EGS/AGU Assembly Society, April 10th 2003, S. A. Josey

Outline

1) Background - The Eastern Mediterranean Transient

2) Possible Causes of the EMT

3) Recent Results from SOC & NCEP/NCAR Datasets

4) Conclusions

Air-Sea Flux Variability in the Eastern Mediterranean and its Influence on Deep Water

Formation

E Med Map 1

Early 1990s Picture

The Eastern Mediterranean Transient

Repeat hydrographic surveys in 1987 and 1995 revealed sharp increase in salinity of deep water in Eastern Med (Roether et al., 1996).

Analysis of T-S properties of water revealed that location of deep water formation had shifted from Southern Adriatic to Aegean Sea.

Salinity Sep 1987 Salinity Jan 1995

The Signature of the EMT in Chlorofluorocarbons

Signal of new deep water formation also present in elevated levels of CFC-12.

1987

1995

Isolines of CFC-12 along a section across the Ionian basin in 1987 and 1995 (Roether et al., 1996).

E Med Map 2

Late 1990s Picture

What Caused the Transient?

Various possible causes suggested:

1) Change in wind-driven circulation (Samuels et al., 1999).

2) Increase in salinity due to multi-decadal reduction in river runoff (Boscolo and Bryden, 2001), reduced Black Sea outflow (Zervakis et al., 2000).

3) Extreme heat loss during severe winters of early 1990s.

4) Reduced precipitation during 1989-1993 (Theocharis et al. ,1999).

Relative contributions from 3) and 4) resolved using SOC and NCEP/NCAR datasets (Josey, 2003, JGR-Oceans, accepted.)

Contribution of Wind Forcing to the EMT

Found increased exchange of LIW at Cretan Arc Straits with enhanced deep water formation in the Aegean and outflow into the Levantine / Ionian basins.

Samuel et al. (1999) examined effects of intensified winter wind stress over Aegean in late 80s/early90s in a model study.

Salinity cross-section across Levantine and Ionian south of Crete.

1980-1987

1988-1993 High salinity Aegean outflow.

Impact of Heat Flux and E-P Anomalies

Wu, Haines and Pinardi (2000) used Med version of the GFDL modular ocean model to show that deep water formation in E Med can be triggered by reduction in SST of 1-2o C and increased E-P of 10-15%.

Prior to cooling After 8 years of cooling

Temp.

Salinity

Cross sections south of Crete New (warmer and more

saline) deep water

Combined Effect of Surface Forcing and River Runoff

Boscolo and Bryden (2001) stress the role of long term (1960s-1980s) reduction in river runoff which erodes low salinity intermediate waters.

Find major deep water formation events in simple mixed layer model forced with observed fluxes and long term increase in E-P to simulate runoff changes.

T-S Diagram for Aegean Sea deep water from 1987-1995: open squares-

observations, filled circles-model.

1987

1995

Issues Raised at CIESM Workshop

Origin and evolution of the EMT reviewed at a CIESM workshop in March / April 2000. Report noted that:

a.) Relative contributions of anomalously dry years and cold winters in triggering EMT need to be resolved.

b.) Connection of EMT to larger scale atmospheric anomalies (NAO?) needs to be established.

Attempted to address both of these issues.

Climatological Mean Winter Forcing

Climatological winter (Nov-Feb) mean net heat flux and net evaporation (E-P) in the Mediterranean Sea.

Components of the Winter Forcing

Wind stress and specific humidity (g/kg)

Latent heat flux (W/m2) Shortwave flux (W/m2)

Evaporation (m/month) Precipitation (m/month)

SOC Winter (Nov-Feb) Flux Components and Met Variables

Winter Heat and Freshwater Flux Anomalies

Time series of the anomalous net heat flux and net evaporation (E-P) in the Aegean Sea during winter (Nov-Feb).

Heat Flux Freshwater Flux

1987 Cruise 1995 Cruise

Density Flux Calculation

Monthly mean estimates of the thermal and haline contributions to the density flux determined from:

Salinity fields from the MEDAR dataset (M. Rixen).

€

Fρ =−ρ αQNet

ρcP−βS

E - P

(1-S/1000)

⎛

⎝ ⎜

⎞

⎠ ⎟

Thermal term Haline term

Impact on the Ocean Buoyancy Loss

Time series of the total, thermal and haline density flux anomalies in the Aegean Sea in winter.

Total Density Flux

Haline Density Flux

Thermal Density Flux

Thermal and Haline Components of the Density Flux

SOC NCEP/NCAR

Total

Thermal

Haline

Anomalous density flux (units kg m-2 s-1) averaged over winter 1991-92 and 1992-93.

Is The Summer Important?

Winter Annual

Total

Thermal

Haline

Time series of annual and winter mean density flux anomalies for the Aegean Sea….winter dominates.

Source of Heat Flux Variability

Composite plots of heat flux anomalies for severe winters of 1991-92 and 1992-93.

Net Heat (Wm-2) Latent (Wm-2)

Sensible (Wm-2) Longwave (Wm-2)

Shortwave (Wm-2)

Effects of Northerly Flow

Composite plot of latent heat flux and meteorological variable anomalies for severe winters of 1991-92 and 1992-93.

Latent (Wm-2)

Sensible (Wm-2)

Wind stress and sea level pressure (mb)

Atmospheric humidity (g/kg) Sea-air humidity difference (g/kg)

Strong latent heat flux anomalies driven by intense northerly flow which brings cold, dry air over the Aegean.

Can E-P Increase Explain EMT Salinity Change?

Roether et al. (1996) require increase in E-P of 20 cm/yr over 1988-1994 to explain EMT salinity increase.

Tsimplis and Josey (2001) found 10 cm/yr using reduced Cretan Arc Straits outflow.

1980 -1987 1988-1994 Increase

SOC 83 cm/yr 87 cm/yr 4 cm/yr

NCEP/NCAR 80 cm/yr 82 cm/yr 2 cm/yr

Annual Mean E-P for the entire Eastern Mediterranean Basin

Observed change (2-4 cm/yr) is a small proportion of required change (10-20 cm/yr). Implies internal salinity redistribution important.

The Long Term Context

Winter heat flux anomalies from NCEP/NCAR for 1950-2002

Winters with intense cooling

Aegean Sea EMT

The Long Term Context

Winter heat flux anomalies from NCEP/NCAR for 1950-2002

Winters with intense cooling

Aegean Sea EMT

Southern Adriatic

Recent Deep Water Formation in the Southern Adriatic

Hydrographic section across S Adriatic, March 2002, Bruno Manca (personal comm.). S Adriatic in completely new state, with deep ventilation from open-ocean convection and water from the Northern Shelf.

Are the Anomalies caused by the NAO?

Are the Anomalies Caused by the NAO?

Answer: No. Possibly related to separate mode of variability instead.

Scatter of the anomalous net heat flux against the NAO index (right) and London-Black sea index (right).

r2=0.05 r2=0.52

Is a Different Mode Responsible?

Rogers (1990) identified four main modes of sea level pressure variability in the North Atlantic. Some similarity between his second mode, termed the East Atlantic pattern (EATL), and the composite formed from extreme

Aegean winter heat loss.

January SLP difference between years when the rotated principal component scores exceed 1 standard deviation from mean in period 1899-1986 (from Rogers, 1990).

Summary

Relative contributions of heat and freshwater flux anomalies to density changes at the time of the EMT analysed using the SOC climatology and NCEP/NCAR reanalysis.

Density changes dominated by heat flux, freshwater flux term is an order of magnitude smaller.

Driven by anomalously strong northerly airflow over the Aegean in winters 1991-92 and 1992-93 which is not related to the NAO.

Possibility of earlier event in the mid-1970s (?).

SOC Climatology

Source Data: 30 million ship meteorological reports globally over period 1980-93.

Method: Fluxes estimated from the reports using various semi-empirical formulae .

Mediterranean Version : Revised fields generated using Med basin specific longwave formula (Bignami et al., 1995) and aerosol correction (Gilman and Garrett, 1994).

Basin Mean Heat Budget

Use of Mediterranean radiative flux formulae leads to better agreement with basin mean heat flux (-3 to-7 Wm-2) from exchange at Gibraltar.

Remaining difference most likely due to latent heat flux.

Shortwave Longwave Latent Sensible Net

Original SOC 199 -63 -87 -7 42

Med SOC 185 -85 -87 -7 6

Winter Latent Heat Loss 2002-2003

NCEP/NCAR reanalysis shows strong latent heat loss in Aegean last winter. Possible consequences for deep water production….

Latent heat loss of

140 to160 Wm-2.

Issues for CLIVAR

Theocharis

The Pre-EMT View of the Circulation

Schematic of Circulation from M-R?

Annual Mean Fields