2002: Ocean-Atmosphere Connections

Team Members: Leon Abbo, Akinwale Olaleye, Jason Van Sluytman, Clinton Byrd, Adam Greenbaum, Imani Marshall. Meghan Conk, Satanya McLaughlin, Ron Miller and Gavin Schmidt

Introduction:

How well do ocean-atmosphere models simulate general features of the ocean circulation?

In order to advance our understanding of the Earth's climate and predict future changes due to human influence, atmospheric and ocean processes are mathematically simulated in computer modeling programs called General Circulation Models (GCMs). The world's oceans and atmosphere are major transporters of heat energy around the globe. Oceanic and atmospheric circulation patterns include El Nino, the circulation of the Gulf Stream, rain fall distribution, and temperature variation around the world. By comparing the climate variables associated with these phenomena, such as salinity, ocean temperature, precipitation, and ocean currents between observation and the model, we can understand how well these phenomena are represented by the model.

Forecasts of future climate based upon GCMs contain a great deal of uncertainty related to the quality of the GCM. Scientists at GISS are actively involved in testing the capability of the GISS GCM and using the results of their analysis to make improvements. The new GISS atmospheric GCM has been coupled to various ocean GCMs. These "coupled' models will be used to predict ocean uptake of heat captured by anthropogenic greenhouse gases, along with the resulting regional patterns of the ocean response.

As a prerequisite to these calculations, the project team will evaluate the coupled model's ability to simulate general features of the ocean circulation, such as the Gulf Stream, thermohaline circulation, El Nino, and water mass trajectories. This will be done by comparing the model simulations to observational data. This study is a first attempt to see how well the most recent and advanced coupled models at GISS represent the observed ocean and atmosphere. The results of this study will be used to identify necessary model improvements.

Research Objectives and Questions

Specifically, the team will examine the time-averaged behavior of model simulations subject to constant forcing: i.e. input of solar energy and atmospheric concentrations of greenhouse gases are held constant. The guiding question is whether the coupled model can simulate the major features of ocean behavior. Other questions the team will attempt to answer:

• Is year-to-year variability of sea surface temperature (SST) consistent (both in location and magnitude) with satellite and in situ estimates?
• Does the model exhibit El Nino variations in SST and tropical rainfall?
• Does the fidelity of the simulation increase with the resolution of the ocean model?
• Is the north-south overturning within Atlantic Ocean represented with the proper strength by the model?
• Do the modeled water masses (e.g. ocean water sinking off the coast of Antarctica) exhibit distinct values of temperature and salinity characteristic of the observed values? (Water beneath the ocean surface has a distinct temperature and salinity identifying the location where it left the surface. The ability of the model to simulate water masses reveals its ability to depict observed inputs of heat and freshwater within the surface layer and downstream trajectories of water descending into the deep ocean.)
• Is the ocean input of fresh water (e.g. precipitation over evaporation, river runoff) realistic in the model?

In addition, the effect of the ocean circulation upon the atmosphere will be addressed by comparing the coupled model behavior to that of an atmospheric GCM, where SST is prescribed to have the same climatological value year after year.

Work Plan: Research Tasks and Deliverables

Week 1: Introduction to the GISS computing environment. Overview of the ocean circulation and the associated natural phenomena studied in this project. Readings: Essentials of Oceanography, Chapter 7: Ocean Circulation. Mathematical definitions of anomalies, means, and variance will be studied through examples from past team results: how was the statistic used appropriate to the scientific question being addressed.

Compute a mean for the 50-year run of Gavin's (4×5 resolution) and Nick's (2×2 resolution) coupled models. Compare this to the time-mean to the Reynolds blend of in situ and satellite observations. Compute a standard deviation with coupled model SST and compare that to the data. The aim is to see how well the model simulates the climatology of observed SST and its regional variability.

Week 2: Evaluation of model simulations of El Nino. Why are the coldest SST's within the tropics located within the equatorial Pacific where the solar input is largest? Does the model's simulation of El Nino improve with increased resolution? We will compare the oceanic and atmospheric behavior of ENSO in two GISS coupled models with observed SST (Reynolds) and rainfall (the Xie-Arkin blended satellite and rain gauge data set).

Week 3: Fidelity of the model simulations of temperature and salinity within ocean basin (e.g. Pacific, Atlantic, and Arctic) as a function of depth and latitude. Can the observed water masses be identified within each model, e.g. the North Atlantic deep water and Antarctic deep water? This can be inferred by plotting temperature and salinity data (see Essentials of Oceanography Figure 7-24). Is the north-south overturning represented with the observed strength and location within the model? This can be addressed by plotting the overturning stream function which is indicative of which direction the water is flowing. Is the ocean carrying the correct amount of heat toward higher, colder latitudes compared to (limited) estimates from the literature? The location of where water sinks below the surface depends on both temperature and salinity. (Discussion of why salinity is important to climate variability on decadal time scales.) Compare model values of excess precipitation, over evaporation and river runoff to observations. Rainfall, evaporation and run-off will be plotted as a function of latitude in order to relate the surface flux of freshwater upon the surface salinity.

Week 4: (Time permitting) Compare the behavior of the model atmosphere according to whether the ocean surface is represented by an ocean GCM or the prescription of SST. In particular, compare the variability of surface air temperature. Is it larger in regions where ocean dynamics is especially active (e.g. the Gulf Stream, deepwater formation regions)?

Week 5: Prepare for final presentation and team paper.

Science Understandings

General patterns of ocean and atmospheric circulation

Features of ocean circulation such as the Gulf Stream, thermohaline circulation, El Nino, and water mass trajectories

Relationships between variables relevant to this study e.g. ocean temperature, ocean currents, ocean depth, salinity, evaporation, river runoff, precipitation

Capabilities and limitations of the GISS General Circulation Models and Coupled Models used in this study

General knowledge about remote sensing techniques to measure climate data

Concept of a climate forcing: greenhouse gases, solar energy

Mathematical Skills

Calculating climatologies, anomalies, standard deviation, and variance

Technical Skills

Unix to access, manipulate and organize data

Fortran to modify programs to perform statistical analysis

GISS Nmaps program for plotting scientific programs

Readings

Essentials of Oceanography, Chapter 7