How Unusual Has Been the Temperature and Precipitation this Fall?

 Several folks have asked about this autumn.  Has it been drier and warmer than normal?

Let's check it out by considering a representative location:  Olympia.

Below is a plot of cumulative precipitation at this location from September 1 to yesterday.  Green is this year, and the brown line is normal (the climatological values).

September was close to normal, but October has been quite dry.

No wonder I have been able to bike to work without rain gear!

Temperature is shown below, with the blue bar showing the daily highs and lows and the brown band showing the normal range.  Red indicates record highs.  

September was very close to normal, and October has been modestly above normal.

But let's get a better historical picture.  

Below is a plot of the Sept 1-October 21 precipitation totals for the past 50 years at Olympia.  A trend line for the period is also shown.

This year's precipitation is low, but many years were lower

Interestingly, precipitation increased over the period, so you can't blame climate change!

The average temperature for the entire (Sept 1-Oct 21) period was very close to normal ( this year was the 25th warmest year out of 50 years).

However, this is evidence of long-term warming, with about a 2°F warming over the past 50 years, very consistent with warming found in many studies.

What about the outlook for the rest of the month?  

The excellent European center model is predicting cooler-than-normal conditions (blue) over much of the Northwest (except for NE Washington).

In contrast, the European center model is going for drier-than-normal conditions over the Cascades and Olympics over the next week.

Biking should be relatively dry for the remainder of the month.

Important Milestone: The Current El Nino Has Hit Its Peak Early

The very strong El Nino appears to have hit its peak earlier than predicted a few months ago, suggesting a substantial weakening later this autumn and winter.

Let's begin by looking at the sea surface temperature differences from normal (called anomalies) for two critically important areas in the central tropical Pacific:  the Nino 3.4 and 4 areas (see map below).

The magnitude of El Nino events is usually judged by the sea surface temperature anomaly in the Nino 3.4 area... and I show the latest values below (from NOAA Climate Prediction Center).  The magnitude of this El Nino appears to have leveled off over the last few weeks, around 2.5 °C above normal.

The fact that the sea surface temperatures are pretty much unchanged during the past few weeks is illustrated by the sea surface temperature anomalies on October 1 and October 19 (see below).

Hardly changed.

 

To understand the future of an El Niño event, it is useful to look at the upstream region (Nino 4) to gain insight into the future evolution of temperature in the Nino 3.4 region during subsequent months.

Why look upstream?   Because the sea surface warming tends to move eastward with the Kelvin waves I described in my previous blog.

The Nino 4 temperature anomalies (from climatological conditions) are shown below.   The Nino 4 temperatures rose rapidly from April to late June and then declined rapidly in July and August.

This is consistent with the rapid rise in Nino 3.4 temperature a few months later, from July through September.

NOAA runs complex atmosphere/ocean models for seasonal prediction.  NOAA's primary model  (the CFSv2) is predicting a rapid decline in the temperature in the Nino 3.4 area starting right now!   

If that prediction is correct, it means we are now at the El Nino peak.

Other modeling systems are less rapid in the decline, such as the GFDL Spear system (see below), but all suggest we are near the maximum. 

In summary, it appears that we are now at the peak of the current El Nino, and the latest forecasts are suggesting a fairly rapid decline.   

However, there is a lot of uncertainty in such forecasts because of the highly convective (e.g., thunderstorm) nature of tropical meteorology, and because the interaction between the ocean and atmosphere is often poorly modeled.

Lots to keep graduate students busy over the next decade!

The implications of the upcoming decline in El Nino will be the topic of a future blog.