In this section we discuss changes to the modern land/sea mask to simulate sea level low stands at the Last Glacial Maximum (LGM), and extending continental glaciers across North America. There are many reasons why paleo-climate modelers may be interested in changing the land/sea mask, but changing the mask is a multi-step process. Examples of changing the land/sea mask include:
- simulate sea level change
- add land ice over North America, covering Hudson Bay
- open/close straits (e.g., Bering Strait, Isthmus of Panama)
- remove Hudson Bay for pre-Pleistocene simulations
The process of altering the modern land/ocean mask is complicated and we offer these general guidelines:
- Raising or lowering sea level requires changing the land/sea mask along the continental margins and creating new coupler masking files. Raising sea level requires that land cells be reclassified as ocean; conversely, lowering sea level requires that ocean cells be reclassified as land cells. In general, changing cells from ocean to land is more straightforward than turning land into ocean.
- We do not recommend changing ocean bathymetry or depth (KMT) when simulating changes in sea level (other than to make localized changes that are essential to your science question: e.g., closing the Bering Strait, or opening the Isthmus of Panama). Model results will be very sensitive to ocean depth. If you decide to change ocean depth you should also run a similar control experiment with your modified bathymetry to test your model sensitivity to these changes.
- In general, we do not recommend changing seafloor relief (other than to make localized changes that are essential to your science question: e.g., closing the Bering Strait, or opening the Isthmus of Panama). Ocean model results will be very sensitive to seafloor terrain, biasing the climate signal you may be interested in. If you change ocean basin relief you should also run a similar control experiment to test your model results.
When you change the land/sea mask (i.e., re-assigning ocean cells as land cells) you need to create a new binary KMT file. The tool change_kmt.ncl provides a template for making simple changes to the present day land/sea mask. The script will produce a new binary KMT file that will be read by the ocean model.
If you modify the land/sea mask you will also need to create new coupler mapping files.