompanies the new dense water formation, is shown to be broadly consistent with observation in regions where new deep water is present. However, there is no
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outflow of new dense water west of Crete, suggesting insufficient water is formed in the Cretan Sea to force an overflow at Antikithera, or a problem with the resolution of the topography. The role of wind forcing, as investigated by Samuel et al. (1999), is found to be secondary to that of the buoyancy forcing in the current model. However, the model suggests that stronger northerly winter winds for the period 1988– 1993 do promote the southward transport of cool north Aegean water as far as the Cretan Sea, and lead to slightly improved water properties in the deep eastern basin. In reality, the wind and buoyancy fluxes are not decoupled as in the present model. Increased wind strength should account for not only increased momentum fluxes, but also increased buoyancy fluxes via increased heat loss and evaporation. Increasing wind strength also provides extra mechanical energy for deep mixing, but it is likely that the buoyancy fluxes will be dominant in producing the deep water in the area of interest in this study. While the qualitative performance of the model is good, a comparison of the total salt increase in the eastern basin with that estimated from observation reveals that the model changes are not large enough. For example, hydrographic data suggest a net increase of 4 –5 1013 kg of salt in the eastern basin between 1987 and 1995 (Klein et al., 1999). The model records at most half this increase, which indicates that there is insufficient net evaporation in the eastern basin in the model. A relaxation to more realistic salinities (e.g., Brankart and Pinardi, 2001) may produce some improvement in the salt budget, but the impact of changing freshwater budgets on the model would be more easily addressed using flux boundary conditions (e.g., Myers and Haines, 2000). In conclusion, the model results capture a number of important features of the observed eastern Mediterranean transient, including deep CFC12 distributions and vertical redistributions of salt. The results suggest an important role for both freshwater and heat fluxes in driving the event. There remain important outstanding questions involving the transient, such as the ultimate source of the extra salt observed in the basin. A second important issue to be resolved in the current model is the lack of new deepwater overflow to the west of Crete. Improved models of the Mediterranean bathymetry and improved forcing data offer ways to address such questions.
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Acknowledgements The work was inspired by discussions at a Commision Internationale pour l’Exploration Scientifique de la mer Me´diterrane´e (CIESM) workshop in Trieste in March 2000. We are grateful to Birgit Klein for kindly providing the observational data shown in Fig. 3 and to Wolfgang Roether for the CFC12 concentrations. The work was funded by the European Community MAST programme under grant MAS3-CT96-0051 (MATER) and MAS3-CT98-0189 (MEDNET—see http://www.met.ed.ac.uk/mednet/). References Adcroft, A., Hill, C., Marshall, J., 1997. Representation of topography by shaved cells in a height coordinate ocean model. Mon. Weather Rev. 125, 2293 – 2315. Brankart, J.-M., Pinardi, N., 2001. Abrupt cooling of the Mediterranean Levantine intermediate water at the beginning of the 1980s: observational evidence and model simulation. J. Phys. Oceanogr. 31, 2307 – 2320. Brasseur, P., Beckers, J.-M., Brankart, J.-M., Schoenauen, R., 1996. Seasonal temperature and salinity fields in the Mediterranean Sea: climatological analyses of an historical data set. DeepSea Res. 43, 159 – 192. Bryan, K., 1969. A numerical method for the study of the circulation of the world ocean. J. Comp. Phys. 4, 347 – 376. Gill, A.E., 1982. Atmosphere – Ocean Dynamics Academic Press, New York. Josey, S.A., Kent, E.C., Taylor, P.K., 1999. New insights into the ocean heat budget closure problem from analysis of the SOC air – sea flux climatology. J. Clim. 12, 2856 – 2880. Killworth, P.D., Staniforth, D., Webb, D.J., Patterson, S.M., 1991. The development of a free-surface Bryan – Cox – Semtner ocean model. J. Phys. Oceanogr. 21, 1333 – 1348. Klein, B., Roether, W., Manca, B., Bregant, D., Beitzel, V., Kovacevic, V., Luchetta, A., 1999. The large deep water transient in the eastern Mediterranean. Deep-Sea Res. 46, 371 – 414. Klein, B., Roether, W., Manca, B., Theocharis, A., 2000. The evolution of the eastern Mediterranean climatic transient during the last decade: the tracer viewpoint. CIESM Workshop Abstracts, Trieste. Lascaratos, A., Roether, W., Nittis, K., Klein, B., 1999. Recent changes in deep water formation and spreading in the eastern Mediterranean Sea. Prog. Oceanogr. 44, 5 – 36. Malanotte-Rizzoli, P., Manca, B., Theocharis, A., Brenner, S., Bu¨ zsoy, E., 1999. The eastern Mediterranean in the dillon, G., O 80s and in the 90s: the big transition in the intermediate and deep circulations. Dyn. Atmos. Oceans 29, 365 – 395. Marullo, S., Santoleri, R., Malanotte-Rizzoli, P., Bergamasco, A., 1999. The sea surface temperature field in the eastern Mediterranean from advanced very high resolution radiometer (AVHRR) data: Part II. Interannual variability. J. Mar. Syst. 20, 83 – 112.
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