Cloud Feedback Key to Marine Heatwave off Baja California

Abstract

Between 2013 and 2015, the northeast Pacific Ocean experienced the warmest surface temperature anomalies in the modern observational record. This "marine heatwave" marked a shift of Pacific decadal variability to its warm phase and was linked to significant impacts on marine species as well as exceptionally arid conditions in western North America. Here we show that the subtropical signature of this warming, off Baja California, was associated with a record deficit in the spatial coverage of co-located marine boundary layer clouds. This deficit coincided with a large increase in downwelling solar radiation that dominated the anomalous energy budget of the upper ocean, resulting in record-breaking warm sea surface temperature anomalies. Our observation-based analysis suggests that a positive cloud-surface temperature feedback was key to the extreme intensity of the heatwave. The results demonstrate the extent to which boundary layer clouds can contribute to regional variations in climate. Plain Language SummaryThe northeast Pacific Ocean experienced a "marine heatwave" between 2013 and 2015. This was characterized by the highest surface temperatures ever recorded in a vast swath of the ocean from near the Gulf of Alaska to off the coast of Baja California. The unprecedented warming event was linked to significant impacts on marine life and a severe drought in western North America. We analyze satellite data to show that the heatwave was associated with a record decrease in the typically high cloudiness over an area of the Pacific off Baja California that is roughly half the size of the contiguous United States. Such a deficit in cloud cover coincided with a large increase in the amount of sunlight absorbed by the ocean surface, resulting in extremely warm temperatures. Our findings suggest that a reinforcing interaction (or positive feedback) between clouds and ocean surface temperature can strongly contribute to significant and difficult-to-predict changes in marine climate.

The Role of Clouds and Surface Heat Fluxes in the Maintenance of the 2013–2016 Northeast Pacific Marine Heatwave

“…This yielded a near‐climatological SST tendency during the MHW and led to the unprecedented persistence of the warm SST anomalies. Our results also show that mechanisms that start a MHW are not necessarily the same as the combination of oceanic and atmospheric mechanisms that maintain the anomalous SSTs during a MHW or contribute to the decay of a MHW (Bond et al, ; Myers et al, ).…”

“…There was also a seasonal signature in sensible heat flux anomalies with the highest positive anomaly in September‐October‐November (4.4 W/m 2 ) and a negative anomaly in December‐January‐February (−3.3 W/m 2 ; Table ). Anomalously weak winds were observed during the MHW (Bond et al, ; Gentemann et al, ; Myers et al, ) and could be partially responsible for the latent and sensible heat flux anomalies during the MHW. The combination of turbulent and radiative heat fluxes yields the net flux from the ocean surface, which in this case was dominated by turbulent flux anomalies.…”

“…Many studies of the NE Pacific atmosphere–ocean system have observed concurrent increases in SSTs, decreases in low cloud fraction, and increases in incoming solar radiation at the ocean surface, suggestive of a positive SST‐cloud feedback (Bellomo et al, ; Clement et al, ; Norris et al, ). In the subtropical Pacific off the coast of Baja California during the same MHW, Myers et al () observed a positive SST‐cloud feedback and concluded that it did indeed play a role in the persistence of the anomalous SSTs. However, they also found that further north in the NE Pacific (40–50°N and 140–160°W) from January 2013 to December 2013, there was no evidence of a positive SST‐cloud feedback during the onset of the MHW (Myers et al, ).…”

“…In the subtropical Pacific off the coast of Baja California during the same MHW, Myers et al () observed a positive SST‐cloud feedback and concluded that it did indeed play a role in the persistence of the anomalous SSTs. However, they also found that further north in the NE Pacific (40–50°N and 140–160°W) from January 2013 to December 2013, there was no evidence of a positive SST‐cloud feedback during the onset of the MHW (Myers et al, ). The NE Pacific MHW offers a convenient natural experiment to study whether or not a SST‐cloud feedback helped maintain the MHW in the midlatitudes—a region that was not been the focus of most previous work.…”

“…The NE Pacific 2013/2016 MHW (hereafter the NE Pacific MHW) is one of the most well‐studied MHW events (Bond et al, ; Di Lorenzo & Manuta, ; Gentemann et al, ; Hartmann, ; Hobday et al, ; Jacox et al, ; Myers et al, ; Oliver, Perkins‐Kirkpatrick, et al, ), making it a good case study for understanding the maintenance mechanisms of MHWs. A MHW is defined as a period when SSTs exceed an upper threshold, typically the 90th percentile relative to local climatology, for 5 days or longer (Hobday et al, ).…”