The surface ocean is much warmer than normal this year as summer begins off the North Island of New Zealand. Sea Surface Temperatures, or SST, have ramped up quickly over the last three months from October to December 2025.
A good way to see if this year is unusual is to calculate the temperature anomalies. To answer the question “How much warmer is this year than normal”, we can compare current temperatures with the average temperature at the same time of year and at the same place. In this case, the average temperatures are calculated over over the period 2003 to 2014. The map of SST anomalies is a map of the difference in temperature between now (or any other date) and the average SST between 2003 and 2014. These differences are usually quite small, and they can be positive or negative. Everyone knows that some years are warmer or cooler than normal. A warmer than average year will produce a map with positive SST anomalies, while a cooler than average year will give a map of negative SST anomalies. The anomalies in the ocean are usually a couple of degrees C warmer or cooler, which translates to about plus or minus 2 degrees C. It’s convenient to plot these temperature anomalies on a map as either red (warmer than normal) or blue (cooler than normal).
Just as temperatures in the ocean around New Zealand vary with region, the SST anomalies also vary with region. The map in Figure 1 shows that SST was about 1 degree C warmer than normal (red colour) around Northland in October, while it was about 1 degree C cooler than normal (blue colour) in Hawkes Bay and off the Manawatu coast. At the same time, SST in the Bay of Plenty was close to normal (whitish colour).
Anomalies also vary with time, and this is where things get interesting. If you compare the October SST anomaly map in Figure 1 with the November SST anomaly map in Figure 2, you can see that the warm anomalies have spread. It’s particularly noticeable that the Bay of Plenty was was about 1 degree C warmer in November compared to October. There is also obviously warmer than normal water off Wairarapa and Taranaki in November (Figure 2) compared to October (Figure 1). Even more striking is the change in Hawkes Bay which changed from having SST cooler than normal (blue in Figure 1) to becoming warmer than normal (red in Figure 2).
The most striking change in SST anomalies is shown in the map for early December (Figure 3). The strong red colour indicates that the SST anomalies were up to 4 degrees C warmer than normal over much of the ocean off the norther half of the North Island. This is a very strong warming signal, indicating an unusually warm start to the summer game fishing season. If these conditions persist they will create what is called a marine heatwave.
The SST anomalies suggest that large pelagic fishes tracking warm conditions may arrive early this season. You may well want good SST maps. We can help with that.
