Supporting Information for The Active Role of the Ocean in Transient Climate Sensitivity

Transcription

1 GEOPHYSICAL RESEARCH LETTERS Supporting Information for The Active Role of the Ocean in Transient Climate Sensitivity Oluwayemi A. Garuba 1 Jian Lu 1, Fukai Liu 2, and Hansi A. Singh 1 Contents of this file 1. Text S1 2. Figures S1 to S6 Text S1 1. Passive tracer formulation. Two passive tracers mimicking ocean temperature are used in the fully-coupled and partially-coupled experiments. They both have units of temperature and are forced with surface heat fluxes. Corresponding author: O. Garuba, Pacific Northwest National Laboratory, Richland, Washington, WA 99354, USA. (Oluwayemi.garuba@pnnl.edu) 1 Pacific Northwest National Laboratory, Richland, Washington, USA 2 Ocean University of China, Qingdao, China

2 X - 2 GARUBA ET AL.: OCEAN S ACTIVE ROLE 1.1. Surface-forced anomaly tracer (P ) The surface flux forced anomaly tracer, P, emulates the surface-forced temperature anomaly T F, which at the initial time is zero but grows with time as the surface heat flux anomaly increases. Therefore, P is initialized at zero and forced at the surface with heat flux anomaly F. Its evolution can be written as: DP Dt = F (1) where D Dt = t +v (i.e, the total derivative following the ocean circulation, v = v+v ), overbars represent the control variables, and primes denote anomalies from the control. Thus the evolution of P is the same as the surface-forced temperature anomaly component. Surface heat flux at the concurrent time in the control experiment is prescribed as input for P, then F is computed at each coupling time step as the difference between surface heat flux from the coupler and the prescribed control surface heat flux F Ocean dynamics-forced anomaly tracer ( P ) The ocean dynamics-forced anomaly tracer, T O, is obtained through the implementation of the tracer P (same as the P formulation in Xie and Vallis [2012] ). T O is the temperature anomaly forced by redistribution of the control-experiment background temperature due to the circulation anomalies in the perturbed experiment. Accordingly, the tracer P emulates the control experiment temperature in the perturbation experiment. Therefore, it is equal to the full control-experiment ocean temperature T at initialization of the CO 2 - quadrupling experiment, and forced at the surface with the control experiment surface heat flux F. Its evolution following CO 2 -quadrupling can be written as:

3 GARUBA ET AL.: OCEAN S ACTIVE ROLE X - 3 D P Dt = F (2) On the other hand, the T evolution in the control experiment is D T Dt = F (3) where D = + v, where v indicates the circulation of the control simulation. The Dt t difference between the operators in equations 2 and 3 show that P and T are advected by two different circulations, v and v. P is initialized to be the same as T, but P grow to deviate from T with time due to the circulation anomaly (v ). If we define T O = P T in equation 2, and take the difference between equation 2 and 3, we will have the equation governing the evolution of the ocean-dynamics-forced temperature anomaly, T O: DT O Dt = v T (4) P is also forced with the surface heat flux at the concurrent time in the control experiment. The ocean dynamics-forced temperature anomaly is computed as the difference between the tracer P and T. Both tracers P and P are introduced in the fully-coupled and partially-coupled simulations, with the initializations described above. References Xie, P., and G. K. Vallis (2012), The passive and active nature of ocean heat uptake in idealized climate change experiments, Climate Dynamics, 38 (3-4), , /s

4 X - 4 GARUBA ET AL.: OCEAN S ACTIVE ROLE Figure S1. The decomposition of the SST response to CO2-quadrupling in the partiallycoupled experiment into the surface-forced (SST P, panel a) and ocean dynamics-forced (SST Opart, panel b) components, and their sum (panel c). The similarity between the sum (c) and the actual SST response (SST, panel d) supports the validity of the tracer approach in the partially coupled experiment.

5 GARUBA ET AL.: OCEAN S ACTIVE ROLE X - 5 Figure S2. Surface air temperature (T S) evolution in the fully-coupled experiment (top row); in the partially-coupled experiment (passive component; middle row); and the difference between the fully- and partially-coupled experiments (active component; bottom row). Shown for the annual mean over years 20 to 50 (left column) and years 51 to 150 (middle column), and the difference between these two time periods (right column).

6 X - 6 GARUBA ET AL.: OCEAN S ACTIVE ROLE Figure S3. Same as Figure S2, but for SST anomalies.

7 GARUBA ET AL.: OCEAN S ACTIVE ROLE X - 7 Figure S4. Mean ocean dynamics-forced surface temperature anomaly patterns (average over years 1 to 50), in the fully-coupled experiment, (SST O, top), and the partially-coupled experiment, (SST O part, bottom). These are computed by subtracting the tracer P from the control experiment surface temperatures T (see section for tracer formulations above).

8 X - 8 GARUBA ET AL.: OCEAN S ACTIVE ROLE Figure S5. Mean sea surface temperature response in the slab experiments: slab 4xCO2 (top), slab P OHU (middle) and slab A OHU (bottom).