NCEP cooler in July

The Moyhu NCEP/NCAR index (based on reanalysis) was down in July, 0.164°C from 0.204°C in June. And June was fairly cool. July was close to April. It started and ended fairly warm, with a big cold dip in the middle. But I'm not sure surface indices will follow, as I'll explain.

The native anomaly period I've chosen is 1994-2013, which is a period where the reanalysis seems most reliable. I now, in the left column of the table, and below the month averages, show averages relative to GISS and NOAA bases. This uses those indices to fill the gaps (details here); it helps because it allows for the relative differences between months in one period vs another. Here are those tables:

1951-80 (GISS) base NCEP GISSlo Jul 0.715 NA Jun 0.741 0.8 May 0.813 0.76 Apr 0.698 0.74 Mar 0.851 0.9

1901-2000 (NOAA) base NCEP NOAAlo Jul 0.722 NA Jun 0.758 0.88 May 0.835 0.86 Apr 0.719 0.78 Mar 0.867 0.9

The NCEP index has been generally under-predicting the surface data recently. I analysed its performance in earlier years here, and it tracked surface measures well. Is something different now?

I think the explanation is somewhat similar to the effect noted by the recent paper of Cowtan et al on GCMs and surface indices. Like GCMs, reanalyses return the air temperature of a near surface layer. I use sig995, which is basically 0.995 atmospheric, or an altitude of about 40 m. Surface indices use SST over ocean. This is a proxy for the air temperature, which they would like to use but have sparse measurements. It won't be perfect.

Recently, warm temperatures have been driven by rising SST. I did a detailed breakdown here to show this, but it has been the general trend for the last year or more. This year, Feb/Mar warmth was especially in Asia (land), but that faded and since has been dominated by SST. I think the air-based reanalysis may be lagging, and surface measures could be higher for a while.

As said, NCEP July was fairly close to April, so those are a reasonable guess for July - ie GISS 0.74°C, NOAA 0.78°C. But I wouldn't be surprised to see them a little higher.

Update. You can see the map for July anomalies as a globe here. Here is a map in the GISS style (and to 1951-1980 base) I use for the surface report. A lot of warmth in the E Pacific and Mediterranean region. Cool in N Europe and W Russia. Generally moderate in N America, but warm in the Arctic islands.