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Atmosphere-Ocean coupling

An atmosphere-ocean coupled regional climate model for the Mediterranean region

Alberto Elizalde

The projections from the last IPCC report show a decrease on precipitation and increase on temperature all over the Mediterranean region. This statement is well known, but it is not clear the combined effects this could have in other processes like the water cycle.

Some of the atmospheric and oceanic processes in the region have a scale of tens of kilometers that are poorly reproduced by current global climate models (GCMs) due to their coarse resolution. For this reason, regional climate model (RCMs) should be used (Figure 1).

Figure 1: Orography of ECHAM5 (~200 km horizontal resolution, left) and REMO (~25 km, right)

Modeling the Mediterranean region is a challenging task due to its specific characteristics, such as:

  • Its complex orography. It blocks and steers the wind flow. This affects the atmospheric circulation and produces local phenomenons, for example, specific areas of cyclogenesis with its corresponding storm formation. This induces large precipitation for certain regions.
  • In the southern part of the region, the Hadley Cell subsidence is present and, in the north, the influence of the North Atlantic Oscillation is received. The regional climate is also connected with African and Indian Monsoons.
  • The Mediterranean sea is a semi-closed sea connected with the Atlantic by the Gibraltar Strait. It acts as a moisture source and a heat reservoir for the region. Evaporation is larger than precipitation; this water deficit is balanced with fresh water coming from river runoff and relative fresh water from the Black sea inflow but essentially from the Gibraltar Strait inflow. The narrow straits play a key role in the water budget assessment.

For a good comprehension of Mediterranean processes, atmospheric and oceanic components should be analyzed together. Up to now, atmosphere-ocean studies of the Mediterranean region were carried out using coupled GCMs (AOGCMs) with a very bad representation of its climatic processes due to theirs course resolution. Other studies, where RCMs were used, only atmosphere or ocean compartment were represented. Recently, only in a few cases, interactive atmosphere-ocean RCMs (AORCM) have been used.

This work studies the water cycle in a closed system using an AORCM. For a complete representation, the Black sea is included in the ocean model, and a hydrologic scheme is used to interactively assess the river discharge. This strategy was never used before and it represents the state-of-the-art of the Mediterranean climate modeling (Fig. 2).

Figure 2: Coupled system configuration. Exchange variables and coupling

Three models developed at Max Planck institute for Meteorology are coupled: the atmosphere regional climate model REMO (0.22°x0.22°), the ocean model MPI OM in a regional version (~11 km x 11 km) and the Hydrological Discharge (HD) model for the rivers routing (0.5°x0.5°).

Technical description of the coupled model

This research is supported by the CIRCE project framework "Climate Change and Impact Research: the Mediterranean Environment" (Contract: 036961). But It is also planned to use this strategy in future investigations.


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