Note: Bob wrote to me with this caveat:
Caution: Those Hovmollers will bring you back to the 60s if you stare at them too long.
Guest post by Bob Tisdale
Hovmoller graphs are used in some discussions of climate variability. Many times they’re used when illustrating surface and subsurface processes that take place during ENSO events. And for those who aren’t familiar with them, they can look like a flashback to the pop art of the 1960s.
When used for variables such as SST anomalies of two portions of the Low Latitudes of the Pacific, the Hovmollers can help to show the upward step changes that result from significant El Nino events.
EASTERN PACIFIC LOW LATITUDE SST ANOMALY HOVMOLLER
Figure 1 is a time-latitude plot of Eastern Pacific Low Latitude SST anomalies (30S to 30N, 178W-70W) from January 1982 to July 2009. The x-axis is time (same as a time-series graph), the y-axis is latitude, and SST anomalies are color coded. This Hovmoller plot is available through the NOAA Earth System Research Laboratory (ESRL) Physical Sciences Division (PSD) website linked later in this post.
The significant El Nino events of 1982/83, 1986/87/88 and 1997/98 stand out in red in the tropical latitudes, and the subsequent La Nina events show up in purples and blues. The lesser (secondary?) El Nino events that formed in groups after the 1986/87/88 and 1997/98 El Nino are also obvious. And for those who aren’t aware of the timing and magnitudes of ENSO events, I’ve grafted a time-series graph of NINO3.4 SST anomalies to the time-latitude plot in Figure 2.
Note 1: The NINO3.4 SST anomalies in 1993 are not classified as a full-fledged El Nino. They rose into El Nino ranges (above 0.5 deg C) but did not remain there long enough to classify it as an El Nino event.
Note 2: Refer to my post “Similarities of the Multiyear Periods Following Significant El Nino Events Since 1970” for a discussion on the El Nino events that appear to be secondary to the significant ones of 1972/73, 1986/87/88 and 1997/98.
Note 3: The notation “3RM” in the right-side of the Hovmoller title block stands for 3-month running mean.
Note 4: The coordinates used by the NOAA/ESRL/PSD for the East Pacific (178W-70W) includes all of the Gulf of Mexico, part of the Caribbean, and a small portion of the North Atlantic.
Figure 3 is the Time-Series graph of the SST anomalies for the area of the Eastern Pacific (30S to 30N, 178W-70W) illustrated by the Hovmoller plot in Figure 1. The linear trend line shows that SST anomalies for the Low Latitudes of the Eastern Pacific have not risen over the past 29 years. If fact, there has been a very slight decline.
WESTERN PACIFIC LOW LATITUDE SST ANOMALY HOVMOLLER
A Time-Series graph of the western counterpart of Pacific Low Latitude SST anomalies are shown in Figure 4. A typical description of that dataset might read, The Western Pacific Low Latitude SST anomalies (30S-30N, 120E-180E) show a great deal of annual variability. Over multiyear spans, they rose sharply from 1980 to 1999 and have declined slightly since then.
A linear trend line, Figure 5, gives the dataset the appearance of a noisy constant rise in SST anomalies.
But the Hovmoller of SST anomalies for the Western Pacific Low Latitudes, Figure 6, illustrates something entirely different. It clearly shows that, after the 1997/98 El Nino, SST anomalies in Western Pacific rose in one step. SST anomalies greater that 0.7 deg C (Illustrated in Red) appear very infrequently before 1998. But after 1998, SST anomalies greater that 0.7 deg C are common. The El Nino event of 1986/87/88 also caused an upward step change in Western Pacific Low Latitude SST anomalies, but it’s difficult to see since it was smaller in magnitude. The eruption of Mount Pinatubo in 1991 also lowered SST anomalies for a few years. This masks the step change in 1988 and emphasizes the rise in 1994 and 1995, which is a rebound from the drop caused by volcanic aerosols.
In Figure 7, a NINO3.4 SST anomalies time-series graph has been spliced to the time-latitude plot of the Western Pacific Low Latitude SST anomalies to show the timing of the ENSO events.
Figure 8 combines the time-latitude plot and time-series graph of SST anomalies for the Low Latitudes of the Western Pacific. To highlight the step changes, I’ve added average SST anomalies for the periods before and after the significant El Nino events of 1986/87/88 and 1997/98. From January 1982 to December 1987, the average SST anomalies were -0.04 deg C; from January 1988 to December 1997, they were 0.05 deg C; and from January 1998 to July 2009 the SST anomalies for the Low Latitudes of the Western Pacific were 0.34 deg C.
Note 5: The processes associated with significant ENSO events that caused the step changes illustrated in this post are the same as those shown in:
Note 6: Time-latitude plots of global TLT anomalies from RSS were used to illustrate the step changes in TLT anomalies caused by the significant El Nino events of 1986/87/88 and 1997/98. Refer to:
“RSS MSU TLT Time-Latitude Plots… …Show Climate Responses That Cannot Be Easily Illustrated With Time-Series Graphs Alone”
Figure 9 illustrates the Hovmoller graphs as downloaded from the NOAA/ESRL/PSD webpage:
Specifically, this link:
I took the liberty of splitting them for this post.
The SST anomalies of the Low Latitudes of the Eastern Pacific mimic NINO3.4 SST anomalies, and they present a slightly negative trend. But there are upward step changes in the Western Pacific Low Latitude SST anomalies caused by the El Nino events of 1986/87/88 and 1997/98, confirmed by the Hovmoller plot, and the SST anomalies for this area have a substantial positive trend. Combine the two datasets and the result is a curve, Figure 10, that clearly shows the influence of ENSO, but has a positive trend. This is the same effect the East-Indian and West Pacific Oceans, which also exhibit the ENSO-induced step changes, have on global SST anomalies.
In “Evolution of El Nino-Southern Oscillation and Global Atmospheric Surface Temperatures”, Trenberth et al (2000) state in their Conclusions, “Although it is possible to use regression to eliminate THE LINEAR PORTION of the global mean temperature signal associated with ENSO, the processes that contribute regionally to the global mean differ considerably, and THE LINEAR APPROACH LIKELY LEAVES AN ENSO RESIDUAL.” [Emphasis added.]
As illustrated in this post and in those linked, that residual accounts for most if not all of the global TLT and SST warming since the late 1970s. Climate scientists attempt to attribute the residual to anthropogenic causes, when it is clearly a result of significant El Nino events.