Alfred Wegener Institute for Polar and Marine Research
On the Mass Budget of the North Atlantic Ocean
M.Wenzel and J.Schr¨oter
Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
mwenzel@awi-bremerhaven.de jschroeter@awi-bremerhaven.de
Introduction
The mass budget of the North Atlantic Ocean is studied with a global cir- culation model that conserves mass instead of volume, i.e. fresh water is exchanged with the atmosphere via precipitation and evaporation and in- flow by rivers is taken into account. The mass is redistributed by the ocean circulation. Furthermore, the oceans volume changes by steric expansion with changing temperature and salinity.
Recent volume changes are monitored successfully by altimetry. However, the corresponding mass changes - or bottom pressure variations - can be estimated only using secular changes in the geoid provided e.g. from the GRACE mission since 2003. But these data are still not accurate enough.
To distinguish between mass variations and steric effects in the measured volume changes of the ocean a global data assimilation experiment was performed. For this satellite altimetry referenced to the GRACE geoid is assimilated together with a set of oceanographic data into an OGCM, that offers the ability to estimate the single contributions to sea level change,
the steric (thermosteric, halosteric) and the non-steric effects (local fresh water balance, mass redistribution) seperately. The model has a 2o ×2o ho- rizontal resolution, 23 vertical layers and a ten day timestep. Nine years (1993-2001) of respective TOPEX/Poseidon sea surface height anomalies are assimilated into the model. In addition the SHOM98.2 mean sea surface relative to the GRACE geoid (GfZ) as well as sea surface temperatures and ice cover information from Reynolds (2002) are assimilated into the model.
Furthermore background information from the Levitus WOA98 is used.
To adjust the model to the data the adjoint method is employed. The con- trol parameters of this optimization are the models initial temperature and salinity state as well as the forcing fields (windstress, air temperature and surface freshwater flux). For verification the models bottom pressure an- omalies are compared to the geoid variations derived from the GRACE mission.
Sea Level Evolution
∂
∂ t ζ = P – E + R freshwater flux + ∇ · Z
ζ−H
~ v dz divergence + Z
ζ−H
1 α
∂α
∂ T
S,p
∂
∂ t T dz thermosteric effect + Z
ζ−H
1 α
∂α
∂ S
T,p
∂
∂ t S dz halosteric effect
+ A
h∆ ζ subgrid processes
SSH / Bottom Pressure
1993 1994 1995 1996 1997 1998 1999 2000 2001 -6
-4 -2 0 2 4 6 8 cm
TOPEX/POSEIDON model
rms = 1.88
area mean SSH anomaly / North Atlantic
1993 1994 1995 1996 1997 1998 1999 2000 2001 -3
-2 -1 0 1 2 3 4 cm
thermosteric halosteric non-steric
SSH components / North Atlantic
2002 2003 2004
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5
cm
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5
mbar
GRACE (data) bottom pressure (model)
geoid anomaly vs. bottom pressure anomaly
North Atlantic
2002 2003 2004
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
cm
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
mbar
GRACE (data) bottom pressure (model)
geoid anomaly vs. bottom pressure anomaly
Global Ocean
0.5
0.1
0.1 0.1
0.2
0.0
0.4
0.0 -0.1
0.1 -0.2
-0.3
0.1
0.1 0.2
0.1
0.0 0.1
0.0
90W 70W 50W 30W 10W 10E 30E EQ.
10N 20N 30N 40N 50N 60N 70N
EQ.
10N 20N 30N 40N 50N 60N 70N local linear trend
[1993-2001]
contour interval:
0.1 mbar/year
bottom pressure anomaly
0.4
0.6
0.6 0.7 0.8
0.9
1.0
0.5 0.3 0.4 .3
0.9 1.1
1.0
0.6 0.7
0.80.9
0.7
0.7
0.9
90W 70W 50W 30W 10W 10E 30E EQ.
10N 20N 30N 40N 50N 60N 70N
EQ.
10N 20N 30N 40N 50N 60N 70N mean annual cycle
[1993-2001]
contour interval:
0.1 mbar
amplitude
bottom pressure anomaly
240 240
240
240
240 230 240
2
280 250 250 250
220
260 270
27
260
2
90W 70W 50W 30W 10W 10E 30E EQ.
10N 20N 30N 40N 50N 60N 70N
EQ.
10N 20N 30N 40N 50N 60N 70N mean annual cycle
[1993-2001]
contour interval:
10 days
phase
day of maximum value
bottom pressure anomaly
The upper left figure shows the North Atlantic mean sea sur- face height anomaly (between 19oN and 65oN) as estimated by the global model. It fits the altimeter measurements well, although the amplitude of the annual cycle is underestimated as compared to the TOPEX/POSEIDON data (black curve).
Aside the strong seasonal cycle that is mainly caused by ther- mosteric effects (upper right figure, black curve) there is al- so a positive linear trend visible that is due to the halosteric part (blue curve). The volume changes through mass variations (non-steric part, red curve) are much smaller in amplitude and exhibit only a negligible positive trend. The comparison of the corresponding bottom pressure anomalies (mean annual cycle) to the geoid variations estimated from the GRACE mission (in cm watercolumn analog, lower left figure) shows that the model again underestimates the seasonal amplitude and that it leads the measurements by about two month. Especially for the phase the comparison gets even worse when looking more lo- cally, but it gets better when looking at larger area means, e.g.
the global ocean (lower right figure). However this compari- son should be treated with caution because the GRACE data are still rather preliminary.
For the North Atlantic part of the world ocean the three plates on the right show from top to undermost: the bottom pressu- res linear trend as well as the amplitude and the phase of the mean annual cycle. The bottom pressure trend is decorrelated from the sea level trend and cannot be observed by altimitry. It varies substantially locally with an increase in mass in the sub- tropical gyre and a decrease in the Labrador Sea. Such large scale model predictions should be detectable by the GRACE satellites once the measurements have been fully analysed. The annual cycle of the bottom pressure is fairly coherent over the North Atlantic basin. Its maximum is in August and the highest amplitudes are found along the North American coastline.
North Atlantic Mass Balance
5 -5
-15
25 -20 15
10
30 5
0 10
5
-10 -15
-10
5
-5
-35 5 5
5 5 20
-25
1 -40
0 2
10 5 10
15
-5
0 10
-10
90W 70W 50W 30W 10W 10E 30E EQ.
10N 20N 30N 40N 50N 60N 70N
EQ.
10N 20N 30N 40N 50N 60N 70N temporal mean
[1993-2001]
contour interval:
5*10-9 m/s 0.47Sv
0.04Sv
0.30Sv 0.05Sv
-0.18Sv
surface fresh water flux
1993 1994 1995 1996 1997 1998 1999 2000 2001 -3
-2 -1 0 1 2 3 Sv
surface-650m 650m-bottom total
Atlantic mass transport across 65N
1993 1994 1995 1996 1997 1998 1999 2000 2001 -0.6
-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 Sv
65N C-G 19N
to MED surface balance
0.0004
-0.474 -0.045 -0.303
0.035 -0.181 0.0004
North Atlantic mass balance (Sv):
1993 1994 1995 1996 1997 1998 1999 2000 2001 -30
-20 -10 0 10 20 30 Sv
surface-1300m 1300m-4000m 4000m-bottom total
Atlantic mass transport across 19N
In the nine year mean the mass balance of the North At- lantic between 19oN and 65oN is closed (upper left figure).
The net inflow across 65oN is balanced mainly by the net outflow across 19oN and the mass loss by evaporation. The inflow through the Canadian Archipelago and the loss to the Mediterranean Sea give only minor contributions to the balance. Nevertheless the total mass of the North Atlan- tic is not constant. There is a seasonal cycle in the balance with an amplitude of about 0.05 Sv (lower left figure, black curve).
The temporal variability of the mass flux through the sur- face (lower left figure, blue curve) is highly anti-correlated with the North Atlantic horizontal transport divergence (r = −0.98). For the single contributions of the total mass balance the correlation is highest (by absolut value) bet- ween the surface flux and the total transport across 19oN (green curve, r = −0.86), while there is no correlation bet- ween the Denmark Strait overflow (upper right figure, red curve) and the mid-depth outflow across 19oN (blue curve in lower right figure, r = 0.01).
