Meteorologische Zeitschrift, Vol. 9, No. 1, 59-66 (February 2000) © by Gebrüder Borntraeger 2000 Article

5

Closing the water and heat cycles of the Baltic Sea

ANDERS OMSTEDT1 and ANNA RUTGERSSON2

1 Swedish Meteorological and Hydrological Institute, Norrköping, Sweden - University of Uppsala, Sweden (Current affiliation: SMHI, Norrköping, Sweden)

(Manuscript received June 15, 1998; accepted April 12, 1999)

Abstract

The objective of the present paper is to analyze the water and heat cycles of the Baltic Sea. The closure equations for the water and heat cycles are formulated and the appropriate fluxes are calculated using the ocean model PROBE-Baltic forced by meteorological fields, river runoff and sea level data from the Kattegat. The time period considered is from November 1980 to November 1995. In the closing of the water cycle it is clear that river runoff, net precipitation (precipitation minus evaporation), in- and outflows through the Baltic Sea entrance area are the dominating flows. From the ocean model it is illustrated that the long-term water balance is consistent with the salinity in the Baltic Sea and that the net precipitation is positive during the studied period. For the closing of the heat cycle, the net heat loss to the atmosphere from the open water surface, as an annual mean, is in close balance with the solar radiation. The dominating fluxes in the net heat loss to the atmosphere are the sensible heat flux, the latent heat flux and the net long wave radiation. The heat flux from water to ice also needs to be included in the modeling efforts. Heat flows associated with precipitation in the form of rain and snow can, as annual means, be neglected as well as the heat fluxes associated with river runoff, solar radiation through the ice and ice advecting out through the Baltic Sea entrance area. The total annual mean heat loss from the water body is in close balance with the annual change of heat storage in the water and the net heat exchange through the Baltic Sea entrance area is small. This illustrates that the Baltic Sea thermodynamically responds as a closed ocean basin.

Zusammenfassung

Ziel dieses Artikels ist es, die Kreisläufe von Wasser und Wärme in der Ostsee zu untersuchen. Schließungs­gleichungen für den Wasser- und Wärmekreislauf werden aufgestellt und die zugehörigen Flüsse mit dem Ozeanmodell PROBE-Baltic berechnet, das mit meteorologischen Feldern, Abfluss aus Flüssen und Meereshöhendaten aus dem Kattegat angetrieben wird. Der betrachtete Zeitraum liegt zwischen Novem­ber 1980 und November 1995. Bei der Schließung des Wasserkreislaufes wird klar, dass Abfluss, Netto-Niederschlag (Niederschlag minus Verdunstung) sowie Ein- und Ausströmung durch die Dänischen Straßen die dominierenden Flüsse sind. Mit Hilfe des Ozeanmodells wird verdeutlicht dass die Langzeit-Wasserbilanz konsistent mit dem Salzgehalt der Ostsee ist und dass der Netto-Niederschlag während des Unter­suchungszeitraums positiv ist. Zur Schließung des Wärmekreislaufs ist der Nettowärmeverlust der offenen Wasseroberfläche als Jahresmittel nahezu im Gleichgewicht mit der Solarstrahlung. Der Netto-Wärmeverlust des Ozeans an die Atmosphäre wird dominiert durch sensible und latente Wärmeflüsse sowie durch die Netto-Langwellenstrahlung. Der Wärmefluss vom Wasser in das Eis muss in die Modellierung ebenfalls einbezogen werden. Wärmeflüsse verbunden mit Niederschlag in Form von Regen und Schnee kann, als Jahresmittel, ver­nachlässigt werden, ebenso wie die Wärmeflüsse durch den Abfluss der Flüsse, Sonnenstrahlung durch das Eis und Eistransport aus der Ostsee heraus. Der mittlere gesamte Jahreswärmeverlust des Wasserkörpers ist nahezu im Gleichgewicht mit der jährlichen Änderung der Wärmespeicherung im Wasser, wobei der Netto-Wärmeaustausch durch die Dänischen Straßen gering ist. Dies zeigt, dass die Ostsee sich thermodynamisch wie ein abgeschlossener See verhält.

1 Introduction

Does the Baltic Sea gain or lose fresh water from the at­mosphere? Does the Baltic Sea gain or lose heat from the atmosphere? How much water and heat leave the Baltic Sea through the entrance area? These are questions es­sential for the BALTEX program (BALTEX, 1995) and require water and heat cycle studies. As the water and

' Corresponding author: Anders Omstedt, Swedish Meteorological and Hydrological Institute, S-60176 Norrköping, Sweden, e-mail: anders.omstedt@smhi.se

heat balances are closely linked they cannot be studied in isolation, but require consistent methods where both models and measurements are used. The importance of using the Baltic entrance area for closing the water and salt budgets was probably first realized by KNUDSEN (1900) in his now classical study. Several other water balance studies have followed that line and a review is given by JACOBSEN (1980), see also HELCOM (1986) and STIGEBRANDT (1995). One early study of heat bal­ance of the Baltic Sea was due to WALLERIUS (1932). By calculating the different components of this balance

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60 A. Omstedt and A. Rutgersson: Water and heat cycles of the Baltic Sea Meteorol. Z, (9), /, 2000

Table 1: Estimated annual mean volume flows for the Baltic Sea (order of magnitude). The flows are positive when going into the Baltic Sea. For an explanation of the variables see section 2.2.

Qi Qo Qo-Qi (P-E)AS Qr Qice Qrise (m3s~') (m3s_l) (m3s_l) (m3 s-1) (m3 s-1) (m3 s_l) (m3 s~')

105 -105 -104 103 104 -102 -101

he concluded that the heat exchange is mainly balanced locally between the atmosphere and the water body and heat fluxes due to river runoff and fluxes through the en­trance area are small. This work was later supported by JERLOV (1940). A review of the early heat studies over the Baltic Sea can be found in KULLENBERG (1981 ) and HENNING (1988).

The importance of considering the transports in the ocean during air-sea heat flux studies has been con­sidered by ISEMER et al. (1989) when studying large scale air-sea interaction. The heat budgets of the Arctic Mediterranean and the Nordic Seas have also been ana­lyzed using estimates of ocean heat transports as closure constraints (SLMONSEN and HAUGEN, 1996).

In this paper we illustrate the need of introducing ocean models in water and heat cycle studies. Ocean models provide the boundary condition for the atmo­sphere models, with high resolution in space and time, in terms of sea-surface temperature and ice. But ocean models are also important tools for the closure of the wa­ter and heat cycles as the closure of the BALTEX bud­gets is only possible in the entrance area of the Baltic Sea.

We apply the model PROBE-Baltic (OMSTEDT and NYBERG, 1996), and the water and heat cycles are cal­culated and analyzed using this model and observations from meteorological stations, water levels from the Kat­tegat and river runoff. A 15 year time period, from November 1980 to November 1995, is considered.

In Section 2, the theoretical background and details of calculations are given. The results are then presented and discussed in Section 3 and compared with some ear­lier studies. Finally a summary and some conclusions are given in Section 4.

2 Theory

2.1 Background

The direct coupling between the water and heat cycles are through the turbulent latent heat flux, which cools the sea surface and removes fresh water from the Baltic Sea through evaporation, but several other water and heat fluxes also need to be considered. The water bal­ance (Figure la) is mainly controlled by:

1. The in- and outflow between the Baltic Sea and the Kattegat.

2. The river runoff.

3. The net precipitation (precipitation minus evapora­tion).

The volume changes due to land uplift and ice advec-tion through the Baltic entrance area are much smaller (see Table 1 for order of magnitude estimates). As a long-term mean the water flows balance, but on shorter time scales this is not the case. The change in volume storage may in fact show both interannual and decadal variability and needs therefore to be considered in water balance studies.

In the calculations we have used the PROBE-Baltic model and considered the reduction of evaporation due to sea ice (OMSTEDT et al., 1997). The in- and out­flows were also calculated and the river runoff data were taken from observed monthly means (BERGSTRÖM and CARLSSON, 1994). The water balance for the Baltic Sea was evaluated by comparing measured and calculated

(P-E)A,

Q<

Ai 1-Ai

0

Ai 1 —A î

0

Figure 1: The main components in the water (a) and heat (b)

cycles of the Baltic Sea water body.

Meteorol. Z, (9), 1, 2000 A. Omstedt and A. Rutgersson: Water and heat cycles of the Baltic Sea 61

Table 2: Estimated annual mean heat fluxes for the Baltic Sea (order of magnitude). The fluxes are positive when going from the water to the atmosphere.

Fn F° Fi F; Fprec Fsnow Fice F, F0 - F, (Win-2) (Wm-2) (Win-2) (Wm~2) (Wm-2) (Wm-2) (Wm"2) (Wm-2) (Win-2)

102 -I02 10° -10° I0-1 10"' -10"' IQ"1 10°

mean salinities in the Baltic proper. The results illustrate the importance of considering the net precipitation to the Baltic Sea (OMSTEDT and AXELE, 1998). The heat balance of the Baltic Sea water body (Figure lb) is mainly controlled by:

1. The net heat flux from the open water surface to the atmosphere (equal to the sum of the sensible heat, the latent heat, and the net long wave radiation).

2. The solar radiation to the open water surface.

3. The heat flux between water and ice.

25 inflow, T

long term mean

outflow, T = 9.0 long term

O 15

1993 1993.5 1994 1994.5 1995 1995.5 1996 1996.5 1997 Year

0.5 [ b.

I 0.4 «,

> UT 1 , ' . < " / » , c , » u

£ i <, 1 (0 i Z -0.11 CC I

i ' -°'2[ (Wlon,«™^0-17

-0.3 Fo-F|

-0.4 F « 0.22, F = 0.05

-O.5' ' 1 1 1 ' 1

1993 1993.5 1994 1994.5 1995 1995.5 1996 1996.5 1997 Year

Figure 2: a) Measured in- and outflow temperatures from 5

m depth in the Öresund, b) corresponding monthly mean heat fluxes. The fluxes are transformed into heat fluxes across the

surface area of the Baltic Sea.

4. The solar radiation through the ice.

5. The heat fluxes associated with the in- and outflows through the Baltic Sea entrance area.

In Table 2 an order of magnitude estimation is given for the different heat fluxes, where all fluxes are transformed into heat fluxes across the surface of the Baltic Sea. The heat balance above sea ice is not dealt with as we only consider the water body. As a long-term mean the sum of the heat fluxes balance (if no trend), but on a shorter time scale this is not the case. The change in heat storage may show both interannual and decadal variability and needs to be included in heat balances studies.

2.2 Closure equations

In water and heat cycle studies we are often dealing with small differences between large numbers. To correctly model these small, but essential, differences it is impor­tant to use several methods of checking the results. The idea of closing the water and heat balances is therefore of great importance as it allows us to get some addi­tional information that could be evaluated and thus im­prove our modeling results. As the atmosphere does not have any natural lateral boundaries, regional closing can only be done by considering net outflows from drainage basins or straits. In the case of the Baltic Sea and its drainage basin, the closing section is in the entrance area. The reason is that if we know the fresh water in­put (river runoff and net precipitation) to the Baltic Sea, this volume flow as a long-term mean needs to pass the entrance area. Imagine the problem one would have to face if trying to close the water balance in the middle of a drainage basin or in the central part of the Baltic Sea!

The closure equation for the water volume reads:

As~T7 — Qi~ Qo+ (P—E)A s+ Q r— Qice+ Qrise (2.1 ) at

where A s is the surface area of the Baltic Sea, zs the horizontal mean water level of the Baltic Sea, As^ the change in volume storage, Qj and Q„ the in- and out­flows through the Baltic entrance area, P and E the pre­cipitation and evaporation rates, Q, the river runoff, Que

the ice volume loss through the entrance area and (2rise the volume flow associated with land uplift.

In the closure equation for the heat we consider the heat balance of the water body as follows:

dH , - = {F i-F0-F l o s s)A s (2.2) at

62 A. Omstedt and A. Rutgersson: Water and heat cycles of the Baltic Sea Meteorol. Z, (9). /, 2000

Table 3: A comparison between some estimates on the long-term mean water balance of the Baltic Sea. The flows are positive when going into the Baltic Sea and averaged over a 15 years period.

AUTHOR Qi

(m3 s"1) Q„

(m3 s ')

Q,, - Q, (m3 s ')

Qr (m3 s"1)

(P-E)AS

(m3s"1) Storage change

(1113 s"1)

Present paper 40919 -57 866 -16 947 15 141 '> 1 868 62

JACOB et al., run 1 -19 089 15 316 3 773

JACOB et al., run 2 -19216 15 760 3 456

HELCOM Table 1 lg -14 938 13 829 1 259 149

') From BERGSTRÖM and CARLSSON (1994)

where H is the total heat content of the Baltic Sea, F, and F„ the heat fluxes associated with in- and outflows and Fioss the total heat loss from the water surface (note that the fluxes are positive when going from the water to the atmosphere). F/„.„ reads:

F,oss = (\-Ai)(Fn+Fn+MFi+Fi)+Fice+Fr (2.3)

where

Fn = F/, + Fe + F) + Fprec + F snow (2.4 )

The different terms in Equations (2.3) and (2.4) are de­noted as follows: A-, is the ice concentration, F/, the sensi­ble heat flux, Fe the latent heat flux, F/ the net long wave radiation, Fprec and Fsnow the heat fluxes associated with precipitation in the form of rain and snow respectively, F" the solar radiation to the open water surface, F'v is the heat flux to the ice, Fj solar radiation through the ice, Fice the heat flux associated with ice advection out from the Baltic Sea, F, heat flux associated with river runoff.

2.3 Details of calculations

In the PROBE-Baltic model, the Baltic Sea is divided into 13 sub-basins, and the properties of each sub-basin are calculated with horizontally averaged, time-dependent, advective-diffusive conservation equations

Figure 3: Calculated annual mean heat transports through the

Belt Sea and the Öresund.

of temperature, salinity and momentum, and conserva­tion of volume and ice. Turbulent exchange coefficients are calculated from a turbulence model and each sub-basin is coupled to surrounding sub-basins through hor­izontal flows. A detailed description of the model is given in OMSTEDT and NYBERG (1996) and OMST­EDT and AXELL (1998) and will not be repeated here. The model was run for a 15-year period, from Novem­ber 1, 1980 to November 1, 1995. and the initial pro­files were approximated from available oceanographic measurements. Forcing data were based upon meteoro­logical fields from seven synoptic weather stations, river runoff and precipitation data and sea level data from the Kattegat. The different flows in the water and heat cycles were calculated with the model and integrated over the whole Baltic Sea inside the Öresund and the Belt Sea. The evaporation rate in Equation (2.1) was calculated from the latent heat flux and reduced by the ice con­centration as in OMSTEDT et al. (1997) and the volume flows associated with ice advection out from the Baltic Sea (Qice) and associated with the land uplift (£>;•,v) were neglected (see Table 1 ).

In the heat balance. Equations (2.2)-(2.4), the heat fluxes associated with river runoff (/>), ice advection out of the Baltic Sea (Fice), precipitation (Fim,c) and snow (Fsnow) were neglected (see Table 2). and all other lluxes were calculated and integrated over the whole Baltic Sea. The total heat loss from the water body. F/„vv, was thus calculated based upon the atmospheric forc­ing data, the calculated sea surface temperatures and the calculated ice concentration. The parameterization of the different heat lluxes follows OMSTEDT (1990) and will not be repeated here. The total heat content, (H = f f pcpTdzdA, where p is water density, cp the specific heat of water and T the water temperature), was calculated by vertically integrating the heat con­tent in each sub-basin and horizontally integrating over the whole Baltic Sea inside the Danish Straits. The heat fluxes associated with in- and outflows were taken from the model by adding up the (luxes between the Belt Sea and the Arkona Basin, and between the Öresund and the Arkona Basin. Using the model we can thus cal­culate all terms in the water and heat cycles. This is a clear improvement over earlier studies where one or two terms were often calculated as rest terms from the clo­sure equations.

Meteorol. Z, (9), 1, 2000 A. Omstedt and A. Rutgersson: Water and heat cycles of the Baltic Sea 63

Table 4: A comparison between some estimates of the long-term mean heat balance of the Baltic Sea. The fluxes are positive when going from the water to the atmosphere.

AUTHOR Fi, (Wm"2)

Fe (Wm"2)

Fi (Wm-2)

Ff (Wm"2)

F' 1 W (Wm-2)

Ft-CW m 2)

Floss (Wm-2)

Fa-Fi (Wm-2)

Present 7 35 43 -89 3 -0 -1 1 JACOB et al. run 1 7 32 44 -88 -5 JACOB et al. run 2 11 36 44 -86 5

HENNING Table 12 18 39 -41') 16

Net radiation (F/+F")

3 Results

3.1 Long-term means

The results for the calculated water balance are given in Table 3. From the table we can see that the net precip­itation in the present calculations is positive and about 2000 (nv's-1). In Baltic Sea studies il have often been assumed that the net precipitation could be neglected. With new and probably quite accurate calculations of the river runoff (BERGSTRÖM and CARLSSON, 1994) it is easier to state that more fresh water, compared to the river runoff is needed. An error of 2000 (m3s_l) in

E 60

Month

E 150

9? 100jc

Month

Month

Figure 4: Monthly means (based upon 15 years calculation) of the sensible heat flux(FA), the latent heat flux(Fe), the net

long wave radiation(F; ), the net heat loss (F„) from the open sea and the solar radiation to the open water surface(-FJ°, note

the sign). F/0„ is the total heat loss from the Baltic Sea water surface defined according to Equation 2.3.

the fresh water input changes the salinity of the Baltic Sea about 1 permille (OMSTEDT and AXELL, 1998) and if neglected must be regarded as a large error in the wa­ter balance. It should be noted that when estimating the net precipitation to the Baltic Sea the whole region, and not only the Baltic proper needs to be considered. In the calculations by OMSTEDT et al. (1997) the long term net precipitation in the Baltic proper is quite small com­pared to that in the gulf regions (the Gulf of Riga, the Gulf of Finland and the Gulf of Bothnia). The present water balance calculation is also compared with some other estimates. JACOB et al. (1997) discussed three runs using models for the atmosphere. In the first run (run 1), the global climate model ECHAM4 (STENDEL and ROECKNER, 1998) was applied using climatological sea surface temperatures for a period of 7 years. The second run (run 2), used the same model for a period of 10 years but with observed sea surface temperatures from 1979 to 1988. As pointed out by JACOB (1998, pers. com.) the third run was simulated using a high resolution limited area model. The sea surface temperatures were obtained from a global coupled atmosphere-ocean model simula­tion which employed an unrealistic land-sea distribution in the Baltic region. Therefore the results from run 3 will not be included in the present comparison.

From an oceanographic point of view it is quite clear that the meteorological water balance discussed by JA­COB et al. (1997) gives too much net precipitation over the Baltic Sea, implying that the salinity of the Baltic Sea should be reduced by about 1 permille. The differ­ences between the present calculation and the HELCOM (1986) study, can partly be explained by the fact that the 1980s was the wettest decade during the last 70 years, a period included in the present study but not included in the HELCOM ( 1986) study.

The corresponding estimates on the long-term mean heat fluxes are shown in Table 4. At a first look it is surprising that the heat fluxes in the different studies are so similar in magnitude. Studying the total heat loss from the Baltic Sea (F[oss) the results, however, show greater differences. From a simple heat estimation, as­suming a volume flow of 50 000 (m3s~l) through the Baltic Sea entrance area, a net heat loss of 16 (Wm-2), 5 (Wm-2) and 1 (Wm-2) corresponds to a tempera­ture difference between the in- and outflowing water of 28 (°C), 9 (°C) and 2 (°C) respectively. The estimate by

64 A. Omstedt and A. Rutgersson: Water and heat cycles of the Baltic Sea Meteorol. Z, (9), /, 2000

HENNING (1988), also pointed out by STIGEBRANDT (1995), is clearly an overestimation, which is also true in the runs discussed by JACOB et al. (1997). It is inter­esting to notice that already in the early stage of analyz­ing temperature and salinity data from the Baltic Sea it was possible to state that the net heat transport in the in-and outflowing water to the Baltic Sea is small (WAL-LERIUS, 1932).

In run 1 discussed by JACOB et al. (1997) and in the present calculations, the sign is negative indicating a net outflow of heat from the Baltic Sea. As a first guess one would think that the heat flux should go into the Baltic Sea. However, the total amount of water, and therefore the heat content, is larger for the outflowing water, in­dicating that the Baltic Sea loses heat. Assuming that the in- and outflowing water have the same temperature and equal to 8 (°C), the heat loss from the Baltic Sea is equal to 1.5 ( Wm~2), which is close to the present calcu­lation. In Figure 2a, some direct measurements from the Öresund are illustrated. The time period is from 1993 to 1997 and we have defined the in- and outflow tempera­tures based upon the in- and outflow calculations. From the figure we can notice only minor differences between the temperatures, and with a long-term mean temper­ature difference of 0.7 (°C), bringing slightly warmer water out of the Baltic Sea. The corresponding heat flux out of the Baltic Sea is illustrated in Figure 2b. Again the heat exchange through the Öresund is small. Mea­surements of the heat flow through the Belt Sea have not been analyzed in the present paper, but the calculation indicates that the net long-term heat transport through the Belt Sea is less than through the Öresund. The inter-annual variations of the modeled heat transport in and out through the Danish Straits are illustrated in Figure 3. The net heat transport is close to 1 (Wnr2) and shows rather small interannual variations.

3.2 Seasonal and interannual variations

The seasonal variations in the fluxes between the at­mosphere and the open sea as well as the total heat loss from the Baltic Sea surface are illustrated in Fig­ure 4. The seasonal variation in Fioss is between -180 to +155 (Wm~2). All fluxes show a strong seasonal cy­cle with an input of heat (negative F[oss) to the Baltic Sea starting after March to end of August (Figure 4c). The solar radiation then dominates over the net heat loss (Figure 4b), and the sensible heat flux (Figure 4a) also adds heat to the Baltic Sea during part of that period. The latent heat flux and the net long wave radiation are cooling the Baltic Sea during all seasons.

The interannual variation of some of the main fluxes are shown in Figure 5. The annual means of the net heat loss and the solar radiation in the open water area are in close balance (Figure 5b). However, quite large inter­annual variations can be noticed in the latent heat flux (Figure 5a) and the heat flux from the water to the ice

(Figure 5c). The interannual variation of the latent heat follows the results by OMSTEDT et al. (1997), illustrat­ing that the evaporation rates are reduced during cold winters as during 1985 to 1987. During severe ice con­ditions almost the whole Baltic Sea is ice covered and the time period when ice is present is prolonged. This influences also the annual mean heat flux from the water to ice, which increases during severe ice winters, Figure 5c.

The annual variation of the heat loss from the Baltic Sea is plotted in Figure 6, were also the variation of the different terms in the total heat budget is shown. The an­nual means of the total heat loss from the surface are almost balanced by the calculated change in total heat content. The long-term means of loss and change in heat content were -1 and 0 (Wm~2), respectively. The calcu­lated net outflow of heat was I (Wm~2), thus balancing the net heat loss. The figure illustrates that the annual mean heat loss from the water surface almost balances the total annual heat content change in the Baltic Sea and that the heat exchange through the Baltic entrance area is small. Thus the Baltic Sea is in local balance with the atmosphere and works thermodynamically as a closed ocean basin.

3.3 Implications

As has been stressed earlier in the paper the closing of the water and heat cycles of the Baltic Sea drainage basin is only possible if the water and heat balances of the Baltic Sea water body are considered. The main reasons being that we then can rely on observations of water volume and heat flows through the entrance area and changes in heat and salt contents within the Baltic Sea. In the water and heat balance studies within the BALTEX research we have not yet reached a consen­sus on how these calculations should be done and what kind of fluxes should be included. One of the aims of the present paper is therefore to suggest the appropri­ate closing of the water and heat cycles. One difference with the present approach from that of meteorological calculations as those discussed by JACOB et al. (1997), is the treatment of sea ice. In the present work, the fluxes between the atmosphere and the water surface are all re­duced by sea ice and the heat loss during winter seasons is not the heat loss between ice and atmosphere, but be­tween the water and the ice. The difference between the two approaches is illustrated in Figure 7, where the heat losses are plotted for the cases with and without sea ice. The difference between the two calculations is about 5 (Wm~2), with a difference in sign as well. When sea ice is included the long-term total heat loss becomes nega­tive, indicating a net heat gain to the Baltic Sea. In the case when sea ice is not considered, the Baltic Sea loses heat, implying the need for an import of heat through the entrance area. This difference could be one possible ex­planation, why the calculations discussed by JACOB et al. ( 1997) differ from the present.

Meteorol Z, (9), /, 2000 A. Omstedt and A. Rutgersson: Water and heat cycles of the Baltic Sea 65

1982

1980 982

1980 1982 1984 1986 1988 1990 1992 19S Year

Figure 5: Annual means of the sensible heat flux(F/,), the latent heat llux(FP|. the net long wave radiation(f)), the net heat loss

(Fn) from the open sea, the solar radiation to the open water

surface(-Fs°, note the sign), the heat flow between water and ice(FB,) and the solar-radiation penetrating the ice(Fsf).

Figure 6: The interannual variation of the Baltic Sea heat bud­

get (Equation 2.2) based upon annual mean values. The fully

drawn lines represent the total heat loss from the Baltic Sea wa­

ter surface (F/oss), the dashed lines the annual mean change in

heat storage and the dotted lines the difference be­

tween the out- and inflow heat (F0 — F|). Both annual and long-term means are given in the figure.

4 Summary and conclusions

In the paper we have analyzed the water and heat cycles of the Baltic Sea. Emphasis is placed on using the Baltic Sea entrance area as the closing section, and considering the heat balance of the water body. We have illustrated the need of using an ocean model in the water and heat cycle studies of the Baltic Sea and its drainage basin. The model provides a clear and consistent picture of all the details in the budgets involved and makes closing possible. The long-term means, seasonal and interannual variations of the heat budget, from a 15-year integration using the Baltic Sea model by OMSTEDT and NYBERG (1996), have been studied.

In the closure of the water cycle it is clear that river runoff, net precipitation (precipitation minus evapora­tion), in- and outflows through the Baltic Sea entrance area are the dominating volume flows. For the closure of the heat cycle, the net heat loss to the atmosphere from the open sea area, as an annual mean, is in close balance with the solar radiation to the open water. The dominat­ing fluxes in the net heat loss to the atmosphere from the open sea are the sensible heat flux, the latent heat flux and the net long wave radiation. The heat flow from water to ice also needs to be included in the modeling efforts. Heat flows associated with precipitation in the form of rain and snow, as annual means, could be ne­glected as well as the heat fluxes associated with river runoff, solar radiation through the ice and ice advecting through the Baltic Sea entrance area. The total annual mean heat loss from the Baltic Sea surface is in close balance with the annual change of heat storage in the water and the heat flows in and out from the Baltic Sea are small. This illustrates that the Baltic Sea thermody-namically responds as a closed ocean basin.

1980 1982 1984 1986 1988 1990 1992 19! Year

Figure 7: The total heat loss (F/oss) from the Baltic Sea water

surface with and without considering sea ice in the calculations.

Note that positive values imply that the water body loses heat, while negative values imply heat input from the atmosphere to

the Baltic Sea.

66 A. Omstedt and A. Rutgersson: Water and heat cycles of the Baltic Sea Meteorol. Z, (9), 1, 2000

The main conclusions from the paper are summarized as follows.

1. From the ocean model it is illustrated that the long-term water balance is consistent with the salinity in the Baltic Sea and the net precipitation is positive and of the order of 103 (m3s_1) during the studied period.

2. As a long-term mean the Baltic Sea is almost in ther-modynamical equilibrium with the atmosphere. The dominating fluxes, as annual means, are the sensible heat, the latent heat, the net long wave radiation, the solar radiation to the open water and the heat flux between water and ice.

3. Heat fluxes of less importance, as annual means, are the fluxes associated with river runoff, precipi­tation, solar radiation through the ice, ice advection out from the Baltic Sea and probably the net heat ex­change through the Baltic Sea entrance area.

4. In water and heat cycle studies for the BALTEX program, the response of the salinity and the heat content of the Baltic Sea are important parameters to study when evaluating and improving the atmo­sphere, ocean and river runoff models.

Acknowledgments

This work was carried out within the BASYS-SP6 project with funds from the European Commissions Pro­gram MAST III (Contract No. MAS3-CT96-0058, DG 12-D), the Swedish Environment Protection Agency and the Swedish Meteorological and Hydrological Institute. We would like to thank Anders Stigebrandt for some in­spiring ideas during the First Study Conference on BAL­TEX, 1995, to Barry Broman for making the measured in- and outflow temperatures from Öresund available and to Lars Axell, Daniela Jacob, Raj Murthy and Gösta Walin for valuable comments on an earlier draft.

References

BALTEX. 1995: Baltic Sea Experiment BALTEX. Initial im­plementation plan. - International BALTEX Secretariat 2, GKSS Research Center, Geesthacht, Germany, 84 pp.

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