Warm waters back at the Jersey Shore + revisiting the upwelling

Now over a week ago, we posted about the cold water temperatures observed at many New Jersey beaches, due to SW winds persisting for almost 3 weeks and leading to intense coastal upwelling. Water temperatures reached a reported low of 55°F at the Atlantic City Steel Pier, compared to average temperatures of about 69-70°F.

However, those relatively frigid water temperatures are mostly gone. The sea surface temperature (SST) image on July 18 from Rutgers below shows all the blues along the NJ coastline gone, replaced with warmer, near 80°F water.

The coastal upwelling “disappeared” because the persistent SW winds relaxed over the past week or so. The figure below shows a feather plot of winds from June 1 to July 17, 2013 from the Tuckerton, NJ WeatherFlow station. Winds are plotted in direction towards. I denoted with the light blue vertical lines the start and end of the upwelling period, and the peak in the upwelling event in darker blue on July 7. The upwelling correlates very well with the period of strong SW winds. After July 13, SW winds relax and even rotate to the north or northwest at times, producing a downwelling circulation that strongly counteracted the upwelling.

Compare this time series of winds to last year over the same period, in the figure below. Last year, there were really no persistent periods of SW winds that would have produced any strong upwelling events. Several times N or NW winds were observed.

For a moment, let us explore some of the modeling that is being conducted at Rutgers. The figure below shows a new SST product developed by the COOL group at Rutgers; this product captures the cold upwelled water not only along the NJ shore but upwelling occurring south of Long Island, east of the Delmarva Peninsula, and even east of Virginia and northern North Carolina. This image is from July 8, right around the peak of the upwelling event.
 Next, we turn to a global SST product that is readily used as input into regional weather models, such as the North American Mesoscale (NAM) weather model. This SST product shown below (for the same date) is called the Real-Time Global (RTG) SST and has a horizontal resolution of about 9.25 km. Compare this to the Rutgers SST product which has a 2 km horizontal resolution. Notice that the RTG SST product does NOT capture the coastal upwelling in the Mid Atlantic Bight like the Rutgers SST product does. This could be due to lower resolution as stated above, or more likely the fact that the product tags and removes any cold water as clouds, thus removing any upwelling centers. In contrast, the Rutgers SST product retains upwelling centers by using a coldest dark pixel compositing technique, because light pixels are known to be clouds whereas dark pixels are known to be the ocean. Sea surface temperatures (and SST gradients) have a significant impact on resulting near-surface winds, such as the sea breeze circulation. The figure below is weather radar reflectivity in Clear Air Mode from the Fort Dix radar in central NJ. On the figure, I have denoted a sea breeze front with a dashed line that was propagating inland just west of Long Beach Island at about 5 pm local time on July 9.
 The question remains: what happens when a model run is performed with the Rutgers SST product, and then with the RTG SST product? We can answer this simply by comparing the operational NAM forecast (12km horizontal resolution) to the Rutgers University version of the Weather Research and Forecasting (RUWRF) model run (3km horizontal resolution) at the same time. NAM uses RTG SST without the upwelling and RU-WRF uses Rutgers SST with the upwelling.

The figure below shows winds at 10m from the RUWRF model run at the same time as the weather radar image shown above. Again, I have indicated with a dashed line the model’s representation of the sea breeze front, which shows good agreement with the weather radar image at that time. The RUWRF model does an excellent job at reproducing the inland propagation of the sea breeze front on this day, which was likely affected by the cold coastal waters from the upwelling event.
 What does the NAM model show for this same time? The figure below plots NAM 10m winds at this time. No sea breeze front over NJ is apparent in this model run, nor any hours before or after. It is clear that the combination of a lower resolution weather model (NAM) with an SST product that doesn’t resolve upwelling (RTG) produces a less realistic simulation of the sea breeze circulation over NJ, at least for this case.
Finally, will coastal upwelling and cold surface waters at the beach return to NJ, or will waters remain warm for beachgoers for the weekend? Winds are already beginning to turn back to the SW, and forecasts indicate SW flow for Friday and Saturday and into Saturday night, when they shift to the W for Sunday and eventually to the E into Monday and Tuesday of next week. So a weak upwelling event could return for the weekend but it looks like it won’t last long into next week.

-Greg Seroka