Land and Sea Temperature: South West Australia

This year, the south-west of Western Australia has recorded some unexpectedly low temperatures.  Has this been due to rainfall, cloud, winds, or the cooler than normal Leeuwin Current and Sea Surface Temperatures in the South West Region?

In this post I examine maximum temperature and rainfall data for Winter in South-Western Australia, and Sea Surface Temperature data for the South West Region, all straight from the Bureau of Meteorology’s Climate Change time series page .

All temperature data are in degrees Celsius anomalies from the 1961-90 average.

Figure 1 is a map showing the various Sea Surface Temperature monitoring regions around Australia.

Fig. 1

sst-regions

The Southwest Region is just to the west and southwest of the Southwest climate region, and winter south westerlies impact this part of the continent first.  2016’s winter has seen maxima drop sharply.  In fact, it was the coldest winter since 1993:

Fig. 2:  Southwestern Australia Winter TMax Anomalies

sw-tmax

There is a relationship between rainfall and Tmax- as rain goes up, Tmax goes down, so here south west rainfall is inverted and scaled down by 100:

Fig. 3:  TMax and Rain:

sw-tmax-rain

The next plot shows TMax and the South West Region’s Sea Surface Temperature anomalies (SST):

Fig. 4:  TMax & SST:

sw-tmax-sst

Again, related: both have strong warming from the 1970s.  Next I check for whether there was a real change in direction in the 1970s, and if so, when.  To do this I use CuSums.

Fig. 5:  CuSums of Winter TMax and SST compared:

sw-tmax-sst-cusums

Both have a distinct change point: 1975, with SST warming since, but TMax appears to have a step up, with another change point at 1993 with strong warming since.  Rainfall however shows a different picture:

Fig. 6:  CuSums of Winter Rainfall

sw-rain-cusums

Note the major change at 1968 (a step down: see Figure 3), another at 1975 with increasing rain to the next change point at 2000, after which rain rapidly decreases.

I now plot TMax against rainfall and SST to see which has the greater influence.  First, Rain:

Fig. 7:  TMax vs Rain:

sw-tmax-vs-rain

100mm more rain is associated with about 0.5C lower TMax, but R-squared is only 0.22.

Fig. 8:  TMax vs SST:

sw-tmax-vs-sst

A one degree increase in SST is associated with more than 1.1C increase in TMax, and R-squared is above 0.51- a much closer fit, but still little better than fifty-fifty.

TMax is affected by rain, but more by SSTs.

I now look at data since the major change points in the 1975 winter.  The next three figures show trends in SST, Rain, and TMax.

Fig. 9:  Trends in SST:

sw-sst-trends

Warming since 1975 of +1.48C/ 100 years.

Fig. 10:  Trends in Rainfall:

sw-rain-trends

Decreasing since 1975 at 89mm per 100 years (and much more from 2000).

Fig. 11:  Trends in TMax:

sw-tmax-trends

Warming since 1975 at +2.14C per 100 years.

Detrending the data allows us to see where any of the winters “bucks the trend”.  In the following plots, the line at zero represents the trend as shown above.

Fig. 12:  SST Detrended:

sw-sst-detrended-75-to-16

Fig. 13:  Rainfall Detrended:

sw-rain-detrended-75-to-16

Fig. 14:  TMax Detrended:

sw-tmax-detrended-75-to-16

Note that SST in 2016 is just below trend, but still above the 1961-90 average.  Rainfall is only slightly above trend, and still below average.  However TMax is well below trend, and well below average, showing the greatest 12 month drop in temperatures of any winter since 1975.

My conclusions (and you are welcome to comment, dispute, and suggest your own):

  • Maximum temperatures in winter in Southwestern Australia are affected by rainfall, but to a much larger extent by Sea Surface Temperature of the South West Region.
  • The large decrease in winter temperature this year cannot be explained by rainfall or sea surface temperature.  Cloudiness may be a factor, but no 2016 data are publicly available.  Stronger winds blowing from further south may be responsible.
Advertisements

Tags: , , ,

3 Responses to “Land and Sea Temperature: South West Australia”

  1. waclimate Says:

    How about minima?

    At Perth Metro, the six months from May to October had the fourth coolest min since 1897 and max were 53rd coolest (based on Perth Regional Office 9034).

    At Perth Airport, min were 10th coolest and max 11th coolest since 1945 opening. ACORN adjusts those figures so that min were the 11th coolest and max 6th coolest since 1910.

    My calcs indicate September at the 15 south west ACORN stations recorded the coldest mean temp ever for the month. The BoM’s September 2016 Monthly Climate Summary for WA lists 31 weather stations that had their coldest ever average minimum and eight that had their coldest ever average maximum.

    Most indicators point to the greatest cooling in min, although both min and max were down significantly. Rainfall of any quantity doesn’t seem to have done its usual job of min up, max down or vice versa, suggesting it was cold southerly winds blowing off the 1-2C below average SST.

    November in Perth is set to have an average min of 13.6C compared to Metro’s average since 1993 of 14.3C, with a mean max of 27.7C compared to the station average of 26.6. November rainfall will be 14.4mm compared to an average 23.7mm, so the rain-temp correlation seems to be back to normal.

    Three of the first five days of December in Perth are forecast as partly cloudy but both min and max over the five days are expected to be about 4C below the full monthly average … way too early as a monthly indicator and just Perth rather than the south west, but still suggesting that cold air from below average SST will (hopefully) create a fairly comfortable summer.

    The cold seas started emerging off WA’s west coast in February and are still dominant across Australia’s southern coast. NOAA SST maps suggest it’s been decades since we last had such cold water over such a wide area for so long.

    As predicted by the CSIRO in the wake of last year’s strong El Nino, the Leeuwin Current seems to have brought little heat down from the tropics to warm Southern Ocean waters. However, this isn’t what happened after the equally strong 1997/98 El Nino, hinting that something else is at play.

    As you’ve noted before, Ken, the next six months will be interesting.

  2. kenskingdom Says:

    Hi Chris
    TMin next. I looked at TMax because that’s supposed to be most affected by rain. TMin is entirely different. Sneak preview: TMin has been cooling since 1975, and 2016 is only a little below trend and nowhere near as cold as several previous winters (2001, 2006, 2008, 2010). According to Acorn of course. And spring data might be interesting.
    KS

  3. waclimate Says:

    Hi Ken … yes, I agree that south west min have another influence at work that’s seen them cool in recent decades – most probably ~12% lower rainfall and thus less nighttime cloud cover to trap heat.

    If you scroll way down at http://www.waclimate.net/september-coldest-2016.html, you’ll see I’ve compared 1897-1917 min at Perth RO with Perth Metro, showing that average Perth nights since 1994 have been cooler than a hundred years ago from May to September.

    I look forward to your min data since 1975 (which is very close to 1972 metrication … hmmm). The south west deep freeze didn’t happen till September this year and Perth temps suggest the thaw is now (November) complete. Kalgoorlie looks like having a mean 1.5C above the November average since 1939 (whereas the Sep mean was 2C below average).

    I suspect the spring data will show the south west with below average mean, min mostly responsible, but nothing spectacular because above average Nov will balance well below average Sep.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s


%d bloggers like this: