Assessment of ENSO and other variability in 104

[islasimpson]

@phillips-ad @swrneale here's a place to discuss ENSO skill and other aspects of variability in the new runs

[swrneale]

The easiest thing to do first is for me to add a ridiculous screenshot that @megandevlan and I looked at for the evolution of ENSO between 98 and 109. Bottom line it's complicated and data insufficient, but promising!

[swrneale]

But this is the 104 20th C and obe for the same period - pretty good.

[megandevlan]

Another metric, this is looking at the correlation of SST anomalies and the Nino 3.4 index within the Nino 3.4 box at various lag//lead times. It looks like 104 B1850 (peach dashed line) and the historical run (red dashed line) are two of the better performing simulations. Still showing somewhat of a too-regular ENSO pattern with elevated correlations well ahead of and after ENSO events, but certainly an improvement on other simulations.

[phillips-ad]

Looking at 104 in the CVDP, there were a couple metrics that gave me pause. First, the standard deviation of SST in the Tropical Pacific for DJF:

compared to that seen in 99 and obs:

I haven't seen such limited variability in the tropical Pacific SSTs in any prior CESM3 run. Here is a histogram of the nino3.4 standard deviations by month for 104LT/MT (observational estimates = orange lines):

compared to those from 92/98/99 (and earlier CESM3 runs):

[islasimpson]

I'm trying to reconcile this standard deviation plot with Rich's. Rich's is variance and this is standard deviation but it seems they are different. A standard deviation of 0.7 in January for the new runs in these plots would give a variance of 0.7 times 0.7 which is something more like 0.5 whereas RIch's variance plots are showing something more like 0.8. Maybe these are plotting different things aside from that?

[phillips-ad]

Two thoughts: Rich's is from the 104 historical, mine is from the piControl. And mine has been quadratically detrended prior to the calculation. There is a positive trend of .5->1C across the nino3.4 region in 104LT over the period I am looking at.

[swrneale]

A few things. Mine is just a linear de-trend, but unlikely to be the difference think. Also Adam is using 1979-2023 for the monthly std plots, when there is significant more variability than earlier in the century (granted that is pre-satellite) whereas I am using the whole period for the 104 HIST comparison. And yes there is no doubt the HIST simulations look more active than the 1850 for #104.
It is just really damn hard to say what the 'typical' observed behavior is because of the multi decadal variations in obs., and the model. Saying all that, I think the biggest problem that remains for CESM3 is the too long-lasting events.

[jfasullo]

I also have concerns about the CESM3 simulated patterns of ENSO teleconnections and attach a related figure below. The top two panels are CESM3 (98b). The mid left is GPCP, right is CESM2. The bottom ones are CESM3 biases versus each. Note the weakness of the NH ITCZ bias in the eastern Pacific and the strength in the SH. I see the same issue for radiative flues and across all members of CESM3. I suspect the issue is related to the equatorial dry zone bias.

[phillips-ad]

The nino3.4 autocorrelation looks better w/112 (upper right panel) compared to the 104 variants (2nd row), and looks more similar to the CESM2-piControl (bottom 2 rows):

The Tropical Pacific variability looks better in 112 as well:

The ENSO teleconnections are also slightly improved, and the Tropical Pacific dry slot has also improved slightly. Good news!
CVDP Comparison 104-112

[jfasullo]

Unfortunately I'm not seeing a big improvement in ENSO teleconnections in 112 (top right) - and still much lower than CESM2 (mid right). There is some noise in this metric, so its hard to be definitive, but the overall score I find of 0.78 (not shown) is still much less than CESM2 (0.84) for which the 2 sigma range is 0.02 based on CESM1-LE. See also the poor Z500* teleconnections attached.

[megandevlan]

It looks like the western extent of SST anomalies has grown in 112 as well (at least for the highest autocorrelation region):

The minimum variance of the monthly Nino3.4 index has increased a bit, but the maximum is still pretty low:

[megandevlan]

It looks like part of the discrepancy is the period looked at. I'm using years 1-90 and the Nino 3.4 autocorrelation looks fairly problematic:

But if I use years 6-50 as in @phillips-ad nice postage-stamp plot above, things aren't nearly as concerning.

[jfasullo]

Great point - for all metrics it helps to provide some range of uncertainty against the CESM2 LE when possible. ENSO in particular has considerable internal noise.

[phillips-ad]

Man, adding another 46 years to 112 definitely changed things. (Good catch on my analyzation years @megandevlan!) So I just extended my 112 data through year 96, and ran a CVDP comparison where I specified the last 80 years of 112 as the reference, 77 80-year slices of the CESM2 piControl, and 8 overlapping 80 year slices from observations. The nino3.4 autocorrelation is as previously showed of course, but I'm adding this as it shows the spread in CESM2 (top panel) and observations (bottom panel):

112 isn't really oscillating and is well outside the observational spread.

The ENSO spatial composites show very little response in the North Pacific with little deepening of the Aleutian Low (upper left plot is 112, the other 3 panels are random CESM2 slices):

That phenomenon may be linked to a weaker precipitation response. Here are difference plots for the ENSO precipitation spatial composite between 112 and CESM2 (top) and observations over land (bottom).

Looking at the top plot, CESM2 is much wetter (drier) in the Central Tropical Pacific (Western Tropical Pacific).

[swrneale]

I agree with @phillips-ad here definitely. CESM2 had about the right remote response with too strong nino3.4 SSTA amplitudes. CESM3 seems to have a better SSTA amplitude but with much weaker teleconnections. We need to look at the precip response more closely I imagine.

[islasimpson]

Given that in the past we had seen issues with the upper tropospheric westerlies in the Pacific sector, I had been meaning to check what that's currently looking like since it could impact ENSO teleconnections. Happy to report it looks good! No problems there.